Grain-oriented surface crystallization of lanthanum borosilicate and lanthanum borogermanate glasses

Under appropriate conditions (high temperature gradient, rapid heating and following prolonged isothermal treatment, suppression of bulk crystallization, polished glass surface, etc.) the La2O3-B2O3-XO2 (X = Si, Ge) systems glasses are crystallized in preference on the surface with formation of perfect textures based on needle-shaped stillwellite-like crystals of the LaBXO5 composition. Lanthanum borogermanate (LBG) glasses exhibit more intense texture-forming ability than lanthanum borosilicate (LBS) ones. Glass ceramic textures with a degree of grain orientation up to 0.9 in the LBG system are obtained. The pyroelectric coefficient values of the best of synthesized LBG textures are as high as 0.5–1.5 nC cm−2 K−1 in a wide temperature range. A poor crystallization ability of the LBS glasses at temperatures less than 1000°C supposed to be used in preparing both thin polar crystalline films on the glass surface for nonlinear optical applications and ferroelectric (pyroelectric) glass ceramics by the powder technology.