Symposium EN10 : Thermoelectric Materials, Devices and Applications

This talk will discuss our recent work to simulate electron and phonon thermoelectric properties based on the density-functional theory, including electrical conductivity, Seebeck coefficient, electronic thermal conductivity and phonon thermal conductivity. Main challenges are simulation of scattering among carriers by phonons, impurities and alloy. For electron transport simulations, the electron-phonon interactions, as well as electron-impurity and electron-alloy scatterings are computed from first-principles to obtain electron relaxation times based on Fermi’s golden rule. The energy dependent relaxation times are then used in the Boltzmann transport theory to obtain the electrical conductivity, Seebeck coefficient and electronic thermal conductivity. For phonon transport, the anharmonic force constants are derived from first-principles and used to compute phonon relaxation times. The energy dependent mean free paths are computed for both electrons and phonons. After validating the simulation on well-characterized materials such as Si and GaAs (EPL, 109, 57006, 2015; PRL, 114, 115901, 2015; PNAS, 112, 14777, 2015; PRB, 95, 075206, 2017), we moved on to simulate thermoelectric materials such as half-Heuslers, chalcogenides and several alloy systems. We reveal that 1) large power factors often seen in half-Heuslers can be attributed to their non-bonding orbitals at the band edge, which can be protected by the crystal symmetry; 2) Dirac-like band structure allows electron mean free path filtering that can significantly enhance the Seebeck coefficient. These results led to deeper understanding of thermoelectric transport in existing materials, and also point to new directions for improving existing materials via nanostructures, as well as for discovering new material systems. This work is supported by S³TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.

The multi-component materials of chemical bond hierarchy, exhibiting the part-crystalline part-liquid (PCPL) state, have recently been proposed to be emerging candidate of thermoelectric materials. These materials contain at least two different types of sublattices, one crystalline and another one strongly disordered or liquid-like, leading to extremely low lattice thermal conductivity. This talk presents a survey on the general characteristics of the thermal transport in the PCPL materials. We also develop a molecular dynamics (MD) approach to simulate the complex thermal transport process. We compare the results in Green-Kubo method and Boltzmann transport theory to elucidate the thermal conductivity of PCPL materials by using empirical interatomic potentials fitting to the liquid-like thermoelectrics like Cu₂Se. The contribution to thermal transport from each structural component, i.e. the rigid-crystalline, strongly disordered, and/or liquid-like parts, are respectively analyzed. Relationship to the minimum thermal conductivity is also discussed.

High-throughput explorations of novel thermoelectric materials based on the Materials Genome Initiative paradigm only focus on digging into the structure-property space using nonglobal indicators to design materials with tunable electrical and thermal transport properties. As the genomic units, following the biogene tradition, such indicators include localized crystal structural blocks in real space or band degeneracy at certain points in reciprocal space. However, this nonglobal approach does not consider how real materials differentiate from others. Here, we successfully develops a strategy of using entropy as the global gene-like performance indicator that shows how multicomponent thermoelectric materials with high entropy can be designed via a high-throughput screening method. Optimizing entropy works as an effective guide to greatly improve the thermoelectric performance through either a significantly depressed lattice thermal conductivity down to its theoretical minimum value and/or via enhancing the crystal structure symmetry to yield large Seebeck coefficients. The entropy engineering using multicomponent crystal structures or other possible techniques provides a new avenue for an improvement of the thermoelectric performance beyond the current methods and approaches.

Higher Manganese Silicide materials have the advantage of environmental-friendly, low-cost, and stable. Cobalt has one more valence electron than manganese, thus the carrier concentration can be decreased when manganese is replaced by cobalt. In addition, the lattice distortion can also be induced. Thus, the lattice thermal conductivity could be reduced too. We adopted arc melting, high temperature annealing and SPS sintering techniques to get the MnSi_1.72 and the Co-doped samples (1%-4% substitution proportion). After measuring the properties, we found that the substituted Co obviously decreases the electric conductivity and lattice thermal conductivity while the Seebeck decreases slightly, leading to a zT of 0.44 when the Co amount is 4%. When the Co amount is over 4%, CoSi₂ impurity phase is observed in the materials.

BiCuSeO-based materials are promising thermoelectrics for intermediate temperatures, primarily due to their ultralow lattice thermal conductivity. The intrinsically low carrier mobility in these materials, however, largely limits further improvements of their thermoelectric properties. In this talk, we show that the electrical transport properties of these materials can be enhanced by increasing the chemical bond covalency in the Cu-Se layer, and by rationally utilizing the multi-valley electronic band structure. High thermoelectric figure of merit ZT values of 1.2-1.3 can be achieved at 873 K. All samples were synthesized by nonequilibrium self-propagating high-temperature synthesis (SHS) processes. The resulting hierarchical structural features lead to lattice thermal conductivity values close to the amorphous limit.

The development of thermoelectric oxide materials like Ca₃Co₄O₉ and Nb:SrTiO₃ as an alternative to SiGe compounds for high temperature applications has attracted large interest in recent years. In comparison to other heavy metal and toxic elements containing thermoelectric materials, complex metal oxides comprise extremely high chemical and temperature stability along with low toxicity and high abundance of the constituent chemical elements. The encouraging thermoelectric properties of Ca₃Co₄O₉, the coupling of the magnetic moments of the Co spins and the quasi-two-dimensional electric transport properties can lead to novel functionalities and applications in thermoelectric and -magnetic devices. In order to build useful devices using thin films of this complex metal oxide integration into silicon technology electrical contacts with minimum electrical resistance are decisive criteria. Moreover, due to high chemical reactivity of Si with oxygen and silicide forming metals like cobalt a diffusion barrier is needed which ideally enables epitaxial growth of Ca₃Co₄O₉ and Ir thin films on a (001)-Si substrate.

We show that Ca₃Co₄O₉/Ir electrical contacts with very low electrical resistivity can be fabricated. Ca₃Co₄O₉/Ir electrical contact pairs were formed by growing a conductor track of Ca₃Co₄O₉ on crossing Ir conductor tracks. The potential drop at individual Ca₃Co₄O₉/Ir electrical contacts was determined by combining four-wire resistance measurements eliminating Ca₃Co₄O₉ conductors track resistances. The I-V characteristics show a slight diode like asymmetry and the corresponding contact resistivities were found to be between 1.6 and 3.6 mΩcm².
Secondary ion mass spectrometry depth profiles show an approximately 5 nm thick layer of IrO_x formed during PLD on the Ir conductor tracks by indiffusion of oxygen.

XRD pole figures of the Ca₃Co₄O₉/Ir/YSZ/Si-substrate and Ca₃Co₄O₉/YSZ/Si-substrate reveal a 12-fold in-plane rotational symmetry on the (001)-Ir and (001)-YSZ buffer layer, but rotated by 15°. This leads to high symmetry grain boundaries with low electrical resistivity where the Ca₃Co₄O₉ track crosses the edge of the Ir contact conductor tracks towards the electrically insulating YSZ buffer layer. This epitaxial relationship can be explained by energetically preferred growth directions of the pseudo hexagonal CoO₂ sublayers in monoclinic Ca₃Co₄O₉ on the cubic (001)-YSZ and (001)-Ir surface. This symmetric in-plane orientation between the charge carrying CoO₂ sublayer domains results in minimal in-plane resistivity of the Ca₃Co₄O₉ thin film. In addition, we show that the influence of the azimuthal orientation on the temperature dependent Seebeck coefficient of the Ca₃Co₄O₉ thin film is imperceptible.

Thermoelectric materials, used to convert thermal into electrical energy, present a promising route for renewable energy generation. The range of applications for thermoelectrics is broad, with industries from manufacturing to the automotive likely to benefit from efficient recycling of waste thermal energy.¹ The dimensionless figure of merit for thermoelectrics, ‘ZT’, depends on both electronic and thermal transport properties, with a material considered promising if its ZT exceeds ~1.5. Unfortunately, despite over 50 years’ development, the champion thermoelectric materials, such as Bi₂Te₃, show lack-lustre performance and are costly to produce due to their reliance on tellurium.² Significant research effort has been spent attempting to produce oxide based thermoelectrics due to their earth-abundance, chemical stability and dramatically reduced costs. However, all attempts to produce high performance n-type oxide thermoelectrics have failed, often due to their high lattice conductivity which limits obtainable ZT.³

Standard packages now exist for calculating ZT from an electronic band structure, with the results being dependent on two major approximations: a fixed lattice thermal conductivity and electronic chemical potential (Fermi level). Typically, the Fermi level is assumed without knowledge of the true response of the material to defects and doping, which can lead to incorrect predictions of high ZT capability.

In this work, we have used rational chemical design to pinpoint a series of layered oxides that should exhibit degenerate n-type conductivity, whilst still possessing very low lattice thermal conductivity.⁴ We employ state of the art methods to calculate the lattice thermal conductivity, using many-body perturbation theory to capture phonon-phonon scattering processes. We also use rigorous defect chemistry analysis, performed using hybrid density functional theory, to explicitly consider the intrinsic and extrinsic defect behaviour and obtain a physical and realistic doping density and Fermi level. Combining these methods, we have predicted the largest ZT of any oxide thermoelectric material previously reported and provide guidance on the growth conditions to enhance thermoelectric power conversion.


References

1. L. E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 321, 1457–1461 (2008)
2. M. W. Gaultois, T. D. Sparks, C. K. H. Borg, R. Seshadri, W. D. Bonificio, and D. R. Clarke, Data-driven review of thermoelectric materials: performance and resource considerations, Chemistry of Materials 25, 2911–2920 (2013)
3. G. Tan, L-D. Zhao, and M. G. Kanatzidis, Rationally designing high-performance bulk thermoelectric materials, Chemical Reviews 116, 12123–12149 (2016)
4. Alex M. Ganose, W. W. Leung, Adam J. Jackson, R. G. Palgrave, and David O. Scanlon, Submitted (2017)

Highly anharmonic bonding, quantified by the Grüneisen parameter, can lead to low glass-like lattice thermal conductivity and is therefore desirable in thermoelectric materials. However, most investigations of anharmonic effects on lattice thermal conductivity emphasize phonon-phonon scattering, while overlooking the impact of anharmonicity on bond strength and sound velocity at elevated temperatures. The elastic moduli of most thermoelectric materials decrease gradually with increasing temperature due to thermal expansion (increased bond length leads to weaker bonds). Although a correlation between the Grüneisen parameter and the slope of the elastic moduli versus temperature has long been recognized, this relationship and its consequences have not been systematically investigated, and are rarely accounted for when modeling thermal conductivity. In this work, we combine high-temperature resonant ultrasound spectroscopy and in-situ X-ray diffraction to characterize the temperature-dependent elastic constants and anisotropic thermal expansion in several classes of thermoelectric materials (e.g., Zintl antimonides, Cu₂ABTe₄ stannites, Ge_xSb₂Te_3+x alloys, and others). We have observed materials with high Grüneisen parameters and rapid softening at high temperature, as well as cases in which the lattice actually stiffens despite increasing average bond length. At both extremes, the temperature-dependence of the bond strength is found to have a significant impact on the speed of sound, lattice thermal conductivity and ultimately the performance of these materials.

Layered cuprous chalcogenides such as Cu₂S, Cu₂Se and Cu₂Te have attracted significant attention recently due to their ‘Phonon-Liquid-Electron-Crystal’ like thermoelectric behavior. Among these three compounds Cu₂Se has been reported to exhibit a high figure-of-merit but it lacks stability. Hence in the present work Cu₂Te which has a relatively better thermal stability compared to Cu₂Se has been explored and its figure-of-merit enhanced by doping both the cation and anion simultaneously. Li- and Se-doped Cu₂Te, Cu_2-xLi_xTe_1-ySe_y alloys have been synthesized by a simple, conventional arc melting process. The resulting alloy ingots were characterized without subjecting to any intermediate annealing process. The alloys have two polymorphic phases-a orthorhombic super structure and a hexagonal phase corresponding to P3m1 space group. The hexagonal form however is found to be predominant in all the alloys. Morphologically, the phases have a platelet like layered nanostructure with the plate-like grains oriented in random directions. The alloys exhibit a degenerate semiconducting behavior in the range 300 K to 1000 K. The high temperature electrical resistivity varies from 0.3 mΩcm to 1.4 mΩcm depending on the type and extent of doping. The Seebeck coefficient of all the alloys increases with increasing temperature with the high temperature value in the range 30 µVK^-1 to 135 µVK^-1. All the alloys have a positive Seebeck coefficient indicating that holes are predominant charge carriers. The highest power factor achieved is 16 µW^-1cm^-1K^-2 for the alloy with Li-0.1 and Se-0.03 substitution. The thermal conductivity of this alloy decreases to 1.6 W^-1m^-1K^-1 at highest temperature resulting in a figure-of-merit of 1.0. An interesting aspect of these alloys is that even at these temperatures they do not exhibit onset of bipolar conduction indicating the robustness of charge carriers.

Recent publication shows that Cu₂Se have a peak zT value above 2.0 at 400 K during the second-order phase transitions. One of the reason for such high zT is the great reduction in lattice thermal conductivity, which is strongly supported by the giant reduction in the measured thermal diffusivity. In order to understand the mechanism of such thermal diffusivity reductions, we built a model for the heat transport during phase transitions. The result shows that the reduction in thermal diffusivity is mainly attributed to the coupling between phonons and phase transition. Such large reduction is only occurred when the phase transition is fast. In experiment, the thermal diffusivities and phase transitions of Ag₂Se, Cu₂Se, and Cu₂S were measured. The dramatic thermal diffusivity reductions were only observed in Cu₂Se, and Cu₂S because they have fast phase transitions, which is consistent with our model.

Cu12+xSb4S13(x = 0.0, 0.5, 1, 1.5) and Cu12+xSb4S12Se (x = 0.5, 1, 1.5) compounds are synthesized by conventional solid state reaction followed by spark plasma sintering and their thermoelectric properties were investigated. The results reveal that the intrinsically low thermal conductivity of polycrystalline Cu12+xSb4S13 materials can be further reduced to 0.25 W.m−1.K−1 with the aid of exsolution process. Furthermore, we realize a substantial power factor enhancement for Cu12+xSb4S13(x = 0.5, 1, 1.5) via Se solid solution. By properly balancing electrical and thermal transport, a maximum zT of 1.1 associated with a power factor of 1.2 mW.m-1.K-2 at 723 K for Cu13.5Sb4S12Se is reported. Finally, we demonstrate that zT of mineral based thermoelectric materials can be greatly improved by synergistic integration of band structure engineering and exsolution process.

The state-of-the-art thermoelectric power system for space applications has typically been based up on either Si_1-xGe_x alloys or PbTe/TAGS for the past 50 years. Although reliable and robust, the thermoelectric performance of these systems remains low with a system level conversion efficiency of ~6%. In recent years, complex materials such as n-type La_3-xTe₄ and p-type Yb₁₄MnSb₁₁ have emerged as new high efficiency, high temperature thermoelectric materials with ZTmax on the order of 1.2 at 1275 K. The high performance of these complex structures is attributed to their 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 thermopower values near their peak operating temperatures. Computational modelling indicates that the conduction band of La_3-xTe₄ is dominated by the La d-orbitals. Introduction of states near the Fermi level could potentially lead to a significant enhancement of the electronic transport properties. Praseodymium telluride (Pr_3-xTe₄) is a La_3-xTe₄ analogue with 3 f-electrons (whereas La has none). Density functional theory (DFT) calculations indicate that the f-electrons lead to a sharp peak in the conduction band edge near the fermi level. In order to verify the theoretical calculations, we utilized a mechanochemical approach to synthesize Pr_3-xTe₄ with various Pr:Te vacancy concentrations. The powders were compacted using spark plasma sintering (SPS) and the compacts were characterized using X-ray diffraction, scanning electron microscopy, and electron microprobe analysis. The temperature dependent electrical resistivity, Seebeck coefficient, and thermal conductivity will be presented and discussed.

It is well known from growing binary semiconductors that at least two different AB semiconductors can be produced d with A-excess or AB with B-excess have distinctly different properties: one possibly being n-type and the other could be p-type. IFor the discovery of new functional semiconductors, these multiple, distinct states of the same nominal composition expand the space of materials to investigate. In thermoelectrics for example, researchers have been examining hundreds of nominally single phase materials for decades in search of, for example, n-type Zintl compounds with predicted high thermoelectric efficiency. The discovery of high performance n-type Mg₃Sb₂, only recently, highlights the importance of examining all the distinct thermodynamic states by identifying the phase boundaries (Mg-excess as well as Sb-excess in this case) we call phase boundary mapping. Futher examples in CoSb₃ skutterudites and complex Zintl phase Ca₉Zn_4+xSb₉ will be given.

High power factors sparked significant experimental efforts to synthesize SrTiO₃ with high thermoelectric efficiencies; it has become one of the most heavily studied n-type oxide thermoelectric materials. A complex band structure is believed to be responsible for the high power factors. Despite persistent efforts, the experimentally realized thermoelectric figure of merit in these materials is less than 0.5, even at temperatures as high as 1000 K. In this work, post-processed electronic structure calculations are used to elucidate how the quazi-2D electronic structure of SrTiO₃ emerges from Ti_d-O_p molecular orbitals. This chemical view offers an intuitive understanding of the complex band structure. A band model, that highlights the 2D nature of the band structure, is developed to model electronic transport in n-type SrTiO₃ single crystals. This model, implemented with acoustic phonon scattering, explains the temperature and carrier dependent effective mass of SrTiO₃, and the high power factors with high effective valley degeneracy. Based on this robust electronic transport model, the figure of merit at optimal doping conditions is evaluated as a function of lattice thermal conductivity, the last free parameter in the performance of SrTiO₃ as a thermoelectric.

Experimental evidence shows that grain boundaries are responsible for the thermally-activated conductivity in some thermoelectric materials, such as Mg₃Sb₂. Existing grain-boundary models using the Matthiessen’s rule on the carrier scattering rate fail to explain the thermally-activated conductivity in n-type Mg₃Sb₂-based materials. We establish a model describing the carrier conductivity (σ) and Seebeck coefficient (S) of polycrystalline thermoelectric materials. The key factor is to treat the depletion region induced by the grain boundary as a secondary phase, which takes into account the relatively larger depletion width in semiconductors, as compared with classical metals. The model is successfully applied to explain both the temperature dependency (i.e. σ-T) and energy dependency (i.e. log|S|-logσ) of Mg₃Sb₂-based compounds. We discuss how the model can be extended to other thermoelectric materials.

Recent discoveries of new phenomena due to interacting Dirac fermions across vastly different energy and length scales have led to a fascinating convergence between condensed matter physics and high energy nuclear physics. Dirac/Weyl semimetals have a linear dispersion that leads to the electrons near the Fermi energy behaving like Dirac fermions. Topological materials, such as ZrTe₅ Dirac semimetal_, hold promise of transmitting and processing information and energy in new ways. Many of the topological materials originate from the thermoelectric compounds. In this presentation, I will present our studies on the transport properties of Dirac/Weyl semimetals, with a view on thermoelectric applications. Dirac dispersion can give rise to large thermopower in a magnetic field and the Nernst effect. Combined with an ultrahigh carrier mobility, Dirac/Weyl semimetals may be exploited for thermomagnetic refrigeration.

We demonstrate a van der Waals Schottky junction defined by crystalline phases of multilayer In2Se3. Besides ideal diode behaviors and the gate-tunable current rectification, the thermoelectric power is significantly enhanced in these junctions by more than three orders of magnitude compared with single phase multilayer In2Se3, with the thermoelectric figure-of-merit approaching ∼1 at room temperature. Our results suggest that these significantly improved thermoelectric properties are not due to the 2D quantum confinement effects but instead are a consequence of the Schottky barrier at the junction interface, which leads to hot carrier transport and shifts the balance between thermally and field-driven currents. This “bulk” effect extends the advantages of van der Waals materials beyond the few-layer limit. Adopting such an approach of using energy barriers between van der Waals materials, where the interface states are minimal, is expected to enhance the thermoelectric performance in other 2D materials as well.

Toplogical insulators (TI) represent a new quantum state of matter which is characterized by peculiar edge of surface states and expect to observe new physical phenomena that have never been observed in other system.
We report transport studies, Shubnikov-de Haas oscillations (SdH) and thermoelectric properties on Bi₂Te₃ topological insulator thin layers and wires.
Perfect single crystalline of Bi₂Te₃ layers with thickness 10- 20 μm were prepared using the mechanical exfoliate method by cleaving thin layer from bulk single crystal Bi₂Te₃ samples [1]. Bi₂Te₃ microwires in glass coating with diameter d= 10- 20 μm were prepared by the Ulitovsky- Taylor method [2]. X- ray studies showed that the Bi₂Te₃ layers were single- crystal and the plane of the layers was perpendicular to the C₃ trigonal axis. The microwire core is in general polycrystal consisting of big disoriented single crystal blocks.
From experimental data on SdH oscillations at temperature of 2.1- 4.2 K, cyclotron effective mass, Dingle temperature and the quantum mobility of charge carriers are calculated. The high quantum mobilities m_q~13000cm²/V*sec. were determined from SdH oscillations in longitudinal (LM) (H||I) and transverse (H⊥I) magnetic fields (TM) up to 14 T at 2.1 K in layers and are substantially higher than in the bulk.
It was revealed, that the value of phase shift SdH oscillations has made 0,5 both in parallel, and in perpendicular magnetic fields in Bi₂Te₃ layers and wires. It is known, that phase shift is connected with Berry’s phase which is the integrated characteristic of orbit cyclotron curvature and the electron dispersion and is characteristic surface state.
The unique surface properties, transport, thermoelectric and thermopower measurements observed in these objects (layers and wires TI) contributes the new complex approach to thermoelectric device (thermogenerators and thermocoolers) fabrication.
This work was supported by Institutional project 15.817.02.09A.

