Thermal expansion of glass–ceramics in the system BaO/Al2O3/B2O3

Abstract Glasses in the system BaO/Al2O3/B2O3 with and without the addition of platinum were melted. In one sample series, the BaO-concentration was varied while the ratio [Al2O3]/[B2O3] was kept constant. In another sample series, the [BaO]/[Al2O3]-ratio (= 0.9) was kept constant and the B2O3 concentration was varied. The samples were thermally treated at 720 °C for 24 h and subsequently at 780 °C for 4 h. In most thermally treated samples, the crystalline phase BaO · Al2O3 · B2O3 occurred. At some compositions, the platinum-doped samples showed larger concentrations of the crystalline phases. The most remarkable property of the obtained glass–ceramics is their zero or negative thermal expansion coefficient. Here, notable differences were observed: samples with fine grained microstructures showed thermal expansion coefficients approximately zero up to temperatures of around 80 °C. By contrast, samples with coarser microstructures and large spheroidal crystals exhibit negative expansion coefficients up to temperatures of around 280–375 °C. The thermal expansions of these samples were close to those of the mean thermal expansion of the unit cell of the BaO · Al2O3 · B2O3 phase. The thermal expansion of the fine grained samples was approximately equal to that of the crystallographic a-axis of the BaO · Al2O3 · B2O3 phase.