Thermoelectric properties of n-type half-Heusler compounds (Hf_0.25Zr_0.75)_1-xNb_xNiSn

A series of Nb doped (Hf_0.25Zr_0.75)_1-xNb_xNiSn (x = 0-0.03) samples were synthesized and studied by arc-melting the elements to first form ingots, then ball-milling the ingots to obtain fine powders, and finally hot-pressing the fine powder to form bulk samples. Instead of the conventional Sb doping on the Sn site of the n-type half-Heusler, it was found that Nb is also an effective dopant. When the carrier concentration is optimized at 2.2% Nb, a power factor of ∼47 μW cm^-1 K^-2 is achieved at and above 600 °C, and a peak ZT ∼0.9 is achieved at 700 °C with Nb doping from 1.8 at% to 2.2 at%. An output power density of ∼22 W cm^-2 and a leg efficiency of ∼12% are calculated for the Nb doped samples.