Using a first-principles cluster expansion, we shed light on the solid-state phase diagram and structure of the recently discovered high-performance thermoelectrics, Pb-Ag-Sb-Te alloys. The observed nanoscale inhomogeneities are shown to be coherent precipitates of ordered (Ag)m(Sb)n(Te)m+n phases, such as AgSbTe2, all immiscible with rocksalt PbTe. The solubility is high for PbTe in AgSbTe2 and low for (Ag,Sb)Te in PbTe (8% vs. 0.6% at 850 K). The differences in the phonon spectra of PbTe and AgSbTe2 confirm that the inhomogeneities enhance the thermoelectric performance.
By performing first principles calculations combined with Boltzmann transport equations, we calculate the thermoelectric power factor (PF) of Bi-Sb-Te and Bi-Te-Se ternary alloys as a function of alloy composition ratio, carrier concentration, and temperature. The point defect formation energy calculations also perform to search potential n-type dopant candidates in ternaries.
Half-Heusler alloys (MgAgSb structure) are promising thermoelectric materials. RNiSn half-Heusler phases (R=Hf, Zr, Ti) are the most studied in view of their thermal stability. The highest dimensionless figure of merit (ZT) obtained is ~1 in the temperature range ~450-900oC, primarily achieved in nanostructured alloys. Through proper annealing, ZT~1.2 has been obtained in a previous ZT~1 n-type (Hf,Zr)NiSn phase without the nanostructure. There is an appreciable increase in the power factor, decrease in charge carrier density, and increase in carrier mobility. The findings are attributed to th…
We present a comprehensive first-principles investigation of optical, transport, and thermoelectric properties of pure and doped hexagonal Si$_x$Ge$_{1-x}$ alloys based on density-functional theory calculations, the Boltzmann transport equation, and the generalized quasi-chemical approximation to obtain alloy averages of electronic properties. At low temperature, phase decomposition into the hexagonal elementary crystals is thermodynamically favored, but around and above room temperature random alloys are predicted to be stable. While hexagonal Si has an indirect band gap, the gap of hexagonal…
Interest in high entropy alloy thermoelectric materials is predicated on achieving ultralow lattice thermal conductivity $κ\sub{L}$ through large compositional disorder. However, here we show that for a given mechanism, such as mass contrast phonon scattering, $κ\sub{L}$ will be minimized along the binary alloy with the highest mass contrast, such that adding an intermediate-mass atom to increase atomic disorder can increase thermal conductivity. Only when each component adds an independent scattering mechanism (such as adding strain fluctuation to an existing mass fluctuation) is there a bene…
Structural and thermoelectric properties of metallic and semiconducting Sb2Te3 are reported. X-Ray diffraction and Raman spectroscopy studies reveal that semiconducting sample have higher defect density. Nature and origin of possible defects are highlighted. Semiconducting Sb2Te3 hosts larger numbers of defects, which act as scattering center and give rise to the increased value of resistivity, thermopower and power factor. Thermopower data indicates p-type nature of the synthesized samples. It is evidenced that the surface states are often mixed with the bulk state, giving rise to metallicity…
We report on the high-temperature (350 K <T< 1000 K) thermoelectric properties of the ternary compounds LuNiSb (cubic) and LuNiSn (orthorhombic) and a composite material (LuNiSb)$_{0.5}$(LuNiSn)$_{0.5}$. The electrical conductivity in LuNiSn is metallic, while it is semiconducting-like in LuNiSb. The Seebeck coefficient reaches -5.5 $μ$V/K at 700 K for the former compound and 66 $μ$V/K at 607 K for the latter one. The composite sample (LuNiSb)$_{0.5}$(LuNiSn)$_{0.5}$ constructed from orthorhombic matrix with cubic inclusions combines the electrical conductivity of LuNiSn with the thermoelectri…
Half-Heusler (HH) phases have garnered much attention as thermally stable and non-toxic thermoelectric materials for power conversion. The most studied alloys to date utilize Hf, Zr, and Ti as the base components. These alloys can achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies have advanced the thermoelectric performance of HH alloys by employing nanostructures and novel compositions to achieve larger ZT, reaching as high as 1.5. Herein, we report that traditional alloying techniques applied to the conventional HfZr-based half-Heusler alloys can also lead to excep…
We report magnetoresistive properties of direct and indirect band gap Bismuth-Antimony (Bi-Sb) alloys. Band gap increases with magnetic field. Large positive magnetoresistance (MR) approaching to 400 % is observed. Low field MR experiences quadratic growth and at high field it follows a nearly linear behavior without sign of saturation. Carrier mobility extracted from low field MR data, depicts remarkable high value. Correlation between MR and mobility is revealed. We demonstrate that the strong nearly linear MR at high field can be well understood by classical method, co-build by Parish and L…
