Preparation and thermoelectric properties of Sc-doped Ti_1–<i>x</i>NiSb half-Heusler alloys

Abstract

The nominal composition TiNiSb with 19 valence electrons is demonstrated to be composed of off-stoichiometric half-Heusler phase and impurities. In this work, the Ti_1–<i>x</i>NiSb (<i>x</i> = 0, 0.10, 0.15, 0.20, 0.25) samples are prepared by ball milling and spark plasma sintering. The single-phase Ti_0.9NiSb sample, deviating from the theoretical composition Ti_0.75NiSb base on 18-electron rule, is obtained, which might be ascribed to the small defect formation energy of Ti filling the vacancy as well as our ball-milling preparation method. With the single-phase Ti_0.9NiSb sample used as the base material, a small amount of Sc is used to partially replace Ti in order to further reduce the carrier concentration. Thus, the Ti_{1–<i>x</i>–<i>y</i>}Sc_<i>y</i>NiSb (<i>x</i> = 0.10, 0.15; <i>y</i> = 0.03, 0.05) samples are designed to investigate the effect of Sc doping on the thermoelectric properties. The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analysis confirm the single-phase nature of the Ti_{1–<i>x</i>–<i>y</i>}Sc_<i>y</i>NiSb samples. Energy-dispersive X-ray spectroscopy (EDS) results indicate that the actual compositions of the Ti_{1–<i>x</i>–<i>y</i>}Sc_<i>y</i>NiSb samples are consistent well with their nominal compositions, and all elements are distributed uniformly in the sample. Moreover, the doping of Sc can increase the content of Ti vacancy while maintaining the single-phase structure, which could be attributed to the higher binding energy between Sc and Sb because the electronegativity of Sc is less than that of Ti. Both the substitution of Sc for Ti and the increase of the Ti vacancies significantly reduce the carrier concentration, which decreases from ~13.6 × 10²¹ cm^–3 for Ti_0.9NiSb to ~3.4 × 10²¹ cm^–3 for Ti_0.8Sc_0.05NiSb. The reduced carrier concentration results in greatly increased Seebeck coefficient, therefore the Ti_0.8Sc_0.05NiSb sample achieves a power factor as high as 17.7 μW·cm^-1·K^-2 at 973 K. Although the lattice thermal conductivity of Sc-doped sample increases slightly due to the reduction of electron–phonon scattering and the enhancement of chemical bonds, the total thermal conductivity decreases dramatically due to the electronic thermal conductivity decreasing greatly. Finally, the Ti_0.8Sc_0.05NiSb sample reaches a <i>ZT</i> value of ~0.42 at 973 K, which is 180% higher than that of Ti_0.9NiSb sample. Despite the fact that the thermoelectric performance of our sample is still inferior to those of the state-of-the-art off-stoichiometric 19-electron half-Heusler alloys, this work demonstrates that the thermoelectric performance of Ti_1–<i>x</i>NiSb can be further improved by non-isoelectronic doping.