There are many technical and economic challenges associated with the commercialization of new materials technology. This is particularly true for thermoelectrics for mid- to high-temperature waste heat recovery applications, be it for stationary or vehicle implementation. In this talk I will outline some of the thermomechanical performance requirements for the thermoelectric materials and modules for automotive applications. I will describe the research progress of our materials development team in terms of thermoelectric performance improvement, module development and conversion efficiency. I will also discuss recent developments in the application of melt-spinning in combination with spark-plasma-sintering as a potentially high throughput method for the production of skutterudite materials. The thermoelectric performance and mechanical properties of materials made by this method will be high-lighted. The work to be described here is a part of a broad R&D effort to demonstrate the commercial viability of a high temperature capable thermoelectric generator for fuel economy improvement in a passenger vehicle.

There is an increasing need for recovery and utilization of waste heat and much attention is now focused on a thermoelectric (TE) effect as a solution technology. To find efficient TE materials is important, however, it is difficult because the multiple parameters (Seebeck coefficient:S, electric resistivity:ρ and thermal conductivity:κ) are involved and these parameters are conversely related to each other in terms of the carrier density. Therefore, the novel scenarios which can break the trade-off among the TE parameters are seriously needed. An off-diagonal TE effect is focused as a new trend of thermo-electric conversion, which essentially develops in the tilted multilayer. In this material, we can see the unique relation between vertical ΔT and consequent transverse TE voltage. This fact manifests that, with this scheme, we can control the heat and the current flow independently. Recently we have reported the TE power generation in a tubular thermoelectric generator (TTEG) made of the tilted Bi0.5Sb1.5Te3/Ni multilayer using the ΔT between tube inside and outside based on the off-diagonal TE effect.1)
Here, we report the breaking of the trade-off between ρ and κ not by the conventional carrier doping but by the material porosity and layer tilting angle of the layers. By introducing the porosity into the Ni layer, we can independently reduce the κ without giving the negative effect on the transverse current flow. Furthermore, by tuning the tilt angle appropriately, we can make the favorable situation for TE property of thermally insulative and electrically conductive. As a result, we experimentally observed the breaking of trade-off between ρ and κ and significant improvement of generated power by a factor of three (7.9 W) under the small temperature difference of ΔT = 85 K near room temperature. These results reveal the resolving the well-known conflict between ρ and κ and provide the new guiding design concepts for the effective TE materials based on the off-diagonal TE effect. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan.
1) A. Sakai et al, J. Electron. Mater. DOI: 10.1007/s11664-012-2355-4 (2012)

Solar thermophotovoltaic (STPV) devices have the potential to overcome the Shockley-Queisser limit by converting solar radiation to a narrow-band thermal emission matched to the spectral response of a photovoltaic (PV) cell [1], [2]. However, limiting the influence of non-idealities of the individual STPV components (absorber/emitter/PV) through an understanding of the highly coupled energy conversion process is needed to approach high efficiencies. Although reported TPV efficiencies (thermal-to-electric) have exceeded 10% [3], measured overall STPV conversion efficiencies are below 1% [4], [5]. One of the most prohibitive aspects of this system can be the thermal transfer efficiency due to the high temperature operation of the device.
In this work, we designed an experimental system for characterizing high-temperature planar STPVs, schematically shown in Figure 1, aimed to bridge the gap between potential and measured STPV performance. We show how increasing the emitter-to-absorber area ratio (AR) can compensate for non-ideal spectral selectivity on the absorber side. This concept was implemented in planar devices through a process of seeded growth of vertically aligned multi-walled carbon nanotube forests on smooth tungsten surfaces (shown in Figure 2). The TPV-side of the system is composed of a one-dimensional Si/SiO2 photonic crystal emitter (1D PhC) paired with a low band-gap PV cell (InGaAsSb). With a 4:1 emitter-to-absorber area ratio (AR 4), we experimentally demonstrate a two-fold increase in thermal transfer efficiency (relative to AR 1) and a significant boost in STPV performance leading to efficiencies exceeding 2%.
[1] H. Nils-Peter and W. Peter, "Theoretical limits of thermophotovoltaic solar energy conversion," Semicond. Sci. Technol. , vol. 18, no. 5, pp. 151-157, 2003.
[2] W. Shockley and H.J. Queisser, "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells," J. Appl. Phys., vol. 32, no. 3, pp. 510-519, 1961.
[3] C.J. Crowley, N. A. Elkouh, S. Murray, and D. L. Chubb, "Thermophotovoltaic Converter Performance for Radioisotope Power Systems," in AIP Conf. Proc., 2005, vol. 746, pp. 601-614.
[4] A.S. Vlasov et al., "TPV Systems with Solar Powered Tungsten Emitters," in AIP Conf. Proc., 2007, vol. 890, pp. 327-334.
[5] A. Datas and C. Algora, "Development and experimental evaluation of a complete solar thermophotovoltaic system," Prog. Photovolt: Res. Appl., Available: doi: 10.1002/pip.2201.

Thermoelectric energy recovery is an important technology for recovering waste thermal energy in high temperature industrial, transportation and military energy systems. Thermoelectric (TE) power systems in these applications require high performance hot-side and cold-side heat exchangers to provide the critical temperature differential and transfer the required thermal energy to create the power output. Hot-side and cold-side heat exchanger performance is typically characterized by hot-side and cold-side thermal resistances, Rh,th and Rc,th, respectively. The heat exchanger performance determines the hot-side temperature, Th, and cold-side temperature, Tc, conditions when operating in energy recovery environments with available temperature differentials characterized by exhaust temperatures, Texh, and ambient temperature, Tamb. This work focused on analytically defining a crucially important design relationship between (P/Pmax) and (Rh,th / Rc,th) in TE power generation systems to determine the optimum ratio of (Rh,th / Rc,th) maximizing TE system power. A sophisticated integrated TE device / heat exchanger analysis was used, which simultaneously integrates hot- and cold-side heat exchanger models with TE device optimization models incorporating temperature-dependent TE material properties for p-type and n-type materials, thermal and electrical contact resistances, and hot side and cold side heat loss factors. This work examined the (P/Pmax) versus (Rh,th / Rc,th) relationship for system designs employing single-material TE couple legs and segmented TE couple legs with various TE material combinations, including bismuth telluride alloys, skutterudite compounds, and skutterudite / bismuth telluride segmented combinations. This work defined the non-dimensional functional relationships and found the optimum thermal resistance condition:
(Rh,th / Rc,th)opt → infin;
created the maximum power output in TE optimized designs for various TE material combinations investigated. These functional relationships were investigated for various electrical contact resistances, differing thermal loss factors, and at various hot-side/cold-side temperature conditions. This work showed that the non-dimensional functional relationships were invariant under these differing conditions. It also was determined that a condition of (Rh,th / Rc,th) = 1 creates power output far below the maximum power condition. The (P/Pmax) - (Rh,th / Rc,th) relationship also dictated certain temperature profile conditions, defined by the parameter, (Th - Tc) / (Texh - Tamb), which were directly associated with design points in this relationship including maximum power points. The value of (Th - Tc) / (Texh - Tamb) was generally less than 0.5 at maximum power conditions in TE energy recovery designs using TE materials investigated here.

As the efficiency of thermoelectric materials continues to improve, there is potential to directly convert solar energy to electricity using solar thermoelectric generators (STEGs). Prior demonstrations of STEG technology have achieved limited efficiencies (~5%) due to the low Carnot efficiency from a small temperature drop and the limitations of the zT for the materials used.[1] Achieving STEG efficiencies that are competitive with existing solar energy conversion systems will require significantly larger temperature differences, and thus, the use of concentrated sunlight. Both the sunlight-to-heat conversion and the thermal minimization of deleterious heat loss must be optimized in the same system. By combining state-of-the-art TEGs developed at JPL with NREL&’s solar testing capabilities and Colorado School of Mines&’ STEG engineering, this project has modeled and designed a STEG prototype that should be capable of achieving 15% solar-to-electrical conversion efficiency.[2]
To produce a high-efficiency, high-temperature STEG, several state-of-the-art technologies must be integrated, such as optical management, a selective absorber and/or optical cavity, the TEG itself, and heat management. We will discuss an overview of the project, the design of the integrated system, as well as the fabrication of a prototype device and ‘on-sun&’ testing results from NREL&’s high flux solar furnace. Equally important for this technology is the development of a cost model to determine the economic competitiveness and possible application niches for STEG technologies. We will discuss the results of an economic analysis that has been carried out to understand the cost issues for large-scale manufacturing of thermoelectric materials towards electrical energy generation.
[1] D. Kraemer, et al., High-performance flat-panel solar thermoelectric generators with high thermal concentration, Nat. Mater., 2011, 10, 532-538.
[2] T. Caillat, et al., Progress Status of the Development of High- efficiency Segmented Thermoelectric Couples, Nuclear and Emerging Technologies for Space, 2012.

Misfit-layered cobaltite Ca3Co4O9 is considered as good p-type thermoelectric material in high temperature region (950 - 1100 K), while half-Heusler (HH) Ti0.3Zr0.35Hf0.35CoSb0.8Sn0.2 is high performance p-type material at temperatures below 950 K. In this work, oxide Ca3Co4O9 is segmented with HH using an electrically conductive adhesive and brazing joining technique. The thermoelectric properties of the component materials as well as the interfacial resistance at high temperatures were characterized as a function of temperature up to 1100 K, and the results are discussed in details.

The field of Thermoelectrics is increasingly contributing to the development of sustainable energy management. At present, scientists are more focused on thermoelectric (TE) material development, but the TE module design procedure is still in a relatively virgin state. A prevalent TE module design techniques is to introduce the same current through the p and n leg of the thermoelectric generator (TEG) and changing the current density of each element by changing the area of both legs. This technique leads to the TE module architecture for the most efficient configuration for both p and n legs. In the current paper we are applying this technique in a novel way to design a TE module producing higher power output and/or reducing the size of the module comparing to the values given by the original technique. Our studies indicate that for some combinations of p and n material properties, optima yielding significant material savings can be determined. Here we have done the study in the high temperature range, focusing more on power output than the efficiency of the TEG. A range of material properties were studied to identify how these affect the final power output and the module architecture of the TEG. This study indicates that the Figure of Merit of the materials is not the only or main concern when selecting a material for a TEG, but rather a more comprehensive evaluation of the individual material properties.

Zintl phases such as Yb₁₄MnSb₁₁ have been demonstrated as high performance high temperature thermoelectric materials with ZTs as high as 1.4 at 1275 K. The high ZT in this phase is attributed to inherently low glass-like lattice thermal conductivity due to its unique anionic covalent bonding and structural complexity. This system is a part of the larger family “14-1-11” Zintl phases with the A₁₄MPn₁₁ stiochiometry (A = Ca, Ba, Sr, La, Yb, Eu, M= Mn, Al, Cd, Ga, In, Zn, Pn=P, As, Sb, Bi). A significant amount of work has been conducted in order to improve upon the electronic properties of this system with very little improvement on the ZT of Yb₁₄MnSb₁₁ system. Preliminary results from both computational studies and recent experimental work on doping on the cation and metal site has revealed some insight in role of these elements in the structure and has led to methods of improving the electronic properties of the system by increasing the covalency of the anionic substructure via doping. Computational and experimental results on the 14-1-11 system will be presented and discussed.

One of the greatest challenges associated with optimizing thermoelectrics for use in waste heat recovery is the interconnectivity between the thermal conductivity (k), electrical conductivity (σ), and Seebeck coefficient or thermopower (α) in these materials. Nanostructuring candidate thermoelectric material systems offers a method for decoupling these properties, allowing the ability to adjust these parameters independently. In this talk, we will present our approach to developing p-type materials for optimized thermoelectric efficiency through fabrication of composites consisting of single-walled carbon nanotubes (SWCNTs) dispersed in a polymer matrix. Polymer-based composites offer benefits over the highest performing inorganic thermoelectrics including lower cost, material flexibility, simple fabrication techniques, and decreased toxicity. We will discuss fabrication and post-fabrication treatments of the SWCNT thin films and polymer:SWCNT composites and the benefits offered by nanostructuring these architectures to optimize the dimensionless figure-of-merit, ZT. Finally, we will present the suspended membrane technique we use to measure all three properties (σ, α, and k) that define the ZT of these nanostructured constructs in a unified measurement geometry^1-3.
1. Rubina Sultan, A.D. Avery, G. Stiehl, and B.L. Zink, Journal of Applied Physics105, 043501 (2009).
2. B.L. Zink, A.D. Avery, Rubina Sultan, D. Bassett, and M.R. Pufall, Solid State Communications150, 514-518 (2010).
3. A.D. Avery, Rubina Sultan, D. Bassett, D. Wei, and B.L. Zink, Physical Review B Rapid Communications83, 100401(R) (2011).

: Progress is summarized for the US Department of Energy Supported Automotive Thermoelectric Generator and Thermoelectric Air Conditioner/Heater Projects The Department of Energy initiated the application of thermoelectric generators (TEGs) to vehicles in 1994. This TEG was built by Hi-Z Technologies evaluated on a dynamometer test stand then tested successfully installed on a fully loaded Heavy Duty Diesel truck on the PACCAR test track for the equivalence of 550,000 miles. Today every major automobile manufacturer is investigating thermoelectric applications. The US Department of Energy is supporting the development of production prototype TEGs with teams headed by Gentherm and GM to integrate TEGs to directly convert engine waste heat to electricity in the BMW X6, the Ford Lincoln MKT and the Chevy Suburban. These first generation TEGs will provide a nominal 5 percent improvement in on-highway fuel economy by allowing the alternator to be downsized by at least 1/3. The 2nd generation TEG is planned to replace the alternator and provide a nominal 10 percent improvement in fuel economy.
DOE/NETL conducted a competitive procurement for automotive thermoelectric air conditioners/heaters (TE HVAC) development and selected teams headed by Ford and GM.. Current air conditioners use the R134a refrigerant gas, which produces 1300 times the "Greenhouse Gas Effect" of carbon dioxide (CO2), the primary "Greenhouse Gas". Approximately 41 Million Metric tons of CO2 equivalent (CO2e) are released to the atmosphere in the US annually from air conditioner The TE HVACs are candidates to eliminate refrigerant gases from vehicles. Preliminary analysis indicates that with TE HVAC a single occupant can be made comfortable using about 630 Watts whereas current compressed refrigerant gas air conditioners typically use 3,500 to 4,500 Watts. The TE HVAC uses design advantages afforded by Thermoelectrics as a dispersed or zonal system wherein only the occupants are cooled/heated, not the whole cabin. As TE HVAC is a DC electrical system it only requires a switch to go from the cooling mode to heating. The Zonal System will consist of a thermoelectric seat, thermoelectric units in the overhead and dashboard, A&B pillars focused on each occupant. There will be a cooling loop with a dedicated radiator. TE HVAC is design is vehicle specific. In this program there are the Cadillac SRX, the Chevy Volt and the Ford Fusion. The latter 2 will also have TEGs.

The non-toxic and light-weight (~2.0 g/cm3) material Mg2Si is one of the dominant candidates for a thermoelectric (TE) material that could enable devices to operate at temperatures ranging from 500 ~ 900 K. In order to realize an Mg2Si TE power generator, information about its mechanical properties such as strength, elastic moduli and hardness are required, in addition to a study of its thermoelectric capabilities. The current status for Mg2Si is aimed toward TE module fabrication and appropriate system integration techniques for automotive applications. The basic mechanical properties of Mg2Si, such as Young&’s modulus, bending strength and fracture toughness, are needed in order to design TE modules and TEG systems. For the fabrication of practical TE modules, Sb-doped Mg2Si is known to be durable and offers ZT values that are tentatively sufficient at elevated temperatures, but it will be necessary to incorporate a metallic binder for the scalable fabrication of sintered pellets because of the difficulties that occur in sintering when Sb is incorporated.
In this study, we examine 3- or 4-point bending tests, ultrasonic tests, and nanoindentation tests on Mg2Si specimens of (i) Sb-doped sintered pellets for mass-production processes (ii) Sb-doped sintered pellets with the incorporation of a metallic binder, (iii) co-doped sintered pellets. Regarding mass-production processes for sintered pellets, specimens with diameters of more than 50 mm have already been achieved, and have been advanced to TE chip fabrication, though homogeneous TE characteristics and mechanical properties are needed to ensure spatial uniformity of the sintered pellets. For specimens incorporating metallic binders and multiple dopants, over-dosage can lead to a decline in the mechanical toughness, although mixing of metallic binders such as Cu, Zn and Ni and double doping with Sb+Zn were effective in enhancing ZT values up to unity in previous works. For the cases (i) to (iii) mentioned above, we report here the special mechanical properties of sintered pellets in terms of tensile strength, Young&’s modulus and Vickers hardness with reference to the TE characteristics of the Seebeck coefficient, electrical conductivity, and thermal conductivity.

Lead telluride-based materials continue to be amongst the best thermoelectric materials for mid-to high-temperature power generation applications. However, concerns regarding lead toxicity and tellurium abundance have led us to consider alternate materials with properties similar to the lead-based compounds. The chalcopyrite semiconductor family consists of several compounds of general chemical formula I-III-VI2 or II-IV-V2, where the roman numerals indicate the respective elemental groups in the periodic table. This family spans a wide range of electronic band gaps (ranging from half to one and a half electronvolts), and some preliminary studies indicate thermoelectric figure of merit in excess of unity at 850K. Here we present results of our investigations of the thermoelectric properties of CuInTe2, CuGaTe2, and solid solutions of these compounds doped with Zn. Samples were synthesized by vacuum melting of stoichiometric ratios of the starting elements, followed by annealing. Sample purity was checked by x ray diffraction, and when verified the samples were ball milled in a high energy vibratory mill and hot pressed in an argon atmosphere. Final densities were measured by the Archimedes' method to assure sufficient densification. We observe a strong decrease in resistivity with the addition of Zn, indicating efficient p-type doping by this element, accompanied by an increase in thermoelectric power factor. This is verified by measurements of carrier concentration with a standard 5 probe hall measurement. Low temperature thermal conductivity was also slightly reduced with zinc doping, and substantially reduced with gallium substitutions, giving an increase in ZT at room temperature from both processes. High-temperature results for the thermal conductivity and thermoelectric figure of merit for these alloys will be reported. We have also synthesized compositions in the I-III-VI2 family using Se and S on the Te site and Fe on the In/Ga site. In particular, compounds of composition CuFeS2, which is that of the natural mineral chalcopyrite, are of interest for earth-abundant alternatives to traditional thermoelectric materials, and our efforts to synthesize and optimize these compounds will be presented.

Sustainability and waste heat recovery has gained public & scientific communities&’ attention for the past decades to reduce the impact of fossil fuels and their byproducts as well as fluctuations in oil prices due to the crises in the Middle East. Thermoelectric (TE) materials are among the alternative energy harvesting materials intensively studied that are able to directly interconvert between heat and power/electricity, having no moving parts and no //low carbon footprint. In the intermediate temperature region of 300-600 oC skutterudites, in particular CoSb3 based TE materials have relatively high TE figure of merit for energy harvesting/power generation. To commercially mass produce TE devices for energy harvesting p- and n-type material are needed with best matching chemical and mechanical characteristicsqualities. qualities. Iron is one of the promising dopant candidates for p-type skutterudite material, therefore a series of skutterudite samples with the general stochiometricgeneral formula FexCo1-xSb3 are investigated. To improve the power factor - and the TE figure of merit, further compositions with the general formula of NiyFexCo1-x-ySb3-zTez have also been fabricated and studied.
The investigated materials have been synthesized via a solution chemistry method, which is rather promising for large-scale synthesis of TE nanopowders. The method involves chemical co precipitation of the iron, cobalt, and nickel, antimony and tellurium precursor compounds of iron, cobalt, and nickel, antimony and tellurium. Further thermochemical process of calcination and reduction were performed to produce the desired chemical composition. For materials characterization, XRD, SEM and EDX analysis have been performed to investigate their phase purity as well as chemical composition. Temperature dependent transport property evaluations have been performed and presented in detail along with the structural and physiochemical characterization results.
Acknowledgments: This work has been funded by EC-FP7 program under NEXTEC project and in part by the Swedish Foundation of Strategic Research - SSF.

Recently, our group has demonstrated that semiconducting ribbons with nanometer-sized lamellar structure consist of silicon and chromium silicide (CrSi₂) nanocrystals could be obtained by melt spinning of alloys with eutectic composition, Cr_0.149 Si_0.851. The size of the domain of the lamellar structure is about 12-30 nm. The thermoelectric property of the ribbon was compared with that of the alloy with micrometer-sized lamellar structure prepared by arc melting. The thermal conductivity of the ribbon reduces from 35 to 12 W/mK at room temperature. The power factor values of these samples, on the other hand, were nearly equal to each other. This result indicates that fabricating nanometer-sized lamellar structure in thermoelectric materials is a promising method to achieve high thermoelectric figure of merit values.
The objective of this study is to fabricate Si-based high performance bulk thermoelectric materials assembled from the lamellar nanocrystals by using a spark plasma sintering (SPS) method. During conventional sintering, grain growth occurs and it is a real challenge to keep the nano-sized structures after the sintering stage. To overcome such undesirable grain growth behavior, a number of studies have been conducted. Recently, many studies experimentally found that sintering at high pressures and low temperatures can yield full densification of nanocrystalline powders with minimal grain growth. Hence we apply the high pressure sintering technique to compact the nanostructured ribbon without significant grain growth.
The starting ribbons were prepared by melt spinning of the alloys of which composition was Cr_0.149 (Si_0.99B_0.01)_0.851 (B was added as the acceptor dopant). The ribbons were sintered by SPS at normal condition (150 MPa and 940 °C) and high pressure condition (330 MPa and 600 °C). To evaluate the structural change after sintering, we observed the samples by scanning electron microscopy. After the sintering of the normal condition, we observed significant grain growth of Si and CrSi₂ crystals. In contrast, the nano-ordered lamellar structures were preserved in the pellet sintered at the high pressure condition. The thermolectronic properties of these pellet will be discussed.

Intermetallic clathrates based on the group 14 elements Si, Ge, and Sn are known to have a high potential as thermoelectric materials. This study focuses on a subset of such compounds in the systems A-Cu-Sn and A-Zn-Sn (A = K, Rb, Cs). The two new series of tin-based clathrates have formulaeA₈Cu_2.67Sn_43.33 and A₈Zn₄Sn₄₂ (A = K, Rb, Cs), respectively, and crystallize in the cubic type-I structure (space group Pm-3n). The framework of these cage compounds is made up by 46 atoms on the three sites 24k, 16i and 6c. Both 24k and 16i sites are exclusively occupied by tin, while the 6c site is where the statistical substitution of Sn with Cu or Zn metal atoms occurs. The cages in the structure are filled completely with alkali metals. Structure refinements based on single x-ray diffraction data indicate that in the larger tetrakaidecahedral cages, the larger Cs atoms remain at the center, whereas the smaller K and Rb atoms are displaced off-center. Thermoelectric property measurements were carried out on single crystals of A₈Cu_2.67Sn_43.33 with A = K and Cs and A₈Zn₄Sn₄₂ with A = K and Rb. The results of temperature depending measurements of the Seebeck coefficients (300 - 500 K) and electrical resistivity (100 - 400 K) will be presented and compared.

We report the thermoelectric properties of Pb doped Mg3Sb2 (Mg3Sb2-xPbx; 0 le; x le; 0.3) Zintl compounds, in wide range of temperature from 323 to 773K. These compounds were synthesized using a single-step process, reaction sintering employing Spark Plasma Sintering of the blended powder of their constituent elements in stoichiometric proportion. A single-phase solid solution of Mg3Sb2-xPbx for x < 0.3 was observed. However, with increasing Pb concentration in Mg3Sb2 (Mg3Sb1.7Pb0.3), we see precipitation of Pb in Mg3Sb2 matrix which is due to its limited solubility. Temperature dependent behaviour of electrical conductivity of Mg3Sb2-xPbx (0 le; x le; 0.3) shows an increasing trend with temperature, which is a typical characteristic of a semiconductor. The electrical conductivity of single-phase Mg3Sb2-xPbx increases monotonically with Pb doping and exhibiting a semiconductor to metal transition at x=0.3. Despite an increasing electrical conductivity due to Pb doping, no significant decrease in Seebeck coefficient was noticed. Interestingly, doping by heavy metal Pb in Mg3Sb2 Zintl compound promotes reduction in thermal conductivity and thereby increases the thermoelectric figure-of-merit to a value of 0.83 at 773K in Mg3Sb1.8Pb0.2 compound. The reduction in thermal conductivity has been attributed to mass fluctuations and grain boundary scattering. The granular structure is revealed by transmission electron microscopic imaging. The relatively high value of thermoelectric figure-of-merit, abundance of Mg, Sb and Pb combined with the one single-step synthesis designed by us could make this material a promising cost-effective option for power generation from waste heat.
*Corresponding author: [email protected], [email protected] (DKM)

Among the potential materials for thermoelectric applications, Higher Manganese Silicides (HMS) MnSix (with x around 1.75) exhibit interesting figures-of-merit at intermediate temperatures (573K to 873K). Moreover, the figure-of-merit can be improved by germanium doping [1]. The optimization of the elaboration of such alloys needs the knowledge of the ternary Ge-Mn-Si system and of its constitutive binaries.
The germanium-manganese system has been experimentally studied [2] but no Calphad description is available yet. After a critical review of the literature concerning the phase diagram and the thermodynamic properties, a thermodynamic description of the Gibbs energy of the phases is performed using the Calphad method. The liquid phase is described with an associated model and the variation to the stoichiometry of the solid phases is taken into account.

Mg₂Si and its doped compounds are well known as promising thermoelectric materials. Several research teams have investigated the Mg₂(Si,Sn) based ternary solid solutions and their thermoelectric properties, reporting high figure of merits (ZT). Nevertheless, the reports dealing with the in-homogeneities of these materials are scarce over the published literature. In-homogeneities may occur at different scale lengths; at atomic scale (as dopant and alloying), at nano-scale (as nano-inclusions and nano-crystals), at meso-scale (as grains of different composition) and at macro-scale (as dopant modulated structures and dopant grated materials). In the present study, a comparative study is attempted, dealing with the in-homogeneities monitored in the cases of Mg₂Si, Mg₂(Si,Sn) based ternary compounds as well as the quaternary solid solution based on Mg₂(Si,Sn,Ge). Structural in-homogeneities were monitored by using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) equipped with EDS analyzer, X-Ray diffraction (XRD) analysis as well as conventional Fourier transform infrared spectroscopy (FTIR) in the reflectivity mode. The Mg₂Si-Mg₂Sn alloys currently investigated are probably, in the equilibrium state, two-phase alloys with Sn-poor and Sn-rich phases. The presence of these phases was verified by SEM/EDS studies, indicating that samples present a mosaic-like structure as well as XRD analysis. The existence of the two phases was also confirmed by TEM as well as conventional IR studies. All TEM, SEM and XRD observations show Si-rich and/or Sn-rich areas in Mg₂Si_1-xSn_x system suggesting the appearance of a phase separation. Moreover, since in all three cases doped materials by either Bi or Sb were subjected to the present study, free carrier concentration becomes a key parameter, as it generally hinders the sample in-homogeneities caused by compositional (or) doping gradient, or local microstructure. Therefore, besides the aforementioned techniques, a non-destructive, reflection-based, mid infrared micro-spectroscopic mapping analysis was also applied, towards identification of position-sensitive structural in-homogeneities. This was achieved through the coupling of an infrared spectrometer to an infrared microscope and offers the unique opportunity of studying small samples with extended spatial resolution. Fitting analysis of the collected IR spectra yields, among others, the plasmon frequency, which was used as probe for carrier in-homogeneity and consequently doping differentiation based on its dependence on the carrier concentration. Micro FTIR identified the presence of dopant in-homogeneities in most cases of doped samples. Dopant in-homogeneities were also partially confirmed by SEM-EDS.

