SrTiO₃ glass ceramics - Part I Crystallization and microstructure

This is the first in a series of two papers describing the crystallization and dielectric properties of glass-ceramics derived from a particular strontium titanium aluminosilicate glass composition. This first paper concerns the development of crystalline phases and microstructure of glass-ceramics prepared under various crystallization conditions. In the following paper, the dielectric properties of these glass-ceramics are described and correlated with the characterization results. Perovskite strontium titanate (SrTiO₃) was the primary crystalline phase in glass-ceramics crystallized over the temperature range of 800 to 1100° C. At crystallization temperatures below 950° C, SrTiO₃ formed with a spherulitic or dendritic growth habit. X-ray diffraction suggested that the SrTiO₃ crystallized in a perovskite-like "precursor" phase which transformed to perovskite SrTiO₃ with further crystallization time. However, electron diffraction indicated that this "precursor" phase was cubic perovskite SrTiO₃. At higher crystallization temperatures, perovskite SrTiO₃ was present as individual crystallites without evidence of the spherulitic habit. The crystallization of SrTiO₃ was followed by that of other phases, the hexacelsian and anorthite forms of SrAl₃Si₂O₈, and the rutile and anatase forms of TiO₂. The crystallization sequence and microstructure of the glass-ceramics were determined by the competition for strontium and titanium between the crystallizing phases, SrTiO₃ and SrAl₂Si₂O₈, and TiO₂.