Phase evolution by thermal treatment of equimolar cobalt–magnesium cordierite glass powders

The structural evolution with annealing time at temperatures between 900 and 1100 °C of loosely compacted equimolar cobalt–magnesium cordierite glass powder was reported. The glass, with composition MgCoAl4Si5O18, was prepared by melting a glass precursor, previously synthesized by a semiwet method, at 1650 °C. Field emission scanning electron microscopy (FESEM) of isothermally heated glasses at 900 and 950 °C revealed increased phase separation with thermal treatment. X-ray diffraction (XRD) results of these powders indicated that the first crystalline phase formed was μ cobalt-containing cordierite which transformed to α cordierite with longer annealing. At the beginning of the μ to α transformation, mullite and cobalt–magnesium aluminate spinel were detected but on further annealing they disappear. Infrared (IR) spectroscopy after long annealing at 1100 °C showed that some ordering for aluminium and silicon in tetrahedral sites had taken place, indicating some transformation to the orthorhombic crystalline form, β, of cordierite. FESEM examination revealed that the μ-cordierite grew by a dendritic mechanism along the particle surface and that the nucleation and growth of α-cordierite occurs within the μ-cordierite dendrites. Finally, after short annealing at 1100 °C the fully crystallized microstructure developed was α-cordierite with columnar or linear features.