Dimagnesium silicide (Mg2Si) is an eco-friendly material useful for thermoelectric power generation using waste heat of temperature range (600-900 K). To improve the thermoelectric performance of the Mg2Si compound, we made the Al-added compounds under magnesium-rich condition (with 67.0 at% of Mg) using a liquid-solid phase reaction and using a pulse discharge sintering. The thermoelectric performance of each sample containing Al of 0 to 2.0 at% was measured during 50 h air exposure with temperature difference. The temperature difference was given by contacting the hot side of a sample with a hot plate held at 773 K and by contacting the cold side with a heat sink with cooling fan. The electrical resistivity of the Al-free sample increased with exposure time by internal oxidation. All the Al-added samples kept the low resistivity during the air exposure test. We confirmed the resistance to deterioration in thermoelectric performance of the Al-added samples during air exposure with temperature difference.

Mg2Si-based alloys have attracted much attention as they are composed of cheap, abundant, and non-toxic raw materials, have a high figure of merit and the lowest density amongst all efficient thermoelectrics. Mg2Si1-xSnx series has been found to be the most favorable in terms of thermoelectric energy conversion. Enhancement of the thermoelectric performance of Mg-Si-Sn ternary compounds can be accomplished by and adjustment of the Sn content, via thermal conductivity reduction, as well as the Bi doping content, via power factor increase. In this work, Mg2Si0.6minus;x-ySnxBiy (0le;xle;0.4 and 0le;yle;0.05) series were fabricated by a two-step solid-state reaction method, followed by hot pressing. The materials were characterized by Seebeck coefficient, electrical and thermal conductivity at temperature range of 300-800K. The carrier concentration was also followed by Hall Effect measurements. The thermoelectric performance in this system is high and ZT value in the range of 1.3-1.4.

Alkali bismuth chalcogenides are promising for thermoelectric applications as suggested by previous works. The K2Bi8Se13 compounds were grown from melt as polycrystalline ingots or from Bridgman technique as highly-oriented polycrystalline ingots. However, even if high ZT is achieved, the mechanical properties of such ingots can be major problem on the thermoelectric devices assembly. Powder techniques have been recently applied on K2Bi8Se13-xSx series in order to fabricate, for the first time, hot pressed pellets. The pellets had high density and performed ZT of about 0.5 at 800K.
In this work, we apply powder techniques on K2Bi8Se13-based materials. The materials were ball milled and hot-pressed in order to fabricate pellets of high density and mechanical strength. The hot pressing conditions were selected based on the previously applied statistical Design of Experiments optimization approach. The effect of the ball milling and the sintering process on the thermoelectric properties is discussed.

Thermoelectric (TE) generators can directly generate electrical power from waste heat, and thus could be an important part of the solution to future power supply and sustainable energy management. The efficiency of TE materials is quantified by a dimensionless figure of merit ZT. To enhance ZT, it is important to reduce the lattice thermal conductivity with maintaining the high electrical conductivity. Recently, cage like compounds with phonon-glass and electron-crystal properties such as filled-skutterudite compounds have attracted attention as high-performance TE materials. In these compounds, the guest atoms in the cages of the host framework lattice rattle and reduce the lattice thermal conductivity. Our group has focused on Gd₅Si₃P_y which has a cage-like structure. It has been reported that the host framework compound: Gd₅Si₃ shows a metallic behavior, while the P-filled compound: Gd₅Si₃P shows a semiconductor behavior [1]. In the present study, the TE properties of polycrystalline bulk samples of Gd₅X₃Sb_y (X = Si, Ge, y = 0, 0.2, 0.4, 0.6, 0.8, and 1) were examined. The host framework compounds: Gd₅Si₃ and Gd₅Ge₃ showed a metallic behavior with relatively low thermal conductivity; at room temperature the thermal conductivity values were 4.00 and 3.66 Wm^-1K^-1 for Gd₅Si₃ and Gd₅Ge₃, respectively. The TE properties not only of the host framework compounds but also of the Sb-filled compounds will be discussed.
[1] M. Yahia et al., J. Solid State Chem., 179 (2006) 2779.

Filled skutterudites are the most promising candidate materials for thermoelectric (TE) applications in the temperature range around 500-800 K. Recent years, CoSb3-based skutterudites filled by group 13 elements (Ga, In, or Tl) have received much attention due to their excellent TE performance. In particular, the n-type Tl-filled one: Tl0.25CoSb3 exhibits ZT = 0.9 at 600 K, due to its remarkably reduced lattice thermal conductivity [1]. In comparison with the n-type skutterudites, the p-type skutterudites show low ZT. Therefore, p-type skutterudites with high ZT should be developed. Recently, our group has reported that the Tl-filled CoSb3-based skutterudites in which one fourth of Co site is substituted by Fe, i.e. TlxFeCo3Sb12 show p-type behavior with relatively high ZT of 0.36 at 723 K [2]. Although the Fe/Co substitution leads to p-type behavior, the Fe/Ni combination might be more attractive than the Fe/Co combination in the view point of material&’s cost. In the present study, polycrystalline samples of TlxFe2.5Ni1.5Sb12 (0 le; x le; 1.0) were synthesized and the TE properties were examined in the temperature range from room temperature to 773 K. The phase states of the samples were identified by XRD and FE-SEM/EDX analysis at room temperature. All samples contained the skutterudite phase as the major phase. In the samples of 0 le; x le; 0.4, the orthorhombic (Fe,Ni)Sb2 phase and elemental Sb existed as minor phases in addition to the skutterudite phase. On the other hand, the samples of x ge; 0.6 contained almost no impurity phases. All samples exhibited relatively low thermal conductivity. The thermal conductivity decreased with increasing the Tl content. For example room temperature values of the thermal conductivity of Fe2.5Ni1.5Sb12 and Tl0.6Fe2.5Ni1.5Sb12 were 3.88 and 2.02 Wm-1K-1, respectively. The effect of the Tl content on the TE properties of the samples will be discussed in detail.
References
[1] A. Harnwunggmoung, K. Kurosaki, H. Muta, and S. Yamanaka, Appl. Phys. Lett. 96, 202107 (2010).
[2] Donghun Kim, Ken Kurosaki, Yuji Ohishi, Hiroaki Muta, and Shinsuke Yamanaka, APL materials, in press (2013).

The nonstoichiometric SmS_x compound (x=0.842-1.333) can be synthesized via a direct reaction between Sm metal and sulfur powder. Figure of merit (ZT) value of SmS_x increases with decreasing x in SmS_x. ZT value of SmS_x reaches the maximum of 0.863 for SmS_0.965 ⁽¹⁾ .
In this study, SmS_x powder was synthesized through a solid phase reaction between Sm₂S₃ and SmH₃, which are more stable and tractable. Then, SmS_x sintered compacts were fabricated by pulse electric current sintering of the obtained SmS_x powder. Commercially available α-Sm₂S₃ and SmH₃ powders are mixed in different molar ratios to synthesize SmS_x powder with various compositions(x = 0.667 - 1.071). The powder mixture is then placed in a BN crucible and heated in vacuum of 4×10 ^-3 Pa at 1273 K for 3 hrs. Next, the obtained SmS_x powder is sintered by a pulse electric current sintering apparatus in vacuum (7×10 ^-3 Pa) for 1 hr, at an applied pressure of 50 MPa, and temperature ranging from 1273 to 1773K.
XRD analyses indicated that the obtained powders contained no free Sm and S, even when the Sm ratio was higher than that of the stoichiometric composition. When the mixing ratio of Sm to S was 48.3 : 51.7 (SmS _1.071), the obtained powder was identified to be the mixture of SmS and Sm₃S₄. In Sm rich region (x lE; 1), the characteristic peaks of Sm₃S₄ were not observed. However, each obtained SmS_x powder contained a trace amount of Sm₂O₂S. In XRD patterns of sintered compacts, semiconductor-, metallic - phase SmS_x and a trace amount of Sm₂O₂S were detected. Furthermore, the crystal grain size of sintered compact tended to increase with an increase in the Sm ratio. The power factor was measured at temperature ranging from room temperature to 600 K. The results revealed that the power factor increased with an increase in the Sm ratio, reaching 580mu;W K^-2 m^-1 for SmS_0.667 sintered compact.
(1) A.Golubkov et.al., Inorganic Materials, 39(2003), 1251-1256.

The thermoelectric properties of Th₃P₄ - type La₂S₃ (γ - La₂S₃) single crystal depend on its composition. For example, the figure-of-merit (ZT) of LaS_1.465 reaches 0.29 at 1200 K ⁽¹⁾ . Furthermore, ZT of LaS_1.48 polycrystal reported to be 0.64 at 1273 K ⁽²⁾ . γ - La₂S₃ single crystal with a melting point of 2368 ± 30 K was synthesized by using the Bridgman method. In order to prevent the dissociation of sulfur, the crystal growth of La₂S₃ was conducted in a sulfur gas stream. However, its high melting point requires expensive apparatus and high energy cost. Therefore, we attempted to reduce the growth temperature of La₂S₃ crystal. In this study, we employed a flux method to fabricate γ - La₂S₃ single crystal. SnS (melting point: 1153 K) was used as the flux because it does not react with a rare earth sulfide to form intermediate products ⁽³⁾.
2 g of a mixture of SnS and β - La₂S₃ in a molar ratio of 0.96 : 0.04 were placed in a high-density graphite crucible with a smooth surface. This crucible was put into the one end of a quartz ampoule (diameter: 25 mm ; length: 140 mm). The ampoule was sealed in vacuum (< 4.0×10^-4 Pa) and heated at 1373 K for 12 hrs to homogenize the La₂S₃ - SnS melt. Subsequently, the temperature at another side of the ampoule was decreased to 1343 K and this condition was maintained for 3 - 6 days. After heating, the ampoule was cooled down to room temperature in a furnace.
In order to elucidate the growth mechanism, La₂S₃ - SnS melt after 3days was quenched. Reddish needle - like crystals (length: ~4.2 mm) and angular crystals (length: ~1.5 mm) were observed on the surface of the quenched - flux lump. Cross-sectional observations of the quenched-flux lumps revealed that these crystals grew from the surface to the inside of the lumps. EDX analysis revealed that molar ratio of La : S : Sn in the reddish crystal was 1 : 1.55 : 0.01. After 6 days heating, only reddish crystals remained in the carbon crucible. Single-crystal X-ray diffraction analysis revealed that reddish crystals took tetragonal β - La₂S₃ (La₁₀S₁₄O) structure.
We have previously reported the synthesis of γ - La₂S₃ single-phase sintered compacts from a mixture of β - La₂S₃ and 2wt% Ti powder by using a pulsed electric current sintering apparatus ⁽⁴⁾. In this study, we attempt to synthesize γ - La₂S₃ single crystals by adding a small amount of TiS₂ powder into SnS flux.
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Recently exceptional thermoelectric properties have been discovered in the oxyselenide BiOCuSe mainly as a result of its “soft” chemical bonds and layered structure. The performance can be effectively optimized either by introducing Cu vacancies or by doping alkaline-earth elements. A series of Cu-deficient BiOCuδSe (0.925 le; δ le; 1.0) have been synthesized by conventional solid-state reactions in sealed quartz tubes and then characterized by powder X-ray diffraction, high-resolution electron microscopy, synchrotron X-ray absorption spectroscopy at various edges and electrical transport measurements. It is found that the conductivity increases with the concentration of Cu vacancies, and thanks to the descent Seebeck coefficient, an amazingly high power factor exists in BiOCu0.950Se 80 mu;W/cmK2 around 45 K. A side-by-side comparison between these phases and doped (Bi1-xBax)OCuSe (0 le; δ le; 0.15) will be presented.

The binary narrow band gap semiconductor PbSe combines several attractive features for potential thermoelectric applications such as a favorable electronic valence band structure in similarity to that of PbTe. PbSe is much less studied compared to PbTe even Se is more abundant and has longer-term price stability than Te. We have investigated fundamental physical properties of PbSe-based systems as candidates for high ZT thermoelectric materials for power management through use of wasted heat sources. We present a systematic study with respect to the synthesis, characterization and thermoelectric effects of Sb in the lattice of PbSe with PECS samples for introducing mesoscale-structured grains. The experimental data indicate that not only the donor action of Sb is very effective but also addition of few percent of Sb into PbSe generates nanoprecipitates that confirmed by TEM. These meso- and nano-structures produce a large reduction of the lattice thermal conductivity compared to pristine PbSe. The electrical conductivity, thermoelectric power, thermal conductivity, and charge carrier density as a function of temperature were measured. High ZT values were achieved at high temperature.

The high temperature thermoelectric (TE) figure of merit (ZT) of Cu2Ga4Te7 has been reported recently. However, the TE properties of Cu2In4Te7 with the same defect zinc-blende structure (space group : F-43m) with Cu2Ga4Te7 have scarcely investigated. Here, we show the high temperature TE properties of Cu2In4Te7. Polycrystalline high density bulk samples of Cu2In4Te7 were prepared by direct reaction of Cu2Te and In2Te3. The TE properties , i.e. , electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were examined in the temperature range from room temperature to 700 K. Cu2In4Te7 exhibited relatively high thermoelectric figure of merit, ZT (= S2T/ρκ, where T is the absolute temperature) of 0.3 at 700 K, with the S, ρ, and κ values of +394 mu;VK-1, 62.1×10-5 Omega;m, and 0.61 Wm-1K-1, respectively.

Thermoelectric materials, which convert heat into electrical energy, may provide a solution to increased energy efficiency through waste heat recovery. To date, most commercial thermoelectric materials are too inefficient to be a viable option for most waste heat applications. This research proposes to investigate the fabrication and characterization of nanostructured III-V semiconductor thermoelectric materials to increase the performance of existing technology.
In order to improve thermoelectric material efficiency, either the lattice thermal conductivity must be lowered or the thermoelectric power factor must be increased. This research will focus on the latter by modifying the density of states of the semiconductor material and studying the effect of quantum confinement on the material&’s thermoelectric properties. Using focused ion beam milling, nanostructured cantilevers are fabricated from single crystal wafers. An all-around gate dielectric and electrode are deposited to create a depletion region along the outer core of the cantilever, thus creating an inner conductive core. The Seebeck coefficient can then be measured as a function of confinement by varying the gate voltage. This technique can be applied to various material systems to investigate the effects of confinement on their thermoelectric properties.

PbTe is a well-known high temperature thermoelectric system capable of reaching ZT=1.8 or higher when doped with appropriate secondary phases such as PbSe, PbS, or SrTe. Reaching high ZT values requires concurrent reduction of the lattice thermal conductivity via incorporation of nanoparticles to scatter phonons while simultaneously altering the band structure to enhance the power factor. Previous reports have described the band structure of the high ZT PbTe-PbSe system, but analysis of the matrix and evidence of nanostructuring is lacking since PbTe-PbSe is generally considered a solid solution. Herein we report on the nanostructuring of PbTe-PbSe systems previously unexamined in the literature. Transmission electron microscopy (TEM) and local electron atom probe (LEAP) tomography characterization of the nanoscale precipitates show the formation of nanoprecipitates with varying alloy composition in the (PbTe)_1-x(PbSe)_x system. The nanostructure and thermoelectric properties of Na doped p-type and Sb doped n-type doped samples are compared with the undoped solid solution to highlight the effects of different dopants in (PbTe)_1-x(PbSe)_x. The origin of the high ZT in these systems will be discussed with respect to the nanostructure of the system as well as the band structure.

In4Se3-x compound is a potential thermoelectric material for its comparably low thermal conductivity among all existing ones. While most studies focused on improving In4Se3-x thermoelectric properties by adjusting selenium or other dopants concentrations, in the current study, it was found that for a fixed initial In/Se ratio, the resulting In/Se ratio varied significantly for different melting and annealing duration times, which also resulted in surprisingly different thermoelectric properties as well as fracture surface morphologies. Specimens in this study were processed in a sequence of melting, annealing, pulverizing, and sintering. The elemental In and Se pellets were mixed at a fixed ratio and sealed in a quartz tube before melting and annealing. Different holding time (tmelt) during melting, and annealing duration times (tanneal) were employed to synthesize In4Se3-x ingots, which were then pulverized and sintered using spark plasma sintering (SPS).
The phase characterization and composition analysis of all specimens were carried out using X-ray diffraction (XRD) and X-ray fluorescence (XRF), respectively. The electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ) and ZT of all specimens were measured in the temperature range of 350 to 650 K. The fracture surface morphologies were observed using scanning electron microscopy (SEM) equipped with energy dispersive X-ray detector (EDX).
The XRD results showed that the major phase of all specimens was In4Se3 phase. According to the XRF measurements, the In/Se ratios for specimens were 4:2.61 (In4Se2.61) when melted for 24 hours and annealed for 48 hours, 4:2.54 (In4Se2.54) when melted for 48 hours and annealed for 48 hours, and 4:2.47 (In4Se2.47) when melted for 48 hours and annealed for 96 hours, respectively. As the melting and/or annealing duration times increased, the content of Se decreased. The electrical conductivity of specimen In4Se2.47 reached 80 S/cm at 650 K, and the thermoelectric power factor S2σ of which largely exceeded the ones reported in literatures. Specimen In4Se2.61 showed the lowest thermal conductivity among the three. However, specimen In4Se2.47 with the highest electrical conductivity and power factor had the highest ZT of 0.66 at 650 K. A further investigation of the fracture surfaces showed that specimen In4Se2.47 had dispersed larger bulk grains consisting of less than 100 nm thick layers, while the other two specimens had uniformly distributed fine grains. The heterogeneous grain distributions as well as the nanosize layers were likely contributing to improve the overall ZT of In4Se3-x compounds. The current research showed that the thermoelectric properties of In4Se3-x were largely dependent upon the choices of processing parameters, which could alter both the final chemical compositions and microstructures of the polycrystalline compounds.

The recent demonstration of high-efficiency, high-temperature thermoelectric devices has motivated the investigation of thermoelectric generators (TEGs) for terrestrial energy generation applications. Solar thermoelectric generators (STEGs) powered with concentrated solar energy have potential for use as primary energy converters or as topping-cycles for more conventional concentrated solar power (CSP) technologies. Considering a three-stage TEG based on current record modules from the Jet Propulsion Laboratory, modeling suggests thermoelectric efficiencies of 18% could be experimentally expected with a temperature gradient of 1000 - 100°C.
Prior demonstrations of STEG devices have relied on low levels of optical concentration, but to reach efficiencies that make STEGs cost-competitive with current energy generating technologies requires higher levels of concentration (200 - 300 Suns). We have modeled the STEG as two subsystems: a TEG, and a solar absorber that efficiently captures the concentrated sunlight and limits radiative losses from the system. The TEG subsystem is modeled using thermoelectric compatibility theory; this model does not constrain the material properties to be constant with temperature, enabling more accurate predictions of TEG performance under real conditions. Unlike radioisotope generators or waste-heat conversion devices, the design of a STEG requires careful consideration of radiative heat transfer on the hot side of the TEG, both in terms of incident illumination and blackbody losses. We have developed a model that combines thermal equivalent circuit modeling with optical-ray tracing to design a receiver cavity specifically for STEG applications that enables an overall solar to electric efficiency of 15%. We will discuss the integration of thermal, optical, and thermoelectric modeling to design an optimized STEG system. Additionally, preliminary results on the thermal and electrical performance of a prototype STEG system will be discussed.

Most of researches in thermoelectrics have been focused on the improvement in the thermoelectric figure of merit, which is a material&’s intrinsic index representing its conversion efficiency. While conversion efficiency is important, in certain cases such as low quality waste heat recovery, the price per watt, i.e. $/W, serves as a critical parameter. Since thermoelectrics is a solid-state energy conversion device, minimizing material consumption is essential for reducing device cost. In this sense, thin-film is ideal, but, due to the low thermal resistance of the thin film, i.e. negligible temperature drop, ΔT, across the thin film, power output is small. In this talk, we present new device architecture, what we called the spacer-inserted-thermoelectric device (SITED), which minimize material consumption yet enhance power output. Using an electrically non-conducting spacer along with the thin film, large temperature difference has been achieved. Therefore, more than an order of magnitude increase in power output out of same amount of Bi-Te was achieved using the SITED. Both temperature drop and power output, W/kg, were normalized by mass of Bi-Te. We achieved this enhancement by separating thermal path to electrical path while both electrical charges and thermal energy share the same path in the conventional device architecture. We also applied this device to harvest body-heat and experimentally demonstrated its usability as an energy conversion device for recovering low quality heat.

Bipolar thermal conductivity has been experimentally observed in many advanced thermoelectric materials. This effect is detrimental to the thermoelectric performance of materials. We have developed a numerical method of calculating bipolar thermal conductivity in heavily doped semiconductors, which allows us to determine the magnitude, and temperature dependence of bipolar thermal conductivity in a variety of thermoelectric materials, including the Bi2Te3 compounds, Si-Ge alloys, skutterudites, and etc. We find that the bipolar thermal conductivity in heavily doped semiconductors is predominantly controlled by the minor carrier conduction, in contrast to other thermoelectric properties. We will discuss methods of minimizing the bipolar thermal conductivity in materials.

Organic thermoelectrics provide significant advantages over their inorganic counterparts such as mechanical flexibility, low-cost synthesis, and solution processability over large areas. While carbon nanotubes (CNTs) have been described as effective fillers for polymers, development of high performance CNT-based thermoelectrics has been impeded due to the lack of a thorough understanding of the fundamental physics of their interacting components (i.e. CNTs, polymers, phonons, and electrons). In this study the seebeck coefficient, and thermal and electrical conductivity of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) nanocomposites based on three different multi-walled CNTs (MWCNTs) -viz., pristine, graphitized, and nitrogen doped- were measured. The different MWCNT types retained identical diameter and length resulting in a similar nanocomposite structure at any CNT loading when prepared under the same condition. MWCNTs accounted for 5, 35, and 60 %vol. of different nanocomposites. Nanocomposite structure was controlled by varying the duration of ultrasonication of MWCNTs in an aqueous solution of PEDOT:PSS, where three distinguishable dispersion states of low, medium and high CNT entanglement were achieved at each MWCNT loading. For all samples studied, the electrical conductivity of nanocomposites increased with improved dispersion and higher CNT loading. However, the Seebeck coefficient was not proportional to either CNT loading or dispersion state, suggesting that carrier mobility is more important than carrier density to obtain higher thermopower. The thermal conductivity of samples remained almost unchanged due to phonon scattering at the CNT/polymer and CNT/CNT junctions. Despite electrically conductive CNT networks where a more conductive and dispersed CNT filler guarantees a higher electrical conductivity, thermoelectric efficiency of CNT based nanocomposites should be optimized with respect to doping, and CNT type, loading and dispersion quality. We expect that the crucial outcome of this study can be used to guide the rational design of the next generation highly efficient and tunable organic thermoelectric materials.

ABSTRACT
Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we present a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT~1.6 at 923K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering both, electronic structure and band alignment across nanostructure-matrix interface: Utilizing density functional theory (DFT) for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS and ZnSe, we infer favorable valence band alignments between PbSe and compositionally-alloyed nanostructures of CdS1-xSex/ZnS1-xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (light hole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power factor.
ACKNOWLEDGMENTS
This work were supported as part of the Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0001054 (LDZ, SL, SH, CIW, XZ, CU, CW, TPH, VPD and MGK).

Thermoelectric phenomena at high-temperatures have been typically studied in the context of energy conversion using large-scale devices¹, and electronic transport at small-scales has been mostly studied in the lower temperature range (< 400 K) where the behavior of the charge carriers can be understood by examining the potential energy profiles. Thermoelectric studies at high-temperatures (~1000 K), where the thermal energy of the carriers in semiconductors becomes comparable to their potential energy, and under extreme thermal gradients (~> 1 K/nm), which arise at small scales and lead to uncommon electrothermal phenomena, have been limited due to difficulties in performing controlled experiments. Here we demonstrate the importance of thermoelectric effects for high-temperature small-scale electronics through experiments on self-heating of current-carrying highly-doped silicon micro-wires which show melting consistently starting at one end of the wires. Local melting in the wires leads to thermal gradients on the order of 1 K/nm. The extreme asymmetry in melting reveals the significance of energy transport by drift of thermally generated minority carriers and their recombination downstream. This minority-carrier generation-transport-recombination process is a distinct behavior in highly-doped semiconductors under extreme thermal gradients at high temperatures and opposes the direction of the electronic-convective heat flow which dominates in semiconductors at lower temperatures and in metals. These results are directly applicable to emerging electrothermal nanoscale devices such as phase-change memories² which utilize local self-heating for non-volatile data storage or computation, and in which thermal gradients even greater than those in our experiments are expected.
1. DiSalvo, F. J. Thermoelectric cooling and power generation. Science 285, 703-706 (1999).
2. Lankhorst, M. H. R., Ketelaars, B. W. & Wolters, R. Low-cost and nanoscale non-volatile memory concept for future silicon chips. Nature Materials 4, 347-352 (2005).

Chevrel phases with the general formula MxMo6X8 (M: metal; X: S, Se, Te) have considerable potential as high-temperature thermoelectric materials. The host structure of a Chevrel phase consists of stacked Mo6X8 clusters. The guest metal atoms M fill the cavities between the Mo6X8 clusters. In this study, we have enhanced the thermoelectric efficiency in Chevrel phase sulfides by tuning the M content and extending the cluster from Mo6S8 to Mo9S11. Mo6S8 cluster-based MxMo6S8 (M: Cr, Mn, Fe, Ni, Cu, Pb) powders were prepared by reacting appropriate amounts of M, Mo, and MoS2 powders at 1273-1523 K for 8-12 h in vacuum. Mo6S8 cluster-based K1.10Mo6S8.2, Mo6S8 and Mo9S11 cluster-based K1.85Mo15S19, Mo9S11 cluster-based K2.15Mo9S11, and Mo6S6 chain-based K3.00Mo6S6 powders were synthesized by reacting appropriate amounts of Mo, MoS2 and K2MoS4 powders at 1413-1473 K for 8 h in vacuum. K2MoS4 powder was prepared by sulfurizing K2MoO4 powder with CS2 gas at 673 K for 8 h. All the samples were then consolidated by pressure-assisted sintering at 1223-1423 K for 1-2 h under a uniaxial pressure of 30-40 MPa in vacuum. In MxMo6S8 the carrier concentration can be tuned by changing the M content so as to optimize the thermoelectric power factor. In K-filled systems, the lattice thermal conductivity of K2.15Mo9S11 seems to be less than that of K3.00Mo6S6, K1.10Mo6S8.2, and K1.85Mo15S19. The highest ZT value of 0.4 was found in Cu4.0Mo6S8 at 950 K.

Starting with a historical review of the Hicks-Dresselhaus papers of the 1990s, the developments of nanothermoelectricity will be reviewed, closing with personal perspectives about where the thermoelectrics field might be heading in the future.

Proven state-of-practice “heritage” thermoelectric (TE) materials used in extremely reliable radioisotope power systems (RPS) exhibit only modest dimensionless figure of merit (ZT) values, resulting in relatively system-level conversion efficiencies of 6 to 6.5%. These heritage materials have been known since the late 1950&’s and 1960&’s, so that even the recently developed multi-mission radioisotope thermoelectric generator (MMRTG) builds upon 40-year old thermoelectric converter technology. However, there is great potential for large gains in performance thanks to recent advances in materials synthesis, the discovery of novel complex structure compounds, the ability to engineer with increasing precision micro- and nanostructure features coupled with improved scientific understanding of electrical and thermal transport in such engineered materials and the means to perform in-depth theoretical simulations with fast turnaround time.
In the last few years, sustained NASA-funded collaborative research on high temperature TE materials has already resulted in doubling TE couple level conversion efficiency from about 7.0 - 7.5% up to 15% at beginning of life. Current efforts are aimed at achieving even higher conversion efficiencies, in excess of 20%, in the next few years. The research areas include structurally complex refractory compounds, design engineering of composite materials to decouple and optimize electrical and thermal transport properties and compositional tuning guided by computationally intensive first principles theoretical calculations. We report on recent performance testing of proof-of-principle high efficiency advanced segmented couples and modules.

