Yoichi Nishino

The half-Heusler TiNiSn alloy is a promising candidate for thermoelectric power generation, capable of directly converting waste heat into electric power, due to its high thermoelectric performance over a wide temperature range from 500 to 1000 K. However, the thermoelectric performance of p-type TiNiSn is much lower than that of its n-type counterpart. Here, we demonstrate that Hf substitution in the p-type half-Heusler TiNi0.8Co0.2Sn alloy significantly enhances thermoelectric performance within the miscibility gap region of the binary TiNiSn-HfNiSn phase diagram. The Seebeck coefficient, which is below 80 μV/K for TiNi0.8Co0.2Sn, is substantially improved to 170 μV/K in Ti0.5Hf0.5Ni0.8Co0.2Sn at 650 K, owing to modifications in the electronic band structure induced by Hf substitution. Moreover, the lattice distortion and point defects introduced by Hf substitution effectively reduce thermal conductivity, from 5.0 W/mK for TiNi0.8Co0.2Sn to 3.3 W/mK for Ti0.5Hf0.5Ni0.8Co0.2Sn at 900 K. Consequently, the thermoelectric figure of merit (ZT) significantly increases from 0.03 for TiNi0.8Co0.2Sn to 0.26 for Ti0.5Hf0.5Ni0.8Co0.2Sn at 800 K. This enhancement of thermoelectric performance in p-type TiNiSn enables the construction of thermoelectric modules composed solely of half-Heusler TiNiSn based alloys, which are expected to exhibit high thermal stability useful for long-term thermoelectric power generation.

We investigated Heusler-type Ru₂VAl, a candidate material for next-generation thermoelectric conversion, by first-principle calculations of its thermoelectric conversion properties and direct experimental observations of its electronic structures, employing photoemission and infrared spectroscopy. Our results show that Ru₂VAl has a wider pseudogap near the Fermi level compared to Fe₂VAl. Accordingly, a higher thermoelectric conversion performance can be expected in Ru₂VAl at higher temperatures.

The electronic structure of Fe_2−xRe_xVAl compounds is investigated using bulk‐sensitive hard X‐ray photoelectron spectroscopy (HAXPES) measurements and electronic band calculations to elucidate the origin of the improvement in their thermoelectric properties. The HAXPES results show that, with the substitution of Re atoms for the Fe site, a new Re 5dstate appears near the Fermi level, which leads to a change in the electronic structure with a gradual narrowing of the pseudo‐gap. The decrease in the pseudo‐gap width with Re substitution is found to be related to the increase in thep‐type Seebeck coefficient and the shift in its peak temperature to the lower temperature side.

Improvement of p-type thermoelectric performance for Heusler-type Fe₂V_1.08−yTi_yAl_0.92 alloys with y = 0–0.34 has been investigated, focusing on the valence electron concentration (VEC) effect for tuning the Fermi level across the center of the pseudogap via Ti doping, combined with a constructive modification of the electronic structure around the pseudogap through the off-stoichiometric V/Al composition change. The Seebeck coefficient changes its sign from negative to positive at around VEC = 6.0 due to Ti doping, and the peak value reaches approximately 120 μV/K at 350 K for y = 0.22. As a result of a drastic reduction in the electrical resistivity at 300–600 K, the power factor enhances up to 3.7 × 10⁻³ W/mK² at 300 K for y = 0.30. The thermal conductivity increases with Ti doping to 15.5 W/mK at 350 K for y = 0.16, because of an increased stability of the L2₁ structure for VEC closer to 6.0, but then decreases to approximately 12 W/mK for y = 0.34. While the dimensionless figure of merit ZT = 0.13 is achieved at 500 K for y = 0.34, the codoping of Ta (Fe₂V_0.69Ti_0.34Ta_0.05Al_0.92) leads to a further enhancement of up to ZT = 0.18 at 500 K.

This study set out to examine the flash sintering of thermoelectric materials. A finite element method analysis of electric current activated/assisted sintering with an insulating mold suggested that the use of a sufficiently high Joule-heating rate, such as >10 000 K/s, relative to the thermal diffusion rate of the mold material, can realize uniform sample heating. Based on the analysis results, the sintering of Sb₂Te₃, a typical thermoelectric material, was conducted with a current feed duration of 1 s, using a sintering apparatus that had been specially devised for flash-sintering. Although a slight reduction in the density and degree of grain alignment was observed, the crystallographic phase was unaffected by the rapid heating, with the thermoelectric performance of the Sb₂Te₃ sample prepared by the 1-s sintering being comparable with that of a sample that had been conventionally sintered at 773 K for 3 min. This significant reduction in the sintering time can result in the energy consumption for the sintering process being decreased to less than 1% of that required for conventional current sintering. This significant energy-conservation effect of the flash sintering should be appealing from a practical point of view, especially for the fabrication of those thermoelectric materials used for power generation and aimed at energy recovery.