Glass-ceramics in the system BaO/TiO2(ZrO2)/Al2O3/B2O3 and their thermal expansion

Glasses in the system BaO/TiO2(ZrO2)/Al2O3/B2O3 were melted from the raw materials with and without the addition of platinum. They were crystallized in two steps at 720 and 780 °C. TiO2 does not act as nucleating agent in this system, but widely prevents crystallization. Samples additionally doped with platinum show the crystallization of BaAl2B2O7. However, samples containing equimolar quantities of TiO2 and ZrO2 showed the crystallization of this phase without the addition of platinum. The thermal expansion coefficients of samples with TiO2-concentration ⩽7 mol% or TiO2/ZrO2-concentrations ⩽5 mol% exhibit zero or even slightly negative thermal expansion coefficients.