High thermoelectric performance of MgAgSb-based materials

•Phase purity is essential to achieve good ZTs in MgAgSb-based materials.•ZTs of ~0.7 at 300 K and ~1.2 at 450 K were achieved in MgAg0.97Sb0.99.•ZTs of ~1 at 325 K and ~1.4 at 450 K were achieved in MgAg0.965Ni0.005Ag0.99.•MgAg0.965Ni0.005Ag0.99 is as good as Bi0.4Sb1.6Te3 in the same temperature range.•Good ZTs in MgAg0.965Ni0.005Ag0.99 is extremely important for power generation.

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 MgAg0.97Sb0.99 and MgAg0.965Ni0.005Sb0.99. 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 20 nm. 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 ~0.7 W m−1 K−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 475 K. Our discovery promises a new class of thermoelectric materials with high thermoelectric performance between 20 and 250 °C.

Graphical abstractBecause of the higher power factor from the higher phase purity, and lower thermal conductivity due to the small grain size and point defects of vacancies and antisites, we achieved ZT of ~0.7 at 300 K and ~1.2 at 450 K for MgAg0.97Sb0.99. By further alloying the Ag site using a very small amount of Ni, e.g., MgAg0.965Ni0.005Ag0.99, we obtained ZT of ~1 at 325 K and ~1.4 at 450 K. Such ZTs are comparable with those of the best Bi0.4Sb1.6Te3 in the same temperature range. Considering Bi0.4Sb1.6Te3 has been the only material with ZT~1 since 1950s, achieving a new material with higher ZT in the same temperature range could be extremely important.Figure optionsDownload full-size imageDownload as PowerPoint slide