[1] D. Teweldebrhan, V. Goyal and A. Balandin, Nano Lett., 10(4), 1209–1218 (2010).
[2] A. Nikolaeva, D. Gitsu, L. Konopko, M. Graf, and T. Huber, Phys. Rev. B, 77, 075332 (2008).

Significant progress has been made on searching for good thermoelectric materials in the past twenty years. Recently we have made good progresses in finding some new half-Heusler materials with good thermoelectric properties and also improving the properties of some existing half-Heusler materials. For Zintl materials, we discovered that Mg vacancy was the root cause for the strong electron scattering by the ionized impurities causing low electrical conductivity, so we were able to drastically suppress the ionized impurity scattering through replacing a very small amount of Mg by Nb, Fe, Co, Hf, Ta, etc., leading to a higher peak ZT of ~1.8 and drastically higher average ZT of ~1.2 in the temperature range from 300 K to 800 K. These materials may find their potential applications in the mid-high temperature heat sources for power generation.

The lecture will provide a short overview on the development of complex thermoelectric materials and information on the research field of multifunctional perovskite-type ceramics and Heusler-type intermetallics gaining importance for future thermoelectric technologies.
High temperature thermoelectric applications require tailoring of thermoelectric materials for different temperature levels. For the high temperature side of a converter oxides are generally the material of choice while at the low temperature side intermetallic compounds and chalcogenides are the better performing materials. The good performance of all those materials can be explained based on their suitable and tuneable band structure, stability, adjusted charge carrier density and mobility of e.g. strongly correlated electronic systems. These properties are predicted by theoretical concepts based on a fundamental understanding of the composition-structure-property relation to adjust the composition, structure and size of the crystallites in tailored scalable synthesis procedures.

Motivated by high peak zT values reported 13 years ago for the (Ti,Zr,Hf)NiSn Half-Heusler system, Half-Heusler materials are identified as a promising materials class for thermoelectric applications. We investigate the TiNiSn sub-system, which does not rely on expensive Hf.
From phase boundary mapping, we find that the thermoelectric TiNiSn Half-Heusler phase shows a narrow solubility range on the Ti-Ni-Sn phase diagram primarily in the range of excess Ni that can be approximated as TiNi_1+xSn, where x is temperature dependent with 0 ≤ x ≤ 0.06 at 1223 K. Four phase boundary compositions with different Ni contents associated with four three-phase regions are identified. We characterize the thermoelectric properties of these stable compositions and find significant difference between Ni-rich and Ni-poor phase boundary compositions of TiNiSn, which amounts up to 41%, 58%, and 25% difference in Seebeck coefficient, lattice thermal conductivity, and thermoelectric figure of merit respectively. This explains the large discrepancy of literature data on the thermoelectric properties of TiNiSn within a unified phase diagram framework.
We demonstrate that Ni-rich TiNiSn results in a narrower band gap using the Goldsmid formula, which we interpret to be due to the formation of an impurity band from interstitial Ni in the forbidden gap as previously suggested. Interstitial Ni atoms scatter both electrons and phonons, with the latter effect being much stronger, thus a lower lattice thermal conductivity compensates the decrease in electron mobility leading to a high zT value of 0.6 at 850 K for intrinsic Ni-rich TiNiSn. With Sb doping, the carrier concentration in these stable boundary compositions can be tuned but the distinct features in their transport properties remain unchanged. A maximum zT value of 0.6 was also achieved at 850 K for intrinsic Ni-poor TiNiSn upon Sb doping.
We further present results on the integration of n-type TiNiSn Half-Heusler material into 36-legs modules in conjunction with p-type FeTiNb Half-Heusler material including their multi-physics modelling-assisted matching and integration into a heat exchanger mounted and tested in the exhaust gas system of a sport utility vehicle.

Y. Tang, X. Li, L.H.J. Martin, E. Cuervo Reyes, T. Ivas, C. Leinenbach, S. Anand, M. Peters, G.J. Snyder, C. Battaglia, Energy & Environmental Science, in press

D. Landmann, Y. Tang, D. Widner, R. Huber, R. Widmer, C. Battaglia, in preparation

Half-Heusler compounds are one of the most promising candidates for thermoelectric materials for automotive and industrial waste heat recovery applications. In this talk, we will give an overview about our recent investigations in the material design of thermoelectric half-Heusler materials. Since the price for Hafnium was doubled within the last 2 years, our research focusses on the design of half-Heusler compounds without Hafnium. We will present a recent calculation on ZT per € and efficiency per € for various materials followed by our very promising results for n-type half-Heusler compounds without Hafnium resulting in 20 times higher ZT/€ values, which reduces the cost of TE materials used in a commercial TEG by 90%, entering an economical meaningful scenario. We will show how we adapted our knowledge from the n-type materials to design p-type Heusler compound without Hafnium exhibiting similar thermoelectric properties. We will present how we used phase separation to design thermoelectric highly efficient nano-composites of different single-phase materials. Since any high temperature TE material will only be suitable for the mass market if the material production and the module production is industrial upscalable, we will comment on various upscaling approaches, their challenges, and how one could tackle these challenges.
These results strongly underline the importance of phase separations as a powerful tool for designing highly efficient materials for thermoelectric applications that fulfill the industrial demands for a thermoelectric converter. Finally, we will discuss if and how the rather new topological materials and Weyl materials could have an impact in the thermoelectric material science and especially in the thermoelectric application scenarios.

Thermoelectric oxides are considered as promising materials because of their durability against high temperature in air, low cost for producing and non-toxicity etc. Thermoelectric modules using p-type Ca_2.7Bi_0.3Co₄O₉ (Co-349) and n-type CaMn_0.98Mo_0.02O₃ (Mn-113) have been produced using Ag paste to form junctions. In order to enhance the conversion efficiency of the modules, repetition of hot-forging was attempted to prepare the Co-349 bulks. The power factor of the sample prepared by three repetitions of hot-forging is 1.4-2.9 fold higher than one-time hot-forging. The out-put power of the thermoelectric module composed of Co-349 and Mn-113 devices is enhanced by twice. The maximum power density of the module was increased to 0.72 W/cm² against the total cross-sectional area of the devices at 1073 K of the heat source temperature (T_H) by water cooling at 293 K (T_c).
The durability against high temperature, heat cycling, and vibration of the oxide modules was investigated quantitatively. Life time tests have been carried out for the oxide modules up to 1073 K of T_H by water circulation at 293 K under the air atmosphere. No degradations in both generated power are observed up to 1073 K of T_H. The durability against heat cycling was investigated between 873 and 373 K of T_H in air. The maximum out-put power is kept constant during 1000 times of the heat cycling. The vibration test assumed to be used on automobiles was carried out for the oxide thermoelectric module at room temperature. The change of contact resistance at the junctions between before and after the vibration test was measured. Great changes tend to be observed near the four corners of the module.
The air-cooled thermoelectric units have been developed using heat pipes. The maximum out-put power reaches 2.2 W at 823 K of the heat source temperature. The power generation can be shown by lighting LED lamps, charging the smart phone and portable TV, and wireless transmission of data and moving images by the temperature sensor and web camera, respectively using the combustion of natural gas or firewood as the heat sources.

We propose a model of a thermoelectric generator (TEG) based on composite material obtained by sintering of diamond nanoparticles [1]. The effect of electrons drag by ballistic phonons is used to increase the useful conversion of heat into electric current. The effect of the thermal resistance of the boundaries between the graphite-like and diamond-like phases of the composite is used to reduce the ineffective heat spread. It is predicted, that such TEG can possess a record value of thermoelectric parameter, ZT, up to 3,5 [2]

We calculate an optimal thickness of sp² layers that occurs between diamond nanoparticles during the process of sintering (Fig. 1). The thicker are sp² layers, the larger is conductivity of the composite and the smaller is thermal conductivity, which is good for thermoelectric conversion. However, if sp² layers are thicker than a phonon mean free pass, the effect of electrons drag by ballistic phonons is reduced, the electrons drag by phonons comes to a diffusive regime and thermoelectric parameter drops drastically. We estimate the phonon mean free path in the sp² region. It turns out to be approximately 5 nm. Then we deduced that optimal thickness of sp² layers l is l = (L/2) cos(π/6), where L is an initial mean size of nanodiamonds. Hence, the optimal thickness of sp² layers is l ≈ 1.1 nm.

We calculate thermal resistance of the composite with optimal structure taking into account thermal resistance of the boundaries between the graphite-like and diamond-like phases. The thermal resistance of such boundaries arises because electrons transferring heat in the metal do not transfer it through the interface, but are involved in the heat transfer only at a certain distance from it [3]. The thermal resistance of the boundaries crucially restricts thermal conductivity of such composites thus increasing thermoelectric parameter.

An existence of an optimal volume ratio between graphite-like and diamond-like phases of the composite is predicted and obtained experimentally. The maximum value of the thermoelectric coefficient exceeds its minimum values of 5 μV/K for graphite by 20 times – but not by 1000 times as it is expected. Probably, this is explained by a failure in creating the TEG of the optimal design.

The authors are grateful to A.Ya. Vul’ for his attention to this work. A.P. Meilakhs and E.D. Eidelman are grateful to the RSF (Project 16-19-00075) for support. B.V. Semak is grateful to the Russian Foundation for Basic Research (Project 16-03-01084a).

References
[1] Eidelman E. D., Meilakhs A.P., Semak B.V., Shakhov F.M. Journal of Physics D: Applied Physics, In Press.
[2] Eidelman E. D., Meilakhs A.P. Nanosyst.: Phys. Chem. Math. 7, 919-924 (2016).
[3] Meilakhs A.P., Eidelman E. D. JETP Lett. 100, 81-85 (2014).

Continuously operating thermo-electrochemical cells (thermocells) are of interest for harvesting low-grade waste thermal energy because of their potentially low cost compared with conventional thermoelectrics. However, realizing high areal power densities and high temperature operation has been problematic. Pt-free thermocells devised here provide an output power of up to 12 W m^-2 for an inter-electrode temperature difference (ΔT) of 81 °C, which is six-fold higher power than previously reported for Pt-free planar thermocells operating at ambient pressure. The previous record power output, normalized to (ΔT)², for an organic electrolyte thermocell operating above 100 °C, has been tripled. The advances leading to this performance include the use of: 1) multifunctional inter-electrode thermal separators, 2) improved electrolytes, 3) inexpensive carbon fiber textiles as vascular electrodes, and 4) multi-pin or fin electrodes. Transitioning from conventional single-leg thermocells to arrays with n-type and p-type legs produces 2.18 V from a ΔT of only 21 °C, which enables the practical charging of capacitors for energy storage.

Thermoelectric materials that can be utilized for direct conversion between heat and electricity have drawn worldwide interests for decades. Bismuth telluride (Bi₂Te₃) is the best thermoelectric material for electronic cooling and power generation using low-temperature waste heat, whose property enhancement has great impact on applications. Mixing SiC nanoparticles into the p-type BiSbTe matrix is effective for its thermoelectric property enhancement; a high dimensionless figure of merit (ZT) value up to 1.33 at 373 K is obtained in Bi_0.3Sb_1.7Te₃ incorporated with only 0.4vol% SiC nanoparticles of 30 nm in diameter. Such ZT enhancement by SiC dispersion is also found in n-type BiTeSe, but only happens below 450 K and then ZT decreases at higher temperatures. It is found that SiC dispersion decreases the carrier concentration in n-type BiTeSe by strongly changing the amounts of charged point defects. The decreased carrier concentration shifts the maximum ZT value to lower temperatures, and limits the high temperature ZT values due to minor carrier excitation. Further self-tuning carrier concentration by Cu/I doping effectively solves this problem and realizes a ZT plateau at 473-573 K. In addition to ZT improvement, the mechanical strength of both p-type BiSbTe and n-type BiTeSe are dramatically enhanced by SiC nano-dispersion, which is advantageous for thermoelectric devices’ manufacturing and application.

Hybrid materials consisting of conducting polymers in conjunction with inorganic nanostructures have been proposed for thermoelectric applications near room temperature. The performance of thermoelectric materials are typically discussed in terms of the dimensionless figure of merit ZT = S²σκ^-1T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the temperature. Improvements in the figure of merit can be realized by having a large Seebeck coefficient and electrical conductivity, while simultaneously limiting the thermal conductivity. In practice, this can be challenging because each of these variables are interrelated by the carrier concentration. Hybrid composites are attractive because they can enhance the thermoelectric performance via the intrinsic low thermal conductivity of the polymer, utilizing nanostructuring to improve phonon scattering, and through energy filtering effects. In addition, these hybrid materials systems also offer advantages related to their solubility characteristics, whereby they can be utilized in high-throughput, solution processable manufacturing routes. Most of the hybrid thermoelectric materials that have been reported in the literature have been p-type, owing to difficulties in n-type doping of conducting polymers in conjunction with the nature of the applied nanocrystals. As a result, there is a strong drive to compliment the advances in hybrid p-type materials with new n-type materials, since both types are required for module development. In this contribution, we explore our recent developments in the synthesis of new hybrid materials, where chalcogenide nanowires encapsulated in poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are utilized as templates for the growth of a variety of compounds, such as silver-, bismuth-, and lead-based chalcogenides. We have found that we are able to directly control the stoichiometry of our materials during synthesis, such that we can effectively tune our composites from p-type to n-type. We utilize X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photoemission electron microscopy (PEEM) to detail the development in the structural and morphological properties of our materials and relate these modifications to the overall thermoelectric performance. Ultimately, we aim to develop high-performance composites for low-cost room temperature thermoelectric applications.

As interest in green energy is increasing, there have been many studies on thermoelectric generator (TEG) that convert waste heat into electricity. To evaluate the TE characteristics, the figure of merit ZT value is used, which is defined as ZT = σS²T / k. Where σ is conductivity, S is seebeck coefficient and k is thermal conductivity. σS² is expressed by a power factor. The higher the ZT value, the higher the power factor and the lower the k value, the better the TE material. Traditionally, bismuth telluride-based inorganic materials have been studied because of their high ZT values. However, wearable TEG using body heat has been actively researched meaning that solid and bulk inorganic materials are not suitable. Therefore, conductive polymer is studied recently which is suitable for wearable devices because of its flexibility, transparency, low cost fabrication and solution process.
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT : PSS) is one of the most promising organic materials because it shows the best performance and stability compared to other organic materials. PEDOT: PSS is a mixture of PEDOT and PSS. PEDOT : PSS is suitable for the solution process because it is dissolved in water. PEDOT : PSS is basically thin film through spin coating. Because it has low thermal conductivity but the power factor is lower than that of inorganic materials, there have been many studies to improve the power factor of PEDOT : PSS. In many researches, conductivity can be greatly increased by adding solvent PEDOT PSS. Most of polar organic solvents with high boiling point like DMSO, EG, PEG, MeOH and H2SO4 are used as solvents. Recently, Kim et al. obtained high TE property of ZT = 0.42 by adding DMSO into PEDOT: PSS. However, these studies did not consider any change in mechanical properties with solvents addition. To find the optimal wearable TEG material, both thermoelectric and mechanical properties should be considered simultaneously.
We compared the mechanical properties of PEDOT PSS with various solvents and measured the thermoelectric properties according to strain. According to this study, the H2SO4 treatment method, which is known to achieve high conductivity, is not suitable for wearable TEG materials because the PEDOT: PSS is too brittle and the flexibility is very low. On the other hand, a solvent such as PEG had highly flexible PEDOT: PSS thin film with favorable thermoelectric properties.

Bismuth Antimony Telluride (Bi,Sb)₂Te₃ alloys are the most widely used thermoelectric bulk materials near room temperature, developed in 1950s. Nevertheless, wide use of applications using (Bi,Sb)₂Te₃ alloys are yet constrained because of the low thermoelectric conversion efficiency. In order to enhance the thermoelectric conversion efficiency, low total thermal conductivity of the alloy is required, which can maintain the temperature difference across a material. The total thermal conductivity of (Bi,Sb)₂Te₃ alloys is divided into three components in respect to its physical nature, including electronic, bipolar, and lattice thermal conductivity. Since the electronic thermal conductivity is simply proportional to electrical conductivity under Wiedermann-Franz law, the bipolar and lattice thermal conductivities should be minimized to reduce the total thermal conductivity. Bipolar thermal conductivity can be engineered by controlling band structures, such as carrier concentration or bandgap, and the lattice thermal conductivity can be reduced by introducing various defect structures enhancing phonon scattering. Herein, we analyzed the bipolar and lattice thermal conductivies of (Bi,Sb)₂Te₃ alloys with defect structures, including point defects (0 dimension, 0D), dislocations (1D), grain boundaries (2D), or nano-sized metal inclusions (3D), by using Debye-Callaway model and a single parabolic band model based on Boltzmann transport. Then, the lattice thermal conductivity depending on the density of each defect was estimated based on the analysis, providing materials design rule for reducing thermal conductivity of (Bi,Sb)₂Te₃ alloy. Furthermore, the influence of multiple defects on frequency-dependent phonon scattering was evaluated in order to properly design multi-defect structure.

A new metal telluride Ba₃Ag₃InTe₆ was synthesized by solid-state reaction. This compound adopts a new two-dimensional structure constructed by AgTe₄ and InTe₄ tetrahedra and Ba²⁺ cations. The AgTe₄ tetrahedra form a puckered layer and the InTe₄ tetrahedra form a zig-zag chain dangling from both edges of the layer. The material possesses a narrow band gap estimated to be around 0.48 eV by UV-vis-NIR absorption spectra. The electronic band structure reveals a direct band gap at the G point of face centered primitive Brillouin zone. Ba₃Ag₃InTe₆ is a p-type semiconductor with high Seebeck coefficients about 325 uV/K at 400 K. The electrical conductivity of 9.4 S/cm and the thermal conductivity of 0.35 W/mK give a ZT value of 0.11 at 400 K for the undoped sample. The density of states (DOS) analysis shows that the p-type hole transport is mostly achieved through the layer consisting of AgTe₄ tetrahedra.

The n-type half-Heusler alloy ZrNiSn has been well studied because of its high power factor but its high thermal conductivity leads to low figure-of-merit. Hence in the present work an attempt has been made to reduce the total thermal conductivity by a combination of substitutions. Four different ZrNiSn based half-Heusler alloys ZrNiSn_0.95Ge_0.05, ZrNiSn_0.9Ge_0.1, Zr_0.75Ti_0.25NiSn_0.97Si_0.03 and Zr_0.75Ti_0.25NiSn_0.97Si_0.02Sb_0.01 have been synthesized by vacuum arc melting of pure elements. X-ray diffraction together with Rietveld refinement shows that all the alloys are essentially single phase which has a F4m cubic structure. Scanning electron microscopy shows large grains and a uniform chemical composition with very little loss of elements. Electrical resistivity shows a weak temperature dependence indicating a degenerate semiconducting behavior, 0.5 mΩcm to 6.4 mΩcm. The carrier concentration is found to be ~ 10²⁰ cm^-3. The Seebeck coefficient increases with increasing temperature and shows a bipolar behavior for T > 700 K. The ZrNiSn_0.95Ge_0.05 alloy has the highest Seebeck coefficient of 148 µVK^-1 and a power factor of 2.9 mWm^-1K^-2. The thermal conductivity has a very low temperature dependence indicating significant electronic contribution. The ZrNiSn_0.95Ge_0.05 alloy has a thermal conductivity of ~ 6 Wm^-1K^-1 while the Zr_0.75Ti_0.25NiSn_0.97Si_0.03 alloy has the lowest thermal conductivity of ~ 3.2 Wm^-1K^-1. The ZrNiSn_0.95Ge_0.05 alloy exhibits the highest figure-of-merit of 0.45 at 1023 K.

Advances in thermoelectric(TE) technology have shown promise in the extension of TEs to consumer electronics and wearable energy harvesting devices. While the TE performance of the material is of obvious importance, certain mechanical properties need to be optimized while maintaining as much of the energy conversion efficiency as possible.
A (Bi₂Te₃) nanowire/SWCNT composite material was developed in our research group and has exhibited excellent n-type thermoelectric properties that are required for extension of TEs to wearable devices. In addition to conversion performance, flexibility and durability are highly desired qualities, especially for wearable TEs applications. For the preliminary experiments, Bi₂Te₃ nanowires/CNTs composite was drop-casted onto a flexible polyimide substrate and was analyzed by performing various mechanical tests that could affect the TE properties of the material. The electrical conductivity was measured using the van der Pauw method with an automated four-point probe. Multiple bends in 100 count increments were performed and the conductivity was remeasured. The electrical conductivity of the samples is shown to decrease gradually with each cycle, as expected, yet maintains 70-80% of the initial conductivity after thousands of cycles. Based on the previously obtained results, we will further investigate on film thickness effects and different substrate effects on mechanical properties of composite films. The different types of mechanical testing will be conducted using a Bose dynamic mechanical analysis machine. The knowledge obtained from this research will be beneficial to designing flexible thermoelectric devices using hybrid composite films.

Group IV Tellurides are formidable functional materials, and lead tellurides are no exceptions. This simple rocksalt-type compound is widely known thermoelectric (TE) materials with excellent performances, among other applications. Recently [1], we have shown that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe, which is good for TE devices. Moreover, it was also shown that by alloying this material with Se, the electronic conductivity of the alloys is also enhanced [2]. But, it is not clear if the same occurs when alloying PbTe with Sn. We show, in this work, our theoretical results for the stability, vibrational and dielectric properties of Pb_1-xSn_xTe alloys. The calculations were carried out by using the Density Functional Theory, gradient conjugated techniques, and the plane-wave pseudopotential method (VASP and abinit codes). The alloys were described by both the Virtual Crystal and the Generalized Quasi-Chemical Approximations. Our results show that, while their lattice parameters obey Vegard’s rule, their bulk moduli, phonon frequencies and dielectric constant do not. Based on this feature, we have detected that when increasing the Sn concentration x, the anharmonic LA-TO coupling enhances and reaches its maximum for x ~ 0.70. This corresponds to the maximum value for the dielectric constant as well, and this alloy formation is stable from 600 to 800 K. Consequently, the obtained lattice contribution to the static dielectric constant is higher, when compared with both PbTe and SnTe bulk values, showing that the alloy can behave better as TE device than their bulk counterparts. We acknowledge support from FAPEMIG (grant CEX APQ 02695-14).

