Rational design and synthesis of Cr_1–x Te/Ag₂Te composites for solid-state thermoelectromagnetic cooling near room temperature

Abstract

Materials with both large magnetocaloric response and high thermoelectric performance are of vital importance for all-solid-state thermoelectromagnetic cooling. These two properties, however, hardly coexist in single phase materials except previously reported hexagonal Cr1-xTe half metal where a relatively high magnetic entropy change (-ΔS M) of ~2.4 J kg-1 K-1 @ 5 T and a moderate thermoelectric figure of merit (ZT) of ~1.2×10-2 @ 300 K are simultaneously recorded. Herein we aim to increase the thermoelectric performance of Cr1-xTe by compositing with semiconducting Ag2Te. It is discovered that the in-situ synthesis of Cr1-xTe/Ag2Te composites by reacting their constitute elements above melting temperatures is unsuccessful because of strong phase competition. Specifically, at elevated temperatures (T > 800 K), Cr1-xTe has a much lower deformation energy than Ag2Te and tends to become more Cr-deficient by capturing Te from Ag2Te. Therefore, Ag is insufficiently reacted and as a metal it deteriorates ZT. We then rationalize the synthesis of Cr1-xTe/Ag2Te composites by ex-situ mix of the pre-prepared Cr1-xTe and Ag2Te binary compounds followed by densification at a low sintering temperature of 573 K under a pressure of 3.5 GPa. We show that by compositing with 7 mol% Ag2Te, the Seebeck coefficient of Cr1-xTe is largely increased while the lattice thermal conductivity is considerably reduced, leading to 72% improvement of ZT. By comparison, -ΔS M is only slightly reduced by 10% in the composite. Our work demonstrates the potential of Cr1-xTe/Ag2Te composites for thermoelectromagnetic cooling.