Cold sintering and thermoelectric properties of Ca3Co4O9 ceramics

In this study, Ca3Co4O9 ceramics were successfully prepared using a cold sintering process at significantly low temperatures of approximately 150 degrees C. This new procedure was used to produce Ca3Co4O9 ceramics, in particular, as this material is extremely sensitive to the synthesis and processing owing to its partial decomposition into Co3(O4) or Ca2Co2O5 as a secondary phase. Ca3Co4O9 powder was prepared through a solid-state reaction and calcined at 900 degrees C. For the cold sintering process, the prepared powders were uniaxially pressed into 12 mm diameter discs of approximately 1 mm thickness, at 350 MPa, while the temperature was isothermally maintained at 150 degrees C for 1 h. Subsequently, selected "as prepared" ceramics were re-sintered at 900 degrees C for 6 h or 24 h. The results indicated that bulk densities greater than 74% of the theoretical density were obtained for the cold sintered ceramics. Compared to conventionally sintered ceramics, the cold sintered ceramics exhibited greater electrical conductivity (6459 S/m) and reduced thermal conductivity (0.672 W/m K) at 600 degrees C. The highest Figure of Merit value obtained was 0.12 for the cold sintered ceramics, which is thrice that of conventionally sintered ceramics. The cold sintering process was observed to be efficient in increasing bulk densification and improving the thermoelectric properties of Ca3Co4O9 ceramics compared to the conventional sintering method.