High thermoelectric figure-of-merit in Sb₂Te₃/Ag₂Te bulk composites as Pb-free p-type thermoelectric materials

We investigated the thermoelectric properties of Sb₂Te₃/Ag₂Te (ST/AT) composites with ST/AT molar ratios of 1/1, 2/1, 4/1, 8/1, 16/1, and 32/1. The extrinsic composites, synthesized by wet ball milling of two separate powders of Sb₂Te₃ and Ag₂Te, can be differentiated from the intrinsic composites by high temperature phase separation because of the low temperature synthesis process. The thermoelectric properties of the composites show a systematic behaviour of decreased electrical and thermal conductivities with increasing Ag₂Te dispersion concentration. The composite with ST/AT ratio = 1/1 exhibits an extremely low lattice thermal conductivity with a high power factor over a wide temperature range, resulting in a high ZT value of 1.5 at 700 K, which is a significantly enhanced value of ZT compared with those of other Pb-free p-type chalcogenide thermoelectric materials.