We explore a material design strategy to optimize the thermoelectric power factor. The approach is based on screening the band structure changes upon a controlled volume change. The methodology is applied to the binary silicides and germanides. We first confirm the effect in antifluorite Mg2Si and Mg2Ge where an increased power factor by alloying with Mg2Sn is experimentally established. Within a high-throughput formalism we identify six previously unreported binaries that exhibit an improvement in their transport properties with volume. Among these, hexagonal MoSi2 and orthorhombic Ca2Si and …
We investigated six heavy element bismuth-based 18-VEC half-Heusler alloys CoTiBi, CoZrBi, CoHfBi, FeVBi, FeNbBi, and FeTaBi by first principles approach, in search of better thermoelectric prospects. The motivation is driven by expected lower thermal conductivity and the recent discovery of CoZrBi-based materials. Significantly, our calculated power factor values of all the systems show an increment of $\sim$40\% in comparison to the reported \textit{p}-type CoTiSb. We propose that doping at Bi-site, on account of electronic features, will be helpful in achieving the proposed power factor val…
High lattice thermal conductivity in half-Heusler alloys has been the major bottleneck in thermoelectric applications. Disordered half-Heusler alloys could be a plausible alternative to this predicament. In this paper, utilizing first-principles simulations, we have demonstrated the low lattice thermal conductivity in two such phases, NbFe$_{0.5}$Ni$_{0.5}$Sn and TaFe$_{0.5}$Ni$_{0.5}$Sn, in comparison to well-known half-Heusler alloy TiCoSb. We trace the low thermal conductivity to their short phonon lifetime, originating from the interaction among acoustic and low-lying optical phonons. We r…
By using ab initio electronic structure calculations here we report the three new full Heusler alloys which are possessing very good thermoelectric behavior and expected to be synthesized in the laboratories. These are Fe2 ScP, Fe2 ScAs and Fe2 ScSb compound. First two compounds are indirect band gap semiconductors and the last one shows semimetallic ground state. The value of band gap of Fe2 ScP and Fe2 ScAs is 0.3 and 0.09 eV, respectively. These compounds show the presence of flat conduction bands along Γ - X-direction suggesting for the large electron like effective mass and promising for …
We screened six cobalt-based 18-VEC systems CoVSi, CoNbSi, CoTaSi (Si-group) and CoVGe, CoNbGe, CoTaGe (Ge-group) by the first-principles approach, with the motivation of stabilizing these orthorhombic phases into the cubic symmetry -- favorable for thermoelectrics. Remarkably, it was found that the Ge-group is energetically more favorable in the cubic symmetry than the hitherto orthorhombic phase. We account the cubic ground state of the Si-group to the interplay of internal pressure and covalent interactions. The principle of covalent interactions will provide an insight and could be vital i…
Half-Heusler alloys have been one of the benchmark high temperature thermoelectric materials owing to their thermal stability and promising figure of merit ZT. Simonson et al. early showed that small amounts of vanadium doped in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change with the increased density of states near the Fermi level. We herein report a systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta) as prospective resonant dopants in enhancing the ZT of n-type half-Heusler alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was fou…
Calculations using density functional theory (DFT) were performed to investigate the structural, dynamical, mechanical, electronic, magnetic, and thermoelectric properties of VTiRhZ (Z = Al, Ga, In) alloys. The most stable structure of these alloys was found to be the type-I configuration. Using GGA-PBE functional, VTiRhGa, and VTiRhIn alloys are predicted as half-metallic ferromagnets with a 100% spin-polarization and a total magnetic moment of 3μB, which is promising for spintronic applications. The thermoelectric properties and lattice thermal conductivity of VTiRhZ alloys were obtained usi…
The thermoelectric power (TEP) measurements have been carried out to investigate the changes in the electronic structure associated with the intermartensitic and martensitic transitions in Ni$_{2+x}$Mn$_{1-x}$Ga (0 $\le$ x $\le$ 0.19). The samples have been characterized by a.c. magnetic susceptibility measurements. The correlation between the TEP and the microstructural changes sensitive to the increasing Ni content in Ni$_{2+x}$Mn$_{1-x}$Ga has been investigated. The changes in the density of states as reflected in the temperature variation of the TEP are consistent with the conclusions draw…