About 60% of the total energy used in the world are lost as heat and most of this energy is present in fluids at temperatures below 200°C. A current challenge is to create systems that can recover some of this energy at relatively to supply devices such as wireless sensors for example. Thermoelectric generators can transform heat flux into electricity using the Seebeck effect. Organic materials are a very promising alternative to the toxic and scarce Bi2Te3 for harvesting energy from heat sources below 200 °C.
We developed new poly(3,4-ethylenedioxythiophene) (PEDOT) -based thermoelectric materials. Chemical treatments allow us to reach high conductivity (above 2500 S/cm). In order to improve the moderate Seebeck coefficient (13-18µV/K) of pristine PEDOT based materials, we studied a simple wet redox process which enables us to reach 160 µV/K for PEDOT:PSS layers.
In addition to this study, processing methods, as well as dopants were investigated. Drop-casting, spin-coating or spraying methods lead to materials with roughly the same properties. New processing methods were developed to transfer micrometric thick films of PEDOT:PSS to any substrates and to fabricate millimetric-thick legs of PEDOT:PSS materials.
We will also present heat flux sensors fabricated from these materials to demonstrate their interest in functional devices.

It is proposed that the best thermoelectric efficiency can be achieved when the charge transport is through a single energy level [1,2]. Since then, there have been considerable interest in the studies on atomic and molecular junctions. Seebeck coefficient (S) of atomic and molecular junctions is described as a function of dT/dE, where T and E represent transmission function and energy, respectively [1]. As the Fermi energy level (EF) of the electrode is usually located between the transmission peak related to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) level, the sign of S can indicate the energy level of the EF relative to the HOMO or LUMO levels [3].
In this study, we investigate the effect of the electrode material, gold (Au) and nickel (Ni), on the thermoelectric voltage of C60 and benzenedithiol (BDT) molecular junction.
The thermoelectric voltage of the molecular junctions was measured with a home build scanning tunnelling microscope (STM). Temperature difference between the tip and the substrate, was given by controlling the substrate&’s temperature with a Peltier device. Si diode temperature sensors were used to monitor the temperatures of the tip and substrate. The STM tip was brought close to the substrate until the threshold current value was reached. Then, the voltage difference between the tip and the substrate was measured during the retraction of the tip.
We obtained negative S for for Au-C60-Au and Ni-C60-Ni junctions. The result indicates that charge transport for C60 is through the LUMO level when using either Ni or Au as electrode. In the case of BDT molecule, positive and negative S is obtained when using Au and Ni electrode, respectively. We have investigated the result using first-principle calculation. We found that spin splitting of the HOMO level of BDT occurs when Ni electrode was used.
1. J. A. Malena, S. K. Yee, A. Majumdar, and R. A. Segalman, Chem. Phys. Lett. 491, 109-122 (2010).
2. G. D. Mahan and J. O. Sofo, Proc. Nat. Acad. Sci. USA 93 (15) 7436 (1996).
3. M. Paulsson and S. Datta, Phys.Rev.B 67, 241403 (2003).

With the lattice component of thermal conductivity approaching its amorphous limit, the improvement of power factor, i.e. through band structure engineering, becomes important for the further enhancement of zT. In this work, we report orbital interaction as a useful handle to engineer the band structure and transport property based on density functional theory (DFT) calculations. The strength of orbital interaction has been successfully tuned with hydrostatic strain and chemical composition. Specifically for IV-VI semiconductors, i.e., lead/tin/germanium chalcogenides, we find that the convergence of carrier pockets not only displays a strong correlation with the s-p and spin-orbit coupling, but also coincides with the enhancement of power factor. Moreover, it appears necessary to treat holes and electrons differently due to a symmetry-induced repulsion within the conduction bands. For holes, one can either apply appropriate compressive/extensive strain or split the anion p and cation s orbitals to converge the valence band. For electrons, a high conduction band degeneracy can be achieved by selecting cations with small spin-orbit coupling or by applying compressive strains to invert the band structure, which eliminates the band repulsion. At last, we discuss useful alloying strategy attained from this work, which may provide enticing opportunity for optimizing the available IV-VI thermoelectric materials. In sum, our results suggest a new path to engineer the band structure, i.e., towards high band degeneracy for enhanced thermoelectric properties.

New and novel ways of enhancing thermoelectric power of earth abundant materials in the temperature range of 100 to 1000 ^oC are being devised for conversion of waste heat and the heat generated by solar radiation and nuclear fuel into electricity. This conversion is critically dependent on the material parameters such as the Seeback coefficient (S), electrical conductivity (κ), temperature (T) and thermal conductivity (σ) which define the dimensionless figure of merit zT (= S²σT/κ). In Low dimensional structures comprising of thin film superlattices, nanowires and vertical nanocomposite films quantum confinement effects in the paradigm of one electron band theory are expected to increase thermopower. Additional contributions to thermopower emanate from the spin entropy carried by electrons under a thermal gradient. One interesting class of carrier confined systems which is promising for high temperature thermoelectric applications is of perovskite oxide based thin films and superlattices because appropriate doping of these materials can enhance the spin and orbital contributions to thermopower. Here we present an overview of novel ways to boost thermopower in SrTiO₃ based 2D electron gas systems such as LaAlO₃ - SrTiO₃ and LaTiO₃ - SrTiO₃ interfaces. Delta doping of these interfaces with fractional layers of 3d transition metal (Tm) oxides LaTmO₃ results in giant increase of thermopower. The resistivity measurements give evidence for strong Coulomb correlations in the δ-doped heterostructures. We present model calculations based on modified Heikes formula which attribute the thermopower enhancement to the increased spin and orbital entropy originating from the LaTmO₃ sub-monolayer. These effects have been established by measuring thermopower in high magnetic fields.

Besides bulk applications that are well-established in industry, thin film technology moves into focus. Self-sustaining sensors and on-chip cooling represent the most emerging fields of application. As valid for the conventional techniques, enhancing ZT would open new prospects in commercial usage. For thin films, the fabrication of superlattices enables a unique type of nanostructuring. To sustain electronic behavior from layer to layer, subsequently deposited materials must have a structural similarity. XRD and transport measurements have shown that the Half-Heusler alloys TiNiSn and Zr0.5Hf0.5NiSn are appropriate in doing so. Increased cross-plane ZT in this material system is expected by additional interface scattering. Depressed thermal conductivity has been measured by 3omega method. The measurement was performed by a home-made assembly using a passive circuit and a lock-in amplifier. Reliable Seebeck measurements could be performed up to 550 K. In-plane measurements performed with commercial systems have exhibited power factors up to 0.772 mW/(K2m). We gratefully acknowledge financial support by priority programme SPP1386 of the DFG.

Due to their large power factors, high temperature stability, large constituent Earth abundance, and ability to be doped n- and p-type, the semiconducting Half Heusler compounds are an attractive family of materials for thermoelectric applications. However, despite the body of transport studies on bulk polycrystalline samples, very little work has explored the fundamental transport and electronic band dispersions of single crystalline or epitaxial thin film Half Heuslers. In this presentation, we report the growth of the semiconducting Half Heuslers CoTiSb and NiTiSn by molecular beam epitaxy (MBE). The films are grown on InAlAs/InP(001) and MgO(001) respectively and are epitaxial and single crystalline, as confirmed by low energy electron diffraction (LEED), X-ray diffraction (XRD), and scanning tunneling microscopy (STM). Temperature dependent transport studies show that the films are semiconducting, with electron mobility and background carrier density among the highest and lowest reported to date for semiconducting Half Heusler compounds (mu; = 500 cm2/Vs and n = 1018 cm-3 for CoTiSb at room temperature, mu; = 16 cm2/Vs and n = 3 × 1020 cm-3 for NiTiSn) [1]. Angle resolved photoemission spectroscopy (ARPES) measurements confirm that CoTiSb is semiconducting, with bulk Γ-X and Γ-M dispersions in general agreement with theory. However the CoTiSb (001) surface has surface states within the bulk bandgap.
Nanocomposites of the Full Heusler Ni2TiSn inclusions within a Half Heusler NiTiSn matrix have also been grown by codeposition of Ni1+δTiSn with excess Ni. Despite the large lattice mismatch (2.9%) between the Ni2TiSn and NiTiSn, the Ni1+δTiSn films remain epitaxial for compositions in excess of δ > 0.5. Transport measurements show that the addition of Ni increases the net carrier concentration, and transmission electron microscopy and I-V measurements of Ni2TiSn/NiTiSn(001) heterostructures are used to explore the structural and electrical properties of the interface. These Full Heusler / Half Heusler nanocomposites show promise for phonon scattering in thermoelectric applications.
This work was supported by the ARO, ONR, and NSF.
[1] J. K. Kawasaki, T. Neulinger, R. Timm, M. Hjort, A. A. Zakharov, A. Mikkelsen, B. D. Schultz, and C. J. Palmstroslash;m, J. Vac. Sci. Tech. B. 31, 04D106 (2013).

Thermal conductivity is typically considered to be a material property independent of the size of the sample. A thickness-dependent cross-plane thermal conductivity in semiconducting or insulating thin films is, however, commonly observed. This thickness dependence is often attributed to an interface resistance in series with the intrinsic film resistance, which reduces the apparent thermal conductivity with decreasing thickness. In this presentation, we examine the intrinsic thickness dependence of thin films in the absence of interface resistance. We will show that the thermal conductivity of the film itself (assuming no change in film quality) can have a strong thickness dependence due to ballistic transport of the heat carrying phonons and electrons. By decreasing the thickness of the film to tens of nanometers, one can reduce the thermal conductivity by an order of magnitude. One might expect a thickness dependence when the sample thickness is on the order of the mean-free-path of the bulk material. Our theoretical analysis shows that this is true for thermal conduction by electrons, but not by phonons. We find that the electronic thermal conductivity decreases by more than 10% of the bulk value when the film thickness is approximately 10 times the electronic mean-free-path in the bulk, as expected. For phonons, however, we find that the lattice thermal conductivity decreases by more than 10% of the bulk value when the film thickness is approximately 10³ times the phonon mean-free-path in the bulk, a surprising result. Moreover, in the case of silicon, we determine that the thermal conductivity should only saturate to its bulk value at a thickness of ~1 cm. These results are obtained using detailed thermal modeling incorporating quasi-ballistic transport (Landauer formalism), a full band description of the phonon states (density functional theory), and experimentally-calibrated parameterization of the mean-free-path. This work sheds light on the quasi-ballistic nature of thermal transport in nano-scale and even bulk materials, and suggests a possible route to experimentally extract the contributions of different phonon mean-free-paths to the thermal conductivity.

Nanoparticles of ZnO with a uniform particle size of ~15nm were successfully synthesized by forced-hydrolysis. These nanoparticles were densified by spark plasma sintering (SPS) with dopant elements of Al, Ga, Sn, Ce and Gd etc. Chemically homogeneous and well crystallized samples of the sintered nanocomposites were observed by Scanning Electron Microscopy and X-ray Diffractometer. Distinctive defects and secondary phases (ZnAl2O4, ZnGa2O4, ZnSn2O4 etc.) were detected on a nano scale (Grain size of 15nm to 25nm). The as-showed nano structures not only suppressed the lattice thermal conductivity to a level ~4 times lower than micron-grain ZnO bulks at RT, but also maintained the electrical properties comparable to that of micron-grained samples. The samples showed a ZT of 0.3 to 0.4 at 1223K which gives promise to high temperature thermoelectric application. The thermoelectric properties were also measured after thermal cycles in order to discuss the reliability of doped ZnO for real application under different atmospheres.

Printing is one way to produce easily and very cheap large quantities of thin film thermoelectrics for macroscopic devices. Although various printing technologies are available today thermoelectric inks are rare. In cooperation with different institutions a complete printing process of thermoelectrics is investigated. The prepared ink has to meet several criteria for the printing process. First of all, appropriate thermoelectric particles must be found. Bismuth telluride is widely known as a good thermoelectric material for room temperature applications and used in bulk as well as thin film material. Different solvents for dispersing the particles and compositions were investigated as well as some additives to improve the dispersed phase. The relevant parameters like particle size distribution, surface energy and viscosity were determined. The inks were printed on suitable substrates and annealed to remove the solvent. Furthermore, some thermoelectric data of thin films are shown.
Electrochemical deposition is another cost-efficient method to grow thermoelectric thin films on large substrates. The development and optimization of electrochemical deposition procedures of Bi2Te3 thin films were investigated. A temperature controlled electroplating setup for 4 inch substrates with a peristaltic pump for electrolyte circulation during the electrochemical deposition was used. N- and p-type Bi2Te3 thin films could be successfully deposited on 4 inch substrates. In addition, simultaneous temperature dependent measurements of Seebeck coefficient and electrical conductivity were made on a measurement platform developed at the Fraunhofer IPM. Furthermore, measurement results of the thermal conductivity are shown.

Inorganic materials with ABX stoichiometry that often crystallize in half-Heusler structure constitute an important class of thermoelectrics. Remarkably, rapid transition from basic synthesis and characterization science these compounds to thermoelectric module applications have already been demonstrated. Finding new half-Heusler materials with yet higher ZT is a promising future development strategy that is complementary to nanostructuring or alloying of the existing materials. Here we present a systematic combined theoretical/experimental high-throughput study aimed to identify previously unreported ABX materials, some of which may have half-Heulser structure with a potential for thermoelectric applications.
The results of systematic search across databases and literature indicate many feasible ABX materials are unreported, for example 29 out of 45 materials in the V-IX-IV 18-electron family. Theoretical calculations reveal that 8 of these V-IX-IV materials are thermodynamically stable, including 4 new materials with half-Heusler structure. Thin film combinatorial synthesis experiments using sputtering, x-ray fluorescence and x-ray diffraction confirm that one of these materials TaCoSn is stable in the predicted half-Hesuler structure. Thiese experimental results validate our high-throughput approach to systematic discovery of unreported ABX materials [1].
We applied this approach to screen a relatively unexplored group of ABX materials, with 8 instead of 18 electrons, for new stable materials with half-Heusler structure. Out of the discovered 235 stable materials, 16 crystallize in the half-Heusler structure. Half of these 16 materials are semiconductors and thus have potential for thermoelectric applications [2]. Synthetic efforts have also validated one of the newly discovered 8-electron materials AgYGe in the predicted crystal structure.
Finally, we developed an experimental high-throughput temperature-dependent instrument for thermoelectric power factor mapping, with the goal to quantify thermoelectric performance of the newly discovered materials. The instrument is capable of simultaneously measuring electrical conductivity and Seebeck coefficient as a function of position on the sample up to 400-500C temperatures. As a test case, the instrument has been demonstrated to be operational on the example of Co3O4-ZnO and Co3O4-NiO combinatorial sample libraries with intentional continuous composition spreads [3]
In summary, the high-throughput approach to discovery and optimization of novel thermoelectric materials is promising for accelerated development of ABX half-Heusler and other novel thermoelectric materials.
This work is supported by the US Department of Energy, Office of Science, Basic Energy Sciences, as a part of the Energy Frontier Research Center “Center for Inverse Design”.
[1] A. Zakutayev et al JACS, 10.1021/ja311599g (2013)
[2] X. Zhang et al AFM 22, 1425 (2012)
[3] A. Zakutayev et al RSI 84, 053905 (2013)

Thermally Regenerative Electrochemical Cycle (TREC) was developed about half a century ago for harvesting thermal energy. This strategy is based on the temperature dependence of electrochemical potential. For a half reaction, A + n e- --> B, the temperature coefficient is defined as
alpha = dV/dT = ΔS(A,B)/nF
where V is the electrochemical potential, T is temperature, n is the number of electrons transferred in the reaction, F is Faraday&’s constant, and ΔS(A,B) is the partial molar entropy change for the half cell reaction in isothermal condition. This effect indicates that the voltage of a battery depends on temperature; thus a thermodynamic cycle can be constructed by discharging the battery at T1 and charging back at T2. If the charging voltage at T2 is lower than the discharging voltage at T1, net energy is produced by the voltage difference and it originates from heat absorbed at the higher temperature, similar to a thermomechanical engine whose theoretical efficiency is limited by Carnot efficiency. The use of traditional TREC systems is often impractical, as such systems must often be operated at conditions that are incompatible with many processes in which heat recovery would be useful.
In this talk, new electrochemical systems for heat recovery are presented and discussed. We experimentally demonstrate high efficiency for heat-to-electricity conversion. This new system has applications in a variety of systems in which waste heat recovery is desired.

Engineering thermal transport in nanomaterials is crucial for thermoelectric devices. In fact, many nanostructured materials including Silicon nanowires, thin films and nanoporous materials have shown remarkably low thermal conductivities [1,2,3]. However, modeling phononic thermal conductivities is possible only in a few cases, where the phononic dispersion curves and scattering times are known. We developed a general computational framework, based on the Boltzmann Transport Equation, where the only input parameter is the bulk thermal conductivity accumulation function, which is a property that can be directly obtained by experiments [4]. The method avoids the discretization of the frequency spectrum, which can be computationally expensive, especially for complex unit cell materials where the number of phonon polarizations is higher. Furthermore, the developed method allows the computation of thermal transport from nano to macro scale thanks to a hybrid Boltzmann/Fourier solver [5]. By applying the method to nano-porous Silicon, including the aligned and disordered pore arrangements, we show its simplicity and computational effectiveness compared to a frequency-dependent approach. The presented method could have great potentials for the prediction of thermal transport in complex and unexplored shapes of nanomaterials, unlocking the high-throughput screening of thermal transport for thermoelectric devices.
[1] J.-H. Lee et al. NanoLetters 8, 3750 (2008).
[2] J-H. Lee et al. APL 91, 223110, (2007).
[3] Jen-Kan Yu et al. Nat. Nanotech, 5, 718 (2010).
[4] A. J. Minnich. Physical Review Letters, 109, 205901 (2012)
[5] JM Loy, JY Murthy, D Singh - Journal of Heat Transfer, (2013)

Characterization of the thermoelectric properties of films is of high interest because thru nano-structuring, the figure of merit (ZT) may be enhanced. The figure of merit depends of three properties: the electrical conductivity, thermal conductivity and Seebeck coefficient. Suppression of the thermal conductivity and/or enhancement in the Seebeck coefficient leads to a higher figure of merit. As these feats are being attempted, the importance of robust, versatile, and fast measurement techniques is paramount for the advancement of the field. The scanning hot probe method allows for characterization of the thermal conductivity and Seebeck coefficient of both bulk and thin film materials. This work emphasizes characterization of films at room temperature, specifically super lattices and nanowires. This method utilizes a tip from a scanning thermal microscope (SThM), locally heating the sample and causing a temperature gradient to be induced on the sample surface. Heat diffusion models allow for the calculation of the temperature rise on the sample and the thermal resistance of the film, from which the Seebeck coefficient and thermal conductivity of the film can be readily obtained.

We have prepared [(MSe)1+δ]m[TiSe2]n intergrowth compounds where M = Bi, Pb and Sn containing rotational misorientation between respective layers that are not accessible via traditional synthetic approaches by utilizing nucleation-limited reactions in compositionally modulated solid state precursors. Comparing the measured properties of the ferecrystal [(PbSe)1+δ]1[TiSe2]2 to it's crystalline misfit layered compound analog, we have found that the electrical conductivity is a factor of five larger and the Seebeck coefficient has increased to 90 mu;vK-1 in the ferecrystal from the 50 mu;vK-1 found in the crystalline misfit layer compound. The simultaneous increase in conductivity and Seebeck coefficient is unusual, especially as the ferecrystal is a more disordered material than the crystalline misfit layer compound. We will present structural information and electrical properties of several series of ferecrystals where m is held constant and n is varied, and where n is held constant and m is varied. We will also present data on the formation and properties of [(M1-xM'xSe)1+δ]m[TiSe2]n intergrowth compounds where M and M' are Bi, Pb and Sn. Systematic changes in carrier concentration allow us to infer the position of the constituent energy levels and hence the systematic changes observed in conductivity, Seebeck coefficient and thermoelectric power. We will discuss the extent to which composition, local structure, and nanostructure can be simultaneously controlled in these materials, enabling potential pathways to preparing nanostructured materials with specific thermoelectric properties.

Thermoelectric materials are attractive for realizing ecofriendly solid-state refrigeration, and waste heat recovery and harvesting. Besides obtaining high thermoelectric figure of merit ZT materials, tailoring the electrical and thermal transport properties of interfaces of these materials with metals is crucial for high performance device applications. Here, we describe the nexus between interface chemistry and the electrical and thermal conductances of n-type Bi2Te3 and p-type Sb2Te3 contacted with Cu, Ni, Ti and Ta. Our results reveal that the thermal interface conductance, measured by pump-probe thermoreflectance spectroscopy, is insensitive to the majority charge carrier type. Cu-contacted interfaces show the highest values that are about tenfold higher than the other interfaces. In contrast, we find that Ni- and Ta-contacted p-Sb2Te3 show tenfold higher electrical conductances than their interfaces with n-type materials. The interfacial electrical properties are insensitive to the majority carrier type for Cu and Ti contacts, and the former shows the lowest electrical conductance. Rutherford backscattering spectrometry and X-ray diffractometry analyses reveal that all the metals studied to form Te-containing interfacial phases. We also observe rapid Cu diffusion into both n-Bi2Te3 and p-Sb2Te3, conducive for thermal conductance, but deleterious for electrical transport. The difference in the electrical transport behavior across interfaces between Ni and Ti with n-Bi2Te3 and p-Sb2Te3 correlate with large compositional changes at metal/p-Sb2Te3 interfaces and the lack thereof for metal/n-Bi2Te3. Based upon these findings, and annealing experiments, we describe a phenomenological model revealing the connection between interface chemistry and the thermal and electrical transport properties. Our results will be important for designing metal contacts to thermoelectric devices.

Natural mineral-based compounds of the tetrahedrite family of nominal composition Cu12-x(Zn, Fe, Ni)xSb4S13 have been found recently to exhibit good thermoelectric properties. The highest zT values reported for Ni and Zn doped tetrahedrites are 0.7 and 0.9, respectively. DFT calculations show that Ni and Zn doping can have different effects on the electronic structure of these compounds. With Ni doping, the band structure remains metallic, since the unoccupied minority spins from Ni atoms give rise to a new d-electron hole level, residing at the top of the valence band. Zn doping, on the other hand, provides additional electrons to fill the hole levels in the valence band, thereby moving the Fermi level into energy gap. Therefore, we have performed studies of Ni and Zn co-doping, with the aim of moving the Fermi level close to the flat Ni d bands at the top of the valence band in order to obtain higher thermopower. Our preliminary results indicate that the thermopower of co-doped samples is enhanced, but at the expense of reduction in the electrical conductivity. However, the thermal conductivity was also reduced by impurity scattering from co-doping. Overall, the highest zT value obtained in this study is in excess of unity, a more than 50% enhancement over that of the best pure-Ni doped samples.

Engineering nanoscale interfaces is a requisite for harnessing electrical and thermal transports within nanostructured materials, especially those destined for thermoelectric applications requiring an unusual combination of low thermal conductivity and electrical resistivity. The conventional approach is to empirically search for materials with large Seebeck coefficient S, and small thermal conductivity κ and resistivity ρ. Optimizing such a relationship quickly reduces the space of available materials, and it is well known that most good thermoelectrics are small-band-gap semiconductors or semi-metals. Assuming that the total thermal conductivity can be written as the sum lattice and the electronic thermal conductivities. One approach to decrease the thermal conductivity without significantly affecting the electronic properties is based on the use of nanoparticles or nanoscale superlattice systems.
The concept we pursue here on tungsten oxides is based on an intrinsic, layered nanostructure defined by crystallographic shear planes (CS) as structure motifs. The CS are characteristic structural features in reduced early-transition- metal oxides. For tungsten oxide, WO3-x, a representative of the Magneli phases, {102} and {103}. We report the thermoelectric properties of spark plasma sintered Magneli phases WO2.90 and WO2.722. The crystallographic shear planes, which are a typical feature of the crystal structures of Magneli-type metal oxides, lead to a remarkably low thermal conductivity for WO2.90. The figures of merit (ZT = 0.13 at 1100 K for WO2.90 and 0.07 at 1100 K for WO2.722) are relatively high for tungsten-oxygen compounds and metal oxides in general. The electrical resistivity of WO2.722 shows a metallic behaviour with temperature, while WO2.90 has the characteristics of a heavily doped semiconductor. The low thermopower of 80 µV/K at 1100 K for WO2.90 is attributed to its high charge carrier concentration. The thermoelectric performance for WO2.90 compared to WO2.722 originates from its lower thermal conductivity, due to the presence of crystallographic shear and dislocations in the crystal structure. We show that low-cost thermoelectric materials based on the use of intrinsically nanostructured materials rather than artificially structured layered systems lead to strongly reduced lattice thermal conductivity.
This work demonstrates that SPS can be used for creating additional scattering centers on the nanoscale. By adding small amounts of Ta2O5, the lattice thermal conductivity of WO2.90 decreases steadily with the formation of (W)-Ta-O scattering centers. Additionally, HRTEM studies reveal the presence of dislocations, oxygen vacancies and Wadsley-defects in the pellets, which also contribute to a low thermal conductivity. Scattering centers on the atom scale (defects) and nanoinclusions lead to an improvement of the lattice thermal conductivity and values of κlat. = 2 W m-1 K-1 were observed. This leads to an enhancement of the zT.

Forming solid solutions has long been employed in thermoelectric research because the effective lattice thermal conductivity reduction it brought. There are other influences on transport properties that need to be kept in mind when looking for better thermoelectric materials. For instance, the mobility of carriers will also be reduced due to the scattering of carriers from atomic disorder. Lead chalcogenide alloys offer a great opportunity to study and characterize such influences. In n-type (PbTe)1-x(PbSe)x alloys, the atomic disorder only provides scattering of phonons and electrons without changing the band structure. Through a carefully designed study both of such scattering observed are explained with proper models and their magnitude characterized, which lead to a simple criteria that gives rough estimate of whether a certain system of solid solution would be thermoelectrically beneficial or not. One step further than this simplest example is the n-type (PbSe)1-x(PbS)x alloys where the band effective mass also changes with the degree of anion site disorder. Such influence can be approximated with a simple linear relationship and when this is taken into account the transport properties observed can again be understood with the same models previously introduced. The criteria when applied for this (PbSe)1-x(PbS)x, predicted the change of zT on alloy composition in addition to an weighed average of maximum zT found in PbSe and PbS. One should keep in mind that in a lot of well-known example of thermoelectric solid solutions the band structures actually change with alloy composition. A complete understanding of alloying on thermoelectric properties in these systems needs a comprehensive consideration of the influences from atomic disorder in solid solutions as described above and that from the change of band structure, i.e. band engineering. This is demonstrated here using an alloy system based on PbSe as example.

Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation and offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat. The key to the development of advanced TE technologies is to find highly efficient TE materials. In current commercial materials, the zTs are limited to values around unity. Recently, several novel concepts have been proposed to enhance the efficiency of TE materials and laboratory results suggest that high zT values can be realized in several families of bulk materials. In this presentation, the thermoelectric properties of bulk copper chalcogenides (Cu2-δX) are reported in a wide temperature range, where X could be S, Se, or Te. We will show these materials possess extremely abnormal thermoelectric properties with very low thermal conductivity and good thermoelectric figure of merit. In particularly Cu2-δSe is taken as an example to demonstrate the abnormal properties of the low temperature phase, the high temperature phase, and the transition states during the phase transitions. The physical mechanisms behind these abnormal thermoelectric properties will also be discussed to show the possibility of realization of ultrahigh thermoelectric figure of merit.

Development of the novel materials where charge and heat transport are partially de-coupled is a key factor for the next generation of thermoelectric materials. New bulk compounds with high density of states near Fermi level and low thermal conductivity are required for efficient thermoelectric energy conversion. We propose to use layered silicon arsenides as a base for new thermoelectric materials. We have shown that large alkali metal cations can be successfully intercalated into interlayer space in the crystal structure of these arsenides. Re-arrangement of the silicon arsenic layers leads to the formation of large channels where alkali metal cations are situated in a disordered fashion. Moreover, silicon arsenic layers exhibit high tolerance towards different substitutions in the silicon positions, thus allowing optimization of the charge carrier concentration of the synthesized materials. Synthesis of new materials, their crystal and electronic structure as well as thermoelectric properties will be discussed.

