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Less-expensive and abundantly available Hf-free half-Heusler (HH) alloys are promising candidates for mid-temperature thermoelectric (TE). In the present work, we combine experimental outcomes with...

High thermal conductivity and exorbitant cost of Hf has for a long time limited the prospects of half-Heusler (HH) alloys for applicability in thermoelectric (TE) energy conversion devices. This work demonstrates the implication of nanostructuring and efficacy of p-type acceptor dopant in (Ti,Zr)CoSb based HH alloys for enhancing the figure of merit (ZT) while eliminating the use of Hf. A series of (Ti,Zr)CoSb1−x(Si,Sn)x HH composition was synthesized using arc-melting and consolidated employing spark plasma sintering (SPS). The optimal doping of acceptor dopants, namely, Si and Sn significantly improves the power factor and strengthens the phonon scattering resulting in an enhanced TE performance with maximum ZT of 0.26 and 0.5 at 873 K, obtained for TiCoSb0.8Sn0.2 and ZrCoSb0.8Sn0.2, respectively. For further optimization, microstructural modifications by fine-tuning of the Ti to Zr ratio induces strain field effects and mass fluctuation in (Ti,Zr)CoSb0.8Sn0.2 compositions, which remarkably introduces additional phonon scattering resulting in maximum ZT ∼ 0.8 at 873 K for the best performing Zr0.5Ti0.5CoSb0.8Sn0.2 compound. The current study provides a better understanding of p-type dopants in HH materials by which prospective high TE performance can be obtained in low-cost Hf-free p-type (Ti,Zr)CoSb half-Heusler alloys.

ZrCoSb based half-Heusler (HH) alloys have been widely studied as a p-type thermoelectric (TE) material for power generation applications in the mid-temperature regime. However, their intrinsically high thermal conductivity has been found to be detrimental for the improvement in their thermoelectric figure-of-merit (ZT), which presently is far below unity. In the current work, a state-of-the-art ZT ∼1.1 at 873 K was realized in an optimized composition of nanostructured Zr1-xHfxCoSb0.9Sn0.1 HH alloys by employing compositional modulation i.e. grain-by-grain compositional variations, which leads to a substantial increase in its power factor coupled with a concurrent decrease in its thermal conductivity. Significant reduction in the phonon mean-free-path is observed on Hf substitution, which is comparable to the average crystallite size (∼25 nm), thus leading to a very low thermal conductivity of ∼2.2 W m−1K−1 at 873 K, which is amongst the lowest reported in HH alloys. The TE device characteristics, estimated using cumulative temperature dependence model for quantitative evaluation of TE performance, yielded an output power density of ∼10 Wcm−2 with a leg efficiency of ∼10% in the optimized composition of nanostructured Zr1-xHfxCoSb0.9Sn0.1 HH alloys, which is comparable to the reported efficiencies of other state-of-the-art TE materials. Keywords: Half Heuslers, Iso-electronic, Waste heat recovery, Mid-temperature, Efficiency, Compositional modulation

Nanostructuring has rejuvenated great interest in thermoelectric (TE) based power generation by enabling enhanced performance in nano-grained TE materials over their bulk counterparts. In ZrNiSn based half-Heusler (HH), a promising n-type TE materials, we examine the prospects of nanostructuring by synthesizing nano-crystalline iso-electronically substituted n-type Zr1-xHfxNiSn HH alloys to achieve a state-of-the-art TE figure-of-merit ZT ∼ 1.2 at 873 K, which is ∼20% higher than its bulk counterparts and corresponds to high conversion efficiency of ∼9% with output power density ∼7 Wcm−2, estimated using cumulative temperature dependence model. Enhanced phonon scattering at nano-scale grain boundaries and crystal defects, arising from nanostructuring and mass defect fluctuation in Hf substituted ZrNiSn alloys resulted in significantly reduced thermal conductivity. It was found that the partial substitution at Zr site with its heavier homologue Hf causes variation in the carrier effective mass and carrier concentration, which at lower Hf concentration results in an enhancement in the power factor.

CrSi2 is recognized as potential thermoelectric material for mid-temperature energy generation applications owing to its high temperature chemical stability coupled with its cost-effective and non-toxic constituent elements. However, its thermoelectric performance has been reported to be limited owing to its high thermal conductivity, which is reported to dominate by its lattice counterpart. In the present studies, we realize a state-of-the-art (ZT)max ∼ 0.32 at 673 K in an optimized nanocomposite composition of CrSi2/7.5 wt%SiGe, synthesized using spark plasma sintering of bulk CrSi2 dispersed with SiGe nanoparticles (crystallite size ∼12 nm). The incorporation SiGe nanoparticles in bulk CrSi2 resulted in a significant reduction in its thermal conductivity owing to enhanced scattering of heat-carrying phonons by a high density of nanoscale interfaces. Concurrently, the power factor of the nanocomposite was also found to increase due to an increase its carrier concentration and mobility on dispersion of SiGe nanoparticles in the CrSi2 matrix. Thus, the favorable tuning of the electrical and thermal transport properties led to a ZT∼ 0.32 which is ∼125% higher than its pristine counterpart. The as-synthesized pristine and nanocomposites were characterized employing X-ray diffraction and field emission scanning electron microscopy, based on which the enhancement in their thermoelectric properties has been discussed.

