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

Among the state-of-the-art thermoelectric materials, Cu2S has attracted much attention because of its nontoxicity, high abundance, and "phonon-liquid electron-crystal" characteristics. It is found the thermoelectric performance of Cu2S 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 gamma-phase and tetragonal delta-phase is formed in the HPRT Cu2S. Meanwhile, a strained lamella structure with a large number of dislocations is found within the monoclinic gamma-Cu2S 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 Cu2S 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 Cu2S, much higher than that of 0.19 for melting combined with plasma activated sintering processed Cu2S.