Highly efficient thermoelectric cooling performance of ultrafine-grained and nanoporous materials

Nanograins and pores, as two common microstructural defects, are capable to impede phonon transport. However, thus far, the stability of nanograins at elevated temperature and the feasibility of porosity in boosting the figure of merit ZT are still concerned in thermoelectrics. Herein, we report that a specifically designed microstructure mainly consisting of ultrafine grains within the nanocrystalline regime and randomly distributed pores in α-MgAgSb gives rise to an ultralow lattice thermal conductivity ∼0.46 W m⁻¹ K⁻¹ at 300 K that breaks the limit of the estimated minimum value. Associated with a slightly deteriorated electrical performance, an unprecedented performance, ZT ∼0.94 at 300 K and ZT_ave ∼1.16, are realized. Benefiting from optimized α-MgAgSb in this work and Mg_3.2(Bi, Sb)₂ material to fabricate a Peltier module, we can achieve a high maximum temperature difference ΔT_max ∼52 K and a maximum coefficient of performance COP ∼8.3 at hot-side temperature T_h = 300 K, outperforming these previously reported non-Bi₂Te₃ modules. This is, to our knowledge, the first demonstration of high performance thermoelectric modules mediated by the utilization of ultrafine grains and nanoporous structures, which provides a swift pathway to accelerate the wide employment of thermoelectric cooling technology in modern society.