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
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[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, 2