Enhanced thermoelectric performance in the p-type half-Heusler (Ti/Zr/Hf)CoSb_0.8Sn_0.2 system via phase separation

Abstract

A novel approach for optimization of the thermoelectric properties of p-type Heusler compounds with a C1b structure was investigated. A successful recipe for achieving intrinsic phase separation in the n-type material based on the TiNiSn system is isoelectronic partial substitution of Ti with its heavier homologues Zr and Hf. We applied this concept to the p-type system MCoSb0.8Sn0.2 by a systematic investigation of samples with different compositions at the Ti position (M = Ti, Zr, Hf, Ti0.5Zr0.5, Zr0.5Hf0.5, and Ti0.5Hf0.5). We thus achieved an approximately 40% reduction of the thermal conductivity and a maximum figure of merit ZT of 0.9 at 700 °C. This is a 80% improvement in peak ZT from 0.5 to 0.9 at 700 °C compared to the best published value of an ingot p-type half-Heusler compound. Thus far, comparable good thermoelectric p-type materials of this structure type have only been realized by a nanostructuring process via ball milling of premelted ingot samples followed by a rapid consolidation method, like hot pressing. The herein-presented simple arc-melting fabrication method reduces the fabrication time as compared to this multi-step nanostructuring process. The high mechanical stability of the Heusler compounds is favorable for the construction of thermoelectric modules. The Vickers hardness values are close to those of the n-type material, leading to good co-processability of both materials.

Synthesis and Thermoelectric Properties of Ni-Doped ZrCoSb Half-Heusler Compounds

“…Half-Heusler compounds with 18 valence electron count (VEC) per unit cell have been widely studied as one of the promising thermoelectric materials working in the middle temperature range due to their narrow bandgap and high Seebeck coefficients, such as MCoSb, MNiSn (M = Ti, Zr, Nb, and Hf), et al [10][11][12][13][14] Among these compounds with 18 VEC, MCoSb based half-Heusler compounds (M = Ti, Zr, Nb, and Hf) have been extensively investigated including the substituting at M, A or B sites and many studies have achieved high thermoelectric properties. Yuan et al synthesized the ZrCoSb 1−x Sn x by substituting Sn on Sb sites and the maximum ZT value of 0.52 at 973 K was obtained [15].…”

Three-dimensional multimodal imaging and analysis of biphasic microstructure in a Ti–Ni–Sn thermoelectric material

“…[20][21][22] This holds true in p-type Ti 1−x−y Zr x Hf y CoSb as well. 23 Given the importance of phase separation in the most advanced half-Heusler based thermoelectrics, it is critical to understand the microstructure in order to relate composition to properties, as recognized in previous studies.…”

Enhanced power factor and reduced thermal conductivity of a half-Heusler derivative Ti9Ni7Sn8: A bulk nanocomposite thermoelectric material

“…These materials have a decent Seebeck coefficient with moderate electrical conductivity due to combined feature of a narrow energy gap and a slight shift of Fermi level above the top of the valence band. 8 Reports show that both n-type 9-12 and p-type [13][14][15][16] with exceptionally large power factor can exist in such compounds and hence may help in making a compatible module for thermoelectric devices. Despite all these favorable properties, the main drawback in this class of thermoelectric materials is the very large thermal conductivity in comparison to other state-of-the-art TE materials [17][18][19][20][21][22] which hinders to yielding a descent thermoelectric figure of merit, ZT ¼ a 2 rT j , where r is the electrical conductivity, a is the Seebeck coefficient, j is the total thermal conductivity, and T is the absolute temperature.…”

“…These three physical parameters a, r, and j are interrelated in such a way that modification to any of these adversely affects the other and hence limits the overall enhancement in ZT. 23 In the recent years, several strategies such as doping, 9,15 solid solution alloying, [12][13][14] and nanostructuring 10,16 in HH compound have been adopted to disrupt heat carrying phonons to significantly reduce their j. Recently, full-Heusler (FH) inclusions within the p and n type HH compounds have been produced by several groups [24][25][26][27][28][29][30] by adding excess Coconcentration in p-type MCoSb (where M ¼ Ti, Zr, Hf) and Ni-concentration in n-type MNiSn (where M ¼ Ti, Zr, Hf).…”