High thermoelectric figure-of-merit in Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$/Ag$_{\mathrm{2}}$Te bulk composites as Pb-free $p$-type thermoelectric materials

We investigated thermoelectric properties of the Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$/Ag$_{\mathrm{2}}$Te (ST/AT) composites with molar ratios of ST/AT $=$ 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 Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$ and Ag$_{\mathrm{2}}$Te, are differentiated with intrinsic composites by high temperature phase separation in that it is low temperature synthesis process. The thermoelectric properties of the composites show systematic behaviour of decreased electrical and thermal conductivities with increasing Ag$_{\mathrm{2}}$Te dispersion concentration. The ST/AT $=$ 1/1 composite exhibits extremely low lattice thermal conductivity with high power factor over a wide temperature range, resulting in high \textit{ZT} value 1.5 at 700 K, which is the significantly enhanced value of \textit{ZT} compared with other Pb-free $p$-type chalcogenide thermoelectric materials.