Cu2Se shows a second order superionic phase transtiion at 410K. The coupling of a continuous phase transition to carrier transport in Cu2Se over a broad (360-410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of zT (to 0.7 at 406 K). The use of entropy and critical fluctuations for enhanced thermopower and phonon scattering could lead to new engineering approaches for thermoelectric materials with high zT and new green applications for thermoelectrics.

Wide band gap semiconductors like the quaternary chalcogenides of the stannite or kesterite type have proven to be good thermoelectric materials for the mid-range temperature regime. Several compounds have been investigated for their thermoelectric properties. The cation-ordered, distorted variants of the sphalerite structure leads to inherently low lattice thermal conductivities (below 1 W / (Km) at 700 K), while the structural robustness of this class of compounds opens the possibility of precise doping for a control over carrier concentration, e.g. resulting in a maximum ZT of 0.95 at 850 K for Cu₂ZnSn_0.90In_0.10Se₄.¹
Recently we have investigated the influence of different kinds of substitutions in the quaternary stannite type compound Cu₂ZnGeSe₄. While isoelectronic cation²- and anion³-substitutions leads to a reduced thermal transport through local anisotropic structural disorder and mass fluctuation, the aliovalent doping⁴, however, enabled electronic optimization of the carrier concentration and leads to a nano-sized phase segregation which results in reduced thermal conductivity. Furthermore, these studies reveal the presence and illustrate the control of a phase transition, which is correlated to the melting of the copper sublattice. Due to the molten sublattice, a phonon-liquid-like behavior is reached, resulting in decreased thermal conductivity. In addition to the variable temperature XRD and MAS-NMR measurements to investigate the phase transition, we will discuss the influences of the different substitutions on the transport properties and give an outlook on how to further optimize the thermoelectric figure of merit in this class of compounds.
¹X. Y. Shi, F.-Q. Huang, M.-L. Liu and L-.D. Chen, Appl. Phys. Lett.2009, 94, 122103.
²W. G. Zeier, Y. Pei, G. Pomrehn, T. Day, N. Heinz, C. P. Heinrich, G.J. Snyder, W. Tremel, JACS2013, 135, 726-32.
³C. P. Heinrich*, T. Day*, W. G. Zeier, G. J. Snyder, W. Tremel, submitted.
⁴W. G. Zeier, A. LaLonde, Z. M. Gibbs, C. P. Heinrich, M. Panthöfer, G. J. Snyder, W. Tremel, JACS2012, 134, 7147-54.

PbTe is well known as a thermoelectric material with high efficiency, which works in intermediate temperature range. In order to get the high thermoelectric efficiency, we need the large Seebeck coefficient, electric conductivity, and the low thermal conductivity in the thermoelectric materials. Because PbTe is a narrow gap semiconductor, it has the large electric conductivity. At the same time, the large Seebeck coefficient can be expected from the fact that the band structure has high degeneracy of band extrema. Additionally the thermal conductivity that is another important factor for the thermoelectric efficiency is small because PbTe has heavy electron.
But Pb is toxic and environmental unfriendly material. In this sense, Pb should be substituted with other candidates, which are confortable for the environment. Here, we propose that GeTe can be the alternative material of PbTe. Basically, the thermoelectric efficiency of GeTe is low because Ge is lighter than Pb. To improve thermoelectric efficiency of GeTe, we will suggest two ways in this talk.
One way is to use phonon scattering by vacancies. The thermal conductivity can be controlled easily by forming Ge vacancies. We have calculated the formation energy and transition levels of defects in GeTe. Our calculation results show that it is able to introduce the vacancies of Ge in GeTe, and they will not be harmful carrier trap. So the vacancies of Ge may be available as a factor of phonon scattering. GeTe has possibilities of reducing the thermal conductivity.
Another way is to combine GeTe with AgSbTe₂ to displace Pb to Ag and Sb in GeTe. AgSbTe₂ shows high thermoelectric efficiency in itself. GeTe and AgSbTe₂ can make the solid solution (GeTe)_x(AgSbTe2)_1-x called TAGS. GeTe and AgSbTe₂ have rock-salt structures; therefore, TAGS is able to form layered structure. The experimental data have shown that TAGS represents good thermoelectric conversion efficiency. GeTe and AgSbTe₂ show low and slightly better thermoelectric efficiency, respectively. However, the combination of them shows very high efficiency. The origin of the high thermoelectric efficiency has never understood and discussed in theoretical point of view. In this talk, our purpose is to clarify the mechanism of the high efficiency by first principles calculations and Bloch-Boltzmann equation.

[1]G. D. Mahan, Solid State Physics, 51(1997)81
[2]S.B.Zhang, Su-Huai Wei, Alex Zunger, and H.Katayama-Yoshida, Phys. Rev. B 57(1997)9642
[3]Glen A. Slack, Solid State Physics, 34(1979)1

In order to achieve waste heat recovery using thermoelectric systems, thermoelectric materials showing high conversion efficiency over wide temperature range and high resistance against oxidation are indispensable. A silicide material with good n-type thermoelectric properties and oxidation resistance has been discovered. The composition and crystal structure of the silicide are found out Mn₃Si₄Al₂ (abbreviated as 342 phase) and hexagonal CrSi₂ structure, respectively. Element substitution of Mn with 3d transition metals is succeeded. Enhancement of Seebeck coefficient is observed in a Cr-substituted sample. The maximum dimensionless thermoelectric figure of merit ZT is 0.3 at 573 K in air for the Mn_2.7Cr_0.3Si₄Al₂ sample [1]. However, when the simple substances of Mn, Si, and Al are use as the starting materials, the large amount of secondary phases are formed after arc melting because of oxidation of Al. This makes reproducibility of thermoelectric properties of the samples low. It will be a serious problem for mass production of the 342 devices. Using binary alloys, Mn-Al or Si-Al, as the starting materials of Al are effective to prepare single phase 342 samples, because the oxidation of Al can be prevented. The silicide thermoelectric modules consisting of 14 pairs of the legs have been fabricated using the higher manganese silicide (MnSi_1.75) and non Cr substituted Mn₃Si₄Al₂ devices as p- and n-type legs, respectively. Power generation and durabilty tests of the modules have been carried out. Deterioration of the generated power was observed at temperatures of the hot side higher than 773 K in air. On the other hand, no decrease in the generated power was detected for longer than 3 days in vacuum.
[1] R. Funahashi, et. al., J. Appl. Phys, 2012, 112, 073713.

Sb-doped Mg₂Si_1-xSn_x has been reported as competitive n-type thermoelectric material for operation in the mid-temperature range (300 - 550°C). Not only does it exhibit promising thermoelectric properties (with a figure of merit reported close to unity), but it also has the advantage of consisting of earth abundant and non-toxic elements. Chemical disorder and local lattice distortion have been observed in the Mg₂Si_1-xSn_x solid solution phase, which leads to enhanced phonon scattering and consequently reduced thermal conductivity. However, it is difficult to achieve homogeneous Mg₂Si_1-xSn_x alloys due to the phase separation into Mg ₂Si-rich and Mg₂Sn-rich domains, which comes from the fact that the pseudo-binary Mg₂Si - Mg₂Sn system possesses a peritectic transformation and a miscibility gap.
In this work, we investigated how the phase distribution and microstructure of Mg₂ Si_1-xSn_x alloys can be controlled by rapid quenching and how it modifies the thermal and electrical conductivities. Several (Sb-doped and undoped) Mg₂ Si_1-xSn_x starting compositions were prepared by HF melting and processed by melt spinning. Morphological and compositional studies performed by FEG- SEM and EPMA showed a progressive change in microstructure in the cross plane of the ribbon. The structure varies from a columnar supersaturated solid solution in the area of the surface in contact with the spinning wheel to a refined dendritic microstrure of Mg₂Si surrounded by the Mg₂Sn-rich phase where the cooling rate is weaker. Computational thermodynamic is applied to interpret the solidification process. Rapidly solidified samples were densified by spark plasma sintering and their transport properties and thermoelectric performances were characterized and compared to as-cast materials in order to investigate the effect of microstructure refinement induced by high undercooling conditions.

Mg2Si0.3Sn0.7 solid solutions stimulate our interests with excellent thermoelectric figure of merit ZT due to the convergence of the conduction bands which causes very high carrier effective mass, power factor and average ZT values within 300-800K. Results indicate that, Bi acts as the effective n-type dopant which increases the electron density and power factor, reduces the lattice thermal conductivity, and thus significantly enhances the ZT value of the compound. The Seebeck coefficient of Bi doped Mg2Si0.3Sn0.7 solid solutions can be well simulated by the single parabolic band model with an effective mass of 2.45 me while a rigid band approximation is utilized to resolve its temperature dependence within the extrinsic conduction range. This confirms that the two conduction bands are nearly degenerate and behave as a single band regarding the electrical transport in these compounds. These features of the conduction band structure result in Bi doped Mg2Si0.3Sn0.7 solid solutions having the optimum power factor among all Mg2Si1-xSnx compounds and within a wide temperature range, 4.0-4.7 mWm-1K-2 in 400le;Tle;800K when Bi doping amount y and electron density are within 0.01-0.03 and 1.65-2.78×1020 cm-3, respectively. Thermal transport measurements show that Bi doping can remarkably reduce the lattice thermal conductivity (κph) and the room temperature κph of the sample with y=0.04 decreases by 14% as compared with the pure one. Fitting using the Debye approximation reveals that the reduction in κph at low temperatures results from the strengthened point defects scattering. In addition, κph above room temperature of Mg2Si0.3Sn0.7 is primarily under the influence of acoustic phonon scattering. Due to the degenerate nature of conduction bands and the much improved power factor in the whole temperature range as well as the reduced κph, Bi doped Mg2Si0.3Sn0.7 with y=0.015 possesses the highest ZT of 1.3 at 700K and the average ZT value of about 0.9 between 300-800K. Further optimization in the electrical properties through doping or alloying in n-type Mg2Si0.3Sn0.7 offers a possibility for additional optimization of ZT values.

A Mg2Si consisting of abundant and non-toxic elements, and shows good n-type thermoelectric properties in the temperature range from 500 to 773 K. This is suitable thermoelectric material, especially for mobile applications such as automobiles because it is composed of light weight elements.
Non-doped Mg2Si has been reported to show n-type conduction, which makes it difficult to get good p-type conductor especially at high temperature. However, recent first principle calculation suggests that n-type conduction of non-doped Mg2Si is originated from the Mg-excess preparation condition. One way to check the possibility of this prediction is to grow high quality epitaxial Mg2Si films at relatively low temperature under the not-Mg-excess condition that is different from the conventional condition. Moreover, Mg2Si epitaxial films over hundred nanometer of thickness have not been reported due to the high volatility of the Mg.
In this study, we have succeeded to grow (111)-oriented epitaxial Mg2Si films with 500 nm in thickness at 573 K on (001)Al2O3 substrates by RF magnetron sputtering method. This epitaxial Mg2Si films indicate p-type conduction and the band gap of 0.77 eV, which estimated from temperature dependency of the conductivity. This value agrees with the single crystal&’s data of 0.6~0.8 eV. Seebeck coefficient at room temperature was above 500 mu;V/K and decreased with the temperature, and remained the positive value even at 773 K, which indicates the p-type conduction. These results show the possibility to get good thermoelectric property with p-type conduction for Mg2Si prepared under not-Mg-excess condition.

Magnesium silicide (Mg2Si) is a candidate for use as a thermoelectric (TE) material at operating temperatures ranging from 600 to 900 K. Mg2Si has some appealing features, such as the abundance of its constituent elements in the earth&’s crust and the non-toxicity of both Mg2Si and its processing by-products. This suggests it is safe to handle and can be used in practical devices, and results in freedom from concern with regard to the increasing regulations on hazardous substances. Previous studies reported that incorporation of metallic binders such as Al, Cu, Zn and Ni to the Mg2Si is effective to enhance sinterability. However, residual thermal stresses induced due to difference in the coefficient of thermal expansion between Mg2Si and electrodes is a serious problem for structural reliability of the TE devices in practical use. It was reported that large thermal stress should be a cause of cracks appears in the TE elements. In order to design and manufacture reliable TE devices, mechanical properties of the Mg2Si containing metals should be understood clearly. Furthermore, temperature dependence of the properties is also important, because the TE elements will be operated at high temperature.
Based on such backgrounds, we focused on the Mg2Si containing metal binders in this study, and the relationship between those Young&’s moduli and the amount of the binders was investigated at room temperature at first. The Mg2Si samples were sintered by Plasma Activated Sintering, and the Young's modulus was obtained by nanoindentation tests and ultrasonic measurements. Then, the temperature dependence of the Young&’s modulus and of the coefficient of thermal expansion up to 800 °C was measured using thermal mechanical analysis (TMA) and dynamic mechanical analysis (DMA), respectively. In addition, by utilizing those obtained material properties, finite element analysis were carried out to examine the distribution of the thermal stresses induced in the TE elements. The analytical results should be important data to understand the thermal and mechanical behavior of the Mg2Si during operation, and to design the practical TE devices.

Because of its power generation capability, its abundance, its non-toxicity, and its light weight, the significance of Mg2Si as a thermoelectric (TE) material has recently come to the fore. While a value of 1 for ZT is typical for n-type Mg2Si, its TE properties need to be improved for it to become a major TE material. It is well known that doping Mg2Si with appropriate impurity elements can enhance the TE performance, increase the power factor and/or decrease the thermal conductivity. Al, Bi and Sb are well known donor impurities for Mg2Si. However, Bi-doped matrices deteriorate rapidly at elevated temperatures, and while Al-doped Mg2Si is easy to process, it gives only a moderate ZT of ~0.7. In terms of good TE properties and durability, the current prominent dopant for Mg2Si is Sb, even though Sb-doped Mg2Si is difficult to sinter. Stable, substitutional impurity elements in Mg2Si are needed in order to progress the TE characteristics and ensure long lifetime operation at elevated temperatures. To further enhance the TE capability of Mg2Si, the device fabrication process requires that any element used as an impurity is thermodynamically stable, electrically active in substitutional sites, and can easily be adapted for manufacturing processes.
To examine the effects of impurity doping, samples of Mg2Si doped with various impurities, such as Sb, Al, Zn, Co and Ta, were prepared using an all-molten polycrystalline synthesis process. The samples were examined with regard to their power factor and thermal conductivity. For some combinations of impurities, the resistivity of the TE chip was maintained during aging tests at ~900 K for >1000 h, and the scalability and reproducibility of the plasma sintering process were unsurpassed. Temperature-variable Hall measurements were also performed to understand the basic electrical characteristics of undoped and doped Mg2Si. Some anomalous deterioration mechanisms in the Mg2Si TE chips prepared using the all-molten source material and consequent sintering process of the manufacturing-type process were revealed at elevated temperatures. The mechanical properties of Mg2Si, such as Young&’s modulus, the bending strength, and fracture toughness, were examined by bending tests, indentation fracture toughness tests, and ultrasonic tests, in order to gain an understanding of the structural reliability of the TE chip and TEG module. To further increase the TE power generation characteristics of Mg2Si, it is believed that the use of nanostructured material in the sintered chips reduces thermal conductivity; however, there is concern that oxidation is enhanced during pulverization of the Mg2Si. With the current manufacturing-type process using the all-molten Mg2Si source, we found no evidence of the formation of MgO during pulverization down to powder sizes of 100-300 nm.

One unexpected feature of the TE power of oxides containing magnetic transition metal lies in the quasi T-independent S(T) curve over a large T range. For instance, consistently with the theory of the strongly correlated electron system [1], well above the temperature of the quasi particles formation, the Sconstant behaviour of SrRuO₃, an itinerant ferromagnet, can be approximated by the generalized Heikes formula by taking into account the spin entropy term [2]. The SPS ceramic of CoS₂, an itinerant ferromagnet crystallizing in the pyrite structure has been revisited [3]. Although an electron magnon term is needed to reproduce the S(T) data in the ferromagnetic state (TC122K) as for some oxides, S shows a T dependent metallic regime at high T (T>>TC) which contrasts with the quasi T-independent S in the itinerant ferromagnet SrRuO₃. A similar T-dependent regime is indeed observed in the case of the quadruple perovskite ruthenate Ca_1-xLa_xCu₃Ru4O₁₂, a Pauli like metallic paramagnet [4]. The large value for CoS₂ together with its low resistivity leads to PF1mW.m^-1.K^-2 for TC-1.K^-2 at 50K. A comparison between CoS₂ and SrRuO₃ will be made to enlight the similarities/differences between these 3D materials.
References:
[1] X. Deng et al, Phys. Rev. Lett. 110, 086401 (2013),
[2] see for instance: Y.Klein et al, Phys. Rev. B 73, 052412 (2006),
[3] S. Hébert et al, J. Appl. Phys, 114, 103703 (2013),
[4] N. Hollman et al, Phys. Rev. B 87, 155122 (2013).

Among the potential materials for thermoelectric applications, Higher Manganese Silicides (HMS) MnSix (with x around 1.75) exhibit interesting figures-of-merit at intermediate temperatures (573K to 873K). Moreover it appears that the figure-of-merit can be improved by germanium doping [1]. The optimization of the elaboration of such alloys needs the knowledge of the ternary Ge-Mn-Si system and of its constitutive binaries.
Literature data of the Mn-Si system [2] is analyzed and discordances are pointed out. First principles calculations are performed to clarify the enthalpies of formation of the intermetallic phases. Especially the enthalpies of formation of the various possible structures of the MnSix are discussed. On the basis of these new data, a thermodynamic description of the Gibbs energy of the phases is performed using the Calphad method. The system Ge-Mn is also assessed using the Calphad method for the first time.
For both these systems, liquid phases are described with an associated model and the variation to the stoichiometry of the solid phases is taken into account.
Finally a thermodynamic description of the ternary system is suggested. Especially the solubility of germanium in MnSix is modeled.
References
[1] I. Aoyama, M.I. Fedorov et al., Jpn. J. Appl. Phys. 44(12), 8562-8570 (2005)
[2] A.B. Gokhale and R. Abbaschian, Bull. Alloy Phase Diagr. 11(5), 468-480 (1990).

Thermoelectric devices that convert waste heat directly into electricity have potential to reduce greenhouse gas emissions and provide a cleaner mode of generating electricity. The performance of thermoelectric materials can be gauged by examining their figure of merit, ZT, which is given by zT=(S^2 σT)/(κe+κl). Here, S is the Seebeck coefficient of the material, σ is the electrical conductivity of the material, and κe and κl are the thermal conductivities of the material from electronic and lattice contributions, respectively. Enhancing the zT values of materials beyond the current state-of-the art requires enhancing their electrical conductivities, while simultaneously lowering their thermal conductivities. Recent studies indicated that nanostructuring of materials, and thereby selectively reducing their κl serves as a route for enhancing their zT values. Of the many forms of nanomaterials, single-crystalline nanowire form is known to offer the possibility of enhanced charge transport. Hence, materials in nanowire form are ideal for the fabrication of next-generation thermoelectric devices. Furthermore, a material useful for the fabrication of thermoelectric is magnesium silicide (Mg2Si). It is non-toxic, comprised of only abundantly available elements, and inexpensive.
Fabrication of bulk thermoelectric modules using Mg2Si nanowires as the fundamental building blocks requires the following: Strategies for the mass production of pure Mg2Si nanowire powders, and strategies for the large-scale assembly the synthesized nanowire powders. Additionally, any large-scale strategy developed for the integration of nanowire powders should characteristically ensure that the nanowire-nanowire interfaces in the assemblies are oxide-free. Synthesis of Mg2Si nanowires by direct reaction schemes, via the supply of Mg and Si through the vapor phase onto substrates, is very difficult. This difficulty is owed to the high vapor pressure and consequently low condensation coefficient of magnesium, coupled with its high propensity for oxidation. In a previous work, we reported an alternate strategy for the formation of Mg2Si nanowires, the phase transformation of pre-synthesized silicon nanowires into Mg2Si nanowires (Kang et al., Materials Letters, 100, 106-110, 2013). Vapor phase supply of magnesium onto silicon nanowires led to the formation of polycrystalline Mg2Si nanowires in that study. In this context, this talk will focus on alternate experimental routes for the phase transformation of single-crystalline silicon nanowires into single-crystalline Mg2Si nanowires. In addition, strategies for welding the nanowires so as to assemble them on a large-scale, while also obtaining oxide-free nanowire-nanowire interfaces, will also be presented in this talk.

Materials that exhibit large absolute Seebeck coefficients (the constant of proportionality that quantifies the conversion of an applied temperature difference into an electric potential), in addition to other optimal transport properties, are considered candidates for use in thermoelectric applications. Comparative measurements of this parameter, especially at high temperature, have proven challenging due to the diversity of instrumentation and the lack of standardized measurement protocols. We present measurements of the Seebeck coefficient as a function of contact geometry (2- and 4- probe arrangement) under both steady state and transient thermal conditions of the differential method, using a custom developed apparatus capable of in situ comparative measurement. The measurement of surface temperature by contact (a prevalent high temperature design) is influenced by intrinsic thermal errors. Application of a sensor to the surface of the sample modifies the thermal interaction of the contacted surface with the environment, inducing a parasitic thermal transfer between the sample and the sensor that perturbs the local temperature field. We demonstrate experimentally that the contact geometry, dependent on the thermal interface, is the primary limit to high accuracy, while the measurement technique, under ideal conditions, has little influence on the measured Seebeck coefficient. The off-axis 4-probe contact geometry, as compared to the 2-probe, results in a greater local temperature measurement error that increases with temperature, thereby overestimating the Seebeck coefficient. Our results illustrate the likely outcome of commonly adopted measurement practices currently employed among both commercially available and custom developed instrumentation. As such, the stated conclusions are expected to be qualitatively applicable to guide researchers in developing reasonable uncertainty limits.

Complex oxides are excellent candidates for use in thermoelectric (TE) energy harvesting devices. The key challenge is optimization of the TE figure of merit (ZT). Despite the high thermal conductivity present in most oxides, the power factor (S^2 σ ) of materials like doped SrTiO3 rivals that of bismuth telluride. Recent theoretical studies have suggested that magnetic co-doping could facilitate enhancements in the ZT. For example, vanadium (V) has been shown via full-potential density functional theory to enhance ZT up to 28% [1] for V concentrations up to 25 at% in bulk niobium-doped strontium titanate (SrTiO3). In this study, co-doping was achieved by alternate ablation of vanadium or iron and Nb-doped SrTiO3 targets. The samples were then annealed to enhance crystallinity. Our preliminary results suggest that the co-doping can be utilized to enhance the thermal transport characteristics. In this talk, we will show the results of aberration corrected analytical microscope (TEM/STEM) combined with energy dispersive X-ray spectroscopy EDS that is used to characterize the V- Nb and Fe co-doped thin films along with thermal transport measurements.
1. K. Ozdogan,DOI:10.1063/1.4714541

Environmentally friendly thermoelectric materials are believed to be a promising technology with respect to waste heat energy conversion. A good thermoelectric material needs to demonstrate a low electrical resistivity and thermal conductivity and have a high thermoelectric power. Ca₃Co₄O₉ is one of the promising materials among all others oxides with its high thermo-power, low thermal conductivity and resistivity values at high temperatures. For any real practical use in power generator, the ZT needs to be higher than 1. One of the ways for optimizing both these layers must be very effective to improve their conversion efficiencies by dopants and substitutions at the Ca-site or at the Co-site of some elements. Polycrystalline Ca₃Co₄O₉ and the transition metals-doped Ca_3-xCr_xCo₄O₉ (x=0.01, 0.03, 0.05 and 0.07) thermoelectric samples have been prepared by citrate sol-gel process and their thermoelectric properties have been carefully studied at temperatures from 300 K up to 1150 K. Microstructural characterizations have demonstrated that all the Cr has been incorporated the Ca₃Co₄O₉ structure and no secondary phases have been investigated. Apparent density measurements have shown that all samples are highly similar, with densities around 96 % of the theoretical one. FE-SEM observations have shown that all samples possess very similar microstructure and are composed by plate-like grains with similar sizes. A decrease of the measured resistivity from 0.00 to 0.07 Cr substitutions with a very similar behaviour has been investigated with the values of 6.5 and 2.2 m#8486;cm at 1100K, respectively. Thermal conductivity measurements of doped (for x=0.07: 1.11 W/mK) and undoped (for x=0.0: 0.91 W/mK) samples showed no significant difference in value and nearly steady trend as a function of temperature was observed. The values of the Seebeck coefficient increase with temperature, with similar behaviour for all the samples except for the 0.03 Cr-doped one. The room temperature values increase with increasing the Cr contents from ~ 120 µV/K for the undoped sample to ~ 130 µV/K for the 0.03 Cr-doped one. The maximum Seebeck coefficient value (~ 193 µV/K) at 1100 K, higher than the obtained for the undoped sample (~ 154 µV/K), is obtained for the sample with 0.03 Cr-substitution. Cr-doped samples indicate that Cr addition does not affect, in a great extent, the Ca₃Co₄O₉ conduction band. The improvement of thermoelectric performance is most significant in 0.03 doped sample due to the significant change in thermopower and resistivity. The substitution of Cr for Co results in increasing of thermopower slightly but decrease of electrical resistivity, which could be attributed to the decreasing of carrier concentrations. The improvement in both parameters along with low thermal conductivity leads to higher ZT value of 1.39 than the usually obtained in literature.

Ca3Co4-xMxO9 thin films (M = Cu and Zn, and 0 le; x le; 0.3) with the thickness around 100 nm have been prepared by pulsed laser deposition (PLD) method. Extensive characterization revealed that these thin films were well crystallized with c-axis orientation. It was found that both Cu- and Zn-substitution can reduce the electrical resistivity by the increased carrier concentration. However, the deterioration of Seebeck coefficient by Zn-substitution was much less then Cu-substitution. As a result, the highest power factor among Zn-substituted thin films reached 0.79 mWm-1K-2, which was higher than 0.76 mWm-1K-2 of the best Cu-substituted samples and 0.65 mWm-1K-2 of the pure Ca3Co4O9 thin film.

For realizing a sustainable society, it is necessary to develop the renewable energies with stabilized dispersion type power supply system. Therefore, thermoelectric (TE) power generation from exhaust heat is attracted as stabilized co-generation system combined with heat and electrical power. The discovery of Na_xCoO₂ (p-type) opens to develop TE oxide materials in this decade. Nb-doped SrTiO₃ (n-type) also show excellent TE properties. Tungsten bronze type oxide (TB) such as Sr_xBa_1-xNb₂O₆ (SBN) and Ba₂NaNb₅O₁₅ (BNN) are well known for electro-optic crystal. Recently, n-type thermoelectric properties of reduced SBN single crystal is firstly reported about typical Seebeck coefficient (S) and electrical conductivity (σ) for c-axis. Therefore, crystallographic orientation for TB as TE ceramic application is expected to increase dimension less figure of merit (ZT).
Here, we report about TE properties of non-stoichiometric BNN utilizing Harman method for ZT and thermal conductivity (κ) estimation, and heat transfer analysis by finite element method (FEM).
In experimental, BNN were prepared from BaCO₃, NaCl and Nb₂O₅ powders at 1100^oC-1350^oC for 12h by solid state reaction. Needle-like (Ba,Na)Nb₂O₆ (N-BNN) particles were synthesized at 900^oC-1100^oC for 4 h in closed crucible. Mixture of BNN and N-BNN was made by tape casting for in-plane orientation, and was sintered at 1340^oC for 12h. The orient BNN was reduced at 1050^oC-1100^oC for 2-5h in CO atmosphere. Temperature dependence of S and that of σ for reduced BNN were measured from 20^oC to 300^oC by conventional two probe method and four-terminal method, respectively. ZT and κ of BNN were measured by using Harman method. The measurement probe with BNN was analyzed by finite element method (FEM: ANSYS Mechanical APDL 14.5). The heat transfer and radiation effect for κ in BNN was analysed using heat (q) from electrical furnace, heat transfer coefficient, and radiation ratio.
In results, S and σ were measured about -260 mu;V/K and 17.5 S/cm with weak temperature dependence, respectively. The behaviour of S was associated with small polaron. PF (=S²σ) was estimated to be 1.2x10^-4 W/mK². ZT of BNN was measured as 0.06 by Harman method, and κ was estimated 0.54 W/mK at 200^oC in low radiation state. In order to use Harman method with precise ZT and κ measurements in high radiation state, thermal loss (dq) analysis was also carried out by using FEM. In the presentation, the approaches of crystallographic orientation and reduction for BNN will be emphasized about S in view of small polaron hopping enhancement. In addition to dq, thermal conductance (K) will be analyzed for measurement state of Harman method in order to calibrate radiation effect for κ of BNN.
This study is partly supported by Green Education program, Regional Innovation Strategy support program (MEXT), and JSPS KAKENHI Grant-in-Aid for Scientific Research (C).

