Reactions between calcium- and strontium-substituted lanthanum cobaltite ceramic membranes and calcium silicate sealing materials

The reaction between Ca- and Sr-substituted LaCoO3 and CaSiO3/Ca2SiO4 has been studied by electron microscopy (SEM and TEM) and X-ray diffraction. The aim was to investigate the chemical stability of these materials as a model system for, respectively, a membrane and a sealing material in dense oxygen-permeable membrane systems. Sintered powder mixtures of the two materials were analyzed to gain information about coexistent phases in the CaO/SrO-La2O3-CoO-SiO2 system. The estimated phase composition along the CaSiO3-LaCoO3 line has been worked out. The chemical aspects of glass-ceramic sealing of dense perovskite membranes were studied by making diffusion couples. LaCoO3 was found to be kinetically more stable to calcium silicate than Ca- and Sr-substituted LaCoO3. The results also revealed that Ca2SiO4 is a more suitable sealing material than CaSiO3, due to lower reactivity. Thus, the stability of the membrane/sealant interface was observed to be quite sensitive to the O:Si ratio of the calcium silicate: 3 < O:Si < 4 will give rise to good sealing properties and moderate reactivity to the membrane material. A suitable material is a two-phase material with O:Si close to four, e.g. an orthosilicate glass-ceramic material with small amounts of metasilicate or disilicate glass.