Thermoelectric high ZT half-Heusler alloys Ti1−x−yZrxHfyNiSn (0 ≤ x ≤ 1; 0 ≤ y ≤ 1)

Although thermoelectric materials with a figure of merit, ZT, higher than 1.2 have gained considerable interest in electric power generation, little is hitherto known on the influence of severe plastic deformation on thermoelectric performance. Severe plastic deformation is one of the elegant techniques to furnish ultrafine grained microstructure with a high level of point, linear and surface defects. Particularly the phonon scattering on these defects is used as a main route to reduce thermal conductivity in thermoelectric materials and -as a consequence -increasing ZT.The present article provides an overview on the achievements of the various techniques of severe plastic deformation to gain high figures of merit in the thermoelectric materials, commonly used in energy conversion devices, such as skutterudites, clathrates, Heusler phases, and bismuth tellurides.For all skutterudites, high pressure torsion, as one of the major techniques of severe plastic deformation processes, is a great tool to either enhance ZT of hot pressed samples or to directly produce fast and easily high ZT thermoelectric bulks. A still unsurpassed highlight is the enhancement of ZT from 1.6 to almost 2 (Sr 0.09 Ba 0.11 Yb 0.06 Co 4 Sb 12 ) at 825 K.Whilst for thermoelectric clathrates so far little success was reported, high pressure torsion treatment of Heusler and Half-Heusler phases in some cases was able to boost ZT (VTa 0.05 Fe 2 Al 0.95 , rising ZT from 0.22 to 0.3) although the absolute ZT increases are still disappointing. For p-and n-type Half-Heusler alloys (NbFeSb-and TiNiSn-based) at least three temperature cycles are necessary to gain a thermally stable state, indicating that obviously the introduced defects and structure changes are more resistant against heat treatments than in case of skutterudites.Bismuth tellurides of type V 1 (VI) 1 and/or V 2 (VI) 3 (V, VI denote the group elements) have been already deformed by high temperature pressure or high temperature extrusion before the SPD methods were known, for the sake of improving ZT at least with temperatures 300500 K, at most fighting with the condition to achieve a high electrical conductivity because of the strong anisotropy in these materials. By starting with ball milling followed by high temperature pressing at not too high temperatures, not only the electrical conductivity could be kept large but also the lattice thermal conductivity was diminished such that figures of merit up to ZT = 1.4 at T = 373 K were achieved. This value could be reached by many of the ECAP experiments published so far, although only across the sample long axis because of lattice anisotropy. First applications of HPT did not reach that ZT level as either the conditions of texture could be not fulfilled, or, above all, the processing rates and/or temperatures were too high. Recent investigations not having involved SPD found the importance of the lattice defects' specific phonon scattering efficiencies, especially that of dislocations, and by introducing them in sufficiently high densities,...

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“…All samples, arc-melted, annealed, BM and HP, were prepared as described in detail in Ref. 39). For all three HH alloys (Fig.…”

Section: Half-heusler Alloysmentioning

confidence: 99%

The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides

2019

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“…Finally, we remark that there are many reports of phase separation in Ti x Zr y Hf 1−x−y NiSn samples [68][69][70]. This naturally affects the local compositions and thus the mean lattice thermal conductivity of phase-separated samples.…”

Section: Effect Of Alloyingmentioning

confidence: 99%

Lattice thermal conductivity ofTixZryHf1−x−yNiSnhalf-Heusler alloys calculated from first principles: Key role of nature of phonon modes

et al. 2017

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In spite of their relatively high lattice thermal conductivity κ , the XNiSn (X = Ti, Zr, or Hf) half-Heusler compounds are good thermoelectric materials. Previous studies have shown that κ can be reduced by sublattice alloying on the X site. To cast light on how the alloy composition affects κ , we study this system using the phonon Boltzmann-transport equation within the relaxation time approximation in conjunction with density functional theory. The effect of alloying through mass-disorder scattering is explored using the virtual crystal approximation to screen the entire ternary Ti x Zr y Hf 1−x−y NiSn phase diagram. The lowest lattice thermal conductivity is found for the Ti x Hf 1−x NiSn compositions; in particular, there is a shallow minimum centered at Ti 0.5 Hf 0.5 NiSn with κ taking values between 3.2 and 4.1 W/mK when the Ti content varies between 20% and 80%. Interestingly, the overall behavior of mass-disorder scattering in this system can only be understood from a combination of the nature of the phonon modes and the magnitude of the mass variance. Mass-disorder scattering is not effective at scattering acoustic phonons of low energy. By using a simple model of grain boundary scattering, we find that nanostructuring these compounds can scatter such phonons effectively and thus further reduce the lattice thermal conductivity; for instance, Ti 0.5 Hf 0.5 NiSn with a grain size of L = 100 nm experiences a 42% reduction of κ compared to that of the single crystal.

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“…Recent achievements of ZT1.2 in some familiar n-type RNiSn phases [2,15,16] and ZT1.5 in new p-type FeNbSb phases in the high temperature range of 600 o C to 800 o C [6] have given promise to the prospect of higher TEG efficiency. For the n-type alloys, Chen et o C for 30 min under 60 MPa.…”

mentioning

confidence: 99%

Half-Heusler Alloys for Efficient Thermoelectric Power Conversion

Chen

Zeng

Tritt

et al. 2016

Journal of Elec Materi

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Half-Heusler (HH) phases (space group F43m, Cl b ) are increasingly gaining attention as promising thermoelectric materials in view of their thermal stability 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 the samples close to their melting temperatures. Introduction of mass fluctuation and scattering centers play a key role in the high ZT measured, as shown by the reduction of thermal conductivity and increase of thermopower. Based on computation, the power conversion efficiency of a n-p couple module based on the new n-type (Hf,Zr,Ti)NiSn particles-in-matrix composite and recently reported high-ZT p-type HH phases is expected to reach 13%, comparable to that of state-of-the-art materials, but with the mentioned additional materials and environmental attributes. Since the high efficiency is obtained without tuning the microstructure of the HalfHeusler phases, it leaves room for further optimization.

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Thermoelectric high ZT half-Heusler alloys Ti1−x−yZrxHfyNiSn (0 ≤ x ≤ 1; 0 ≤ y ≤ 1)

Cited by 173 publications

References 52 publications

The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides

The Effect of Severe Plastic Deformation on Thermoelectric Performance of Skutterudites, Half-Heuslers and Bi-Tellurides

Lattice thermal conductivity ofTixZryHf1−x−yNiSnhalf-Heusler alloys calculated from first principles: Key role of nature of phonon modes

Half-Heusler Alloys for Efficient Thermoelectric Power Conversion

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