Thermoelectric devices have been prepared from 100-200 alternating layers of SiO2/SiO2+Au superlattice films using Magnetron DC/RF Sputtering. Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used to determine the stoichiometry of Si and Ge in the grown multilayer films and the thickness of the grown multi-layer films. SEM and EDS have been used to analyze the surface and composition of the thin films. High energy Si ion beam has been performed using the AAMU Pelletron ion beam accelerator to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and increase the cross plane electrical conductivity to increase the figure of merit. We characterized the thin film devices on their thermoelectric and optical properties.

Keywords: Ion bombardment, thermoelectric and optical properties, multi-nanolayers, figure of merit.
Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-1158862, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF-12-1-0063 and National Nuclear Security Admin (DOE/NNSA/MB-40) with grant# DE-NA0001896, NSF-REU with Award#1156137.

We prepared thermoelectric generator devices from 100-200 alternating layers of SiO2/SiO2+Ge superlattice films using Magnetron DC/RF Sputtering. Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used to determine the stoichiometry of Si and Ge in the grown multilayer films and the thickness of the grown multi-layer films. SEM and EDS have been used to analyze the surface and composition of the thin films. The 5 MeV Si ion bombardments have been performed using the AAMU Pelletron ion beam accelerator to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and increase the cross plane electrical conductivity to increase the figure of merit. Impedance spectroscopy has been used to characterize the multi-junction thermoelectric devices. In addition to thermoelectric properties, we also will be showing our findings on optical properties of the multilayered thin film systems.

Keywords: Ion bombardment, thermoelectric and optical properties, multi-nanolayers, figure of merit.
Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-1158862, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF-12-1-0063 and National Nuclear Security Admin (DOE/NNSA/MB-40) with grant# DE-NA0001896, NSF-REU with Award#1156137.

Bi-Te (bismuth-telluride) based alloy has attractive applications for power generation and device cooler due to its good thermoelectric properties. BiSb-Te (p-type), among several thermoelectric materials, has been intensively studied due to their high figure of merit, ZT, at room temperature. However, thermoelectric properties of the material are heavily dependent on the processing conditions that control microstructure changes in BiSb-Te materials.
P-type BiSb-Te films were co-sputtered on the Si substrate at room temperature from a BiSb alloy and tellurium targets in a radio frequency magnetron sputtering system. The thin film samples were annealed at 200 oC for various durations (1, 4, 8 and 12 h) in vacuum atmosphere. After annealing, The microstructure evolution of the BiSb-Te films was analyzed by using TEM, XRD and pole figure analyses. Bi2Te2 phase type of nano-crystalline microstructure was observed in as-deposited BiSb-Te thin film and a big grain growth is prompted after post-annealing. The results of the electrical and thermoelectric characterization reveal that the microstructure evolution profoundly impacted thermoelectric properties of the samples. Seebeck coefficient and power factor of the films were improved after the post-annealing process. The ZT value of the BiSb-Te films arises from 0.2 for the as-deposited sample to nearly 1 for the 12 h annealed film.

Magnesium silicide (Mg2Si) has attracted much attention in view of its potential application in thermoelectric generators, infrared sensors, and solar photovoltaic cells. Recently, Mg2Si thin films have significant interest for applications in thin film devices on flexible substrates and hybrid devices such as thermoelectric generator and solar photovoltaic cells. On the other hand, fewer reports were published about fundamental properties of Mg-Si thin films.
In this work, we report that the electrical conduction type is experimentally found changing near the stoichiometric composition of Mg2Si thin film. In order to understand the fundamental properties of Mg2Si thin films, Mg-Si thin films were systematically studied using combinatorial approach by RF co-sputtering. RF co-sputtering was used to fabricate continuous composition gradient on quartz glass at 573K with Mg and Si targets. The background pressure before introducing Ar gas was 1.0E-5 Pa and the working pressure was 5.0 Pa during the deposition. X-ray photoelectron spectroscopy (XPS) measurements of the grown films indicated continuous composition gradient. Mg and Mg2Si peaks were detected in the XRD pattern of Mg-Si film for the Mg-rich area. With decreasing Mg content, Mg peak became smaller and Mg2Si peaks were increased in height. In the area of stoichiometric compositions for Mg2Si, Mg peak vanished and there was a significant increase in height of Mg2Si peak. Further decrease of Mg composition caused decrease of Mg2Si peaks. Electrical resistivity and thermoelectric voltage were used to characterize the films in each area. In Mg-rich area, resistivity is very small for the presence of metal Mg and the conduction type is n-type. In the area near the stoichiometric composition of Mg2Si, electrical resistivity and thermoelectric voltage were increased dramatically, and then a transition of electrical conduction type from n-type to p-type occurred at the point where the strongest peak of Mg2Si appeared in the XRD pattern.
We are going to analyze the origin of the transition of conduction type from n-type to p-type by XPS, FTIR and Raman measurements and also first-principle calculations of Mg2Si.

In quest of clean energy to complement present and future energy demand and to reduce the carbon footprint generated by the conventional fossil fuel, alternate energy options such as photovoltaic, thermoelectricity, piezoelectricity, wind energy, geo-thermal, infrared energy harvesting, etc. is gaining significant attention. Clearly photovoltaic, hydrothermal and wind energy are the front runners. At the same time, there is abundance of thermal difference exists between the outside environment and inside room in civil and other infrastructures (example: submarines in deep ocean). It is well known that room heating and cooling consume significant amount of power (nearly 20% of whole global energy consumption). Therefore, in the recent past we have demonstrated thermoelectricity generating windows using nano-materials embedded window glasses with potential of 300 watts of power generation from 100 ft² when a temperature gradient of 20 °C exists between the outside environment and inside the room. We ball-mill the commercially available thermoelectric micro-powders to nanostructure them, next by using hot pressing inside custom made mold we give them cylindrical (5 mm long, 2 mm diameter) thermopile shapes. We compare such techniques with thin film deposition (sputtered) based thermoelectric generator formation in the edge of window glasses. Next to advance such development further, we report that in northern territorial countries we can use the same technology and can offset some energy consumption. Additionally, we also show that low cost thermoelectric materials such as copper or nickel wire can be spin cast inside the mold of masonry concrete and slabs to generate substantial amount of power from the whole building infrastructure. Especially rapid build-up of high rise building provides opportunity to embrace such technology. We have also investigated the role of environmental impact such as rain and show that by spin coating transparent silicone we can mitigate any short circuiting or such issues. Next, we also show that usage of transparent conductive oxide or 2D atomic crystal structural materials like graphene can be used to overcome challenges associated with opaqueness formed by embedding thermoelectric nano-materials inside window glasses or such. We also investigate the possibility of integrating a concentrator with vertically aligned thermopiles to use them in a double-pane glass. We complement our studies with modeling and theoretical analysis. Therefore, we provide a comprehensive picture of a very exciting mass-scale clean thermoelectric energy harvesting technology from the naturally abundant thermal gradients.

Highly mismatched alloys have been predicted to exhibit enhanced thermoelectric properties. Here we report on transport properties of one such system, nitrogen-doped ZnTe epitaxial layers on GaAs (100). Hall effect, electrical resistivity, and Seebeck coefficient measurement were performed between 5 K - 300 K for samples with a room temperature hole concentration of 0.34 - 2.16 × 10¹⁹ cm^-3. Significant phonon drag thermopower reaching 1.5 - 2.5 mV K^-1 was observed. Fermi-Dirac statistics was used to analyze the transport parameters of ZnTe:N films assuming a single parabolic band. The power factor demonstrates a measurable improvement with increasing nitrogen concentration.
This work is supported by the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0000957.

We investigate magnetic doping as a possible route to enhance the thermoelectric behavior of SrTiO3-based materials, using first principles calculations within full-potential density functional theory. We study the effects of V/Nb co-doping on the local electronic structure and demonstrate for Co doping a particularly high figure of merit, where the temperature at which the maximum value appears can be tailored by means of the Co concentration. Doping of Pr on the Sr site also shows a positive effect on the thermoelectric performance. The high temperature stability of doped SrTiO3 in O environment and an easy tunability of the electronic states under substitutional doping combined with a substantial improvement of the figure of merit opens great potential for materials optimization by magnetic doping.
References:
Journal of Applied Physics 111, 054313 (2012)
Applied Physics Letters 100, 193110 (2012).

The electronic structure and thermoelectric properties of strained (biaxial and uniaxial)
Sr0.95Pr0.05TiO3 and SrTi0.95Nb0.05O3 are investigated in the temperature range from 300 K to
1200 K. Substitutions of Pr at the Sr site and Nb at the Ti site generate n-type doping and
thus improve the thermoelectric performance as compared to pristine SrTiO3. Further enhancement
is achieved by the application of strain, for example of the Seebeck coefficient by 21 % for
Sr0.95Pr0.05TiO3 and 10 % for SrTi0.95Nb0.05O3 at room temperature in the case of 5 % biaxial
strain. At 1200 K we obtain figures of merit of 0.58 and 0.55 for 2.5 % biaxially strained
Sr0.95Pr0.05TiO3 and SrTi0.95Nb0.05O3, respectively, which are the highest values reported for rare
earth doped SrTiO3.

Oxide ceramics with the nominal composition La1-xCaxCoO3 (0le; xle; 0.12) were prepared using a solid-state reaction method. Their structural and morphological properties were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Transport properties were evaluated in the temperature range between 100 and 290 K using Seebeck coefficient S(T) and electrical resistivity ρ(T) measurements. S(T) showed positive values, suggesting a p-type material, and decreased with the Ca content. The electrical resistivity, measured using a four-probe DC method, revealed semiconducting behavior; ρ(T) decreased with the Ca content and reached minimum values close to 10 mOmega;-cm at room temperature. From S(T) and ρ(T) data the thermoelectric power factor was calculated, which reached maximum values close to 10 mu;W/K2-cm. Thus, these ceramics are promising materials for use in thermoelectric devices for room-temperature applications.

Samples of La0.7Sr0.3Co1-xMnxO3-d (0 x 0.12) (LSCoO-Mn) were grown by citrate sol-gel method, their transport properties were studied in the temperature range between 100 and 290K as a function of temperature and the manganese content. Seebeck coefficient (S) is positive over the measured temperature range and its magnitude increases with the manganese content up to values close to 140 mu;V/K. Electrical resistivity (ρ) goes from metallic to semiconducting behavior as the manganese level increases, at room temperature ρ(T) exhibit values less than 3 mOmega;-cm. From S(T) and ρ(T) experimental data it was possible to calculate the thermoelectric power factor, PF. This performance parameter reaches maximum values close to 30 mu;W/K2-cm. The observed behavior in the transport properties become these ceramics potential thermoelectric materials, which could be used in thermoelectric applications based on oxide ceramics.

We report high thermoelectric performance in p-type polycrystalline BiCuSeO, a layered oxyselenide composed of conductive (Cu2Se2)2- layers alternately stacked with insulating (Bi2O2)2+ layers. Electrical transport properties of BiCuSeO system can be significantly improved by substituting Bi3+ by Ca2+. These materials exhibit a large positive Seebeck coefficient of ~ 330 mu;VK-1 at 300K, that could originate from the “natural superlattice” layered structure and the corresponding large effective mass that is suggested by both electronic density of states and carrier concentration calculations. After Ca doping, enhanced electrical conductivity coupled to moderate Seebeck coefficients leads to power factor value of ~4.74 mu;Wcm-1K-2 at 923K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature ranges from 300K (~0.9 Wm-1K-1) to 923K (~0.45 Wm-1K-1). These intrinsically low thermal conductivity values possibly result from the weak chemical bonds (Young&’s modulus, E ~76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, γ ~1.5). In addition to optimizing the power factor, Ca doping reduces the lattice thermal conductivity, as confirmed by both experimental results and Callaway calculated model. The combination of optimized power factor and the intrinsically low thermal conductivity gives a high ZT of ~0.9 at 923K for Bi0.925Ca0.075CuSeO.

Large efforts have been made in the last years to improve thermoelectric characteristics such as the Seebeck coefficient (S) and figure of merit (ZT) of various materials, in which telluride, antimonide, and some silicide compounds are the populars. Meanwhile, new material searching taking consideration to material diversification need to be encouraged, so that the range of applications would be broadened and devices could applied under a wide range of environments. In this study, conduction properties and thermoelectric properties of oxide glass based conductive materials were investigated. Here we demonstrate conduction properties and thermoelectric properties of vanadate glass of V₂O₅-P₂O₅-Fe₂O₃-CuO based conductive oxide materials. V₂O₅-P₂O₅ glasses are considered to have layered structure comprise VO₄, VO₅ and PO₄ polyhedra, that reveals semiconductor properties due to hopping conduction between V⁴⁺ and V⁵⁺ ions. The glass has relatively low transition temperature (Tg), softening point (Ts) and crystallization temperature (Tc) around 300°C. The melt-quenched and milled raw glass powder was applied to α-Al₂O₃ substrates via paste form by adding solvent of diethylene glycol monobutyl ether acetate and cellulose type binder. Prepared film samples were then heat treated for glass softening and crystallization after volatilization and combustion of organic additives. Thermal treatments were conducted in air and in controlled reductive atmosphere such as vacuum condition a temperature range of 300°C to 550°C. The crystallized phase altered from V₂O₅ to VO₂ and V₂O₃ or which coexist with composite oxides comprise V⁵⁺ or V⁴⁺ and V³⁺. The resistivity and the Seebeck coefficient of crystallized glass films reached 10^-2 Omega;cm and minus;130 mu;V/K according to the alteration of the crystalline phase. It is noteworthy that conduction mechanism changes that those semiconductor materials could controlled to be not only n-type but also p-type although using the same starting material. These conduction characteristics would be elaborated in the presentation as well as their crystallographic informations.

Many modules have a high contact resistance, which usually occurs at the interface between Na_xCoO₂ and Ag electrodes. Contact resistance at the interface between the semiconductor and the metal electrode is a serious problem in many TE modules. An oxide-metal junction notably causes a high contact resistance. Accordingly, some reports on the fabrication of metal oxide-based TE modules show improvement in electrical contacts. In particular, Na_xCoO₂ has a high contact resistance because it forms an insulated layer of NaHCO₃ and Na₂CO₃ that are produced in a chemical reaction with carbon dioxide and water in air on the surface. For example, electrodes made of silver pastes containing different oxide powders have been reported that improve the electrical contacts. In this study, we tried to improve the interface resistance between Na_xCoO₂ and Ag sheet electrodes by connecting these materials with the spark plasma sintering (SPS) technique. Unlike the conventional powder sintering method, SPS makes it possible to prepare ceramics at low temperatures and over a shorter duration by charging the intervals among powder particles with electrical energy and applying a momentary highly energized spark electrical discharge. In addition, there was an example of a report that stated that SPS can break the thin layer of alumina, making it possible to sinter aluminum powder, which is technically difficult to do ordinarily. In this paper, the interface resistance between Na_xCoO₂ and Ag sheet electrodes connected by SPS is compared with that connected with Ag paste. In an experiment, the interface resistance of a sample treated by SPS decreased less than 1/600 times compared with that between Na_xCoO₂ and Ag paste. It is thought that the NaHCO₃ and Na₂CO₃ insulated layer is decomposed through the application of a large value of applied DC current by using the SPS technique.

According to the conventional thermoelectric material, metal oxides have been considered to unsuitable for thermoelectric material.
Because many of metal oxides have a low mobility.
However, in recent years, metal oxides with a unique electronic structure has attracted attention as a thermoelectric material.
Original LaOCuS with a layered structure is known as a transparent p-type natural superlattice semiconductor.
First of all, We have performed ab initio total energy calculation on native point defects to investigate the origin of p-type conductivity in ‘undoped&’ original LaOCuS.
We have found that sulfa interstitial and copper vacancy have relatively low formation energies and they are the relevant defects in LaOCuS.
It is also found that sulfa interstitial induces deep levels, while copper vacancy induce no deep levels in the band gap. Our results indicate that copper vacancy plays an important role in the p-type conductivity.
Dimensionless figure of merit ZT is expressed by a function of the band gap and B factors. B factor is expressed by the effective mass and degenerate of band extrema.
Original hole doped LaOCuS have 8 valley of degeneracy. As a result, LaOCuS of Original has a high power factor. In this paper, for this system, by applying the Block Layered Concept, we make a super lattice. A result of the improved two-dimensional nature strong systems, evaluate whether now What thermoelectric properties compared to LaOCuS the Original, performing Material Design in this system.
Estimation of the thermoelectric power is receiving much attention from the viewpoint of computational material design (CMD) for thermoelectric equipment to utilize wasted heat and to control the temperature accurately. On the basis of the first-principles band structure calculation, the Boltzmann equation is applied to estimate the transport coefficient.
Applicability of the Boltzmann equation itself and its constant-relaxation-time approximation is, however, questionable for actual materials, and especially for strongly correlated electron systems. In this session, we examine the applicability to LaO(CuS)(CuTe) which is a strong localized system.

This talk is about the role of grain size of doped strontium titanate oxide (SrTiO3, STO) and Ruddlesden-Popper STO ceramics on the thermoelectric properties, aiming to improve ZT of STO based materials to transform these oxides into high temperature TE solutions.
The increasing need for clean, sustainable energy sources to meet the exponentially growing energy demands of the world has compelled researchers and scientists worldwide to look for new power generation strategies. Industrial, automobile and household waste heat is an untapped, underutilized and abundant energy resource. In this scenario, Thermoelectric (TE) energy generation technology that directly converts heat to electric power by utilizing the Seebeck effect is being looked upon. The efficiency of the thermoelectric generator (TEG) depends on the material&’s figure of merit (ZT) which is a dimensionless quantity that relates the thermal and electrical conductivity: ZT = (S^2 σ/κ) T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is thermal conductivity, and T is the absolute temperature. The best ZT TE materials are metallic alloys, based on highly toxic components (lead, bismuth and tellurium) that pose considerable environmental concerns and with limited temperature working ranges (300 C). Alternative non-toxic high temperature stable materials with high TE efficiency are required.
Nb-doped Strontium Titanate (STO) is one of the promising oxides materials for the n-type component of a TEG. Intrinsically high Seebeck coefficient, large electrical conductivity and high melting temperature are the fundamental properties of this doped material system for TE application. Though the power factor (S2σ) of doped STO is comparable to conventional TE materials, its ZT is meagre due to its large κ.
Nano-structuring and use of the natural superlattice, Ruddlesden-Popper (RP), phases of STO has been reported to lower the value of κ. However the effect of grain size on κ has been seldom reported. Hence, in this work we studied the influence of grain size of doped STO and RP-STO ceramics on the thermoelectric properties, stating a possible way to improve ZT of STO based materials and transforming STO based material into high temperature TE solutions.
The samples were prepared by the conventional solid-state reaction technique and the grain-size reduction was carried-out by attrition milling. Structural and microstructural characterization were carried-out by XRD and electron microscopy (SEM and TEM). The morphology and chemical composition were assessed by Coulter Counter, TEM and EDS. The electrical resistance was measured using a DC 4-probe technique up to 1000K, Seebeck coefficient was measured by ZEM-3 and the thermal conductivity was measured using a Laser Flash technique up to 1000K. Results are discussed in the framework of microstructure influence on the reduction of thermal conductivity, preserving the high power factor of doped STO.

Thermoelectric material systems have many unique advantages such as silent operation, time reliability, and dimensional scalability. Most recently, researchers Song et al. [1] found that MnO2 nanoparticles show a giant Seebeck coefficient of S = 20 mV/K, which is 100 times higher than bismuth telluride, one of the best TE materials. Song et al.[1] concluded the paper claiming that the giant S is related to the surface density of the electronic states (DOS). However, no figure of merit measurements have been reported so far. In this project, we present preliminary results of the figure of merit as a function of particle electrical resistance. Varying the packing density of the particles controlled this electrical resistance. Commercially available 60-230 mesh MnO2 particles were hand-ground using a mortar and pestle for 0, 15, 30, 45 and 1 hour (S4, S5, S6, S7 and S8, respectively). Although the particle sizes were non-homogeneous throughout each sample (ranging from the nanometer to micrometer size), the average particle sizes were determined to be 142, 127, 78, 17 and 12 µm for S4, S5, S6, S7 and S8 respectively. For measuring the figure of merit, we used the Transient Harman method and obtained values in the range of 0.12 to 0.18 for each sample over a particle resistance range of from 10 to 80 Omega;. The Seebeck coefficient varied for each sample, with an average generally around 300 µV/K. This preliminary data also shows that these particles obey the Wiedemann-Franz Law, displaying a linear relationship between the electrical and thermal conductivities. The Lorentz numbers of our particles are within 1 to 2 orders of magnitude higher than the nominal Lorentz number (i.e., L=2.44E-8 WOmega;K-2).

Hybrid organic-inorganic materials form a growing field of research that can offer many exciting prospects for the improvement of material properties by making it possible to combine some of the properties typically found in organic and inorganic materials, e.g. flexible electronics. For the purposes of optimizing the performance of thermoelectric materials, the very different nature of the organic and inorganic constituents in hybrid materials could potentially be exploited to achieve higher ZT values for instance by hindering the propagation of phonons through the material. In order to realize this potential though, nanostructuring approaches are needed, and the atomic layer deposition (ALD) technique is ideally suited for this purpose. By combining ALD with molecular layer deposition (MLD), hybrid organic-inorganic thin films can be fabricated with precise control over the thin film structure, enabling the design of a variety of nanostructures.
In this work, the ALD/MLD technique was employed to deposit thin films of Al- or P-doped ZnO with single layers of one of three organic molecules repeating periodically to create a hybrid superlattice structure. 1,4-aminophenol, oxydianiline and hydroquinone were used in the deposition of the organic layers. The precursors used for the inorganic part of the deposition consisted of diethyl zinc (l), trimethyl aluminum (l) and trimethyl phosphate (l) for the cations, and H2O (l) was used as the oxygen source. The ratio of ZnO and organic layers in the films was varied between 199:1 and 1:1, while the inorganic dopant concentration ranged from 0 to 6.67 at.%.
The structural properties of the films were determined using X-ray diffraction and reflection techniques. The X-ray reflection data strongly indicated the presence of a superlattice structure in most of the hybrid films. The organic constituents in the thin films were analyzed with a FTIR spectrometer, which clearly showed that the films contain benzene rings and other organic groups associated with the organic molecules used in the study. The effects of the inorganic dopants and organic layers on the films&’ thermoelectric properties were investigated by measuring the Seebeck coefficient and electrical resistivity of the films at low temperatures. The organic layers had a clear effect on ZnO&’s TE properties, with most results indicating a decrease in the material&’s carrier concentration, although at low concentrations the organic layers seemed to cause a slight increase in carrier concentration. The combined effect of the inorganic dopants and the organic layers on the TE properties was also found to be slightly non-linear.

There exist a vast number of uninvestigated ternary and quaternary materials that could be potential high-efficiency TE materials. Traditional approaches to investigating such compounds involve tedious synthesis and characterization procedures that can be time-consuming and costly. Here we discuss a device and methodology which allows for the rapid measurement of both the electrical resistivity ρ and the Seebeck coefficient α of thermoelectric materials, at a fixed temperature of T asymp; 300K. Using non-traditional resistivity measurements based on the van der Pauw technique combined with rapid, room-temperature thermopower measurements such as those described here, a reliable and time-efficient means of gauging the power factor α2/ρ of newly synthesized thermoelectric materials is achievable. The viability of the apparatus and method has been tested by measurements of the thermoelectric properties of a fractional-filled skutterudite Yb0.2In0.1CoSb3, a Marlow Industries n-type Bi2Te3, and Si80Ge20P2.5. Finally, the efficacy of the van de Pauw technique for measuring the resistivity of thermoelectric materials has been verified, and when combined with the system for rapidly measuring the room-temperature thermopower of thermoelectric materials, can be used as a viable and time-efficient means of determining the power-factor of new thermoelectric materials.

Absolute Seebeck coefficients of reference materials such as lead, copper and platinum are essential for accurate measurements of Seebeck coefficient in thermoelectric materials. Absolute Seebeck coefficient of reference materials above room temperature is determined from Thomson coefficient using a Kelvin relationship in a separate experiment with a typical relative measurement through the temperature ranges of interest. The Thomson coefficient can be determined by measuring the change of the temperature when DC current passes through a sample at two different temperature points. The most accurate reference data have been obtained for lead up to 550 K, tungsten from 273 K to 1600 K, and platinum from 273 K to 1800 K based on the DC method. While prior analysis focused on the DC excitation to calculate Thomson coefficient from experimentally measurable quantities such as dimensions, thermal conductivity, temperature gradients, and input voltages of a sample, we have analyzed the heat conduction in a single metal wire when AC current the root-mean squared (rms) value of which is equivalent to DC passes through the sample. We find our expression to derive Thomson coefficient is simplified by calculating the ratio of the steady-state temperature between AC and DC at sufficiently high frequency, since the Thomson effect is suppressed at AC mode. Our expression requires four parameters such as temperature gradient in a sample, rms value of AC voltage, temperature changes of the sample on AC and DC, therefore, prevents the information of thermal conductivity and length of the sample that a conventional DC method requires. Our analysis on AC-DC method would suggest the improvement of accuracy for the determination of absolute Seebeck coefficient above room temperature.

Characterization of the thermoelectric properties of films is of high interest because thru nano-structuring, the figure of merit (ZT) may be enhanced. The importance of robust, versatile, and fast measurement techniques is paramount for the advancement of the field. To date, much work has emphasized in-plane characterization of thermoelectric film materials, and less attention has been focused to the challenging characterization of cross-plane properties. The photothermoelectric method allows for the simultaneous characterization of both in-plane and cross-plane thermal conductivity and in this presentation we show how it can be extended to Seebeck coefficient characterization of thin film thermoelectric materials. This work emphasizes characterization of films at room temperature.

The development of novel nanostructured materials has become increasingly important to produce high-ZT thermoelectrics for energy harvesting and solid-state cooling; yet these materials generally exhibit limited elastic compliance and thus cannot be readily integrated to fabrics and curved surfaces. On the contrary, it has been shown that nanowire networks can be stretched and folded extensively without failure. Nanowire network-based thermoelectric materials, however, have not been realized so far. This poster presents our recent work on the fabrication and properties of electrodeposited indium-antimony (InSb) nanowire networks, which can be assembled on compliant polymeric substrates to form flexible thermoelectric devices. We also address the use of bimetallic Ni-InSb nanowires, which can be magnetically aligned to improve reversible sliding in networks, as well as nanotwinned InSb nanowires, which exhibit increased elastic limit and reduced thermal conductivity, compared to single crystalline nanowires. High-ZT nanowire-network materials could enable flexible thermoelectric devices for a wide range of applications.

Many applications have emerged for conducting polymers since the discovery of polyacetylene in the 70s, such as OLEDs, sensors, transistors, solar cells and, more recently, thermoelectric devices. These types of organic materials offer many advantages in comparison to inorganic materials, such as: low production cost methods, easy chemical modification, low cost of raw materials, good mechanical properties, not toxicity and low thermal conductivity (around 0.2 W/m K). In the last years, many efforts have been devoted to the increase of the thermoelectric efficiency of these materials, usually measured in terms of ZT, the dimensionless figure of merit. Typically, in polymers like Polyaniline and PEDOT:PSS, ZT values around 10E-5-10E-3 have been obtained [1-2]. Recently, Tosylate has been used instead as counter ion in PEDOT and, optimizing the doping level, a good ZT value around 0.2 have been observed [3-4]. Good ZT values have been also found for PEDOT:ClO4 in bulk form (ZT=0.03-0.04). The nanostructuration of these materials can be the key to develop high efficiency thermoelectric materials. In particular, we have shown that it possible to obtain PEDOT:ClO4 nanowires using anodic alumina templates. The figure of merit has increased slightly as compared to the bulk counterpart (ZT>0.05).

