Thermoelectric Properties of Off-Stoichiometric Ti-Ni-Sn Half-Heusler Systems

The Ti-Ni-Sn half-Heusler compound exhibits a high power factor but relatively low figure of merit due to its high thermal conductivity. In this paper, we propose an effective and inexpensive way to reduce the thermal conductivity, and independently increase the power factor. To this end, we have systematically synthesized off-stoichiometric Ti-Ni-Sn half-Heusler compounds and introduced Y-Sb dilute co-doping at Ti-Sn sites. Excess Ni introduces interstitial defects in the half-Heusler crystal and results in significant reduction of the thermal conductivity, which drops below 3 W/mK at room temperature. In addition, the Y-Sb dilute co-doping at Ti-Sn sites improves the power factor while the thermal conductivity remains reasonably small. As a result, the figure of merit at room temperature is eight times larger than that of nondoped Ti-Ni-Sn.