Effect of Suppression of Grain Growth of Hot Extruded (Bi0.2Sb0.8)2Te3 Thermoelectric Alloys by MoS2 Nanoparticles

Nanostructured bulk materials are regarded as a means of enhancing the performance of thermoelectric (TE) materials and devices. Powder metallurgy has the distinct advantage over conventional synthesis that it can start directly from nanosized particles. However, further processing, for example extrusion, usually requires elevated temperatures, which lead to grain growth. We have found that introduction of semiconductor nanoparticles of molybdenum disulfide (MoS₂), a well-known solid lubricant, suppresses grain growth in bismuth telluride-based alloys, thus improving the extrusion process. Scanning electron microscope images show that adding MoS₂ particles at concentrations of 0.2, 0.4, and 0.8 wt% to p-type (Bi_0.2Sb_0.8)₂Te₃, under otherwise identical extrusion conditions, reduces average grain size by a factor of four. Scherer’s formula applied to x-ray diffraction data indicates that average crystallite sizes (～17 nm) of powders are not significantly different from those of alloys extruded with MoS₂ (～18 nm), which is in stark contrast with those for conventional alloy (Bi_0.2Sb_0.8)₂Te₃ extruded under the same conditions (～80 nm). Harman measurements of TE properties reveal a decrease of the thermal conductivity accompanied by reduction of the room-temperature figure of merit (ZT) from 0.9 to 0.7, because of a lower power factor. Above 370 K, however, the performance of alloys containing MoS₂ surpasses that of (Bi_0.2Sb_0.8)₂Te₃, with reduction of the thermal conductivity which is more significant at temperatures above the cross point of the ZT values.