The anomalous Nernst effect (ANE), a heat-charge conversion mechanism based on a magnetic metal, has been extensively studied for application in thin thermoelectric devices. The low magnetization and fabrication at room temperature are important for flexible ANE thermoelectric devices on plastic substrates. Therefore, we investigated the ANE in ferrimagnetic GdxFe100-x films with low magnetization in this study by systematically varying the Gd composition x. Although the Gd33Fe67 film with a Gd composition close to magnetic compensation composition exhibited very low magnetization, an ANE coef…
Thermoelectric (TE) materials research plays a vital role in heat-to-electrical energy conversion and refrigeration applications. Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures (100-150K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples consisting of micron-size grains. The achieved ZT is nearly 50 percents higher than polycrystalline averaged single crystal ZT of ~0.4, and it is also significantly higher than ZT of less than ~0.3 measured below 150K in Bi-Te alloys commonly us…
Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties o…
Half Heusler (HH) thermoelectric alloys provide a wide platform to choose materials with non-toxic and earth abundant elements. This article presents an ab-initio theoretical evaluation of electrical and thermal transport properties of three Bismuth-based most promising thermoelectric alloys, selected out of 54 stable HH compounds. These are brand new compounds which are recently proposed to be stable (Nature Chem. 7, 308 (2015)) and may have interesting properties. The calculated band structure of the three compounds, namely HfRhBi, ZrIrBi and ZrRhBi, served as a hint for their promising ther…
Utilizing a material in thermoelectric applications requires a mechanical, thermal, and lattice stability as well a high figure of merit (ZT). In this work, we present the structural, electronic, magnetic, mechanical, thermodynamic, dynamic, and thermoelectric properties of CoFeYGe (Y = Ti, Cr) quaternary Heusler compounds using the density functional theory (DFT). The calculated mechanical properties and phonon dispersions reveal that the structures of these compounds are stable. Both CoFeCrGe and CoFeTiGe compounds show a ferromagnetic and ferrimagnetic half-metallic behavior with band gaps …
Structural, electronic, and transport properties of ScNiSb, ScPdSb, and ScPtSb were investigated from first principles. Electronic band structures derived within the fully relativistic MBJLDA approach were compared with those obtained from the standard GGA calculations. All the compounds studied exhibit indirect narrow band gaps (0.24-0.63 eV). The effective masses of hole-like carriers are relatively small (0.27-0.36), and decrease with an increasing atomic number of the transition metal component. The carrier relaxation time, required for realistic calculations of the electrical conductivity…
Thermal variation of structural property, linear thermal expansion coefficient, resistivity, thermopower and power factor of polycrystalline Bi1-xSbx (x=0.10, 0.14) samples are reported. Temperature-dependent powder diffraction experiments indicate that samples do not undergo any structural phase transition. Rietveld refinement technique has been used to perform detailed structural analysis. Temperature dependence of thermal expansion coefficient is found to be stronger for Bi0.90Sb0.10. Also, power factor for direct band gap Bi0.90Sb0.10 is higher as compared to that for indirect band gap Bi0…
Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering …
Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first principles
The thermoelectric properties of 54 different group 4 half-Heusler (HH) alloys have been studied from first principles. Electronic transport was studied with density functional theory using hybrid functionals facilitated by the $\mathbf{k} \cdot \mathbf{p}$ method, while the temperature dependent effective potential method was used for the phonon contributions to the figure of merit $ZT$. The phonon thermal conductivity was calculated including anharmonic phonon-phonon, isotope, alloy and grain-boundary scattering. HH alloys have an ${\it XYZ}$ composition and those studied here are in the gro…
Realizing record-high transverse thermoelectric figure of merit at room temperature in artificially tilted multilayers based on high power factor NiFe alloy
National Institute for Materials Science（物质材料研究机构）；University of Tsukuba（筑波大学）；The University of Tokyo（东京大学）；Resonac Corporation（瑞萨科株式会社）
Using the density functional theory (DFT) in combination with Boltzmann transport theory, the influence of Mg concentrations (x) doping on the thermoelectric properties of Hg1-xMgxSe ternary alloys was systematically investigated. The generalized gradient approximations of Perdew-Burke-Ernzerhof (GGA-PBE) have been used to illustrate the exchange correlation potential. Various thermoelectric transport parameters, such as the Seebeck coefficient (S), the thermal conductivity over relaxation time, the electrical conductivity over relaxation time, the power factor (PF) and the figure of merit (ZT…