1. H. W. Leite Alves, et al., Phys. Rev. B 87, 115204 (2013).
2. Y. Pei, et al., Nature 473, 66 (2011).

Today’s modern soldier relies on a dozen electronic gadgets, from standard gear, such as radios, GPS units, and night-vision goggles, to improvised explosive device (IED) jamming system and mine detecting device, all requiring electrical power, for successful mission. In a typical 72 hour mission, requiring average power of 20 W, a soldier carries 70 individual batteries corresponding to 12.7 kg of rechargeable military batteries or 8.2 kg of primary batteries. Batteries account for 20% of the weight a soldier carries in battlefield. To unburden the soldiers by lightening the battery load, thermoelectric generator (TEG) can be a good candidate as a military portable power sources because it has the inherent advantages of quiet operation, few moving parts, and compact and lightweight construction. Particularly, thermoelectric systems can be easily designed to operate small heat sources and small temperature differences regardless of soldier’s mission time and climate change. In this study, we developed a military portable TEG with a maximum output power of 10 W and a weight of 1.2 kg at a designed temperature difference of 140°C. A TEG is composed of four Bi-Te thermoelectric modules (TEMs) connected in series and water cooling. A variety of environmental tests were carried out to investigate the robustness of TEG under the environmental conditions simulated deployment of military soldier use, which were categorized into high temperature (storage, operation), low temperature (storage, operation), humidity, vibration, shock, and transit drop according to MIL-STD-810F. A developed TEG exhibited a reliable electrical performance and no mechanical damage after all of environmental tests. This durability test data of TEG can provide good environmental test criteria in designing the robust TEG as a military portable power sources.

We investigated the effect of graphene quantum dots (GQDs) on the thermoelectric properties of free-standing poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (.PEDOT:PSS) films. The electrical conductivity and Seebeck coefficient of the film containing 0.50 vol% GQDs are 164.60 S/cm and 34.85 µV/K, compared with 22.50 S/cm and 27.72 μV/K, respectively, for the pristine PEDOT:PSS film without GQDs. The power factor (PF) increased up to 22.37 μW/mK², which is ~13 times higher than that (1.73 μW/mK²) of the pristine film through the selective dedoping of PEDOT by the chemical treatment. Thus, the improved PF is due to the optimized charge carrier concentration and increased Hall mobility by the morphological and structural evolution

A material’s thermoelectric efficiency is represented by its figure of merit, zT. In order to maximize zT, the electrical conductivity and Seebeck coefficient of a material must be simultaneously increased. Materials with anisotropic crystal structures are of particular interest as they present a method of decoupling the Seebeck coefficient from the electrical conductivity.
The focus of the research are Ca₅M₂Sb₆ (M = Al, Ga, or In) Zintl compounds, containing covalently-bonded MSb₄ tetrahedral polyanions that resemble infinite double chains. Density functional theory predicts light band mass and improved thermoelectric performance in the direction parallel to the MSb₄ chains. Verifying this effect experimentally requires single crystals of sufficient size. Synthesis of pure-phase polycrystalline Ca₅M₂Sb₆ (M= Ga, or In) and subsequent flux growth using either Sn, GaSb, or InSb flux was used to obtain single crystals up to 2 mm in length. The crystals were found to grow preferentially along the c-axis (parallel to the tetrahedral chains), leading to needle-like morphologies. The grown crystals were analyzed using single crystal X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy to determine phase purity and crystal structure. The electronic and thermal properties were measured parallel to the long axis of the needles, in the direction predicted to have highest zT.

The observation of the spin-Seebeck effect has opened a new direction to spin current generation and manipulation. This alternative thermal approach of spin injection and thermoelectric energy conversion is believed to be energy-efficient. The spin-Seebeck effect is attributed to phonon-driven spin redistribution and magnon thermal transport. This has led to significant interest in spin-mediated thermal transport and spin-phonon interactions. The spin-phonon interactions and magnon transport can be understood by studying the spin-mediated thermal transport behavior in ferromagnetic and semiconductor thin films. In this work, we present the magneto thermal transport behavior characterization of ferromagnetic (Ni₈₀Fe₂₀) and semiconductor (p-Si, n-Si) thin films and their interfaces.

Cu₂(Te_1-x,Se_x) bulks with x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0 were prepared via powder metallurgy method, including high energy ball milling, cold compacting and inert gas annealing. The entire process was shortened to <20 hours compared to the traditional method which might cost several days. The results showed that Cu₂(Te_0.9,Se_0.1) inherited the four phase transitions of Cu₂Te from room temperature up to 800 K. When x=0.1, the introduced Se would cause the formation of Se containing nano-scale particles covering Se free matrix, remarkably suppressing the thermal conductivity of the material. An enhanced ZT of ~0.9 at 1000 K could be obtained in Cu₂(Te_0.9,Se_0.1) bulks.

The thermal conductivity χ, electrical conductivity σ and thermopower α in foils of Bi_1-xSb_x alloys in semimetallic and semiconducting states in the temperature range 4.2-300 K were experimentally studied. The foils of Bi_1-xSb_x alloys were obtained by the method of high-speed crystallization of a thin layer of melt on the inner polished surface of a rotating copper cylinder.
High crystallization rates v = 5*10⁵ m/s ensured a uniform distribution of the components in the volume. The thickness of the foils was 10-30μm with the texture 1012 parallel to the plane of the foil and the C₃ axis coinciding with the nominal foil. The semimetal-semiconductor transition is observed in Bi_1-xSb_x foils at x> 0.03%Sb, as in bulk single crystals of the corresponding composition
It is shown that the thermal conductivity of semimetallic Bi-3%Sb foils in the low-temperature range (T <10 K) is two orders of magnitude smaller, and in semiconductor (Bi-16%Sb) it is an order of magnitude smaller than in bulk samples of the corresponding composition. The effect is interpreted from the viewpoint of decreasing the phonon drag effect in the low-temperature region due to both surface scattering and scattering at grain boundaries of the foil texture. From the dependences ρ(T), α(T), χ(T), the thermoelectric efficiency of foils was calculated in the temperature range 5-300 K. It is established that at 100 K the thermoelectric efficiency ZT in semiconductor Bi_1-xSb_x foils is 2 times higher than for bulk samples with crystallographic orientation similar in foils, which can be used in low-temperature thermoelectric energy converters.
This work was supported by Institutional project 15.817.02.09A

Molybdenum disulphide (MoS₂) has attracted much attention in thermoelectric application because of its high carrier mobility and tunable electronic and thermal properties which can be tailored by controlling the number of layers. Bismuth telluride (Bi₂Te₃) and antimony telluride (Sb₂Te₃) are the most efficient thermoelectric materials at the room temperature. In the present study, we have prepared nanocomposite samples of Bi₂Te_3, (Bi₂Te₃/MoS₂) and Sb₂Te₃ , (Sb₂Te₃/MoS₂) using MoS₂ nanoflakes. The effect of incorporating MoS₂ nanoflakes on electronic and thermoelectric properties Bi₂Te₃/MoS₂ and Sb₂Te₃/MoS₂ nanocomposite samples have been studied. The value of ZT was calculated to be 0.77 and 0.48 for Bi₂Te₃/MoS₂ and Bi₂Te₃ samples, respectively, at room temperature. Similarly, ZT value of 0.18 and 0.28 was calculated at 427 K for Sb₂Te₃ and Sb₂Te₃/MoS₂ samples, respectively. Both nanocomposite samples show higher ZT values as compared to the corresponding pristine samples. In the case of Bi₂Te₃/MoS₂, the enhancement in ZT is due to decrease in thermal conductivity, whereas, in Sb₂Te₃/MoS₂ it is due increase in the power factor. This difference can be attributed to the difference in behavior of Bi₂Te₃/MoS₂ and Sb₂Te₃/MoS₂ interfaces. Kelvin probe force microscopy (KPFM) has been employed to determine the surface potential values of the pristine and nanocomposite samples. The above study shows that the surface potential value at Bi₂Te₃/MoS₂ interface is lower by 300 mV as compared to Bi₂Te₃ and in case of Sb₂Te₃/MoS₂ surface potential is observed to be 150 mV lower as compared to Sb₂Te_3. This decrement of the surface potential value shows higher work function at the interface in comparison to the pristine sample. The interface energy barrier in Bi₂Te₃/MoS₂ and Sb₂Te₃/MoS₂ nanocomposite samples is expected to modify electron/hole transport and phonon scattering.

Thermoelectric energy harvesters that convert any source of heat into electricity, are gaining attention due to the rapid increase of power needs of the internet of things. At a given temperature the efficiency of TE devices is determined by a figure of merit (ZT). In order to achieve a high ZT it is necessary to decrease thermal conductivity while maintaining a high power factor (electrical conductivity times the square of Seebeck coefficient) to attain the phonon-glass-electron-crystal regime. Si/Ge superlattices (SL) have been extensively investigated as a TE material for over 30 years. Thermal transport in such SLs is significantly diminished due to phonon scattering at interfaces [1]. Introduction of interstitial defects is a viable approach to introduce additional scattering mechanism to further reduce thermal conductivity and thus improve ZT in SLs [2]. However, understanding of electronic transport in SLs with interstitial defects is critical for development of TE devices with high efficiency. In this work we investigate the effect of interstitial defects on electronic transport in Si/Ge SL by employing first principle DFT calculations in conjunction with semi-classical Boltzmann transport theory. Interstitial defects introduce additional energy levels and strain in the system. To understand the effect of additional levels we investigate electronic transport in bulk silicon with 1.56% of commonly occurred interstitial defects: Ge, C, Si and Li placed in different symmetry locations of the lattice. Interstitials lead to the formation of additional deep and/or shallow energy levels depending on both the guest species type and the symmetry location. Upon comparison of the electronic transport coefficients of all different silicon-interstitial systems we observe that 1.56% of Ge interstitial defects placed in hexagonal sites provide the best improvement of ZT by a factor of 17 with reference to the bulk value. In a parallel study of ideal Si/Ge SLs of varying periods, we demonstrate that the electronic transport properties can be tuned by applying external strain. At higher carrier concentrations positive strain (tension) in the in-plane direction of the SL leads to significant improvement of the Seebeck coefficient. Finally, we introduce interstitial defects in Si/Ge SL to determine how additional energy levels and defect-induced strain can be used to tailor electron transport in superlattices.
1. S.-M. Lee, D. G. Cahill, and R. Venkatasubramanian, "Thermal conductivity of Si–Ge superlattices", Appl. Phys. Lett. 70, 2957 (1997).
2. P. Chen, N. A. Katcho, J. P. Feser, Wu. Li, M. Glaser, O. G. Schmidt, D. G. Cahill, N. Mingo, and A. Rastelli, "Role of Surface-Segregation-Driven Intermixing on the Thermal Transport through Planar Si/Ge Superlattices", Phys. Rev. Lett. 111, 115901 (2013).

Manipulation of thermal transport (pursuing ultra-high or ultra-low thermal conductivity) is on emerging demands, since heat transfer plays a critical role in enormous practical implications, such as efficient heat dissipation in nano-electronics and heat conduction hindering in solid-state thermoelectrics. It is well established that the thermal transport in semiconductors and insulators (phonons) can be effectively modulated by structure engineering or materials processing. However, almost all the existing approaches involve altering the original atomic structure, which would be frustrated due to either irreversible structure change or limited tunability of thermal conductivity. Motivated by the inherent relationship between phonon behavior and interatomic electrostatic interaction, we comprehensively investigate the effect of external electric field, a widely used gating technique in modern electronics, on the lattice thermal conductivity (). Taking two-dimensional silicon (silicene) as a model system, we demonstrate that, by applying electric field (E_z = 0.5 V/Å) the thermal conductivity of silicene can be reduced to a record low value of ~0.091 W/mK, which is more than two orders of magnitude lower than that without electric field (19.21 W/mK). Fundamental insights are gained from the view of electronic structures. With electric field applied, due to the screened potential resulted from the redistributed charge density, the interactions between Si atoms are renormalized, leading to the phonon renormalization and the modulation of phonon anharmonicity through electron-phonon coupling. Our study paves the way for robustly tuning phonon transport in materials without altering the atomic structure, and would have significant impact on emerging applications, such as thermal management, nanoelectronics and thermoelectrics.

Semiconducting single-wall carbon nanotubes (s-SWNTs) are expected to have high Seebeck coefficient and high electrical conductivities [1,2]. However due to the presence of metallic SWNTs, polymer wrapping, and dopants, the effective Seebeck coefficient of materials based on s-SWNTs have been measured to be much lower [3]. In this work, we present a comprehensive study of the effect of temperature and doping (both n- and p-type) on the thermoelectric transport in ultra-pure (>99.9 %) s-SWNT networks. We measure the highest electron and hole Seebeck coefficients for polymer-free s-SWNT networks, over the 80 to 600 K temperature range.

We use nanoscale on-chip thermometry to measure the electrical conductance and Seebeck coefficient of ultra-pure s-SWNT networks (5-7 nm thick) as a function of Fermi energy by back-gating the network, from electron- to hole-transport regime. We fabricate large numbers of devices based on an electrical thermometry technique [1]. First, we pattern and deposit Ti/Pt electrodes on 300 nm SiO₂/p++ Si substrates. Next, the s-SWNT are solution-deposited onto the chips. We remove traces of polymer from the SWNTs and subsequently pattern and etch the s-SWNTs into 10–50 µm scale channels over the electrical thermometers.

We measure the Seebeck coefficient and electrical conductance of these networks under vacuum while varying the back-gate voltage over temperatures ranging from 80 K to 600 K. Due to the high quality of the s-SWNTs, the networks have high electrical on/off ratios of 10⁶. The SWNT films transition from p-type to ambipolar transport above 450 K. Beyond 550 K, we measure both high hole and electron thermopower, reaching of up to ±550 μV/K, which is a record for a SWNT network. We attribute our results to the minimal amount of SWNT bundling, low polymer residue, and very few metallic SWNTs in our networks. Using physical models of the thermal, thermoelectric, and electrical properties of both individual SWNT and the junction between s-SWNTs, we produce a deeper understanding of the fundamental thermoelectric transport in these networks.

[1] J. P. Small et al., Phys. Rev. Lett., 91, 256801 (2003)
[2] N. T. Hung et al., Phys. Rev. B, 92, 165426 (2015)
[3] A. D. Avery et al., Nat. Energy 1, 16033 (2016)

Demands on high-quality two-dimensional (2D) chalcogenide thin films are growing due to the findings of exotic physical properties and promising potentials for device applications. However, the difficulties in controlling epitaxy with defect density and an unclear understanding of van der Waals epitaxy (vdWE) for 2D chalcogenide film on the substrate have been major obstacles for the further advances of these materials. In this research, we demonstrate new scalable approaches enabling the vdWE of 2D chalcogenide films on 2D and 3D substrates. As a proof of concept, highly-crystalline bismuth antimony telluride thermoelectric thin-films were epitaxially grown on 2D (graphene) and 3D (α-Al₂O₃) substrates by pulsed laser deposition. It was elucidated that the vdWE growth mechanism of these films on 2D and 3D substrates is utterly governed by the surface reaction of the substrate with chalcogen. In particular, this peculiar vdWE renders the high-quality 2D chalcogenide film with superior carrier mobility and low defect density comparable to single crystal. Furthermore, exceptionally low thermal conductivity were observed in these vdWE films.

The strategy of alloying in the Mg₃Sb₂-Mg₃Bi₂ thermoelectric compound has originally been understood mostly as a means to reducing the thermal conductivity. However, it is evident from electronic transport properties that alloying also has a significant impact on the band structure. To fully understand the optimum alloy composition for thermoelectrics, it is necessary to model both the p- and n-type compounds as a function of the Mg₃Sb₂ vs. Mg₃Bi₂ composition. We establish a model for the optimum alloy composition and find that the electronic property enhancement accounts for about 50 % of the benefits from alloying. We discuss how the Mg₃Sb₂-Mg₃Bi₂ alloying impacts the band structure in terms of the band gap, mass, and convergence, which are the essential features that should be considered for band engineering in this material.

The thermal conductivity (k) of silicon thin films can be reduced by additional phonon scattering at boundaries of the thin film. Though the lower k makes thermal management in electronics devices challenging, it is promising for an enhancement of the power factor (for thermoelectric devices) in silicon nanostructures, which has been demonstrated in recent experimental and theoretical studies. Beyond nanostructuring, mechanical strain impacts both the electron and phonon transport in nanostructures. This work focuses on using strain engineering to reduce thermal conductivity in order to further improve the thermoelectric figure of merit (ZT) in sub-40-nm silicon nanofilms. While past simulations showed an impact of strain on thermal transport in semiconductor films, there is not yet a conclusion on its impact on ZT due to the conflicting simulation results. Thus, here, we systematically measure the size-, strain-, and temperature- dependent thermal conductivity to elucidate the strain-dependent phonon transport in strained silicon nanostructures. In addition to the impact of strain on thermal transport in silicon nanofilms, we evaluate the potential impact on ZT.

Reliability characteristics of thermoelectric module (TEM) depends on case-by-case application and availability. A military soldier’s portable thermoelectric generator (TEG) mainly consists of thermoelectric modules, heating parts, cooling parts, and power circuit. Thermoelectric module is a key factor to determine the mechanical and electrical durability of a military portable TEG. From the life cycle environmental history of the TEM as an application of soldier’s portable power sources, the main natural and induced environmental stresses were categorized into high temperature, low temperature, humidity, vibration and shock. According to MIL-STD-810F, reliability tests of TEM were successfully carried out under the environmental conditions simulated deployment of a military solider use. In this study, Bi-Te thermoelectric module was used to evaluate its environmental and mechanical reliability under the military environmental conditions. The bismuth-telluride (Bi-Te) TEM after environmental tests exhibits a slight decrease of maximum output power about 3 ~ 6%. A humid atmosphere is more harmful to the electrical performance of TEM as an application of portable power generator, and the TEM should be encapsulated by metal housing with an inert or vacuum atmosphere or sealing its outer perimeter to prohibit it from water contact. The Bi-Te TEM after environmental tests exhibits no visual evidence of mechanical damage. The Bi-Te TEM after mechanical tests (vibration and shock) exhibits no degradation of maximum output power, and no visual evidence of mechanical damage, such as cracking or deformation. This reliability test data of TEM can provide good design factors in designing a military portable thermoelectric generator.

We have recently demonstrated subwavelength thermoelectric nanostructures designed for resonant spectrally selective absorption, which creates large localized temperature gradients even with unfocused, spatially-uniform illumination to generate easily measureable thermoelectric voltages[1]. We have shown that such structures are tunable and are capable of highly wavelength - specific detection, with an input power responsivity of up to 38 V/W, referenced to incident illumination, and bandwidth of nearly 3 kHz, by combining resonant absorption and thermoelectric junctions within a single membrane-suspended nanostructure, yielding a bandgap-independent photodetection mechanism. We report results for both resonant nanophotonic bismuth telluride – antimony telluride structures and chromel – alumel structures as examples of a broad class of nanophotonic thermoelectric structures useful for fast, low-cost and robust optoelectronic applications such as non-bandgap-limited hyperspectral and broad-band photodetectors.

Probing thermal states in such nanophotonic systems has traditionally come with the caveat that the measurement technique itself alters the thermal state of the system. For example, AFM thermal probes, which come within nanometers of the surface, may radiatively alter the system and are also limited by thermocouple error. Platinum RTD thermometers, with wires comparable to the size and thickness of the nanophotonic device itself, alter the thermal profile of the system. Non-contact, far-field techniques such as Fourier transform infrared spectroscopy, are limited in collection area by the thermal wavelength which is often larger than the nanophotonic device being measured. Additionally, it is not possible to obtain sub-Kelvin temperature resolution by collecting thermal radiation.

We have developed a non-invasive measurement technique which uses the nanophotonic materials themselves as thermometers. We combine noise thermometery, which measures the absolute temperature of the electrons within the nanophotonic material, with thermoelectric measurements of the nanophotonic devices, which allow us to observe the temperature rise in a nanophotonic wire array with probes as far as 100 microns away from the center of light absorption in a nanophotonic thermoelectric device. We predict a temperature rise of several Kelvin within the nanophotonic structures, and test these predictions using room temperature, kHz noise thermometry to get better than 0.5 Kelvin error in temperature.

[1] Mauser, K. M., et al, “Resonant Thermoelectric Nanophotonics”, Nature Nanotechnol., 2017, 12, 770-775.

(GeTe)_mSb₂Te₃ alloys have been previously shown to be excellent thermoelectric material with figure of merit >2 when fully optimized. The (GeTe)_mSb₂Te₃ superlattice can be visualized as m layers of GeTe inserted into the center of each Sb₂Te₃ slab, which expands the initial unit cell of Sb₂Te₃ to include long-range ordered 3D blocks with vacancies between the blocks. The (GeTe)_mSb₂Te₃ superlattice exhibits a phase transition from rhombohedral (R-3m) to cubic rock salt (Fm-3m) at high temperature, similar to GeTe. This reversible phase transition is accompanied by abrupt changes in electrical and optical properties, enabling applications in phase-change memory devices. However, even though the structural and thermal properties of these materials have been studied in some depth, the effect of the phase transition on bond strength and phonon transport properties has not been studied. In this study, we combine high temperature X-ray diffraction and high-temperature resonant ultrasound spectroscopy to measure the lattice parameters, elastic moduli and sound velocity in (GeTe)_mSb₂Te₃. We find that the elastic moduli and speed of sound increase gradually with increasing temperature up to the phase transition, then exhibit a final sharp increase upon transforming to the rock salt structure after which the elastic moduli begin to decrease. Our results suggest that with increasing temperature, the ordered vacancy layers diffuse gradually into the surrounding distorted rock salt matrix, increasing the interlayer bond strength, thus leading to the anomalous temperature-dependence of the thermal conductivity.

