Effect of Al2O3 on phase formation and thermal expansion of a BaO-SrO-ZnO-SiO2 glass ceramic

A glass from the system BaO-SrO-ZnO-Al2O3-SiO2 was synthesized and the crystallization behavior was studied using differential scanning calorimetry, X-ray diffraction, and scanning and transmission electron microscopy. The main phase precipitating is a Ba1-xSrxZn2Si2O7 solid solution with very low thermal expansion. The formation of this phase leads to a decrease of the coefficients of thermal expansion of sintered and afterwards crystallized powder compacts. The addition of Al2O3 leads to the formation of another phase at the early stage of crystallization, which disappears after longer heat treatments or at elevated temperatures. For comparison, a stoichiometric powder with the composition Ba0.5Sr0.5Zn1.9Si1.9Al0.2O7 was prepared via a solid-state reaction in order to verify whether or not Al3+-ions can be incorporated into the crystal structure. This phase does not exist as a single-phase material, which means that the addition of Al2O3 leads to the formation of small concentrations of secondary phases.