Concentrating Solar Power (CSP) technologies use mirrors to collect and focus sunlight onto a heat exchanger that conveys the solar-derived energy to a heat transfer fluid and ultimately to a thermal power cycle. CSP technologies have been around for decades, originally developed in parallel with photovoltaic (PV)-based systems as a lower cost alternative at the utility scale. Today, PV and CSP are nearly equivalent on a cost basis, and the value of CSP is closely tied to its ability to incorporate low-cost thermal energy storage at the utility scale, allowing solar power plants to produce electricity day and night. The use of thermal energy storage to decouple the solar resource from power generation greatly improves power generation flexibility, allowing CSP plants to complement PV facilities by increasing output in the early evening hours as PV plants, which generally don&’t include storage, come offline.
Many thermal energy storage configurations have been envisioned for CSP generation. These are generally categorized by the manner in which the energy is stored (e.g. sensible, latent, or thermochemical) and the temperature at which the storage is required, which is tied to the operating point of the thermal power cycle. Today, the research direction in CSP is toward higher temperature power cycles (> 600#730;C to increase overall efficiency) and toward lower cost storage systems, which typically requires using inexpensive materials or increasing the overall storage capacity per mass of storage media. Many of the challenges, and opportunities, related to thermal energy storage for CSP exist at the intersection of materials science and systems engineering. That is, in order for CSP technologies to meet future techno-economic goals, thermal energy storage media having favorable properties (storage capacity, reactivity, and cost) must be developed and matched with systems engineered to efficiently transfer thermal energy in and out of storage. This may, in principle, seem straightforward and in fact solutions exist for storage up to about 565#730;C; however, moving beyond this temperature range leads rapidly to challenges related to the stability of the storage media, and to its reactivity with other system components. Combine this consideration with scale (gigawatt-hours per facility) and a typical thirty year plant design lifetime, and the development of thermal energy storage for utility-scale CSP becomes an interesting, and challenging endeavor.

Thermal energy storage enables low-cost dispatchable power production from renewable energy sources. Latent heat thermal energy storage (LHTES) is particularly attractive for its high energy density and nearly isothermal operation, and is ideally suited for systems such as Solar Thermoelectricity via Advanced Latent Heat Storage (STEALS), which integrates LHTES near 600 C with thermoelectric power generation from concentrated solar power. Dispatchable power generation is achieved in STEALS by use of a thermal valve (currently under development) that is capable of controlling heat flow between the LHTES and the thermoelectric generator subsystems. A strong connection between materials science and mechanical engineering have been used to address the challenges of integrating LHTES into STEALS, and solutions to these challenges are given in this present work.
In selecting a LHTES material, three primary issues must be addressed: material thermal performance, corrosion mitigation, and overall system cost. Thermal performance properties of energy density (heat of fusion) and thermal conductivity have been determined through literature review and differential scanning calorimetry measurements. 200 hour corrosion experiments have been used to inform initial down-selection of LHTES materials, and further 3000 hour experiments have been used to demonstrate long-term stability with containment and heat transfer materials. Finally, both the cost of the LHTES material itself as well as the impact of the material thermal performance and corrosivity on the costs of other system components have been investigated. Using these results, a time domain system-level thermal model has been developed to consider the system configurations necessary to achieve high performance with different LHTES materials, with several insightful results. Salt materials with low thermal conductivity require extensive and costly heat transfer enhancements for exergetically efficient heat transfer, but may be compatible with low-cost standard engineering materials. Metal alloys with higher thermal conductivity may be used with much simpler heat transfer design, but corrosion experiments demonstrate that they require careful selection of containment materials or use of costly barrier coatings. By considering the impact of LHTES materials on the full STEALS system cost, this study has been able to evaluate the true merit of several candidate materials and demonstrate their viability when integrated into a full system design.

This study evaluates the performance of passive thermal management system of high power Li-ion batteries in a cold environment. Two different phase change materials (PCM) consisting of pure paraffin and paraffin-graphite composites are considered and compared with a battery module in absence of PCM material. Battery modules are subjected to different cold time periods representing short and long vehicle stops during winter. Battery performance parameters such as capacity, power, and temperature along with thermophysical properties of each module are recorded, calculated and compared for different scenarios.
Results show that in spite of low thermal conductivity of paraffin wax as PCM, after short period stops, it improves battery module performance. Battery module will stay two times warmer than the module without PCM and retains its nominal capacity. On the other hand paraffin wax becomes detrimental after long cold soak. Warming rate is slower by 29% which cause 16% decrease in capacity retention compared to the module with no PCM. Increasing thermal conductivity of PCM based module using a commercial graphite/paraffin composite was shown to be disadvantages at both short and cold stops. After short stops battery warm up is as fast as unsupported module. Furthermore after long cold soak it keeps battery cold enough to lower its capacity by 17%. The unexpectedly poor performance of commercial paraffin-graphite composite PCM is thought to come from excess pressure in a battery cell. It is anticipated that by removing excess pressure, commercial PCM module resemble a module with no PCM with ability to warm up slightly faster. Lumped capacitance analysis showed that thermal diffusivity of commercial PCM module is identical to the module with no PCM and is 33% larger than paraffin PCM module.

Isomerization-based solar thermal fuels (STFs) have recently re-emerged as attractive materials for solar harvesting and energy storage due to rapid progress in the computation of their photo-switching properties. These materials harvest solar radiation and store its energy within their bonds, following which they may release it as heat. New material strategies have been identified through density functional theory (DFT)¹ computation and verified experimentally, resulting in energy storage densities comparable with lithium-ion batteries.² Here we report joint computational and experimental studies on polymer STF materials that are compatible with solution processing into thin films and exhibit sufficient heat release for solid state applications. This work paves the way for solid state applications where solar energy may be stored and controllably released in an on-demand fashion for heating applications.
Derivation of a suitable monomer for STF applications first involves the computation of the expected heat release based on criteria that enable subsequent polymerization. Here, we employed azobenzene derivatives with additional functional groups and directly calculated the enthalpy associated with their trans to cis photoisomerization upon absorption of ultraviolet photons. We synthesized both polymer and monomer species, and compared their heat release properties. It was found that the favorable properties of the monomer translate well (70%) to the polymer, and agree with our computational results for the monomer species. Given the potential of storing > 1 eV per molecule, this presents an attractive avenue for achieving high energy densities in solid state.
Subsequently, we developed a methodology for casting solid-state films from common organic solvents. Importantly, we show that polymerization is required to make films of controllable thickness and uniformity, enabling films of tens of nanometers to several microns in a single step. We demonstrate that despite possible rigid confinement in the solid state, these films are capable of being charged and readily result in isomerization of the azobenzene species under ultraviolet radiation. Furthermore, we demonstrate that these films can be cycled with minimal degradation, and offer energy storage with a half-life of over 50 hours at room temperature. These findings suggest that STF polymers present an attractive platform for energy harvesting and storage, where heat release is required in solid-state applications.
(1) Kolpak, A. M.; Grossman, J. C. Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels. Nano Lett.2011, 11, 3156-3162.
(2) Kucharski, T. J.; Ferralis, N.; Kolpak, A. M.; Zheng, J. O.; Nocera, D. G.; Grossman, J. C. Templated Assembly of Photoswitches Significantly Increases the Energy-Storage Capacity of Solar Thermal Fuels. Nat Chem2014, 6, 441-447.

An application of phase transitions that is gaining popularity in recent years is the use of latent heat for the storage of thermal energy. In particular the solid-liquid transition is used. Selection of materials and modeling of applications requires a good knowledge of the thermal properties of these materials. Apart from the phase transition temperature (defining the temperature region of application), the thermal storage capacity (defining the efficiency) and the conductivity (defining the speed of interaction with the environment) are needed. As the thermal storage capacity is quantified by the enthalpy and, specifically, the latent heat of the materials, a technique capable of measuring the enthalpy is necessary.
Classical adiabatic scanning calorimetry [1] and the recently developed Peltier-element-based adiabatic scanning calorimetry (pASC) [2] are the only available calorimetric techniques that allow the direct measurement of the temperature dependence of the enthalpy, thus discriminating between true latent heats and pre-transitional enthalpy changes at first-order phase transitions.
An often-suggested group of materials are linear alkanes of medium length. In this paper, we concentrate on results of the melting behavior of different linear alkanes as well as on the different rotator phases occurring in the solid phase just below the melting temperature in several of the n-alkanes. These compounds show a narrow melting transition with a large latent heat, accompanied by several smaller transitions.
In practice, pure alkanes are not always suitable as PCM, and instead, mixtures of alkanes are used to tune the thermal properties (in particular the transition temperature) and to suppress the tendency to supercool. We present new data for several commercially available PCMs. Here as well as for the pure alkanes extensive comparison is made with enthalpy results obtained by other techniques, e.g. differential scanning calorimetry (DSC), showing the superior resolution of pASC for the determination of the transition parameters.
[1] J. Thoen, in Heat Capacities: Liquids, Solutions and Vapours, E. Wilhelm, T. M. Letcher, Eds., Royal Society of Chemistry: London (2010), pp 287-306.
[2] J. Thoen, J. Leys, C. Glorieux, Patent Application PCT/BE2011/000042, patent pending.

The capital cost of many thermoelectric generator (TEG) systems, including those used for vehicle waste-heat recovery and for CSP bottoming cycles, are often dominated by the cost of the cold-side heat exchanger, where cost-scaling “laws” for heat exchangers are not favorable. This necessitates re-thinking TEG architectures, fabrication, and even materials themselves. Following the cost scaling “laws” to their logical conclusion suggests materials with thermal conductivities < 0.5 W/m-K, such as polymers. Polymers afford new fabrication techniques and architectures that are inaccessible to many inorganic thermoelectrics. Polymer thermoelectrics also offer new transport physics dominated by electron-phonon coupling that lead to enhanced power factors making them competitive with their inorganic counterparts. However, suitable n-type polymers have limited compelling device demonstrations and therefore hinders the realization of a high-power density, all-polymer TEG. This talk will begin with the cost-scaling motivation that governs TEGs. From this analysis, we identigy new TEG deice architectures for waste heat recovery. These architectures require the development of new materials. Therefore next, I will discuss our progress in developing n-type polymer thermoelectric thin-films with conductivity exceeding 20 S/cm and Seebeck coefficients comparable in magnitude to PEDOT:PSS, making our n-type polymer one of the best-in-class. Finally, I will present our new radial device architecture and the results of our polymer TEG device performance, where the device is passively cooled with natural convection. This circumvents the heat exchanging cost-scaling problem and provides a compelling demonstration for the potential of polymer TEGs for bottoming cycles and low temperature waste-heat recovery.

Fabricating thermoelectric (TE) modules, especially those capable of producing significant power, is of vital importance. The traditional TE modules are made of rigid and expensive inorganic materials with moderate scalability. Recent demonstrations of solution-processible organic materials with ZTs approaching unity suggest new, scalable thermoelectric modules. These solution-processible organic materials are able to form stable ink-like formulations which can be deposited as TE thin films via printing techniques, such as screen printing and inkjet printing, which drastically improves the scalability of fabrication. However, further design is necessary to make TE modules with the hundreds of legs necessary to achieve appropriate open circuit voltages in a compact package and to fully take advantage of heat source through the TE module with thin film geometry. To demonstrate the great scalability of fabricating TE modules with organic materials as well as explore the module design that effectively uses thermal gradient in a compact package, here we use solution-processible p-type PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) and n-type perylene diimide derivatives as the “inks” as well as nickel foil and apply cost-effective screen printing to large-scale pattern full-scale modules. The meters long bands with flexible units were wound into compact centimeter size TE rolls which show great power output with a thermal gradient along their longitude direction. In this presentation, we will demonstrate a prototype with 144 pairs of p-n junctions composed of PEDOT:PSS (p) and nickel foil (n) can produce an open circuit voltage of over 0.42 V and power output of over 70 µW at a temperature difference of 100 K. With a voltage step-up chip in the circuit, these modules were used to power red, green and blue LEDs.

Owing to advantages of light weight, low cost, flexibility, eco-friendly feature and ease of processing, organic thermoelectric (TE) materials and devices based on conjugated polymers are attracting more and more attention in the past several years.¹ Great progress has been achieved for p-type conducting polymers and their composites, especially for poly(ethylenedioxythiophene) polystyrenesulphonate (PEDOT-PSS) with an ultimate ZT of 0.42 at room temperature.² However, much less effort has been devoted to n-type conducting polymers, which have lagged behind their p-type counterparts. Considering that both p-type and n-type conducting polymers with high power factors are required in viable TE modules, development of comparable n-type conducting polymers is essential to obtain high-performance TE generators.³
The first solution-processable n-type TE material based on pyromellitic diimide (PyDI)-acetylene homopolymer with Seebeck coefficient (S) of 40 mu;V K^-1 was reported by our group recently.⁴ The power factor of 50 to 100 mu;W/mK² and ZT > 0.1 were achieved from the composite of PyDI polymer with weak n-type dopant SnCl₂.⁵ To obtain higher conductivity and Seebeck coefficient, a series of n-type Donor-Acceptor (D-A) or Acceptor-Acceptor (A-A) imide-functionalized conjugated polymers with larger cores and stronger electron affinity compared to that of PyDI polymers were designed and synthesized. Introduction of strong planar electron-poor unit to the diimide conjugated polymer main chain facilitates lowering LUMO level and strengthening intermolecular interaction, which gives rise to improved air stability and higher electron conductivity. The electron mobilities up to 0.12 cm² V^-1 s^-1 in air were obtained from top-gate OFETs using these diimide conjugated polymers. Moreover, these polymers display Seebeck coefficient up to -2500 mu;V/K for the pristine films in air. When all-acceptor planar conjugated polymer was doped with a strongly electron donating solid, the conductivity was increased by orders of magnitude and a high Seebeck coefficient was maintained. Furthermore, electron paramagnetic resonance signals were observed in doped forms of these polymers. All-acceptor n-type planar conjugated polymers with observable radical character will open a new path for obtaining high performance solution processable n-type TE materials.
References:
1. (a) Poehler, T. O.; Katz, H. E. Energy Environ. Sci .2012, 5 (8), 8110. (b) Zhang, Q.; Sun, Y.; Xu, W.; Zhu, D. Adv. Mater. 2014, 26 (40), 6829.
2. Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Nat. Mate.r 2013, 12 (8), 719.
3. McGrail, B. T.; Sehirlioglu, A.; Pentzer, E. Angew. Chem. Int. Ed. 2015, 54 (6), 1710.
4. Kola, S.; Kim, J. H.; Ireland, R.; Yeh, M.-L.; Smith, K.; Guo, W.; Katz, H. E. ACS Macro Lett. 2013, 2 (8), 664.
5. Ireland, R. M.; Liu, Y.; Guo, X.; Cheng, Y.-T.; Kola, S.; Wang, W.; Jones, T.; Yang, R.; Falk, M. L.; Katz, H. E. Adv. Sci. 2015, 2, 150015.

Thermoelectrics are a viable solid-state technology to convert waste heat into useable electrical power. Conducting polymers have entered the thermoelectric landscape in the last five years as promising materials, but the Seebeck coefficients of conducting polymers have been low in comparison to conventional inorganic thermoelectrics. The combined thermoelectric and thermogalvanic effect of electrons and ions in a mixed conductor shows great promise for power factor enhancement. Recent investigation of ion conduction within conducting polymers has resulted in huge, albeit short-lived, Seebeck coefficients, demonstrating the need for design rules through chemical design. We demonstrate a rational method to vastly increase the lifetime of the ionic Seebeck effect in the conducting polymer PEDOT:PSS through the addition of carefully selected ion conductors and electrode materials. We characterize the thermoelectric effect in the pure ion conducting system, and demonstrate the role of humidity in controlled the overall power factor. We then incorporate the ion conductor into the PEDOT:PSS, and demonstrate that the thermoelectric effect of this new mixed conductor is larger than just either the pure electronic phase or ionic phase, resulting in a synergy between the ionic and electronic mixed conduction. The applicability of this effect is then surveyed in a device by measuring the power output over time and compared to that of PEDOT:PSS. Finally, we propose a theoretical explanation to discuss the thermogalvanic effect in pure ion conductors and extend this to mixed conductors, so that fundamental design rules can be proposed for rational design of mixed conductors for harvesting waste heat.

In an effort to create a paintable/printable thermoelectric material, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled nanotubes (DWNT) were alternately deposited from aqueous solutions using a layer-by-layer assembly. This unique combination resulted in increased carrier mobility, originating from strong π-π interactions between PANi, DWNT and graphene. A 40 quadlayer thin film (470 nm thick), comprised of a PANi/graphene/PANi/DWNT repeating sequence, exhibits electrical conductivity (σ) of 1 X 10⁶ S/m and a Seebeck coefficient (S) of 130 µV/K, producing a thermoelectric power factor (S²σ) of 1825 µW/(m^.K²). This is the highest value ever reported for a completely organic material. This exceptional performance is attributed to synergy within the three-dimensional conjugated network, with PANi-covered DWNT bridging gaps between graphene sheets, which increases the electrical conductivity and Seebeck coefficient due to improved carrier mobility. In all respects, the LbL approach reported here provides a unique way to fabricate nanostructured hybrid composites with great potential to improve the performance of TE-driven devices. These water-based nanocoatings could be painted or printed anywhere there is a thermal gradient present (e.g., exhaust pipes, cell phones, or even clothing that harnesses body heat).

Thermoelectric materials require a unique set of fundamental thermal and electronic transport properties in order to function efficiently. For instance, it is desirable to minimize thermal conductivity as much as possible in order to attain high figure of merit. One very successful approach to achieving this condition is the so-called phonon-glass-electron-crystal (PGEC), in which localized phonon modes of guest atoms in cage-like structures, such as skutterudites and clathrates, induce strong phonon scattering. Here we show that PGEC-like behavior and high thermoelectric figure of merit can be achieved even in materials which exhibit no obvious cage-like structure: the family of compounds based on tetrahedrite. By combining thermal and electronic characterization with theoretical calculations, x-ray and neutron probes,we show that tetrahedrites exhibit unique structural and bonding aspects that give rise to PGEC-like behavior, strong lattice anharmonicity, and resulting minimal thermal conductivity. In the presence of a favorable band structure, tetrahedrites can exhibit thermoelectric properties rivalling that of PbTe, and thus can offer the potential of a low-cost, environmentally benign material for use in thermoelectric power generators on a large scale. The unique structural features giving rise to PGEC-like behavior in tetrahedrite may occur more generally in other materials and thus could provide a new route to the design of high efficiency thermoelectrics.

Lead chalcogenides have historically been the top performing thermoelectric material for high temperature applications, but recently tin selenide was found to have a thermoelectric performance higher than what has been achieved by lead chalcogenides. Tin selenide&’s extraordinary performance was due to its layered crystal structure and the low thermal conductivity resultant from it. Selenium is a relatively expensive element and it would be advantageous to replace it with the more earth abundant element sulfur. Fortunately tin sulfide adopts the same crystal structure as tin selenide and is a promising thermoelectric material. Sodium and silver have been used recently as dopants for tin sulfide, but were only able to reach a carrier concentration around 10¹⁸ carriers/cm³. New dopants are needed to reach the ideal carrier concentration of 10¹⁹ carriers/cm³ to fully optimize tin sulfide. In this study we prepared both polycrystalline and single crystalline tin sulfide with several different dopants and their performances are compared.

The complex interactions between solid-state excitations, such as phonon-phonon, phonon-electron, and phonon-magnon, are often responsible for unusual material properties. In this work, we have investigated the role of phonon anharmonicity and electron-phonon coupling in SnSe and Mo₃Sb_7-xTe_x to understand the origin of high thermoelectric performance in these materials. We have performed extensive inelastic neutron and x-ray scattering measurements of phonons in SnSe and Mo₃Sb_7-xTe_x , mapping the four dimensional phonon dispersion surfaces as well as the density of states. Our first-principles density functional theory simulations, coupled with experimental measurements, reveal that the large anharmonicity in SnSe originates from a bonding instability involving Sn 5s electrons. Moreover, our detailed experimental and theoretical study of Mo₃Sb₇ reveals the importance of electron-phonon coupling in this material. Doping with Te leads to transfer of extra electron to Mo, and moves the Fermi surface to the top of the valence band. The change in Fermi surface topology and decrease of screening (from suppression of the electron-phonon coupling) causes phonons to stiffen markedly. Our measurement of acoustic dispersions quantifies the impact of this effect on the thermal conductivity.
Neutron and x-ray scattering measurements were supported by the US DOE, Office of Basic Energy Sciences, as part of the S3TEC EFRC (SnSe) and through the Office of Science Early Career Research Program (Mo₃Sb₇). Simulations were supported through the Center for Accelerating Materials Modeling of SNS Data (CAMM), funded by the US DOE, Office of Basic Energy Sciences.

It was recently found that SnSe has unprecedented high thermoelectric figure of merit (ZT) at 923 K, which is believed to be closely related to its structural phase transition from Pnma to Cmcm at about 800 K. It is desirable to decrease the temperature where SnSe shows the highest ZT value for practical purposes, as it is relatively closed to the melting point. Based on ab-initio molecular dynamic simulations, we have directly observed that the structural phase transition temperature of SnSe can be effectively controlled by applying an external hydrostatic pressure. By density functional investigations of the effects of uniaxial stress, we find that although the Cmcm phase quickly becomes energetically favorable over Pnma at T = 0 K, the Cmcm phase is dynamically unstable with soft phonon modes. The influences of stress on the electronic structures and the electrical transport properties have also been investigated.

In the context of materials design and high-throughput computational searches for new thermoelectric materials, the need to compute electron and phonon transport properties renders direct assessment of the thermoelectric figure of merit (zT) for large numbers of compounds untenable. While the state-of-the-art theoretical and computational approaches can deliver desired accuracy in assessing the transport properties of real materials, their application is currently limited to relatively simple systems and to case-by-case studies. In this talk I will discuss integrated theory-computations-experiment efforts in developing a robust set of materials descriptors that are computationally tractable, thereby allowing for high-throughput materials searches, but at the same time sufficiently accurate to provide reliable predictions. To do so, we pursue somewhat different route compared to more traditional high-throughput computational approaches that are constructed predominantly around electronic transport properties. We start from Boltzmann transport equations and the thermoelectric quality factor (β), a quantity that depends solely on intrinsic materials properties such as charge carrier mobility (mu;_o), DOS effective mass (m_DOS) and lattice thermal conductivity (κ_L). Next, to overcome the limitations associated with direct calculations of mu;_o and κ_L we develop semi-empirical models, motivated by the classic theories for electron-phonon and phonon-phonon scattering rates, by combining only the quantities readily available from standard first-principles calculations with the large body of available experimental data. These models of transport properties are then combined into a semi-empirical descriptor termed β_SE, which is demonstrated to correctly identify known thermoelectric materials. At the end, I will discuss the advantages and disadvantages of semi-empirical approaches in searching for new candidate thermoelectrics, as well as the routes to assess the dopability of candidate materials, which will ultimately provide a more complete set of search criteria. This work is supported by NSF-DMR program.

In order to improve thermoelectric properties, adding filler or substitutional atoms is a promising method. Skutterudites - thermoelectric material candidates - inherently have a high electrical conductivity, but difficulty of obtaining figure of merit, ZT greater than unity. The incorporation of filler or substitutional atoms into skutterudites can increase electrical conductivity and decrease thermal conductivity by means of scattering phonons. We have performed density functional theory-based calculations to investigate the structural, electrical and thermal properties of Ca and Ce double-filled Fe₄Sb₁₂ and Co₄Sb_12-2xTe_xGe_x compounds (x = 0, 0.5, 1, 2, 3 and 6). In both systems, the filler (i.e. Ca/Ce-filled Fe₄Sb₁₂ compound) and substitutional atoms (i.e. Co₄Sb_12-2xTe_xGe_x compound) lead to a decrease in lattice constant. In addition, the pnicogen rings in CoSb₃ are structurally distorted as Te/Ge substitution concentration increases. It indicates a deviation from cubic symmetry for the compound. Besides an increase in electrical conductivity, a transition from direct to indirect band gap semiconducting behavior occurs. Phonon dispersion relations show that the lattice thermal conductivities of both compounds are dominated by acoustic modes. For Co₄Sb_12-2xTe_xGe_x compound, x = 3 has the lowest phonon dispersion gradient, resulting in the lowest lattice thermal conductivity. We will discuss the computational results in comparison with experimental data.

CoSb₃-based filled skutterudite has emerged as one of the most viable candidates for thermoelectric applications in automotive industry. However, the scale-up commercialization of such materials is still a challenge due to the scarcity and cost of constituent elements. Here we study Ce, the most earth abundant and low cost rare-earth element as a single filling element and demonstrate that by solubility design using a phase diagram approach, the filling fraction limit (FFL) x in Ce_xCo₄Sb₁₂ can be increased more than twice the amount reported previously (x = 0.09). This ultra-high FFL (x = 0.20) enables the optimization of carrier concentration such that no additional filling elements are needed to produce a state of the art n-type skutterudite material with a zT value of 1.3 at 850K before nano-structuring. The earth abundance and low cost of Ce would potentially facilitate a widespread application of skutterudites.

Extensive work has been done searching for new efficient thermoelectric materials for conversion of waste heat into electricity. Herein, we report the synthesis and characterization of binary tetrel arsenides, TtAs (Tt = Si, Ge). SiAs and GeAs are isostructural compounds that crystallize in the monoclinic space group C2/m (No. 12). They have a layered crystal structure with the surface of each layer terminating in As atoms. Within the layer, Tt-Tt pairs are surrounded by 6 As atoms forming a distorted octahedron. Each layer is formed through edge-sharing octahedra rotated 90 and 180#730; with respect to its neighbor. TtAs can be doped with other tetrel elements forming Tt_1-xTt&’_xAs (Tt, Tt&’ = Si, Ge, and Sn), creating structural distortions, which leads to an additional reduction in thermal conductivity. Details pertaining to the crystal and electronic structures and transport properties will be discussed.

