Microstructure and Thermoelectric Properties of Mechanically Alloyed Zr_(0.5)Hf_(0.5)Ni_(0.8)Pd_(0.2)Sn_(0.99)Sb_(0.01)/WO_3 Half-Heusler Composites

摘要

Bulk Zr_(0.50)Hf_(0.50)Ni_(0.8)Pd_(0.2)Sn_(0.99)Sb_(0.01) half-Heusler (HH) alloys containing various concentrations of WO_3 inclusions were prepared using a combination of mechanical alloying and hot pressing. The microstructures of hot pressed pellets of the synthesized composites were investigated using transmission electron microscopy (TEM) and their thermal and electronic transport properties were measured from 300 K to 750 K. The electrical conductivities of the synthesized composites containing low concentrations (up to 5 wt%) of WO_3 inclusions is slightly lower than that of the bulk matrix. Increasing the concentration of WO_3 to 10 wt% results in a large increase (30%) in the electrical conductivity. The thermopower slightly decreases with increasing concentrations of WO_3 inclusions up to 5 wt% and drastically drops (33%) for the composite with 10 wt% WO_3 inclusions. Using electron microscopy and powder X-ray diffraction data, we attribute the observed large increase in the electrical conductivity and drastic decrease of the thermopower for the composite with 10 wt% WO_3 to the formation of highly conductive Zr_xWO_3 inclusions within the matrix arising from partial reaction between the WO_3 and the HH matrix. The lattice thermal conductivity of the composites increases with increasing WO_3 content, presumably due to the formation of interconnected precipitates of thermally conductive Zr_xWO_3 and HfO_2 phases within the HH matrix.