Synthesis and thermoelectric properties of (Ti,Zr,Hf)(Co,Pd)Sb half-Heusler compounds

(Ti,Zr,Hf)(Co,Pd)Sb half-Heusler bulk materials have been synthesized using a high-frequency induction melting method followed by the spark plasma sintering densification process. The thermoelectric transport properties have been investigated as a function of temperature. It is found that the electrical conductivity of TiCo1-xPdxSb (x = 0.01; 0.03; 0.05) systematically increases with increasing Pd-doping ratio, x, but x > 0.05 degrades the power factor. Furthermore, keeping the optimal x = 0.05, the Zr and Hf substitution on the Ti site has been employed to improve the electrical properties and lower thermal conductivity in the (Ti,Zr,Hf)(Co0.95Pd0.05)Sb samples. As a result, the dimensionless figure of merit (ZT) of 0.23 is attained for the Ti0.3Zr0.35Hf0.35Co0.95Pd0.05Sb sample at 873 K, an increase ∼130% over that of TiCo0.95Pd0.05Sb.