The intermetallic clathrates first discovered in 1965 [1] attracted attention of chemists and physicists due to the fascinating structural features, especially the formation of large cavities within the three-dimensional framework [2]. These cavities may be also un-occupied (empty clathrates [3]). The coexistence of the different bond kinds (inhomogeneity of the bonding) is one of the reasons for reduced thermal conductivity and opens also the possibility to tune the charge carrier concentration, which makes these materials interesting for thermoelectric applications [4]. The suitable combination of the electronic and phononic transport in clathrates for thermoelectric application was recognized and proven quite fast [5,6]. One of the challenges on the way to an application is the understanding of the low thermal conductivity of this family of materials. One possible mechanism is associated with the vibrations (‘rattling’) of the filler atoms within the cage-like crystal structure (e.g. [7,8]). Recently, the phonon filtering mechanism was proven by the inelastic neutron scattering experiments [9,10].
[1] C. Cros et al. C. R. Acad. Sci. Paris 260 (1965) 4764.
[2] M. Pouchard, C. Cros. In: The Physics and Chemistry of Inorganic Clathrates, Springer, 2014, p 1.
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[10] P.-F. Lory et al. Nature Comm. 8 (2017) 491.

Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 Wm/K) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon–phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon–phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga–N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for applications in energy conversion such as thermoelectrics. Our study offers fundamental understanding of phonon transport in ML GaN within the framework of BTE and further electronic structure, which will enrich the studies of nanoscale phonon transport in 2D materials and shed light on further studies.

Thermoelectric (TE) materials, which could directly convert temperature difference into electricity, are attracting increasing attentions among energy harvesting technologies. Generally, the performance of TE materials is evaluated in terms of a dimensionless figure of merit, ZT=S²σTκ⁻¹, where Z is the figure of merit, T is the absolute temperature, S is the thermopower (≡Seebeck coefficient), σ is the electrical conductivity and κ is the sum of the electronic (κ_ele) and lattice thermal conductivities (κ_lat) of a TE material. In addition to reducing κ_lat, enhancing S²σ, which is regarded as power factor (PF) is also a promising strategy.
Two-dimensional electron system (2DES)–carrier electrons are confined within a narrow layer (the thickness < de Broglie wavelength, λ_D)–is known as one of efficient strategies to achieve an enhanced PF because it could promise an enhanced S without reducing σ. ^[1,2] Since the degree of S-enhancement strongly depends on the two-dimensionality of 2DES, a conducting material with longer λ_D would be efficient to enhance PF if the carrier electrons are confined within a defined thickness layer.
Recently, we found that with increasing x in SrTi_1−xNb_xO₃, carrier effective mass (m^*) exerts a reducing tendency from 1.1m_e to 0.7m_e, when x increases across x = 0.3 point.^[3] So 2DES of 1 u.c. layer thick SrTi_1−xNb_xO₃ (x > 0.3) is hypothesized to exhibit greatly enhanced S due to its longer λ_D and correspondingly stronger two-dimensionality.
Here we report the TE properties of oxide 2DESs, [N unit cells SrTi_1−xNb_xO₃|11 unit cells SrTiO₃]₁₀ superlattices (1 ≤ N ≤ 12, x=0.2−0.9), in which the λ_D of x > 0.3 is ~5.2 nm while that of x ≤ 0.3 is ~4.1 nm. The S-enhancement factor (S_obsd./S_bulk) of the 2DES for x=0.8 was ~1000%, while that for x=0.2 and 0.3 were 400−500%, clearly indicating that two-dimensionality can be enhanced by using a conducting material with longer λ_D. As a result of precise control of N and x, PF of the superlattice (N=1, x=0.6) exceeded ~5 mW m⁻¹ K⁻², which is double of the optimized bulk SrTi_1−xNb_xO₃ (PF~2.5 mW m⁻¹ K⁻²). The present results might be fruitful to design efficient TE materials with 2DES.

References
[1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993).
[2] H. Ohta et al., Nature Mater. 6, 129 (2007); Nature Commun. 1, 118 (2010); Adv. Mater. 24, 740 (2012).
[3] Y. Zhang, H. Ohta et al., J. Appl. Phys. 121, 185102 (2017).

Manipulating heat conduction is an appealing thermophysical problem with enormous practical implications, which requires insight into the lattice dynamics. Although lone-pair electrons have long been proposed to induce strong phonon anharmonicity, no direct evidence is available from a fundamental point of view and the electronic origin still remains untouched. Besides, strain engineering is one of the most promising and effective routes towards continuously tuning the thermal transport properties of materials due to the flexibility and robustness. However, previous studies mainly focused on quantifying how the thermal conductivity is affected by strain, while the fundamental understanding on the electronic origin of why the thermal conductivity can be modulated by mechanical strain has yet to be explored.
In this study, we perform comparative study of thermal transport in two-dimensional group III-nitrides (h-BN, h-AlN, h-GaN) and graphene. Although the monolayer group III-nitrides possess similar planar honeycomb structure with graphene, their thermal conductivity is substantially lower and the root reason cannot be intuitively attributed to the mass difference. We then establish a microscopic picture to connect phonon anharmonicity and lone-pair electrons. Direct evidence is provided for the interaction between lone-pair electrons and bonding electrons of adjacent atoms based on the analysis of orbital-projected electronic structures, which demonstrates how nonlinear restoring forces arise from atomic motions and lead to strong phonon anharmonicity. The microscopic picture of lone-pair electrons driving strong phonon anharmonicity provides coherent understanding of the diverse thermal transport properties of the monolayer group III-nitrides compared to graphene. Furthermore, the thermal conductivity (κ) of planar monolayer group III-nitrides is unexpectedly enlarged by up to one order of magnitude with bilateral tensile strain applied, which is in sharp contrast to the strain induced κ reduction in graphene despite their similar planar honeycomb structure. The anomalous positive response of κ to tensile strain is attributed to the attenuated interaction between the lone-pair s electrons around N atoms and the bonding electrons of neighboring (B/Al/Ga) atoms, which reduces phonon anharmonicity. The microscopic picture for the lone-pair electrons driving phonon anharmonicity established from the fundamental level of electronic structure deepens our understanding of phonon transport in 2D materials and would also have great impact on future research in micro-/nanoscale thermal transport such as materials design with targeted thermal transport properties.

Dopants play a very important role in engineering semiconductor materials. They can strongly influence the phonon scattering processes and thereby the thermal conductivity. We have recently shown how Boron, when substituted in place of Carbon in 3C-SiC, acts as a “super-scatterer” and exhibits resonant phonon scattering which is one to two orders of magnitude higher than Nitrogen and other defects [1]. This opens a new path to tailor thermal conductivities where required values range from very low in thermoelectric materials to very high in power electronics applications.
While the mass difference caused by Boron and Nitrogen is the same when substituting Carbon, it is the large perturbation in the 2^nd order inter-atomic force constants (IFCs) which leads to a resonance. This large IFC perturbation is the result of a small lattice distortion accompanied by a change from tetrahedral to threefold symmetry around the Boron atom. In order to understand the physics behind and the factors causing resonance in semiconductors, we explored such symmetry breaking lattice distortions with the help of a simple 1D mono-atomic linear chain. We found that small lattice distortions emanating from two or more close energy minima in potential energy surface lead to a very large IFC perturbation resulting in resonant phonon scattering. Such a behavior is characterized by a peak in the trace of imaginary part of the T matrix (which is closely related to the scattering rates) and reflection coefficient approaching unity.
Finally, we also show how a similar distortion by Boron in diamond causes an equally large IFC perturbation but does not result in resonant scattering. We demonstrate how a large value of the casual Green's function is required in addition to a large IFC perturbation.
We acknowledge support from EU Horizon 2020 grant 645776 (ALMA) –www.almabte.eu
[1] A. Katre, J. Carrete, B. Dongre, G. K. H. Madsen, and N. Mingo, Physical Review Letters 119, 075902 (2017).

When phonons encounter a region of crystal containing a gradient in the properties that dictate equilibrium phonon radiance, there must exist additional phonon collision processes in order to satisfy the principle of detailed balance [1]. Despite much research in this area, there is no widely accepted heat transport model across dissimilar materials. Here we introduce a simple form for this additional collision operator for use in Boltzmann transport simulations (BTE) of phonons using the relaxation time approximation. In particular, we develop a partially diffuse boundary condition that leads to zero boundary resistance in the case of imaginary interfaces, i.e. between two identical materials, and to complete diffuse scattering in the case of a hard wall. The method has been developed within OpenBTE [2], a recently introduced platform for multiscale phonon size effects in materials with complex geometries. Molecular dynamics (MD) simulations based on the Green-Kubo relation are also employed to study phonon transport across interfaces at the atomic level. Based on MD simulation findings, phonon-phonon scattering, phonon mean free paths, phonon lifetimes and transmission coefficients across the interfaces are calculated. We will also discuss possible routes for coupling MD simulations with the BTE.

[1] E. S. Landry and A. J. H. McGaughey. Physical Review B 80.16 (2009): 165304.
[2] www.openbte.org

Heat management in modern electronic devices is becoming increasingly important with escalating computing demands for fast data processing in a broad range of applications ranging from embedded smart cameras to artificial retinas. In order to optimize energy transport in multi-component devices, it is fundamental to characterize the impact of energy dissipation near interfaces to global transport characteristics. When the device size reaches the nanoscale, scattering at interfaces dictate the device performance and the functionality, since the characteristic dimensions of the devices approach electron and/or phonon mean free paths. Additionally, dimensional reduction significantly modifies the phonons in the nanostructure inducing dramatic changes in their dispersion relation and altering density of states. Bulk mode based description often fails to explain observed heat transport properties in nanostructures [1]. Thus, a complete treatment of thermal transport in a multi-component system requires solving the complex interplay between dimensional confinement and interface scattering. In this work, we investigate phonon transport properties of layered Si/Ge superlattice (SL) configurations with imperfect interfaces. The existence of a secondary periodicity in SLs suggests that bulk-like phonons will not exist in short-period superlattices. Instead, phonons related to the secondary periodicity are the vibrational mode of interest [2]. We employ classical molecular dynamics (MD) and lattice dynamics techniques to analyze superlattice phonons that develop in the transition from an isolated interface to periodic superlattices. We employ non-equilibrium MD to investigate phonon mean free paths in the Si and Ge subsystems and characterize the effect of dimensional confinement on phonon propagation [3]. A similar numerical method yields thermal conductivities of Si and Ge subsystems as well as the interface thermal conductance [4]. Knowledge of phonon MFPs combined with the thermal conductance values quantifies the impact of interface on phonon transport. We determine the extent of disruption of the superlattice phonons due to interfacial imperfections by investigating imperfect interfaces that contain defects, such as vacancies and interstitials. Interfacial defects can have a strong influence on both the electronic structure and charge-carrier scattering near imperfect interfaces. We investigate the effect of interstitial defects on electronic transport in Si/Ge SL by employing first principle DFT calculations with semi-classical Boltzmann transport theory. Our work illustrates the aspect of carrier size effects in multilayered systems and highlights the effect of interfacial structural characteristics on global energy transport in multi-component systems.
1. S. Kwon et al, Nanoscale, 8 (27) 13155 (2016). 2. S. C. Huberman et al, Phys. Rev. B, 88, 155311 (2013). 3. K. Sääskilahti et al, 90, 134312 (2014). 4. P. K. Schelling et al, 65, 144306 (2002).

Structural complexity can lead towards materials with low thermal conductivities (κ) – one of the key requirements for efficient thermoelectric and phase change materials. In polar chalcogenides and pnictides, asymmetric coordination environments and complex structural motifs, e.g. polyanionic networks, layers or channels can be obtained through a charge transfer from the cation to the anionic framework and through the formation stereoactive lone pairs. By including transition metals interesting magnetic properties such as spin frustration or mixed valence can arise and sharp features in the electronic density of states can be introduced close to the Fermi level influencing electronic structure and physical properties.
Several examples of complex compounds will exemplify our work towards a better understanding of the relation between bonding and properties in these rich families of intermetallics.

Thermal-electricity conversion is one of the most promising routes to harvest heat and convert it as easily storable and deliverable electric energy. Significant progresses have been made since the discovery of Seebeck effect in 18211, particularly, the figure of merit zT approached a record high value in 20142. However, for thermoelectric devices, high average zT values (zTave) over the operating temperature range is more important as it is directly related to the conversion efficiency (η). Approaching highly stable and repeatable ultra-high zTave for Te-free materials has been historically challenging over the past century though exciting progresses with zTave well above 1.10 was made recently3, 4. Here, through synergistic band engineering strategy for single crystalline SnSe, we report a series of record high zTave over a wide temperature range, approaching ~ 1.60 in the range from 300 K to 923 K in Na-doped SnSe0.9S0.1 solid solution single crystals, with the maximum zT of 2.3 at 773 K. These ultra-high thermoelectric performance derive from the new multiple valence band extrema near the band edges in SnSe0.9S0.1 and the shift of Fermi level towards the multi-valley bands through Na doping which introduce additional carrier pockets to attend electrical transport. These effects result in an optimized ultrahigh power factor exceeding 4.0 mWm-1K-2 in Sn0.97Na0.03Se0.9S0.1 single crystals. Combined with the extremely lowered thermal conductivity attributed from the intrinsic anharmonicity and point defect phonon scattering, the series of ultra-high zTave and a record high calculated conversion efficiency of 21% over a wide temperature range are approached.

Thermoelectric devices are promising clean energy technologies that use waste heat to generate electricity. SnSe has recently attracted attention due its large peak thermoelectric figure of merit, ZT ~ 2.5 at 923 K in single crystals and large temperature average ZT_device~1.3 in Na doped single crystals.^1-3 Here, ZT = S²T/ρk, where S is the Seebeck coefficient, ρ is the electrical resistivity, k is the sum of the lattice (k_lat) and electronic thermal conductivity (k_el) and T is the absolute temperature. The outstanding thermoelectric performance of SnSe is largely based on its low k_lat, which has proved controversial with large variations in reported values.

In this contribution, we present our results on the link between microstructure and ZT in polycrystalline ingots and on our investigation into the link between the crystal structure and thermal properties.^4-6

Polycrystalline samples were synthesized using solid-state reactions and hot pressing. These samples showed strong “V-shape” texturing of the SnSe platelets with marked differences in measured thermal conductivities depending on the ingot (0.6 ≦ k_lat ≦ 1.6 W m^-1 K^-1).⁴ Ingots with larger and more oriented SnSe platelets afford thermoelectric power factors (S²/ρ) = 0.9 mW m^-1 K^-2 at 750 K, and ZT>1 at ~850 K in p-type polycrystalline SnSe. Callaway fitting suggests that lower k_lat values are linked to an increased amount of disorder in the ingots, which we attribute to changes in the microstructure.⁴

A variable temperature neutron powder diffraction (4-1000 K) was undertaken to investigate the evolution of the crystallographic structure. Distortion mode analysis was used to reinvestigate the Pnma-Cmcm phase transition.⁵ This reveals significant Sn motions perpendicular to the SnSe layers, which broaden the phase transition. This was complemented by heat capacity measurements to probe the lattice dynamics.⁶ The data could be satisfactorily fitted using two Debye modes with Θ_D1 = 345(9) K and Θ_D1 = 154(2) K. The energies of these modes are found to scale with the bond strengths of the short and long bonds in the crystal structure. The presence of two lattice energy scales is reminiscent of the classical Phonon Glass Electron Crystal materials with weakly bound rattling atoms. This suggests that searching for materials with widely diverging bond distances is another possible route towards discovering good thermoelectric materials.

References:
1. L. D. Zhao et al., Nature, 2014, 508, 373.
2. L. D. Zhao et al., Science, 2016, 351, 141-144.
3. K. L. Peng et al., Energy & Environmental Science, 2016, 9, 454-460.
4. S. R. Popuri et al., Journal of Materials Chemistry C, 2016, 4, 1685-1691.
5. S. R. Popuri et al., in-preparation, 2017.
6. S. R. Popuri et al., Applied Physics Letters, 2017, 110, 253903.

Research and development on electronic devices having flexibility, not the functions of today's electronic devices, is active. Stretchable heaters with flexibility and elasticity can be utilized in various fields such as flexible displays, surgical instruments, sensors, soft robotics, smart clothes and so on. Because the stretchable heaters will undergo various forms of mechanical deformation, it is necessary not only to deform together with the mechanical deformation at this time but also to build a new construction. Therefore, in order to implement stretchable heaters, research on structure is important. In this study, in order to maintain conductivity by mechanical deformation, the structural characteristics of the electrode based on the embossed pattern were evaluated. In particular, the nano-structured pattern and the model of increasing/decreasing compressed thin film material were investigated. The von Mises stress and strain distribution were analyzed and the results are graphically depicted.

Thermoelectric devices provide a reliable, non-toxic, solid-state solution to energy lost as waste heat. However, there are relatively few materials with high efficiency in the mid-temperature regime (200 – 500 °C), where a high thermoelectric figure of merit (zT) material could impact low grade waste heat recovery, such as in industrial processes, and small-scale power conversion, for example in mid-temperature solar thermoelectric generators (STEGs). Yb_2-xA_xCdSb₂ (A = Eu, Ca) is an ideal p-type Zintl phase and a new addition to materials with efficient energy conversion (as indicated by the figure of merit, zT). Yb_2-xEu_xCdSb₂ has potential in the optimal mid-temperature STEG operating range and results in a zT at 523 K of 0.67. The significant zT stems from the high Seebeck coefficient and extremely low thermal conductivity of this material due to its complex phonon dispersion and point scattering by defects. The Yb-Eu system has an optimized thermal conductivity at or below its minimum theoretical value. The synthesis, structure, and bonding of this system will be presented on these new thermoelectric materials that have significant potential for future improvement.

Zintl compounds are attractive as thermoelectric materials owing to their favorable charge transport properties and low lattice thermal conductivities. We have computationally assessed the potential for thermoelectric performance of 145 Zintl compounds and predicted that many of these Zintls, if doped with electrons (n-type), can outperform the hole-doped (p-type) materials. However, almost all known Zintl thermoelectric materials are exclusively p-type, including Yb₁₄MnSb₁₁, Sr₃GaSb₃, and Ca₅Al₂Sb₆. Only recently, we have reported relatively high thermoelectric performance (zT~1) in two n-type Zintls, KAlSb₄ and KGaSb₄. To facilitate the search for new n-type Zintl thermoelectric materials, we have identified a simple empirical rule that correlates the average oxidation state of the anion (An_ox) and the dopability (p- vs. n-type) of the material. Within Zintl pnictides, compounds with An_ox < -1 are p-type while compounds with An_ox = -1 can be doped n-type. The dopability of a material is intimately related to its defect chemistry. In Zintl compounds with An_ox < -1, as in the case of LiZnSb, Ca₅Al₂Sb₆, and CdSb, n-type doping cannot be achieved due to the presence of large concentrations of electron-killing defects, most commonly the cation vacancies. We demonstrate that in Zintl compounds with An_ox = -1, such as KGaSb₄, KSb and CdAs₂, the electron-killing defects are present in negligible concentrations such that extrinsic n-type doping can be achieved without charge compensation. Using An_ox as a tool for predicting dopability, we have screened the Inorganic Crystal Structure Database (ICSD) and identified candidate Zintl compounds that can be potentially doped n-type. Our defect calculations for these candidate materials confirm that these compounds can indeed be doped n-type.

Two-dimensional (2D) van der Waals (vdW) heterostructures have shown multiple functionalities with great potential in electronics and photovoltaics. After scanning many 2DvdW structures, we propose WSe₂-MoSe₂-WSe₂ vdW heterostructure with scandium electrodes as a high performance thermionic energy conversion device. We characterize the device performance using first-principles GW calculations. The proposed device is found to have a room temperature equivalent figure of merit of 1.2 which increases to 3 above 600 K. A high performance with cooling efficiency over 30% of the Carnot efficiency above 450 K is achieved. Due to the large interfacial thermal resistance, thermionic efficiency can be larger than that of a thermoelectric material of same ZT.

Since the recent discovery of n-type Mg_3.2Sb_1.5Bi_0.49Te_0.01 with high performance, n-type Mg₃Sb₂-based compounds are attracting considerable interest. In this study, we demonstrate how to improve the overall figure-of-merit zT value by tuning the amount of excess Mg. Originally, as Mg interstitials were considered to be responsible for n-type behavior, a substantial amount of nominal Mg (x=0.2) in Mg_3+xSb_1.5Bi_0.49Te_0.01 was added to ensure the n-type property.¹ However, a thermodynamic investigation has shown that such a large amount of excess Mg is not necessary as long the amount is above a minimum threshold for n-type conduction.² Here we investigate the effect of minimizing the amount of excess Mg added to synthesize the n-type material. We find a significant reduction in thermal conductivity, leading to an increased zT. We discuss the experimental aspects of controlling the nominal Mg composition.

1 H. Tamaki, H. K. Sato and T. Kanno, Adv. Mater., 2016, 28, 10182–10187.
2 S. Ohno, K. Imasato, S. Anand, H. Tamaki, S. D. Kang, P. Gorai, H. K. Sato, E. S. Toberer, T. Kanno and G. J. Snyder, Joule, 2017, just accepted.

