Synthesis of Nb0.8Hf0.2FeSb0.98Sn0.02 and Hf0.25Zr0.25 Ti0.5NiSn0.98Sb0.02 Half-Heusler Materials and Fabrication of Thermoelectric Generators

In this study, half-Heusler (HH) thermoelectric materials Nb0.8Hf0.2FeSb0.98Sn0.02 (p-type) and Hf0.25Zr0.25 Ti0.5NiSn0.98Sb0.02 (n-type) were synthesized using induction melting and spark plasma sintering. For alloying, a conventional induction melting technique was employed rather than arc melting, for mass production compatibility, and the thermoelectric properties of the materials were analyzed. The maximum dimensionless figures of merit (zT(max)) were 0.75 and 0.82 for the p- and n-type material at 650 degrees C and 600 degrees C, respectively. These materials were then used to fabricate generator modules, wherein two pairs of p- and nlegs without interfacial metal layers were brazed on direct bonded copper (DBC)/Al2O3 substrates using a Zrbased alloy. A maximum power of 0.57 W was obtained from the module by applying a temperature gradient of 476 degrees C, which corresponds to a maximum power density of 1.58 W cm(-2) when normalized by the area of the material. The maximum electrical conversion efficiency of the module was 3.22% at 476 degrees C temperature gradient. This value was negatively affected by the non-negligible contact resistivity of the brazed interfaces, which ranged from 6.63 x 10(-9)Omega m(2) to 7.54 x 10(-9)Omega m(2) at hot-side temperatures of 190 degrees C and 517 degrees C, respectively. The low electrical resistivity of the HH materials makes it especially important to develop a brazing technique for ultralow resistance contacts.