The interest in developing highly efficient Hf-free Half-Heusler (HH) thermoelectric (TE) materials is rejuvenated by the growing concerns on their cost and scalability. Despite being the most expensive element among other HH constituents, Hf has remained an essential ingredient for reducing the intrinsically high lattice thermal conductivity normally observed in HH alloys. This work demonstrates a nanocomposite approach as an effective paradigm for enhancing the thermoelectric figure-of-merit (ZT) in Hf-free HH alloys, which otherwise presently is far below unity. The optimized n-type (ZrNiSn)1−x(ZnO)x HH compositions exhibit a state-of-the-art ZT ∼ 1.0 at 873 K. The synthesised HH nanocomposites, comprising of uniformly-dispersed semiconducting ZnO nano-inclusions and in-situ formed insulating ZrO2 precipitates within the ZrNiSn HH matrix were studied for temperature dependent TE properties. These multi-dimensional secondary phases in the HH matrix act as additional scattering centres for heat-carrying phonons and energy barriers for energy filtering of low-energy and low-mobility carriers, thus simultaneously reducing the thermal conductivity and elevating the power factor. The synergistic scattering of phonons and energy filtering of electrons results in a ZT ∼ 1.0 at 873 K for optimised (ZrNiSn)1−x(ZnO)x HH nanocomposite, which corresponds to ∼70% enhancement over its pristine n-type ZrNiSn HH counterpart. The present work demonstrates exciting possibilities for similar HH systems that feature enhanced thermoelectric performance via. the nano-composite approach.

Despite Hf-free half-Heusler (HH) alloys being currently explored as an important class of cost-effective thermoelectric materials for power generation, owing to their thermal stability coupled with high cost of Hf, their figure-of-merit (ZT) still remains far below unity. We report a state-of-the-art figure-of-merit (ZT) ∼ 1 at 873 K in Hf-free n-type V-doped Zr1–xVxNiSn HH alloy, synthesized employing arc-melting followed by spark plasma sintering. The efficacy of V as a dopant on the Zr-site is evidenced by the enhanced thermoelectric properties realized in this alloy, compared to other reported dopants. This enhancement of ZT is due to the synergistic enhancement in electrical conductivity with a simultaneous decrease in the thermal conductivity, which yields ZT ∼ 1 at 873 K at an optimized composition of Zr0.9V0.1NiSn, which is ∼70% higher than its pristine counterpart and ∼25% higher than the best reported thus far in Hf-free n-type HH alloys. The enhancement of the electrical conductivity is due to...

We report an enhancement in the thermoelectric performance of Cu2SnSe3 alloy on Pb doping, owing to a sharp increase in its power factor. The powder XRD pattern of all samples of Cu2Sn1−xPbxSe3 (0≤x≤0.03), prepared using solid state reaction, exhibited a cubic structure with a space group of F43m. The results show that temperature dependent electrical resistivity, ρ(T) increases with increasing temperature thereby demonstrating that the samples display heavily doped semiconducting nature, which could be satisfactorily described by small polaron hopping model in the whole temperature range of measurement for all the samples. Both the resistivity and the Seebeck coefficient are reduced with 2 vol% Pb doping. The thermal conductivity of all the samples reduces with increasing temperature. Despite a decrease in Seebeck coefficient the power factor shows an increase on Pb doping, owing to a sharp surge in the electrical conductivity which results in an enhanced ZTmax ~0.64 at 700 K for an optimized composition of Cu2Sn0.98Pb0.02Se3, which is nearly twice the value of the corresponding undoped counterpart.