Much of our recent work has focused on the exploitation of low-dimensional structural units for the design of high-performance thermoelectric materials containing earth-abundant elements. This has led us to investigate materials of general formula A₃B₂S₂ (where A = transition series element; B = main-group element) that adopt the shandite structure. The shandite structure contains two-dimensional kagome-like layers of metal atoms, capped by sulphur atoms. The layers are stacked in an ABC sequence and are linked by main-group metal atoms located in inter-layer sites of trigonal-prismatic coordination geometry. Co₃Sn₂S₂ exhibits the unusual combination of a metal-like resistivity and high Seebeck coefficient. We have recently shown that substitution of tin by indium leads to a significant enhancement of the thermoelectric properties, leading to a maximum figure-of-merit, ZT =0.32 for Co₃Sn_1.6In_0.4S₂ at 673 K.
We describe here the impact of electron and hole doping on the thermoelectric properties of Co₃Sn₂S₂, effected by substitution at the transition-metal (A) site, through preparation of the series Co_3-xM_xSn₂S₂ (M = Fe, Ni). In the case of nickel substitution a complete solid solution exists over the compositional range 0 le; x le; 3, whereas iron substitution occurs over the more limited range x < 0.7. Electron (Ni) and hole (Fe) doping have contrasting effects on the thermoelectric properties. With increasing nickel substitution, the metal-like resistivity falls systematically as the Fermi level is raised. However, the concomitant reduction in |S| leads to reduced power factors, offsetting reductions in thermal conductivity, and lowering ZT. By contrast, iron-substituted phases show an increase in ZT over that of the end-member, Co₃Sn₂S₂. Whilst electrical resistivity rises systematically with iron content, showing a factor of two increase between x = 0 and x = 0.6, an increase in |S| leads to power factors which, depending on temperature and composition, are comparable to, or exceed, that of the end-member phase. Moreover, the marked decrease in thermal conductivity on nickel substitution, leads to an increase in the figure of merit; ZT exceeding 0.2 at 573 K. The contrasting nature of these changes will be discussed in the context of the calculated band structure of the shandite phases.

Recent developments in solar-thermal power generation aim as well to achieve higher temperatures to increase the efficiencies of the power cycles as to store the solar energy to enable baseload power generation from a transient energy source.
Thermochemical redox processes are an option to store large amounts of solar energy in a compact storage system. The enthalpy effects of these reversible chemical reactions can be exploited. Oxides of multivalent metals in particular, capable of being reduced and oxidized under air atmosphere with significant heat effects are perfect candidates for air-operated Concentrated Solar Power plants since in this case air can be used as both the heat transfer fluid and the reactant (O₂) and therefore can come to direct contact with the storage material (oxide).
Examples are manganese oxide and cobalt oxide cyclic redox schemes. They can be cycled in the range of 800-1000^oC.
Porous ceramic structures like honeycombs and foams are favorable for heat exchange applications, the idea of employing such structures either coated with or entirely made of a redox material like Co₃O₄, as a hybrid sensible-thermochemical solar energy storage system in air-operated Concentrated Solar Power plants has been set forth and tested.
At first, small-scale, redox-inert, cordierite foams and honeycombs were coated with Co₃O₄ and tested for cyclic reduction-oxidation operation via Thermo-Gravimetric Analysis.
To improve the volumetric heat storage capacity of such reactors, ceramic foams made entirely of Co₃O₄ were manufactured. Such foams exhibited satisfactory structural integrity and were comparatively tested vs. the “plain” Co₃O₄ powder and the Co₃O₄-coated, cordierite supports under the same cyclic redox conditions.
A thermochemical storage system prototype was modeled, built and implemented for the first time in an existing concentrated solar power facility, DLR&’s solar tower in Juelich, Germany.

Two-step thermochemical water splitting has become a promising route to generate hydrogen from water with solar energy. One particularly attractive approach uses ceria as the reactive medium due to its rapid reaction kinetics, which has been attributed to its nonstoichiometric behavior and fluorite structure. Rapid material kinetics raise the possibility that the overall reaction rate may be limited, not by material kinetic factors (which can then be considered to be infinitely fast), but rather by the thermodynamic capacity of the reactant gas to modify the oxide oxidation state. This condition is the gas-phase limited regime. In this contribution, the oxidation temperature is varied from 800 °C - 1500 °C with a highly porous (~78% porosity) sample of ceria under flow of a specific flowrate of 600 mL g^-1 of 20% steam in argon. We demonstrate a transition from material kinetic limitation at low oxidation temperatures to gas-phase limited kinetics at high oxidation temperatures. Gas production profiles during oxidation in the gas-phase limited regime are shown to be in agreement with a numerical model. In addition, decorating the ceria sample with Rh particles is shown to enhance gas production kinetics in the material-limited regime, while no kinetic enhancement is observed in the gas-phase limited regime, confirming the interpretation of the rate behavior.

The principal motivation driving development of ferroic caloric effect materials is the promise of efficient energy transduction, an important application of which is refrigeration. Ferroic materials exhibiting first-order phase transitions can provide a material basis for refrigeration cycles; however these materials are marked by hysteresis, which greatly reduces the availability of refrigeration work in such cycles. Here, we present a method of analysis for hysteretic giant magnetocaloric effect (GMCE) materials in an arbitrary cycle, which combines a Preisach model for rate-independent hysteresis with a non-equilibrium thermodynamic analysis of phase transformations in order to evaluate refrigeration work and energy dissipation. These terms are related through simplified but physically-meaningful descriptors to a given GMCE material's magnetic and thermal properties, demonstrating the method's use as a materials analysis and design tool.
We demonstrate application of this approach on a single-crystal off-stoichiometric Heusler alloy (Ni₄₅Co₅Mn_36.6In_13.4 at.%), for which we calculate the maximum obtainable refrigeration work and corresponding energy dissipated per cycle for different temperature (T) - magnetic field (H) cycles. Using this methodology, we: 1) Calculate materials performance metrics for irreversible hysteretic cycles under realistic external magnetic field constraints (B_max = 1.5 T, 5 T), 2) Evaluate the relative performance of magnetic Ericsson cycles (alternating iso-thermal, iso-field legs) and magnetic Brayton cycles (alternating isentropic, iso-field legs) under the same magnetic field constraints, and 3) Explore the role of hysteresis behavior, by analyzing materials performance metrics as a function of a simplified two-parameter hysteresis model. We will follow by discussing utilization of this approach to analyze and design superior energy transduction materials.

The synthesis of geometrically tunable nano-size hollow silicate particles (NHSPs) using water-based poly (methacrylic acid) sodium salt (NaPMA) colloidal nano-aggregates as soft-templates, tetraethyl orthosilicate (TEOS) as a silica precursor and ammonia hydroxide (NH4OH) as a catalyst by simple sol-gel reaction was carried out. Tuning the nano-size diameter of NHSPs between 60 nm and 200 nm and decreasing the shell thickness from 40 nm down to 18 nm were achieved by only varying the concentration of NaPMA and the amount of TEOS, respectively. The thermal insulating feature of NHSPs was demonstrated by coating NHSPs on the glass substrate (C-NHSPs), a ~29.9 % difference of thermal conductivity as compared to glass substrate, which was evaluated by non-contact approached using the Xenon flash method. C-NHSPs was done by simple sequential adsorption of poly_ (allylamine hydroxide) (PAH) followed by deposition of NHSPs on the cleaned glass substrate surfaces using an automated dip coater. The dip coated samples (C-NHSPs) showed good thermal insulating properties as well as excellent transparency in the visible range.

Renewable energy is increasingly penetrating the worldwide energy landscape, due to increasing energy demand in the face of global climate concerns. Concentrating solar power (CSP) is one technology for large-scale generation of electricity. However, the inherent intermittent availability of solar energy is inconsistent with the current paradigm of on-demand electricity. Furthermore, the available daily solar resource is out of phase with the demand curve. Thermal storage can help alleviate this uncertainty in CSP; thermal energy can be stored while the system is on-sun, and released at other times to accommodate transients and peak demand. Such storage is currently implemented with molten salts wherein the energy is stored in the form of sensible heat. However, next-generation CSP plants (power towers) are expected to operate at higher temperatures (> 800 °C) in order to maximize efficiency. These temperature requirements are inconsistent with molten salts which have an inherently low energy density and a tendency to decompose above 800 °C. Thermochemical energy storage (TCES), in which heat is stored as both sensible heat and in the form of chemical bonds, is a potential technology to achieve the energy storage requirements of next-generation CSP.
Metal oxides that undergo reversible reduction/oxidation (redox) reactions are of interest for TCES because of their thermal stability, fast redox kinetics, and potential high energy densities. Mixed-ionic/electronic conducting (MIEC) perovskites with the general formula ABO_3-δ, where δ represents oxygen nonstoichiometry, have been recognized as candidates for high-temperature (> 900 °C) TCES due to their facility for cyclic endothermic reduction and exothermic oxidation. In this presentation, the synthesis, characterization, and thermodynamic properties of several promising MIEC perovskite oxides will be discussed. It will be demonstrated that these materials are stable at temperatures exceeding 1000 °C, redox cyclable, and exhibit promising thermal storage densities, with total mass-specific enthalpies on the order of 1000 kJ/kg.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000. This material is based upon work supported by the U.S. Department of Energy SunShot Initiative under Award Number DE-FOA-0000805.

Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb_8-xW_9+xO₄₇ (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a “phonon-glass electron-crystal” through a layered nanostructure defined by intrinsic crystallographic shear planes. The thermoelectric properties of the tetragonal tungsten bronzes Nb_8-xW_9+xO_47-δ (0 < x < 5) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO₂ were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb₈W₉O_47-δ was found stable at 973 K and a pO₂ of asymp; 10^-15 atm. The oxygen nonstoichiometry δ can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity r and the Seebeck coefficient S decreased. For x = 2, r reached 20 mWcm at 973 K, combined with a Seebeck coefficient of approximately -120 µV/K. The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W/(Km), attributed to the complex crystal structure. The thermoelectric figure of merit zT of the investigated samples was > 0.1 for x = 4 at 973 K. The control of the oxygen non-stoichiometry δ opens a second independent optimization strategy for tetragonal tungsten bronzes.

The discovery of novel materials with high thermoelectric performance has historically been led by experimentalists. However, to facilitate the search for thermoelectric materials, we can look to high throughput first principles calculations for guidance. A computationally determined metric, β, has been related to the thermoelectric figure of merit, zT, and is the basis of the study herein. In this presentation, we give an overview of the β parameter and present the application of β to oxide materials. From an initial database of over 700 compounds, a down-selection to a handful of initial candidates was conducted through a combination of β and synthesizability/stability considerations. For a series of ternary oxides composed of alkaline earth and p-block metals (e.g. Ba₂SnO₄, SrIn₂O₄, and Ba₃In₂O₆), we discuss synthesis techniques and explore the thermoelectric properties, measured from room temperature to high temperature. These are then compared to the computational predictions in order to further refine the β parameter model. Finally, the challenges associated with the prediction of properties for oxides and how the knowledge we have gained can be applied going forward will be summarized.

The layered structured bismuth copper oxytelluride (BiCuTeO) is a promising p-type oxychalcogenide material for thermoelectric power generation due to its low thermal conductivity (~0.6W/mK) and low energy gap (Eg=0.4eV). Based on the theoretical predictions[1], the power factor of BiCuTeO could be enhanced by optimizing the carriers concentration, which also can be tuned by varying copper vacancies. In the present work, we investigated the experimental details of the thermoelectric properties of both Bi and Cu deficiencies, and also studied the influence of oxygen composition in BiCuTeO system. The single phase BiCuTeO was prepared by solid state synthesis method, which was confirmed by X-ray diffraction (XRD) and X-Ray Fluorescence (XRF) techniques. In the environment of Cu deficiency, Bi vacancy led to enhanced Seebeck coefficient by decrease in carrier concentration, and also reduction in thermal conductivity by defect scattering. A significant enhancement (46%) in the thermoelectric figure of merit zT has been achieved by Bi vacancy in Cu defficient BiCuTeO. Moreover, the effect of variation in oxygen composition would be discussed.
Reference:
1. Zhao, L.-D., et al., BiCuSeO oxyselenides: new promising thermoelectric materials. Energy & Environmental Science, 2014. 7(9): p. 2900-2924.

The study of lead chalcogenides nanostructures has gained a lot of attention in recent years, particularly thrust by possible applications in solar cells and thermoelectric applications. PbTe, PbSe and PbS nanostructures were prepared by using cost effective matal-ion reduction chemical method tuning at different pH, temperature & reaction time. The synthesized PbTe nanocubes and sheets, PbSe nanosheets and PbS nanoparticles were characterized by X-ray Diffraction (XRD), Field Emission- Scanning Electron Microscope (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM) with Selective Area Electron Diffraction (SAED) and X-ray Photoelectron Spectrometer (XPS) measurements. These nanostructures have the same structures as bulk expect line broadening due to the quantum size effect which was confirmed by X-ray diffraction (XRD). The elemental mapping performed using FESEM which shown 1:1 atomic ratio of Pb and Te. XPS analysis confirmed the presence of Pb (+2) state and Te (-2) state. The distribution of shape, size and phase was further confirmed by HRTEM. Such kind of different dimensionality helps to reduce the thermal conductivity by enhancing the phonon scatterings at grain boundaries without effecting the electronic transport properties, which eventually enhances the Figure of Merit of thermoelectric materials.
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Key Words: Lead chalcogenides, chemical reduction method, pH, Reaction time, XRD, FESEM, HRTEM & XPS.

We investigated the dependence of film thickness and grain size on the out-of-plane thermal conductivities of single-phase Sb and Te films, which were prepared by electron-beam evaporation at room temperature. The thermal conductivities of these films were measured at room temperature using four-point-probe 3-omega method. For this study, we prepared 50-, 100-, 200-nm-thick Sb and Te thin films. From the measured thermal conductivities, we evaluated that the average thermal conductivities of the single-phase Sb and Te thin films were 5.9-10.2 W/(m#8901;K) and 0.8-1.2 W/(m#8901;K), respectively, at room temperature. This result clearly shows the fact that the thickness and grain size of each thin film strongly affect the modulation of its thermal conductivity at room temperature.

The thermoelectric properties of segmented nanosheets and nanowires [1] are studied by means of the Kubo-Greenwood formula and a real-space renormalization plus convolution method [2]. The tight-binding and Born models are respectively used for the calculations of electronic and lattice thermal conductivities [3]. Given that the thermoelectricity is improved around the band edges, we study in this work the thermoelectric figure of merit (ZT) of several designed periodic segmented nanosheets, whose ZT reaches to four times larger than those obtained from full periodic ones. Furthermore, an additional enhancement of ZT is found in nanosheets with a few number of layers. For quasiperiodically segmented nanosheets, whose segments are ordered following the Fibonacci sequence, an even larger ZT is obtained. Such enhancement of ZT observed in segmented nanosheets is strongly related to the reduction of its thermal conductivity caused by the scattering at segment interfaces. Moreover, the quasiperiodicity reduces the density of long-wavelength phonons and they are responsible of the lattice thermal conduction at low temperatures. In conclusion, the periodic and quasiperiodically segmented nanosheets under design could be good candidates for the thermoelectric applications.
This work has been partially supported by UNAM-IN113714. Computations were performed at Miztli of DGTIC, UNAM.
[1] S. C. Andrews, et al., Chem. Sci.2, 706 (2011).
[2] V. Sanchez and C. Wang, Phys. Rev. B70, 144207 (2004).
[3] C. Wang, F. Salazar and V. Sanchez, Nano Lett.8, 4205 (2008).

There are lots of research on thermoelectric power generation in order to promote effective utilization of a waste heat energy. Mg₂Si has attracted much attention as a thermoelectric element because of its high thermoelectric conversion efficiency and low environmental load. However, the Mg₂Si crystal is subjected to be oxidized under a relative high working temperature at around 600 °C in the air. Therefore, the Mg₂Si is necessary to be coated by an oxidation resistant layer for a long time utilization. In this study, a silicate glass-ceramics coating was prepared on the Mg₂Si. The appropriate glass composition in Na₂O-K₂O-CaO-Al₂O₃-SiO₂ system was selected from the viewpoints of the thermal expansion coefficient of the resulted glass ceramics, the relative low sintering temperature and the high chemical durability. The silicate glass was prepared by a conventional melt-quenching method. The glass slurry was prepared by grounding the glass into the fine particles and dispersing them in ethanol. The glass layer was formed on the Mg₂Si sintered sample by a dip coating of the slurry, and the coating layer was subsequently sintered at 700 °C. The crystallization behavior and the micro texture of the coatings were examined by XRD measurements and SEM observations. The main crystalline phase of the coating was kalsilite (KAlSiO₄). The coating exhibited some small voids on its surface but there was no cracking and exfoliation after sintering because the difference of the thermal expansion coefficients between the Mg₂Si and the coated glass ceramics was small. These results indicate that the prepared glass-ceramic is a good candidate as the thermal resistant coating on the Mg₂Si.
This work was supported by Future Pioneering Projects “Research and Development of Thermal Management Materials and Technology” entrusted by New Energy and Industrial Technology Development Organization (NEDO).

Current research efforts on the development of thermoelectric materials have been heavily focused on chalcogenide compounds for high-temperature applications including power generation and waste heat recovery. On the other hand, there are significant demands for materials development for environment-friendly cooling applications that need to be met. A few recent studies reported thermoelectric properties for the FeAs₂ marcasite compound, which has a potential for becoming a good thermoelectric material for low-temperature cooling applications. The compound can be more environment-friendly and more economically viable than other competing materials, for the composition does not involve rare and expensive element like Te or Pt.
In this study, we investigate thermoelectric properties of the FeAs₂ marcasite compound by first-principles calculations. Electronic band structures and density of states are constructed from DFT (density functional theory) calculations, from which electrical properties, including the Seebeck coefficient and the electrical conductivity, are estimated. At the same time, vibrational characteristics are investigated through DFPT (density functional perturbation theory) calculations, from which the thermal conductivity is estimated using semiempirical formulae and the Grüneissen parameters of the compound obtained at the level of the QHA (quasi-harmonic approximation). Combining these properties, the thermoelectric figure of merit (ZT) of the FeAs₂ marcasite compound is provided, and the feasibility of the compound for the use in practical cooling applications is discussed based on the results.

In recent years, bismuth telluride (Bi₂Te₃) and bismuth selenide(Bi₂Se₃) is widely studied as a new materials for topological insulators (TI). Besides, bismuth telluride has been well known for its excellent thermoelectric properties with high performance at room temperature.Ternary Bi₂Te_3-xSe_x system has been recently emerged as a promising candidate for thermoelectric application. In this work, we have grown p-type Bi₂Te_3-xSe_x nanostructure on soda lime glass (SLG) substrate using MBE technique.We have synthesized c-axis oriented Bi2Te3-xSex on SLG substrate. Effect of Na ion diffusion into Bi2Te3-xSex lattice has been studied and discussed based on Raman, SEM and Seebeck coefficient measurements.The use of SLG as substrate has facilitated the Na ion diffusion into Bi₂Te_3-xSe_x that results in the p-type conduction and the formation of nanostructure. Remarkably, large enhancement of the Seebeck coefficient has been demonstrated in this Bi₂Te_3-xSe_x/SLG up to ~450 mV/K at x = 1.52 as compared with that grown on sapphire (~100 mV/K). Our analysis reveals that the enhancement is attributable to the reduced carrier concentration owing to the Se doping in Bi₂Te₃ and the formation of abundant grain boundaries in this nanostructured material. Our results have provided an attractive platform for high performance thermoelectric materials.

Niobium oxide (Nb₂O₅) and silver (Ag) composites with different amounts of incorporated Ag (up to 19.2 wt%) were synthesized using a spark plasma sintering (SPS) method. The electrical conductivity (σ) increased with increasing Ag amounts, whereas the Seebeck coefficient (S) increased then decreased. It is plausible that the highly conductive Ag particles, introduced between the Nb₂O₅ grains through mechanical milling of the Nb₂O₅ and Ag₂O as raw materials, contributed the increases in both σ and S. However, upon further increasing the amount of Ag, the Ag particles became connected with each other and acted as bypasses for carrier transport, thus increasing σ and decreasing S. The thermal conductivity (κ) increased with the Ag content; however, the rate of increase was small when the Ag particles were not connected with each other. Our findings demonstrate that the introduction of a metal into an insulator by SPS causes it to become a thermoelectric material.

The latent heat storage is one of the most efficient ways to store thermal energy. It provides a higher storage density with a smaller temperature difference between the stored and released heat than the sensible heat storage method.
Being one of the energy saving related applications, phase change materials (PCM) are used to design thermal energy storage systems for recovery of waste heat, discontinuous energy production, and improved insulation materials. To be suitable for energy storage, a PCM should possess large phase change enthalpy, phase change temperature adapted to a given energy storage system, reproducible phase change, and a limited subcooling effect. All these parameters can be determined by calorimetric techniques and several examples on both latent and sensible heat storage materials studies will be given.
The thermal behavior of large heterogeneous samples with different shapes (polyolefins encapsulated in polymer beads) was studied by Differential scanning calorimetry, which simulates the actual conditions of use. The latent heat of fusion of a mixture of paraffins and a dicarboxilic acid was measured after several phase changes to evaluate another important parameter - cycling stability.
The experimental results obtained for another type of thermal energy storage materials, hydrated inorganic salts, will be presented in order to assess the quantity of heat, which can be stored for a certain temperature range. The latent heat of fusion and the heat of dehydration were measured and analyzed using highly sensitive DSC technique. Thermal conductivity measurements on phase change materials were performed using interfacial, non-destructive TCi technology.

Recently there have been growing concerns over global warming and the possible depletion of fossil fuels as these become used more extensively with the increase in energy consumption. One of the solutions to these problems is the application of thermoelectric (TE) technology which can convert waste heat directly into electricity.
This technology is used to produce clean energy by utilizing the waste heat emitted from automobiles, industrial furnaces and so on. Of the many thermoelectric materials, we focused on Mg₂Si. Mg₂Si has many favorable attributes, such as the abundance of its constituent elements in the earth&’s crust, and the fact that the manufacturing processes are non-toxic, in addition to it being a lightweight material. Many studies have been conducted in order to bring Mg₂Si into use as a thermoelectric material. For example, studies on electrode formation and depositing oxidation resistant coatings for high temperature applications have been done. It is difficult to obtain satisfactory interfacial conditions between Mg₂Si and the different materials deposited on it, as the surface of Mg₂Si is oxidized in the fabrication process. For the technologies described above Mg₂Si needs to be in contact with different materials, such as glass and metal, therefore, it is necessary to clean the surface of Mg₂Si in order to obtain satisfactory interfacial conditions before preparing oxidation resistant coatings or electrodes. Moreover, little research on chemical etching for cleaning the surface of Mg₂Si has been conducted because Mg in Mg₂Si is highly reactive. Therefore, the best conditions for chemical etching have not yet been identified for Mg₂Si. The aim of our research is to find the best etching conditions and surface morphologies of Mg₂Si after chemical etching. As possible etchants, both alkaline and acid liquids were examined for several crystalline conditions of the Mg₂Si samples, which were prepared by the plasma activated sintering method and the all-molten growth method. To find the best etching conditions we varied the type of etchant, etching time, temperature and concentration focusing primarily on the etch rate. In consequence, a practicable alkaline etchant was selected for Mg₂Si. In this report, we describe the variations in the conditions of the etched surfaces as a result of varying the type of etchant and the etch conditions. The surface morphology was analyzed using a conventional Scanning Electron Microscope (SEM), high-resolution filed-emission SEM/Energy Dispersive X-ray (EDX) Spectroscopy, and an Atomic Force Microscope (AFM).

Two-dimensional crystals such as graphene and MoS₂ have attracted for future nano-electronic and optoelectronic devices. Among them, a phosphorene, which is one monolayer of layered black phosphorus, has attracted much attention because it is a p-type 2D material as the counterpart of the n-type MoS₂. Phosphorene has high carrier mobility and a direct band gap of 0.3 eV for bulk and 1.1 eV for single layer. Due to tunable electronic transport with layer thickness, phosphorene is expected as a potential p-type 2D material for application to nano-devices. Recently, phosphorene has been reported as a promising candidate for thermoelectric applications by a theoretical prediction.
In this study, we have investigated the thermoelectric properties (Seebeck coefficient, electrical and thermal conductivities) of phosphorene using the micro-fabricated thermoelectric measurement platform. All the parameters were characterized in the temperature range of 50 to 500 K. At 300 K, Seebeck coefficient and electrical conductivity were measured to be 485 mu;V/K and 1800 S/m, respectively. And the thermal conductivity was measured to be 8.3 W/m#8901;K at 300 K under the steady-state heat flow based on Fourier law. The thermoelectric enhancement of phosphorene is discussed in view of size effect, i.e., diffusive thermopower, suppression of the lattice thermal conductivity caused by surface scattering, and surface transport. Finally, it is demonstrated to fabricate a thermoelectric generator based on 2D materials.

Design and fabrication of micro/nanosize hollow silicate particles (MNHSPs) with varied sizes and morphologies are of great interest especially in developing an optical and new generation of thermal insulation materials (TIM) for buildings. The key point is to develop a simple / faster with eco-friendly route with sustainable / stable hollow silicate particles of different sizes (micro and nano) and geometry (shell thickness) as well as to characterize their physico-chemical with thermal physical properties with an aim of applying them for optical and thermal energy efficiency applications. This is essential since buildings constitute a substantial part in reducing the total global energy consumption. Hence, developing a simple-environmentally friendly process for MNHSPs in TIM applications is the goal.