Thermoelectric materials which can directly convert waste heat into electrical power and vice versa, have potential to improve our society energy efficiency. Clathrate compounds are good thermoelectric materials due to their unique structural motif with three-dimensional host frameworks encapsulating guest atoms in large oversized cages. The “rattling” behavior of guest atoms in the cages results in the low thermal conductivity of clathrate compounds. An enclathration of small trivalent rare-earth cations was predicted to enhance the power factor of clathrate and overall thermoelectric performance. Ba₈Cu₁₆P₃₀ clathrate, which exhibits the smallest size of the pentagonal dodecahedral cages among all clathrates, was chosen to be a clathrate host for the La and Ce rare-earth guests. The unambiguous proofs of incorporation of rare earth elements into cages were proved by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning-transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances the hole mobility and concentration which results in the drastic increase in the thermoelectric performance.

The performance of thermoelectric materials is mainly governed by the materials’ electrical and thermal conductivity properties and a number of new materials and structures have been exploited in order to optimize the energy conversion efficiency. Especially, nanostructure engineering via dopants, precipitates or phase/twin/grain boundaries is found to be effective in increasing the conversion efficiency by reducing the thermal conductivity. However, a direct correlation of these nanostructures to the material’s property is yet to be elucidated. Nowadays, with the rapid development of aberration-corrected transmission electron microscopy (TEM), the resolution of electron microscopes takes a leap forward to sub-angstrom and sub-eV, which allows a direct access to a material’s structure and chemical composition at an atomic scale.
In this talk, we present the atomic and nano structure of layered thermoelectric material AgCrSe₂, By using the state-of-the-art aberration-corrected electron microscopy, we characterized its structure and found that it has an aperiodic stacking of AgCrSe₂ unit cells and bilayer Ag atoms in the order of …-(AgCrSe₂)_m-Ag-Ag-(AgCrSe₂)_n-Ag-Ag-…, in which the interface between AgCrSe₂ and bilayer Ag atoms are incoherent and the average distance between these interfaces is below one unit cell. According to the Anderson localization theory, these high-density aperiodic incoherent interfaces would lead to the localization of phonon and thus result in the extremely low lattice thermal conductivity. In addition to the stationary structure characterization, the heat-driven dynamic behavior of the thermoelectric materials is another important aspect to investigate, as the performance of the thermoelectric materials is usually temperature-dependent. In-situ Cs-corrected TEM technique is an ideal tool for directly probing the local structural change of layered thermoelectric materials with ultrahigh resolution. We have conducted in-situ heating experiment on various layered thermoelectric materials, where we are able to monitor the structural evolution as a function of temperature. By correlating the change in the microstructure and their bulk thermoelectric properties, we gain insight of the origins of their extraordinary high ZT performance.

The figure of merit of thermoelectric materials is the key parameter describing its suitability for solid state cooling/heating applications and/or power generation. This parameter is defined by three underlying properties: thermal conductivity, electrical conductivity and Seebeck coefficient.
It is first shown how the electrical conductivity and Seebeck coefficient of thin films thermoelectric materials are simultaneously measured with one instrument allowing testing of such specimens with ease. Handling and preparation have been considerably simplified: no tool kit, no wiring and no measurement of the distance between the thermocouples required.
Secondly it is also shown how the transport properties, i.e. thermal diffusivity and thermal conductivity, are measured with an ultra-fast laser flash method. This method is particularly applicable to very thin specimens (nano/micrometer range), whose heat diffusion times (ratio of square of the thickness over thermal diffusivity) are very small. The ultra-fast flash method is primarily characterized by very short laser pulses, and measurement of the temperature excursion with the thermoreflectance technique.

Scanning thermal microscopy (SThM) is a powerful tool for the characterization of micro- and nanoscale material properties. For instance, SThM can be used as a high resolution and high throughput screening tool for combinatorial materials or nanostructured materials with inhomogeneous properties. Despite their popularity, microscale SThM probes have been demonstrated to simultaneously measure thermal conductivity and thermopower on few thermoelectric materials. Through extensive study we have found that the conventional resistive wire microprobe has several limitations, including high thermal contact resistance, low measurement repeatability and sensitivity, and the inability to establish electrical contact through nanoscale oxide layers. These limitations greatly limit the range of samples that can be measured while the latter renders microscale Seebeck coefficient measurement impossible with traditional sample preparation and handling.

Here we present the development of a scanning thermal probe based on heated resistive wire with novel support structure. The support structure allows unprecedented probe-sample contact force, lowering thermal contact resistance, which, in turn, significantly increases thermal measurement sensitivity when compared to the conventional commercial probe. The enhanced thermal sensitivity allows measurement of a wide range of materials with thermal conductivity beyond 20 W/m*K. The higher contact force also grants reliable electrical contact on samples with oxide layers tens of nanometers thick. The spatial resolution of the newly developed probe is experimentally shown to be ~2 µm. The probe is demonstrated with quantitative thermal conductivity and Seebeck coefficient mapping on two thermoelectric materials, each with oxide layers up to 25 nm thick: combinatorial Ti-Ni-Sn film and proton-irradiated Hf_0.25Zr_0.75NiSn_0.99Sb_0.01. The irradiation-induced change in material properties is mapped as a function of depth from the irradiated surface and compared to the vacancy concentration profile derived from theory.

Materials with ultralow thermal conductivities have a wide range of applications [1]. It is generally accepted that nanocomposite formation can produce very low thermal conductivity, better than the alloy and amorphous limits [2]. In this work we present a detailed analysis of the role of dimensionality in reducing phonon conductivity of ultrathin nanocomposites. Our method uses a recently developed semi-ab-initio technique [3] based on a combination of density functional peturbation theory [4], third- and fourth-order elastic anharmonic terms in a crystal Hamiltonian expressed in terms of a temperature-dependent Grüneisen's constant, a quasi-harmonic approximation, and the linearized phonon Boltzmann equation [5]. Our numerical results reproduce the experimentally measured [6] phonon conductivity results for bulk Si and Ge in the wide temperature range 5-1500 K. Our cross-plane conductivity result for the ultrathin planar superlattice Si(11Å)Ge(11Å)[001] is in good agreement with reported experimental measurements [7]. From our computed results, we draw the conclusion that at and above room temperature the in-plane and cross-plane thermal conductivities in this planar superlattice geometry are, respectively, at least five and ten times lower than the lower of the two bulk conductivities (viz. in bulk Ge). It is also found that the formation of ultrathin nanowire and nanodot superlattice structures (Si inserts in a Ge host) produces conductivity results lower than that obtained for the ultrathin planar superlattice structure. A detailed analysis of these findings will be presented. The role of sample size and doping levels on the reduction of the conductivity will also be discussed.

[1] W. Kim, R. Wang and A. Majumdar, Nanotoday 2, 40 (2007).
[2] S.-M. Lee et al, Appl. Phys. Lett. 70, 2957 (1997).
[3] I.O. Thomas and G. P. Srivastava, Submitted for publication.
[4] S. Baroni et al, Rev. Mod. Phys. 73, 515 (2001).
[5] G. P. Srivastava, The Physics of Phonons (Taylor and Francis, New York, 1990).
[6] C. J. Glassbrenner and G. A. Slack, Phys. Rev. 134, A1058 (1964).
[7] H. T. Huxtable et al, ASME IMECE2002-34239, pp 1–5 (2002)

To reduce thermal conductivity is a key goal in the design of high-performance thermoelectric materials. It is well known that lattice thermal conductivity of thin films and nanowires can be significantly reduced by surface roughness. Usually, this phenomenon is explained and modelled by the diffuse scattering of phonons at the surface. However, experiments have shown that the thermal conductivities of some electrolessly etched [1] and intentionally roughened VLS-grown [2] silicon nanowires are lower than their Casimir limit (i.e., fully diffuse case). Several different models have been proposed to explain the mechanism of the ultra-low thermal conductivity, for example, the additional reduction caused by the amorphous or oxide layer on the surface of nanowires revealed in the Molecular Dynamics (MD) simulations [3], multiple incoherent scattering events caused by the rough surface modeled explicitly in Monte Carlo simulation [4], and multiple coherent backscattering from correlated surfaces [5]. While all these mechanisms have been shown to contribute the low thermal conductivity, the key mechanism and to what extend the thermal conductivity can be reduced using surface roughness have not been fully studied.
In this work, the reduction mechanism of surface roughness on the thermal conductivity is investigated by studying silicon thin films with different types of structured surface using MD simulations. Based on the phonon dispersion calculated by Lattice Dynamic method, it has been found that in addition to diffuse scattering, the resonance hybridization between the disordered rough surface and the bulk thin film plays an important role in reducing thermal conductivity. The phonon group velocities in the thin film are significantly reduced by resonance hybridizations, and the phonon energy is trapped near the rough surface in the resonance modes. This mechanism has been observed in thin films with regularly patterned nano-pillars on the surfaces, which have been proposed recently as promising thermoelectric materials, due to their reduced thermal conductivity and uncompromised electric conductivity [6]. We have found, via MD simulations of thin films regular pillared surface and the disordered rough surface, that the reduction in thermal conductivity is comparable for the two types of structures. This implies that it is possible to design practically feasible surface “roughness” to achieve ultra-low thermal conductivity.

[1]. A. I. Hochbaum, et al., Nature 451, 163 (2008).
[2]. J. Lim, et al., Nano Lett. 12, 2475 (2012).
[3]. Y. He, and C. Galli, Phys Rev Lett, 108, 215901 (2012)
[4]. L. N. Maurer, et al., Appl Phys Lett, 106, 133108 (2015)
[5]. J. Sadhu and S. Sinha, Phys Rev B, 84, 115450 (2011)
[6]. B. L. Davis, M. I. Hussein, Phys Rev Lett, 112, 055505, (2014).

Thermoelectrics has a potential to recover usable energy from waste heat and capture handy energy from the environment. The largest amount of heat are wasted in the low temperature region ~ 150°C [1], because the conventional systems lost the recovery power for such low temperature heat. Thermoelectrics, however, can maintain the energy convergent efficiency to exceed the conventional systems in the efficiency for the low temperature region [2]. Differing from inorganic thermoelectric materials, organic thermoelectric materials can provide a further value: flexibility. The thermoelectric figure of merit has also been dramatically improved to 0.42, recently [3]. The Seebeck effect, however, generates only several ten µV/K. To drive electric devices, we need to pattern and connect more than one hundred thermoelectric cells as a thermoelectric module.
Herein, we utilized photolithography to fabricate organic π-type thermoelectric modules. We can simply fabricate one-leg thermoelectric modules via printing the bottom electrodes, organic thermoelectric materials, and the upper electrodes which electrically connect with the next bottom electrodes. However, the organic one-leg thermoelectric modules cannot generate electricity because the upper electrodes connecting with the next bottom electrodes conduct the heat and kill the temperature difference for the thermoelectric generation. To maintain the temperature difference by separating the upper and bottom electrodes, we fabricated π-type thermoelectric modules via fulfilling p-type and n-type thermoelectric materials into photolithographically patterned resist molds. Overall, we re-arrange the well-established fabrication processes, such as photolithography, fulfilling, and electrode deposition, to emergently fabricate the organic π-type thermoelectric modules.
To achieve 250 mV to drive a booster circuit, we designed a module pattern, 13 × 13 cells in 40 × 40 mm². We fulfilled p-type and n-type thermoelectric materials based on poly(3,4-ethylenedioxy thiophene) polystyrene sulfonate [3] and tetrathiafulvalene 7,7,8,8-tetracyanoquinodimethane salt [4], respectively. When the single π unit reaches 3 mV, the designed module can drive electric devices with a booster circuit. In the presentation, we report the details of the optimized thermoelectric materials and the module performances.


References
1. Shindo, T., Nakatani, Y., Oishi, T. Toshiba Review (in Japanese) 63, 7–10 (2008).
2. Vining C. B., Nat. Mater. 8, 83–85 (2009)
3. Kim, G-H., Shao, L., Zhang, K., Pipe, K. P. Nat. Mater. 12, 719–723 (2013)
4. Bubnova, O., Khan Z. U., Malti, A., Braun, S., Fahlman, M., Berggren, M., Crispin, X. Nat. Mater. 10, 429–433 (2011)

In this study, we report on the fabrication of pressure/ temperature/ strain sensors and all-solid-state flexible supercapacitor based on a single common thermoelectric material of polypyrrole-coated graphene foam (PPy/GF) with a porous structure. For simultaneous detection of pressure and temperature without interference, a dual-mode sensor was designed where the changes of current and voltage could be measured, respectively, upon application of those impacts. The fabricated dual-mode sensor showed a high performance with a high sensitivity, fast response/recovery, and high durability over 10,000 cycles of pressure loading. In particular, the pressure was monitored using the thermoelectric voltage induced by the simultaneous increase in temperature via finger touch on the sensor. Additionally, a resistor type strain sensor fabricated using the same PPy/GF could detect the strain up-to 30%. In addition to those sensor decives, a flexible supercapacitor as a power supply could be fabricated with the electrodes of PPy/GF using its high surface area and psudocapacitance and gel-type electrolyte, exhibiting high electrochemical performance. Furthermore, an integrated system of such fabricated multifunctional sensors and supercapacitor showed the successful operation of the sensors with the power of the supercapacitor. This study clearly demonstrates that proper choice of a single functional material enables fabrication of both active multifunctional sensors for pressure, temperature, and strain, and supercapacitor as their power supply with a high potential application in wirelessly powered wearable devices.

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi₂Te₃ alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material’s performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides using a cold deformation. The cold deformation determines the electron density via the formation of intrinsic point defects. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way.

In the present study, Bi₂Te₃ thin films and bulk nanocomposites, with varying concentration of Graphene (G), Silicon(Si) and Carbon (C) have been synthesized. The G, Si and C phases have been introduced inside grains and at the grain boundaries to enhance the thermoelectric performance of the Bi₂Te₃ nanocomposites. The effect of concentration of G, Si and C secondary phase segregated along Bi₂Te₃ crystallite boundaries on electrical and thermal properties of Bi₂Te₃ nanocomposite has been investigated. The effect of different nanoinclusions on growth and structural properties has been discussed in detail. The effect of concentration on the thermal conductivity of Bi₂Te₃ nanocomposites at nanoscale level was investigated using scanning thermal microscopic studies. The value of thermal conductivity for the composite samples was determined using modified Parker’s method. A commercial SThM system was modified by incorporating a microcontroller driven microhotplate. The radial thermal conductivity of Bi₂Te₃ and Bi₂Te₃:Si at around 70 °C is calculated to be 1.15 W/mK and 0.57 W/m K, respectively. Incorporation of optimized concentration of Si resulted in change in electronic properties due to modification in crystallite orientation, and phonon transport due to the presence of a secondary conducting phase along Bi₂Te₃ crystallites. This resulted in higher electron transport and increased phonon scattering leading to enhanced ZT ~ 1.4 for Bi₂Te₃:Si and ZT ~ 0.92 for Bi₂Te₃:G composite samples. Enhanced value of ZT for Bi₂Te₃:G sample in comparison to Bi₂Te₃:C sample highlights the advantage of using 2D materials at interfaces for increased phonon scattering without affecting the electron transfer. The present study presents a novel route for simultaneous control of phonon as well as electron transport to decouple the unfavorably coupled thermoelectric parameters. Further this study is important for establishing the role of secondary phase along crystallite boundaries leading to enhanced thermoelectric performance.

With the increasing demand for electricity and rising environmental awareness, emerging green energy has gradually been taken seriously in the world. Among this green trend, thermoelectric is one of them. In energy conversion process, the energy mainly lost in the form of heat flow. By using thermoelectric device, the waste of the heat can be reused. In this study, one-dimensional Sb-doped homologous In₂O₃(ZnO)_m nanowires has been formed by the In ions treatment on the Sb-doped ZnO. The well-aligned Sb-doped homologous In₂O₃(ZnO)_m nanaowires array is designed to to be the thermoelectric device. Unlike most of the papers just investigating the thermoelectric properties by a single nanowire, we tests the thermoelectric properties of the device as a sheet by forming well-aligned Sb-doped homologous In₂O₃(ZnO)_m nanoarrays and compare thermoelectric properties with the pristine zinc oxide nanowire arrays, Sb-doped ZnO nanowires arrays and pure homologous In₂O₃(ZnO)_m nanowires arrays. Finally, we will investigate the mechanisms of the changes of thermoelectric properties resulted from the data.

Energy harvesting using thermoelectric devices has been very attractive because they can directly convert heat into electricity and vice versa. The thermoelectric performance is described by a figure-of-merit, ZT=S²σT/k, where S, σ, T and k are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. To improve a ZT value, it is required to increase the Seebeck coefficient and electrical conductivity and to reduce the thermal conductivity. However, this is very challenging because the electrical conductivity and thermal conductivity are coupled together with Wiedeman-Franz law, k=σTL, where L is Lorenz number.
GeTe thermoelectric material has a high carrier concentration due to a Ge vacancy, leading to the high electrical conductivity and low Seebeck coefficient. Therefore, to increase Seebeck coefficient, it is necessary to suppress the carrier concentration by doping element with three valence electrons. GeTe-based thermoelectric materials have a characteristic herringbone structure with an alternating bright and dark contrast, resulting from domains with different polarities caused by cubic-to-rhombohedral phase transformation. A herringbone structure is beneficial to increasing a phonon scattering, leading to reduction of thermal conductivity. Thus, if we can control the herringbone structure in GeTe, the thermoelectric performance will be improved.
In this work, we investigated the microstructure and thermoelectric properties of GeTe-based materials. Especially, the effects of doping elements on the herringbone structure were examined in detail. In this presentation, we will discuss them.

Recent research revealed that magnetic semiconductors, such as CuFeS₂ and Mn-doped CuGaTe₂ with chalcopyrite structure, could be considered as promising power-generation materials due to their excellent transport properties [1, 2, 3]. Here we report the preparation and thermoelectric properties of Cu₄Mn₂Te₄, which is antiferromagnetic with T_N = 50 K. [4]
Cu₄Mn₂Te₄ adopts a spinel-related structure. Each unit cell contains eight formula units (Z = 8). The Te ions form a cubic closest-packing (ccp) with Cu occupying half of the tetrahedral sites and Mn half of the octahedral sites. When temperature is over 723 K, Cu and Mn will statistically occupy half of the tetrahedral sites and half of the octahedral sites, respectively. [5] Thermoelectric properties measurements show that Cu₄Mn₂Te₄ displays an electrical conductivity 2500 Ω^–1cm^–1 and Seebeck coefficient 20 μV K^–1 at 325 K [6]. Its thermoelectric performance might be further improved through electron doping.
In this work, we have prepared various modifications of Cu₄Mn₂Te₄ by directly reacting the elements followed by spark plasma sintering (SPS). And then investigated the effects of the contents of extra Mn and In on thermoelectric properties. All samples presented are mainly composed of the Cu₄Mn₂Te₄ phase, as observed from powder X-ray diffraction (XRD). According to the thermoelectric transport properties measured, we demonstrate that the thermoelectric figure-of–merit for Cu₄Mn₂Te₄ could be enhanced through the addition of excessive Mn or In. Benefiting from the increased power factor and reduced thermal conductivity, zT is improved by 125% from 0.12 to 0.27 through excess Mn doping in Cu₄Mn_2+xTe₄, further to 0.52 in Cu_4-yIn_yMn₂Te₄ with In₂Te₃ precipitates, and finally to zT = 0.65 in Mn\In co-added Cu_4-yIn_yMn_2+xTe₄ at around 680 K. This value (zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides. It is worth noting that the effective mass of the carriers for samples with excessive Mn are estimated to be around 1.93m₀, which perhaps is responsible for the high Seebeck coefficient and power factor of the samples. Another interesting feature is their low thermal conductivity values, which could be understood based on the low Debye temperature, a very low speed of the sound and a high Grüneisen parameter. [6]
Thus we believe that chalcogenides with spinel\spinel-related structure have great potential for future application and exploring magnetic semiconductors is a novel direction for developing thermoelectrics.
This work was supported by JST CREST Grant Number JPMJCR15Q6, Japan.

References
[1] Ang, R., et al., Angew. Chem. Int. Ed. 2015, pp. 12909-12913.
[2] Ahmed, F., N. Tsujii, and T. Mori, J. Mater. Chem. A, 2017, pp. 7545-7554.
[3] Mori, T., Small, 2017, in press, doi: 10.1002/smll.201702013.
[4] Plumier R., et al., Materials Science Forum, 1994, pp. 687-694.
[5] Lotgering, F. K. et al., J. Phys. Chem. Solids, 1972, pp. 2071-2078.
[6] Guo, Q., et al. submitted.

Understanding thermal transport in wide band gap (WBG), AlGaN/GaN heterostructures grown on Si can improve the reliability of emerging power electronic devices and enable new device architectures for sensing, controlling and harvesting thermal energy. Since the highly conductive, two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is based on built-in polarization fields (not doping) and is confined to few-nanometer thicknesses, its charge carriers exhibit much higher mobilities in comparison to their doped counterparts.¹ This can lead to enhancements in the Seebeck coefficient that can be exploited for the realization of monolithically-integrated AlGaN/GaN thermoelectric-based sensors and energy harvesters. However, the impact of temperature and AlGaN/GaN film structure on the thermoelectric behavior of the 2DEG has yet to be fully mapped.

Here, we examine the Seebeck coefficient of suspended, gated AlGaN/GaN 2DEG heterostrucures over a wide temperature range (50 to 300 K) with varying GaN buffer layer thickness for the first time. This allows us to control the roughness of the AlGaN/GaN interface where the 2DEG forms. For rough interfaces, we observe a linear increase in Seebeck coefficient with temperature, typical of its “diffusive” nature. However, for pristine interfaces (RMS roughness ~ 1 nm), we observe a Seebeck coefficient enhancement in a broad temperature range (50 to 150 K). Such enhancements are usually attributed to phonon drag effects, where non-equilibrium phonons deliver excessive momenta to the electrons since phonon-phonon scattering is suppressed at low temperature.² This scenario would lead to the Seebeck coefficient and thermal conductivity peaking at approximately the same temperature.³

Contrary to the expected behavior, experimental measurements of thermal conductivity in our samples show that the thermal conductivity peak is at least >50 K higher, thus the enhancement cannot be reconciled by phonon drag alone. Furthermore, the phonon drag effect is usually suppressed for samples with high carrier densities (> 10¹⁹ cm^-3), which are present in our samples.² Thus, we uncover that the observed effects can be attributed to a combination of phonon drag and energy filtering due to inelastic scattering of electrons in pristine AlGaN/GaN 2DEGs. These effects are inhibited for the rough samples due to dominant roughness scattering. Overall, the results are essential for understanding energy harvesting and sensing utilizing WBG materials, over a wide temperature range.

