Suppression of thermal conductivity without impeding electron mobility in n-type XNiSn half-Heusler thermoelectrics

We outline a strategy to improve the thermoelectric performance of n-type XNiSn based half-Heusler alloys through Cu doping into vacant tetrahedral sites. A comprehensive combination of structural characterisation and modelling is employed to discriminate the competing mechanisms for thermoelectric enhancement. During synthesis a mineralising effect occurs that improves the homogeneity of the alloying elements Ti, Zr and Hf, and promotes grain growth, leading to a doubling of the electron mobility. In the formed materials, Cu is a strong n-type dopant, like Sb, but occupies the interstitial site and strongly enhances phonon scattering without diminishing carrier mobility (in contrast to interstitial Ni). Simultaneous alloying with Ti, Zr and Hf serves to minimise the thermal conductivity via regular mass disorder and strain effects. A best electronic power factor, S-2/rho, of 3.6 mW m(-1) K-2 and maximum ZT of 0.8 at 773 K were observed for a Ti0.5Zr0.25Hf0.25NiCu0.025Sn composition, enabling promising device power densities of similar to 6 W cm(-2) and similar to 8% conversion efficiency from a 450 K gradient. These findings are important because they provide new insight into the mechanisms underpinning high ZT in the XNiSn system and indicate a direction for further improvements in thermoelectric performance.

我们概述了一种通过将铜掺杂到空位四面体位置来提高n型XNiSn基半赫斯勒合金热电性能的策略。采用结构表征和建模的综合方法来区分热电增强的竞争机制。在合成过程中会产生一种矿化效应，该效应提高了合金元素Ti、Zr和Hf的均匀性，并促进了晶粒生长，使电子迁移率提高了一倍。在形成的材料中，铜是一种强n型掺杂剂，与锑类似，但占据间隙位置，在不降低载流子迁移率的情况下（与间隙镍相反）强烈增强了声子散射。同时与Ti、Zr和Hf合金化通过规则的质量无序和应变效应使热导率最小化。对于Ti₀.₅Zr₀.₂₅Hf₀.₂₅NiCu₀.₀₂₅Sn成分，在773 K时观察到最佳电子功率因数S²/ρ为3.6 mW m⁻¹K⁻²，最大ZT为0.8，从而在450 K的温度梯度下可实现约6 W cm⁻²的有前景的器件功率密度和约8%的转换效率。这些发现很重要，因为它们为XNiSn系统中高ZT的支撑机制提供了新的见解，并为进一步提高热电性能指明了方向。