Crystallization and properties of crystallized product of a Li<sub>2</sub>O-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>-F glass

Abstract

Crystallization of a glass having the composition SiO2 63.2, Al2O3 20.1, MgO 2.6, MgF2 8.2, Li2O 5.9% in weigt and some properties of the crystallized glass were investigated.The results are summarized as follows;1) Small unknown crystals of about 80 Å in size were already found in the original glass, which might have precipitated during cooling of the melt. Heat treatment of the glass caused first the precipitation of another kind of unknown crystal and subsequently the precipitation of the main crystal, that is, β-quartz solid solution. It was conjectured that the presence of the former crystals might have facilitated the crystallization of the latter crystal in the bulk of the glass.2) Crystallization of β-quartz solid solution at 680°C and 700°C was easier at the surface than in the interior of the samples. At 740°C, however, little difference was observed in the amount of the precipitated β-quartz solid solution between the surface and the interior supposedly because of a rapid in increase of the amount of the crystal throughout the sample. β-quartz solid solution precipitated at 700°C transformed into β-spodumene solid solution at higher temperatures than 950°C.3) The bending strength of the crystallized glass increased somewhat with time of heat-treatment at 680°C and 700°C, while no increase was observed at 840°C. The bending strength of the glass heated at 700°C for 5hours and then further heat-treated at 800°C, 900°C, 950°C or 1000°C for 3 to 8 hours was slightly smaller than that of the glass subjected to the only heat-treatment at 700°C. The bending strength decreased markedly upon heating up to 1050°C.4) The thermal expansion coefficient of the crystallized glass was relatively low (3-7×10-7/°C) when the glass was heat-treated at 800°C or 900°C after the heat treatment at 700°C for 5 hours and relatively high (23×10-7/°C) when heat treated at 950°C. The thermal expansion coefficient decreased again slightly (18×10-7/°C) when the glass was heat-treated at 1000°C and decreased markedly (0×10-7/°C) when heat-treated at 1050°C.