Microwave heating synthesis and thermoelectric property characterization of highly dense Ca3Co4O9 bulk

Abstract The present study is focused on synthesis and sintering of calcium cobaltite (Ca3Co4O9) by solid-state reaction of calcium carbonate (CaCO3) and cobalt oxide (Co3O4) using microwave heating at 850, 900, and 950 °C. Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) results show a weight loss of ~26% due to the decomposition of CaCO3 in the temperature range of 600–820 °C, resulting into single phase of Ca3Co4O9 which remains stable up to 920 °C. The Ca3Co4O9 phase is observed in samples heated at 850 °C and 900 °C, whereas at 950 °C, Ca3Co4O9 is transformed to Ca3Co2O6 as identified by X-Ray Diffractometry. 98.76% of relative density is obtained by microwave sintered sample at 950 °C with 1hr holding time. Changes in synthesis temperature did not show any effect on the Seebeck coefficient (119–155 μV/K). The specimens sintered at 900 °C exhibit a low resistivity (150-160 μΩ-m) and better power factor (94–145 μW/mK2) in the 35–500 °C temperature range.