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 cry stallization, polished glass surface, etc.) the La2O3-B2O3-XO2 (X = Si , Ge) systems glasses are crystallized in preference on the surface wi th formation of perfect textures based on needle-shaped stillwellite-l ike crystals of the LaBXO5 composition. Lanthanum borogermanate (LBG) glasses exhibit more intense texture-forming ability than lanthanum bo rosilicate (LBS) ones. Glass ceramic textures with a degree of grain o rientation up to 0.9 in the LBG system are obtained. The pyroelectric coefficient values of the best of synthesized LEG textures are as high as 0.5-1.5 nC cm(-2) K-1 in a wide temperature range. A poor crystall ization ability of the LBS glasses at temperatures less than 1000 degr ees C supposed to be used in preparing both thin polar crystalline fil ms on the glass surface for nonlinear optical applications and ferroel ectric (pyroelectric) glass ceramics by the powder technology. (C) 199 7 Elsevier Science S.A.