Effect of alkali metal Oxides on viscosity and crystallization of the MgO-Al2O3-SiO2 glasses

MgO-Al2O3-SiO2 glasses were prepared by using TiO2 and ZrO2 as nucleation agents, and 2% (wt%, the same as below) content of alkali metal oxide was added. The viscosity of MAS glass melt was investigated by the rotating crucible viscometer, and the melting temperature and activation energy for viscous flow of the melt were derived by the Arrhenius equation. After melting and casting, the glass samples were crystallized in the temperature range from 880 to 1145 °C. The prepared glass-ceramics presented transparent to opaque with the increasing of crystallization temperature, which had been investigated by X-ray diffraction and field emission-environment scanning electron microscope. The results showed that the melting temperature of glass containing 2% Na2O was about 1585 °C. The MgAl2O4, ZrTiO4 and the sapphirine crystalline phases precipitated from matrix glass successively as the crystallization temperature raising. When 1% Na2O was replaced by Li2O, the melting temperature of the glasses decreased to 1508 °C, meanwhile, the main crystal phase changed from MgAl2O4 to high-/low-quartz solid solution, therefore, the crystallization behavior was uncontrollable and the transparent glass ceramic cannot be obtained. As 1% K2O substituted for Na2O, the melting temperature decreased to 1573 °C. The adding of K2O could broaden the crystallization temperature range of preparing transparent glass-ceramics. After crystallized at the range from 900 °C to 1000 °C, the fine spinel (MgAl2O4) crystals which had average size about 25 nm precipitated from matrix glasses, and the specimens kept transparent.