Phase evolution and thermoelectric performance of calcium cobaltite upon high temperature aging

Crystal phase transformation and their nanostructure evolutions were studied for baseline thermoelectric ceramic Ca3Co4O9, and for a sample aged at 1283 K, which is significantly higher than the Ca3Co4O9 decomposition temperature of 1199 K. X-ray diffraction from both samples only reveals peaks from Ca3Co4O9 phase. TEM examination reveals that in the aged sample, CaO nano-phase exists between neighboring Ca3Co4O9 nano-lamella grains. Moreover, a small amount of Co3O4 and Ca2Co2O5 phase were also observed in the aged sample. The Ca2Co2O5 phase does not exhibit the same lamella nanostructure as Ca3Co4O9 and displayed few crystal defects. Except for Ca3Co4O9, Ca2Co2O5, and Co3O4, no other Ca–Co–O cobaltite phase, such as Ca3Co2O6, was found in the aged sample. Electrical and thermal properties of both baseline and aged sample were measured over the temperature range of 320–1033 K.