Effect of synthesis methods on the Ca3Co4O9 thermoelectric ceramic performances

Three different synthesis methods producing nanometric grain sizes, coprecipitation with ammonium carbonate, oxalic acid, and by attrition milling have been studied to produce Ca3Co4O9 ceramics and compared with the classical solid state route. These three processes have produced high reactive precursors and all the organic material and CaCO3·have been decomposed in a single thermal treatment. Coprecipitation leads to pure Ca3Co4O9 phase, while attrition milling and classical solid state produce small amounts of Ca3Co2O6 secondary phase. Power factor values are similar for all three samples, being slightly lower for the ones produced by attrition milling. These values are much higher than the obtained in samples prepared by the classical solid state method, used as reference. The maximum power factor values determined at 800 °C (~0.43 mW/K2 m) are slightly higher than the best reported values obtained in textured ones which also show much higher density values.