References:
¹ O. Ambacher et al., J. Appl. Phys. 87, 334 (2000).
² J. Zhou et al., Proc. Natl. Acad. Sci. 112, 14777 (2015).
³ G. Wang et al., Phys. Rev. Lett. 111, 046803 (2013).

Graphene has attracted a lot of interest due to its excellent properties and potential for practical applications. Graphene has superior electrical conductance and mechanical properties, though its lack of an intrinsic bandgap limits its applications. Recently the thermoelectric power (TEP), described by the Seebeck Coefficient (S), of this material has been studied in order to apply its thermoelectric properties to photodetection or energy harvesting applications. Additionally, because the TEP depends on the material’s band structure, it is a powerful tool to help characterize the electronic structure of a material. Graphene has been shown to have improved characteristics when it is suspended from the substrate. Despite this, there has been little research into the thermoelectric properties of suspended graphene.

The TEP is the ability of a material to convert a temperature gradient into a voltage. Efficient power generation is shown by the figure of merit, ZT = σS²T/K, where σ is the electrical conductance, T is the temperature, K is the thermal conductance, and S = ΔV/ΔT. A high ZT value indicates a higher performance of the thermoelectric material. To study and improve the thermoelectric properties of graphene in this research, an established suspended device structure is used for graphene grown via chemical vapor deposition (CVD).

Graphene was grown via a CVD process by placing a copper substrate into a low pressure furnace with an Ar, H₂ mixture and ramping the temperature to 1000C. Then CH₄ was introduced for 10 minutes, after which the furnace was allowed to cool. The graphene was then spin-coated with PMMA before etching the substrate with ammonium persulfate. The graphene was then wet transferred onto a pre-patterned low stress SiN_x wafer. The wafer was then patterned again such that a platinum heater and gold contacts were added, and that the graphene and islands became suspended.

Fabricated suspended devices with graphene were then studied using a closed circuit refrigeration system under high vacuum. The device is first calibrated for the R-T correlation of the platinum resistors on each island for use as thermometers. This process is done by using an I-V slope to find the resistance at each temperature point. A temperature gradient can be created and measured across the islands by using a relatively large current on one of the resistors. Measuring the resulting voltage across the graphene at the same time allows S to be found. The electrical conductance of graphene can also be found using the same device structure. Preliminary studies in this research have shown excellent thermal insulation of the device indicating that the heat is being transferred via thermal conduction instead of radiation. Finally this research will discuss results from gold nanoparticle covered graphene samples to see if any improvements on S or ZT occur.

The ever-growing usage of graphene and different forms of carbon nanomaterials including carbon nanotubes (CNTs) in advanced devices has called for a variety of innovative methods to develop them. Spin-on-CNT is an alternative, low-cost approach to fabricating CNTs into devices. Spin-on-CNT was deposited on two different substrates, glass and SiO₂, by adopting four different recipes, leading to four different CNT concentrations. Raman and SEM characterization techniques have been used to verify the soundness of the process and to further corroborate the CNT structure in the deposited layer. The initial results indicate that the uniformity and the area of spin-coated CNT films need to be carefully controlled by modifying the spin-coating speed and concentration of the solution.

The recent use of hybrid ABX₃ perovskites where A is a molecular species and X is a monatomic halide has led to a revolution in advanced solar cells and has sparked significant interest in the synthesis and characterisation of various species of perovskite with unique electrical properties. In addition, the development of perovskites where both A and X are molecular is a source of great interest in the field of advanced materials, particularly the well-known functional properties of the X = formate perovskites including multiferroicity, and their complex responses to pressure and temperature changes. The total number of perovskite families with distinct molecular X-sites is low, so the synthesis of perovskites with novel molecular X anions is of special interest. We present a new family of hybrid perovskites synthesised with X = hypophosphite ([H₂POO^-]), a phosphorus analogue of the formate ligand that is cheap, easily accessible, and non-toxic. We demonstrate the rich crystal chemistry of this family, having the composition [Am](Mn)(H₂POO)₃, with Am = guanidinium, formamidinium, imidazolium, triazolium, and DABCOnium. Their wide range of thermal phase transitions were thoroughly characterised by single-crystal X-ray diffraction, and we show the contrasting antiferromagnetism of the guanidinium phase and the paramagnetism of the formamidinium phase arising from pronounced perovskite layer shearing, behaviour not seen in other molecular perovskite families.

Thermoelectric (TE) devices, which can directly convert a temperature gradient to electricity, have been studied for several decades with inorganic semiconductors such as Bi₂Te₃. Although the performance in terms of the figure-of-merit, ZT, has been improved sufficiently for being used in energy harvesting, the practical utilization of the TE devices are limited to a few applications due to the high cost of materials and a bulky form of the devices. To overcome those disadvantages of inorganic TE devices, organic TE materials have attracted attentions for the next-generation TE devices. The organic TE materials can be synthesized easily with low cost, and using these materials TE devices can be fabricated with various shapes and high flexibility. Moreover, the performance of organic TE devices comparable to the inorganic TE devices has been reported recently. However, studies on the characterization of the output power and module design are still far apart from the main research topics in organic TE devices, which are important for practical use of the TE devices. In this work, we characterized and compared the output power characteristics of various organic TE devices based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The Seebeck coefficient of the PEDOT:PSS thin films were measured to be about 14–17 μV/K depending on the device conditions. We also analyzed the internal electrical properties of the devices by the impedance spectroscopy. We believe that the results are meaningful to develop the organic TE devices and to practically utilize them. In the presentation, we will show the detailed results and analysis of the PEDOT:PSS TE devices.

Thermoelectric materials are the heart of radioisotope thermoelectric generators, which are the main power system for space missions such as Voyager I, Voyager II, and the Mars Curiosity Rover. However, materials currently in use (i.e. SiGe or PbTe based materials) enable only modest thermal to electric conversion efficiencies of 6.5%, warranting the development of material systems with improved thermoelectric performance. One material of interest previously examined at JPL is lanthanum telluride (La_3-xTe₄), a high-temperature n-type material. La_3-xTe₄ possesses a defect structure where the La³⁺ vacancies control the carrier concentration, and this structure possesses an inherently low thermal conductivity. With an optimized vacancy concentration, zT values of 1.2 are achievable at 1275 K. Here we present a study of the thermoelectric properties of neodymium telluride (Nd_3-xTe₄), another rare earth telluride with a similar structure to La_3-xTe₄. Density functional theory (DFT) calculations predicted a 100% increase in the Seebeck coefficient due to alteration of the band structure, suggesting an increase in zT compared to La_3-xTe₄. The high temperature electrical resistivity, Seebeck coefficient, and thermal conductivity were measured and reported for Nd_3-xTe₄ at various vacancy concentrations.

Polycrystalline composites with a nominal composition of Ca_0.95Gd_0.05MnO₃/Ag were grown using the solid state reaction method. Thermoelectric properties were studied using Seebeck coefficient S(T) and electrical resistivity ρ(T) measurements. These transport properties were studied in the temperature range between 85 and 300K. The thermoelectric performance of was improved as a consequence of the decreasing of its electrical resistivity with Gd doping and Ag adding. The composite exhibits maximums values for the thermoelectric power factor, PF, close to 8 μW/K²-cm. However, the values decrease when the Ag is above 5% wt , because the Seebeck coefficient is deteriorated by the presence of large amounts of Ag particles. Thus, the behavior observed in the transport properties become these composites promising materials for use in thermoelectric devices for low-temperature applications.

An ability to modify the propagation of acoustic phonons – the main heat carriers in semiconductor and electrical insulator materials – has important implications for thermoelectric devices and thermal management of electronics [1]. The thermal transport can be strongly affected in nanostructured or doped materials via the changed phonon – boundary and phonon – point defect scattering rates. However, the thermal conductivity can also be altered via the changes in the phonon group velocity [1]. In this presentation, we show on the example of neodymium (Nd) doped sapphire (Al₂O₃), that substitution of Al atoms with much heavier Nd atoms results in a noticeable decrease in the acoustic phonon group velocity. The samples selected for this study had different amount of Nd ion substitution (x= 0, 0.1 wt% and 0.25 wt%) [2]. The acoustic phonon spectra for each sample were measured directly using the Brillouin-Mandelstam spectroscopy (BMS) at room temperature [3]. All spectra were excited with a continuous wave solid-state diode-pumped laser operating at 532 nm wavelength. The measurements were carried out at 180^ο backscattering configuration. The scattered light was collected by the same lens and directed to the six-pass tandem Fabry-Perot interferometer. The polarization of the incident beam was in the plane of the axis normal to the sample and the direction of the incident beam (p-polarized). Our BMS results clearly show that with the increase in the Nd doping, the frequency of both longitudinal acoustic (LA) and transverse acoustic (TA) phonon modes, at fixed phonon wave-vector, decreases, indicating the change in the phonon group velocities. The phonon velocity modification is in line with the preliminary thermal conductivity data. Our results suggest that phonon spectrum engineering via substitutional doping can become an important tool for tuning the thermal conductivity even of the bulk materials. This capability complements a conventional approach of changing thermal conductivity via the phonon scattering rates, and it can have important implications for thermoelectric energy conversion.

This work was made possible by the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award # SC0012670. AAB also acknowledges National Science Foundation (NSF) grant #1404967 on defect engineering in materials.

[1] A. A. Balandin and D. L. Nika, “Phononics in low-dimensional materials,” Material Today, 15, 266 (2012).
[2] S. Ramirez, et al., “Thermal and magnetic properties of nanostructured densified ferrimagnetic composites with graphene - graphite fillers,” Materials Design, 118, 75 (2017).
[3] F. Kargar, et al., “Direct observation of confined acoustic phonon polarization branches in free-standing semiconductor nanowires,” Nature Communications, 7, 13400 (2016).

The potential benefits of new thermoelectric materials can be drastically reduced by a failure to optimally dope the material to its zT maximum point. When analyzing experimental data of an arbitrary material, the important question is: Can we determine the optimal doping concentration to reach the maximum zT in the given material? A procedure to do so under the assumption of parabolic energy bands has recently been reported [1]. In this paper, we use a technique that makes use of the full, complex band structure obtained by DFT simulation and apply it to experimental results for CuAlO₂.

An integrated computational and experimental study using the 2H phase of CuAlO₂ as a model material is presented. The 2H phase of CuAlO₂ has gained interest as a promising metal oxide candidate for high temperature p-type thermoelectric applications [2-4]. Using a method similar to one that has recently been reported but without assuming parabolic energy bands, we will show how a complex TE material can be computationally assessed and how experimental data can be analyzed using first-principles informed calculations to answer the question posed above.

Experimentally, 2H phase CuAl_1-xFe_xO₂ (0≤x≤0.5) nanobulk [5] were synthesized using solid-state methods, and their thermoelectric properties will be measured using temperature dependent characterization tools. Specifically, electrical properties will be obtained by Van der Pauw method, thermal conductivity will be measured by Laser Flash Method using Netzsch LFA 567, an the Seebeck will be obtained by a home-built high temperature system. From there we can extract the Fermi level and subsequent theoretical doping concentration that maximizes zT for this material. Optimized doping concentrations will be achieved using Al and Fe as dopants through solid-state methods to achieve the optimized zT as predicated using our computational approach. The experimental data will be compared to the modeling work to refine our model for better experimental design guidance. Full band, first principles informed calculations will be compared to calculations that use parabolic energy bands [1], and the differences will be discussed.

[1] S. D. Kang and G.J. Snyder, arXiv:1710.06896v1, 2017.
[2] X. G. Zheng, K. Taniguchi, A. Takahashi, Y. Liu, C. N. Xu: Appl. Phys. Lett. Vol. 85 (2004),
p.1728
[3] W. T. Liu, Y. Y. Luo, Z. T. Liu, Z. M. Wei, "Density Functional Theory Study of P-Type
Transparent Conducting 2H-CuAlO2 Oxide", Applied Mechanics and Materials, Vol. 252,
pp. 263-266, 2013
[4] Y. Feng, X. Jiang, E. Ghafari, B. Kucukgok, C. Zhang, I. Ferguson, and N. Lu, “Metal Ox- ides for Thermoelectric Power Generation and Beyond,” Adv. Comp. Sci., 2017.
[5] Q. Hao, D. Xu, N. Lu, and H. Zhao, “High-throughput predictions of nanoporous bulk
materials as next-generation thermoelectric materials: A material genome approach,”
Phys. Rev. B, 93, 205206, 2016.

Spin transport between a magnetic insulator and a normal metal is an area of active research within the spin physics community to further the realization of spintronic devices. We investigate picosecond spin-currents across Au/TmIG and Au/YIG interfaces in response to ultrafast laser heating of the Au metal. In the picoseconds after heating, large interfacial spin currents occur due to a temperature imbalance between electrons and phonons in the metal, and magnons and phonons in the magnetic insulator. We utilize four different optical probes to develop a complete picture of the heat and spin transport in Au/TmIG and Au/YIG. Magneto-optic Kerr effect measurements of Au at a wavelength of 800 nm detects the spin accumulation in the normal metal that results from interfacial spin-currents. Magneto-optic Kerr effect measurements of the YIG/ TmIG at 400 nm monitor the ultrafast decrease in the magnetic moment of TmIG/YIG due to heating from Au electrons. Finally, thermoreflectance measurements at 450 and 950 nm monitor the temperature evolution of the Au electrons and phonons, respectively. Together, these measurements allow us to estimate the magnitude of the transport coefficients responsible for the longitudinal spin-Seebeck effect in these systems.

Up-converting thermographic phosphors are of interest due to specific advantages for temperature measurement applications. Typically, infrared excitation stimulates visible fluorescence only from the target phosphor and not the surrounding medium. This is in contrast to ultraviolet excitation which may also produce interfering luminescence from cells and other biological tissue in the vicinity, for instance. When traversing a material, usually infrared losses due to scattering and absorption are less than for ultraviolet wavelengths. An example is human skin.
This investigation follows logically from earlier efforts incorporating thermographic phosphors into thin samples of PDMS and Aerogels. Thin samples of phosphor-doped PDMS are compliant and their function as a reusable temperature sensor has been previously demonstrated by the authors. Layered phosphor/PDMS/Aerogel composites are being investigated for heat flux sensing. For maximum utility and understanding; physical, optical, and thermal properties are characterized over a wide temperature range.
Lanthanide-doped up-converting phosphor composites with increasing concentrations were synthesized for this study and fully characterized as a function of temperature. The excitation/ emission characteristics of the powder alone and the prepared composite thin films were investigated between -80 C and +80 C in an environmental chamber and the decay behavior of each sample type was measured. Thin film composites were prepared using spin-coating techniques and the excitation/ emission behavior was characterized as a function of film thickness also. Here, the authors report on decay behavior and spectra as a function of temperature and emission intensity of the composite films as a function of excitation wavelength. Results were compared with powder –only parameters and are reported here.

The Internet of Things (IoT), coupled with advanced analytics, is poised to revolutionize manufacturing maintenance and efficiency. However, a practical route to powering these many IoT devices remains unclear. In this work, flexible thermoelectric generators (TEGs) were fabricated from low cost, screen printed silver and nickel inks before being integrated into a novel radial form factor device based on commercial steam pipe insulation. Through improved matching of internal resistances and series/parallel design, this 420-junction TEG device produced 308 µW of power at a temperature difference of 127 K. This was sufficient to power a temperature sensing circuit with wireless communication capabilities. We have demonstrated that, after an initial 4 hours of charging, the TEG device could power a standard RFduino microcontroller for 10 minutes while sending temperature readings every 30 seconds via Bluetooth Low Energy (BLE) to a cell phone. Additional optimization and scaling could further increase system efficiency and provide a viable route to powering an industrial wireless sensing network (WSN).

A ferrofluid is a suspension in which nano-sized magnetic particles are dispersed in colloid solution. Since the ferrofluid is ferromagnetic, the particles do not settle even in a force field such as gravity or magnetic field, and the flow of liquid can be controlled by the magnetic field. When a magnetic field is applied, the magnetic volume force forms a vortex field, which affects the heat transfer characteristics. In this study, heat transfer characteristics of ferrofluids in microchannel were investigated using the commercial code, COMSOL Multiphysics. Numerical results of the local Nusselt number, magnetophoretic force, and temperature distribution were obtained in various configurations of microchannel, and results were graphically depicted with various flow conditions.

Material Selection is very important in modern engineering design process due to great diversity in the properties of modern materials. Today thousands of materials are available for engineers to select the best required material among these, for their design. One may use online material data base like MatWeb or Granta’s Design CES selector software for material selection. In this study research is conducted for the selection of materials for radiant plaques of gas heaters by using the Granta’s Design CES Edupak software. Radiant (Infrared) gas heaters are generally used for the purpose of cooking food and warming rooms, halls etc. to achieve human comfort. The plaques used in these heaters are usually made of ceramics like glasses, cordierite, alumina etc. In this work candidates of materials for these plaques are selected using the Granta’s Design CES Edupak software.

The study of thermoelectricity in molecular junctions is of fundamental interest for the development of various technologies including cooling and heat-to-electricity conversion. Recent experimental progress in probing the Seebeck effect of molecular junctions has enabled studies of the relationship between thermoelectricity and molecular structure. However, observations of Peltier cooling—a critical step for establishing molecular-based refrigeration—have remained inaccessible. Here, we report direct observations of Peltier cooling in molecular junctions [1]. By integrating conducting-probe atomic force microscopy with custom-fabricated picowatt-resolution microdevices, we created an experimental platform that enables the unified characterization of electrical, thermoelectric and energy dissipation characteristics of molecular junctions. Using this platform, we studied gold junctions with prototypical molecules (Au–biphenyl-4,4′-dithiol–Au, Au–terphenyl-4,4′′-dithiol–Au and Au–4,4′-bipyridine–Au) and revealed the relationship between heating or cooling and charge transmission characteristics. We expect these advances to stimulate studies of both thermal and thermoelectric transport in molecular junctions where the possibility of extraordinarily efficient energy conversion has been theoretically predicted.
[1] L. Cui et al., Peltier cooling in molecular junctions, Nature Nanotechnology (2017). doi:10.1038/s41565-017-0020-z

Binary and ternary semiconductor chalcogenides especially from IV-VI group have been very attractive because of their large-scale applications in science and technology. Lead Selenide (PbSe) is important for its narrow bandgap (E_g = 0.27eV), high carrier mobility and dielectric constant. It crystalizes into fcc cubic structure with Fm3m space group. All these characteristics make it unique and important for applications in solid state devices. 10µm long PbSe nanowires have been synthesized using template assisted electrochemical deposition in 80 nm pores of polycarbonate membrane. The synthesized nanowires were irradiated with 120 MeV Au⁺⁹ ions at Inter University Accelerator Centre (IUAC), New Delhi, India to study post irradiated induced changes in structural, optical and electrical properties as well as surface morphology using X- ray diffraction, UV- visible, I-V characterization and scanning electron microscopy (SEM) respectively. Electrical study was performed to study the change in resistivity of the nanowires upon swift heavy ion irradiation. The change in efficiency of the thermoelectric material is characterized by figure of merit ZT. Thus, irradiation induced changes in ZT factor of nanowires have also been evaluated to study the efficiency. The detailed results will be discussed during the presentation.

Temperature dependent X-ray diffraction and Rietveld structure refinements on Na_xSi₁₃₆ clathrates with x = 1.1, 5.5, 12.6, 17.3, and 21.9 have been performed. Although the linear coefficient of thermal expansion (293 K < T < 423 K) of Si₁₃₆ (x = 0) is only about 20% larger than that of the ground state a-Si (diamond structure), the linear coefficient of thermal expansion of Na_xSi₁₃₆ monotonically increases by more than 150% upon filling the framework cages with Na atoms in Na_xSi₁₃₆ (0 < x < 24), ranging from a = 2.6 × 10^-6 K^-1 (x = 0) to 6.5 × 10^-6 K^-1 (x = 24) by only varying the Na content, x. Taken together with heat capacity and bulk modulus data, the dramatic increase in thermal expansion can be attributed almost entirely to an increase in the mode-averaged Grüneisen parameter by a factor of more than 2 from x = 0 to x = 24, indicating the Na guests not only contribute low energy lattice vibrations in the phonon spectrum, but also significantly influence the character of the phonons as well. Due to the ability to fully vary the guest content, Na_xSi₁₃₆ (0 < x < 24) clathrates offer a useful model system to investigate the effects of the guest atoms on the lattice dynamics in clathrate materials. In particular, effects of the guest content can be experimentally discerned from the effects of differing framework or guest composition in studies of different clathrate compounds.

Solid state thermoelectrics (TE) devices exhibit great prospects for power generation by harnessing waste heat, thus offering an effective solution to the ever rising need for sustainable energy worldwide. However, conflicting material characteristics and parasitic losses arising during fabrication of these TE devices pose a formidable challenge. In the present work, we demonstrate a time efficient fabrication of n & p thermo-elements by Spark Plasma Sintering using Ti electrical contact from high performance half-Heuslers TE materials having a ZT value of ~ 1 for both n-type Zr_0.5Hf_0.5Ni_1-xSn and p-type Zr_0.5Hf_0.5Co_1-xSb_0.9Sn_0.1 (x = 0 - 0.04) at 873 K. A very low specific contact resistivity of the as-sintered HH/Ti thermo-elements better than 10 mΩ.cm² for both n-type and p-type thermo-elements was realised. Fabricated efficient n-type and p-type thermo-elements, with strong and stable metallic interconnects display the promising aspects of the present methodology for large-scale, economical and time efficient fabrication of a TE device.

