Influence of hot pressing temperature on thermoelectric properties of type‐I clathrates

Abstract

It is well known that the process variables for sample synthesis affect the electrical, magnetic, mechanical, etc. properties of the final product. In this study, we present the influence of the hot pressing temperature on the thermoelectric properties of a type‐I clathrate of nominal composition Ba8Cu4.5Ge35.5Si6. The samples are prepared in a high‐frequency furnace, annealed, and subject to a long‐time ball milling. Finally, the nanopowders from ball milling are hot pressed at different temperatures. All hot pressed samples are characterized by X‐ray diffraction, energy dispersive X‐ray spectroscope, scanning electron microscopy, and transport property measurements. The results show that lower hot pressing temperatures are desired to prevent the nanopowders from coarsening, but are unfavorable for the densification: Samples hot pressed at lower temperatures have smaller crystallite sizes, larger porosities, and lower relative densities and, consequently, have higher electrical resistivity, and lower thermal conductivity. No significant differences are seen in ZT as the positive effect of reduced thermal conductivity is compensated by the enhanced electrical resistivity. The highest ZT of 0.42 is reached at 873 K for a sample hot pressed at 780 °C.