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-Al 2 O 3 -SiO 2 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 Ba 1−x Sr x Zn 2 Si 2 O 7 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 Al 2 O 3 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 Ba 0.5 Sr 0.5 Zn 1.9 Si 1.9 Al 0.2 O 7 was prepared via a solid-state reaction in order to verify whether or not Al 3+ -ions can be incorporated into the crystal structure. This phase does not exist as a single-phase material, which means that the addition of Al 2 O 3 leads to the formation of small concentrations of secondary phases. [ABSTRACT FROM AUTHOR]