Crystallization kinetics of a low-expansion feldspar glass for dental applications

Previous studies have shown that the crystallization of glasses in the system K2O-Al2O3-SiO2, in the primary field of leucite, is not possible without modification of the composition. Leucite (KAlSi2O6) is used as a reinforcing phase in some compositions for all-ceramic dental restorations. However, because of their higher coefficient of thermal expansion, these materials cannot be veneered with conventional metal-ceramic porcelains. The purposes of this study were to investigate the crystallization behavior of a glass in the system K2O-Al2O3-SiO2 and to evaluate the effect of heat treatment on the crystal size, percent crystallinity, and coefficient of thermal expansion of the material. Ion-exchanged glass powder was prepared by mixing the glass with rubidium nitrate and heat-treating at 450 degrees C for 4, 8, 24, or 48 h. Bars were made from these powders and baked under vacuum at 1038 degrees C for 2 min. The bars made from the powder ion-exchanged for 48 h were further heat-treated for 4 h at either 800 degrees C, 850 degrees C, 900 degrees C, 950 degrees C, or 1038 degrees C. X-ray diffraction analyses showed cubic leucite was the only crystalline phase in the specimens made from the powders ion-exchanged for 4, 8, 24, or 48 h and baked at 1038 degrees C for 2 min. Further heat treatment for 4 h at either 800 degrees C, 850 degrees C, 900 degrees C, 950 degrees C, or 1038 degrees C promoted the growth of cubic leucite. In addition, a second phase identified as tetragonal rubidium-leucite was present in the specimens heat-treated for 4 h at 1038 degrees C. SEM observations showed that all specimens made from the powders ion-exchanged for 48 h exhibited small spherical crystals dispersed in a glassy matrix. The percent crystallinity ranged from 18.9% to 42.9% and the average particle size was between 0.64 to 1.18 microns. The coefficients of thermal expansion ranged between 8.076 and 8.788 x 10(-6)/degree C.