In recent years, the significance of magnesium silicide (Mg₂Si) as a thermoelectric (TE) material has come to the fore. Among the reasons for this are its power generation capability, its abundance, its non-toxicity, and the fact that it is lightweight. These features are helping to broaden the application of Mg₂Si in the field of thermoelectric devices and are encouraging the development of systems for waste heat recovery. An important aspect of Mg₂Si is its capability for being doped in order to modify its electrical conductivity, thermal conductivity and durability at elevated operating temperatures. We have adopted an all-molten synthesis process to fabricate polycrystalline Mg₂Si, allowing thermally-stable doping control to be easily done in terms of the type of dopant, site substitution and concentration. To manufacture a thermoelectric power-generation tip, a Plasma Activated Sintering method was carried out using pulverized polycrystalline Mg₂Si with a powder size of 25~75mu;m.
As possible n-type impurities in Mg₂Si, we have attempted to incorporate the main group elements Bi and Sb (Group 15), and Al (Group 13) as donor impurities by substituting them for Si and Mg, respectively. As a result, Bi and Sb contributed to a reduction in thermal conductivity, and Al an increase in power factor; however, the thermal stability of Bi and Al in Mg₂SI was insufficient in the power generation temperature range. Sb produced some interesting thermoelectric doping characteristics, while the process reproducibility, especially for sintering, was poor for production. Thus, we looked for elements possessing the same stable oxidation states as Bi and Sb (+3 and +5) to substitute for Si. We used the transition metal elements Co, Nb, Nd, Sm, and Ta. However, the results obtained by doping with these transition metals were comparable to or poorer than the thermoelectric properties for those doped with the Sb.
In previous studies, when we used Mg alloy, which contains Al and Zn as constituent elements, as the source of Mg for synthesizing Mg2Si, the Zn component altered the wettability of the molten Mg2Si. Although Zn is expected to be isoelectric in Mg2Si, it had the effect of reducing the thermal conductivity, and improving the sintering ability and thermal durability. Then we examined the effects of co-doping with Sb, Al, and Zn. We investigated samples of Mg₂Si doped with Sb+Zn, Sb+Al, Al+Zn and Sb+Al+Zn. Currently, the Sb+Zn doped samples show the best thermoelectric performance in our research. We will report, in detail, on the thermoelectric behavior of these n-type impurities in Mg2Si, and present the most promising n-type doping conditions for Mg₂Si

Thermoelectric (TE) technologies which produce the electric power from waste heat, have been expected as one of the potential candidates for the next generation energy sources. Mg₂Si (Magnesium silicide) has been expected as the material used for this technology due to lightweight, abundance of its constituent elements and its non-toxicity.
Because thermoelectric conversion devices are subjected to various thermo-mechanical loadings such as mechanical vibrations and thermal stresses during operation, evaluation of the mechanical properties of Mg₂Si is necessary for designing reliable TE modules. Furthermore, Mg₂Si is used at 873 K in the view of thermoelectric properties, however, the effect of heat exposure at 873 K on the mechanical properties of Mg₂Si has not been sufficiently investigated. In the present study, therefore, mechanical properties of Mg₂Si after heat exposure at 873 K were evaluated. The effect of holding time on the strength was also investigated.
The specimen with the dimension of 3 × 4 × 38 mm was cut from as-sintered specimens with the diameter of ~50 mm and the thinness of ~12 mm. The specimens were polished and heated in Ar atmosphere at 873 K for 100 h, 250 h, 500 h, respectively. The materials were characterized by a optical microscope (OM), and X-ray diffraction (XRD).
Four-point bending test was carried out on as-sintered and heat-exposed specimens to obtain bending strength at room temperature. After the tests, fracture surface was observed using a scanning electron microscope (SEM). Stress-strain curves showed linear elastic behavior and specimen was fractured at the maximum stress. This behavior was independent of the holding time of heat exposure.
Bending strength of as-sintered specimen was 50 MPa on average. That of heated for 100 h is almost equal to as-sintered specimen, however, it increased after the heat exposure for 250 h. Based on the experimental results, the effect of microstructure on mechanical properties of Mg₂Si will be discussed.

ZnO is a very promising semiconductor material in the fields of light emitting diodes, photodetectors, and solar cells due to its large band gap and high exciton binding energy. Moreover, ZnO-based materials (Al-doped or Al and Ga co-doped ZnO) have attracted much attention as an n-type thermoelectric material. Al-doped ZnO has been known to show a high electrical conductivity and low thermal conductivity compared with those of undoped ZnO. However, ZnO doped with Al₂O₃ at more than 1 at.% usually contains ZnAl₂O₄ as impurity phase because of the high reactivity between ZnO and Al₂O₃ at high temperature. In this study, in order to achieve good thermoelectric properties with improved thermal stability, we tried to introduce metallic Ag particles in the ZnO matrix. If the size of the Ag particles is of the order of several nm, it is expected to increase the electrical conductivity owing to metallic conduction in Ag and to decrease the thermal conductivity by the interfacial phonon scattering. Moreover, Ag is stable at high temperature (the melting point of metallic Ag is 1235 K).
First, we synthesized Zn_1-xAg_xO nanocomposites by using soft-chemical precursors. The Zn_1-xAg_xO (x = 0, 0.01, 0.02) precursors were prepared by using solution method. Zn(NO₃)₂middot;6H₂O, AgNO₃ ([Zn]+[Ag] = 60 mmol), and 480 mmol of urea were dissolved in 150 mL of deionized water. The precursors were obtained from the solution after heated at 90 #730;C for 6.5 h. The Zn_1-xAg_xO nanocomposites were synthesized by calcining the precursors at 300 #730;C for 3 h in air. Ag/ZnO sintered bodies were obtained by sintering the pressed pellet of the Zn_1-xAg_xO nanocomposites at 1100 #730;C for 5 h in air. The heating and cooling rates were 200 #730;C/h.
The crystal phase of all the calcined nanocomposites was confirmed by XRD as the hexagonal phase of ZnO with broader peaks compared with pristine sintered samples. In the case of Zn_0.98Ag_0.02O, no peak shift was observed for the ZnO phase, and additional peaks assigned to metallic Ag appeared. The XRD peaks of the ZnO phase in the sintered samples became much sharper than those in the calcined ones, and no byproducts were observed except metallic Ag.
The electrical conductivity of the sintered samples increased with increasing temperature, being 0.8 (x = 0) and 0.6 (x = 0.02) S/cm at 1273 K. The Seebeck coefficient was -400 #822; -600 mu;V/K. The thermal diffusivity and thermal conductivity virtually unchanged regardless of the Ag concentration within the range we have examined. Consequently, the thermoelectric properties of Ag/ZnO sintered bodies (x le; 0.02) were almost similar to those of pure ZnO. These results imply that the Ag concentration is still too low to engineer the thermoelectric properties. Further investigation of the thermal and electrical properties of ZnO with higher Ag concentrations (x > 0.02), and observation of the microstructure of the Ag/ZnO sintered bodies are currently underway.

The conversion of heat to electricity through thermoelectric devices may play a key role in the future for energy harvesting. In order to reach that goal, more efficient thermoelectric materials are needed. For this purpose, type-I thermoelectric clathrates have been intensively investigated in the last decade. A important development in this area has been achieved with the successful introduction of trivalent rare-earth elements into the cages of this class of materials.¹ We carried out an investigation, using first-principles calculations, of the structural, electronic, vibrational, and thermoelectric properties of the hypothetical charge-balanced compound La₂Ga₆Ge₄₀. Our main interest is to evaluate the direct effects of introducing La³⁺ guest ions into the Ge₄₆ structure, without the extra complication of excessive Ga-Ga bonds. Our calculations indicate that partial introduction of La³⁺ inside the Ge₄₆ dodecahedra (2a Wyckoff positions) and Ge₄₆ tetrakadecahedra (6d Wycoff positions) , with Ga substitution for Zintl charge balance, can lead to metastable structures. The compounds with lowest formation energies are those with the La³⁺ guest ions at the 2a positions. The results for these more stable structures show that electronic states near the top of the valence band and near the bottom of the conduction band in these materials are very sensitive to the Ga positioning among the three inequivalent cage sites. Furthermore, our results indicate that the charge distribution disorder induced by the Ga atoms is capable of promoting an off-center displacement of the La³⁺ ions placed in the 2a site. The Ga atoms distribution also affects significantly the vibrational properties, and consequently, the lattice thermal conductivity and the figure of merit.
1. A. Prokofiev, A. Sidorenko, K. Hradil, M. Ikeda, R. Svagera, M. Waas, H. Winkler, K. Neumaier and S. Paschen, Nature Mater., 12, 1096 (2013).

Recent thermoelectric measurements on two-dimensional transition metal dichalcogenide thin films (MoS₂) reveal a low thermal conductivity and high Seebeck coefficient, however, few results are reported for similar bulk samples. Since nanostructuring has proven to be an effective approach in reducing lattice thermal conductivity and improving efficiency of thermoelectric materials in bulk materials, we are investigating molybdenum sulphoselenide (MoS_xSe_2-x, x = 0, 1, 2) nanocomposite samples fabricated by a ball milling and hot press method. Temperature-dependent thermoelectric transport properties as a function of S doping, grain size and hot pressing temperature will be discussed.

Thermoelectric materials for renewing waste-heat into useful electrical energy have been investigated widely to enhance its properties which are expressed as dimensionless figure of merit, zT. Materials should have high value of Seebeck coefficient and electrical conductivity and low thermal conductivity to attain high zT value. Nanostructuring materials into superlattices, nanowires and nanodots would enhance zT value mainly by quantum-confinement effects and phonon scattering at interfaces. In this work, we investigated quantum confinement effect of PbSe nanoribbons with different widths fabricated and aligned by photolithography which is well-known as LPNE(lithographically patterned nanowire electrodeposition) method. Nanoribbons fabricated in this way do not need to be dispersed and aligned for measurements. Electrodeposition process was conducted from aqueous solutions containing SeO₂ and lead complex with EDTA (ethylenediaminetetraacetic acid) with the control of concentration for stoichiometric composition. Cyclic voltammetry was used for finding the appropriate deposition potential range. The widths of nanoribbons were controlled by adjusting the electrodeposition time. The reason why PbSe was utilized here is it has relatively large bohr radius which makes investigating quantum confinement effect easier. Through correlating measured Seebeck coefficient and the width of PbSe nanoribbons, we could investigate quantum confinement effects of PbSe. Moreover, by utilizing the field-effect transistor (FET) patterned with nanoribbons, we could also manipulate chemical potentials or carrier concentrations of PbSe nanoribbons by changing the gate voltage of field-effect transistor. The deposited ribbons were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. Detailed results not only quantum confinement effects but also the composition, electrical, and thermoelectric properties correlated with size and chemical potential would be presented.

Thermoelectric (TE) power generation in the range of mid to high temperature is the focus of considerable attention because of the potential for environmentally benign and cost-effective conversion of waste heat to electricity. A single crystalline form of p-type SnSe has recently shown an exceptionally high ZT of ~2.6 at 923 K along the b-axis due to a highly anharmonic bonding. Since then n-type ones as well as p-type SnSe materials have been quite actively investigated for practical applications. It has been also reported that the remarkable ZT in n-type SnSe could be expected based on first-principles calculation. However, it is quite difficult to make n-type SnSe because the formation of Sn vacancy, acting as electron acceptors, in SnSe is thermodynamically stable. Thus, we investigated the effect of doping elements and doping amount for n-type SnSe materials. In this study, we present detailed investigations of electrical and thermal transport measurements as well as structural data on n-type SnSe based materials. The n-type SnSe based materials is successfully realized and its origin will be discussed.

Much of our understanding of thermal properties of hard condensed matter relies on the assumption of harmonic phonon eigenstates and quasiparticles. A microscopic understanding of thermal transport is of broad interest for the design of efficient energy materials, and requires going beyond harmonic phonon dispersions, to rationalize phonon scattering mechanisms that lead to finite phonon mean-free-paths. Neutron and X-ray scattering experiments can map excitations throughout reciprocal space efficiently, and provide critical insights into phonon scattering mechanisms, including anharmonicity, electron-phonon coupling, and scattering by defects or nanostructures. Such microscopic information about phonon mean-free-paths is useful to design more efficient thermoelectric materials, for example. In addition, first-principles simulations of the dynamical structure factor, including anharmonic effects, enable the rationalization of complex experimental datasets. I will present results from our recent investigations of phonons and thermal transport in several important thermoelectric materials [1-3], focusing on chalcogenide systems, in particular on the peculiar lattice dynamics of the strongly anharmonic and strongly anisotropic compound SnSe.
[1] O. Delaire, J. Ma, et al., Nature Materials 10, 614 (2011).
[2] J. Ma^*, O. Delaire^*, A. F. May, et al., Nature Nanotechnology 8, 445 (2013).
[3] C.W. Li, O. Hellman, J. Ma, A.F. May, H.B. Cao, X. Chen, A.D. Christianson, G. Ehlers, D.J. Singh, B.C. Sales, and O. Delaire, Physical Review Letters (2014).
Funding from US DOE BES MSED, Office of Science Early Career Award, and as part of the S3TEC EFRC.

Thermoelectric energy conversion systems, capable of generating electricity from waste heat sources, play an important role in energy efficiency and thermal management applications. The efficiency of thermoelectric materials and devices is evaluated by the dimensionless figure of merit (zT), defined as zT=(S²s/k)T where S, s, k and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Achieving high zT in thermoelectric materials has been problematic because it requires a combination of high power factor (S²s) and low thermal conductivity, a difficult task as all these properties are interdependent and often follow unfavorably opposing trends. One promising approach to overcome this dilemma involves synthesis of materials with complex crystal structures, low lattice thermal conductivities, and high electronic conductivities. This has led to an increased computational effort to design highly efficient materials from first principles. However, the main challenge to synthesizing these designer thermoelectrics lies in developing techniques capable of producing large quantities of materials that meet the practical requirements for a thermoelectric device, particularly, the need for scalability at relatively low-cost, high zT, and thermal stability at elevated temperatures.
In this study, we show that self-assembly through solvothermal/hydrothermal chemistry provides both excellent control over morphology making it possible to reduce thermal conductivity via grain-boundary scattering, and excellent control over impurity doping allowing incorporation of a wide range of the periodic table. As a proof-of-concept, we will discuss the synthesis of (Pb_(1-x)Sm_xS)_(1-y) (Ag₂S)_y complex thermoelectric structures and demonstrate that this technique allows doping of Sm and Ag in the host PbS structure while preserving the control over morphology and crystallite size. Additionally, due to the relatively large Bohr radius of PbS, quantum confinement effects become observable at room temperature by using soft and hard templates or surfactant structure directing agents. We emphasize that the self-assembly method can be applied to different class of materials and future studies can lead to promising new thermoelectric materials.

Doping induced band convergence has been demonstrated as an efficient way of enhancing the performance of thermoelectrics. However, this effect is usually composition dependent, namely, better band convergence requires higher doping fraction. We demonstrate here a wide range of solid solutions between SnTe and MnTe with a maximum figure of merit ZT approaching ~1.3 at 900 K. The heavy alloying of MnTe significantly increases the Seebeck coefficient of SnTe to a greatest value of ~230 mu;V/K by effectively pushing the light- and heavy-hole bands closer in energy. Meanwhile, the phonon transport of SnTe is considerably inhibited due to the strong point defect scattering by Mn substitution for Sn within the solid solution along with the considerable interfacial scattering by MnTe nanoprecipitates when Mn exceeds its solubility limit of ~9 mol.%, leading to a lattice thermal conductivity as low as 0.9 Wm^-1K^-1. The high performance achieved in Mn-doped SnTe together with its non-toxic constituent elements makes it a good candidate for high temperature thermoelectric power generation.

The idea of exploiting critical phenomena to enhance thermoelectric performance has been put forward based on observations of the superionic phase transition of Cu_2-xSe [1-2]. Thermopower increases rapidly around the phase transition, contributing to the increase in the figure-of-merit (zT). This increase has been associated with critical fluctuations [1-3] by assuming a continuous phase transition. We show that, however, the phase transition should be discontinuous by considering the symmetry relations. The apparent critical phenomena observed near the transition are well explained without assuming continuity of the transition. We will discuss how the enhancement in zT should be interpreted in terms of future thermoelectric research.
[1] D. R. Brown et al., Phase transition enhanced thermoelectric figure-of-merit in copper chalcogenides. APL Materials,1 (5), 052107 (2013).
[2] H. Liu et al., Ultrahigh thermoelectric performance by electron and phonon critical scattering in Cu₂Se_1-xI_x. Advanced Materials, 25 (45), 6607 (2013).
[3] G. D. Mahan, The Seebeck coefficient of superionic conductors. Journal of Applied Physics 117 (4), 045101 (2015).

The superionic conductor Cu_2-δSe has been shown to be a promising thermoelectric at higher temperatures, which exhibits a maximum zT of 1.6 at 1000 K, making it a competitive and cheap alternative for PbTe, the leading thermoelectric material in the hundreds of kelvin temperature range. Due to a small, intrinsic copper deficiency δ, copper selenide (Cu_2-δSe) is a p-type semiconductor with a band gap of 1.23 eV. Above 410 K, copper selenide transforms to a Cu⁺ conducting phase, wherein the copper ions are mobile and described as “liquid like” because their diffusion coefficient is about 10^-5 cm²s^-1, comparable to the value for water molecules in liquid water. This superionic state results in extremely low lattice thermal conductivity values as low as 0.4 W(Km)^-1 in the superionic phase and in turn a high figure of merit. Starting with the superionic Cu_2-δSe, many other superionic copper and silver selenides have been found to be interesting thermoelectrics; for instance the argyrodites Cu₇PSe₆, Ag₂Se, CuAgSe, and quaternary copper chalcogenides Cu₂MM&’Q₄ (M=Zn, Fe,..; M&’ = Zn, Ge, Q = S, Se, Te) which also exhibit interstitial copper ions and even show band convergence. While copper selenide has potential as a thermoelectric material at high temperatures, its ionic conductivity has caused long-term stability problems for use in thermoelectric generators. This spurs interest in the properties of copper selenide below the superionic phase transition at 410 K, where the ion migration would not be of concern.
In this work we present the potential of copper selenide to achieve a high figure of merit at room temperature, if the intrinsically high hole carrier concentration can be reduced. Using bromine as a dopant we show that reducing the charge carrier concentration in Cu_2-δSe is in fact possible. Furthermore we provide profound insight in the complex defect chemistry of bromine doped Cu_2-δSe via various analytical methods and investigate the consequential influences on the thermoelectric transport properties. Here we show, for the first time, the effect of copper vacancy formation as compensating defects when moving the Fermi level closer to the valence band edge. These compensating defects provide an explanation for the often seen doping inefficiencies in thermoelectrics via defect chemistry and guiding further progress in the development of new thermoelectric materials.

Agyrodite Ag₈GeTe₆ belongs to a short but growing list of mixed conductors in which promising thermoelectric performance and super-ionic conduction coexist. In this talk we will focus on the lattice dynamics of Ag₈GeTe₆ between 2 K and 400 K. The results of X-ray diffraction, neutron diffraction, heat capacity, and thermal conductivity measurements corroborated strong anharmoncity, lattice instabilities, and unusual phonon density of states. These results in their totality help in the understanding of (i) the very low and amorphous-like lattice thermal conductivity; and (ii) how the material evolves through 4 consecutive phase transitions within a narrow span of 100 K to set the stage for the coexistence of super-ionic conduction and promising thermoelectric performance at elevated temperatures.

FeS₂, also known as iron pyrite or “fool&’s gold” has been widely investigated because of its interesting characteristics that made it suitable for various energy applications, from solar cells, to batteries. Regarding solar cell applications, its unique properties are a high adsorption coefficient allowing it to absorb most of the visible light at thicknesses lower than 100 nm, and its price as it is the most common sulfide mineral. Despite these advantages, making viable solar cells based on iron pyrite proves to be a daunting task. Many defects arise during and after the synthesis and cause the low performances of the cells. One of the defects results in the metal like conductivity of the FeS₂ films. The idea was to use this defect and investigate the thermoelectric properties of the FeS₂ nanoparticles. The nanoparticles were synthesized by rapidly injecting in a flask containing iron (II) chloride dissolved in hexadecylamine, elemental sulfur dissolved in diphenylether. The native hexadecylamine ligands were replaced by (NH₄)₂S ligands to improve the electrical properties of the material. The nanoparticles were dried in vacuum, and crushed to make a powder. The powder was then pressed to make a pellets. The thermoelectric properties of the pellets were measured. The seebeck of the FeS₂ showed a p type conductivity but its electrical conductivity was low resulting in a figure of merit in the order of 10^-3. To increase the electrical conductivity, FeS₂ was doped with cobalt. The nucleation temperature of CoS₂ is lower than the nucleation temperature of FeS₂, making it difficult to synthesize Fe_1-xCo_xS₂ using hot-injection method. The doping was therefore done in two parts. First a nucleation process involving only Fe and sulfur precursors, followed by a growth process at a temperature lower than the nucleation temperature of CoS₂, where Co, Fe, S precursors were introduced in the flask. The Fe_1-xCo_xS₂ nanoparticles with different concentrations of Co (5, 10, 15 and 20 percent) were characterized using XRD and SEM-EDX. The cobalt doped FeS₂ nanoparticles were also dried, crushed and pressed to make pellets and their thermoelectric properties were measured. As expected the conductivity of the nanoparticles switched from p-type to n-type after introduction of cobalt. The electrical conductivity gains two orders of magnitude, while the thermal conductivity just slightly increased. The figure of merit gained two order of magnitude. More work needs to be done to further improve the figure of merit but it is the highest reported figure of merit from FeS₂ nanoparticles reported so far to the best of our knowledge.

Heusler (1-2-1) and half-Heusler (1-1-1) alloys have been shown to provide a promising base for well performing thermoelectric materials and a rather robust characteristics for applications as thermoelectric generator. While the figure of merit ZT of half-Heusler alloys reached already values well above one, the overall performance of the full Heusler alloys is still lacking such values mostly because of the very large thermal conductivity found in this family of compounds.
The Heusler system Fe₂VAl is a Zintl phase, i.e., the Fermi energy E_F is placed right at a narrow gap of the electronic density of states and thus adding or removing electrons shifts the system either towards electron- or hole dominated transport [1]. Moreover, the power factor a = 5.4 mW/mK² [2] near room temperature is comparable to that of Bi₂Te₃-based systems and the Seebeck effect is largest below 100°C. This scenario could be capable to finally exchange expensive Bi₂Te₃-based systems by such full Heusler materials with a melting temperature around 1500°C.
In this contribution we show how doping and substitutions on the various lattice sites of the Heusler structure modifies the electronic transport in the system and, accordingly, how thermal conductivity decreases due to scattering of the heat carrying phonons on point defects. Additionally, we demonstrate how a substantial reduction of grain sizes by ball milling and severe plastic deformation positively influences thermal conductivity and thus improves the thermoelectric figure of merit.
Work supported by the Christian Doppler Laboratory for Thermoelectricity.
References:
[1] T. Tsuji, R.O. Suzuki, K. Ono, J. Jpn. Inst. Metals 63, 1435-1442 (1999).
[2] H. Kato, M. Kato, Y. Nishino, U. Mizutani and S. Asano: J. Jpn. Inst. Metals 65, 652-656 (2001).

Thermoelectric materials have seen great improvement over the last two decades. The goal of many material optimizations is to independently tune the electrical property and the thermal transport. One commonly-used approach is to create nanocomposite, in which nano-sized grain boundaries and precipitates strongly scatter phonons but have less effect on electrons. The ability of tuning phonon transport without affecting electrons relies on the fact that typical electron mean free paths are much smaller than the phonon mean free path. As a result, nanostructures with sizes in between can scatter phonons more frequently than intrinsic phonon scatterings, while the scatterings of electrons are still dominated by the intrinsic processes. The rational design of the nanostructures will require an advanced knowledge of the mean free path distribution of both electrons and phonons. Recently, the phonon mean free paths were calculated using the first principles approach without any adjustable parameters. This provides a detailed and accurate description of how much each phonon mode contributes to the thermal transport. Obtaining the same information for electrons, however, has so far mainly relied on approximations to the electron scattering processes, such as the constant relaxation time model or the deformational potential approximation. First principles calculations of electronic transport properties were recently done in silicon and reasonable agreements with experiments were shown. The developed approach, however, does not properly account for the long ranged forces induced by the polar optical phonon and therefore is less accurate for polar materials. In this work, we study the electronic transport properties in a practical thermoelectric material - ZrNiSn, taking into account the long ranged forces induced by the polar optical phonon. We expect that the mean free path distribution obtained for both electron and phonon transport can provide the guidance on how this material and similar half-Heusler materials can be optimized to maximally reduce the thermal conductivity while keeping the electronic properties, therefore enhancing the thermoelectric efficiency.

We present an advanced thermal battery for electric vehicles that can deliver efficient and compact heating and cooling to the passenger cabin with minimal drain to the electric battery. To realize such a concept, we used an adsorption-based approach that relies on the development of advanced nanomaterials and a tailored system design to maximize heat and mass transport and accordingly achieve high thermal power and energy densities. In this work, we describe our development of a high performance thermal battery. First, we modifed microporous zeolites to maximize the adsorption capacity with water, and to consequently increase the thermal energy density. We tailored the micropores and surface chemistry of the NaY zeolites using ion-exchange. Meanwhile, we developed three-dimensional graphene tubes with void channels embedded with the zeolites to enhance both the thermal conductivity and vapor transport, and hence, increase the thermal power density without sacrificing the overall adsorption capacity. These thermal binders were characterized, where there was ~400% improvement in thermal conductivity and a ~30% in vapor transport rate with the composite compared to zeolites alone. We also demonstrated a power density of 900 Wh/kg and an energy density of 330 Wh/kg with the composites in a custom experimental setup. Finally, we show the promising performance with such materials in a proof-of-concept adsorption bed design. Our results provide useful insights and opportunities for the development of thermo-adsorptive batteries for electric vehicles as well for other applications that require both high power and energy density thermal energy storage.

Water adsorption became more important for many engineering applications including thermal energy storage, desalination, and water harvesting. To develop such applications, it is essential to understand interactions and energy required for adsorption/desorption processes of porous material-adsorbate systems. In this work, we present thermodynamic model and experimental technique using conventional DSC and TGA systems to characterize the enthalpy of adsorption/desorption of zeolites and metal-organic frameworks (MOFs) with water as an adsorbate. With the proposed method, we present experimental characterizations of adsorption enthalpies of recently developed MgY zeolite and MOF-801 as a function of both uptake and temperature. Our study shows that type I zeolites have adsorption enthalpies 2 to 3 times higher than the latent heat in an initial uptake region, while adsorption enthalpy for MOF-801 was nearly constant in wide uptake range.

