High thermoelectric performance of MgAgSb-based materials

Bismuth telluride has been the only thermoelectric material with thermoelectric figure-of-merit (ZT) of 1-1.5 in the temperature range of 20 and 150°C since its discovery in 1950s. It has been primarily used for cooling even though power generation has also been attempted for hot side of 250°C and cold side of ~20°C. Here, we report our discovery of comparable ZT values in MgAgSb-based MgAg_0.97Sb_0.99 and MgAg_0.965Ni_0.005Sb_0.99. The materials are made by ball milling powders of elements in a two-step process and hot pressing the powders into dense bulk samples with grains smaller than 20nm. The small grain size together with point defects including vacancies and antisites in the structure are the main reasons for the very low thermal conductivity of ~0.7Wm^-1K^-1 at room temperature. The ZT values at room temperature are close to 1 and increase with temperature to a maximum of ~1.4 at 475K. Our discovery promises a new class of thermoelectric materials with high thermoelectric performance between 20 and 250°C.