Properties Control by Solid Solution Substitution for Vacancy-Site in Half-Heusler ZrNiSn-Based Thermoelectric Materials

Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. We are focusing on the half-Heusler compound ZrNiSn, a well-known excellent n-type thermoelectric material, which can be used at around 1000 K to directly convert high temperature waste heat into clean electrical energy. Recently, thermoelectric power generation is expected to use as battery-free IoT applications such as data sensing and transmission system aiming to utilize waste heat lower than 500 K. The present Author’s group found that thermoelectric properties of ZrNiSn can be converted from n-type to p-type by the solid solution substitution of Co and Ir for the vacancy-site, one fourth of all the lattice points, which is a characteristic of Half-Heusler. The understanding for this important finding remains phenomenological, and it is necessary to elucidate mechanistically, for achieving the new thermoelectric materials design concept. For a given amount of substitution for the vacancy-site, Ir is more effective than Co in enhancing p-type values of Seebeck coefficient. Additionally, it was elucidated that lattice thermal conductivity can effectively be reduced according to the solid solution effect substituting for the vacancy-site, owing to the enhancement of phonon scattering. Here, we have noticed of the chance that solid solution formation should affect mechanical properties through solid solution hardening or softening depending on the situation of the vacancy-site substitution.<br/>Objective of the present work is to understand the effects of solid solution substitution for the vacancy-site on thermoelectric properties, and as well as on mechanical properties, for the half-Heusler ZrNiSn. For the desirable thermoelectric module design, it is beneficial to design both n- and p-type materials based on the same compound since physical and chemical properties are expected to be similar each other in n- and p-type materials, which may reduce efforts of complicated care against damages due to such as thermal stress and oxidation. Nearly single-phase Half-Heusler Zr(Ni,M_<i>x</i>)_1+<i>x</i>Sn alloys (M = Co, Ir) were fabricated by the directional solidification using optical floating zone melting. In our previous research, the amount of Ir substitution fraction <i>x</i> was higher than 0.20 up to 0.43 in the chemical formula Zr(Ni,Ir<i>_x</i>)_1+<i>x</i>Sn, and a peak temperature range showing the maximum of Seebeck coefficient was at around 1000 K. In the present work, for example, we decreased Ir substation amount ranging from <i>x</i> is 0.04 to 0.10, with a prediction that peak temperature of Seebeck coefficient could be shifted toward the low temperature direction based on the previous observation. Consequently, the peak temperature of Seebeck coefficient shows the tendency to be shifted from 1000 K down to about 380 K as the Ir substitution decreases from 0.20 to 0.04, while the maximum value of Seebeck coefficient slightly decrease from 115 to 95 μV/K. This result should be attributed to the change of electronic structure regarding narrow band gap and the Fermi level. On the other hand, regarding mechanical properties, it was found that Half-Heusler ZrNiSn shows higher hardness in nano-indentation measurement at 300 K and higher 0.2% proof stress at 1273 K in compression test than Heusler ZrNi₂Sn. In the case of Co substitution for the vacancy-site, propensity of the solid solution softening has been observed while the amount of Co substitution increases from Half-Heusler to Heusler compositions.