Salt hydrates are of interest as thermal energy storage (TES) materials due to their large specific and volumetric enthalpy of fusion at melting temperatures appropriate both for capture and reutilization of low-grade waste heat as well as for many thermal management applications. Despite this, many salt hydrates suffer from considerable undercooling, which has limited practical application. Promoting heterogeneous nucleation in liquid to solid phase transformations decreases undercooling and can dramatically improve isothermal as well as constant-cooling rate solidification kinetics. However, the design of material-specific nucleation catalysts remains non-trivial, especially for phases with complex structures; historically, very few nucleation catalysts have been identified for PCMs which both significantly reduce undercooling and remain stable over a large number of melt-freeze cycles.
Here, we investigate heterogeneous nucleation in the technologically important TES material lithium nitrate trihydrate (LNH), and demonstrate a close correlation of undercooling with lattice mismatch between planes of closely packed coordination polyhedra in a number of potential nucleation catalysts. This result supports extending the planar matching model for nucleation catalyst design to more complex structures by focusing on lattice matching of planes containing closely packed coordination polyhedra. In particular, copper hydroxyl nitrate hydrate (CHNH) has a very small lattice mismatch for the orientation (010)_LNH||(100)_CHNH and [100]_LNH||[010]_CHNH, and decreases undercooling by up to 66% over previously known nucleation catalyst phases. CHNH has a layered structure which delaminates along (100)_CHNH, maximizing the number of potential nucleation sites for LNH along this lattice-matched plane and potentially contributing to the nucleation catalyst activity of this phase. Mixtures of LNH/CHNH are demonstrated to be quite stable despite large numbers of cycles (N > 900 cycles), and aging at elevated temperature for extended periods of time (t > 250 days).

Organic/inorganic hybrid halide perovskites, formed by substituting the cations A of ABX$_3$ halide perovskites with certain organic cations, may be used for solar thermoelectric applications. In this work, we systematically study three lead-free hybrid perovskites, i.e., methylammonium tin iodide CH$_3$NH$_3$SnI$_3$, ammonium tin iodide NH$_4$SnI$_3$, and formamidnium tin iodide HC(NH$_2$)$_2$SnI$_3$, by first-principles calculations. We find that in addition to the commonly known motif in which the corner-sharing SnI$_6$ octahedra form a three-dimensional network, these materials may also favor a new motif formed by alternating layers of the SnI$_6$ octahedra and the organic cations. These two motifs are nearly equal in free energy and are separated by low barriers. Interestingly, the layered structures feature many flat electronic bands near the band edges, potentially improving the thermoelectric performances. Calculations within a semiclassical model indicate that the layered structures may indeed feature high thermoelectric figure of merit $zT$, thereby suggesting further investigations on this promising structural motif of the hybrid halide perovskites.

Thermoelectric materials allow direct conversion of heat into electricity and vice versa, offering us a variety of technologically attractive applications including power generators, temperature sensors, and temperature regulators. However, due to the speed limitation of conventional electrical methods, it still has been a great challenge to manipulate and trace the thermoelectric transients at high speeds, thus restricting their use in rather slow devices. Elucidating the ultrafast dynamics in thermoelectric transport is important not only in terms of basic physics point of view but also in applied physics point of view, which may take our thermal management performance to the next level.
Here, using an optical method based on femtosecond laser pulses, we reveal that thermoelectric conversion essentially occurs at high speeds in the picosecond time scale. The method we employed here is terahertz emission spectroscopy. This method allows us to elucidate the ultrafast charge dynamics in solids by identifying the underlying mechanism of terahertz emission introduced by ultrashort laser pulses. In this study, we have employed this technique on layered cobaltite Ca_xCoO₂ thin films and examined the ultrafast carrier dynamics involved with thermoelectric transport. The terahertz emission properties from the Ca_xCoO₂ films were highly unique, which cannot be explained by the well-established mechanisms based on photoconduction and nonlinear optical processes. Instead, we demonstrate that terahertz emission from the Ca_xCoO₂ films is thermoelectric in origin, and further, discuss that the terahertz emission property can fully be understood by considering the unique tensorial nature of the Seebeck coefficient, which is offered by the special incline-orientation of our film. Correlating the picosecond-electrical transient with the tensorial thermoelectric transport gives direct evidence of the Seebeck effect arising in the picosecond time scale. Moreover, revealing such feature in this standard thermoelectric material highlights the generality of the ultrafast Seebeck effect. We also discuss that the tensorial thermoelectric effect offers much larger photo-thermoelectric response than the ordinary thermoelectric effect where the Seebeck coefficient is generally treated as a scalar. The results not only provide new insights into the ultrafast dynamics in solids but also have potential applications in various new ultrafast optoelectronic devices including bias-free terahertz sources and high-speed bolometers based on thermoelectric conversion.

Thermoelectric (TE) generators can directly convert heat energy into electrical energy. To achieve high efficiency, TE materials are required to have high temperature thermal and chemical stability, high mechanical strength, and enhanced electrical properties. Oxide TE materials, particularly thin film ZnO satisfy these requirements and have been considered as promising high temperature TE material compare to conventional semiconductor alloys, such as Bi₂Te₃, PbTe, and SiGe. In this paper, we studied the defect structure of ZnO thin films on its room temperature TE properties, grown by metal organic vapor deposition (MOCVD). The structure and overall surface morphology of the samples were characterized by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, the optical, electrical, and thermal properties of the samples were examined by photoluminescence, van der Pauw hall-effect, and thermal gradient methods, respectively. The results will provide new insights on how point defect and grain boundary of thin film ZnO affect on their TE properties.

Novel high performance thermoelectric (TE) materials provide a promising way for environmentally friendly, efficient power generation, as well as other practical applications such as waste-heat capturing or solid-state refrigeration. In this work, a theoretical and experimental screening was carried out to discover promising TE materials in the quaternary (Mn,Sn)(Se,Te) phase space, with SnTe, MnTe, SnSe and MnSe as corner points. SnTe is one of the frequently studied TE materials, but so far its performance suffers from its metallic band structure. Prior experimental publications have also demonstrated promising results for MnTe [1], and in particular, SnSe crystals [2]. Further improvements of the TE properties in these Sn- and Mn-based binary chalcogenides should be achievable by isoelectronic (Mn,Sn)(Se,Te) alloying, to decrease the thermal conductivity through point defect scattering, and also to tune the electronic structure between semiconducting and metallic. The large number of possible compositions, as well as the size of the associated parameter space, could make the (Mn,Sn)(Se,Te) alloy investigation extremely time intensive. To address this challenge, a high-throughput combinatorial synthesis and characterization was used for an accelerated investigation of solubility limits, electrical conductivity, and thermoelectric transport properties of (Mn,Sn)(Se,Te) alloys.
Thin-film combinatorial libraries have been deposited utilizing composition and temperature gradients via RF co-sputtering from ceramic MnSe, MnTe, SnSe and SnTe targets. The samples were analyzed via spatially-resolved XRF and XRD with respect to their composition and crystal structure. The thermoelectric-related properties have been investigated using 4-point-probe conductivity and Seebeck coefficient measurements. These experimental results were compared with the insights from the first-principles alloy calculations in this materials space. Depending on the precursor combinations and deposition conditions, changes in crystal structure and TE properties were observed. For the SnSe/MnTe composition spread, phase separation into cubic SnTe and cubic MnSe was observed whereas for the SnTe/MnSe composition spread, the solubility limit has been determined to be up to 20% for MnSe into SnTe. Preliminary analysis of the electronic properties of this single-phase region showed no significant increase in resistivity with increasing alloying concentration (2.5x10^-4 to 5 x10^-4 Ohm-cm), which is an important prerequisite for further improvement of the TE properties of this material. The corresponding Seebeck coefficient measurements are in progress and will be reported in this presentation.
¹Xie, W. et al. Journal of Applied Physics, 115, 103707, 1-7, (2014)
²Zhao, L.-D. et al. Nature, 508(7496), 373-7, (2014)

We report a simple method for simultaneous fabrication of nitrogen doped reduced graphene oxide (rGO) and of 3-dimensional porous structure. As preparing polystyrene (PS) sphere terminated by amine group (amine-PS), which is called positive charged PS, we fabricated graphene oxide (GO)/positive charged PS composite via vacuum filtration, and the composite was annealed in Ar atmosphere, depending on the annealing temperature. In this process, the nitrogen doping on reduced GO and fabricating 3-dimensional porous structure were fabricated simultaneously. Generally, the electrical conductivity of porous structure was increased, depending on the amount of amine-PS and annealing temperature. And the electrical conductivity was increased slightly with increasing ambient temperature. In the case of Seebeck coefficient, the character of GO/amine-PS after annealing was changed into n-type, because of nitrogen doping. Moreover, the absolute value of Seebeck coefficient of that was increased, depending on the ambient temperature. Since polystyrene spheres were removed by thermal annealing, the porous structure was obtained, resulting in low thermal conductivity.

Thermionic emission from diamond is a promising route for making thermal energy convertors for use in solar power generation or energy harvesting devices. However, despite much recent progress, the production of high-current-density diamond thermionic emitters remains elusive. The most promising thermionic emission materials reported to date use nanocrystalline or even ultrananocrystalline diamond (UNCD) films, since it is believed that a high sp²/sp³-carbon ratio is essential in improving the electrical conductivity throughout the films. However, there is a trade off with the film thermal conductivity, a high value of which is necessary to transport heat from the heater at the base of the film to its emitting surface.
Electron emission can be further improved by using n-type diamond, doped with phosphorus or sulfur, or n-type UNCD. Lithium has been suggested as a possible alternative n-type dopant due to its potential as a shallow donor in diamond. However, experimentally doping with lithium is difficult to achieve due to the low solubility of Li in diamond, and its relatively high mobility which allows it to diffuse through the lattice at fairly low temperatures. It has been suggested that the unwanted Li diffusion can be prevented by adding substitutional nitrogen together with interstitial Li, with the nitrogen acting as a trap to pin down the Li in the diamond lattice while retaining its n-donor properties. Calculations suggest that the optimum ratio of Li-to-N is 1:4, with LiN₄ clusters acting as shallow donors. To study this, we investigated the incorporation of both lithium and nitrogen while growing microcrystalline diamond in a hot-filament chemical vapour deposition (HFCVD) system. Diamond thin films were grown using a methane/ammonia/hydrogen gas mixture with Li being added as solid LiN₃ and overgrown with diamond. Secondary ion mass spectrometry showed that high levels of Li and N were embedded in the diamond and were situated in close proximity to each other. The crystallinity and morphology of the diamond films were unchanged by the Li/N incorporation. The diamond surface was hydrogen terminated. Although the films remained relatively resistive, the Li was found to enhance the thermionic current density of nitrogen-doped diamond by a factor of two, making it a promising thermionic material. We shall report the results of these studies, plus those from varying the co-dopant species to Li+B, Mg+N and Mg+B in a set of films to investigate the effect upon thermionic emission.

Thermionic energy conversion can provide high efficiencies as the thermal barrier separating emitter and collector electrodes is realized through a vacuum gap. An electron current is released through thermionic emission from an emitter that is opposed by a collector with a lower work function. For an ideal converter a collector work function of 0.5eV could provide an efficiency greater than 50% at an emitter temperature of 1400K which would correspond to a thermoelectric ZT>10. We have demonstrated an ultra-low work function electrode from a phosphorus doped diamond epitaxial film that was prepared on a single crystal diamond substrate by plasma assisted chemical vapor deposition. The electrode was characterized with respect to the Richardson - Dushman formalism where a work function of 0.6eV was obtained by a fitting procedure. While the current in a conventional thermionic converter is governed by the law of thermionic electron emission an enhancement can be effected by utilizing surface ionization effects as described by the law of Saha - Langmuir. Here, a gaseous particle with a suitable electron affinity can become ionized through a tunnel process from occupied states in the emitter if their energy levels are properly aligned. Thus, in addition to a thermionic electron current an ion current can contribute to the total device current and enhance its power output. Nitrogen doped diamond with a work function in the range of 1.4 - 2.6 eV has been shown to generate an ionization current using atomic hydrogen and ammonia that can enhance its electronic counterpart by several orders of magnitude. In a thermionic converter a de-ionization process at the collector completes the molecular mediated charge transfer which was demonstrated for the ultra-low work function phosphorus doped diamond collector. We will discuss the potential of thermionic energy conversion utilizing ultra-low work function collectors as well as surface ionization over a wide temperature range.
This research is supported by the Office of Naval Research through grant # N00014-10-1-0540.

Thermogalvanic cells, also known as thermoelectrochemical cells or simply as thermocells, have recently attracted a lot of attention because of their immense potential in converting low-grade thermal energy to electricity.^1,2 The thermal temperature coefficient of electrode potential (α) in a thermogalvanic cell is directly proportional to the change in entropy (ΔS) of the redox reaction in the cell through the relation (α = part;E/part;T = ΔS/nF), where E is the cell potential, T the temperature, n the number of electrons involved in the reaction, and F the Faraday&’s constant. Although the nature of effects are different, α is often referred to as the Seebeck coefficient. Our recent work has showed that an annular copper-copper sulfate (Cu/Cu²⁺) thermogalvanic cell, which was affixed conformingly onto a hot pipe to simulate car&’s exhaust pipe, produced electricity in the order of tens of mW m^-2.³ While we already have considerable experience with the Cu/Cu²⁺ system,^2,3,4 our present work discusses applicability of a novel binder-free graphene foam (GF) as electrode materials for more conventional ferro/ferricyanide (Fe(CN)₆^4-/Fe(CN)₆^3-) system. The binder-free GF is synthesized by chemical vapor deposition (CVD) method from a porous metal catalyst template that is scalable towards large scale production.⁵ This results in an ultra-lightweight 3D network of monolithic free-standing few-layer graphene that is highly-interconnected, hierarchically-porous, and free of any additional binders. Carbon-based materials have been widely used as electrode materials in numerous energy storage and conversion systems due to their excellent conductivity and remarkable electrochemical activity.⁶ More recently, graphene-based materials, such as graphene powder and reduced graphene oxide, have been introduced as electrode materials for thermogalvanic cell. However, most of these reported materials contain many disordered boundaries and interfaces, leading to surge of electron scattering that ultimately increases internal resistance of the cell. In addition, the overall power density of these materials is considerably reduced by the extra weight of the binders needed to stabilize and interconnect these materials. In contrast, the GF introduced herein allows rapid electron and ion transport due to its seamless interconnected structures, superior electrical conductivity, and exceptional gravimetric surface area. In this talk, new progress on the applicability of our GF electrodes to further improve the power density of the Fe(CN)₆^4-/Fe(CN)₆^3- thermogalvanic cell for real-world applications will be discussed.
[1] T.I. Quickenden and Y. Mua, J. Electrochem. Soc. 142, 3985 (1995)
[2] A. Gunawan et al., Nanoscale Microscale Thermophys. Eng. 17, 304 (2013)
[3] A. Gunawan, Electrochem. Soc. Interface 23, 81 (2014)
[4] A. Gunawan et al., Int. J. Heat Mass Transf. 78, 423 (2014)
[5] K. Xi et al., Nanoscale 6, 5746 (2014)
[6] A.I. Aria and M. Gharib, Langmuir 30, 6780 (2014)

Temperature gradients are generated by the sun and a vast array of technologies and can induce molecular concentration gradients in solutions via thermodiffusion (Soret effect), For ions, this leads to a thermovoltage that is determined by the thermal gradient ΔT across, together with the ionic Seebeck coefficient α_i of the electrolyte. So far redox-free electrolytes have not been explored in thermoelectric applications due to a lack of strategy to harvest the energy from the Soret effect.
Here, we demonstrate the conversion of heat into stored charge, via the ionic Soret effect, in an ionic thermoelectric supercapacitor (ITESC), thus providing us with a new mean to harvest energy from an intermittentheat source. We show that the stored electrical energy of the ITESC is proportional to (ΔTα_i)² and that its α_i and efficiency reach beyond 10 mV/K and 0.6%, respectively. The resulting ITESC can convert and store several thousand times more energy as compared to a traditional thermoelectric generator connected in series with a supercapacitor.

Radioisotope thermal generators (RTGs) are devices which convert heat produced as a result of nuclear decay to useful electrical work. These devices are extensively used in deep-space missions where solar radiation is insufficiently intense and would require an unacceptably large solar array. RTGs typically employ solid-state thermoelectric devices to convert heat to electrical work, but these devices suffer from low thermal efficiency, around 8%. Increasing efficiency will decrease overall mass and reduce payload size.
A novel approach to radioisotope thermal generation based on thermoelectron (a.k.a. thermionic) emission from a wide-bandgap semiconductor material is presented. In a typical thermoelectron energy converter (TEC), an emitter electrode is held at a relatively high temperature and a collector electrode is held at a lower temperature. These electrodes are separated by a finite interelectrode distance and enclosed in an evacuated container. In the case of a thermoelectron RTG, a radioisotope source is held in the vicinity of the emitter, but not in direct thermal contact. Thus, energy is transferred via radiation (alpha or beta) from the radioisotope to the emitter but not via direct thermal conduction. Since the emitter is at a higher temperature than its environment, heat is transferred out of the emitter via three mechanisms: direct conduction through mechanical and electrical contacts, thermal (blackbody) photons, and thermoelectrons. Direct thermal conduction can be minimized, but at low temperatures thermal photons dominate thermoelectrons in terms of the heat transport from a metallic emitter.
In contrast, the low energy portion of the thermal photon spectrum from a semiconductor is truncated due to the bandgap. In this case, heat transport from a thermally isolated emitter electrode is dominated by thermoelectrons over thermal photons at low temperatures. Therefore, a wide bandgap material of sufficient thickness can capture and efficiently convert relatively high energy nuclear radiation (keV to MeV) to a thermoelectron current to perform work in an external electrical load.
A model of device performance will be presented in detail. This model suggests that that for a SiC emitter operating at 1000K, the ratio of the energy carried by thermoelectrons to the energy carried by thermal photons is on the order of 1e13. In contrast, the ratio for a metallic emitter with similar properties would be on the order of 1e-8. Thus, the wide bandgap emitter is a more effective converter of incident radioisotope energy due to the truncation of the thermal photon spectrum.

As a new class of potential mid-temperature thermoelectric materials, quaternary chalcogenides like Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) suffer from low electrical conductivity due to insufficient doping. In this work, Cu-doped CZTSe nanocrystals consists of polygon-like nanoparticles are synthesized with sufficient Cu doping contents. The hot-injection synthetic method, rather than the traditional one-pot method, in combination with the hot-pressing method is employed to produce the CZTSe nanocrystals. In Cu-doped CZTSe nanocrystals, the electrical conductivity is enhanced by substitution of Zn²⁺ with Cu⁺, which introduces additional holes as charge carriers. Meanwhile, the existence of boundaries between nanoparticles in as-synthesized CZTSe nanocrystals collectively results in intensive phonon-boundary scatterings, which remarkably reduce the lattice thermal conductivity. As a result, an average thermoelectric figure of merit of 0.70 is obtained at 450 °C, which is significantly larger than that of the state-of-the-art quaternary chalcogenides thermoelectric materials. The theoretical calculations from the Boltzmann transport equations and the modified effective medium approximation are in good agreement with the experimental data.

Lanthanum Telluride (La_3-xTe₄) synthesized by mechanical alloying and sintering has previously been demonstrated as a promising high temperature thermoelectric material with a figure of merit of approximately 1.1.[1] These materials have previously been reliably produced by mechanical alloying and sintering. We report synthesis of La_3-xTe₄ thin films by electrochemical deposition at low processing temperature and show that the stoichiometry of the deposited films can be controlled through the deposition parameters. Electrochemical deposition was achieved using ionic liquid electrolytes to extend the potential window and prevent oxidation of Lanthanum during deposition. Electrochemical deposition techniques provide a scalable method to produce conformal films at low processing temperatures. We also report the thermoelectric properties of the electrodeposited films. The reported electrochemical deposition technique may also be a viable processing technique for producing other telluride compounds.
1. A. May, J-P. Fleurial and G. J. Snyder, Phys. Rev. B, 78, 125205 (2008).

The efficiency of the thermoelectric materials and devices is measured using the figure of merit, ZT. The figure of merit, ZT, is calculated by multiplying the Seebeck coefficient, squared, by the electrical conductivity and absolute temperature in Kelvins, and dividing it all by the thermal conductivity. Thermoelectric devices were prepared using different multilayered thin film structures as SiO₂ / SiO₂ + Ge/ Ge/ Sb + Ge/ Si/ Si + Ge/Ge / Ge + Si. The prepared thermoelectric devices have been bombarded with 5 MeV Si ions at the different fluences (doses) to form quantum dots (nano dots) in the multilayers to improve the efficiency of the thermoelectric devices. The Seebeck coefficients of the prepared devices from the different multilayer thin films have been measured. After obtaining Seebeck measurements, van der Pauw-four probe resistivity, Hall Effect, density, mobility measurements have been performed. After the samples were bombarded, SEM/EDS data were collected. Our research is conducted to verify that proper fabrication of a nano-scale thermoelectric device will result in a low thermal conductivity measurement, high electrical conductivity measurement, and a high Seebeck coefficient measurement. The reduction in thermal conductivity causes an increase in the value of the thermoelectric figure of merit with yields a high efficient device. In addition to the Seebeck coefficients and electrical conductivity measurement, we do plan to complete the thermal conductivity measurements using ULVAC-Laser-PIT thermal diffusivity system to calculate the efficiency of the fabricated thermoelectric devices.
Acknowledgements
Research sponsored by the Materials Research Laboratory (MRL), National Science Foundation under NSFEPSCOR 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 # W911NF-12-1-0063, U.S. Department of Energy National Nuclear Security Admin with grant# DE-NA0001896 and grant# DE-NA0002687, NSF-REU with Award#1156137.

The Peltier and Seebeck effects are thermoelectric phenomena that are defined by the generation of a temperature- or potential- difference, respectively. This can be used to fabricate reliable solid state cooling (the Peltier cell) or to scavenge waste heat (the thermoelectric energy generator, TEG). The high cost and relatively low efficiency of these devices, however, currently prevents widespread application of TEGs for waste heat harvesting. This is driven, in part, by the co-dependence of electric and thermal conductivities that inevitably limit the efficiency of such devices.
It was recently demonstrated that a thermal gradient applied across a metallic, insulating, or semiconducting magnetic material can result in the generation of a spin polarized current: the spin Seebeck effect. This observation has stimulated a wealth of theoretical output on potential applications such as magnetic heat valves [1], position sensitive heat detectors [2], and spin caloritronic nanomachines [2],[3]. Critically, however, it could be implemented in waste heat harvesting technologies [4],[5]. The advantage of such energy harvesting devices is a new architecture that enables reduced fabrication costs as well as separation of the electric and thermal conductivities.
YIG and Pt are commonly chosen materials for observation of the spin Seebeck effect (or indeed demonstration of waste heat harvesting potential), however, the cost of these raw materials can be prohibitively high. We will present data on a sequence of measurements of magnetic thin films, chosen to minimise cost, where voltage generation of the order of 0.1 mV/K/m has been observed. The impact of the interface quality on the magnitude of the signal will also be discussed.
References:
1) T. Heikkila et al., Phys. Rev. B 81, 100408 (2010)
2) G.E.W. Bauer et al., Nat. Mater. 11, 391 (2012)
3) G.E.W. Bauer et al., Phys. Rev. B 81, 024427 (2010)
4) A. Kirihara. et al., Nature Materials 11, 686-689 (2012)
5) K. Uchida et al., Appl. Phys. Exp. 5, 093001 (2012)

In recent years, large efforts have been made to improve thermoelectric (TE) 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. From this viewpoint, unconventional TE materials such as amorphous oxide based component could advance utilization of renewable energies owing to their suppressed thermal conductivity and atmospheric stability. Furthermore, print formation of the materials should reinforce the future construction of 3-D devices and large area energy regeneration. In this study, synthesis of oxide glass-based printable TE materials, their TE properties, and TE generation of printed devices were investigated. V₂O₅-P₂O₅ glasses are considered to have layered structure comprise VO₄, VO₅ and PO₄ polyhedra, that reveal electric conductivity up to 10⁵ Omega;^-1cm^-1 due to hopping conduction between V⁴⁺ and V⁵⁺ ions ^[1,2]. Here we demonstrate conduction properties and thermoelectric properties of vanadate glass of V₂O₅-P₂O₅-Fe₂O₃-CuO based conductive materials. The milled raw glass powder was applied to α-Al₂O₃ substrates via paste form by adding solvent of diethylene glycol monobutyl ether acetate (BCA; C₁₀H₂₀O₄). Prepared film samples were then heat treated for glass softening and partial crystallization. Thermal treatments were conducted in air, in vacuum, and also in more controlled oxidation/reduction atmosphere at a temperature range of 300°C to 650°C. The valence of vanadium ions altered from V⁵⁺ to V⁴⁺ and V³⁺ by modifying the atmosphere from air to vacuum and more reductive H₂ gas filled. The conductivity and the Seebeck coefficient of crystallized glass films reached 10² Omega;^-1cm^-1 and #65392;130 mu;V/K ^[3]. The conduction properties could be controlled and improved by non-equilibrium annealing that is for example, irradiation of ultraviolet pulsed laser beam or rapid thermal annealing using a IR image furnace. 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. The TE generation using these novel n-type and p-type oxide glass-based materials were verified in this study by developing π-shaped prototype devices on ceramic substrates via printing technique. These TE characteristics of the V₂O₅-P₂O₅ glass-based film correlated with their morphology and crystallographic information would also be elaborated.
[1] E. P. Denton, H. Rawson, J. E. Stanworth, Nature173 (1954) 1030.
[2] G. S. Linsley, A. E. Owen, F. M. Hayatee, J. Non-Crystalline Solids4 (1970) 208.
[3] A. Matsuda, M. Liu, T. Aoyagi, T. Naito, T. Fujieda, M. Yoshimoto, J. Ceram. Soc. Jpn.123 (2015) 1.

In this study, thermoelectric (TE) power generating module has been simulated with a series of 3D #64257;nite element models (FEM) in Ansys environment. Various high-performance TE materials spanning a wide temperature range are utilized in the simulation. The signature material properties, such as the Seebeck coef#64257;cients, electrical resistivities and thermal conductivities are all temperature dependent and extracted directly from the recently published experimental data. The results reveal that TE modules could achieve efficiencies up to 24.7% at ΔT=700 K, by segmenting BiSbTe, MgAgSb, PbTeS and SnSe as both p-type and n-type materials, and the combination of the best p-type TE materials with the best n-type TE materials could yields an efficiency of up to ~17% at ΔT=500 K. In addition, effects of module geometries, such as leg thickness, number of TE p-n pairs, and uneven p-n cross-section area on thermoelectric efficiency have been systematically studied. Due to the fact that the n-type TE materials are universally weaker than their p-type counterparts, asymmetry of p-n leg cross-section (An < Ap) plays an important role in enhancing the overall efficiency and boosting the output power while reducing the intake of heat. Results show that weighted average of electrical resistivities and thermal conductivities over segmented leg materials could accurately predict the right geometrical configuration, i.e. the ratio of An over Ap, that leads to the best performance (efficiency) of a TEG module.

