Optical property of Li2O-Al2O3-SiO2 glass-ceramic crystallized under high pressure

The optical properties of Li2O-Al2O3-SiO2 glass-ceramic, with a near-zero thermal expansion coefficient, crystallised under high pressure were studied. A hydrostatic pressure of 196 MPa was applied by a hot isostatic pressing (HIP) technique during the nucleation process and the crystal growth process. The temperature coefficient of the optical path length, 1/l.dS/dT, decreased by 0.6 x 10 exp(-6)/K, because the temperature coefficient of the refractive index, dn/dT, was decreased by the HIP treatment. The decrease in dn/dT was ascribed to the suppression of the temperature coefficient of the molar polarisability, phi, of the glass-ceramic crystallised by the HIP treatment. The suppression of phi occurs in the crystalline phase during the HIP treatment, based on the fact that the reduced 1/l.dS/dT was maintained even after the relaxation of the densified structure in the glass phase upon thermal annealing. The decrease in the number of lattice defects in the crystalline phase was hypothesised to be a cause of the reduced phi. 12 refs.