Enhancing the thermoelectric performance of cold sintered calcium cobaltite ceramics through optimised heat-treatment

Cold sintering is a promising technology for preparing electronic materials enabling densification at low temperature but rarely employed for thermoelectrics. Herein high-quality Ca_2.7Bi_0.3Co_3.92O_9+δ ceramics were synthesised by a combination of cold sintering and annealing processes. Stoichiometric mixtures of raw materials were calcined once or twice at 1203 K for 12 h in air and then cold sintered at 673 K for 60 min under a pressure of 85 MPa followed by annealing at 1203 K for 12 h or 24 h in air. The effects of the calcination processes and annealing conditions on the thermoelectric performance of cold sintered samples were investigated. By optimising heat-treatment the formation of secondary phases texture development and porosity were controlled leading to enhanced electrical conductivity and reduced thermal conductivity. Consequently at 800 K there was 85% increase in power factor and 35% increase in ZT (value of 0.15) compared to previous studies.