Colloidal synthesis is harnessed for the gram-scale preparation of hexagonal-shaped plate-like Bi2Te2.7Se0.3 particles, yielding nearly 5 g of the product in one experiment. The resultant textured particles are highly crystalline, phase-pure, chemically uniform, and can serve as a starting material for the preparation of bulk thermoelectrics for room temperature applications. The consolidation occurs via spark plasma sintering, which affords nanostructured n-type Bi2Te2.7Se0.3 material exhibiting a high figure of merit ZT ≈ 1 at 373 K with an average ZT ≈ 0.93 (300-473 K). Our experimental and theoretical studies indicate that the high thermoelectric performance is attributed to a favorable combination of the resultant transport properties. Specifically, bottom-up formation of the plate-like particles results in the substantial reduction of thermal conductivity by nanostructuring as observed experimentally and can be ascribed to phonon scattering at grain boundaries and suppressed bipolar conduction. When coupled with high electrical conductivity, which is preserved at the bulk scale as confirmed by ab initio calculations, these factors boost the thermoelectric performance of the as-synthesized n-type Bi2Te2.7Se0.3 bulk nanostructured alloy to the state-of-the-art level. The combination of a newly developed scalable colloidal synthesis with optimized spark plasma sintering constitutes a convenient route to nanostructured bulk thermoelectrics, which is an interesting pathway for the preparation of simple and complex thermoelectric chalcogenides., This work was supported by Portuguese National Funding Agency for Science, Research and Technology (FCT) under the UT-BORN-PT project (UTAP-EXPL/CTE/0050/2017). B. A. K. acknowledges funding of this work by the Robert A. Welch Foundation (grant no. F-1464).

The ternary intermetallic TiCoSb based half-Heusler (HH) alloys are prominent material exhibiting good p-type thermoelectric (TE) performance. In this work, we studied the implication of V and Nb as n-type aliovalent dopants on Ti crystallographic site of semiconducting TiCoSb based HH alloys for attaining higher TE performance in n-type counterparts. The carrier concentration optimization between semiconducting (Ti1−xVxCoSb) and semi-metallic regime (Ti1−xNbxCoSb) had resulted in maximum TE figure-of-merit (ZT) of 0.22 and 0.52 at 873 K for optimized Ti0.85V0.15CoSb and Ti0.85Nb0.15CoSb HH compositions, respectively. These substitutional alloys were synthesized using arc-melting and consolidated using spark plasma sintering, which resulted in biphasic microstructure with in-situ phase segregation of heterogeneously distributed Ti-rich precipitates at all length scales within the HH matrix, are examined by microstructural analysis using X-ray diffraction and electron microscopy. Interestingly, higher power factor and simultaneous reduction of thermal conductivity is observed in both the doping as a result of optimal carrier concentration and enhanced phonon scattering. However, Nb-doped alloys exhibit higher carrier mobility and more significant lattice thermal conductivity reduction, thus establishing n-type Ti1−xNbxCoSb HH alloys as an equally promising counterpart of p-type TiCoSb for mid-temperature TE power generation.

The practical applications of unfilled CoSb3-based Skutterudites for thermoelectric (TE) devices have been limited primarily owing to their bipolar conduction behaviour, which is detrimental for the TE performance. In the present studies, a peak TE figure-of-merit (ZT)max ~ 0.7 at 860 K has been realized with a pioneering (ZT)average ~ 0.34 in p-type CoSb3 co-doped by Fe and Bi at an optimal composition of Fe0.25Co0.75Sb2.995Bi0.005. The synthesis route for the alloy was the combination of arc melting and the spark plasma sintering. The TE properties measurement results suggest that the Fe doping at the Co site in CoSb3 lead to the unipolar p-type conduction while Bi substitution at Sb site helps in reducing the thermal conductivity via enhanced point defect induced phonon scattering. The synthesized samples were characterised for phase, morphology and structure using X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy, based on which some TE properties of the synthesized samples have also been discussed. In addition, the effect of magnetic entropy on the TE performance has also been studied experimentally as well as theoretically. The first-principles based density functional theory has been employed for the theoretical calculation of the electronic band-structure, electrical transport properties and the magnetic moment, which are found to be in fair agreement with the experimental observations.

The concept of nanocomposite/nanostructuring in thermoelectric materials has been proven to be an effective paradigm for optimizing the high thermoelectric performance primarily by reducing the thermal conductivity. In this work, we have studied the microstructure details of nanocomposites derived by incorporating a semi-metallic Bi nanoparticle phase in Bi2Te3 matrix and its correlation mainly with the reduction in the lattice thermal conductivity. Incorporating Bi inclusion in Bi2Te3 bulk thermoelectric material results in a substantial increase in the power factor and simultaneous reduction in the thermal conductivity. The main focus of this work is the correlation of the microstructure of the composite with the reduction in thermal conductivity. Thermal conductivity of the matrix and nanocomposites was derived from the thermal diffusivity measurements performed from room temperature to 150 °C. Interestingly, significant reduction in total thermal conductivity of the nanocomposite was achieved as compared to that of the matrix. A detailed analysis of high-resolution transmission electron microscope images reveals that this reduction in the thermal conductivity can be ascribed to the enhanced phonon scattering by distinct microstructure features such as interfaces, grain boundaries, edge dislocations with dipoles, and strain field domains.

