Bismuth calcium cobaltite thermoelectrics: A study of precursor reactivity and its influence on the phase formation

Bismuth calcium cobaltite [Bi1–xCaO2]2[CoO2]1.7 belongs to a thermoelectric oxide family that is promising due to its high temperature and chemical stability. However, the phase equilibria within the quasiternary Bi2O3–CaO-CoOx system is not described in detail what can be problematic during the synthesis of the phase. Several quasibinary Bi-rich oxides have been described exhibiting moderate melting temperature, which can be tricky for the solid-state preparation of [Bi1–xCaO2]2[CoO2]1.7 (moreover which melts incongruently). Therefore, it is very important to map the phase evolution in the context of the precursor used and final thermoelectric properties. Such a study has not been described in literature for [Bi1–xCaO2]2[CoO2]1.7 yet. Two precursors were prepared using a solid state reaction and a sol-gel process and their phase evolution within 4-step calcination was observed. Then, final samples were sintered ending with either fast or slow cooling. The phase composition, microstructure and thermoelectric characteristics of the final samples are presented. The results show that the phase and microstructure evolution differs from the one published for the BiSrCoO structure analogue. The thermoelectric characteristics varied depending on the used precursors. •Thermoelectric [Bi1–xCaO2]2[CoO2]1.7 oxide ceramics was synthetized.•Solid-state reaction or sol-gel method was used for the precursor preparation.•Phase evolution in BiCaCoO oxide was observed at 4-step calcination and final heating.•Microstructure and thermoelectric characteristics of final samples are presented.