The transport properties of thermoelectric materials, especially the lattice thermal conductivity, are closely related to their fundamental microstructures. It is proved that nanoscale in-situ precipitations from matrix materials can effectively scatter short- and medium- wavelength phonons, thus significantly reduce the overall lattice thermal conductivities. Here, we report that the cooling rate plays an important role on the precipitate size and distribution in both n- and p-type PbTe-CdTe. The microstructures were first investigated by scanning/transmission electron microscopy (S/TEM), and then analyzed using classical nucleation and growth theory. It is concluded that a slow cooling rate leads to an obvious bimodal size distribution of CdTe precipitates; this, combined with lattice thermal conductivity calculations in a modified Callaway model, clearly elucidate the distinct behaviors of lattice thermal conductivities between water quenched PbTe-CdTe samples and air quenched ones, as CdTe fraction increases from 1% , 3% to 5%.

Most of the experimental studies in thermoelectrics are focused on improving the dimensionless figure-of-merit ZT of materials. However, for practical devices in a commercially viable product, mechanically robust samples that can withstand fast thermal cycling and comparable competitive ZT values are needed. We present our study to highlight the improved ZT value of both p (La/Ce double filled) and n-type (Ba, La, and Ce triple filled) skutterudite materials made by hot pressing the nanopowder obtained from ball-milled annealed ingots. Details of electrical and thermal transport properties of samples measured from room temperature to ~550^oC will be presented. The measured mechanical strength of both p and n-type samples based on instrumented indentation technique (IIT) will be discussed. We will also present a brief overview on the development of contact alloys. Finally, we will highlight the temperature-dependent output power density achieved from the skutterudite single couples.

Since the discovery of the spin Seebeck effect (SSE) [1] much attention has been devoted to the study of the interaction between heat, spin and charge in magnetic systems. The SSE refers to the generation of a spin voltage upon the application of a thermal gradient and detected by means of the inverse spin Hall Effect. The SSE has been observed in insulating magnetic materials, opening the possibility to explore magnetic oxides as potential candidates for thermoelectric applications. Therefore the study of the thermomagnetic transport properties is mandatory to distinguish diverse phenomena that can influence SSE signal like anomalous Nernst Effect (ANE) and control them. We report the optimization of the SSE signal in Fe₃O₄/Pt thin films at room temperature by the variation of several degrees of freedom as substrate, thickness and material architecture. In this system the measured voltage is highly dominated by the SSE and ANE accounts for less than 1% [2]. Our results further show that the SSE is strongly affected by the interface parameters. [1] K. Uchida, et al. Nature455, 778 (2008).
[2] R. Ramos, et al. Appl. Phys. Lett. 102, 072413 (2013).

Recently, electrical devices have been required to be cost effective, resource saving, and low energy processing. Printed electronics (PEs) is one of the emerging manufacturing technologies intended to overcome those issues and technological aims. The printing method has attracted a great attention for various applications and components installation, especially electrode or bonding of low temperature packaging technology.
In recent years, the authors have been developing low temperature curing flexible and stretchable isotropic conductive adhesives (ICAs) using several resin materials and silver with different kinds of size or shape for fabrication of electrical devices.
Thermoelectric energy generation (TEG) is one of the utilizing methods of enormous heat energy which is abandoned to the earth. This technology is expected to become an energy harvesting one that can convert the dispersed thermal energy into electrical energy.
Here, we have developed flexible TEG modules for applying to heat sources with the curved surface such as exhaust pipes using ICA packaging technology. The module concept is that a mass of small Bi-Te thermoelectric chips are tightly mounted on a thin film substrate and connected with flexible and stretchable ICAs by printing technique. Fabricated TEG modules with 250 p-n pairs of thermoelectric chips have been evaluated in detail.

The spin-Seebeck effect (SSE), which refers to a spin-current generation from a temperature gradient, is expected as a core technology of spin-driven thermoelectric conversion devices. The device has a simple bilayer structure with a ferromagnetic film (FM) and a paramagnetic film (PM). Here, we experimentally show that, by inserting an ultra-thin Fe layer into between FM layer and PM layer, a spin-driven thermoelectric voltage is improved due to an enhancement of the mixing conductance which determines the efficiency of the spin-current injection from FM layer into PM layer and, accordingly, that of thermoelectric conversion.
Device preparation and characterization were done as follows: Bi-substituted yttrium iron garnet (Bi:YIG) was formed as the FM layer by using a metal organic decomposition (MOD) method. Then, an ultra-thin Fe layer and a Pt layer were deposited on the Bi:YIG layer by sputtering. For the devices with different Fe layer thicknesses (d_Fe), the mixing conductance and the spin-driven thermoelectric voltage were investigated.
As a result of the mixing conductance evaluation by ferromagnetic resonance (FMR) measurement, it was found that magnitude of the mixing conductance increased with increasing the Fe layer thickness, and was saturated at the point of d_Fe = 0.3 nm that corresponds approximately to 1.04 ML. This result is consistent with the previous theoretical work where the mixing conductance increases with increasing d_Fe in case of d_Fe < 1.0 ML and is unchanged for d_Fe > 1.0 ML. In the measurement for the spin-driven thermoelectric voltage, the spin-driven thermoelectric voltage increased with increasing d_Fe in case of d_Fe < 0.3 nm, so that the device with 0.3 nm Fe layer generated 1.7 times larger voltage than the device without Fe layer did, and the voltage finally decreased for d_Fe > 0.3 nm. This voltage peak can be explained with the mixing conductance increase, saturation and the spin-current decay in the Fe layer. For d_Fe < 0.3 nm, the spin-driven thermoelectric voltage increases with increasing d_Fe because of mixing conductance enhancement and smaller spin-current decay in the Fe layer. On the other hand, in case of the d_Fe > 0.3 nm, the voltage decreases due to the spin-current decay with the mixing conductance left unchanged in the Fe layer.

Mg₂Si-based materials are promising thermoelectric materials since they exhibit high thermoelectric performance and consist of abundant and non-toxic elements. For further improvement of the figure of merit of the materials, carrier energy filtering is one of the effective approaches because it can enhance the power factor without increasing the carrier thermal conductivity. To maximize the enhancement of the figure of merit by the carrier energy filtering, increasing the carrier concentration of the materials is important since the enhancement is significant at high carrier concentration level such as 1.0×10²¹ cm^-3 [1]. Sb is one of the typical n-type dopants for Mg₂Si and its solid solutions. While Sb shows good solubility in Mg₂Si (> 30at%), it also creates the Mg vacancies which act as electron accepters at high Sb concentration [2]. Within the typical preparation process such as direct melting and hot pressing, increasing the amount of Sb-dopant in Mg₂Si results in restricted carrier concentration such as 1.1×10²⁰ cm^-3 for 10at% Sb-doped Mg₂Si [3].
In this study, we report Sb-doped Mg₂Si with high carrier concentration prepared by ball-milling and spark plasma sintering. Polycrystalline samples of Sb-doped Mg₂Si are synthesized by solid-liquid reaction of Si, Mg, and Sb. The synthesized samples are ball-milled and sintered at 800°C for 1 min by spark plasma sintering. The carrier concentration of Sb-doped Mg₂Si increases with the amount of Sb-dopant up to 9.3×10²⁰ cm^-3 for 10at% Sb-doping. Though the value is lower than that expected from the Sb doping level (1.6×10²¹ cm^-3), it is much higher than that of conventionally prepared ones [2,3]. On the other hand, non-milled Sb-doped Mg₂Si shows low carrier concentration (1.2×10²⁰ cm^-3 for 10at% Sb-doping). We also checked the dependence of the carrier concentration on the sintering time and found that shorter sintering time resulted in higher carrier concentration. This suggests that the short-time sintering is effective to prevent the creation of Mg vacancies in Sb-doped Mg₂Si and realize the high carrier concentration. Other thermoelectric properties and the thermal stability of Sb-doped Mg₂Si will be also presented.
[1] J.-H. Bahk, Z. Bian, and A. Shakouri, Phys. Rev. B 89, 075204 (2014).
[2] G. S. Nolas, D. Wang, and M. Beekman, Phys. Rev. B 76 (2007).
[3] T. Dasgupta, C. Stiewe, R. Hassdorf, A. J. Zhou, L. Boettcher, and E. Mueller, Phys. Rev. B 83, 235207 (2011).

This report presented a new doping method for the synthesis of p-type Mg₂(Si,Sn)-based thermoelectric materials. The combination of flux method and carbonate-doping produced p-type Mg₂Si_0.4Sn_0.6 materials successfully. The carbonated-doped materials showed comparable electrical conductivities and Seebeck coefficients compared with pure alkaline-metal-doped materials. But the MgO produced during the carbonate reaction had significant influence on the lattice thermal conductivity of the materials. A peak ZT ~0.7 was obtained in the sample with a nominal composition of Mg₂Li_0.025Si_0.4Sn_0.6.

Semi-metals are generally unable to exhibit the sort of asymmetric carrier activation needed to produce useful thermoelectric effects and are therefore rarely seen as candidates for thermoelectric applications. One material that has been shown to be an exception to this rule is CuAgSe, a compound that also exhibits very low thermal conductivity and electrical resistivity, properties that are highly desirable in thermoelectrics. CuAgSe has also been shown to exhibit a thermoelectric figure of merit (ZT) value of 0.10 at 100 K when doped with 10% Ni on the Cu site. This represents an increase by a factor of 5 in ZT when comparing Cu_0.9Ni_0.1AgSe and un-doped CuAgSe samples at similar temperatures. The exact mechanism by which this increase in efficiency results remains unclear. In order to provide some new insight into how exactly this occurs, we have synthesized and measured the thermoelectric properties of a series of CuAgSe samples with Co doping, Cu_1-xCo_xAgSe (x = 0, 0.02, 0.05, 0.1). Temperature-dependent carrier concentration, magnetic and thermoelectric transport properties of CuAgSe as a function of Co doping amount will be discussed.

Amorphous Ge (a-Ge), crystalline Ge (c-Ge), and amorphous Si (a-Si) thin films were deposited on a Ge substrate at different temperatures by magnetron sputtering. We measured thermal boundary resistance (TBR) in Au/Ge/Ge and Au/Si/Ge three-layer samples. The measured TBR in Au/a-Ge/Ge and Au/a-Si/Ge decreased slightly with increasing deposition temperature. The measured TBR values were larger than the values predicted by the di#64256;use mismatch model. Furthermore, it is interesting to note that the measured TBR in Au/c-Ge/Ge was twofold larger than that in Au/a-Ge/Ge. Cross-sectional transmission electron microscopy was conducted to investigate interfacial morphology of the samples. The results indicate that the crystalline state of the deposited thin films play an important role in TBR by modifying phonon density of states and interfacial properties. Our findings are of great importance for applications involving thermal management of micro- and optoelectronic devices, and for the development of thermal barrier coatings and thermoelectric materials with high figures-of-merit.

Thermoelectricity in single molecules is interesting because it is relevant to energy conversion, and provides unique insights into the charge transport mechanism. Experimental and theoretical works of thermoelectricity in molecules have been reported, but these studies focus on systems where tunneling is the transport mechanism. Here we describe an experimental study of thermoelectric effect in double strand DNA using the Scanning Tunneling Microscopy break junction method. By tailoring the DNA sequences, we were able to study thermoelectricity in both tunneling and hopping transport regimes. We found that the thermoelectric effect (Seebeck coefficient) of DNA is highly correlated with the mechanism of charge transport, and can be manipulated by change of molecular length and sequence.

In recent years much interest has been shown in various methods of direct conversion of heat into electricity. Waste heat from hot engines and combustion systems could save billions of dollars if it could be captured and converted into electricity via thermoelectric devices. For this challenging application Linseis has developed a characteristic evaluating instrument for these materials and devices; the LSR -3 “Linseis - Seebeck & Electric Resistivity Unit”. It can determine the thermal power, thermoelectric power, or Seebeck coefficient of a material. It measures the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. The LSR is an all in one device with modular sample holder systems to easily fulfill all the needed requirements.

A new family of coinage metal stuffed compounds, Eu₉Cd_4-xCM_2+x-yo_ySb₉ (o = vacancies, CM = Coinage Metal: Cu, Ag and Au), based on the Ca₉Mn₄Bi₉ structure type will be presented. This family of compounds with low thermal conductivities (less than 2 W/mK) combined with low electrical resistivity (similar to the resistivity of poor metals such as Bi, Yb and Gd) makes them classic cases of “phonon-blocking/electron-transmitting” materials, a criteria necessary for an efficient thermoelectric. In addition to the thermoelectric properties studies, Density Functional Theory (DFT) calculations have been carried out to rationalize the observed electronic properties in this family of compounds. The theoretical calculations show that regardless of the success of Zintl concept in rationalization of the properties, the representation of coinage - metal stuffed Eu₉Cd₄Sb₉ structure as Eu cations encapsulated into polyanaionc (Cd/Cu)Sb network is oversimplified and underestimates the importance of the Eu-Sb bonding interactions. This family of compounds show that the heat transferred by charge carriers can be lower than the quantity derived from the Wiedemann- Franz law, κ_e = LT/ρ (κ_e = electronic component of thermal conductivity, L = Lorenz number, T = temperature, ρ = resistivity) as the Lorenz number cannot be considered as a simple constant value derived from the free electron model. Even alternative methods of Lorenz number calculations such as the single parabolic band (SPB) model or using the Seebeck coefficient turns out to be overestimating κ_e. These unprecedented low L values materials can be a breakthrough to achieve high performance thermoelectric properties in complex metals as zT of a metal where κ_e >> κ_L is given by zT = S²/L. As a result, it could lead to a new mechanism for achieving new efficient thermoelectric materials among metals.

Materials with complex crystal structures are being widely used in thermoelectric applications due to the inherent low thermal conductivity in complex, layered, disordered, rattling, and defect structures. The RE₁₂Co₅Bi (RE=Gd, Er) compounds, closely related to the structurally complex skutterudite family of materials, have recently been recommended via a first-of-its-kind machine learning algorithm as good candidate thermoelectric materials. Pure, polycrystalline samples have been synthesized and the thermoelectric transport properties have been measured. Although the samples are too metallic to have good thermoelectric performance (~15mu;V/K) the materials exhibit highly unusual and unexplained thermal conductivity which increases twofold with temperature rather than decreases. In this contribution we describe the materials recommendation engine as well as recent experiments using high temperature synchrotron diffraction to look for systematic changes to the RE₁₂Co₅Bi structure such as changes to the thermal displacement parameter which could explain the unusual thermal conductivity observed in these materials.

A sustainable solution of the energy crisis requires not only to replace fossil fuels, but also to diversify our energy resources. Thermoelectric materials converting waste heat into useful electrical energy have a great potential as a sustainable and reliable energy source. Recently, we initiated a high-throughput computation project for new thermoelectric materials within the Materials Project (www.materialsproject.org) electronic structure database. By assuming constant relaxation for electron scattering, we revealed a new group of thermoelectric materials, XYZ₂ (X, Y: Rare earth or transition metals, Z: Group VI elements). In this system, we found a large number of candidates including XYTe₂ (X: Sm, Dy, Tm, Er, Ho, Tb, Lu and Y, Y = Ag, Cu) with good stability, intriguing electronic band structures, and favourable thermal transport properties. Here, we will present the crystal structure, thermal behaviour, optical absorption, first principles calculations, and electronic and thermal transport results of TmAgTe₂ and intrinsically doped YCuTe₂ samples. TmAgTe₂ and YCuTe₂ display extremely low measured thermal conductivities of ~0.25 Wm^-1K^-1 at 650 K and 0.5 Wm^-1K^-1 at 800 K, respectively. TmAgTe₂ shows low thermoelectric efficiency (zT ~0.35 at 650 K) due to low charge carrier concentration (~10¹⁷ cm^-3 at room temperature), whereas YCuTe₂ is a promising material (zT ~0.7 at 780 K) for mid-temperature thermoelectric applications.

In spite of the emergence of chemically complex thermoelectric materials, simple binary A₁B₁ materials such as PbTe, SnSe, and GeTe, continue to dominate the highest zT thermoelectric materials. To understand the structure-property relations that drive this propensity, we have computationally assessed the thermoelectric performance of 518 A₁B₁ chemistries in 1508 different structures reported in the Inorganic Crystal Structure Database (ICSD). We have employed a thermoelectric performance metric (β)¹ that combines first-principles calculations and modeled electron and phonon transport to offer a reliable assessment of the intrinsic material properties that govern the thermoelectric figure of merit (zT). In studying the correlations between the material structure, composition and β, we have found that the potential for good thermoelectric performance of A₁ B₁ materials originates from low valent ions in cubic and orthorhombic crystal structures, which is mainly beneficial for charge carrier transport properties. These observations can be explained based on the electronic structure. Additionally, we have identied promising new A₁B₁ compounds, including higher-energy polymorphs, that have the potential to be good thermoelectric materials.
[1] J. Yan, P. Gorai, B. Ortiz, S. Miller, S. Barnett, T. Mason, V. Stevanovic, and E. S. Toberer, "Material Descriptors For Predicting Thermoelectric Performance," Energy Environ. Sci. 8 (2015) 983-994.

Zintl phase compounds can be described as covalently-bonded anion substructures surrounded by highly electro-positive cations exhibiting essential features for thermoelectric applications. By combining first principles electronic structure and Boltzmann transport calculations, we report excellent thermoelectric properties of CdSb and ACd₂Sb₂ (where, A = Ca, Ba, Sr) [1]. The electronic structure shows presence of charge carrier pockets and heavy light bands near the band edges, which lead to large power factor contributing to good thermoelectric performance. Furthermore, we calculate lattice thermal conductivity by solving Boltzmann Transport equation using an iterative method. All the compounds studied here exhibit low lattice thermal conductivity, which are in good agreement with experiments. We further analyse these low values by calculating Grüneisen parameters, Debye temperature and phonon group velocities. The large Grüneisen parameters and low phonon group velocities indicate strong anharmonicity in these compounds, which results into low lattice thermal conductivity. The low thermal conductivity and the excellent transport properties lead to a high ZT value of sim;1.9 in CaCd₂Sb₂ and BaCd₂Sb₂ at moderate p and n-type doping. These results indicate that well optimized Cd based Zintl phase compounds have a potential to match the performance of conventional thermoelectric materials.
1. Tribhuwan Pandey and Abhishek K. Singh, PCCP, 2015 (in press)

Since the 1960&’s, the state-of-the-art power systems for space applications have typically been based up on either Si-Ge alloys or PbTe and TAGS materials. Although reliable and robust, the thermal-to-electric energy conversion efficiency of these systems remains fairly low at only 6.5% with an average thermoelectric figure of merit of about 0.5 across the full temperature differential (1275 to 475 K) available to this technology. A factor of 2 improvement in conversion efficiency is highly desirable to support future space missions. In recent years, complex Zintl phases such as n-type La_3-xTe₄ and p-type Yb₁₄MnSb₁₁ have emerged as practical high efficiency, high temperature thermoelectric materials with peak ZTs of 1.2 at 1275K. The high performance of these materials is attributed to their combination of favorable characteristics such as: semi-metallic behavior due to small band gaps, low glass-like lattice thermal conductivity values due to structural complexity and reasonably large Seebeck values near their peak operating temperatures. Recently, JPL and collaborating institutions have investigated a series of promising p-type Zintl phases. The new Zintl phases include Yb_1.05Zn₂Sb₂, Sr₃GaSb₃ and Yb₁₄MgSb₁₁ and their performance is competitive to that of filled skutterudites with ZTs greater than 1. We will present an overview of recent research efforts at JPL and collaborating institutions on the thermoelectric properties of these new materials as well as provide a first assessment of their suitability for infusion into advanced TE devices.

Heritage radioisotope thermoelectric (TE) generators have been used in a number of NASA space science and exploration missions with extremely reliable high temperature TE couple technologies capable to 7 to 7.5% conversion efficiency and long lifetimes, in excess of 30 years. An increase in both conversion efficiency and specific power by a factor of at least 2 is highly desirable for the next generation of space missions. The Jet Propulsion Laboratory (JPL) has been collaboratively pursuing the development of new promising advanced thermoelectric materials that offer performance increases while maintaining the reliability of heritage technology. Previously we have demonstrated that p-type Yb₁₄MnSb₁₁ and n-type La_3-XTe₄ when segmented with filled skutterudites can achieve 15% thermal to electric conversion efficiency, a factor of 2 improvement over the state of the art couple technologies. Recent work by JPL and collaborating institutions have led to the discovery of new complex p-type Zintl phases with high dimensionless thermoelectric figures of merit (ZT): Yb₁₄MgSb₁₁ (ZT of 1.2 at 1273 K), Sr₃GaSb₃ (ZT of 0.9 at 1000 K), and Yb_1.05Zn₂Sb₂ (ZT of 1.1 at 775 K) that could serve as alternate p-type leg materials with comparable conversion efficiency. The p-type Zintl phases could potentially have several advantages over the previous generation couples in terms of improved chemical and mechanical compatibilities that could facilitate device development and help minimize degradation rates during operation. The TE transport properties of the new p-type Zintl materials need to be validated through a proof-of-principle TE couple level performance demonstration using the full temperature differential of interest, 473-1273 K. The experimental and predicted performance of the couple in terms of electrical resistance, output voltage, current, power output and efficiency will be presented and discussed.

The SKD Technology Maturation program is aimed at improving the efficiency and power output of NASA&’s Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which has been jointly designed and produced by Teledyne Energy Systems, Inc. and Aerojet Rocketdyne. Present work is focused on the optimization of a skutterudite (SKD) thermoelectric couple that will replace those constructed from the heritage thermoelectric materials used in the MMRTG that is currently powering the Mars Science Laboratory rover, Curiosity. This enhanced MMRTG (eMMRTG) is predicted to produce up to 25% more power than its predecessor owing mainly to the increased hot-side temperature of the thermoelectric couples now permitted by the SKD materials. This prediction is a result of modeling using several techniques, such as finite element, the iterative technique, and the reduced current density method. The implementation of these techniques, their results and limitations are discussed. Furthermore, real couples have been produced and characterized for comparison to the various modeling results.

Thermoelectric materials can be used to convert the heat of the sun into electricity. Under one sun of illumination, these solar thermoelectric generators (STEGs) have reached a conversion efficiency of 4.6%. By incorporating optical concentration, STEGs can achieve higher temperatures with lower losses, thus allowing for higher conversion efficiencies. Even though theoretical predictions in literature often report efficiencies of over 10% at moderate to high optical concentration, such high conversion efficiencies have not been experimentally demonstrated. The main challenges for achieving such high performance are to fabricate a thermally stable STEG assembly which requires solar absorbers and thermoelectric generators that perform reliably up to 600 0C. Furthermore, the STEG&’s parasitic losses such as radiative heat losses, and electrical and thermal contact resistance losses need to be minimized. We use segmented thermoelectric generators comprising skutterudite and bismuth telluride compounds for hot-junction operation up to 600 0C. For the solar absorber we use a high-temperature stable Al₂O₃ (W, Ni) Cermet-based spectrally-selective surface on tungsten-coated stainless steel substrate developed by us and also a reference black paint. We demonstrate a record high conversion efficiency of close to 10% at an incident solar flux density of around 200 kW/m² and 7% at 40 kW/m². For the experiments with low incident solar flux densities we investigate the use of an optical metal cavity which can significantly improve the performance of the STEG. This work is supported by the DOE EERE SunShot Initiative under award number DE-EE0005806.

Ttetrahedrites (Cu₁₂Sb₄S₁₃) and colusites (Cu₂₆V₂M₆S₃₂ (M = Ge, Sn)) are promising candidates for environment-friendly and cost-effective thermoelectric materials, because of their high thermoelectric figure of merit ZT and earth-abundant constituents. In this study, we have enhanced the ZT of tetrahedrites and colusites and investigated the conversion efficiency of their single-leg modules.
The samples of tetrahedrites and colusites were synthesized by melting stoichiometric amounts of the constituent elements in evacuated and sealed quartz tubes and then consolidated with diffusion barriers by hot-pressing. The low lattice thermal conductivity (~0.5 W K^minus;1 m^minus;1 at 300 K) in both systems leads to high ZT. A ZT of ~0.8 at 670 K and ~0.7 at 660 K was achieved for tetrahedrites and colusites, respectively. The conversion efficiency of the single-leg modules was measured using Mini-PEM (Advance Riko). The diffusion barrier provides the improved electrical and thermal contacts between materials and electrodes, leading to enhancement in output power and conversion efficiency. Three-dimensional finite-element simulation predicts that the tetrahedrite-based single leg have high conversion efficiency of 7.8% for temperature difference of 375 K.

Due to their low lattice thermal conductivities and their high electrical conductivities, organic semiconductors represent a promising class of solution processable thermoelectrics. It is currently difficult to predict the thermopower of organic semiconductors and their electrical conductivity as a function of doping. We will discuss recent progress from our lab to uncover the role of doping method on the thermoelectric performance of p- and n- type semiconducting polymers. The phase behavior of blends of semiconducting polymers and molecular dopants plays a critical role in their ultimate performance. We find that processing plays and important role in defining the electrical behavior of organic thermoelectric materials. Our results and data from the literature show a striking empirical correlation between the power factor and electrical conductivity of a broad range of semiconducting polymers. This relationship is caused by the similarity of electronic structure of the materials and similar transport pathways in polymeric thin films. We will also discuss the role of structural anisotropy in defining the ultimate figure of merit of organic thermoelectric materials.