Layered cobaltate (Ca2CoO3)0.62CoO2 has attracted great interest due to its unusually large thermoelectric power. The specific combination of highly crystalline and strongly distorted layers is essential for enabling large electronic-conductivity and low thermoconductivity that is associated with efficient phonon scattering. To understand the role of disorder we developed a STEM method that allows us to accurately measure local static and thermal displacement through simultaneous acquisition of multiple annular-dark-field images at different scattering-angles. We determined its complex crystal structure, including the incommensurate displacive-modulation and local atomic vibrations through structural refinement. We show the displacive modulation in the system is dominated in the CoO layer of the Ca2CoO3 blocks separated by the well- ordered conducting crystalline CoO2 layers. Based on the determined structure, our theoretical calculations of three-dimensional phonon-dispersion and scattering suggest that acentrosymmetric displacement and resonance scattering are the main origin of the effectively reduced thermoconductivity in the system. The work was supported by the U.S. DOE, under contract No.DE-AC02-98CH10886.

Thermoelectric research is of increasing importance in the development of technologies for improving vehicular fuel economy and for mitigating green house gas emissions. For waste heat energy conversion applications, oxide materials which have high temperature stability are potential candidate materials. In the Ca-M-Co-O (M=Sr, Zn, La, and Nd) systems, in addition to the well-known Ca3Co4O9 phase (with misfit layered structure) that has excellent thermoelectric properties, and the M-doped phases (Ca,M)3Co4O9 or Ca3(Co,M)4O9 phases, other low-dimensional phases include the homologous series, An+2ConCo&’O3n+3 (where A= (Ca, Sr) and (Sr,Ca)) and Can+2(Co,Zn)n(Co&’,Zn)O3n+3. While the members of the An+2ConCo&’O3n+3 series have reasonably high Seebeck coefficients and relatively low thermal conductivity, the electrical conductivity needs to be increased in order to achieve higher figure of merit (ZT) values. This paper discusses our phase equilibria/structural/property studies of selected cobaltates in the Ca-M-Co-O (M= Sr, Zn, La, and Nd) systems.

We report on substrate dependence of thermoelectric properties for 2% Al-doped ZnO (AZO) thin films. We fabricated ~ 500 nm thick films by means of Pulsed Laser Deposition on SrTiO₃ (STO) and Al₂O₃ single crystal substrates at T_d = 300, 400, 500 and 600 °C keeping an oxygen pressure of 27 Pa. Dense AZO target was irradiated by Nd:YAG laser (266 nm, 10 Hz) which energy density is about 4.2 J/cm² for deposition period of 30 min. Electric conductivity (σ) and Seebeck coefficient (S) were evaluated in the interval T = 300 - 600 K.
The films deposited on Al₂O₃ are epitaxial and fully c-axis oriented showing only (002) peak in XRD patterns, independently of T_d. Films deposited on STO are c-axis oriented for T_d = 300 and 400 °C, though at higher T_da-axis orientation (110 peak) also appears. SEM images revealed the nano-sized grain morphology of all films. σ presents a clear semiconducting behavior on STO, while is almost constant with T for the films grown on Al₂O₃. S decreases with T_d for all substrates. S sign is always negative, confirming the n-type conduction due to Al³⁺ substitution, and his absolute value increases with T_d. At the same T_d, films deposited on STO always shows much higher values of σ, S and S²σ (power factor, PF) in comparison with films deposited on Al₂O₃. For example, at T_d= 300 °C and T = 600 K, σ_STO = 382 S/cm, S_STO = -121×10^-6V/K, PF_STO = 0.55 W/m×K² while σ_Al2O3 = 116 S/cm, S_Al2O3 = -44×10^-6V/K, PF_Al2O3 = 0.02 W/m×K². AZO film has larger epitaxial strain ε_c ~ 18%) and larger concentration of dislocations (N_c ~ 4×10¹¹ cm^-2), while ε_c ~ 6%, ε_a ~ 2% and N_c ~ 1×10¹⁰ cm^-2, N_a ~ 1×10¹¹ cm^-2 for AZO on STO. σ is reported to decrease and S to increase with N [1]: this consideration explains the different behavior of the two films. In order to reach optimal N = 10⁶~10⁸ cm^-2 [1] and further improve the thermoelectric performance, it is necessary to release the epitaxial strain by growing thicker films [2] (1000 nm or more) or inserting buffer layers [3]. Evaluation of AZO thin films deposited on amorphous substrate (fused silica) is in progress and will be reported at the conference.
[1] J. M. Woodall et al, Phys. Rev. Lett. 51 (1983) 1783
[2] J.R Waiting and D. J. Paul, J. Appl. Phys. 110 (2011) 114508
[3] S.H.Park et al, App. Phys. Lett. 91 (2007) 231904
[4] J.S.Tian et al, J. Cryst. Growth 310 (2008) 777

In the last decade oxides have emerged as promising high temperature thermoelectric candidates, particularly perovskites and layered cobaltites because of their flexible structure, high temperature stability and encouraging ZT values (where T = absolute temperature and Z = a parameter which depends on S the Seebeck coefficient, electrical resistivity and thermal conductivity). In order to improve the material performance (by maximizing Z) then it is necessary to increase S whilst reducing thermal conductivity and electrical resistivity. One strategy is to employ microstructural engineering at the nanoscale to increase phonon scattering in order to reduce thermal conductivity. We are employing a combination of experimental and modeling techniques to understand nanostructuring processes (at different levels of complexity and dimensionality) in oxides including systems based on CaMnO3, La2x/3Sr1-xTiO3, ZnO and materials exhibiting self assembly nanostructures. By modelling thermal and electron transport it will be possible to understand the processes in more detail and the relative importance of factors controlling thermal and electrical conductivity in the materials. In addition to conventional XRD and TEM investigations, we will define atom level structures by use of high HRTEM (atom level SuperSTEM2). We will simulate structure-stability relationships using DFT and potential-based methods to evaluate effects of dopants, porosity and nanostructural features on thermal and electron transport to define the most effective strategies to enhance thermoelectric properties. Representative results for the oxides will be presented.

Alternative energy technologies, like conversion of waste heat to electricity using thermoelectric materials, have a big role to play in the coming years to reduce our dependence on fossil fuels. Oxide thermoelectric materials have attracted recent attention due to their decreased lattice conductivity and potentially large electrical conductivity. The scope of this research was to theoretically evaluate the compositional design space between three cobalt containing spinel (AB2O4) end members. Those three end members being: Co3O4, Co2NiO4 and Co2ZnO4. Selection of these particular compositions were based on the favorable performance of the independent end members. Moreover, Co3O4 exhibits a large Seebeck coefficient, while Co2NiO4 its increased electrical conductivity. Therefore, it is reasoned that an intermediate composition may result in optimal thermoelectric performance.

To evaluate the thermoelectric properties of the proposed compositional spread, a first principle density functional theory (DFT+U) approach was implemented to predict the ground state properties for a discretized number of compositions within the design space. The ground state results were then used in an independent electrical and thermal non-equilibrium Green's function (NEGF) model to predict the thin film transport properties. The transport properties include the Seebeck coefficient, electrical conductivity and thermal conductivity for each composition. These transport results were then incorporated into the figure of merit (ZT) calculation which was then overlaid on the compositional space to identify trends of a particular composition with greatest thermoelectric performance. Ultimately, the computational framework developed in this research provides a generic computational approach for evaluating more complex compositions in the goal of discovering new and more efficient thermoelectric materials.

Oxide-based semiconductors, thermally and chemically stable at high temperature, have been regarded as promising thermoelectric candidates for utilizing waste heat generated in a variety of industrial processes. High temperature oxide thermoelectrics would also facilitate solar energy harvesting through solar concentrators. Despite extensive investigation for over 20 years on a few promising oxide ceramics, the ZT values of the oxide ceramics still remain rather low due to the medium electrical conductivity and high thermal conductivity. In this work, we demonstrate how to tune thermoelectric properties of n-type In2O3 and p-type BiCuSeO ceramics by heavy dopant element and nanostructures.

The A site ordered perovskite cobalt oxide Sr3YCo4O10.5 was found to show a ferromagnetic transition at 340 K.[1] The saturated moment is determined by the spin state of the Co3+ ions, which have three spin states of high spin (HS), intermediate spin (IS), and low spin (LS) states. In this compound, the Co3+ ions are on the verge of the spin state transition due to the unique crystal structure, and various perturbations drive this transition. For example, the magnetization is reduced by isovalent substitution of Ca for Sr. Simultaneously, the thermopower increases with the Ca content, while the resistivity is unchanged.[2] These results suggest that the thermopower closely relates to the spin state of Co3+. This relation, however, is obscured by the disorder and/or inhomogeneity due to the substitution.
To clarify the relation between the thermopower and the spin state of Co3+, we have measured the hydrostatic pressure effect of the magnetization and the transport properties. In this study, we find that the thermopower drastically increases from 50 to 100 mu;V/K at 100 K, and the magnetization decreases with a pressure of less than 1 GPa. This result directly evidences that spin entropy of Co3+ ions determines the value of the thermopower in this compound.
[1] W. Kobayashi et al. Phys. Rev. B 72, (2005) 104408.
[2] S. Yoshida et al. J. phys. Soc. Jpn. 78 (2009) 094711.

P-type layered misfit cobaltite Bi₂Sr₂Co₂O_X (abbreviated as BSC-222) are considered as promising oxide materials for thermoelectric applications at high temperatures due to its figure of merit (ZT) value of 0.19 at 973 K and high chemical stability in air. BSC-222 exhibits a layered structure, where CoO₂ layers alternate with Bi₂Sr₂O₄ block layers along the c-axis. In these highly anisotropic systems, previous studies show that an almost independent control of electrical resistivity ρ, Seebeck coefficient S and thermal conductivity lambda; can be obtained. For improving the thermoelectric performances of BSC-222 materials, lowering electrical resistivity can be achieved through the enhancement of the material density, the improvement of grain alignment or the increase of grain size in microstructures of prepared samples (for decreasing carriers scattering at grain boundaries). Hot Pressing is an efficient sintering technique for preparing anisotropic materials with high densification and suitable grain alignment. However, this technique is difficult to use for the production of bulk materials in a large industrial scale.
This study deals with the preparation of high performed BSC-222 thermoelectric materials by the development of a simple and reliable bulk melt processing way, similar to single crystal processing. Starting from a partial melted state and by applying a slow cooling rate, an important grain growth is promoted. The thermoelectric properties of such prepared BSC-222 samples have been investigated and compared with BSC-222 samples prepared by hot pressing technique.
[1] R. Funahashi, et. al., Appl. Phys. Let., 2000, 76 No. 17.

Tellurium alloys have attracted significant attention due to their high performance thermoelectric properties [1]. In particular, nanostructured bulk lanthanum telluride (La3-xTe4) by mechanical ball-milling was reported to exceed the figure of merit (ZT) of 1 at high temperatures near 1300K [2-3]. Since the increased thermoelectric efficiency of nanostructured materials are due to the enhancement of phonon scattering introduced by quantum confinement, thin films (2-D systems) and nanowires (1-D systems) also have generated significant scientific and technological interests [4-6]. Here, we report on the electrochemical synthesis of lanthanum telluride thin films. The thickness of thin films can be controlled by deposition duration. We also report on the process of nanowire fabrication involves first electrodepositing lanthanum telluride arrays into anodic aluminum oxide nanoporous membranes. After dissolving the membranes, the ordered and vertically-aligned nanowire arrays are revealed on the substrate [7]. The electrodeposition of lanthanum telluride thin films and nanowires is performed in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bromide, in ambient conditions. These novel procedures can serve as an alternative means of simple, inexpensive and laboratory-environment friendly methods to synthesize nanostructured thermoelectric materials. Characterization of the morphologies and chemical compositions of the deposited films and nanowires by using scanning electron microscopy, x-ray diffractometry, and energy-dispersive x-ray analysis will be presented. The Seebeck coefficient, electrical resistivity, and thermal conductivity of lanthanum telluride thin films and nanowires will also be presented and compared with those of current state-of-the-art thermoelectric materials.
References:
[1] D. M. Rowe, CRC Handbook of Thermoelectrics, CRC Press (1995).
[2] A. May, J-P. Fleurial and G. J. Snyder, Phys. Rev. B 78, 125205 (2008).
[3] O. Delaire et al., Phys. Rev. B 80, 184302 (2009).
[4] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993).
[5] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 16631 (1993).
[6] M. S. Dresselhaus et al., Adv. Mater. 19 (2007).
[7] S. C. Chi, S. L. Farias and R. C. Cammarata, “Synthesis of Vertically Aligned Gold Nanowire-Ferromagnetic Metal Matrix Composites,” 220th ECS Meeting eds. P. Vereecken, G. Oskam, I. Shao, and J. Fransaer, ECS Trans. 41, 119 (2012).

In thermoelectrics, PEDOT:PSS and silicon have drawn great attention due to their high electrical conductivity (~ 1,000 S/cm) and high Seebeck coefficient (intrinsic, -1,200 µV/K), respectively. So far, studies have been performed using Si nanowires to obtain high power factor (~ 22.8 µW/K²cm) by achieving high Seebeck coefficient of silicon intact, as well as reducing its high thermal conductivity by the nanostructuring.¹ Recently, PEDOT:PSS has also been shown that high power factor can be achieved by improving electrical conductivity further.²
In this research, Si / PEDOT:PSS composite thermoelectric system was developed which has high electrical conductivity (~ 400 S/cm) and high Seebeck coefficient (~ +1,000 µV/K) controlled by varying PEDOT:PSS layer thickness (range: 3 ~ 50 nm) on the 50 nm intrinsic silicon-on-insulator (SOI) wafer. The electrical conductivity of this system exhibited linearly increasing trend, and Seebeck coefficient showed volcano shape as the PEDOT:PSS layer thickness rises. It was revealed that the system has energetic barrier between the two layers that filters cold carriers. The improved Seebeck coefficient and lowered electrical conductivity is a consequence of this charge carrier filtering.³ The maximum power factor of the system reaches ~ 40 W/K²cm. In order to accomplish high figure of merit, high thermal conductivity of silicon layer (~ 50 W/mK) was reduced by utilizing array of silicon nanopillars. The PEDOT:PSS layer is coated on the array of nanopillars, to preserve Seebeck coefficient and electrical conductivity.
References
1. J. Tang, H. -T. Wang, D. H. Lee, M. Fardy, Z. Huo, T. P. Russell, and P. Yang, Nano Lett., 10, 4283 (2010).
2. T. Park, C. Park, B. Kim, H. Shin and E. Kim, Energy Environ. Sci., 6, 790 (2013).
3. M. He, J. Ge, Z. Lin, X. Feng, X. Wang, H. Lu, Y. Yanga and F. Qiu, Energy Environ. Sci., 5, 8356 (2012).

The thermoelectric effect was discovered in the early 1800&’s by Seebeck in studies on metals and minerals. Later, in the first half of the 20th Century, further studies on minerals were undertaken by Haken and Telkes. With the advent of semiconductor physics and technology, effort turned in the latter half of the 20th century to synthesizing compounds with controlled compositions from purified elements, a fabrication strategy that has largely continued to the present day.
Concerns over element toxicity and the low abundance of some elemental components such as tellurium and cobalt, have compelled us to return to the study of earth-abundant materials for thermoelectricity. However, we return to this starting point armed with powerful tools that help us to predict the relevant properties of large families of compounds and natural minerals. Guided by density functional theory calculations of lattice dynamics and electronic structure calculations, we have identified the tetrahedrite family of minerals as potential high performance thermoelectric materials. Importantly, the tetrahedrites comprise the most widespread sulfosalts on Earth. The general composition of tetrahedrite may be written as Cu12-xMxSb4-yAsyS13, where M is mainly iron or zinc. An unusual band structure gives rise to large Seebeck coefficient over a wide range of x, spanning the compositions of tetrahedrite that occur in nature. Meanwhile, large anharmonicity drives the lattice thermal conductivity down to near minimum values. As a result, tetrahedrites can possess dimensionless figures of merit exceeding unity at 400C, comparable or even exceeding that of some of the best synthetic thermoelectric materials in this temperature range. We also show that samples synthesized using natural mineral tetrahedrite as a source material possess similar values of figure of merit. The results suggest a new paradigm for thermoelectric material development. Rather than extracting and purifying elements at great effort and cost, and then recombining them into new compounds at more effort and cost, one can use natural minerals directly as sources for highly efficient thermoelectric materials.

In-situ fabrication of a new half-Heusler (HH) derivative with generic composition Ti9Ni7Sn8 is reported. Structural refinements reveal the material to be a composite with 97.1± 1.2% in its half-Heusler phase, 2.6 ± 0.2% in full-Heusler and with traces of metallic Ti6Sn5. In comparison to the normal TiNiSn HH phase, a significant decrease in thermal conductivity (~40%) and simultaneous improvement in the power factor are observed leading to a figure of merit ZT of 0.32 at 773K, which is 60% higher than in equivalent TiNiSn half-Heusler. The bulk Ti9Ni7Sn8 composite was further nanostructured employing mechanical milling. A large reduction in the thermal conductivity from 2.1 W/mK to 1.8 W/mK at room temperature followed by a further decrease at higher temperatures lead to a ZT of 0.65 at 773 K. The mechanism that facilitates thermoelectric ZT enhancement in this composite is related to nanoinclusion- induced electron injection and electron filtering effects. The undoped Ti9Ni7Sn8 half-Heusler derivative obtained by compositional engineering approach is a very promising strategy and could pave the way to a new generation of high ZT thermoelectric materials.
*Corresponding author. E-mail address: [email protected], [email protected] (DKM)

The compounds (LaS)_1+xTS₂ (T: Cr, Nb) belong to the misfit layered family having a general formula (MX)_1+xTX₂ (M: Sn, Pb, Bi, Sb, rare earths; X: S, Se; T: Ti, V, Cr, Nb, Ta; and x: 0.07 - 0.28) and are built of an intercalated LaS layer responsible for disorder and phonon scattering sandwiched between the TS₂ layer with high carrier mobility responsible for electron transport. Being perfect examples of a phonon-glass-electron-crystal (PGEC) behaviour, these misfit sulfides hold good potential as high temperature thermoelectric materials. In this work, we have successfully synthesized pure (LaS)_1.2CrS₂ and (LaS)_1.14NbS₂ misfit sulfides and studied their high temperature thermoelectric properties. The respective powders were obtained by sulfurizing the oxide powders La_1.2CrO₃ and La_1.14NbO₄ with CS₂ gas carried by an Ar gas flow at 1123 K for 12h. The sulfide powders were then consolidated by pressure-assisted sintering at 1223 K for 2 hours under the uniaxial pressure of 30 MPa in vacuum. For (LaS)_1.14NbS₂, the in-plane direction was found to be preferentially oriented perpendicular to the pressing direction which lead to anisotropic properties with electrical conductivity being higher for the in-plane direction. On the other hand, (LaS)_1.2CrS₂ system did not show any considerable anisotropy. We confirmed a p-type metallic conduction for NbS₂ system and an n-type semiconducting behaviour for the CrS₂ system throughout the temperature range studied. Low thermal conductivity of ~1-2 W/mK was observed for both the systems mainly due to the low lattice thermal conductivity arising from the intercalation of LaS layer into the TS₂ layer. Due to anisotropy, the in-plane thermal conductivity of (LaS)_1.14NbS₂ was found to be 5% higher than out-of-plane thermal conductivity at room temperature which increased to 10% at 973 K. The ZT of 0.15 and 0.1 at 973 K was obtained for (LaS)_1.14NbS₂ in in-plane direction and (LaS)_1.2CrS₂ in both direction, respectively.

Ever-increasing energy demands underscore the need for efficient waste heat recovery, and nanostructured thermoelectric materials are an increasingly attractive means to this end. Recent advances have shown enhanced thermoelectric figures-of-merit[1,2] in materials previously known for their bulk performance; however, these materials are commonly compound semiconductors incorporating rare or toxic elements, limiting their potential and adoption rates. Silicon nanomeshes, which have recently been shown to exhibit reduced thermal conductivity[3] while preserving bulk electrical conductivity[4], have significant potential for use as inexpensive, environmentally-friendly thermoelectrics. While altered phononic band structures in the nanomeshes with respect to hole size and spacing have been proposed as a likely mechanism behind the enhanced thermal insulation, these parameters have yet to be fully explored. Moreover, the thermal properties of a given nanomesh are fixed and usually optimized for a small temperature window. We extend this concept by using strain as an external stimulus to enable property tuning and exploit stress amplification nearby film perforations. For realization of truly tunable, and ultimately optimized, transport phenomena in silicon nanomeshes, a highly flexible experimental platform must first be developed.
Here, we show tunable electrical and thermal conductivity in uniaxially strained silicon nanomeshes, tested in situ via four-point probe measurements and micro-Raman, respectively. Using batch fabrication of freestanding nanomesh films with rationally designed architectures on silicon-on-insulator wafers, we present a unique platform for exploring the effects of both changes in nanomesh geometry and strain state on charge and thermal transport. Results are analyzed beginning with a finite element-level model of carrier mobility in arbitrarily-strained silicon which incorporates scattering, band splitting, and band warping effects; following incorporation of thermal conductivity and Seebeck coefficient models, we extend our elemental approach to a global analysis of tuned transport phenomena with respect to pattern, hole geometry, and strain state. The potential of the strained nanomesh as a viable thermoelectric material in the context of conversion efficiency and tunability will be discussed.
Works Cited
1 Poudel, B. et al. Science 320, 634 (2008).
2 Chen, C.-L. et al. J. Phys. Chem. C 114, 3385 (2010).
3 Hopkins, P. E. et al. Nano Lett. 11, 107 (2010).
4 Yu, J.-K., Mitrovic, S., Tham, D., Varghese, J. & Heath, J. R. Nat. Nano. 5, 718 (2010).

We report recent progress in first-principles computational methods for electron transport and show applications of these methods to half-Heusler compounds. The temperature- and doping-dependent electrical transport coefficients are found by solving the semiclassical Boltzmann transport equation which requires as input energy band dispersions and electron lifetimes. The electronic band structure is computed using the GW method which takes into account the electron correlation effects. The intrinsic contribution to electron lifetimes is derived from electron-phonon coupling matrix elements computed using density-functional perturbation theory and interpolated to a fine mesh in the Brillouin zone using maximally localized Wannier functions. The results of our calculations show quantitative agreement with the available experimental data and give insights into optimizing the composition of half-Heusler compounds in order to maximize their thermoelectric performance.

Thermoelectrics as a direct energy conversion method between heat and electricity is mainly used for electrical power generation and cooling applications.
A large variety of materials, such as intermetallic compounds (e.g. half-Heuslers), silicides and chalcogenides (e.g. PbTe and GeTe) have been investigated as thermoelectric materials due to high ZT values at different temperature ranges. Among these material classes, although currently showing lower ZTs, silicides and intermetallic compounds possess additional advantages due to improved mechanical properties, the ability to operate at higher temperatures and the potential for large scale commercialization, since they are composed of naturally abundant and less toxic elements.
Global trends for improving the thermoelectric efficiency via maximizing the ZT values include, electronic doping optimizations; generation of Functionally Graded Materials (FGMs) with an optimal maximal ZT envelope over a wide temperature range; and nanostructuring formation for reduction of the lattice thermal conductivity. Nanostructures generation can be achieved by nano-powdering using energetic ball-milling followed by a rapid consolidation method such as Spark Plasma Sintering (SPS). Yet, due to the demand for high stability characteristics, required for long operation periods at high temperatures, one approach for avoiding nano-features coarsening and thermoelectric properties degradation, is based on utilizing thermodynamically driven nanostructures, due to physical metallurgy based effects such as spinodal decomposition and nucleation and growth reactions.
All of the mentioned above general trends in thermoelectric will be discussed during the talk. A focus on the related activities in the department of Materials Engineering at BGU will be given.

Mechanical integrity is critical to fabrication and utilization of thermoelectric materials and devices. Although various consolidation techniques, such as hot pressing of powderized materials, may improve the strength to some extent, the inherent brittle nature of many thermoelectric materials calls for further strengthening through extrinsic reinforcement. In the current study, we report enhancement of mechanical properties of bismuth telluride-based materials by introducing multi-wall carbon nanotubes (MWCNTs). Temperature dependent thermoelectric and elastic properties will be discussed along with room temperature fracture strength. While maintaining good transport properties of the base material, a small fraction of MWCNT (e.g. 0.5 wt.%) can significantly increase the fracture strength by as much as 200% and decrease the elastic modulus, which reveals the dual roles of the MWCNT: i) linking the grains to improve strength and ii) generating elasticity mismatch at the grain boundaries.

PbTe is known to be one of the most efficient and most promising thermoelectric materials due to its inherently low thermal conductivity and favorable electronic band structure. While most studies of thermoelectric materials only use transport properties to estimate the band positions, we aim to probe the optical properties as well. Most thermoelectric materials, though, are narrow gap heavily doped semiconductors. Degenerately electron doped semiconductors have their chemical potential levels inside of the conduction band. While necessary for optimum thermoelectric performance, it complicates measurement of the valence to conduction band gap due to occupation of the near gap states. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is used to probe the effect of Iodine doping on the optical absorption edge in PbTe. The shifts in the observed optical band gap (Burstein-Moss shift) are quantified by estimating the chemical potential from transport measurements (Hall effect and Seebeck coefficient). Renormalization, or gap shrinkage with increasing doping, is estimated using standard models. The resulting renormalization is shown to be on the order of the band gap itself when using standard models, however, when a self-consistent model is used a more reasonable value is obtained.

After decades of stagnation in thermoelectric efficiency, going to the nanoscale proved to be a way to dramatically increase efficiency. The main reason is that at the nanoscale, boundary scattering becomes dominant, reducing the thermal conductivity, as shown by Dresselhaus et al. [1]. The thermal conductivity is inversely proportional to the thermoelectric efficiency. In specially designed periodic structures, which study has been started by Yu et al. [2], a coherent effect is expected to reduce the thermal conductivity even more. We investigate the impact of such a patterning on Si. In its bulk state, Si is a very poor thermoelectric material, but with its easy processing and high compatibility, it is believed it will provide an interesting power source for energy autonomous devices. In this contribution, we report fabrication of 1D and 2D phononic crystal (PnC) nanostructures, as well as optical measurement of the 1D crystal thermal conductivity. The thermal conductivity measured by time domain thermoreflectance is 14.5 W/m.K which is only 10% of that of bulk Si.
The investigated PnC nanostructures were fabricated using a top-down approach. The patterns are realized by Electron-beam lithography and transferred to the Si by dry etching. Si is the ideal material for such a process, as well as for future integration on-chip.
We developed and built a time domain thermoreflectance setup designed to monitor the thermal conductivity of structures as small as a few micrometer squares, for a wide range of thermal conductivities. A metal pad, in the middle of our suspended symmetrical structure, is heated by a pulsed laser and its reflectivity is monitored by a CW laser. This measurement is then compared and fitted to a heat transfer model created with COMSOL Multiphysics to obtain the thermal conductivity. The thermal conductivity obtained for the Si 1D PnC is 14.5 W/mK, which is only 10% of that of bulk Si. Consistent measures on structures with different lengths confirm this value. The reduced thermal conductivity in this nanostructure seems to stem from both the surface scattering and phononic effect, but further study is necessary to clearly observe the coherent phononic effect.
Acknowledgments
This work was supported by: the Project for Developing Innovation Systems of the MEXT, Japan; by the JSPS through its Funding Program for World-Leading Innovation R&D on Science and Technology” and through Kakenhi (24656576, 25709090); and by The Murata Science Foundation.
References
[1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993).
[2] J.-K. Yu, Nat. Nanotech 5, 718 (2010).

