Thermal Expansion of BaO · 2B2O3 in the Vitreous and Crystalline States: I. Thermal Expansion and Density of Barium Diborate Prepared from Monolithic Glass of the Same Composition

The thermal expansion and density of vitreous and polycrystalline barium diborates prepared by crystallization of monolithic glasses of stoichiometric composition are investigated in the temperature range 20–700°C. The glass crystallizes in the form of the α and β modifications depending on the heat treatment conditions. The α phase nucleates on the surface of the monolithic sample and grows inward the sample in the form of textured layers. The β phase nucleates in the melt bulk and grows in the form of spherulites. Textured polycrystalline barium diborate in the form of the α phase possesses a pronounced anisotropy of thermal expansion with respect to the direction of crystalline-layer growth. This is associated with the negative expansion along one of the crystallographic axes. The effective temperature coefficients of thermal expansion in the temperature range 20–600°C increase in the order: α phase-glass-β phase. Upon heating above 690°C, the β phase transforms partially into the α phase. This is accompanied by a considerable increase in the volume. The densities of the glass and polycrystalline samples in the temperature range 20–700°C increase in the order: glass-α phase–β phase.