Thermoelectric Properties of Half-Heusler TiCoSb Alloys by Single and Double Alloying with (Fe, Ni)

Half-Heusler TiCoSb is a promising thermoelectric material due to its mechanical robustness and thermal stability, but its efficiency is constrained by high lattice thermal conductivity. This study investigates single and double alloying at the Co site. Samples of TiFe_0.2Co_0.8Sb and TiCo_0.8Ni_0.2Sb were produced by arc melting and hot pressing with additional prolonged heat treatments at 873 K. Additionally, double-alloyed TiFe_xCo_1−2xNi_xSb were produced with x = 0, 0.1, 0.2, 0.3, and 0.4. All samples were tested for their microstructure and thermoelectric properties. Single-alloyed samples with 20 at % Fe or Ni showed significant enhancement in thermoelectric properties, especially electrical conductivity—an order of magnitude higher than pristine TiCoSb—while reducing thermal conductivity by nearly 50%. After prolonged heat treatments at 873 K, these samples tended to oxidize. The CoSb metallic phase was formed in both alloys, indicating that CoSb plays a key role in limiting performance, highlighting the need for a controlled low-oxygen environment. Fe-alloyed samples exhibited stable lamellar structures that preserved low thermal conductivity. With further optimization of the doping strategy, these samples can achieve higher thermoelectric efficiency. Double-alloyed samples, Ti(Fe_xCo_1−2xNi_x)Sb, (0 ≤ x ≤ 0.4), formed a primary single-phase half-Heusler structure with a lattice parameter that increased with the alloying content and secondary phases such as Co, CoSb, and (Ti, Fe)Sb. Double alloying effectively reduced thermal conductivity due to enhanced phonon scattering. The TiFe_0.4Co_0.2Ni_0.4Sb sample provides a promising baseline for advanced doping strategies, with strong potential for substantial ZT enhancement.