Sintering of LaCoO3 based ceramics

The densification, microstructure and phase evolution of near stoichiometric, Co-excess and Co-deficient perovskite La1−xMxCoO3−δ (M=Ca, Sr; x=0, 0.2) powders have been investigated by electron microscopy and powder X-ray diffraction. Sub-micron powders were prepared from nitrate precursors using the glycin-nitrate and the EDTA methods. The sintering temperature was observed to decrease with Ca or Sr substitution. Dense materials with grain size in the order of 3–5 μm have been obtained at 1200°C for near stoichiometric powders. Considerable grain growth was observed at higher sintering temperatures. The presence of other crystalline phases in addition to the perovskite due to Co-excess/-deficiency considerably affects the microstructure and acts as grain growth inhibitors by grain boundary pinning. The volume fraction of secondary phases is particularly large in the case of Co-deficient LaCoO3 due to the formation of La4Co3O10. In non-stoichiometric La0.8Ca0.2CoO3, a liquid phase consisting mainly of CaO and CoO was observed at 1400°C causing exaggerated grain growth. Considerable pore coarsening was observed in Co-excess La0.8Ca0.2CoO3 at 1350°C. The present investigation demonstrates the importance of controlling the stoichiometry of LaCoO3 based ceramics in order to obtain dense materials with well defined microstructure.