Lowering the sintering temperature of calcium manganate for thermoelectric applications

This study combines three different approaches to lower the sintering temperature of Sm-doped CaMnO3 to save energy in production and facilitate co-firing with other low-firing oxides or metallization. The surface energy of the powder was increased by fine milling, sintering kinetics were enhanced by additives, and uniaxial pressure during sintering was applied. The shrinkage, density, microstructure, and thermoelectric properties were evaluated. Compared to micro-sized powder, the use of finely ground powder allows us to lower the sintering temperature by 150 K without reduction of the power factor. By screening the effect of various common additives on linear shrinkage of CaMnO3 after sintering at 1100°C for 2 h, CuO is identified as the most effective additive. Densification at sintering temperatures below 1000°C can be significantly increased by pressure-assisted sintering. The power factor at room temperature of CaMnO3 nano-powder sintered at 1250°C was 445 μW/(m K2). Sintering at 1100°C reduced the power factor to 130 μW/(m K2) for CaMnO3 nano-powder, while addition of 4 wt. % CuO to the same powder led to ∼290 μW/(m K2). The combination of fine milling, CuO addition, and pressure-assisted sintering at 950°C resulted in a power factor of ∼130 μW/(m K2). These results show that nano-sized powder and CuO addition are successful and recommendable strategies to produce CaMnO3 with competitive properties at significantly reduced temperatures and dwell times.