The High‐Pressure Processed Cu2S: Phase Intergrowth with Strained Lamella Leading to an Improved Thermoelectric Performance

Abstract Among the state‐of‐the‐art thermoelectric materials, Cu 2 S has attracted much attention because of its nontoxicity, high abundance, and “phonon‐liquid electron‐crystal” characteristics. It is found the thermoelectric performance of Cu 2 S processed by high‐pressure at room temperature (HPRT) can be greatly improved. However, the underlying mechanism remains unknown due to the complex phase and microstructure formed under high pressure. Herein, the origin of the improved properties is disclosed by investigating the dynamic phase and defect structure evolutions using both ex situ and in situ transmission electron microscopy, and by exploring the high temperature thermal kinetics through differential scanning calorimetry and high temperature Hall test. It is surprising to find that an intergrowth of monoclinic γ‐phase and tetragonal δ‐phase is formed in the HPRT Cu 2 S. Meanwhile, a strained lamella structure with a large number of dislocations is found within the monoclinic γ‐Cu 2 S phase. Due to the defected microstructure and thus the increased carrier concentration, conductivities of electron and phonon are decoupled. Notably, the average lattice thermal conductivity of HPRT Cu 2 S is low in 600–900 K, due to the strongly phonon scattering by nanovoids inside the material. Finally, in 600–900 K, an average ZT value of 0.89 is obtained in the HPRT Cu 2 S, much higher than that of 0.19 for melting combined with plasma activated sintering processed Cu 2 S.