Oxide thermoelectric material Ca3Co4O9 is well known as exhibiting outstanding thermoelectric properties in high temperature as well as high thermal and chemical stability. Its high thermoelectric performance is due to the fact that the nanosheets of CoO2 and the block layers of Ca2CoO3-δ work as good electric conduction path and phonon scattering region, respectively. Li et al. reported that partial Bi-substitution for Ca can improve their thermoelectric properties. [1]
Single crystals of Ca3-xBixCo4O9 (x = 0, 0.1, 0.2, 0.3, 0.4) were made by flux method. We measured the x-ray diffraction pattern, in-plane and out-of-plane resistivity, and in-plane Seebeck coefficient. In x = 0.1 sample, x-ray diffraction pattern corresponds to that of Ca3Co4O9, but in x = 0.2 ~ 0.4 samples, we observed two kinds of (00l) reflection corresponding to Ca3Co4O9 and Bi2Ca2Co2Oy. Although Bi2Ca2Co2Oy shows higher resistivity [2] than Ca3-xBixCo4O9, the in-plane resistivity of low substitution samples (x = 0.1, 0.2, 0.3) is not seriously affected by Bi2Ca2Co2Oy which means that Ca3Co4O9 still dominates the electron transport in low substitutions.
Furthermore, we measured the thermoelectric device performance of a crystal of Ca3-xBixCo4O9 with a combination of n-type ceramic sample of Ca0.9Yb0.1MnO3. By focusing light on the uni-coupled pn-junction, the blackbody radiation from a halogen light is converted into electricity. The voltage-current characteristics were measured and the efficiency was calculated according to Snyder and Ursell. [3] The maximum efficiency of 5% was observed.
The work is supported by ALCA, JST.
Reference:
[1] S. Li et al., Chem. Mater. 12, 2424 (2000).
[2] E. Guilmeau et al., Mater. Res. Bull. 43, 394 (2008).
[3] G.J. Snyder and T. S. Ursell, Phys. Rev. Lett., 91, 148301 (2003).

We investigate the thermoelectric properties of electron and hole doped PtSb2.
Our results show that for doping of 0.04 holes per unit cell (1.5×1020 cmminus;3) PtSb2
shows a high Seebeck coefficient at room temperature, which can also be achieved
at other temperatures by controlling the carrier concentration (both electron and
hole). The electrical conductivity becomes temperature independent when the dop-
ing exceeds about 0.20 electrons/holes per unit cell. The figure of merit at 800 K in
electron and hole doped PtSb2 is comparatively low at 0.13 and 0.21, respectively,
but may increase significantly with As alloying due to the likely opening of a band
gap and a reduction in the lattice thermal conductivity.
300 400

The thermoelectric properties of the layered oxides KxRhO2 (x = 1/2 and 7/8)
are investigated by means of the electronic structure, as determined by ab initio
calculations and Boltzmann transport theory. In general, the electronic structure
of KxRhO2 is similar to NaxCoO2, but with strongly enhanced transport.K7/8RhO2
exceeds the ultra high power factor of Na0.88CoO2 reported previously by more
than 50%. The roles of the cation concentration and the lattice parameters in the
transport properties in this class of compounds are explained.

Thermal energy exchange of the free charge carriers with their environments gives rise to thermoelectric phenomena. Thermoelectric devices typically operate in a large temperature window. The design and characterization of the materials used for the thermoelectric devices require high-temperature electrical characterization. Hall measurements using Van der Pauw technique is a common, practical approach that does not require pattering. We have developed an automated measurement setup for characterization of semiconducting thin films from room temperature to ~450 °C over a wide resistivity range. Resistivity of the film, carrier concentration as well as the Hall carrier mobility are calculated from current-voltage (I-V) measurements performed using a semiconductor parameter analyzer while applying the magnetic field. These measurements complement the measurements performed using the high-temperature Seebeck setup. The details of the measurement technique and results on Si and Ge₂Sb₂Te₅ will be presented.

Wide-area, low-cost, and mechanically flexible thermoelectric generators (TEGs) are strongly desired both for the energy harvesting from the widely spread low-density heat flow and for the basic power source in wearable electronics. In general, the ability of thermoelectric material is evaluated by the non-dimensional figure of merit, ZT=α²σT/κ, where α is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ the thermal conductivity. Considering this definition, there are two possible approaches to achieve high performance TEGs, use of a very high σ and moderate α material, or a very high α and moderate (or even relatively low) σ one. The optimum structure of the TEG becomes thick when the former material is used and thin when the latter is used, assuming that the low σ material also exhibits very low κ. For the application to flexible TEGs, it would be natural to choose the latter case. Since promising thermoelectric materials for this purpose have not been discovered yet, quests with no preconceptions for new classes of thermoelectric materials are required. We have been, therefore, investigating a wide range of organic materials using an originally developed apparatus that can measure the Seebeck coefficients of even extremely high resistance or small volume samples [1].
In this presentation, we summarize the map of Seebeck coefficient and electrical conductivity obtained for various organic and organic/inorganic composite materials, not only from our results but also from reported ones by other groups. Metal-like conductors and high-mobility semiconductors distributed in different areas in the σ-α map. The area of carbon nanotube (CNT) composites overlapped with that of the organic conductors. Among them, properties of organic conductors, such as PEDOT:PSS, are close to those of typical inorganic thermoelectric materials.
There are several exceptional materials that do not belong to these groups. An organic Mott insulator exhibited a very high Seebeck coefficient at around the phase transition temperature neat room temperature. Seebeck coefficients of very pure C₆₀ were extremely large, around 50-120 mV/K. To the best of our knowledge, these values are the highest even among inorganic and carbon materials. CNT/cage-shaped protein composites had a unique nature where the two important parameters, α and σ , were tuned by the electronic structure of the core material in the protein [2]. We believe that organic and composite materials possibly open a novel class of thermoelectric material for future flexible electronics.
[1] M. Nakamura et al., Mat. Res. Soc. Symp. Proc., 1197-D09-07 (2010).
[2] M. Ito et al., presented in this symposium.

Organic semiconductors, such as conjugated polymers and fullerene derivatives, offer intriguing possibilities as near-room temperature thermoelectric materials in part due to their typically low thermal conductivities. Notably, the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) exhibits an exceptionally low thermal conductivity < 0.05 W m-1 K-1. The majority of prior work on organic thermoelectric has focused on hole conducting materials; however, development of all-organic thermoelectric devices requires a viable n-type leg as well. Due to their large band gap(asymp; 2 eV), and small electron affinity (asymp; -3 to -4 eV), extrinsic n-type doping of organic semiconductors to sufficient carrier concentrations for thermoelectric applications (~10^19 cm^-3) is challenging. Here we present studies of the thermoelectric properties of n-type organic semiconductors extrinsically doped with molecular dopants. The systems presented here here are processed entirely in solution, and thus offer potential for fabrication via printing methods. We have studied the electrical properties of the high performance polymer, poly{N,N&’-bis(2-octyl-dodecyl)-1,4,5,8-napthalenedicarboximide-2,6-diyl]-alt-5,5&’-(2,2&’-bithiophene)} (P(NDIOD-T2)), doped by dihydro-1H-benzoimidazol-2-yl derivatives that act as donors; however, we estimate that less than 1% of donors are active due to limited solid solubility of the molecular dopant in the polymer. Nonetheless, we achieve thermoelectric power factors ~ 0.1 µWm^-1K^-2 by extrinsically doping P(NDIOD-T2) to an estimated carrier concentration of 10^18 cm^-3 with these molecular donors, values consistent with prior results for hole-conducting organic semiconductors doped to comparable carrier concentrations. Additionally, we will present results on doping of PCBM, a solution processable fullerene, with a novel fullerene-based molecular donor. We propose design rules for candidate n-type organic thermoelectric materials based on our results.

Suppressing thermal conductivity in Si nanowires (NWs) by increasing surface phonon scattering at the nanoscale has garnered considerable interest in recent years for thermoelectric energy harvesting and cooling applications. Depending on the growth direction, Si NWs can form either coherent twinning superlattices or lengthwise twins with twin boundaries running perpendicular or parallel to the wire axis, respectively. Yet limited information is currently available about the role of twin boundaries on thermal transport characteristics. This talk will present the results of molecular dynamics simulations of thermal conductivity and phonon scattering mechanisms in nanotwinned Si NWs. We find up to 70% reduction in thermal transport in <111>-oriented Si NWs with twinning superlattices compared to <112> NWs with lengthwise twins, due to surface and interface scattering effects. Furthermore, we show that thermal transport decreases, then increases, as the twin boundary spacing decreases, leading to a minimum in thermal conductivity at a critical twin spacing. These findings are critically important to understand low thermal conductivity and high thermoelectric figure-of-merit in low-dimensional nanotwinned systems, given that the growth process directly influences which type of twins is produced.

Quasicrystals possess a low thermal conductivity and its electrical conductivity owns an anomalous temperature and defect dependence, i.e., it grows with increasing temperature or defect content in contrast with most metallic alloys. In this work, based on the Kubo-Greenwood formula, the transport of electrons and phonons in segmented nanowires is studied by means of a real-space renormalization plus convolution method [1]. This method has the advantage of being computationally efficient, without introducing additional approximations, and capable to analyze quasiperiodic nanowires with truly macroscopic length. The tight-binding and Born models are respectively used to calculate the electronic and lattice thermal conductivities [2]. The results of electrical conductivity reveal both metallic and anomalous temperature dependences, as a consequence of the Fermi energy position in the multifractal band structure of quasiperiodic systems. For the periodic chains, we found an analytical solution of the thermoelectric figure-of-merit (ZT), which could be improved by introducing the quasiperiodicity. This enhancement of ZT is also observed in nanowire axial quasiperiodic heterostructures in comparison to ZT of periodic nanowires. Finally, the numerical results of ZT are compared with experimental data.
This work has been partially supported by UNAM-IN107411 and CONACyT-131596. Computations were performed at Kanbalam and NES of DGCTIC, UNAM.
[1] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004).
[2] C. Wang, F. Salazar and V. Sanchez, Nano Lett. 8, 4205 (2008).

Traditional bulk thermoelectrics, which have been around for over half a century, are based on crystalline inorganic materials. However, heat and charge transport in bulk inorganics are fundamentally linked in ways that have limited optimization. Organic materials are becoming an appealing option for thermoelectric applications because their associated transport mechanisms are intrinsically different from those in bulk inorganics. Control over electronic transport through molecular design enables new strategies to break the traditional inverse correlation between the electrical conductivity and the Seebeck coefficient. At the same time, organics have inherently low thermal conductivity rivaling some of the lowest values attainable in inorganics. To create a thermoelectric module both hole and electron transporting legs are necessary, connected electrically in series and thermally in parallel. Several high performance hole-transporting organic materials have been developed, but the design of n-type organics has proven challenging due to a number of issues, including limitations in material processing and chemical stability. Thermoelectric studies of organic n-type systems are scarce. In this presentation, we report our recent discovery of a series of novel, water-soluble perylene diimide based n-type small molecule variants. Not only are these materials easily processable, but their electronic transport characteristics place them in line with the best in class n-type systems. The reported results give credence to a new molecular design strategy for radically improving electrical conductivity without significant changes in the Seebeck coefficient and bring the realization of flexible, water-processable and highly tunable thermoelectric devices closer to reality. Creating such a class of thermoelectrics will enable architectural redesign of traditional modules and extend the utility of these energy conversion devices to currently unattainable frontiers. It should be noted that the reporting of these n-type materials has far reaching implications to areas of organic electronics beyond thermoelectrics.

Bismuth telluride (Bi2Te3) is considered to be best materials for near room temperature thermoelectric (TE) applications. A value of dimensionless figure-of-merit (ZT) is required to be ~3 for a practical application. Recently, many efforts have been made to increase the ZT value of Bi2Te3 materials. It is reported that nanostructured TE materials in low dimensional form show a higher ZT value compared with bulk TE materials. There have been many researches for the synthesis methods of nanostructured Bi2Te3 such as solvothermal method. Since these methods normally require high temperature, high pressure, toxic regent and special equipment, it was difficult to achieve the practical use of TE. What is of vital importance today is to develop a synthesis method in an aqueous solution to synthesize nanostructured Bi2Te3 for a green and low-cost synthesis method.
Various research groups, including our group, reported that nano-structured Bi2Te3 was synthesized in aqueous solution at a near room temperature by chemical reduction of metal complexes. In these methods, ascorbic acid was used as a reducing agent. Although the reduction power of ascorbic acid is lower than that of commonly-used reducing agents such as sodium borohydride and hydrazine. Ascorbic acid (vitamin C) is one of the best reducing agent for the environment. However, the synthesis mechanism of nano-structured Bi2Te3 in aqueous solution remains unclear, especially the reduction process of each metal complex by ascorbic acid is veiled. Therefore, the metal complex conditions and the reduction process of each metal complex were evaluated in this work to elucidate the synthesis mechanism.
Each metal condition during the reduction reaction was determined by electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). The reduction process of metal complexes was electrochemically examined by using cyclic voltammetry.
In the solution containing Bi(NO3)3, Na2TeO3, and ethylenediamine-N,N,N',N'-tetraacetic acid disodium salt (2NaEDTA) as metal precursor salts and complex reagent, Bi-EDTA complex and TeO32- are formed and stable before adding ascorbic acid. It was found that Bi2Te3 could not be synthesized when these metal complexes were electrochemically reduced without ascorbic acid. On the other hand, Bi2Te3 can be electrochemically synthesized after adding ascorbic acid. From analyzing data by ESI-TOF-MS Bi-EDTA complex changed to Bi-ascorbic acid complex and TeO32- is reduced to TeOx ion (x=1 or 2) after adding ascorbic acid. These results mean that ascorbic acid works not only a reducing agent but also a complex agent and play important role on the synthesis of nanostructured Bi2Te3.
In our presentation, detailed results and synthesis mechanism from the viewpoint of electrochemical and ESI-TOF-MS analysis will be introduced. This work was supported by Frontier Research Institute for Interdisciplinary Sciences Tohoku University.

Mg₂Si has attracted much interest as an n-type thermoelectric (TE) material operating in the temperature range from 500 K to 800 K because of its non-toxicity, environmental friendliness, lightness, and its abundance compared with other TE materials. The performance of TE materials is usually denoted as the dimensionless figure of merit, ZT = S² σ T / κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature, and κ is the thermal conductivity. It has been reported that nano-structuring can give rise to large reductions in the lattice thermal conductivity due to the increase in phonon scattering at interfaces between grains. Furthermore, this is expected to raise the ZT of thermoelectric materials. In this study, nano-structured Mg₂Si bulk was fabricated by spark plasma sintering (SPS) from Mg₂Si nano powder, and the thermoelectric performance of the samples was evaluated. A pre-synthesized all-molten commercial polycrystalline Mg₂Si source (undoped n-type conductivity) was pulverized into powder 75 mu;m or less. To obtain nano-sized fine powder, the powder was milled using planetary mill equipment under an inert atmosphere. The Mg₂Si nano-powders were fabricated by high-energy ball milling with zirconia balls or tungsten carbide under dry conditions. In using the zirconia ball, we observed that a little MgO was generated by oxidation and decomposition of Mg₂Si. It is thought that the oxygen was transferred from the zirconia to the Mg₂Si during the milling process. Fine Mg₂Si nano-powder with a mean grain of 500 nm or less was obtained without any impurities (i.e. MgO). The powder was put into a graphite die to obtain a sintering body of Mg₂Si and treated in a vacuum in the SPS equipment. High-density Mg₂Si bulk without cracks was obtained. The thermoelectric properties (electrical conductivity, Seebeck coefficient, and thermal conductivity) were measured from room temperature to 873 K.

The performance of thermoelectric materials can be evaluated in terms of a dimensionless figure of merit, ZT, defined as (S²σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the temperature in Kelvin. To maximize the ZT of a material, a large S, high σ, and low κare required. One means of achieving this is to make nano-structured thermoelectric materials that cause phonon scattering without interrupting the electrical conductivity. As representative thermoelectric materials go, the Bi-Te system has a temperature range as low as room temperature, whereas the Pb-Te system boasts a high ZT value. However, these thermoelectric material systems contain heavy metal and rare metal elements, meaning that production costs and toxicity are problems. Recently, n-type magnesium silicide (Mg₂Si) composed of a non-rare metal and light element has been investigated as a potentially eco-friendly thermoelectric material. It has been reported that this material is an n-type thermoelectric one in a wide temperature range (500-900 K) and it has high thermoelectric performance (ZT = 0.93 at 873 K).
The spark plasma sintering (SPS) method is an effective way to prepare polycrystalline materials with superior density and reduced grain growth. Unlike the conventional powder sintering method, SPS makes it possible to prepare ceramics at low temperatures and in a shorter period of time by sending pulsed high direct current through the powder particles and generating a momentary spark discharges between the particles.
To improve the thermoelectric performance of Mg₂Si under the condition of decreasing thermal conductivity, we tried to fabricate nano-structured Mg₂Si by combining SPS with a high-energy ball milling technique. The pre-synthesis milling was performed with a planetary ball mill and Si powder at a rotational speed of 800 rpm, milling time of 30 min, and using a tungsten carbide pot, balls and planetary ball mill. The milling was conducted at steps of 10 min in order to minimize the amount of material adhering to the pot walls. After each step, the powder sample was removed from the pot walls. In the next step, Mg metal and milled Si powders were mixed homogeneously. The mixing ratio was a stoichiometric composition of Mg₂Si. The powder was then placed in a graphite die in an Ar atmosphere and subjected to SPS under vacuum conditions. Scanning electron microscopy was used to observe the milled Si-Mg powder and synthesized bulk sample. The grain size of the milled powder was distributed in a range from several nm to several tens of mu;m. On the other hand, the synthetic bulk had a grain size ranging from several hundred nm to a few mu;m. The XRD analysis confirmed that the bulk sample was Mg₂Si without any impurities. These results show that nano-structured Mg₂Si bulk without any impurities can be easily fabricated by using SPS and an Si-Mg mixture of nano-sized powder.

The performance of solid-state thermoelectric cooling devices is directly related to device operating figure of merit ZT=S2T/RK, in which S is the sum of Seebeck coefficient of both n-type and p-type semiconductor materials, R is the total electrical resistance, K is the total thermal conductance, with negligible parasitic electrical and thermal effects. It is known that Z is proportional to the maximum cooling and the corresponding input current is determined by the total Seebeck coefficient and electrical resistance. The electrical resistivity, thermal conductivity, and geometric factors of n-type and p-type semiconductor materials need to be optimized to maximize the effective cooling. Silicon nanomesh thin films could present an increased ZT with a decreased thermal conductivity due to increased phonon scattering, and an enhanced Seebeck coefficient due to nanomesh from our previous studies. Hence, thermoelectric cooling could be potentially improved in devices using n-type and p-type silicon nanomesh thin films.
Here we report a unique method of patterning nanomesh features with self-assembled block copolymers on SOI that yields hexagonally arranged features [3]. This nanoscale features are obtained by transferring the self-assembled block copolymer pattern to the underlying silicon device layer by reactive ion etching. Current is supplied from n-type silicon nanomesh thin film to the p-type side, resulting a cooled junction in between. Cooling values are directly measured by an infrared camera in a vacuum environment at room temperature.
[1] T. C. Harman, et al. Quantum Dot Superlattice Thermoelectric Materials and Devices. Science, 297, 2229-2232 (2002)
[2] D. M. Rowe. CRC Handbook of Thermoelectrics. CRC Press (1995)
[3] R. Ruiz, et al. Density multiplication and improved lithography by directed block copolymer assembly. Science 321, 936-939 (2008)

Enhancement of thermoelectric properties through inclusion of nanoscale grain boundaries (so called nanostructuring) has proven to be an effective pathway to creating new materials with accentuated efficiency values. As the field has advanced, the manipulation of nanoparticle size, shape, structure or composition, which are key parameters to control the electrical and thermal conductivity, have also proven essential in improving the overall thermoelectric efficiency. Bi-Sb-Te based materials have traditionally held the record for highest achievable ZT value, even in the realm of nanomaterials. Unfortunately however, tellurium has a scare abundance on the earth, and antimony is highly toxic, making Bi-Sb-Te containing thermoelectrics unsustainable. As an alternative, chalcogenide semiconductors such as CuZnS nanoparticles are composed of abundant elements and are inherently non-toxic. To study the thermoelectric properties of these materials, we first developed a unique thermolysis driven reaction scheme towards the formation of CuZnS nanoparticles with a tunable composition, size and structure. The physical properties of the resulting CuZnS nanoparticles were characterized using techniques such as HR-TEM, XRD, STEM-HAADF, and EDS Elemental Mapping. The thermoelectric properties of the materials were then studied and were correlated to the nanoparticle properties and processing strategy. It was revealed that a unique correlation exists between the particle characteristics and the resulting thermoelectric properties. The presentation will focus on our recent advancements in the development of a new class of sustainable chalcogenide semiconductor for thermoelectrics.

Silicides have found widespread acceptance in semiconductor technology. The first study of silicides as potential thermoelectric materials was reported by E.N. Nikitin where MnSi, MnSi2, CrSi2, and CoSi were found to be promising candidates. These compounds are mechanically and chemically strong and they can be used in hostile environments without any protection.
In this work, the fabrication of nanocomposites through the introduction of metal-oxide nanoparticles into thermoelectric matrix of cobalt silicide and magnesium silicide compounds is presented. The matrix materials, Co-Si and Mg-Si, were prepared through ball milling and solid state reaction and the nano-metal-oxides were introduced by mechanical grinding. The materials were hot pressed for the formation of nanocomposites. The structural and morphological modifications have been studied with Powder X-ray diffraction and Scanning Electron Microscopy. The thermoelectric properties of the materials were studied in terms of Seebeck coefficient, electrical and thermal conductivity at temperature range of 300-1000 K.

Wide-area, low-cost, and mechanically flexible thermoelectric generators are strongly desired for the energy harvesting from the waste heat around our lives. Nevertheless, promising thermoelectric materials for this purpose have not been discovered yet. The performance of thermoelectric conversion is evaluated by dimensionless figure of merit, ZT=α2σT/κ (α, σ, κ and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively). Among many candidate materials, carbon nanotube (CNT) composites are expected to have higher σ and superior mechanical properties for flexible devices. The high κ and small α of CNTs are, however, not desirable for thermoelectric applications.
In this work, we have focused on cage-shaped proteins having semiconductor cores to solve these problems. The protein shell was genetically modified to selectively adsorb to CNTs. Mixing the obtained cage-shaped protein and CNTs made a composite having single molecular junctions bridging adjacent CNTs. The core&’s electronic states were tuned by selecting core materials. Applying a temperature gradient to the composite, Seebeck effect was measured. Both α and σ of the composite increased when ferrihydrite or cobalt oxide was used as a core material, and they decreased when zinc sulfide or zinc selenide was used. These indicate that the thermoelectric performance of the composite was effectively modified by the cores, the volume fraction of which is small. Locally low κ at the CNT/protein/CNT junction might result in a steep temperature gradient at the junction. Then, asymmetric electron (or hole) flow through the conduction (or valence) band of the core particle produces Seebeck effect at the junction. The mechanism of the Seebeck effect at the junction will be discussed according to the energy diagram of the core particles.

Bulk silicon is not considered to be a good thermoelectric material due to high thermal conductivity above 100W/m K at room temperature, nonetheless heavily carrier-doped Si has good thermoelectric power factor for practical usage. However, many recent papers report that thermal conductivities of Si-based nanostructured materials are drastically reduced by enhancement of phonon scattering at grain boundaries, leading to high dimensionless thermoelectric figure of merit ZT.
In previous works, we reported self-organization of a nanostructure by rapid solidification from melt eutectic alloys between Si and CrSi₂. The melt spun Cr_14.9(Si_0.99B_0.01)_85.1 eutectic alloys has aligned lamellar structures consisting of Si and CrSi₂ layers with spacing of 20~50nm, i.e. Si thickness of 10~30nm. The thermal conductivity of the “nano-eutected” Cr_14.9(Si_0.99B_0.01)_85.1 sample is reduced to 12W/mK, which is 1/3 of the arc-melted alloy of the same composition with grain sizes of micrometer order, while the thermoelectric power factor is little affected by the nanostructuring.
The present paper investigates the influence of the melt-spinning conditions on the nanostructure and resulting thermoelectric properties. We performed melt spinning of the Cr_14.9(Si_0.99B_0.01)_85.1 eutectic alloys changing conditions as the nozzle diameter of 0.4~1.0mm, the wheel speed of 10m/s~80m/s and the injection pressure of 0.02~0.04MPa. SEM images show that these conditions strongly influence the spacing of the lamella and forming of coarse grains. The thermal conductivity decreases with decreasing spacing of the lamella. Our result shows the optimization of thermoelectric performance by controlling the nanostructure in a self-organizing process.

Thermoelectric materials have recently attracted a huge amount of scientific attention owing to their capability of converting heat into electricity and vice versa, and thus delivering a solution to global warming. The performance of thermoelectric material is determined by figure of merit ZT, which is defined by ZT = σS²/(κ_el+κ_lat), where σ is the electrical conductivity, S is the Seebeck coefficient, κ_el is the electronic thermal conductivity, and κ_lat is the lattice thermal conductivity. High ZT involves either increasing the electrical conductivity and thermopower or decreasing the thermal conductivity. However, adjustment on any of these individual material properties with the hope to optimize ZT without, simultaneously affecting others is almost impossible due to the interrelations between the material properties. In this study, complex oxide SrRuO₃ and ZnO were brought together for large enhancement in thermoelectric performance. Self-assembled SrRuO₃-ZnO nanostructures were successfully engineered on single crystal SrTiO₃ (STO) (111) substrate by means of pulsed laser deposition. High resolution X-ray diffraction and transmission electron microscopy results indicate that the metallic SrRuO₃ (110) nanopillars were embedded in semiconducting ZnO c-axis oriented matrix. Varying the ratio of SrRuO₃ to ZnO modifies the band structure at Fermi level and thus the carrier concentration of the material system. This method was demonstrated to be very effective in transferring the electrons inside SrRuO₃ nanopillars into ZnO matrix, which explains the metallic-like, weak metallic-like, and semiconducting electrical transport behaviour of the SrRuO₃-ZnO nanostructures of different ratio. Meanwhile, the advantage of high interface-to-volume ratio in the SrRuO₃-ZnO nanostructure system provide an effective means for phonon scattering, leading to the maximum reduction in thermal conductivity. The results of this approach allow the electrical conductivity, thermopower, and the thermal conductivity to be tuned feasibly. Our results show that we successfully enhance ZT by significantly reducing both thermal conductivity and electrical resistivity through the self-assembled SrRuO₃-ZnO nanostructure. Such material system demonstrates to be one of the promising candidates in waste-heat recovery and cooling applications with possibilities for practical technological applications. This opens a new pathway to engineer more potential complex oxide thermoelectric materials in the future.

We present our investigations on the colloidal synthesis and physicochemical properties of the metal telluride nanocrystals such as silver telluride (Ag2Te) and lead telluride (PbTe). Predefined procedures [1, 2] have been adapted for these syntheses. The as-synthesized materials were washed several times with deionized water and anhydrous ethanol, and re-dispersed in toluene. Further investigations have been carried out on drop-casted thin films deposited onto a chosen substrate. The phase composition was assessed by x-ray diffraction (XRD). Field emission scanning electron microscope (FESEM-EDX) analyses reveal nearly stoichiometric Ag2Te and PbTe nanocrystals and their self-assembled nature when deposited on both silicon and carbon tape substrates by drop-casting technique. High resolution transmission electron microscopy (HRTEM) reveals that the prepared Ag2Te and PbTe samples comprise nanoparticles of different sizes ranging from 4-8 nm and 6-15 nm, respectively. Optical properties and interaction of surfactant with the nanocrystal surface were also studied by subjecting the Ag2Te and PbTe samples to UV, FTIR and Raman spectroscopy techniques. Additionally, thermal analysis was done to investigate the thermal stability of the nanocrystals. Analyses will be discussed in-detailed.
References
[1] doi:10.1038/nmat1826
[2] doi:10.1021/ja058269b

We report low-temperature thermoelectric transport properties of p-type nanostructured CeCu₆ samples prepared by a hot press method. Our results show that thermal conductivity is reduced by nanostructuring when compared to single crystal data⁺⁺ and that the Seebeck coefficient increases with hot press temperature. Based on our analysis, reduction in thermal conductivity of the nanostructured CeCu₆ comes from suppression in the electronic rather than the phononic contribution. A ZT value of 0.024 at 60 K was observed for sample hot pressed at 800 ^oC. Our initial results indicate that certain strongly correlated electron heavy-fermion compounds may play a role in solid state cooling applications.
Acknowledgement: This work is supported by the DOD, USAF Office of Scientific Research, MURI Program under Contract FA9550-10-1-0533.
⁺⁺Y. Peysson, et al., J. Magn. Magn. Mater. 54-57, 423 (1986).