Single-walled carbon nanotubes (SWCNTs) are a versatile electronic material being explored as cost-effective, high-performance active materials in a variety of renewable energy applications such as transparent conducting or light-harvesting layers in photovoltaics and inclusions in thermoelectric composites.
We present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a semiconducting polymer matrix. Rational choice of the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. We use a stable charge-transfer dopant to control the density of carriers in the s-SWCNT network, as determined by the bleach of the absorption corresponding to the S₁₁ excitonic transition. The performance of these transparent conducting s-SWCNT composite networks is comparable to neat p-type and n-type s-SWCNT networks doped by either nitric acid or hydrazine treatments. By varying the carrier density we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy.
Consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs, we observe a distinct dependence of the thermopower and thermoelectric power factor on the bandgap (or diameter) of the carbon nanotubes. We have measured a colossal thermopower as high as ~2,500 µV/K (for s-SWCNT networks with very low electrical conductivity) and thermoelectric power factor as large as ~350 µW/mmiddot;K², which are an order of magnitude and a factor of two, respectively, larger than previously reported for SWCNT-based material systems. For carbon nanotubes prepared by high-pressure carbon monoxide (HiPCO) conversion, as we tune the carrier density, we are able to maintain a thermopower above 100 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 40,000 S/m, resulting in a thermoelectric power factor of >300 µW/mmiddot;K². These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors.
We also present a sensitive technique, based on a microfabricated silicon nitride thermal isolation platform, to probe the thermal conductivity in the s-SWCNT networks, allowing us to estimate the thermoelectric figure of merit (zT) for these materials to be ~ 0.1.
These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by tuning the carrier density and/or Fermi energy, and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

In the last 10-15 years, organic materials have become important in optoelectronics, both in solar cells and light emitting diodes and, more recently, in thermoelectricity, especially as thin films, where the polymer chains remain basically in two dimensions, which improves the conductivity. In the last few years, several intrinsically conducting polymers (ICPs) have been successfully used in the field of thermoelectricity and the dimensionless figure of merit ZT (ZT=α²σ/κ, where α, σ and κ are the Seebeck coefficient, the electrical and thermal conductivities, respectively) has been improved several orders of magnitude, until values were very close to those of inorganic materials. Polymers present, in addition, many advantages over inorganic materials: non scarcity of raw materials, lack of toxicity, lower cost of production and many others. The incorporation of conducting nanofillers, as CNTs, can be used to control the conductivity and thus the thermoelectric performance ICPs. In this work, the focus is to provide several routes to increase the thermoelectric efficiency of conducting polymers such as: chemical and electrochemical de/doping or the incorporation of carbon nanofillers to the polymer matrix. Using these methods it is possible to achieve a ZT > 0.2 for ICPs. In addition, a new method for the fabrication of thermoelectric modules (TEG) has been developed using only one type of ICP. As a proof of concept, we have developed a thermal sensor based on poly(3,4 ethylenedioxythiophene) (PEDOT) nanofilms as the source thermoelectric material.

This paper reports Seebeck effects driven by both surface polarization difference and entropy difference by using photoinduced intramolecular charge-transfer states in n-type and p-type conjugated polymers, namely IIDT and IIDDT, respectively, based on vertical conductor/polymer/conductor thin-film devices. We obtain large Seebeck coefficients of -898 mV/K from n-type IIDT and 1300 mV/K from p-type IIDDT when the charge-transfer states are generated by a white light illumination of 100 mW/cm², compared with the values of 380 mu;V/K and 470 mu;V/K in dark condition, respectively. Simultaneously, the electrical conductivities are increased from almost insulating state in dark condition to conducting state under photoexcitation in both n-type IIDT and p-type IIDDT based devices. The large Seebeck effects can be attributed to the following two mechanisms. Firstly, the intramolecular charge-transfer states exhibit strong electron-phonon coupling, which leads to a polarization difference between high and low temperature surfaces. This polarization difference essentially forms a temperature-dependent electric field, functioning as a new driving force additional to entropy difference, to drive the energetic carriers for the development of Seebeck effects under a temperature difference. Secondly, the intramolecular charge-transfer states generate negative or positive majority carriers (electrons or holes) in the n-type IIDT or p-type IIDDT, ready to be driven between high and low temperature surfaces for developing Seebeck effects. Based on co-existed polarization difference and entropy difference, the intramolecular charge-transfer states can largely enhance the Seebeck effects in both n-type IIDT and p-type IIDDT devices. Furthermore, we find that changing electrical conductivity can switch the Seebeck effects between polarization and entropy regimes when the charge-transfer states are generated upon applying photoexcitation. Therefore, using intramolecular charge-transfer states presents an approach to develop thermoelectric effects in organic materials-based vertical conductor/polymer/conductor thin-film devices.

Recent progress in conducting polymer-based organic thermoelectric generators (OTEGs) has resulted in high performance due to high Seebeck coefficients (S), high electrical conductivity (σ), and low thermal conductivity (κ) obtained by chemically controlling the materials&’ redox levels [1]. In addition to improving the properties of individual OTEGs to obtain high performance, development of solution processes for the fabrication of OTEG modules is necessary to realize large thermoelectric voltage and low-cost mass production. However, the scarcity of good candidates for soluble organic n-type materials limits the use of π-leg module structures consisting of complementary elements of p- and n-type materials because of unbalanced transport coefficients that lead to power losses. In particular, the extremely low σ of n-type materials compared with those of p-type materials is a serious challenge [3].
Poly(pyridinium phenylene) (poly(PymPh)) is an n-type conducting polymer reported to have high σ and to be soluble in polar solvents such as water [2]. In this study, poly(PymPh) was tested as an n-type semiconductor for OTEGs. Films of poly(PymPh), which were fabricated by spin-coating, were chemically doped by dipping the films into solutions containing the electron-donating dopant sodium naphthalenide (NaNap). The concentration dependence of the thermoelectric properties were measured using two couple of chromel-alume pair to measure S and four-chromel wires in a four-terminal sensing scheme to measure σ. All sample fabrication processes and measurement were done in nitrogen atmosphere without any air exposure.
Optimized power factors (S²σ) of about 0.81 mu;Wm^-1K^-2 for n-type behavior were obtained in poly(PymPh) doped with NaNap at 100 ^oC, which is the highest power factor reported among n-type soluble conducting polymers [3]. The electronic structures of pristine and doped poly(PymPh) as a function of doping concentration were further explored based on changes in absorption spectra and ultraviolet photoelectron spectroscopy, and the doping mechanism was investigated by X-ray photoelectron spectroscopy.
References
[1] O. Bubnova and X. Crispin, Towards polymer-based organic thermoelectric generators, Energy Environ. Sci., 5 (11), 9345 (2012).
[2] D. Izuhara and T. M. Swager, Poly(pyridinium phenylene)s: water-soluble N-Type polymers, J. Am. Chem. Soc., 131 (49), 17724 (2009).
[3] R. a. Schlitz, F. G. Brunetti, A. M. Glaudell, P. L. Miller, M. a. Brady, C. J. Takacs, C. J. Hawker and M. L. Chabinyc, Adv. Mater., 26 (18), 2825 (2014).

Transverse thermoelectric phenomena such as the Nernst and spin Seebeck effects offer an attractive route toward solid state power generation and temperature control, largely due to the fact that the relevant temperature gradient and corresponding electric field are, by definition, orthogonal. This means that many of the classic material and device constraints associated with conventional longitudinal thermoelectric effects no longer apply, such as the need for both n- and p-type materials, etc. Interest in transverse phenomena has been renewed over the last decade, largely due to the discovery of the spin Seebeck effect in 2008. This discovery has opened up an exciting new arena dubbed “spin caloritronics”, in which thermal transport phenomena can be considered within the context of spin and magnetization dynamics. This makes it possible to consider new ways for enhancing thermoelectric phenomena, e.g. through spin-dependent transport effects. Since bismuth has one of the largest Nernst coefficients of all known materials, as well as inherently strong spin-orbit coupling, this element is an obvious starting point for exploring these possibilities. This has inspired us to synthesize several series of novel bismuth-based materials and measure their magneto-thermoelectric properties, which will be revealed in this talk.

Ni45Co5Mn37In13 is a Heusler compound that develops a predominantly ferromagnetic moment at temperatures above 270K due to a first-order magneto-structural transition in which a a magnetic phase transition is accompanied by a structural phase transition. Experimental data will be given for the heat capacity and heat of magnetization as functions of temperature and magnetic field as the material completes its magneto-structural transition. NbP is a newly-discovered topological Weyl semimetal, which is essentially a 3D analog of graphene. NbP has a large magnetoresistance and an ultrahigh mobility. Thermomagnetic tensor elements, including thermal conductivity, resistivity, thermopower, and Nernst effect, will be reported as functions of temperature and magnetic field for both materials.

The longitudinal spin Seebeck effect (LSSE), in which heat current is parallel to spin propagation, is well established in ferromagnet/normal metal interfaces [1] such as YIG/Pt. We investigate the temperature dependence of LSSE with an intermediate antiferromagnetic layer, YIG/NiO/Pt. We also measure temperature dependence of the spin Hall magnetoresistance (SHMR, ref [2]) of both the YIG/Pt and the YIG/NiO/Pt systems. The temperature dependences of the two effects are compared and interpreted in terms of the temperature dependence of the magnon density of states.

Thermoelectric materials provide a viable means of converting heat directly to electricity especially for waste heat recovery. In 2012, the U.S. Department of Energy estimated that out of 95.1 Quads of energy generated, nearly 58.1 Quads of energy are released as waste heat [1]. This equates to nearly 1.57 Quads of wasted thermal energy for every Quad of usable energy. By recovering even a small fraction of this waste heat through thermoelectric based waste heat recovery, a considerable amount of energy savings can be realized. The limitation of current thermoelectric materials is their meager efficiency. In providing a solution, the following research investigated a new approach to thermoelectric materials through the application of spin-based transport. For this study, a computational approach has been developed to investigate the influence of substitutional doping on the spin Seebeck coefficient in cobalt base spinel oxides.
The thermoelectric figure of merit, ZT, which quantifies the performance of a thermoelectric is hampered due to the limiting relationship of the thermal and electrical conductivity. One of the most recent approaches to circumvent this limitation is to leverage on spin based transport in a spin-polarized material. When a temperature gradient is applied across a spin-polarized material, a charge flux is accompanied with a spin flux. In addition to the charge Seebeck coefficient of the material, accumulation of a specific spin orientation at one end of the material can contribute to the spin Seebeck coefficient. Tailoring the material configuration for optimal spin polarization, creating a difference between the spin-up and spin-down populations can result in significant increase in the spin Seebeck coefficient.
A combination of spin-polarized density functional theory (DFT) calculations were conducted along with one-dimensional non-equilibrium Green&’s function (NEGF) calculations. The spin-polarized DFT calculation is used to provide an estimate of the critical parameter used in the NEGF transport model. Preliminary findings indicated that when Co⁺² or Co⁺³ ions in cobalt spinel material are substituted with transition metal ions like Ni⁺², a resultant net spin-polarization is induced to the spinel lattice. Preferential substitution can improve the spin Seebeck coefficient, which in some cases is reported to be greater than charge Seebeck coefficient. Ultimately this research provides a scope for improving ZT by combining spin Seebeck and charge Seebeck coefficient.
[1] Monthly Energy Review May 2013, DOE/EIA-0035(2013/05). Washington D.C. U.S. Department of Energy, Energy Information Administration, 2013.

The interaction between heat and spin currents has been a very active field of study and highly boosted recently since the discovery of the Spin Seebeck effect (SSE). The SSE refers to the generation of spin voltage as a result of a temperature gradient in magnetic materials. In the case of Fe3O4/Pt system, the spin current is thought to arise from the magnon flow across the thickness of the magnetite film and detected by means of the inverse spin Hall effect in a platinum film grown on top. In the longitudinal SSE configuration, the anomalous Nernst effect (ANE) can contaminate the resulting signal; therefore, a systematic study of the ANE is necessary in order to fully understand the observed voltage.
Magnetite epitaxial thin films with different thickness were deposited on a magnesium oxide substrate by means of pulsed laser deposition in an ultrahigh-vacuum chamber. The quality and properties of the films were checked by X-ray diffraction, X-ray reflectivity, magnetization and resistivity vs temperature and transmission electron microscopy. Anomalous Nernst effect and SSE measurements were performed to careful study the characteristic dependence of thermomagnetic signal on the thickness of the films. The experimental data is in agreement with previously reported models [1,2] where the bulk spin current is calculated macroscopically with the Boltzmann equation for the magnon flow. Following this model the estimation of the magnon diffusion length in magnetite thin films is 40 ± 10 nm.
References
[1] S. M. Rezende et al. Magnon spin-current theory for the longitudinal Spin Seebeck effect. Physical Review B, 89(1):014416, January 2014.
[2] Steven S L Zhang and Shufeng Zhang. Magnon mediated electric current drag across a ferromagnetic insulator layer. Physical Review Letters, 109(August):1-5, 2012.

Herein, we report on syntheses and characterization of new thermoelectric composites from exfoliated Bi2(Te,Se)₃ nanoplatelets by chemical methods. First, we performed a scalable synthesis of surfactant-free Bi₂Te_2.7Se_0.3 (BTS) nanocrystals by chemical exfoliation and subsequent spark plasma sintering to fabricate nanostructured thermoelectric bulk materials. The exfoliated n-type BTS nanoplatelets were shown to transform into nanoscrolltype crystals (5 nm in diameter, 50 nm in length) by ultrasonication. The thermoelectric performance of the BTS nanocrystals was found to be recoverable by minimizing surface oxides by chemical reduction of the exfoliated suspensions. Nanostructured bulk materials, composed of plate-like grains with 50 nm thickness, were prepared by sintering of the ultrasonicated sample using a spark plasma sintering technique. The resulting compound showed drastic reduction of lattice thermal conductivity (0.31 W/mK @ 400 K) due to enhanced phonon scattering at highly dense grain boundaries without deterioration of the power factor (21.0 X 10^-4 W/mK² @ 400 K). The peak ZT value of the present compound (0.8 @400 K) is comparable to that of n-type single crystalline Bi2(Te,Se)3, which is one of the highest among the reported values for n-type materials synthesized by a soft chemical route.(J. Mater. Chem. A, 2013, 1,12791)
Using the exfoliated BTS materials, nanocomposites of n-type thermoelectric BTS and unoxidized graphene (UG) were prepared from Polycrystalline BTS were exfoliated into nanoscoll-type crystals by chemical exfoliation, and were re-assembled with UG nanoplatelets. The composites were chemically reduced by hydrazine hydrate and sintered by a spark-plasma-sintering method. The thermoelectric properties of the sintered composites were evaluated and exhibited decreased carrier concentration and increased thermal conductivity due to the embedded graphene. The peak ZT values for the UG/BTS-US and UG/BTS-EX composites were ~0.8 at the UG concentration of 0.05 wt%.(P hys. Status Solidi RRL, 1-5 (2014)) Based on exfoliation/re-assembly strategy, we suggest a simple and scalable synthesis to prepare Cu-Bi₂Te_2.7Se_0.3 (Cu-BTS) nanocomposites. By precipitating Cu nanoparticle (NP) in colloidal suspension of as-exfoliated BTS, homogeneous mixtures of Cu NP and BTS nanosheet were readily achieved, and then the sintered nanocomposites were fabricated by spark plasma sintering technique using the mixed powder as a raw material. The precipitated Cu NPs in the BTS matrix effectively generated nanograin (BTS) and heterointerface (Cu/BTS) structures. The maximum ZT of 0.90 at 400 K, which is 15% higher comparison to that of pristine BTS, was obtained in 3 vol% Cu-BTS nanocomposite. The enhancement of ZT resulted from improved power factor by carrier filtering effect due to the Cu nanoprecipitates in the BTS matrix.

Novel green microcapsulated phase change materials n-ocadecane and peppermint fragrance scent as a solvent has been prepared through interfacial polymerization of isophorone diisocyanate (IPDI) and hexamethylenediamine (HMDA). The size distribution, morphology and chemical structures of the microcapsules were characterized by laser particle size analyzer, scanning electron microscope (SEM) and fourier transform infrared (FTIR), respectively. The thermal properties of the microcapsules were studied by differential scanning calorimetry (DSC) and thermal gravity analysis (TGA). The results show that the mean size of The MicroPCMs (microcapsulated phase change materials) was in the range of 4.0 to 7.0mu;m and decreased with increasing emulsifier. The MicroPCMs are spherical and their surface is rough. The MicroPCMs without nucleating agents demonstrated high heat storage capacity, but had supercooling phenomena. Through the addition of approximately 8.3 wt% nucleating agents 1-tetradecanol or paraffin into core materials of the MicroPCMs can suppress the microcapsules from supercooling. The encapsulated nucleation agents can enhance the thermal stability of MicroPCMs.

Due to their high ZT values filled cobalt-antimony based skutterudites have proven themselves as very promising thermoelectric materials for generator applications in an intermediate temperature range between 400 and 800 K. For applications, besides the functional thermoelectric properties also the skutterudites&’ mechanical properties play an important role to withstand external mechanical and internal thermomechanical loads during operation. Properties of interest are hardness as well as fracture toughness and resistance to fatigue.
Introducing carbon nano tubes (CNTs) into a host matrix is a promising approach to improve a host material's mechanical stability. The main issues here are achieving a homogeneous distribution of the CNTs and good adhesion between CNT and matrix material. In this work we present the influence of the introduction of multi-walled CNTs on the thermoelectric and mechanical properties of p-type skutterudites. The influence of different CNT concentrations and preparation routes on the resulting composite material's thermoelectric, mechanical and microstructural properties is studied.

Organic semiconducting polymers exhibit low thermal conductivities while maintaining electrical conductivities on the order of that of lightly doped silicon, suggesting they may be potential candidates for thermoelectric power generation and refrigeration. Highly conductive PEDOT films were prepared by solution casting polymerization using finely tuned oxidation solution. Thermoelectric (TE) properties of these PEDOT thin films were systematically investigated with different organic additives. These films exhibited Seebeck coefficient of 52 µV/K and thermal conductivity of 0.47 W m^-1 K^-1 and could be processed as flexible thermoelectric films to generate measureable electrical currents of microamperes at temperatures differences of 13-14 Kelvin which corresponds to the difference between body temperature and an ambient temperature of 25 °C. Experimental details and characterization of the fabricated TE thin films will be presented.

The various wearable devices for real time monitoring of vital information are researched, and part are being put to practice use. For some of those wearable devices, we try to install the thermoelectric (TE) devices as a charge-free power supply. The Mg₂X system (X=Sn,Ge,Si) and related solid solution material are one of the most suitable material for TE applications because of the abundance of constituents, non-toxicity, low density (lightness) and biological safety. In this study, we especially focused on the Mg₂Sn material to be able to expect a high TE characteristic shown in Room Temperature ( RT ) range, and carried out the research of the thin film process to fabricate the TE device on the glass substrate or other flexible substrate. So far in our study, it was a issue that the Mg₂Sn thin film was formed with co-existence of the cubic crystal and hexagonal crystal by the conventional sputtering process. However, the issue was solved by the following thin film process. That is, we have the single phase ( cubic crystal ) Mg₂Sn thin film with the conventional magnetron sputtering process at 550#8451; on the glass substrate followed by the deposition of very thin metal layer of Ag or In at RT with the sputtering which we introduce to try to control the conduction type ; Ag as a p-type dopant and In as a n-type dopant. The thickness of the Ag and In film was less than several 10 nm and the sintered Mg₂Sn compound target was used in this sputtering process. We confirmed the single phase of the Mg₂Sn films with XRD measurement, and the almost uniform distribution of the dopants (Ag, In) in the Mg₂Sn films with the depth profile measurements of XPS. We speculate that the existence of Ag or In metal thin layer on the glass substrate controls the stacking of the film growth in the initial stage and then the dopants ( Ag or In ) are diffused into the film during the following growth process. All the deposited thin films show p-type conduction even in the films doped In. Electrical conductivity ( σ ) is increased with the doping of Ag. In the case of the Mg₂Sn film doped 5 atomic% of Ag, ten times higher compared with undoped one. Despite the prediction from the theoretical point of view, Seebeck coefficient ( S ) is slightly increase on the Mg₂Sn film with the 5 atomic% content of Ag, compared with the undoed one. Power Factor ( = S² σ ) of the Ag doped film shows the 2x10^-4 W/K²/m. Further improvements will be expected by increasing the amount of Ag doping to 10 atomic% with tuning of the Mg/Sn stoichiometry.

Mg₂Si is an abundant, low cost, and nontoxic material considered to be a promising candidate for thermoelectric applications. Here, we report the use of radio frequency (RF) magnetron sputtering to deposit Mg₂Si thin films from a Mg₂Si target at different substrate temperatures, from room temperature (RT) to 300 °C. Furthermore, Sn or Al were incorporated into the thin films by co-sputtering Sn or Al from a second target at substrate temperature of 200 °C. The nominal doping concentration of both elements were 1 at%, 2 at%, 5 at% and 10 at%. By controlling the deposition time from the 2^nd target, the dopant concentration is varied. X-ray diffraction (XRD) and selected area electron diffraction (SAD) are used to identify the thin film phase. Only the thin film Mg₂Si deposited at RT is found to be amorphous, Mg₂Si films deposited at higher temperatures are polycrystalline. The charge carrier concentration and electrical resistivity of undoped and doped Mg₂Si thin films are characterized using Hall measurements using a Hall-bar structure. The hall measurement results suggest that when the amount of Sn dopant increases, the carrier concentration decreases; while in the case of Al dopant, the charge carrier concentration increases with the amount of Al dopant. Rutherford backscattering spectrometry (RBS) of Sn doped Mg₂Si films show the Sn dopant is uniformly distributed through the film and the Sn doping concentration is 0.4 at%, 0.8 at%, 2.0 at%, and 4.1 at%. In addition, the cross bridge kelvin resistor (CBKR) method was used to measure specific contact resistance of sputtered Ti, Ni or Hf contacts on undoped thin film Mg₂Si, which was on the order of low 10^-5 Omega;-cm². The post-annealing impact on changing the specific contact resistance will also be discussed.
This project is partially supported by the II-VI foundation.

Rotatory cement kilns are wide used for the cement industry, into this kind of kilns occurs chemicals reactions while specific raw material is moving through the Kiln, temperature and time are two important parameters to control thus the type and quality of materials used to formulate the cement. The energy source is the combustion gases obtained by the mineral carbon combustion traveled counter flow.
In this study an uniform and homogeneous material bed was considered, this bed was divided into small volumes as “control volumes” which were analyzed as plug flow reactors interconnected in series; for each one of the plug flow reactors equations for conservation and the chemical reaction kinetics are applied.
This work present the results of the parameters estimated such as reaction kinetic parameters, activation energy and pre-exponential factor and behavioral profile of the major chemical species in the Portland cement clinker were. A furnace capable of producing 71 tons/hour and kinetics parameters from the literature as baseline information to the numerical tool were considered thus as historical data of the industrial process.

Electronic properties of transition-metal dichalcogenides (TMDs) (MX₂, where M = Mo, W and X = S, Se) are very sensitive to the applied pressure/strain, causing a semiconductor to metal transition. Using first principles density functional theory calculations, we demonstrate that bulk TiS₂ changes from semi-metal to semi-conducting electronic phase upon application of uniform biaxial strain[1]. This phase transition is responsible for the charge transfer from Ti to S and reduces the overlap between Ti-(d) and S-(p) orbitals. The transport calculations show a three-fold enhancement in thermopower for both p- and n-type TiS₂ due to opening of band gap along with changes in bands dispersion. The electrical conductivity and thermopower shows a large anisotropy due to the difference in the effective masses along the in-plane and out-of-plane directions. We further demonstrate that the enhancement of thermoelectric performance, can also be achieved by doping TiS₂ with larger iso-electronic elements such as Zr or Hf at the Ti sites.
1. Atanu Samanta, Tribhuwan Pandey, and Abhishek K. Singh Phys. Rev. B 90,174301 (2014)

The environmental impact resulting from the use of fossil fuel as an energy source affects the entire globe. Eventually, fossil fuels will no longer be a reasonable source of energy and alternative energy sources will be needed. Thermoelectric materials (TE) that directly convert heat into electricity are a viable option to replace the conventional fossil fuel because they are reliable, cost effective, and use no moving parts. Recently researchers discovered the existence of giant Seebeck coefficient in manganese oxide (MnO₂) powders, which ignited an increased interest in MnO₂-based materials. Now, very little it is known about the Seebeck coefficient of such material in thin film form. In this work we present a systematic structural and thermoelectric characterization of amorphous and crystalline MnO₂ thin films. These films were deposited at room temperature on silicon and sapphire substrates by DC Magnetron Sputtering. The thin films have an average thickness of ~300 nm, and they show amorphous crystalline behavior. Subsequent ex-situ annealing at temperatures of 300⁰ C, 500⁰ C, and 700⁰ C show interesting crystallization behavior. Preliminary Seebeck coefficients will also be presented.

Minority carrier generation at higher temperatures is detrimental to thermoelectric generator (TEG) efficiency as minority carriers (electrons in p-type leg and holes in n-type leg) recombine reducing output power. Significant efficiency (Power/Heat Flux) enhancement could be achieved if minority carriers are extracted from their respective TEG leg (reducing recombination) and then transported to the corresponding majority carrier leg. Synopsys Sentaurus software is used perform 2-D axial symmetric simulations of microscale thermoelectric generators composed of Si_xGe_1-x where efficiency is determined for temperature gradients ranging from 0 to 1200 K.
G. Bakan, N. Khan, H. Silva, A. Gokirmak, “High-temperature thermoelectric transport at small scales: generation, transport and recombination of minority carriers,” Scientific Reports 3, 2724, doi: 10.1038/srep02724 (2013).