Several nanostructuring methods have been demonstrated to produce a variety of nanostructured materials, and these methods are well recognized as effective paradigms for improving the performance of thermoelectric materials. Among the variety of nanostructured materials, bulk nanostructured materials have been shown to be the most promising because they not only have high ZT, but they can also be fabricated in large quantities, unlike many other nanostructured materials, making them desirable for large scale industrial application. In this study, the nanostructuring paradigm is extended for the first time to the bulk Mg3Sb2 and Mg3Sb1.8Bi0.2 Zintl phase compounds, which despite the advantages of price and abundance, so far have been disregarded for thermoelectric research due to low ZT relative to the available state-of-the-art thermoelectric materials. The nanostructuring of bulk Mg3Sb2 and Mg3Sb1.8Bi0.2, employing high energy ball milling followed by spark plasma sintering yields a ZT of ∼0.4 and ∼0.94 at 773 K, which are 54% and 56% higher values, respectively, than their respective bulk counterparts. The enhancement in the ZT of these materials is primarily due to the significant reduction in thermal conductivity caused by phonon scattering at numerous grain boundaries of nanostructured materials. The observed decrease in the thermal conductivity of these bulk nanostructured materials is quantified using a simple model that combines the macroscopic effective medium approach (EMA) with the concepts of Kapitza resistance. The microstructural investigation of these nanostructured materials was carried out employing high resolution transmission electron microscopy (HRTEM).

All scale hierarchical architecturing, matrix/inclusion band alignment and intra-matrix electronic structure engineering, the so called panoscopic approach for thermoelectric materials has been demonstrated to be an effective paradigm for optimizing high ZT. To achieve such hierarchically organized microstructures, composition engineering has been considered to be an efficient strategy. In this work, such a panoscopic concept has been extended to demonstrate for the first time in the case of half-Heusler based thermoelectric materials via a composition engineering route. A series of new off-stoichiometric n-type Zr0.7Hf0.3Ni1+xSn (0 ≤x≤ 0.10) HH compositions have been modified to derive HH(1 -x)/full-Heusler (FH)(x) composite with an all scale hierarchically modified microstructure with FH inclusions within the matrix to study the temperature dependent thermoelectric properties. The structural analysis employing XRD, FE-SEM and HR-TEM of these materials reveal a composite of HH and FH, with hierarchically organized microstructures. In such a submicron/nano-composite, the electronic properties are observed to be well optimized yielding a large power factor; α(2)σ (∼30.7 × 10(-4) W m(-1) K(-2) for Zr0.7Hf0.3Ni1.03Sn) and reduced thermal conductivity (∼2.4 W m(-1) K(-1) for Zr0.7Hf0.3Ni1.03Sn) yielding a high ZT∼ 0.96 at 773 K for composition Zr0.7Hf0.3Ni1.03Sn which is ∼250% larger than the normal HH Zr0.7Hf0.3NiSn (ZT∼ 0.27 at 773 K). The enhancement in ZT of these composites has been discussed in terms of primary electron filtering, electron injection and several phonon scattering mechanisms such as alloy scattering, point defect scattering, and grain boundary scattering. The Bergman and Fel model is used to calculate effective thermoelectric parameters of these composites for comparing the experimental results.

Reducing the lattice thermal conductivity (κL) comprises one of the crucial aspects of thermoelectric research. Ternary intermetallic half Heusler compounds have revealed properties promising for thermoelectric applications. Studies have shown that self doping with Ni in Ni based half Heuslers leads to unprecedented lowering in the κL. Although the underlying physical mechanisms have not been explored in detail, with ZrNiSn as a case study, we experimentally investigate the change in κL with increase in the Ni concentration in Ni based n-type half Heusler alloys. We observe that at excess Ni doping of 3% in the half Heusler lattice, the thermal conductivity reduces by more than 60%. Our density functional theory based analysis on the ongoing phenomena reveals that at ultralow Ni doping, the localized modes of the antisite Ni defect hybridize with the acoustic modes and this plays the most dominant role in scattering of the thermal phonons leading to significant lowering in κL. Our theoretical analysis can be employed for predicting a suitable dopants that may reduce the κL prior to the synthesis of the compound in the laboratory.

We report a half-Heusler (HH) derivative Ti9Ni7Sn8 with VEC = 17.25 to investigate the structural changes for the optimization of high thermoelectric performance. The structural analysis reveals that the resulting material is a nanocomposite of HH and full-Heusler with traces of Ti6Sn5 type-phase. Interestingly, present nanocomposite exhibits a significant decrease in thermal conductivity due to phonon scattering and improvement in the power factor due to combined effect of nanoinclusion-induced electron injection and electron scattering at interfaces, leading to a boost in the ZT value to 0.32 at 773 K, which is 60% higher than its bulk counterpart HH TiNiSn.