Influence of a mechano-chemical treatment on the synthesis and characteristics of p-type thermoelectric Ca3Co4O9 ceramics

The influence of a mechano-chemical (MC) treatment on the formation of the Ca349 phase from a stoichiometric mixture of fine-grained CaCO3 and Co3O4 powders, as well as the microstructural and thermoelectric characteristics of the Ca349 ceramics were studied. With an experimentally obtained maximum of 283 mJ/hit the critical ball-impact energy (Eb) required for the formation of the Ca349 compound was not obtained; however, the MC treatment resulted in complete amorphization and an enhanced reactivity of the starting-powder mixture. This enabled the preparation of single-phase Ca349 ceramics with an un-homogeneous microstructure and a density of 50% after only 4 h of calcination at 760 °C, while the untreated mixture required at least 12 h. Additional sintering at 920 °C resulted in a highly homogeneous and fine-grained microstructure for the Ca349 ceramics with a density of 72%, having an approximately two-times higher ZT of 0.10 at 600 °C in comparison to the previously reported values. Our study showed that the mechano-chemical treatment enhances the formation of the Ca349 phase at a low calcination temperature, which enables the preparation of Ca349 ceramics with a microstructure that leads to improved TE characteristics.