Recently, materials with a smaller atom in an oversized cage-like structure such as filled skutterudites, clathrate compounds, and β-pyrochlore type oxide AB₂O₆, are interested in as a promising candidate for thermoelectric materials. An anharmonic thermal vibration called the “rattling” motion of the caged atom in these materials is believed to scatter phonons efficiently. We have reported that the unconventional relation between the size of the A cations and the thermal diffusivity for defect pyrochlore oxides ATaWO₆ (A = K, Rb, Cs), which suggested enhanced phonon scattering by the rattling behavior. Unfortunately, these oxides were insulating. In this work, we investigated the crystal structure and the thermoelectric properties of defect pyrochlore oxides AFe_0.33W_1.67O₆ (A = K, Rb, Cs) with better electrical properties than those of ATaWO₆.
AFe_0.33W_1.67O₆ (A = K, Rb, Cs) (hereafter denoted by KFW, RFW, CFW) were prepared by using solid state reaction. The samples were characterized by powder XRD study with the Rietveld refinement, the Raman spectroscopy, and their thermoelectric properties were evaluated.
AFeWO₆ with smaller cations within the cage-like space indicated larger displacement from the center position of the cage with the larger isotropic displacement parameter. A bulk sample of KFW was unable to obtain because the sintered pellet cracked, and finally broke down due to the moisture in air. The Raman spectra of the samples were found to have the peaks implying the translational modes of the A-site and B-site cations, and the stretching and bending modes of the BO₆ octahedra. The translational modes of the A-site cation can be considered as “rattling mode”. These results suggested the rattling motion in the defect pyrochlore oxides.
The thermal conductivity showed the order of CFW < RFW over the whole temperature range, unlike ATaWO₆ (A = K, Rb, Cs). However, their values were below 1.0 W/m K, being extremely low among oxides, and lower than that of ATaWO₆ (A = Rb, Cs).
The electrical conductivity of both CFW and RFW was of the order of 10^-4 #822; 10^-3 S/cm, being more than 2 orders higher than that of ATaWO₆. Their Seebeck coefficient were -500 #822; -600 mu;V/m K. The power factor, S²σ, was 2.0 × 10^-7 W/m K for CFW, and 2.2 × 10^-7 W/m K for RFW at 800 #730;C, resulting in the dimensionless figure of merit, ZT, of 2.6 × 10^-4.
These results suggested a probability of independent control of thermal and electrical conductions in β-pyrochlore-type oxides.

Polydopamine (PDA) has attracted much attention in recent years because of its unresolved structures and unique properties such as multiple functional groups, strong adhesion to various surfaces, and self-assembling polymerization under mild conditions. Researches also suggested that carbonized PDA (cPDA) might have a graphene-like structure, which makes it potential a good thermal and electrical conducting material. In our research, we aim to characterize its thermoelectric properties including Seebeck coefficient and electrical conductivity. PDA films are deposited on dielectric substrates; and then heat-treated at various temperatures under inner conditions for carbonization. Compared to the pristine PDA, cPDA films showed dramatic increase in electric conductivity. Seebeck effect was also detected after carbonization. Various characterization techniques were used to study the structures including SEM, TEM, Raman spectroscopy, EDS, XRD, etc., which helped to further our understanding of the structural and chemistry evolution during the carbonization process.

Seebeck effect is widely used for the energy harvesting of the waste heat. In the Seebeck effect, electric power can be generated by the temperature difference between both ends of the thermoelectric materials. However, a low conversion efficiency is caused by heat flux from hot side to cold side of sample. In this paper, we have proposed a new thermal power generation mechanism with no temperature difference. We investigated the band structure of Ba₈Au_xSi_46-x clathrate single crystal synthesized by Czochralski method. The single crystal has gradient of the gold contents along the pulling direction. According to the results of Seebeck coefficient and ab initio calculation, the electrical properties of the Ba₈Au_xSi_46-x clathrate dramatically changed depending on the gold contents. In the case of gold content of lower than 5.33, the Ba₈Au_xSi_46-x clathrate showed a n-type semiconductor. In the case of gold content of higher than 5.33, the Ba8AuxSi46-x clathrate showed a p-type semiconductor. The band gap of the n-type and p-type Ba₈Au_xSi_46-x clathrate were wider than the intrinsic semiconductor. We can successfully synthesize a n-p junction single crystal with the energy band curve generated from the deference of Fermi level between p- and n- type semiconductors. The single crystal was heated under the uniform temperature and we can obtain generated electric voltage of around 0.6 mV at 400#8451;. These results suggested that the obtained electric voltage can be generated from the separation of hole-electron pair excited by heat at the intrinsic part with a narrow band gap along to the energy band curve formed by p-n junction.

Mg₂Si is a promising eco-friendly thermoelectric material because its constituent elements exist in abundance on the earth as non-toxic materials. Improvement in the thermoelectric performance is required for the Mg₂Si-based compounds to use for waste heat energy harvesting. In our previous work[1], we have reported that addition of Al lowered the electrical resistivity drastically and optimum amount of Al improved the thermoelectric performance of Mg₂Si synthesized with a liquid-solid phase reaction method. We also have reported that the Al-doped Mg₂Si compound had the resistance to deterioration in thermoelectric performance of during air exposure with temperature difference[2]. In this study, we attempted to reduce the thermal conductivity by grain refinement for further improvement of the thermoelectric performance. We adopted three kinds of process for grain refinement of the Al-doped Mg₂Si. Two different types of fine Si powders, i.e. the commercial fine powder (lE;5 mu;#65357;) and the powder prepared by milling the commercial coarse powder (lE;75 mu;#65357;) using a planetary ball mill in an argon atmosphere, were prepared and mixed with Mg powder (67 at.%) and Al powder (0.1 at.%) in an argon atmosphere. The Al-doped Mg₂Si compounds were synthesized from these powder mixtures by the liquid-solid phase reaction method in an argon gas flow, and then lightly pulverized in a agate mortar by hand in an argon atmosphere. In the third process, the compound synthesized using the commercial coarse Si powder was milled using the planetary ball mill in an argon atmosphere. By including the Al-doped Mg₂Si powder synthesized using the commercial coarse Si powder for comparison, four kinds of prepared powders were sintered by pulse discharge sintering method under same sintering conditions. All samples prepared via three kinds of process for grain refinement had the microstructure with small grain size. And the samples increased the electrical resistivity, had almost same Seebeck coefficient, and reduced the thermal conductivity. Especially, the sintered sample made from ground Mg₂Si powder was effective for grain refinement and for reduction of thermal conductivity, and had the best thermoelectric performance of ZT=1.15 at 873 K.
[1] T. Itoh, A. Tominaga, T. Jinushi, Z. Ishijima, J. Jpn. Soc. Powder and Powder Metallurgy, 61 (6), 324-328 (2014).
[2] T. Itoh, A. Tominaga, Mater. Res. Soc. Symp. Proc. Vol. 1642, DOI: 10.1557/opl.2014.446 (2014).

Pyroelectric energy conversion has experienced resurgence in interest, largely due to the recent realization of high energy conversion densities from thin-film materials. In this work, we describe the optimization of chemical solution-deposited PZT thin-film systems with properties tuned specifically for pyroelectric applications. A state-of-the-art energy conversion density of 4.28 J/cm³ has been calculated from the measured polarization of this material between 95-140 ^oC under applied electric fields of 100-700 kV/cm. We discuss the validity calculating the conversion density from measured polarizations and compare these calculations to an alternative approach based on combing classical thermodynamics with ferroelectric phenomenology. We also present an alternative measurement technique to approximate the pyroelectric energy conversion cycle that is less sensitive to parasitic effects, such as leakage current, than other techniques.
The pyroelectric effect provides an efficient thermal energy conversion mechanism over a range of operating temperatures. Using synchronous thermal and electric fields, pyroelectric capacitors are charged then thermally discharged around a thermodynamic cycle. The net electrical work is calculated from the area created between two temperature dependent polarizations and two applied electric fields. In our work, we focus on increasing the dielectric breakdown strength of the materials to accommodate for higher fields. For high power, these cycles need to be performed at a high rate. Our solution is to use thin films with low thermal mass and pulsed infrared radiation which gives both high rate and long distance power transfer capabilities. We use a 1.5 µm diode laser source and backside cooling to provide the thermal oscillations. The full demonstration of this concept would use pulsed laser power and thin films to accomplish fast temperature cycling combined with high energy conversion efficiency.

Higher manganese silicides (HMS) have potential as thermoelectric materials. The aim of this study was to prepare single-phase HMS compact by only reactive sintering. The study optimized the sintering conditions and particle sizes of the Mn and Si powders. Particle size of Si was larger than that of Mn, and compounds were sequentially synthesized in the Siminus;Mn binary phase diagram.
The Si powder (average particle sizes of 5, 75, or 150 mu;m) and Mn powder (average particle sizes of 1.5, 10, or 75 mu;m) were mixed in a 1:1.75 mole ratio, and a sintered compact was prepared using a pulse electric current sintering apparatus in vacuum (below 6.0 × 10^-3 Pa) or under 50 MPa at 1173minus;1298 K for 0.3minus;18 ks.
First, the polished surface of the sintered compact cross-section was corroded using an etching solution (HF:HNO₃:H₂O₂ = 1:6:13 volume ratio), and the etched surface was observed using an optical microscope. The MnSi phase surrounded by etch pits, Si, and HMS phases could be differentiated from the analytical results obtained using scanning electron microscopy/energy dispersion X-ray spectroscopy.
Using XRD patterns of the product, the optimum conditions for sintering temperature, sintering time, and particle sizes of starting materials were obtained. When Si powder (average size of 75 mu;m) was mixed with Mn powder (average size of 75 mu;m) for 3.6 ks and various sintering temperatures, the peak intensities of MnSi and Si decreased, but that of HMS increased as the sintering temperature increased to 1273 K. When the sintering temperature was fixed at 1273 K and the sintering time was varied, the peak intensities of MnSi and Si showed a tendency to decrease as the sintering time increased.
Next, the sintering condition was fixed at 1273 K for 3.6 ks, and the particle sizes of the starting materials were varied. Optical microscopy of the etched surface revealed that MnSi phase decreased as particle size of Mn powder decreased in the HMS matrix. When Mn powder (average particle size of 1.5 mu;m, which is the smallest used in the study) was mixed with Si powder (average particle size of 75 mu;m), fine residues of Si were observed on the sintered compact, which were identified as single-phase HMS using XRD. When Mn powder (average particle size of 75 mu;m) was mixed with Si powder (average particle size of 5 mu;m), optical microscopy and XRD patterns confirmed the formation of MnSi phase.
Finally, Mn powder (average particle size of 1.5 mu;m) was mixed with Si powder (average particle size of 75 mu;m), and a sintered compact with a relative density of 92.3% was prepared. The dimensionless performance index (ZT) value of the sintered compact was 0.33 at 813 K. When the combination ratio of Si and Mn powders is finely adjusted in order to reduce the number of Si residues, and when a sintered compact, which was prepared by a primary sintering, is crushed and secondary sintering is performed in order to improve the relative density, the ZT value is expected to improve.

To store and transport hydrogen fuel, it is effective to liquefy hydrogen; therefore, a magnetic refrigerant material with a large specific heat near the liquid hydrogen temperature is required. Near the liquid helium temperature, medium and heavy rare-earth compounds exhibiting large specific heats due to magnetic phase transitions and possessing large total angular momentum quantum numbers are at the level practically required of magnetic refrigerant materials. However, Er₃Ni and HoCo₂ have minimum specific heat values near the liquid hydrogen temperature, and oxysulfides that contain heavy rare-earth elements and possess specific heat peaks have small specific heats near the liquid hydrogen temperature.
Since rare-earth monosulfides are expected to form metallic bonds due to the unpaired electrons in their 5d orbitals and possess high conductivity, our group synthesized non-stoichiometric SmS and elucidated its potential use as a thermoelectric material.
Recently, we discovered that polycrystalline EuS, which also contains a bivalent cation (Eu²⁺), has a large specific heat peak (0.7 Jmiddot;K^-1middot;cm^-3) near 15 K, which is the liquid hydrogen temperature. The present study investigated the following: valence-fluctuation elements of Eu, Yb, and Sm whose valences fluctuate easily due to the temperatures; synthesis of sintered compacts from their single-phase powders; and the specific heat characteristics of the synthesized sintered compacts at very low temperatures.
In many cases, to synthesize rare-earth monosulfides, rare-earth sesquisulfides containing trivalent rare-earth elements are first synthesized from rare-earth oxides, and rare-earth monosulfides are synthesized by reductive reaction from rare-earth metals. In this study, EuS, which was revealed by X-ray diffraction to be single phase, could be obtained only by subjecting Eu₂O₃ powder to CS₂ gas sulfurization. When the sulfurization temperature was above 1073 K, single-phase EuS could be obtained regardless of the sulfurization time. When the sulfurization temperature was 773 K, Eu₃S₄ was obtained. However, by performing heat treatment on the obtained Eu₃S₄, single-phase EuS could be obtained in a CS₂/Ar atmosphere at 973 K or higher, in an Ar atmosphere at 1073 K or higher, and in a vacuum at 873 K or higher. In the case of YbS, Yb₂O₃ powder consisting of particles with small diameters and large specific surfaces was used. Single-phase eta;-Yb₂S₃ was first synthesized at sulfurization temperatures between 923 K and 1073 K. By performing heat treatment of the synthesized single-phase eta;-Yb₂S₃ in a vacuum at 1773 K, single-phase YbS could be synthesized. In the case of SmS, Sm₂S₃ was synthesized by subjecting Sm₂O₃ powder to CS₂ gas sulfurization at or above 1073 K. Subsequently, Sm metal or SmH₃ powder was added to the synthesized Sm₂S₃, and the mixture was heat-treated in a vacuum glove box at 1273 K to obtain single-phase SmS.

An ideal thermoelectric material would possess a large S, high σ, and low κ. Recently, magnesium silicide (Mg₂Si) has attracted much interest as an n-type thermoelectric material because it is eco-friendly, non-toxic, light, and relatively abundant compared with other thermoelectric materials. Al, Bi, Sb, etc. doped Mg₂Si has been reported. However, in many reports, carrier concentration did not agree with the amount of additional dopants. A part of dopants is thought to be localized at the grain boundary, and carriers are generated for localized Mg²⁺ in 4b site.
In this study, we tried to improve thermoelectric performance by doping Ge into the Sb doped Mg₂Si to cause phonon scattering and optimize carrier concentration. A high purity Mg₂Si was synthesized from metal Mg and Sb doped Si-Ge alloy by using spark plasma sintering (SPS) equipment. A bulk of Sb doped Si-Ge alloy was fabricated by using an arc-melting method under vacuum conditions. The bulk alloy was crushed and sieved in a dry box filled with Ar gas. The alloy powder and metal Mg were mixed under inert gas conditions. The mixture was heated in an Ar atmosphere by using the SPS equipment. They were evaluated by using X-ray diffraction and X-ray fluorescence analyses. The carrier concentration of the samples was measured by using Hall measurement equipment. The electrical conductivity and Seebeck coefficient were measured by using a standard four-probe method in a He atmosphere. The thermal conductivity was measured by using a laser-flash system.
Electrical conductivity of the Sb doped Mg₂Si were increased, and the Seebeck coefficient and thermal conductivity of the samples were decreased by the addition of Ge. Regardless of the presence or absence of Ge doping, the thermal conductivity of the samples increased with increasing Sb concentration because electrical conductivity increased with increasing carrier concentration caused by the addition of Sb. Conversely, thermal conductivity decreased significantly by the addition of Ge. It seems that is due to a decrease in the lattice conductivity caused by phonon scattering and a decrease in electrical conductivity caused by a decrease in mobility. From these results, we believe that Ge doping and high Sb doping contribute to decreasing the lattice conductivity. The Ge doped samples had a higher ZT value than the Sb doped sample of the same concentration without Ge because the thermal conductivity of the former was lower. Furthermore, carrier concentration could be controlled, and the Sb solubility limit could be increased up to 0.85% by doping 5at% Ge into Mg₂Si. Considering this result, it seems possible to suppress the internal Mg from entering the 4b site with doped Ge.

We present our results on phonon- and electron transport across nanostructured interfaces and the resulting impact on the thermoelectric properties. For the electron transport we focus our investigations on the energy filtering at grain boundaries [1]. Our results are based on a model that we developed to describe electron transport in nanograined materials. The grain boundaries are described using the model introduced by Seto [2]. It is believed that such barriers can increase the efficiency of thermoelectric materials by energy filtering effects. We conclude that electrostatic barriers play no role for thermoelectric devices. For the phonon transport we use an atomistic Greens function method to investigate the phonon scattering Si isotope-multilayer. Our calculations [3] show that a periodic arrangement of the layer-system cannot decrease the phonon thermal conductivity substantially, whereas a random arrangement of the layer-system can lead to a strong decrease in the phonon conductivity. We also show that small deviations from the periodic arrangement are enough to end up in the random regime.
[1] M.Bachmann, M. Czener and C.Heiliger, Phys. Rev. B 86, 115320
[2] J. Seto, J.Appl. Phys. 46 5247 (1975)
[3] M Bachmann et al 2014 Semicond. Sci. Technol. 29 12400

The alloys (GeTe)_X(AgSbTe₂)_1-X (commonly known as TAGS) are among the best high-temperature thermoelectric materials, largely due to their low thermal conductivity. Here we focus on TAGS-85, which is reported to have a transition from a rhombohedral phase to a cubic phase above ~200 °C. We have found a way to stabilize both of these polymorphs at room temperature by careful adjustment of the synthesis conditions. We study the structural properties of both rhombohedral and cubic TAGS-85 as a function of thermal cycling and reveal a much more complex behavior than previously reported, involving transformations to novel layered structures. The thermoelectric properties of both polymorphs will be discussed with respect to their electrical conductivity, Seebeck coefficient and thermal conductivity.

Effective methods of harvesting, storing and utilizing sustainable energy sources continue to be an important global issue. This has spun a global effort to develop new technologies to mitigate the challenges in developing materials and devices for energy generation that are environmentally benign. In this respect, much effort has focused on transforming thermal energy into electrical energy using solid state thermoelectric generators which generally operate much better at temperatures > 200^oC. However, much of the abundant thermal energy from natural sources (geothermal and solar heating) and many human activities (e.g., industrial waste streams and automobiles) of temperatures < 200^oC are mostly untapped and are left to go waste. To efficiently harvest such low-grade heat as electrical energy, thermocells may become attractive candidates, given their simple design, direct thermal-to-electric energy conversion, continuous operation, low expected maintenance, and zero carbon emissions. Thus far, the highest power conversion efficiency (PCE) of 1.4% relative to the Carnot efficiency has been reported for 0.4 M K₃Fe (CN)₆/K₄Fe(CN)₆ aqueous solution with bucky paper of multi-wall carbon nanotubes as electrodes. We report the use of novel techniques to nanoengineer a solid-state electrolyte composite that pushes the efficiency high that the current state-of-the-art.

Bulk thermoelectric materials have attracted great interest for several decades in the field of energy conversion technology due to their potential in the conversion of waste heat to electrical power. Lead chalcogenide compounds are narrow gap semiconductors which show high performance at room temperature and above, especially in the range 600-850 K. They exhibit good electrical transport properties and low thermal conductivities at high temperature, which is unusual for materials with simple structures (lead chalcogenides adopt the rocksalt structure). Doping with 4f electrons can introduce resonant states just below the Fermi level and enhance the thermopower and hence the thermoelectric figure of merit. Here we investigate the influence of Ce-doping on the thermoelectric properties of PbSe.
We have synthesized Pb_1-xCe_xSe (x=0.00, 0.01, 0.02, 0.03 and 0.05) and examined their structural, magnetic and thermoelectric properties.

The ability to convert excess waste heat into useable energy can significantly help meet the global energy demands. One may capture this waste heat through thermoelectrics devices. In a thermoelectric material, the charge carriers transport both electrical current and heat. Consequently, under a temperature difference (ΔT), a carrier concentration gradient results in a voltage (ΔV), which is related to the Seebeck coefficient, α ~ ΔV/ΔT. One of the challenges lies in finding materials that simultaneously have low thermal conductivity (k), high electrical conductivity (σ), and high Seebeck coefficient (α). Conjugated semiconducting polymers can potentially meet this demand due to their inherent low thermal conductivity and high electrical conductivity through sufficient doping. Here, we report on the critical role of thermal processing and substrate treatments on the enhancement of thermoelectric properties of conjugated polymer thin films. These films were doping using three different mechanisms: acid (toluene sulfonic acid), charge transfer (F₄TCNQ), and vapor (fluorinated-alkyl trichlorosilane). To further elucidate the charge transport mechanism driving the thermoelectric performance, we report on the temperature-dependent measurements of both the Seebeck coefficient and electrical conductivity. Lastly, these thermoelectrics properties will be correlated to the structural and morphological properties of the doped thin-films through various synchrotron X-ray scattering techniques.

To reduce our dependence on fossil fuels and to reduce greenhouse gas emissions, it is important that we improve our energy efficiency. Thermoelectric (TE) technology, which can convert waste heat directly into electricity, is one of the more reliable technologies available. Magnesium silicide (Mg₂Si) has been identified as a promising advanced thermoelectric material and it has some important attributes in that it is lightweight, there is a worldwide abundance of its constituent elements, and it is non-toxic. Moreover, since it has good power generation performance in the mid-temperature (~900K) range, it is expected that it can be applied in the automotive industry or in industrial furnaces.
For practical TE power modules, it is essential that the output power is improved. In particular, for modules consisting of many TE chips, improving the power output of each chip is important. These TE chips are connected together using electrodes deposited on the Mg₂Si. Thus, reducing the resistance of the electrode itself and/or the contact resistance are major issues. Moreover, there are concerns with regard to deterioration or damage to the chips. Therefore, selecting an appropriate electrode material and developing a stable process for joining the chips together are essential in order to obtain high power output and maintain long-term reliability. We took two approaches to the formation of the electrodes. Our evaluation of these electrodes was based on (1) the contact resistance, (2) the linear expansion coefficient, and (3) the durability at elevated temperatures.
A standard process for forming electrodes is electroless Ni plating. Improving the process conditions resulted in good adhesion and low contact resistance (~1x10^-8 Omega;m²). However, the durability at operating temperatures of 873 K was not good. An alternative process is monoblock sintering in which the electrodes are formed during Mg₂Si sintering. By using Ni electrodes formed by monoblock sintering we lowered the contact resistance to ~3x10^-9 Omega;m². We then tried to reduce the contact resistance further by using transition metal silicides such as CoSi₂, CrSi₂, NiSi, and TiSi₂. Reduced contact resistance was obtained when some of these metal silicides were placed between the Mg₂Si and the Ni electrode (<1x10^-9 Omega;m²). However, TE chips sintered by this method showed partial delamination of the electrode due to the inevitable difference in thermal expansion between Mg₂Si and Ni. Moreover, this process is unsuitable for mass production; we therefore started development of a new process.
Another possible method for forming electrodes is to use metallic paste printing. Optimizing the calcination process and the chemical composition, we obtained a low contact resistance (~1x10^-9 Omega;m²), which is comparable to that achieved using the monoblock sintering method. At this stage, this method is seen as being the most promising technique amenable to industrial development.

P-type thermoelectric Bi_0.5Sb_1.5Te₃ nanowires were successfully synthesized in porous anodic alumina template (PAA) via pulsed electrochemical deposition. In order to seek alternative way to increase the their performance during growth, in-situ laser treatment was employed, aiming at introducing crystal structural changes. After the growth, three typical properties for thermoelectric materials, such as electrical conductivity (σ), thermal conductivity (κ) , and Seebeck Coefficient (S), were measured on the device fabricated on PAA by MEMS-like fabrication process. The measured results indicate the elevated performance of power factor (Sσ²) for the laser-treated nanowires, while the thermal conductivity remain the comparable level in both cases. The reason for increased power factor was discussed by characterizing the structural and grain changes during employment of laser. High Resolution Transmission Electron Microscope images clearly show the enlarged grain size and reduced grain boundaries, which are beneficial to improve electrical perfomances by reducing electron scattering during the transport in crystals.

In recent years, nanostructuring has been demonstrated as an effective approach to increase the thermoelectric figure of merit. Typically, these increases are attributed to the low thermal conductivity caused by additional phonon scattering at grain boundaries that reduces the phonon mean free paths. However, this picture has never been experimentally confirmed. In this talk, we will report the first measurements of spectral phonon transmission coefficients at a solid interface. Our results show that low frequency phonons travel nearly unimpeded across the interface while high frequency phonons are nearly completely reflected, which suggests, counterintuitively, that long mean free path phonons may be the dominant heat carriers in nanostructured thermoelectrics. This observation indicates that further increases in thermoelectric efficiency may be possible by disrupting the transport of these low frequency vibrations.

We have used non-thermal plasma produced silicon quantum dots as a feedstock for the production of nanostructured bulk silicon. Continuous flow non-thermal plasma reactors are capable of producing nanopowders with precise control over size[1], structure[2,3], and surface termination[2,3]. The nanoparticle production rate is close to 1 gram/hour even for a lab-scale reactor. We have found that starting with silicon nanocrystals in a narrow diameter range (all the particles within 5 nm and 10 nm in size) we can produce bulk nanostructured silicon with grain size uniformly below 100 nm. Starting from larger powder, which can be easily obtained by tuning the nanoparticle synthesis parameters, results into bulk samples with larger grain sizes. The ability to precisely control nanoparticle size, structure, and surface termination before sintering therefore enables the production of bulk materials with tightly controlled nanostructure. This is relevant for (a) improving our understanding of thermal and electrical transport in such materials and ultimately (b) leading to the development of high efficiency devices for cooling or waste heat recovery. This contribution will focus on the correlations between initial particle size, structure, and surface termination and the properties of the resulting nanostructured bulk silicon material. We have found that controlling the chemical termination of the feedstock nanopowder is critical for achieving dense and mechanically robust samples after sintering. We will provide a comprehensive discussion of the role of grain size distribution on electrical and thermal conductivity. We have found that even a small addition of plasma-produced, ultra-fine (<10 nm) silicon nanocrystal to larger, commercially available silicon powders leads to significant reduction in thermal conductivity and improvement in the thermoelectric figure of merit.
1. Mangolini, L., et al., Nano Letters, 2005. 5(4): p. 655-659.
2. Yasar-Inceoglu, O., et al. Nanotechnology, 2012. 23 255604
3. Lopez, T., et al. Nanoscale, 2014. 6.3 p.1286-1294

The control over interfaces is of critical importance to the thermoelectric properties of nanocrystalline materials. The inclusion of heterostructures and organic-inorganic interfaces in thermoelectric materials have been shown as promising methods for improving ZT, but many syntheses and fabrication methods are restricted in terms of their ability to systematically control material parameters such as interface density, nanoscale phase composition, phase inclusion density, and interface electronic structure. Instead, a bottom-up approach through the assembly of nanoscale building blocks promises to provide insight into various theories of thermoelectric property improvement. Here we discuss an approach to Group IV nanocrystals, including Si, Ge, SiGe alloys by a scalable and general solution synthesis. The elimination of strongly coordinating solvents and ligands allows the normalization of Si and Ge decomposition and incorporation rates to control the relative amounts in the nanocrystals. We will discuss their surface chemistry and processing methods for forming thin films and bulk nanocrystal solids and present data on structure-property relationships for the thermoelectric behavior of these bottom-up assembled materials.

