Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe)x(AgSbSe2)100-x

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit (ZT(avg)) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZT(max)). Herein an ultrahigh ZT(avg) of 1.4 for (GeTe) (80)(AgSbSe2)(20) [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZT(max) of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity (k(lat)), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe2 in GeTe results in k(lat) of approximate to 0.4 WmK(-1) in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (k(min)) of GeTe. Additionally, (GeTe)(80)(AgSbSe2)(20) exhibits a higher Vickers microhardness (mechanical stability) value of approximate to 209 kgfmm(-2) compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.