Thermoelectricity in principle provides a pathway to put waste heat to good use. Motivated by this we investigate thermal and electrical transport properties of two new Fe-based Heusler alloys, FeTaSb and FeMnTiSb, by a first principles approach and semiclassical Boltzmann transport theory within the constant relaxation-time approximation. We find a high power factor of \textit{p}-doped FeTaSb, competitive with best performing Heusler alloy FeNbSb at 1100 K. The obtained power factor of \textit{n}-doped FeMnTiSb at room temperature is higher than that of both FeNbSb and FeTaSb. Remarkably, FeM…
Half-Heusler (HH) phases (space group F43m, Clb) are increasingly gaining attention as promising thermoelectric materials in view of their thermal stability, scalability, and environmental benignity as well as efficient power output. Until recently, the verifiable dimensionless figure of merit (ZT) of HH phases has remained moderate near 1, which limits the power conversion efficiency of these materials. We report herein ZT~1.3 in n-type (Hf,Zr)NiSn alloys near 850 K developed through elemental substitution and simultaneously embedment of nanoparticles in the HH matrix, obtained by annealing t…
To understand the unexpectedly high thermoelectric performance observed in the thin-film Heusler alloy Fe$_2$V$_{0.8}$W$_{0.2}$Al, we study the magnon drag effect, generated by the tungsten based impurity band, as a possible source of this enhancement, in analogy to the phonon drag observed in FeSb$_2$. Assuming that the thin-film Heusler alloy has a conduction band integrating with the impurity band, originated by the tungsten substitution, we derive the electrical conductivity $L_{11}$ based on the self-consistent t-matrix approximation and the thermoelectric conductivity $L_{12}$ due to mag…
High-entropy alloys are characterized by high configurational entropy. Since the discovery of high-entropy alloys (HEA) in 2004, entropy engineering has provided a promising direction for exploiting composition, lattice disorder, band structure, and microstructure effects to advance thermoelectric performance. This review discusses the impact of entropy on thermoelectric properties and looks back at the role of multi-principal-element alloys, a weaker version of HEA, on the development of compositionally complex thermoelectric alloys in achieving high thermoelectric performance. The experiment…
High performance thermoelectric materials for wide-range temperature applications still remains a challenge. In this study, we have produced high-quality homogeneous $In_{0.32}Ga_{0.68}N$ on ZnO substrates, with no phase separation at high Indium content, using metal organic chemical vapor deposition for thermoelectric applications. A record high room temperature figure of merit zT is obtained of 0.86, which is five times larger than that of SiGe, the current state of the art high temperature thermoelectric material. These materials are shown to have a nearly perfect doping concentration to ma…
On the basis of density functional theory and semi-classical Boltzmann theory, we have investigated the structural, elastic, electronic, optical and thermoelectric properties of 18--valence electron count rhodium based half-Heusler alloys focusing on RhTiP, RhTiAs, RhTiSb, and RhTiBi. The absence of imaginary frequencies in the phonon dispersion curve for these system verifies that they are structurally stable. RhTiP is ductile in nature, while others are brittle. The alloys are found to be semiconducting with indirect band gaps ranging from 0.94 to 1.01 eV. Our calculations suggest these mate…
Transverse thermoelectric generation using magnetic materials is essential to develop active thermal engineering technologies, for which the improvements of not only the thermoelectric output but also applicability and versatility are required. In this study, using combinatorial material science and lock-in thermography technique, we have systematically investigated the transverse thermoelectric performance of Sm-Co-based alloy films. The high-throughput material investigation revealed the best Sm-Co-based alloys with the large anomalous Nernst effect (ANE) as well as the anomalous Ettingshaus…
We investigate the thermoelectric properties of some semiconducting Chevrel phases. Band structure calculations are used to compute thermopowers and to estimate of the effects of alloying and disorder on carrier mobility. Alloying on the Mo site with transition metals like Re, Ru or Tc to reach a semiconducting composition causes large changes in the electronic structure at the Fermi level. Such alloys are expected to have low carrier mobilities. Filling with transition metals was also found to be incompatible with high thermoelectric performance based on the calculated electronic structures. …