High thermal conductivity and the exorbitant cost of HH in half-Heuslers (HH) alloys limit the prospects for their applicability in thermoelectric (TE) energy conversion devices. However, by incorporating mass fluctuation and strain field effects in HH alloys, their TE performance can be optimized for which Hf has been used extensively. This work demonstrates the efficacy of a p-type acceptor dopant in (Zr/Ti)CoSb based HH alloys in enhancing the ZT by eliminating the use of Hf. A series of HH composition (Zr/Ti)CoSb_1-x(Si/Sn)_x (x =0 - 0.2) samples were synthesized by conventional arc melting and consolidated employing spark plasma sintering. Both Si and Sn doping were found to dominantly introduce hole carriers in the pristine (Zr/Ti)CoSb resulting in a p-type semiconducting. Optimization of carrier concentration by optimal substitution of acceptor dopants i.e. Si and Sn significantly improves the power factor and increases the phonon scattering resulting in an enhanced thermoelectric performance and a maximum ZT of 0.46 and 0.28 at 873K was obtained for optimally doped ZrCoSb_0.8Sn_0.2 and TiCoSb_0.8Sn_0.2 respectively. For further optimization, microstructural modifications by fine-tuning of the Ti to Zr ratio results in strain field effects and mass fluctuation in the optimised p-type (Zr/Ti)CoSb_0.8Sn_0.2 compositions which remarkably introduces additional phonon scattering resulting in maximum ZT of 0.8 at 873K for Zr_0.5Ti_0.5CoSb_0.8Sn_0.2. The present study provides a better understanding of p-type dopants in Half-Heusler materials by which prospective high thermoelectric performance can be obtained in low-cost Hf-free p-doped half-Heusler (Zr/Ti)CoSb alloys.

Layered metal chalcogenide materials have been paid attention for potential thermoelectric materials due to their intrinsic low thermal conductivity due to their weak atomic boning between layers. Indium selenide InSe crystal is known to have low thermal conductivity in range of ~0.37 – 1.2 Wm^-1K^-1, while their intrinsic carrier concentration is quite low as ~10¹⁴ cm^-3 due to the relatively large bandgap of 1.2 eV. Therefore, InSe-based materials can be good candidates for thermoelectric materials, if the carrier concentration can be increased by proper doping. Here, we investigated the electronic and thermal properties of the series of Si-doped InSe, In_1-xSi_xSe polycrystalline samples. The cation substitution of Si increased electrical conductivity, while decreasing activation energy for the electrical conductivity. The negative Seebeck coefficient increased as Si doping increases, resulting in large enhancement in power factor. The slight reduction of thermal conductivity was also observed by the doping. As results, the thermoelectric figure of merit zT value was expected to higher than 0.2 at high temperature, which can offer possibility for InSe-based thermoelectric materials. The improvement of thermoelectric properties is related with simultaneous increase of σ and S with Si doping, which will be discussed based on the interrelationship between two kinds of electrons contributing to charge transport in InSe; (1) high mobility electrons in the conduction band and (2) low-moblity electrons in the 2D electric subbands.

Thermoelectric (TE) materials enable direct conversion of waste heat into electrical energy. It is a promising area of research, given the current demand for alternative energy technologies to reduce our dependence on fossil fuels. Ceramic oxides are a new and promising class of TE materials because of their high stability at elevated temperatures. Such materials are especially suitable for use in prospective TE power generators because high temperatures are encountered in such operations. This work focuses on pyrochlore ruthenates (A₂Ru₂O_7-x), i.e., lead ruthenate (Pb₂Ru₂O_6.5) and some derivatives, all having defect structures. Pyrochlore ruthenates with Bi, Tl, Y, REE, and Pb, in the A site have been studied widely for their electronic, catalytic and to some extent for their TE properties. TE properties of lead have not yet been studied. The TE properties are characterized by a figure of merit (ZT); ZT=S²σT/κ. S is the Seebeck coefficient, κ thermal conductivity, σ electrical conductivity, and T temperature. Figures of merit depend strongly on κ and S. In this paper we present measurements of κ (RT to 300°C) for lead ruthenate and several derivatives with variable contents of lead in the A- and ruthenium in the B-site, which were prepared by solid-state synthesis and characterized by XRF, X-ray diffraction, SEM/EDX, for composition, structure and phase content.

Increasing performance demands on photodetectors and solar cells require the development of entirely new materials and technological approaches¹. We report on the fabrication and optoelectronic characterization of a photodetector based on optically-thick films of dense, aligned, and macroscopically long single-wall carbon nanotubes doped with hydrazine. Here self-assembly method² was used to the realization of horizontal aligned SWCNTs. A responsivity up to 1.293 V/W was observed in these devices, with a broadband spectral response spanning the visible to the near-infrared. Scanning photocurrent microscopy indicates that the signal originates at the contact edges. We conduct a further study of on the photoresponse of SWCNTs-Au heterojunctions with a focus on the polarimetry dependence of the device and found that SWCNTs with hydrazine doping has a better polarization property than undoped SWCNTs. The results give a good reference for developing efficient, low-cost IR detectors

Energy storage has become a key issue because of the increasing demand for electronic devices, electric vehicles and large-capacity energy storage system. Among the various energy storage systems, lithium-ion batteries have dominated the power sources for portable electronics over the past decades and are now powering electric vehicles. In order to further improve the energy density of lithium-ion batteries, silicon-based anode materials have been actively studied. Silicon has a high theoretical capacity, a low reduction potential, is environmentally benign and is low cost, making it attractive candidate for next-generation lithium-ion batteries. However, the silicon-based materials undergo large volume changes during the alloying/de-alloying reaction, resulting in pulverization of the electrode. In addition, the large volume change results in the continuous breakdown and formation of solid-electrolyte interphase (SEI) layer during the repeated cycling which results in a serious capacity decline during the repeated cycling. To solve these problems, many studies have been carried out by different approaches such as controlling the particle size and morphology, alloying with inert metals, embedding silicon in a conductive material, and applying several functional binders.
In this work, we synthesized Si-based alloy materials composed of Si, Al, Fe and Ti, which could deliver a high specific capacity of 1200 mAhg^-1. They were double-layer coated by porous reduced graphene oxide (porous r-GO) and poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) (PEDOT-co-PEG) to suppress volume change of Si alloy and irreversible reaction of the electrolyte solution with Si alloy. Double-layer coating of Si alloy materials with porous r-GO and PEDOT-co-PEG was confirmed by FE-SEM, HR-TEM, XRD and EDS. The protective double layer formed on the surface of Si alloy materials effectively maintained the electrode structure without severe volume change and significantly improved the cycling stability as compared to pristine Si alloy material. Detailed characterization and electrochemical performance of the pristine, single-layer and double-layer coated silicon alloy materials will be presented.

An increasing demand for high-performance rechargeable batteries for various applications including portable electronic devices, electric vehicles and energy storage systems requires large improvements in the energy densities of the current lithium-ion batteries. Graphite has been widely used as an active anode material in commercial lithium-ion batteries due to its low cost and long cycle life. However, the available capacity of graphite is limited to around 350 mAh g^-1, which is close to its theoretical capacity of 372 mAh g^-1. In order to improve the energy density of lithium-ion batteries, silicon materials have been actively studied as attractive anode materials due to their high theoretical capacities, low reduction potentials and low cost. However, silicon materials suffer from substantial volume changes during repeated cycling, which are highly detrimental to the cycling stability of lithium-ion batteries. The mechanical stresses caused by repeated changes in volume can fracture the electrode, which causes poor electrical contacts between the active materials, electronic conductors and current collector. As a result, a serious capacity decline occurs during repeated cycling.
In this study, we synthesized silicon alloys composed of silicon nanoparticles embedded in inert metal matrix phases, and investigate their electrochemical performance. The resulting silicon alloy particles were coated by different kinds of polymer materials or conductive carbon to a form stable interfacial layer and achieve good capacity retention. The interfacial studies and cycling tests were carried out by electrochemical impedance spectroscopy, XPS, FE-SEM, HR-TEM and galvanostatic charge/discharge cycles. Detailed characterization of surface-modified silicon alloy materials along with their electrochemical performance will be presented

Clean energy technologies like thermoelectric energy generation provide a solution to ease our dependence on fossil fuels. However, the low efficiency of thermoelectric materials find limited applications. Various approaches like grain boundary scattering, substitutional effects, band structure engineering etc. were incorporated to improve the performance to certain extent, yet the phonon-electron interactions in the materials still exist. One way to decouple these interactions is to employ magnetic materials that generate spin currents in the presence of thermal gradient. This new avenue has the potential to extract additional spin voltage thereby increasing the overall Seebeck coefficient of the material. To study this effect, the current research developed a 1-D spin transport model by combining, non-equilibrium Green’s function approach, spin transport theory and first principle calculations based on density functional theory. Using the first principle methods, the fundamental parameters like band gap, effective mass of conduction band edge, lattice parameter and magnetization in the material can be calculated and used in the 1-D spin transport model. The available experimental data for La:YIG was used for validation. In comparison to the experimental data, the spin transport model yielded an error less than 10%. The model developed in this research can be applied to study the spin transport properties of various semiconducting magnetic materials in the presence of thermal gradient.

Photo-thermoelectric (PTE) effect in thin films has emerged as a new realm for photo-detection and energy harvesting from readily available sources. Among the materials, conductive polymers were prepared via solution casting polymerization to afford flexible photo-thermoelectric polymer films. These films generated photocurrent accompanied by photothermal effect upon exposure to a light source. The degree of crystallinity of the polymers (DCP) in thin films was enhanced by controlling the polymerization composition and temperature using polymeric surfactant. The photo-thermolectric output and energy conversion efficiency of the PTE films were largely dependent on the DCP. Both PTE and photocurrent output were correlated to DCP, which provided a working mechanism for PTE effect in polymer film. Broad range NIR absorption, photon to heat, and heat to electric conversion have been realized in one transparent film, which could benefit in exploiting invisible NIR sensors and thin film thermal energy harvesting with a simple structure.

Thermoelectrics as a direct energy conversion method between heat and electricity is mainly used for electrical power generation and cooling applications.

A large variety of materials, such as intermetallic compounds (e.g. half-Heuslers), silicides and chalcogenides (e.g. PbTe and GeTe) have been investigated as thermoelectric materials due to high ZT values at different temperature ranges. Among these material classes, although currently showing lower ZTs, silicides and intermetallic compounds possess additional advantages due to improved mechanical properties, the ability to operate at higher temperatures and the potential for large scale commercialization, since they are composed of naturally abundant and less toxic elements.

Global trends for improving the thermoelectric efficiency via maximizing the ZT values include, electronic doping optimizations; generation of Functionally Graded Materials (FGMs) with an optimal maximal ZT envelope over a wide temperature range; and nanostructuring formation for reduction of the lattice thermal conductivity. Nanostructures generation can be achieved by nano-powdering using energetic ball-milling followed by a rapid consolidation method such as Spark Plasma Sintering (SPS). Yet, due to the demand for high stability characteristics, required for long operation periods at high temperatures, one approach for avoiding nano-features coarsening and thermoelectric properties degradation, is based on utilizing thermodynamically driven nanostructures, due to physical metallurgy based effects such as spinodal decomposition and nucleation and growth reactions.

All of the mentioned above general trends in thermoelectric will be discussed during the talk. A focus on the related activities in the department of Materials Engineering at BGU will be given.

Over the last 4 years GM has led a development effort in collaboration with Delphi Electrical and Safety, Dana Thermal, Eberspaecher NA, Marlow Industries, The University of Michigan, Oak Ridge National Laboratory, Brookhaven National Laboratory, The University of Washington, Jet Propulsion Laboratory, Purdue University and Molycorp to advance thermoelectric generator technology for passenger vehicle applications. In this talk we will highlight the attributes and challenges associated with this new, highly integrated generator design. We will discuss the materials, fabrication and assembly operations developed to achieve a compact, lightweight and low cost system suitable for the automotive industry. The integrated generator design eliminates the use of standalone modules or cartridges in favor of directly integrating the hot side dielectric and metallization as printable coatings that are directly applied to the hot side heat exchanger surface. This eliminates the need for large clamping forces to encourage good thermal contact between the modules and the heat source and greatly decreases the both the hot and cold side heat exchangers’ volume and mass through the elimination of scar weight attributable to structural reinforcement. Oxidation protection and light compression are achieved by hermitic sealing and a partial pressure of inert cover gas. We will also present the thermoelectric materials formulation, processing and transport property evaluation, the system level performance when integrated into a production vehicle and discuss the fuel economy improvements achievable. This work was made possible through generous support from the US Department of Energy’s office of vehicle technology, EERE and NETL.

Flexible thermoelectrics have attracted rapid growing interest for sustainable energy harvesting technology to power flexible electronics, such as wearable devices. The inorganic TE semiconductors are still the most competitive candidates for this technique due to their best efficiencies, although the pristine materials cannot be directly used as their intrinsic brittleness and rigidity. Therefore, great efforts from various interdisciplinary fields have been dedicated to searching solutions to improve the flexibility of conventional inorganic TE materials. However, there remains a struggling against the trade-off between the TE performance and flexibility.
Herein, we present a novel strategy to craft flexible thermoelectric nanocomposites through depositing M₂C₃ (M=Bi, Sb; C=Te, Se) alloys on the freestanding transparent single-walled carbon nanotubes (SWCNTs) scaffold. The nanocomposite reveals highly-ordered and nanoporous characteristics, which consists of (000l)-textured M₂C₃ nanograins grown on SWCNTs with good adhesion and perfect alignment along the M₂C₃<[endif]-->2[endif]-->0> and SWCNTs axis. The freestanding M₂C₃ nanocomposite exhibits remarkable mechanically reliable flexibility over hundreds bending circles, of which the bending deformation diameter could be as high as a few millimeters. Besides low-dimension and porosity effects, the highly-ordered microstructures give vital contributions to the excellent flexibility, especially at large bending curvatures. Large power factors of ~1600 to 1100 μW/mK² are obtained for the Bi₂Te₃/SWCNTs nanocomposite from room temperature (RT) to 473 K. Owing to the high density of defects, such as Bi₂Te₃/SWCNTs interfaces, stacking faults and nanopores, the in-plane thermal conductivity is as low as 0.5 W/mK. Such high power factor and low thermal conductivity give rise to a TE figure of merit (ZT) of ~ 0.9 at room temperature (RT). Our approach opens up a new avenue to fabricate flexible TE materials with high performance, which could have promising applications in flexible electronics.

It is of particular interest to understand the factors that dominates the interface diffusion of metallic contacts on thermoelectric power devices during operation due to the loss of heat-to-electric power conversion efficiency. Au is the metal of choice due to its chemical inertness and high electrical and thermal conductivities. However, Au slowly diffuses into the van der Waals gaps of BiSbTe at operating temperatures degrading the efficiency, which is undesirable for medium to long-term applications. Intermetallic barriers such as nickel and cobalt are currently employed to prevent Au diffusion, but Au is still permeable resulting in a larger contact resistance and a minimal loss of material. Furthermore, the thermoelectric characteristics of such systems have not been reported yet. In this work, a GeNi alloy barrier is explored. We found that GeNi barrier does not alter the electrical conductivity even after long-term studies. EDS studies showed that GeNi effectively stop Au diffusion into the thermoelectric material. BiSbTe nanowire (NW) studies are also presented. In our work, we measured the thermoelectric properties of BiSbTe NW from 25 K to 310 K with Ge/Ni/Au contact and Ni/Au, Au contacts for comparison. The properties of BiSbTe NW remain the same after long thermal aging (8 h annealing at 250 in forming gas) while with only Ni as barrier or without barrier, the electrical conductivity and Seebeck coefficient both degraded. The formation of GeTe barriers the metal diffusion. Ni forms an Ohmic contact with BiSbTe and prevents the eutectic formation of GeAu.

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.

Thermoelectric materials have broad applications in energy harvesting and Peltier cooling, though the typically rigid configuration limits potential device applications, especially where flexible platforms are required. Most explorations of flexible composite-based thermoelectric materials use a conducting polymer thermoelectric to improve the conductivity while sacrificing the thermoelectric performance. Thin films or layers deposited on soft substrates can also provide a flexible solution, though such processing requires high cost deposition systems.

Here we propose a 3D printing method to make thermoelectric threads out of standard high-performance thermoelectric materials as the initial step towards weaving thermoelectric fabrics. Two methods are demonstrated: one employing a 3D printed polymer matrix surrounding the thermoelectric powder, and another whereby the polymer is sintered away, leaving behind a pure thermoelectric polycrystalline thread. Here, we 3D-printed fine n-type and p-type thermoelectric fibers with a diameter less than d = 50 µm by extruding composite thread using nonconductive poly (lactic-co-glycolic acid) (PLGA) as a polymer binder, followed H₂ sintering to remove the binder, and liquid-phase compression to reduce porosity with an excess tellurium at T = 440 C and P = 45 MPa. The thermoelectric powders were produced by high energy ball milling, and the diameter of grains or clusters was c = 0.1~10 μm. The composite fibers were the most flexible, and even the sintered fibers acquired a bend radius of order cm-scale.

Sintering was observed to significantly improve the electric conductivity. For an example, in comparison with the conductivity of the initial 3D-printed p-type composite thread, σ = 1 * 10^-6 S/cm, that of sintered counterpart was almost 8 orders of magnitude greater at σ = 44 S/cm, which is comparable to that of bulk pellets of the same Bi_0.5Sb_1.5Te₃ powder, σ = 120 S/cm. The Seebeck coefficient of sintered samples was preserved, S = 160 µV/K, also comparable to that of bulk materials, S = 200 µV/K. Therefore, the calculated power factor (PF) of p-type fiber was PF = 112.6 µW/mK². With the assumption that the sintered fiber manifests a thermal conductivity similar to the identically processed bulk p-type pellets, κ = 1.1 W/mK, the material figure-of merit of sintered fiber is approximately, zT = 0.03 which is only four times lower than that of bulk pellets, zT= 0.13, at room temperature, T = 300K. With optimally doped p-type material with zT = 0.8, one can therefore project an eventual goal of at least zT = 0.19 for these thermoelectric threads. The relatively lower zT of the threads could be attributed a porous structure after sintering. Preliminary results of a thermoelectric module fabricated by 3D-printing of n-type and p-type fibers will be demonstrated.

In recent years, the thermoelectric properties of SnSe-based bulk materials have received much attention because of their ultra low thermal conductivity. Zhao et al. reported that the thermal conductivity of SnSe single crystal at 923 K is 0.23 W/mK, and the corresponding ZT is 2.6 (Nature. 2014, 508:373). By further studying, it is revealed that its low thermal conductivity is mainly induced by their non-harmonic bonds. Inspired by this fact, we speculate that SnSe₂-based materials should also have low thermal conductivity because of the similar Sn-Se bond and weak van der Waals force between Se-Sn-Se layers. In our case, the point defect was regulated by annealing nanopowders prepared by a wet-chemical method to improve electrical conductivity as well as power factor. By further Te doping, we found that lower thermal conductivity can be obtained, leading to improved thermoelectric properties.
Owing to the fact that Se element would sublimate at high temperature, it is difficult to synthesize pure SnSe₂ samples by means of solid state reaction. Here, polycrystalline SnSe₂ nanopowders were successfully synthesized by a hydrothermal reaction at 200 ^oC. Subsequently, the samples were heat treated at 400 ^oC to regulate the point defects. Finally, the polycrystalline bulk materials were obtained by a plasma spark sintering process. their structure, composition and morphology were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM), respectively. Their electrical conductivity and Seebeck coefficient were measured by ZEM-3 (ULVAC) and CTA-3 (CRYALL). The thermal diffusion coefficient and specific heat were characterized by LFA-457 and DSC-404 (NETZSCH), respectively.
The XRD results indicates that the product of the hydrothermal reaction are mainly SnSe₂ with a little amount of Sn impurity. After heat treating, the impurity is successfully removed. Meanwhile, the XRF characterization presents that the Se atomic percent decreases with the increasing of temperature, suggesting point defect generated during heat treating. Furthermore, SEM observation shows that the morphology on fracture surface is anisotropic, depending on the pressing direction. As a result, the electric conductivity can be optimized to 10³ S/m at 673.15K while the Seebeck coefficient is -400*10^-6 V/K. By Te-doping, the power factor nearly retain unchanged (1.7*10^-4 W/mK² at 673.15 K), but the thermal conductivity decreases from 0.8 W/mK to 0.64 W/mK.
The above result reveals that SnSe₂ is a promising thermoelectric materials at middle temperature range. Our work may also pave a way for looking and developing thermoelectric materials with low thermal conductivity.

