Dependences of phase stability and thermoelectric properties of type-I clathrate Ba8Cu4.5Si6Ge35.5 on synthesis process parameters

Abstract

Thermoelectric materials are actively considered for waste heat recovery applications. To improve the thermoelectric (TE) performance, nanostructuring to reduce their thermal conductivity has been inferred as an effective approach. Here, we present our investigations of the influence of ball milling and hot pressing process parameters on the chemical and thermoelectric properties of the type-I clathrate Ba8Cu4.5Si6Ge35.5. Powders with very small grain sizes could be obtained by ball milling. However, in contrast to other materials, the ultra-fine powders deteriorate the sintering behavior in the hot pressing process. We suggest that this is mainly due to an oxygen-induced local decomposition of the clathrate phase, which might start from the surface of nanoparticles. Partial decomposition also occurs in sintered samples of low bulk density when they are heated under low vacuum conditions. Overall this leads to the loss of Si/Ge and thus to an enhanced Cu content in the remaining clathrate(s), which deteriorates the TE performance in spite of the reduced thermal conductivity. Thus, keeping the composition and structure of the clathrate stable during the nanostructuring process is essential for reaching high TE performance.