Synthesis of Phase-Pure n-Type TiNiSn Half-Heusler Thermoelectric Material Using Reactive Hot Pressing

Owing to its nontoxicity, high-temperature strength retention, and the earth abundance of the starting raw materials, half-Heusler (hH) TiNiSn thermoelectric material has gained popularity for intermediate-temperature (400–1000 K) applications. Apart from increasing the thermoelectric figure of merit (ZT), one of the biggest hurdles for its commercialization is the processing of phase-pure TiNiSn hH alloy in a cost-effective and scalable manner. The present research envisages designing such a processing route for TiNiSn hH alloy by employing reactive hot pressing of Ti, Ni, and Sn metal powders. Mechanically milled Ti, Ni, and Sn powders were hot-pressed at 950 °C for 1 h using an applied pressure of 35 MPa, yielding ≈ 99% volume fraction of hH-TiNiSn phase with a relative density of ≈95%, which is remarkable compared to conventional processing methods involving melting followed by long-term thermal treatment or shorter processing time with multiple steps using expensive techniques. Interestingly, the phase purity scales inversely with the applied pressure during reactive sintering. An increase in applied pressure leads to the precipitation of the full-Heusler (fH) phase within the hH matrix by reducing the nucleation barrier of fH at the hot-pressing temperature. Pore formation and densification in this system are governed by the Kirkendall effect, aided by the large difference in the intrinsic diffusivity of the constituent elements. Phase purity is dictated by the accommodation of Ni nonstoichiometry within the hH composition, controlled by long-range diffusion. The reactive hot-pressed TiNiSn sample achieves a near-record ZT value of ∼1.14, which, to the best of our knowledge, has not been reported previously.