Structural and thermoelectric properties are reported for a specially designed class of A-site substituted perovskite titanates, (Sr_1-x-yCa_xNd_y)TiO₃. Two series of charge doped compounds (y = 0 – 0.2) were synthesized with various A-site Sr-rich or Ca-rich (Sr-poor) concentrations to have a nominally constant tolerance factor at room temperature; i.e., to have the same magnitude of the structural distortion from the cubic SrTiO₃ parent material. These compounds are thus different from typically investigated (Sr_1-yR_y)TiO₃ (R³⁺ = Rare Earths) where the charge doping (band filling) is convoluted with the structural changes affecting the degeneracy of the band structure. Materials were investigated using high resolution neutron powder diffraction as a function of temperature and Nd doping. We determine the room temperature structures as tetragonal I4/mcm and orthorhombic Pbnm for the Sr-rich and Ca-rich series, respectively. Three low temperature orthorhombic structures, Pbnm, Ibmm and Pbcm were also observed for the Sr-rich series; whereas, the symmetry of the Ca-rich series remains unchanged throughout the full measured temperature range. Thermoelectric properties of (Sr_1-x-yCa_xNd_y)TiO₃ were investigated and correlated with the structural variables. We succeeded in achieving a relatively high figure of merit ZT=0.07 at ~400 K in the Sr-rich Sr_0.76Ca_0.16Nd_0.08TiO₃ composition which is comparable to that of the best n-type transition metal substituted SrTi_0.80Nb_0.20O₃ oxide material reported to date. For a fixed tolerance factor, the Nd doping enhances the carrier density and effective mass at the expense of the Seebeck coefficient. Thermal conductivity greatly reduces upon Nd doping in the Ca-rich series. With an enhanced Seebeck coefficient at elevated temperatures and reduced thermal conductivity, we predict that Sr_0.76Ca_0.16Nd_0.08TiO₃ and similar compositions have the potential to become some of the best materials in their class of thermoelectric oxides.

The recent development of novel 2D materials has greatly extended the materials toolsets for fundamental thermal science and engineering. Tin selenide, a promising new thermoelectric material, has recently been reported with extreme low thermal conductivity and record-high figure of merit (ZT = 2.6). However, quantitative understanding of the intrinsic thermal conductivity is still under debating due to materials preparation and characterization challenges. Here, we describe our effort in investigating the thermal transport and phonon scattering mechnisms of the single crystal 2D SnSe material. A novel spectroscopy approach based on ultrafast optics has been developed to understand thermal transport mechanisms down to the nanoscale. The thermal conductivity of high quality single crystalline crystals has been carefully characterized and revealed to be highly anisotropic. We investigate the effects to thermal conductivity from defects, interfaces, and grains. The significant impacts of this research in improving the efficiency of thermal energy conversion and management will also be illustrated.

Thermoelectric performance, gauged by figure-of-merit, is proportional to power-factor and the reciprocal of thermal conductivity. In materials with multi-electronic bands close to the band gap region, power-factor can be increased by enlarging the contribution from the extra band(s). However, the effective mass of the extra band studied so far is commonly much heavier than the primary band, leading to a significantly reduced carrier mobility, which in turn limits the net increase in power-factor. On the other hand, the thermal conductivity of a given material can be decreased by introducing different phonon scattering mechanisms. To further decrease thermal conductivity, additional new phonon scattering sources need to be explored.
In this study,^[1] we employ a light extra band in p-type AgSbTe_2-xSe_x alloys to increase power-factor and introduce a high density of stacking faults to achieve an ultra-low thermal conductivity. Synergistically, we achieve a figure-of-merit over 2.0, which is the record for AgSbTe₂-based systems. Our density functional theory calculations confirmed the existence of such a light band and the power-factor enhancement. Using transmission electron microscopy techniques, we find dense stacking faults. Based on the modeling study on phonon transport by considering various scattering sources, we attribute the obtained ultra-low thermal conductivity to the co-existence of grain boundaries, stacking faults, and point defects, in which the stacking faults account for the larger proportion of the reduced thermal conductivity. The strategy of enhancing the power-factor by engineering extra light band(s) and enhancing phonon scattering through introducing stacking faults opens up a robust pathway to tailor thermoelectric performance.

Reference
[1] M. Hong, Z. C. Chen,* L. Yang, Z. M. Liao, Y. C. Zou, Y. H. Chen, S. Matsumur, and J. Zou* Adv. Energy Mater. 2017 (Accepted)

Transition metal dichalcogenides (TMDs) attract much attention due to their useful technological applications, including energy harvesting. Raman spectroscopy is a powerful technique for characterizing and extracting information regarding the electronic, vibrational and thermal properties of TMDs. We discuss a semi-ab-inito theoretical method [1] for calculating Raman linewidths in the transition metal dichalcogenides MoS₂, WS₂ and MoTe₂ in their 2H bulk and monolayer structures. We present results for the temperature variation (in the range 50 - 1000 K) of the ratio of the widths of other Raman peaks with that of the A_1g peak in bulk samples and with that of the A' peak in the monolayer samples. Our results indicate differing behaviours for MoS₂, WS₂ and MoTe₂. These behaviours will be explained by analysing the roles of anharmonic and isotopic interactions of phonons in determining the linewidths in both the bulk and monolayer systems. Our calculated results for the linewidths of the A_1g and E'_2g modes in bulk MoS₂ are in good agreement with measurements for pristine samples reported in [2,3]. However, our calculated values of the linewidths for the A' and E' modes in monolayer MoS₂ are lower than experimental measurements [2,3,4]. An explanation of the discrepancy between our theoretical results and experimental measurements for all the six systems [2-6] will be attempted.

[1] I.O. Thomas and G. P. Srivastava, Submitted for publication.
[2] H. Guo et al, Appl. Phys. A 122, 375 (2016).
[3] K. Golasa et al, Acta Polonica A 124, 849 (2013).
[4] S. Mignuzzi et al, Phys. Rev. B 91, 195411 (2015).
[5] A. Berkdemir et al, Sci. Rep. 3, 1755 (2013).
[6] M. Yamamoto et al, ACS Nano 8, 3895 (2014).

SnTe is a narrow band gap IV–VI semiconductor with potential for thermoelectric applications [1]. Recently, SnTe-based isovalent/heterostructural alloys, such as ternary SnTe-SnSe and SnTe-MnTe, have been demonstrated to have ZT up to 1.3 [2]. Following up on this work, we used a combination of theoretical and experimental methods to access solubility limits in quaternary Sn_1-yMn_ySe_1-xTe_x alloys [3]. Results from first principle calculations indicate low equilibrium solubility of x,y < 0.05 in Sn_1-yMn_ySe_1-xTe_x alloys, in good agreement with results obtained from bulk equilibrium synthesis experiments, and predict significantly higher spinodal decomposition limits. Experimental combinatorial screening using sputtered Sn_1-yMn_ySe_1-xTe_x thin films showed a remarkable increase in non-equilibrium solubility to over x,y > 0.2. These results were used to guide the bulk synthesis of metastable alloys, with similar high solubility limits achieved in ball milled Sn_1-yMn_ySe_1-xTe_x samples. The ability to reproduce the non-equilibrium solubility levels in bulk materials indicates that combinatorial growth can inform bulk synthetic routes. Furthermore, these results illustrate how computational and combinatorial-growth methods together can be used to study equilibrium and non-equilibrium solubility limits in complex alloys used in thermoelectric applications. Finally, the large difference between equilibrium and non-equilibrium solubility limits in Sn_1-yMn_ySe_1-xTe_x indicates these metastable alloys are attractive for controlled formation of nano-precipitates – a potential route to nanostructured thermoelectric materials with reduced thermal conductivity.
[1] J. Snyder et al, Appl. Phys. Lett., 2016, 109, 42102.
[2] M. G. Kanatzidis et al, J. Am. Chem. Soc., 2015, 137, 11507
[3] S. Siol, A.Holder^, B. R. Ortiz, P. A. Parilla, E. Toberer, S. Lany, A. Zakutayev, RSC Adv., 2017, 7, 24747

Tin chalcogenides have shown promise in applications including energy storage, optoelectronics, photovoltaics, and thermoelectrics. Here, we present a colloidal synthesis strategy to produce tin chalcogenide nanocrystals (NCs) with controllable stoichiometry, vacancies, shape, and crystal structure. Compared with previously reported methods, we use less expensive precursors, such as tin(IV) chloride and sulfur or selenium powder to produce tin (II, IV) chalcogenide NCs. By varying the anion precursors and ligands, we produced tin chalcogenides with different Sn valences, and modified the NCs morphology and size. We prepared nanostructured thin films of these materials by spin-coating, followed by post-treatment to study their thermoelectric properties. Room temperature Seebeck coefficients of -150 μV/K, -126 μV/K, 115 μV/K, 85 μV/K, and 154 μV/K were measured for SnS₂, SnSe₂, SnS, SnSe, and SnTe films. The SnTe thin film with high electrical conductivity and Seebeck coefficient was tested at different temperatures, showing that both electrical conductivity and Seebeck coefficient increase with increasing temperature. This work provides experimental evidence showing the promise of these tin chalcogenide NCs as thermoelectric thin film materials

Efforts worldwide to find viable thermoelectric (TE) materials are intensifying and property enhancement mechanisms are being actively researched [1]. We have discovered that magnetic semiconductors can have enhanced thermoelectric properties, which are indicated to originate from magnetic and carrier-magnon interactions [2-4]. A clear example is given where magnetic Mn-doping into nonmagnetic CuGaTe₂ resulted in a monotonic increase of effective mass, accompanied by appearance of a sizable indicated on-site exchange interaction between the Mn spins and conduction electrons, and significant enhancement of the power factor [5]. I will also present recent results on chalcogenide spinel compounds which had previously been considered to be of poor thermoelectric performance. Unconventional Sb doping into CuCr₂S₄ led to an initial promising ZT value of 0.43 [6]. CuCr_1.7Sb_0.3S₄ is a magnetic semiconductor with ferromagnetic ground state and with an estimated high effective mass of 5.9 m₀ (m₀ =free electron mass). Exciting results have also been obtained for different CuX₂Te₄ (X=magnetic transition metal) compounds and variants, and will be presented. Modules were also constructed from original magnetic sulfides and the performance evaluated. This work is supported by CREST, JST and project members are acknowledged.
References
1) T. Mori, “Novel principles and nanostructuring methods for enhanced thermoelectrics”, Small, in press doi: 10.1002/smll.201702013 (2017).
2) N. Tsujii and T. Mori, Appl. Phys. Express, 6, 043001 (2013).
3) R. Ang, A. U. Khan, N. Tsujii, K. Takai, R. Nakamura, and T. Mori, Angew. Chem. Int. Ed., 54, 12909 (2015).
4) H. Takaki, K. Kobayashi, M. Shimono, N. Kobayashi, K. Hirose, N. Tsujii, and T. Mori, Appl. Phys. Lett. 110, 072107 (2017).
5) F. Ahmed, N. Tsujii and T. Mori, J. Mater. Chem. A, 5, 7545 (2017).
6) A. U. Khan, R. A. R. A. Orabi, A. Pakdel, J. B. Vaney, B. Fontaine, R. Gautier, J. F. Halet, S. Mitani, and T. Mori, Chem. Mater., 29, 2988 (2017).

High performance thermoelectric materials requires ultralow lattice thermal conductivity typically through either shortening the phonon mean free path or reducing specific heat. Beyond these two approaches, we propose a new unique, simple, yet ultrafast solid state explosive reaction to fabricate nano-porous bulk thermoelectric materials with well controlled pore sizes and distributions to suppress thermal conductivity. By investigating a wide variety of functional materials, general criteria for solid state explosive reactions are built upon both thermodynamics and kinetics, and then successfully used to tailor material’s microstructures and porosity. Drastic decrease in lattice thermal conductivity down below the minimum value of the fully densified materials and enhancement in thermoelectric figure of merit are achieved in porous bulk materials. This work demonstrates that controlling materials’ porosity is a very effective strategy and is easy to be combined with other approaches for optimizing thermoelectric performance.

The emergence of inorganic quasi-2D crystals has generated an interest within the material science community due to a diverse range of properties and relative ease of implementation into complex structures. Quasi-2D materials garner considerable attention because of their potential applications in nanoelectronics, optoelectronics, energy storage, and thermoelectrics. Computational identification of quasi-2D materials typically only considers materials with Van der Waals bonding between the layers. Complex variants of Van der Waals materials often do not have a Van der Waals gap (such as CsBi₄Te₆), but are still sought out specifically for properties that stem from their layered behavior. The same techniques used for identifying quasi-2D Van der Waals materials can also be applied to search for layered materials with stronger bonding. In this work, we have identified a group of layered ternary materials which are characterized as having binary layers separated by an ionically-bonded spacer element between the layers. We conducted a high-throughput search through ternary systems within the inorganic crystal structure database (ICSD) and identified over 700 compounds which we classify as ionically-bonded layered materials. As confirmation that we correctly identify such materials, we find that the largest grouping of spacer elements comes from the group I and II elements, which fits with the general understanding that many of these materials have spacer elements such as Cs, Ba, Li, Ca, and La. To assess the elastic stability of these structures, we preformed DFT relaxation and elastic calculations within the plane-wave VASP code. From DFT, we found approximately 45% of the compounds within the set with nonzero bandgap. From the elastic tensor, we assessed the anisotropy of these materials using the universal anisotropy index (A_U), which varied from completely isotropic (A_U ≈ 0) to highly anisotropic (A_U > 10). From the elastic tensor we can calculate the speed of sound for any lattice vector using Christoffel's equations. We use this in conjunction with isotropic material parameters to predict the lattice thermal conductivity using a semi-epirical model that we have previously developed. We find that the majority of these layered compounds are predicted to have low lattice thermal conductivity, with 80% having an average value lower than 10 W/mK and 30% with κ_l lower than 2 W/mK. This ratio holds for both zero-gap and finite bandgap. Within this set of ionically-bonded layered materials, we expect to find many suitable candidates for thermoelectric performance and other applications necessitating low thermal conductivity.

Thermoelectric materials have been investigated for producing green energy through the conversion of waste heat generated from cars and other large machinery into electricity. The efficiency of these materials partially depends on having high electrical conductivity in conjunction with low thermal conductivity. Unfortunately, these two parameters are often coupled. In complex crystals, the Phonon Glass-Electron Crystal concept describes a structure that allows for the decoupling of electrical and thermal conductivities. A new family of Cs_0.16M_xSi_1-xAs₂ (M = Cu, Zn, Ga) compounds has been synthesized. All compounds are isostructural and crystallize in the orthorhombic Pnma space group (No. 62). These compounds consist of (Si/M)₁As₂ layers with Cs cations situated in large channels between the layers. Metals of Cu, Zn, and Ga substitute Si atoms for charge balance and allow properties to be tuned. The crystal structure complies with the Phonon Glass-Electron Crystal concept as the (Si/M)₁As₂ layers encourage electronic conductivity and the rattling Cs ions scatter phonons. Synthesis, structure, and thermoelectric properties for these compounds will be discussed.

The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a bandgap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. In this talk, we study several semimetals including HgTe as an example semimetal with competitive thermoelectric properties. We show theoretically and experimentally that semimetals can achieve large thermoelectric figure of merit. We employ ab initio calculations with hybrid exchange-correlation functional to accurately describe the electronic band structure of several semimetals in conjunction with the Boltzmann Transport theory to investigate their electronic transport properties. We show that semimetals can have Seebeck coefficient values close to those of semiconductors. Their electrical conductivity values are higher than semiconductors and they can have similar thermal conductivity values. We calculate the lattice thermal conductivity of HgTe using first principles calculations and evaluate its overall figure of merit. Finally, we prepare semi-metallic HgTe samples and we characterize its transport properties. We show that our theoretical calculations agree well with the experimental data.

The development of spintronics and spin-caloritronics devices need efficient generation, detection and manipulation of spin current. The thermal spin current from spin-Seebeck effect has been reported to be more energy efficient than the electrical spin injection methods. But, spin detection has been the one of the bottlenecks since metals with large spin-orbit coupling is an essential requirement. In this work, we report an efficient thermal generation and interfacial detection of spin current. We measured a spin-Seebeck effect in Ni₈₀Fe₂₀ (25 nm)/p-Si (50 nm) (polycrystalline) bilayers without heavy metal spin detector. The p-Si, having the centosymmetric crystal structure, has insignificant intrinsic spin-orbit coupling leading to negligible spin-charge conversion. We report a giant inverse spin-Hall effect, essential for detection of spin-Seebeck effect, in the Ni₈₀Fe₂₀/p-Si bilayer structure, which originates from Rashba spin orbit coupling due to structure inversion asymmetry at the interface. In addition, the thermal spin pumping in p-Si leads to spin current from p-Si to Ni₈₀Fe₂₀ layer due to tunneling spin galvanic effect and spin-Hall effect causing spin-orbit torques. The thermal spin-orbit torques leads to collapse of magnetic hysteresis of 25 nm thick Ni₈₀Fe₂₀ layer for a temperature gradient of 20.84 mK across the bilayer specimen.

At the nanoscale, the phonon transport is heavily suppressed by the scattering at the interface [1]. Examples of such interfaces include grain boundaries within a polycrystal or heterostructure interfaces within a nanocomposite. The phonon scattering by these interfaces can reduce the lattice thermal conductivity. When bulk-like electrical properties can still be conserved, improved thermoelectric performance can be achieved in various nanostructured bulk materials.
In numerous molecular dynamics simulations, the interfacial thermal resistance (R_K) of a grain boundary has a strong dependence on the misorientation between two grains [2,3]. Similar crystal-orientation dependence is anticipated for a heterostructure interface but the phonon transport process becomes more complicated. Interfacial atom diffusion and participation of optical phonons are two main concerns in such cases. With film-wafer bonding, high-quality Si-Ge heterostructure interfaces have been obtained with its high electrical conductance [4]. Such interfaces provide an ideal model system for interfacial studies. However, the corresponding thermal study is still lacking.
In this work, a 70-nm-thick Si thin film is hot pressed onto a Ge wafer to represent a Si-Ge heterostructure interface formed in SiGe nanocomposites. The interfacial R_K is measured using the 3w method for varied crystal misorientations. Strains at the bonded interface are measured with Raman spectroscopy to reveal its relationship with R_K, along with interfacial structure characterization by transmission electron microscopy. These detailed interfacial thermal studies can provide important guidance for interface engineering to tune heat transport inside a material or device.
References:
[1] Cahill et al., J. Appl. Phys. 93, 793 (2003).
[2] Cao et al., J. Appl. Phys. 111, 053529 (2012).
[3] Kimmer et al., Phys. Rev. B 75, 144105 (2007).
[4] Kiefer et al., ACS Nano 5, 1179-1189 (2011).

Using the first-principles based density functional tight-binding (DFTB) method with our newly developed Si-Si, Ge-Ge and Si-Ge parameters for thermal properties calculations, the in-plane and cross-plane thermal conductivities of Si/Ge superlattices with ideal interfaces are directly calculated. The calculated in-plane and cross-plane thermal conductivities of Si/Ge superlattices show different trends compared to the previous predictions from the virtual crystal approximation. Moreover, the effect of mass and bond variances on reducing in-plane and cross-plane thermal conductivities are studied separately. We find that mass variance has more profound impacts on the cross-plane thermal conductivities reduction by enhancing the interface phonon scattering. But when the mass variance is absent, the in-plane thermal conductivities of Si/Ge superlattices are unexpectedly lower than the cross-plane ones. These results shed light on new understandings of phonon transport in Si/Ge superlattices.

The development of large-scale, highly-efficient thermoelectric materials has emerged as one of the ultimate goals of nanoscale science and technology, and the last two decades have seen incredible progress in the advancement of thermoelectric materials. The use of superlattices, quantum-wells, nanoparticle assemblies, and nanostructured composite materials have significantly increased the thermoelectric figure of merit, ZT, of many systems. Along these lines, hybrid organic-inorganic nanostructured materials provide unique opportunities for improving thermoelectric efficiency because the electrical properties and thermal properties have the potential to be tuned independently. In this study, we demonstrate control over both the Seebeck coefficient and the type of charge carrier (n-type or p-type) in a series of hybrid 2-dimensional molecule-nanoparticle superlattices. Interestingly, by systematically changing the length and HOMO-LUMO gap of a series of electrically conductive heteroacene-ladder molecules used to interlink gold the nanoparticles in a monolayer array, we are able to measure a crossover in the sign of the Seebeck coefficient, corresponding to a crossover in the sign of the majority charge carrier (from positive to negative). This finding is confirmed with Hall-effect measurements. To understand the origins of this effect we examine the redox properties of the heteroacene molecules and find that the alignment between the chemical potential of the nanoparticles and the HOMO and LUMO levels of the molecule change throughout the series, resulting in electron-based tunneling between nanoparticles for some molecules and hole-based tunneling in others, which in turn results in the different signs of the Seebeck coefficient and Hall voltage. Our findings develop a stronger understanding of charge transport in hybrid molecule-nanoparticle monolayer arrays, establish a novel framework for maximizing the thermoelectric efficiency of these materials, and demonstrate that both electron and hole transport can be attained in these systems, thus opening a new opportunity for creating integrated thermoelectric devices.

Researchers have recently developed processes for synthesizing monolithic Si with improved thermoelectric figure-of-merit ZT. These materials obtain higher ZT through a fine dispersion of nanoscale carbide particles. In this work we elucidate the role these particles play in two processes to increase ZT: (1) scattering of phonons to reduce thermal conductivity, and (2) energy selective scattering of electrons to increase the seebeck coefficient. A multiscale approach is used model the former process. Molecular dynamics simulations were used to quantify inclusions’ scattering cross cross-section for phonons, and this fed to Boltzmann transport simulations to predict the collective effect of the particle dispersion close to the Knudsen regime. To model electron energy filtering a semiclassical Boltzmann transport model was developed and used phenomenologically to predict the optimal energy filtering threshold for improving thermoelectric power factor. Together, these models provide guidance to researchers working to engineering high ZT in bulk Si.

Thermoelectric (TE) power generation is the direct conversion of thermal energy into electrical work. The search for novel TE materials is necessary in the pursuit of higher efficiency TE devices. Layered tetrel pnictides have shown promise as TEs due to their anisotropic crystal structure and weak van der Waals interactions between the layers. The binary GeAs is a p-type semiconductor with a narrow indirect bandgap of 0.57eV and high Seebeck coefficient (~250 µV/K at 300 K). The main drawback of this compound is its poor electrical conductivity due to the relatively low charge carrier concentration. This work investigates the aliovalent doping of the Ge and As sublattice in the GeAs structure. The synthesis, structure, and transport properties of Ga- and Se-doped GeAs will be discussed.