Thermoelectric transport properties of p-type silver-doped PbS with in situ Ag2S nanoprecipitates

Abstract

In this study, a series of p-type Ag-doped PbS compounds were prepared by a vacuum melting combined with subsequent spark plasma sintering process. Ag partially occupies Pb sites acting as an electron acceptor to increase the hole concentration, and also in situ forms Ag2S nanoprecipitates at the grain boundaries which are detectable in thermal analysis and microstructural observations. The two existences of Ag both exert significant influences on the electrical transport properties. With increasing temperature, dissolution of Ag2S nanoprecipitates into the PbS matrix as interstitial Ag gradually decreases the hole concentration. Very low mobility of Ag-doped samples is unexpected at low temperatures, which indicates a strong carrier scattering. From temperature dependent mobility, it can be concluded that neutral impurity scattering (μ ∼ Ta, a = 0) dominates at low temperature (T < 200 K), and energy barrier scattering (a > 1.5) governs until lattice vibrations take over at T > 450 K (a ∼ −2.5). The complex scattering mechanisms at T < 450 K in Ag-doped samples should be originated from the multiple existences of Ag (Ag+, interstitial Ag, Ag2S nanoprecipitates). In particular, Ag doping increases the high temperature electrical conductivity, giving rise to an enhanced power factor of ∼0.8 × 10−3 W m−1 K−2 at 870 K for the 1.5 at% Ag-doped sample. Benefiting from the improved power factor and low lattice thermal conductivity originating from strong lattice anharmonicity, a doubled ZT value of ∼0.6 can be achieved for Pb0.985Ag0.015S in comparison to that of pristine PbS.