Improvements of thermoelectric properties for p-type Cu_1.8S bulk materials: Via optimizing the mechanical alloying process

Polycrystalline bulk Cu_1.8S materials were fabricated using mechanical alloying (MA) and spark plasma sintering (SPS). The microstructure and thermoelectric transport properties were investigated in detail with emphasis on the effect of the mechanical alloying ball-milling process. Cu_1.8S single-phase powders were directly synthesized from the elemental powders. High-density (>95%) p-type Cu_1.8S bulk samples were fabricated using the subsequent SPS process at 773 K for 5 min. A Cu_1.96S second phase was detected in the bulk samples when the ball-milling time was increased to 3 h, and the Cu_1.96S content increased with increasing ball-milling time. The electrical properties and Seebeck coefficient of the Cu_1.8S bulk samples were dependent on the milling time of the starting powders. The Cu_1.8S bulk that was sintered by SPS using the powder treated with a 3 h ball-milling obtained a record-high power factor of 1552 μW m^-1 K^-2 at 773 K for the Cu-S system, which is 40% higher than that of pristine Cu_1.8S. The ZT value of the Cu_1.8S bulk sample that was sintered by SPS using the 12 h ball-milling treated powder reached 0.71, which is 67% higher than the ZT value of the sample that was sintered using the powder with a 1 h ball-milling time.