Negative enthalpy delivering improved thermal stability for Mg2Sn-based thermoelectric power generator

Thermal stability is crucial for the thermoelectric power generation device, but it has not yet gotten enough attention. Herein, we propose the negative enthalpy strategy to improve thermal stability with the assistance of first-principal calculation. It was found that the doping of element Y at the Mg sublattice significantly reduces the mixing enthalpy of Mg₂Sn from -8.0 kJ mol⁻¹ (Mg₂Sn) to -13.8 kJ mol⁻¹ (Mg_1.75Y_0.25Sn). Y increases the formation energy of Mg vacancy in Mg_2-xY_xSn, strengthens the Mg-Sn chemical bond, refines grain size, and reduces saturation vapor pressure. Experimentally, the Y-doped sample Mg_2.05Y_0.01Sn_0.728Ge_0.25Sb_0.022 shows a modulus of 97 GPa, hardness of 4.6 GPa, σ_s of 19 MPa, corresponding to a 9%, 3%, and 28% higher than that of the Y-free sample, respectively. In-situ electrical properties measurement confirms the excellent thermal stability of the Y-doped sample Mg_{2. 05}Y_0.01Sn_0.728Ge_0.25Sb_0.022 at 500 °C for over 3000 min. Ultimately, a high specific power density of 0.9 W cm⁻¹ and conversion efficiency of 7.3% were achieved in the corresponding single-leg device under a temperature difference of 370 °C, representing significant advancements in the Mg₂Sn-based thermoelectric devices.