The demand for green energy increases day by day due to environmental concern and thermoelectric (TE) materials are one of the eco-friendly energy resources. Few authors reported high TE performance in TmCu$_3$Ch$_4$, reaching the figure of merit (ZT) above 2 at 1000K, from first-principles calculations neglecting electron-phonon scattering, spin-orbit coupling effect (SOC), and energy-dependent carrier lifetime. Here, thermoelectric transport properties of TmCu$_3$Ch$_4$ are reinvestigated through considering these parameters, and significant discrepancies are found. The ZT of p-type TaCu$_3$…
On the quest for efficient thermoelectrics, semiconducting behavior is a targeted property. Yet, this is often difficult to achieve due to the complex interplay between electronic structure, temperature, and disorder. We find this to be the case for the thermoelectric clathrate Ba$_8$Al$_{16}$Si$_{30}$: Although this material exhibits a band gap in its groundstate, a temperature-driven partial order-disorder transition leads to its effective closing. This finding is enabled by a novel approach to calculate the temperature-dependent effective band structure of alloys. Our method fully accounts …
n-type Mg3Sb2-based compounds are emerging as a promising class of low-cost thermoelectric materials due to their extraordinary performance at low and intermediate temperatures. However, so far high thermoelectric performance has merely been reported in n-type Mg3Sb2-Mg3Bi2 alloys with a large amount of Bi. Moreover, current synthesis methods of n-type Mg3Sb2 bulk thermoelectrics involve multi-step processes that are time- and energy-consuming. Here we report a fast and straightforward approach to fabricate n-type Mg3Sb2 thermoelectrics using spark plasma sintering, which combines the synthesi…
We investigate the high temperature thermoelectric properties of Heusler alloys Fe2-xMnxCrAl (0<x<1). Substitution of 12.5% Mn at Fe-site (x = 0.25) causes a significant increase in high temperature resistivity (\r{ho}) and an enhancement in the Seebeck coefficient (S), as compared to the parent alloy. However, as the concentration of Mn is increased above 0.25, a systematic decrement in the magnitude of both parameters is noted. These observations have been ascribed (from theoretical analysis) to a change in band gap and electronic structure of Fe2CrAl with Mn-substitution. Due to absence of …
The electronic structures of TixZrx/2CoPbxTex, TixZrx/2Hfx/2CoPbxTex (x = 0.5), and the parent compound TiCoSb were investigated using the full potential linearized augmented plane wave method. The thermoelectric transport properties of these alloys are calculated on the basis of semi-classical Boltzmann transport theory. From the band structure calculations we show that the substitution of Zr,Hf in the Ti site and Pb and Te in the Sb site lower the band gap value and also change the indirect band (IB) gap of TiCoSb to the direct band (DB) gap. The calculated band gap of TiCoSb, TixZrx/2CoPbxT…
We have performed first-principles investigations on the native defects in the full Heusler alloys Co$_2$Ti$Z$ ($Z$ one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that Co vacancies (Vc) in all compounds and the Ti$_\text{Sn}$ anti-site exhibit negative formation energies. The smallest positive values occur for Co in excess on anti-sites (Co$_Z$ or Co$_\text{Ti}$) and for Ti$_Z$. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination …
The discovery of Heusler alloys has revolutionized the research field of intermetallics due to the ease with which one can derive potential candidates for multifunctional applications. During recent years, many half Heusler alloys have been investigated for their thermoelectric properties. The f electron based rare earth ternary half Heusler compound DyPdBi has its f energy levels located close to the Fermi energy level. Other research efforts have emphasized that such materials have good thermoelectric capabilities. We have explored using first principles the electronic band structure of DyPd…
Ternary half Heusler alloys are under intense investigations recently towards achieving high thermoelectric figure-of-merit (ZT). Of particular interest is the ZrNiPb based half Heusler (HH) alloy where an optimal value of ZT = 0.7 at 773 K has been achieved by co-doping Sn and Bi at Pb site. In this work, we identify an excellent ZT of 1.3 in ZrNi1+xPb0.38Sn0.6Bi0.02 (x= 0.03, at 773 K) composite alloy. This is achieved by synergistic modulation of electronic as well as thermal properties via introduction of minor phase of full Heusler (FH) in the HH matrix through compositional tuning approa…
The Seebeck coefficients, electrical resistivities, total thermal conductivities, and magnetization are reported for temperatures between 5 and 350 K for n-type Bi0.88Sb0.12 nano-composite alloys made by Ho-doping at the 0, 1 and 3% atomic levels. The alloys were prepared using a dc hot-pressing method, and are shown to be single phase for both Ho contents with grain sizes on the average of 900 nm. We find the parent compound has a maximum of ZT = 0.28 at 231 K, while doping 1% Ho increases the maximum ZT to 0.31 at 221 K and the 3% doped sample suppresses the maximum ZT = 0.24 at a temperatur…
