Abstract
The effects of additives, such as B2O3, P2O5, ZnO, MgF2 and Fe2O3, on the microstructure evolution under different thermal treatment conditions in a low thermal expansion lithium aluminosilicate glass-ceramic material with commercial-like compositions are studied. It can be found that the effects on micro- and electronic structural development accompanied with different types of additives are varied. Samples are prepared by the standard bulk quench method and heated by one or two-step thermal programs to achieve vitreous to crystalline phase transition. The analytical work is emphasized on the results obtained from these techniques: X-ray powder diffraction, X-ray absorption near edge structure and IR absorption spectroscopy. A few results can be summarized as follows: (1)During the devitrification process, the coordination number of Ti (the nucleation agent) changes from 4 and 5 to 6; (2)For highly crystalline samples with commercial-like composition, Zn ions tend to have spinel-like local environments; while in a simplified composition, Zn ions favor hexagonal ZnO-like surroundings; (3)At low doping level (~<0.2%), most of Fe ions remain in disorder states during devitrification process; at high doping level(~>0.6%), some of these ions migrate into order environments, such as the Li site of the main crystalline phase; (4)The crystallization and main phase transformation temperatures are often lowered for samples with these additives; while these phase change temperatures are increased for samples with high Fe2O3 doping concentration (~>0.6%); (5)The microstructural uniformity becomes irregular as the doping level of P2O5, MgF2 or Fe2O3 is increased. As a result, the flexural strengths of these samples are decreased. The possible mechanisms of changes by the additives in viscosity, stoichiometry and cation field strength are proposed to be responsible for the above phenomenon.