Synergistic effects of Sb doping and nanostructuring on spark plasma sintered ZrNiSn thermoelectrics

Abstract

Nanostructured ZrNiSn and ZrNiSn1−xSbx (x = 0.02 and 0.05) half-Heusler alloys were successfully synthesized via mechanical milling followed by spark plasma sintering. All samples crystallized in the expected MgAgAs-type crystal structure (space group 𝐹-43m) and retained their nanostructured powder morphology post-sintering. Thermoelectric measurements revealed that the mechanical milling effectively reduced lattice thermal conductivity, while Sb doping significantly enhances the electrical conductivity in the ZrNiSn system. However, extending the milling time beyond 5 hours led to reduced performance due to the increased structural disorder without further thermal conductivity decrease. Hall effect measurements yielded a relatively low effective carrier mass of around 0.7-1.3 me, which accounts for the observed reduction in the Seebeck coefficient. The optimal Sb doping level was found to be x = 0.02. At this doping level, a substantial improvement in thermoelectric performance is detected with a maximum thermoelectric figure of merit (ZT) of approximately 0.6 at 600 K (two folds that of the undoped sample) and a 60% improvement over the unmilled ZrNiSn0.98Sb0.02 alloy These results highlight the synergistic effects of controlled Sb doping and nanostructuring via mechanical milling on optimizing the thermoelectric properties of ZrNiSn-based half-Heuslers.