Synthesis and thermoelectric properties of Bi_2.5Ca_2.5Co₂O_x layered cobaltites

Bi–Ca–Co–O polycrystalline materials with a layered structure were prepared. The synthesis of the sintered specimens from two starting compositions, Bi_1.8Ca₂Co₂O_xand Bi_2.5Ca_2.5Co₂O_x, revealed the latter is preferentially formed at high temperature(850 °C). The increase in sintering time was shown by growth of large platelike grains (up to 50 μm in diameter and several micrometers in thickness). The reaction mechanisms during the heat treatment and the preferential formation of the Bi_2.5Ca_2.5Co₂O_x phase were observed by x-ray diffraction, thermogravimetry–differential thermal analysis, and scanning electron microscopy. These techniques supposed the presence of a liquid phase at high temperature, origin of a highly kinetic phase formation, and the growth of large grains. Interestingly the liquid phase reaction promotes an efficient stacking and sliding of grains during hot-forging treatment, and highly (00l) oriented materials were prepared. A relationship between thermoelectric performance, texture strength, and microstructure is clarified.