Band Engineerging And Phonon Engineerging Of High Figure Of Merit Half-Heusler Thermoelectric Materials

Solid-state thermoelectric (TE) technology, which can directly realize the reversible conversion of electricity and thermal energy, offers a promising solution for converting waste heat to useful electrical power. The conversion efficiency of a TE device is limited by the Carnot efficiency and the figure of merit zT of the TE material. Therefore, both high operating temperature and high zT are desirable for high conversion efficiency. Half-Heusler compounds, which have excellent electrical and mechanical properties, good thermal stability and cheap composition elements, have been widely investigated as high temperature TE materials in the recent years. Up to now, the maximum zT of n-type half-Heusler compounds has exceeded 1.0. In order to develop high efficiency half-Heusler TE device, the key is to develop high performance p-type half-Heusler compounds, which have been a big challenge in the past years. In the present work, new p-type Fe(V,Nb)Sb-based half-Heusler heavy-band TE materials with high zT were developed via a band engineering approach. The electron and phonon transport characteristics were respectively analyzed by using the single parabolic band model and Debye-Callaway model. New strategies were designed to optimize the performance of this heavy-band half-Heusler TE system and the zT value was further improved. Based on the new p-type compound and the state-of-the-art n-type half-Heusler compound, a prototype half-Heusler module was assembled through cooperation. The main results are listed as below:1) FeRSb-based half-Heusler compounds with high phase purity were successfully fabricated through the levitation melting and spark plasma sintering technologies. The lattice thermal conductivity of FeVSb is greatly suppressed by Nb alloying, which simultaneously induces large mass and strain field fluctuations, leading to strong alloy scattering of phonons. Furthermore, through Co doping, the power factor and zT of n-type FeVo.6Nbo.4Sb compound were improved. The highest zT of 0.33 was obtained at 650K for Fe0.985Co0.015V0.6Nb0.4Sb.2) Through the first principle calculations, the band structure of FeRSb was investigated. It was found that the valence band of FeRSb has high band degeneracy, indicating high TE performance may be achieved in p-type FeRSb compounds. Then, through high content of Ti doping, a high zT of 0.8 was indeed achieved in the p-type Ti doped FeVo.6Nbo.4Sb compounds. By further analyzing the band structure, it was found that the valence band effective mass of FeNbSb is smaller than that of FeVSb, and FeNbSb has larger band gap. Therefore, by increasing the Nb content in FeV1-xNbxSb solid solutions, a lower valence band effective mass and consequently higher carrier mobility were achieved. Moreover, the degradation of the TE performance at high temperatures were suppressed due to the increased band gap. As a result, a high zT of 1.1 was obtained at 1100K for Ti doped FeNbSb compounds.3) As a typical heavy-band TE materials, p-type FeNbSb compound has distinct characteristics compared with the traditional light-band TE system. For example, large density of state effective mass, high optimal carrier concentration and doping content, etc. By rationally selecting heavier Hf dopant, simultaneous optimization of electrical power factor and thermal conductivity were realized in the p-type FeNb1-xHfxSb compounds, and a high zT of～1.5 was obtained at 1200K, which was one of the highest values among half-Heusler TE materials. Based on the p-type compound and the state-of-the-art n-type ZrNiSn-based compound, an 8×8 prototype half-Heusler TE module was assembled through cooperation, which exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 Wcn-2 at a temperature difference of 655K. These findings highlight the optimization strategy for heavy-band TE materials and demonstrate a realistic prospect of high-temperature modules based on half-Heusler alloys with low cost, excellent mechanical robustness and thermal stability.4) P-type FeNbSb shows intrinsically low carrier mean free path. By combining the sub-microscale grain boundaries, atomic-scale point defects and electron-phonon interaction, hierarchical phonon scattering centers were concurrently introduced into p-type Ti doped FeNbSb compounds, which have almost negligible effect on the carrier transport but contribute to a great reduction in the lattice thermal conductivity. Therefore, a significantly enhanced zT of 1.34 was obtained at 1150K for the Fe1.05Nb0.75Ti0.25Sb with intentionally designed hierarchical scattering centers. The present results highlight the efficacy of hierarchical phonon scattering in improving the performance of heavy-band thermoelectric systems.5) A systematical investigation on p-type Hf, Zr, Ti doped FeV1-xNbx-Sb was performed. It is found that the peak zT of p-type FeV1-xNbxSb increases with increasing Nb content in the matrix, and the maximum zT was found in the p-type FeNbSb compound. This result had significant difference compared with the other TE system, in which the maximum zT was usually found in the solid solution composition. The quality factor B* of FeV1-xNbxSb was estimated, which shows similar changing trend with the peak zT. Further analysis showed that multiple factors, including the single band effective mass, the band gap, and the solubility and doping efficiency of dopants, etc., are all changing with the increased Nb content in the matrix. These results demonstrate that comprehensively understanding the multiple roles of forming solid solution are needed to develop high performance TE materials.