P-type thermoelectric properties of TiNi1–x–yCoySn half-Heusler alloy with reduced interstitial Ni atoms

TiNiSn is an N-type thermoelectric material with low toxicity. The electrical conduction of TiNiSn can be tuned from N-type to P-type by substituting Co atoms at Ni sites. However, the ZT value is lower than that of the N-type TiNiSn, primarily due to its higher electrical resistivity. To decrease the electrical resistivity, it is necessary to increase the carrier density or mobility; however, the absolute value of the Seebeck coefficient decreases, which has a trade-off relationship with the electrical resistivity. To solve this problem, we prepared P-type thermoelectric materials TiNi_1–x–yCo_ySn (0 ≤ x ≤ 0.1, 0.01 ≤ y ≤ 0.05) with reduced interstitial Ni atoms by arc melting and heat treatment process. As a result, we successfully improved the electrical conductivity while maintaining the Seebeck coefficient. The hole carrier density increases as the number of interstitial Ni atoms decreases, and the electrical resistivity decreases by 78 % from 2.78 × 10⁻² Ωcm (for x = 0, y = 0.05) to 6.02 × 10⁻³ Ωcm (for x = 0.03, y = 0.05) at 300 K. The ZT value increases by a factor of 1.5 (ZT_max = 0.18 at 700 K) for TiNi_0.92Co_0.05Sn compared to TiNi_0.95Co_0.05Sn, indicating that the reduction of interstitial Ni atoms is effective in improving the thermoelectric properties of the P-type half-Heusler alloy TiNiSn. The peak ZT of 0.18 remains below the level required for practical implementation; however, this can be further improved by decreasing the lattice thermal conductivity.