On the influence of AgMg precursor formation on MgAgSb microstructure and thermoelectric properties

Abstract

α-MgAgSb has been identified as promising thermoelectric material, with peak performance around 300ºC, filling the gap between low- and high-temperature thermoelectric materials. The complexity of the Mg-Ag-Sb phase diagram and the sensitivity of the MgAgSb synthesis process often leads to impurities appearing in the final samples, which reduce the thermoelectric figure of merit zT. Using a two-step ball milling process, we show that the extent of the AgMg precursor formation has a direct effect on the final composition, mainly with respect to the dyscrasite (Ag3Sb) content. We study how extending the first ball milling step, adding a AgMg sintering step before the second ball milling step or reducing the Ag particle size each affect the overall formation of AgMg. It is shown that lower Ag wt% in said precursor results in lower Ag3Sb content in the final samples, achieving<1 wt% Ag3Sb when Ag<1 wt% (in the precursor). This leads to a 23% improvement of zT between 300 and 563 K and a peak value of ~1.2 at 500 K. This shows that the complete conversion of Mg and Ag into AgMg is the key for high performance α-MgAgSb.