Thermoelectric effects fulfill conversion between heat flow and electrical current, which lead to many applications including waste heat recovery and solid state cooling. Recently, nanostructured thermoelectric materials have been extensively studied, which have demonstrated significant enhancement of the figure of merit or ZT primarily due to the reduction in thermal conductivity. Meanwhile, researchers are also seeking ways to improve the power factor of the thermoelectric materials, and energy filtering effect has been proposed as one possible means. By incorporating metallic nanoparticles into a thermoelectric material, energy barriers are formed at the interface of the two phases due to the band mismatch, which blocks low energy carriers and lead to enhanced power factor when the energy barrier height is optimized. However, few experimental demonstration of intentionally modulated energy barrier height is reported. Here, we proposed to utilize AuAg alloy nanoparticles as the inclusions to the PbSe host, given that the work function of the AuAg alloy is tunable with composition. We demonstrated the colloidal synthesis of 7 nm monodisperse AuAg alloy nanoparticles with different compositions, which are then mixed with PbSe nanocrystals of the same size in solution and processed into binary nanocomposite thin film by spin coating. We showed the dependence of thermoelectric properties of the binary system on both of the concentration and composition of the AuAg nanoparticles, which suggests that AuAg alloy nanoparticles provide a means to modulate the energy filtering effect and optimize the thermoelectric performance of the binary system.

Improvement of the efficiency of thermoelectric materials is important for a number of applications. In this contribution we present experimental results on the thermoelectric figure of merit of a new type of material. PbTe/PbSe super-lattices were deposited via electrochemical atomic layer deposition (eALD) on three-dimensionally structured substrates. The materials were doped with bismuth in order to ensure that the structures were n-type semiconductors to control the electrical resistivity. Oxide growths during the material deposition process were restricted by conducting the deposition process in a nitrogen back-filled deposition chamber. Structural characterization of the material was performed with SEM, EDS and XRF. Electrical resistivity was determined via 4-point resistivity measurements and thermal conductivity via time-domain thermoreflectance (TDTR) measurements. Seebeck measurements were performed with a cross-plane Seebeck characterization system. With these measurements, we report the thermoelectric figure of merit for thin film three dimensionally structured TE materials across large surfaces. Presented results promise cost-effective and higher efficiency thermoelectric materials.

In nanostructured thermoelectric materials, when the size of the constituent materials is smaller than the phonon mean-free-paths, the energy dynamics can be described as truncated superdiffusive Lévy flights instead of conventional Brownian motion. All essential physics of the nondiffusive transport are captured by the fractal dimension and exponential decay length of the stochastic process. We determine these two material parameters experimentally for nanostructured ErAs:InGaAs using transient laser thermoreflectometry. It is shown that while changing the concentration of embedded ErAs nanoparticles by two orders of magnitude (0.1% to 10%) can change the total thermal conductivity by more than a factor of three, the Lévy fractal dimension does not change. Similarity and differences with Lévy glass and superdiffusion of light will be described and Implications for the design of high performance thermoelectric materials will be discussed.

Composites of semiconductors are often considered as a way to yield high thermoelectric figures of merit. The most constraining limitation in this approach is given by the effective medium theory [1,2]. When one considers a composite made from two thermoelectric materials, A and B, in the absence of interactions between them the thermoelectric figure of merit of the composite cannot exceed that of the highest of the figures of merit of either A or B [1]. However, it is possible for thermoelectric power factor of the composite to exceed the highest of the power factors of either A or B [2]. Here we describe a mechanism that can lift this limitation by treating charge and heat flux differently. Silica beads coated with a conducting shell are inserted into a thermoelectric host. Phonons are scattered by the beads but electrons can pass through the conducting shells. In this talk, we will present the theory first, followed by experimental data demonstrating enhancement of ZT in p-type (Bi,Sb)₂Te₃ for room temperature operation.
References:
[1] David J. Bergman and Ohad Levy, J. Appl. Phys.70 6821 (1991)
[2] 2. David J. Bergman and Leonid G. Fel, J. Appl. Phys. 85 8205 (1999)

La_3-xTe₄ has emerged as a new high efficiency, high temperature n-type thermoelectric material. The high performance capabilities of La_3-xTe₄ stem from a complex crystal structure and defect controlled carrier concentration, which respectively result in a low lattice thermal conductivity and favorable values for thermopower and electrical conductivities. With an optimized defect stoichiometry the dimensionless figure of merit, ZT, can attain values as high as ~1.3 at 1275K. Recently, our group has demonstrated a 30% increase in the ZT of La_3-xTe₄ with the addition of 12 vol% Ni inclusions, with ZTs as high as 1.8 at 1275 K. The enhancement in ZT is a result of the inclusions forming a charge funneling network, thereby reducing the electrical resistivity of the composite while leaving the thermal conductivity and thermopower virtually unchanged. Here we present a study investigating the effects of Ni inclusion&’s microstructure on the thermoelectric performance of La_3-xTe₄. Various Ni morphologies, ranging from nanometer to microns in size, were distributed with La_3-xTe₄ matrix powder and then densified to dense compacts using spark plasma sintering (SPS). The thermoelectric properties and the influence microstructure will be presented and discussed.

High-performance bulk thermoelectric (TE) materials often contain scarce or toxic elements. The β-FeSi₂ phase, as one of the few semiconducting silicides, has been studied as a TE material for the medium temperature range, due to its relatively high power factor and low-cost, eco-friendly nature. However, its TE figure of merit (ZT) is still yet too small to make an efficient device. Our approach in aiming to enhance the TE performance has focused on hierarchical nanostructuring across multiple lengthscales. One level of structuring arises from the eutectoid decomposition (α-Fe2Si5 → β-FeSi2 + Si) in the Fe-Si binary system, where the Si microconstituent can self-assemble as quasi-periodic nanorods/dots embedded in the β matrix. We and others have shown this to effectively reduce thermal conductivity. Another level of structural control comes from adding of order 5 at% Ge for the initial melt. This creates extended, parallel eutectic Si_yGe_1-y plates with micron separation, embedded in α-Fe₂Si₅ upon rapid solidification. By isothermal annealing, the α phase again decomposes into eutectoid Si nanorods in β-FeSi₂ matrix with even finer grain size due to the confinement effect of the parallel Si_yGe_1-y plates. This eutectic+eutectoid structure further suppresses thermal conductivity. Additional control over grain size and in-grain microstructure for the ternary system is obtained by combining rapid solidification with subsequent powder processing followed by spark plasma sintering (SPS). This also provides dense bulk material for electronic transport studies. For instance, we ball-milled the rapid solidified Fe₂₈Si₆₈Ge₄ into powders with particle size of 1-5um, and obtained well-sintered pellets by subsequent SPS at 850C. The final structure in this composition shows eutectoid Si_0.9Ge_0.1 nanodots (10nm-500nm in diameter), and the eutectic Si_0.5Ge_0.5 plates (retained from rapid solidification) embedded in the β-FeSi₂ matrix. Such hierarchical nano/meso-structuring is promising in reducing lattice thermal conductivity. In addition, the composition of the eutectoid Si_xGe_1-x nanodots can be modified by local intermixing of Ge or varying its content, which could impact electrical transport across heterointerfaces via band structure engineering. We will discuss preliminary data on electrical and thermal transport for the undoped ternary system. We gratefully acknowledge the support of the II-VI Foundation.

Thermoelectric devices possess enormous potential to reshape the global energy landscape by converting waste heat into electricity, yet their commercial implementation has been limited by their high cost to output power ratio. No single “champion” thermoelectric material exists due to a broad range of material-dependent thermal and electrical property optimization challenges. While the advent of nanostructuring provided a general design paradigm for reducing material thermal conductivities, there exists no analogous strategy for homogeneous, precise doping of materials. Here, we demonstrate a nanoscale interface engineering approach that harnesses the large chemically accessible surface areas of nanomaterials to yield massive, finely-controlled, and stable changes in the Seebeck coefficient, switching a prototypical p-type thermoelectric material, tellurium, into a robust n-type material exhibiting stable properties over months of testing. In contrast to any other conventional doping approach, such as substitutional doping or vapor-phase doping, our carrier modulation leaves the crystalline structure wholly intact, thus eliminating problems that doping traditionally introduces, such as ionized-impurity scattering. Our control over carrier conversion is complete (n-, p-, or ambipolar) and fully corroborated through thorough electrical transport measurements using thin film transistor geometries as well as density functional theory calculations. This methodology is based upon true band conversion or band tuning of materials resulting from nanoscale chemical resurfacing, which effectively dopes the materials, opening up new strategies for low temperature, high-throughput materials processing and enabling facile development and implementation of flexible and customizable, module architectures. These remodeled, n-type nanowires display power factors comparable to their p-type counterparts, and are partnered together to demonstrate the first solution-processed, monomaterial flexible thermoelectric generators. We proceed further to dope these nanowires with small organic molecules to generate hybrid organic inorganic nanocomposites and demonstrate power factors and ZTs surpassing bulk tellurium. More generally, these results suggest that interface engineering of nanostructures holds great potential for driving forth the next generation of hybrid materials for applications in thin-film thermoelectrics, light emitting devices, and electronic logic.

The introduction of complexity into a material system has been a promising methodology for enhancing the thermoelectric figure of merit, as it provides a means of decoupling of the various conflicting components of the figure. Materials may be functionalized such that electronic and thermal transport properties may be individually optimized. One such example is nanolaminate thin films, whereby the anisotropy allows for lattice phonons to be scattered by regular interfaces throughout the stacked material, but electronic transport within a layer proceeds relatively unperturbed or even enhanced due to interactions between constituents. These materials are especially desirable as complex model systems, since the well-defined layering within the film is easily characterized. This allows for the determination of chemical/structure-property relationships within a material as long as systematic chemical/structural changes can be made. A synthetic technique whereby precise control of selenide-based layered precursors has allowed for the creation of nanolaminate materials in several chemical systems with nearly arbitrary layering motifs between constituents. We discuss our recent results in these systems with respect to their use as thermoelectric materials, highlighting recent work in the (MSe)_m(TiSe₂)_n system resulting in the highest reported thermoelectric power factors observed within the broader chalcogenide misfit layered compound material class. This demonstrates that systematic control of nanolaminate structures is a promising approach to understand structure-property relationships and resulting materials properties.

We have performed first-principles calculations for lattice thermal expansion and transport in the bulk of the transition metal dichalcogenides (TMDCs) MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂ and WTe₂ using density functional theory (DFT) and the semi-classical phonon Boltzmann transport equation (BTE) within the relaxation time approximation (RTA). In particular, we have focussed on the out of plane behavior motivated by the work of Chiritescu et. al. on ultralow thermal conductivity in WSe₂ [Science 315, 351 (2007)]. Proper modeling of the lattice thermal conductivity is important for an accurate prediction of the thermoelectric figure of merit and better understanding of potentially high performing novel materials such as van der Waals heterostructures based on TMDCs as well as the role of defects in these kinds of structures. Previous work on the systems under study have not fully acknowledged a treatment that accounts for the sparse as well as anisotropic nature in van der Waals type of solids such as the TMDCs. That may be important when investigating thermal conductivity in more complex cases. To this end, we have conducted a thorough investigation of the mentioned TMDCs based on DFT calculations using a recently published van der Waals density functional (vdW-DF-cx) [Phys. Rev. B 89, 035412 (2014)] in conjunction with anharmonic modeling of phonon lifetimes using third order interatomic force constants that allows for solutions to the BTE within the RTA as implemented in the phono3py code [Phys. Rev. B 91, 094306 (2015)]. We found that our calculations agrees with theoretical expectations as well as with experimental data where available so the methods used are suggested as a promising framework for further investigation of more complex systems with potentially novel thermal properties.

Emerging 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only from their discretized density of states, but especially due to their large effective masses and high carrier mobilities, different from gapless semi-metallic graphene. Here we report a measured powerfactor of MoS₂ as large as 8.5 mW/mK² at room temperature, the highest among all thermoelectric materials and twice that of commercial thermoelectric material Bi₂Te₃. For the first time, the measurement of the thermoelectric properties of monolayer MoS₂ allows us to determine the quantum confined 2D density of states near the conduction band edge, which cannot be measured by electrical conductivity alone. We also use temperature-dependent thermoelectric transport measurements to elucidate the Metal-Insulator phase diagram and extract the transport effective mass for single and bilayer MoS₂. The demonstrated record high powerfactor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.

Various physical properties depending on the thickness of MoS₂ have been in spotlight in the field of nanoelectronics, spintronics, and newly arising valleytronics. MoS₂ also has been studied as a candidate for good thermoelectric material because of its high Seebeck coefficient. Because it is essential to enhance the electrical conductivity of MoS₂ for thermoelectric application, the band gap engineering has been suggested by using the control of layer thickness, pressure, and layer mixing. [1-5]
In this study, we have investigated the control of valley degeneracy in MoS₂ and its effect on thermoelectric properties by using ab-initio calculation. By modulating the layer thickness and external electric field, the valence band maximum (VBM) at Γ and K points and the conduction band minimum (CBM) at K and Σ_min points are differently shifted. The valley degeneracy of VBM is observed in MoS₂ monolayer, while that of CBM in MoS₂ monolayer under the external electric field. By tuning the valley degeneracy, the Seebeck coefficient and electrical conductivity can be separately controlled, and the maximum power factor can be obtained in n-type (p-type) MoS₂ monolayer with (without) the external electric field. We suggest that the transition metal dichalcogenides are good example to investigate the role of valley degeneracy in the thermoelectric properties with the control of interlayer interaction and external electric field strength.
[1] H. Guo, T. Yang, P. Tao, Y. Wang, and Z. Zhang, J. Appl. Phys. 113, 013709 (2013).
[2] C. Lee, J. Hong, M.-H. Whangbo, and J. H. Shim, Chem. Mater. 25, 3745 (2013).
[3] C. Lee, J. Hong, W. R. Lee, D. Y. Kim, and J. H. Shim, J. Solid State Chem. 211, 113 (2014).
[4] D. Wickramaratne, F. Zahid, and R. K. Lake, J. Chem. Phys. 140, 124710 (2014).
[5] W. Huang, X. Luo, C. K. Gan, S. Y. Quek, and G. Liang, Phys. Chem. Chem. Phys. 16, 10866 (2014).

Nanocomposite structure is a promising solution for improving figure of merit ZT, especially for reducing a thermal conductivity (κ) in the thermoelectric materials. We focus on higher manganese silicides, MnSi_δ (δ asymp; 1.7), with high ZT of approximately 1.0 because the peritectic reaction enables to stabilize nanocomposite structures with Si [1]. Also thin film technique is useful because the composition and morphology in the film are well controllable. Especially, it is ease to form nm-scale strcture such as stacked multilayers with artificial interface which is expected to reduce κ by enhancing phonon scattering. In this study, we compared κ of nanocrystallline MnSi_δ with that of MnSi_δ/Si multilayers in which the inserted Si acts as the artificial interface in sputtered MnSi_δ-based thin films.
Samples were prepared as follows. Si/Mn multilayers, [Mn(2.6)/Si(t)]₁₂ (thickness in nm), were deposited by magnetron sputtering on thermally oxidized Si substrates: Si/SiO_x(700nm). Then the samples were annealed at 600°C in a vacuum of 10^-6 Pa for 1 hour. For comparison of κ, Sub.//Si(160) was also prepared. The film composition was determined by inductively coupled plasma method. The out-of-plane thermal effusivity was measured at room temperature by picosecond thermoreflectance method and κ was evaluated from the thermal effusivity by using the density and specific heat of MnSi_δ bulk samples [2]. The crystal structure was characterized by X-ray diffraction (XRD) and cross-sectional transmission electron microsopy (TEM).
The Si/Mn composition ratio was determined to be 1.7 and 2.2 in the sample of t = 6.7 and 8.7, respectively. Only MnSi_δ was observed when t = 6.7 by XRD measurements. Cross-sectional TEM images clarified that MnSi_δ nanocrystals are formed in the annealed multilayer film of t = 6.7. The crystal size of MnSi_δ is several nm which almost corresponds to the initial layer thickness. The evaluated κ of 1.9 W/Km is lower than that of polycrystalline bulk samples of MnSi_δ with the typical crystal size of 100 mu;m [1]. On the other hand, it was revealed that MnSi_δ and Si are alternately layered in the case of t = 8.7. Si in the area adjacent to Mn layer reacts with Mn into MnSi_δ by the annealing process, whereas unreacted Si remains as a layer. The κ of 1.0 W/Km is lower than that of not only the film of t = 6.7 but the Si thin film. Hence this low k is considered to originate from the thermal resistance of the interface between MnSi_δ and Si. From these results, it is clarified that the grain boundary in the nanocrystalline MnSi_δ decrease k compared with the bulk MnSi_δ with mu;m-scale grains and the inserted Si interface in MnSi_δ results in the further decrease of κ. This work was supported by TherMAT, Future Pioneering Projects of METI, Japan.
[1] K. Koumoto and T. Mori (Eds.), Thermoelectric Nanomaterials. Meterials design and applications (Springer, Berlin, 2013).
[2] S. N. Girard et al., Chem. Mater. 26, 5097 (2014).

Nanomaterials, especially nanowires, have been reported to have excellent thermoelectric properties. Experimental and theoretical investigations based on single nanowire devices have demonstrated an increase in thermoelectric efficiencies compared to their bulk counterparts. Few studies have demonstrated up to hundred fold increase in thermoelectric performance of single nanowire devices.
Extension of these properties to macro devices based on nanowires, are crucial for thermoelectrics used in terrestrial applications ( for increasing energy efficiency of existing processes as well as generating renewable energy). However, reports on macro devices based on nanowires are scarce and do not delineate a path for extending enhanced performance of individual nanowires to bulk devices. Fabrication of such devices is met with a variety of challenges, including the assembly of these nanowires in an aligned and welded fashion to utilize the anisotropic nature of nanowires. Also, such welded nanowire networks prevent the formation of oxide interfaces between the nanowires when assembled, which is typical of the current strategies for assembling nanomaterials.
In this context, a simple and versatile strategy for synthesizing large scale, welded and aligned, metal silicide nanowire networks was developed. This strategy involves solid state phase transformation of mixtures of aligned silicon nanowires decorated with silica nanoparticles which are aligned. The nanowires are aligned via a plastic deformation process which utilizes simple shear, equal channel angular extrusion (ECAE). This strategy of welding aligned nanowires, offers precise control over the chemical composition, thermal and electrical transport of the interfaces between the nanowires after their assembly. These aligned and welded nanowire networks offer multiple levers for reducing the lattice thermal conductivities of Mg₂Si, while having no detrimental effect on their electrical properties.
In this talk we report strategies for aligning nanowires via ECAE and welding these nanowires via solid phase transformation to obtain welded and aligned nanowire networks. The thermoelectric performances of these aligned and welded metal silicide nanowires, their relationship to the dimensions of the nanowires and the welds between the nanowires will be discussed. The thermoelectric performances of aligned and welded metal silicide nanowire assemblies will be compared to their unwelded and randomly oriented counterparts.

The widely used density functional theory performs reasonably well for the determination of structural properties of many materials, but fails to predict the electronic structure of semiconductors and insulators. The well-known underestimation of band gaps, makes prediction of effective masses, impurity levels or band alignments often unreliable or even impossible. To overcome deficiencies of the local density or generalized gradient approximation we used one of the higher level electronic structure methods - the revised Heyd-Scuseria-Ernzerhof range-separated hybrid functional [1].
We present ab initio calculations on the structural and electronic properties of the narrow-gap lead chalcogenides PbX (X=S, Se, and Te). Using a hybrid functional approach, we explore the band structures of PbX doped with a series of impurities. Based on the density of states difference between the doped samples and pure host sample, the possibility of improving transport properties is discussed. Finally, the transport properties of these doped systems is calculated within the framework of Boltzmann theory and constant relaxation time approximation.
[1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003), and erratum ibid. 124, 219906 (2006).

Organo-lead halide perovskite (OLHP), as a promising photovoltaic candidate, has also demonstrated great merits for thermal energy conversion due to its ultralow thermal conductivity (0.3~0.5 W/Km) and large Seebeck coefficient (~mV/K) at room temperature. The OLHP single crystal, with much low density of trap states and negligible grain boundaries, possess higher electric conductivity as compared to its polycrystalline thin film. In this work, we prepared methylammonium lead triiodide perovskite (CH₃NH₃PbI₃) single crystal with the size up to sub-centimeter through cooling-assisted solution growth method. The X-ray diffraction characterization indicates the tetragonal phase of the grown single crystal at room temperature. Time-dependent photoluminescence characterization demonstrates that the majority of trap states in the grown crystal are positively charged. Meanwhile, the negative Seebeck coefficient from the grown single crystal with symmetric metal contacts, eg. aluminum, silver, and gold, reveals the n-type semiconducting property. More interestingly, we observe that the thermopower can be significantly increased by a factor of ten (~350 mV/K at 400 K) through introducing interfacial dipoles from a series of high work function metal contacts. This phenomenon has also been verified in perovskite polycrystalline thin film. Furthermore, from the temperature-dependent capacitance-frequency measurements, we observe an increase in the interfacial polarization but a decrease in the bulk polarization as increasing the temperature from 298 K to 400 K, which can be associated with the decrease in trap states density due to the negative charge diffusion across the crystal towards the electrode. In conclusion, our work paves the way for realizing high figure of merit (ZT) through combining organic-inorganic hybrid materials and interface engineering.

Anharmonicities in phonons, apart from the conventional Einstein or Debye- mode harmonic phonons, are frequently observed for amorphous or glass-like materials, and these phonon spectrum peaks are called boson peaks. A frontier topic relating to boson peaks revolves around the fact that they are also observed in single crystals with a void of cage structure. Although the origin of the phonon anharmonicity associated with the boson peaks has been the center of scienti c debate for many years, a clear understanding has not yet been achieved. In the present meeting, we show that all boson peaks can successfully be rationalized in terms of a single uni ed exponential line for a variety of clathrates by employing a new parameter associated with the freedom of space. The intrinsic nature of the boson peaks is described based on the uni ed picture with a help of first principles calculations. Although the origin of the boson peaks appearing in glass-like materials is complex to understand due to the missing information on the real structure, our present understandings give important information applicable to other systems. The van der Waals-type guest-host interactions to be presented in the present study are consistent with the part-liquid concept and useful for thermoelectric materials design.

The Seebeck coefficient is an indispensable physical property in evaluating the potential performance of thermoelectric (TE) materials. The measured Seebeck coefficient is proportional to the Seebeck coefficient of the TE material under test and that of the reference wire materials used in the measurement such as Pt and Cu. Thus, the absolute Seebeck coefficient of the reference material must be determined in a separate experiment. For this reason, superconducting materials are often used as reference wires, since the Seebeck coefficient is zero in the Meissner state. However, this method limits the highest temperature to the superconducting transition temperature Tc. To overcome this technical problem, the absolute Seebeck coefficient above Tc has been determined from the Thomson-coefficient-integration method based on Kelvin relation.
Thomson coefficients are obtained from measurements of small temperature changes in thin metallic wires under DC current in the presence of a temperature gradient. Using the conventional DC method, the most reliable measurements of Thomson coefficient have been performed for thin metallic wires such as Pb, W, and Pt. However, the DC method usually requires accurate prior information of thermal conductivity and the geometrical dimensions. This argument points to the need for developing new measurement concepts.
The present work presents the measured absolute Seebeck coefficient using an improved method for measuring the Thomson coefficient. By using AC and DC current, our method does not require knowledge of the temperature dependence of thermal conductivity and sample dimensions, unlike conventional DC method. We have successfully measured temperature dependence of Thomson coefficient of Pt-wire above room temperature using our AC-DC method so far. We are now developing an adiabatic Thomson-heat cryostat to determine the absolute Seebeck coefficient with the combination of high-Tc superconductors such as YBCO above liquid Nitrogen temperature.

Significant progress has been made on thermoelectric materials in the last decade showing great promise for high thermoelectric conversion efficiency. However, in addition to good and stable thermoelectric materials, good electrical and thermal contacts are essential to reach the high potential in device efficiency. Additionally, electrical and thermal contact resistances can affect the measurements of thermoelectric material properties oftentimes leading to uncertainties that are difficult to quantify. There are methods to measure the electrical contact resistances between a thermoelectric material and a metal electrode. The measurement of a thermal contact resistance is more challenging and typically requires rather complex methods. Here, we report a simple method to in situ measure the thermal contact resistance within a thermoelectric device. We demonstrate the method for a single thermoelectric leg and a thermoelectric couple device.

The growth of the home automation, industrial and medical monitoring applications utilizing wireless sensor networks has spurred new research of energy harvesting power sources. Thermoelectric energy conversion is very effective in harvesting electricity from heat sources with low temperature gradients relative to the environmental temperature. However, the usages of conventional bulk thermoelectric devices have been limited due to their rigidity shape, heavy weight and device fabrication cost. From this kind of circumstance, flexible and sheet type thermoelectric devices by using printing techniques have been studied and expected as one of promising approach to expand the range of use and reduce the manufacturing cost.
We have developed the thermoelectric composites consisting of bismuth telluride particles and organic materials including thermal-resistant polymers and electrical conductive additives for the printing method. Thermoelectric properties of the printed films were investigated as a function of thermal annealing temperature. The lowest cross-plane thermal conductivities of 0.3 W m^-1 K^-1 that is lower than that of bulk bismuth telluride was obtained by annealing at the 410 °C under argon-hydrogen atmosphere. The morphology study of the thin film by SEM indicated that this reduced thermal conductivity could be caused by reducing phonon scattering at the grain boundaries and filling of the boundaries with low thermal conductivity organic materials. In contrast to the thermal conductivity reduction, electrical conductivities of the thin films were improved from 10^-2 to 150 S cm^-1. The electrical conductivity of the thin films probably due to the suppressed contact resistance by filling the conductive additives to the interfaces between the particles. The electrical conductivity of the thin films was in agreement with theoretical value of the composite materials calculated by the Maxwell equation. The ZT values of the p- and n-type Bi₂Te₃ thin films were evaluated to be 1.0 and 0.6, respectively.
A prototype flexible thermoelectric module consisting of p- and n-type patterns over an active area of ca. 7 × 7 cm² was fabricated on the polyimide film using screen printing. The generated output power was 8.8 mW at a temperature difference of 15 #730;C between the head and tail sides of the sheet.