Thermoelectrics (TE) provide an alternative route to convert the solar energy into electrical power. Their efficiency is governed by the material&’s figure of merit defined as ZT = S²σT/(κ_c + κ_lat + κ_bp), where S² σ is the power factor, κ_c, κ_lat and κ_bp stand for the electronic, lattice and bipolar contributions to the thermal conductivity, respectively. Bi₂Te₃ and its alloys are the best TE materials available for room and near room temperature applications. Recently, optimization of the Bi₂Te₃ -based materials performance involves enhancement of the ZT values (ZT >1) mainly by the reduction of the κ_lat component of the thermal conductivity through nanostructuring. Most of these efforts incorporate extensive grinding to create nanograined materials, which then are sintered or pressed into bulk objects. In this work we present thermal conductivity reduction by self-formed inhomogeneities on the nanoscale range -as evidenced by Transmission Electron Microscopy (TEM)- via the incorporation of small amounts of an appropriate second phase within the Bi₂Te₃ matrix. The final compositions, after processing with Spark Plasma Sintering (SPS), have shown large reduction in both κ_lat and κ_bp components relative to the pristine materials, leading to higher ZT values. Due to the desire for the replacement of tellurium in the current TE materials, experiments involving the gradual substitution of Se for Te within the matrix are also presented.

Recently there have been reports of hot carrier thermoelectric response in nanostructured materials like graphene and MoS. We report observing that thermoelectric nanowire junctions detect light. In these experiments we employed devices composed of bismuth nanowire arrays which are capped with a transparent indium tin oxide electrode. The incident surface features very low optical reflectivity and enhanced light trapping. The unique attributes of the thermoelectric arrays are the combination of strong temporal and optical wavelength dependences of the photocurrent. Under infrared illumination, the signal can be completely described by "quasi-equilibrium " thermoelectric effects considering cooling rates given by heat diffusion through the array. The thermal diffusivity is found to be less (by a factor of 3.5) than in the bulk, a result that we discuss in terms of phonon confinement effects. In addition to a thermoelectric response, under visible illumination, we observe a photovoltaic response.
This work was sponsored by the National Science Foundation, Army Research Office and the Boeing Company.

High efficiency thermoelectric materials are important for power generation devices that are designed to convert waste heat into electrical energy. The inefficiency of current thermoelectric materials has limited their commercial use. Possible areas where thermoelectric materials can find their application include the automobile industry, heavy manufacturing industries and places were large combustion engines operate continuously. Waste heat from automobiles, factories and similar sources offers a high-quality energy source equal to about 70% of the total primary energy, but it is difficult to reclaim because its sources are small and widely dispersed. The thermoelectric properties of nanostructured magnetic semiconductors have not been extensively studied so far, for example in materials such as the natural occurring copper iron sulfide, also known as chalcopyrite. The nanocrystalline form of this material has a band gap that enables efficient solar energy conversion, which could also indicate the feasibility of chalcopyrite to be an effective thermoelectric material. By tuning the interaction between carriers and magnetic moments in the chalcopyrite material the power factor and overall efficiency of the material can be enhanced. Chalcopyrite nanomaterials for thermoelectrics offer better sustainability because it is composed of abundant and non-toxic elements. In this study, we have developed a synthetic technique towards copper iron sulfide nanoparticles, which till now have not been created with uniform size, shape, composition and structure. The particles are also highly intriguing from a fundamental perspective because the relative composition of copper and iron in the particles can be tuned, resulting in the ability to manipulate the electronic or band gap properties for the desired application. The synthetic technique and compositional/structural nanoparticle characterization are discussed using techniques such as XRD, XPS, HAADF-STEM, elemental mapping and others. An assessment of the thermoelectric and electronic properties for these materials will also be presented.

The effect of edge free sintering on n-type bismuth telluride elaborated by high energy ball milling and spark plasma texturing are investigated. The known anisotropy of this layered material is responsible for anisotropic transport properties that can be altered or tuned by a fine control of the degree of texture.
Quantitative texture analysis is performed using combined analysis and the corresponding anisotropy factors between the direction parallel and perpendicular to the axis of pressing can be calculated and compare with actual experimental values. This technique also allows the determination of the size and shape of the crystallites or coherent domains.
Together with HRTEM and SEM imaging, the whole study leads to the rationalization of the variation of the transport properties at the different processing steps. While Seebeck coefficient and thermal conductivity remains nearly unchanged, the large decrease in electrical resistivity results in a large increase of the thermoelectric figure of merit zT.
Spark plasma textured samples and their study by X-ray combined analysis might lead to enhance thermoelectric properties in numerous anisotropic compounds by the fine tuning of their degree of texture.

Thermoelectric property of 5 % Indium doped polycrystalline SnTe was investigated from 5 K to 300 K. The carrier concentration and electrical conductivity of matrix SnTe sample is extremely high due to the intrinsic Sn vacancy. The Indium doping on Sn sites pushes the hole carrier concentration to a much higher level while depresses the thermopower greatly at the same time. However, by adding very small amount of extra Te, a sharp increase of thermopower at room temperature was observed. We found that the sample with highest thermopwoer shared very similar carrier concentration with undoped SnTe at room temperature. The significantly improved thermopower should be attributed to the resonant impurity levels introduced by Indium. The figure of merit zT was greatly improved and reached 0.17 at room temperature.

La3-xTe4 is the best performing n-type thermoelectric material for temperatures in the 900 K to 1300 K range, with peak ZT values of 1.2 to 1.4 at 1273 K. Together with 14-1-11 rare earth-based Zintl antimonides, n-type La3-xTe4 is now being integrated into high efficiency segmented couples for next generation space power systems. This material is a defect structure low thermal conductivity compound, where the number of vacancies can reach up to one in nine La atoms. It behaves as a semi-metal for x = 0, and a lightly doped semiconductor for compositions approaching x = 0.33. Previous work has focused on controlling carrier concentration through the number of vacancies, and evaluating the role of vacancies versus atomic substitutions on lattice thermal conductivity variations. First principle electronic band structure calculations show that the La atoms play a crucial role in defining the density of states for La3-xTe4. Recent computational modeling indicates that some atomic substitutions on the La site have the potential for substantially increasing Seebeck coefficient values across a wide range of carrier concentration. Substitutions on the Te site are also of interest, as a means of tuning carrier concentration when (2+) alkaline or rare earth elements are substituted for La, as well as a way to introduce point defect phonon scattering and help improve thermal stability and mechanical properties. Experimental and theoretical results on various new compositions are reported and their high temperature transport properties are compared with the baseline La3-xTe4 compound.

The thermopower (S) and electrical conductivity (σ) in conventional semiconductors are coupled adversely through the carriers&’ density (n) making it difficult to achieve meaningful simultaneous improvements in both electronic properties through doping and/or substitutional chemistry. Here, we discuss the effectiveness of coherently embedded nanostructures in tailoring the density, mobility and effective mass of existing ensembles of carriers within the semiconducting n-type and p-type half-Heusler (HH) matrices. We observed a drastic decrease of the effective carrier density within the half-Heusler nanocomposites at 300K followed by a sharp increase with rising temperature. We proposed that the embedded nanostructures form a potential barrier at the interface with the matrix due to the offset of their conduction band minima (CBM). The energy barrier (deltaE) discriminates existing carriers with respect to their energy by trapping low energy (LE) carriers, while promoting the transport of high energy (HE) ones. This “carrier culling” results in surprisingly large increases in the mobility and the effective mass of HE carriers contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of free carriers resulted in large enhancements of the thermopower whereas; the increase in the mobility minimizes the drop in the electrical conductivity. Using X-ray powder diffraction, electron microscopy, and electronic transports data, we will discussed the mechanism of phase formation and transformation, at the sub-ten nanometer scale, in bulk half-Heusler (HH) matrix and the mechanism by which the embedded nanostructures regulate electronic charge transport within the semiconducting HH matrices. Emphasis will be placed on the n-type Zr0.25Hf0.75Ni1+xSn1-yBiy and Ti0.1Zr0.9Ni1+xSn, and the p-type Ti0.5Zr0.5Co1+xSb nanocomposites.

In this work, a combined heat and power boiler is developed using a thermoelectric generator. We designed and fabricated a high-temperature thermoelectric generator (TEG) using our high-efficiency and low-cost nanostructured half-Heusler thermoelectric materials and device packaging architect. The TEG is designed and packaged such that it can operate at temperature differences as large as 500 oC with heat-to-electricity efficiency greater than 8%. By integrating such TEGs into residential boilers, we developed a combined heat and power (CHP) boilers for buildings. We installed the TEG on the water tubes adjacent to the combustion flame in order to take advantage of the large temperature differences between the flame and the water, and thus a portion of the heat from the combustion gas is converted into electricity with the remaining heat contributing to water heating. A total heat and electricity generation efficiency of 95% can be achieved using condensing boilers. The integrated product will generate electricity of three-times greater values than heat with incremental cost, which can be used to drive the boiler operation and other electricity consumers in the building. CHP boilers significantly reduce overall building electricity consumption and enhance building energy efficiency. CHP boilers will not only reduce building electricity cost by about 50% compared with electricity cost from the grid, but also greatly enhance the building power security during inclement weather or natural/artificial disasters.

Zintl phases and related compounds are promising thermoelectric materials, for instance high zT has been found in Zn4Sb3, Yb14MnSb11, clathrates and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitution and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl and defect chemistry, thereby enabling the rational optimization of zT.
Recent results on new Zintl thermoelectric materials such as AZn2Sb2, Sr3GaSb3 and the chemical influence of M in Ca5M2Sb6 will be discussed.

Lead telluride has been the focus of intense study of the due to its excellent thermoelectric properties. To understand how these properties arise, a detailed understanding of the electronic structure of the material is required. Typically this is achieved by using the techniques of ultra-violet or x-ray photoelectron spectroscopy (UPS and XPS), in combination with rigorous sample preparation to prepare surfaces that are free of carbonaceous contaminants. This can be challenge for a material such as PbTe, whereby the process of cleaning the sample intrinsically alters the electronic properties that you want to study. Recent advances in analyser technology, coupled with high photon intensity synchrotron radiation has allowed the development of a bulk electronic structure sensitive technique (rather than surface sensitive UPS or XPS). Hard x-ray photoelectron spectroscopy (HAXPES) utilises photon energies up to 10 keV meaning that the effective probing depth of the experiment can increase by up to a factor of 10. This means that bulk electronic structure can be determined without the need for typical surface treatments (e-beam anneal or argon ion sputtering). Single crystals of PbTe were cleaved in-situ and measured using the HAXPES technique. The core levels and valence band structure will be presented and discussed. The valence bands will also be compared to state-of-the-art density functional theory calculations. This is the first known HAXPES study of PbTe and future directions and opportunities will be discussed.

Hierarchical architectures have large numbers of nanoscale interfaces among the building blocks of thermoelectric nanocrystals, which may reduce the thermal conductivity (k) more than the electrical conductivity (s) based on the quantum confinement effect and differences in their respective scattering lengths, which benefits the enhancement of the ZT value. As an important IV-VI semiconductor, Lead telluride (PbTe) is one of the most studied thermoelectric materials that operate at medium temperatures (450-800 K), which has been playing a dominant role in the thermoelectric power generation application for more than 50 years, especially in the deep space exploring program.
At present, PbTe synthesis systems are far from being well understood. Without exception, the crystallization mechanism of PbTe hierarchical nanostructures is also rather complex, and until now no unequivocal conclusion has been drawn about whether the formation of this material is through a solid or through a solution route. Therefore, a new and deeper understanding of growth mechanism for the viewpoint of supersaturation should be reconsidered. In addition, there is no report on a controllable assembly of nanowires with branched ultrathin nanorods so far. Simultaneously, there are still a great deal of unanswered questions regarding the relationship between morphology and thermoelectric transport properties.
In this article, a triethanolamine(TEA)-assisted solvothermal method has been devised to synthesize PbTe hierarchical nanostructures in large scale with various shapes including octapodal dendrites with a spiral step hollow cubic center, hopper, and nanowires with branched nanorods by tuning the amount of KOH and the volume ratio of solvent. Systematic variation of the kinetic factors, including the reaction temperature, the duration time, the ratio of source materials, and the KOH concentration, reveals that the morphology depends mainly on the supersaturation degree of the free Pb2+ ions released from Pb(TEA)2+ under elevated temperature. The formation processes of PbTe octapodal dendrites with a spiral step cubic center and nanowires with branched nanorods were believed to follow the screw dislocation-driven growth and in-situ template and secondary nucleation route, according to the extensive experimental data. In addition, the electrical transport properties of the samples consisted of octapodal dendrites, hopper, and nanowires with branched nanorods were shown morphology-dependent. The unusual electrical behavior with temperature clearly indicated that charge carrier scattering at grain boundaries and crystal interfaces played a dominant role. It is envisioned that the tailored synthesis of PbTe hierarchical nanostructures may promise unique opportunities for producing thermoelectric materials with greatly enhanced performance.

In the present study, we investigated fully filled p-type skutterudites CaFexCo4-xSb12 (2le;xle;3.5) through both experimental approaches and theoretical calculations. Samples were successfully synthesized using a combined melt-spin (MS) and spark plasma sintering (SPS) processing technique. Structural analysis confirmed phase-pure samples with high filling ratio (>90%), homogeneous filler distribution and exceptionally low oxygen contamination level. Electrical and thermal transport property measurements revealed large power factor and low thermal conductivity. Maximum ZT of 0.85 was achieved in CaFe3CoSb12 at 800 K. To understand the low lattice thermal conductivity in these compounds, we studied the relation between filler vibrational modes and phonon scattering using an ultrafast reflectance spectroscopy technique. Phonon DOS calculations were also performed. The small size of Ca atoms is suggested to result in weak force constant between fillers and the host, and therefore the relatively low phonon frequency, which is considered to contribute to the reduction in lattice thermal conductivity.

A spin current, a flow of spin-angular momentum, is essential in spintronics. To find versatile methods for generating spin currents, many experimental and theoretical studies have been focused on the interplay between spin, charge, and heat currents. In this stream, a novel thermo-spin effect called a spin Seebeck effect was discovered in 2008 [1].
The spin Seebeck effect refers to the generation of a spin voltage as a result of a temperature gradient in a ferromagnet, which injects a spin current into an attached paramagnetic metal [1-7]. In the paramagnet, this spin current is converted into an electric voltage due to the spin-orbit interaction. Recent studies in spintronics have revealed that the spin Seebeck effect appears not only in metals [1] and semiconductors [3] but also in magnetic insulators [2,4], enabling electric voltage generation from heat flowing in insulators, which was impossible if only conventional thermoelectric technology was used.
In this talk, we report the experimental observation of the spin Seebeck effect in various magnetic insulator/metal junctions and discuss the potential of thermoelectric generation based on spin currents. We also show that the thermoelectric generation based on the spin Seebeck effect has high affinity with a coating technology [6]. The “spin-thermoelectric coating” is characterized by a simple film structure, convenient scaling capability, and easy fabrication.
We thank E. Saitoh, S. Maekawa, G. E. W. Bauer, H. Adachi, J. Xiao, T. Kikkawa, A. Kirihara, and M. Ishida for their support and valuable discussions.
[1] K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa, and E. Saitoh, Nature 455, 778 (2008).
[2] K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E. W. Bauer, S. Maekawa, and E. Saitoh, Nature Mater. 9, 894 (2010).
[3] C. M. Jaworski, J. Yang, S. Mack, D. D. Awschalom, J. P. Heremans, and R. C. Myers, Nature Mater. 9, 898 (2010).
[4] K. Uchida, H. Adachi, T. Ota, H. Nakayama, S. Maekawa, and E. Saitoh, Appl. Phys. Lett. 97, 172505 (2010).
[5] K. Uchida, H. Adachi, T. An, T. Ota, M. Toda, B. Hillebrands, S. Maekawa, and E. Saitoh, Nature Mater. 10, 737 (2011).
[6] A. Kirihara, K. Uchida, Y. Kajiwara, M. Ishida, Y. Nakamura, T. Manako, E. Saitoh, and S. Yorozu, Nature Mater. 11, 686 (2012).
[7] T. Kikkawa, K. Uchida, Y. Shiomi, Z. Qiu, D. Hou, D. Tian, H. Nakayama, X.-F. Jin, and E. Saitoh, Phy. Rev. Lett. 110, 067207 (2013).

It has been well established that intermediate valence compounds show potential as low temperature thermoelectrics due to their large Seebeck coefficients at low temperatures. These materials may find application in cryogenic Peltier cooling devices for applications such as infrared detectors on satellites. This study investigates improvements on current synthesis techniques and the thermoelectric properties of the Yb_1-xSc_xAl₂ solid solution. Here we correlate changes in the Seebeck coefficient with the valence state of Yb as determined using magnetic susceptibility measurements. The compound YbAl₂ is a well-known intermediate valence system, and shows a slightly negative Seebeck coefficient below room temperature. However, substituting Sc into the system induces significant changes to the electronic properties. The Sc atom creates an internal chemical pressure on the YbAl₂ system, forcing the Yb ion to progress from a nearly divalent state to a trivalent Yb state. Magnetic measurements show that the degree of intermediate valency of the Yb ion depends strongly on both temperature and composition, and is correlated with both the magnitude and temperature dependence of the Seebeck coefficient in these compounds. We show that by using chemical pressure we can strongly enhance the magnitude of both the Seebeck coefficient and the thermoelectric power factor, as well as control the temperature at which these effects are maximized. In this way one can tailor the maximum performance of these materials for a given temperature range and type of application.

We present pure PbS thermoelectric materials prepared by solvothermal method followed by spark plasma sintering. The effects of grains with different length scales on the thermal conductivity of PbS samples are studied by transmission electron microscopy and theoretical modeling. We find that a high density of nanoscale grain boundaries can dramatically lower the thermal conductivity, not only by effectively scattering long-wavelength phonons, but also by partially suppressing the bipolar effect. Remarkably, the thermal conductivity at room temperature can be reduced from the 2.53 Wm-1K-1 measured for ingot-PbS (grain size >200 mu;m) to 2.36 Wm-1K-1 for micro-PbS (grain size >0.4 mu;m), and even to as low as 0.85 Wm-1K-1 for nano-PbS (grain size sim;30 nm). Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates is proposed to explain such effective enhancement, and the results show that the high density of nanoscale grains can cause an effective phonon scattering by almost 60%. These findings provide an efficient strategy for developing high-performance thermoelectrics at the intermediate temperature range.

A silicon-germanium alloy (SiGe) is one of the promising candidate materials for thermoelectric (TE) energy conversion. Previous experimental and theoretical studies have reported a significant reduction in thermal conductivity in SiGe, compared to pure Si and Ge. However, the experimentally measured and theoretically predicted thermal conductivities tend to be widely scattered, and the underlying mechanisms for thermal conductivity suppression in SiGe alloy nanostructures still remain uncertain. In this talk, we will present a systematic theoretical analysis of thermal transport in SiGe alloys, particularly the effect of the local distribution of Si and Ge atoms. Nonequilibrium molecular dynamics (NEMD) is adapted to calculate the thermal conductivities of SiGe nanostructures as a function of Si:Ge composition ratio. For this work, we optimize Stillinger-Weber potential parameters for Si-Ge interatomic interactions by fitting to relevant atomic forces from first-principles calculations. Our study highlights the important role played by the local atomic arrangement in determining the thermal conductivity of SiGe; the thermal conductivity tends to be more suppressed as constituent Si and Ge are more randomly distributed. This also suggests the sensitivity of SiGe thermal conductivity to the method of sample preparation, which can explain why existing experimental values are widely scattered. We will also discuss mechanisms underlying the suppression of thermal conductivity in SiGe nanowires with different diameters and Si/Ge atomic arrangements. The fundamental understanding is expected to provide important insight into how to modify Si-based alloy materials to enhance their thermoelectric properties.

Understanding and manipulating the coherent phonon transport in solids is of interest to fundamental study as well as many practical applications including thermoelectrics. Coherent phonon transport in superlattices has been observed experimentally. In this study, we investigate the phonon transmission across Si/Ge superlattices. We incorporate first-principles force constants derived from ab initio density functional theory into Green's function method to determine the phonon transmission function, and use Landauer formalism to obtain the thermal conductance. Coherent phonon transport is studied by keeping the period length fixed while changing the number of periods, and by varying the interface roughness.
This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577

First principles electronic band structure calculations are used to investigate electron behaviors in thermoelectric materials. The Boltzmann transport theory combined with accurately computed Fermi surface properties and models for relaxation time were implemented to evaluate electron transport quantities (electrical conductivity, thermopower and electronic thermal conductivity) [1], determining efficiency of thermoelectric conversion. We employed the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) [2,3], which is particularly well-adapted technique to study the electronic properties in disordered materials (e.g. alloys, impurities, defects), but also to calculate velocity and lifetime of electrons on Fermi surface [4]. As a result, the transport coefficients as well as their derivatives as power factor and the figure of merit ZT, can be theoretically explored in a form of two-dimensional maps vs. carrier concentration and temperature. Such results, although containing adjustable parameters as relaxation time and lattice thermal conductivity, may appear highly useful in experimental optimizations of performance of thermoelectric systems.
We discuss recent results of the aforementioned methodology applied to study some exciting phenomena as band convergence enhancing the thermoelectric efficiency in Mg2(Si-Sn) [1], selected half-Heusler phases near the metal-semiconductor crossovers [5] or influence of impurities and point defects on electronic structure.
This work is supported by the Polish National Science Center (NCN), under the grants DEC-2011/02/A/ST3/00124 and UMO-2011/03/N/ST3/02644.
[1] K. Kutorasinski, J. Tobola, S. Kaprzyk, Phys. Rev. B 87 (2013) 195205
[2] A. Bansil, S. Kaprzyk, P. E. Mijnarends, J. Tobola, Phys. Rev. B 60 (1999) 13396
[3] T. Stopa, S. Kaprzyk, J. Tobola, J. Phys. Condens. Matter 16 (2004) 4921
[4] T. Stopa, J. Tobola, S. Kaprzyk, J. Phys. Condens. Matter 18 (2006) 6379
[5] K. Kutorasinski, J. Tobola, S. Kaprzyk, Phys. Stat. Sol. A (submitted)
[6] J. Tobola, S. Kaprzyk, H. Scherrer, J. Electron. Mater. 39 (2010) 2064

Bulk nanostructured semicondutors (also known as nanocomposites) have been proposed as a new class of efficient thermoelectric materials [1,2]. The basic idea is that by tailoring their crystalline structure at the nanoscale, it is possible to improve thermoelectric properties with respect to pristine materials. This can be reached, for instance, by suppressing phonon transport through grain boundary scattering while keeping electric transport still active. SiGe nanocomposites belong to this class of promising materials, having an addition feature which could be possibly best tuned, namely: the alloy composition.
In this work we perform proof-of-concept numerical experiments addressed to investigate how the thermal conductivity of SiGe nanocomposites depends upon stoichiometry and crystallinity.To this aim we perform approach-to- equilibrium molecular dynamics simulations [3] on large scale atomistic models of nanostrcutured SiGe alloys.
Present simulations show that SiGe nanocomposites can reach thermal conductivities below the alloy limit. Moreover we observe that the nanocomposite thermal conductivity marginally depends on stoichiometry and is largely affected by granulometry.
[1] M. Dresselhas et al., Adv. Mater. vol.19, p.1043 (2007)
[2] A.J. Minnich et al., Energy Environ. Sci. vol.2, p. 466 (2009)
[3] E. Lampin et al. Appl. Phys. Lett. vol. 100, p. 131906 (2012)

The thermoelectric materials PbTe, PbSe, and PbS, as well as their alloys, are commonly doped p-type by adding a small amount of Na to the materials. Maximizing the amount of Na doping can lead to high values of ZT, on the order of 2 for PbTe-PbSe and PbTe-SrTe. In addition, adding Na to PbTe-PbS has been shown to cause dramatic changes in the morphology of PbS nanostructures that form in this system. The amount of Na that can be added to PbTe is limited by the precipitation of a Na-rich phase out of the PbTe matrix. Na is typically added to these materials by either a one-to-one replacement of Pb with Na, or the addition of Na₂Te. These two methods of adding Na to PbTe can be expected to have different solubility limits, and in general, the solubility of Na in PbTe is a function of temperature and alloy composition (i.e. PbTe off-stoichiometry and method of doping). We calculate the single-phase boundary of PbTe in the composition space Na-Pb-Te as a function of temperature and composition to determine the solubility limits of Na in PbTe. We use density functional theory (DFT) to calculate the formation energy of intrinsic and Na-containing defects in various charge states in PbTe. We use these defect formation energies to self-consistently calculate the temperature and chemical potential dependencies of the Fermi energy and defect concentrations in PbTe. Finally, we use the calculated defect concentrations to obtain the PbTe single-phase boundary and identify the maximum amount of Na that can be added to PbTe as a function of temperature and doping method. We find that the Na solubility has a peak along the line PbTe-NaTe, and that 6 times as much Na can be added to PbTe through a one-to-one replacement of Na for Pb than through the addition of Na₂Te to PbTe at 1000 K.

Motivated by the significant interest in metal-semiconductor interfaces and superlattice in energy conversion applications like thermoelectric devices, we developed molecular dynamics-based model that captures the thermal transport role of electronic degrees of freedom. We characterize the effects of electron-phonon coupling rate and degree of spatial location of the interaction on heat transport across the metal-semiconductor interfaces. We find that spatially extended electron-phonon coupling, where electrons on the metal side couple to phonons including both metal and semiconductor atoms, facilitates the energy transfer across the interface between metal and semiconductor and, therefore, reduces the thermal interface resistivity. The results of the MD simulations are consistent with recent experimental laser-pump measurements.

Thermoelectric properties of organic materials are still scarcely investigated, in spite of predictions made even long ago (1) that among organic compounds it is expected to find materials with higher ZT than that of the existing inorganic materials. This is due to the relatively recent development of organic materials with sufficiently high conductivity which in its turn caused a traditional sharp separation between the TE community, focused on inorganic materials, and the organic one, that exploited electronic properties of this class of materials for other applications, such as opto-electronic devices, partially overlooking thermoelectric possibilities of their compounds.
With the present work we want to enlighten the exceptional thermoelectric properties displayed by organic conducting and semiconducting materials which often combine high Seebeck coefficient and low thermal conductivity (2) together with the possibility of cheap and scalable printing technologies, that would lower the device fabrication process costs, for low cost heat-to-electricity conversion.
We have undertaken a systematic study of the interrelationship between thermopower, electrical and thermal conductivity initially using highly conducting thin-films of organic conjugated compounds with a view to controlling, insofar as possible, these three parameters separately. A precise evaluation of the Seebeck coefficient, electrical conductivity and power factors of various organic conducting/semiconducting thin films will be hereby presented. To carry out such investigations, a specially designed instrument has been realized and tested (3). Details about the instrument will be given in this contribution. The thermoelectric properties of several typical organic functional materials have been characterized with respect to microstructures; charge transport and charge carrier concentration and the results of these analyses will be outlined and discussed in this contribution.
The overall goal of the study is the development of high ZT materials for efficient energy conversion together with the realization of a thermoelectric generator (TEG) through advanced printed techniques, capable of delivering cheap, large-area TEG devices.
(1) Casian, A, J. of Thermoelectricity, 3, 5 (1996).
(2) O. Bubnova, et al, Mater. 10, 429 (2011).
(3) D. Beretta et al, to be submitted soon.