Kinetics and Mechanism of Crystallization of Lithium Silicate Glasses

Abstract

Crystallization of the Li₂O⋅(1.5-3.5) SiO₂ glasses were followed by DTA, X-ray examination, optical- and electron-microscopic observations. Mechanism of the crystallization process was proposed in connection with microstructure of glasses.
Generally, crystallization initiated at the glass surface, except Li₂O⋅1.5SiO₂, with zero induction period, and grew at constant rate with time at the temperatures 550°-640°C. Li₂O⋅1.5SiO₂ glass nucleated a number of crystals inside at any given temperature, and the determination of the thickness of crystallized layer was practically impossible.
Crystallization of the glasses proceeded mainly in accordance with the phase diagram. For 1.5 and 1.75SiO₂ glasses, crystalline phases were Li₂O⋅SiO₂ and Li₂O⋅2SiO₂, and for (2.0-3.5) SiO₂ glasses, Li₂O⋅2SiO₂ was the only detectable crystalline phase.
DTA suggested that exothermic peak due to crystallization appeared at higher temperature with increasing SiO₂ content, but (2.5-3.5) SiO₂ glasses had their peaks at exactly same temperature.
Rate of crystallization was largest for Li₂O⋅1.75SiO₂ glass and decreased with increasing of SiO₂ content. However, for (2.5-3.5) SiO₂, rates were substantially identical: 1×10^-7cm/sec at 550°C, 1×10^-6cm/sec at 600°C and 3×10^-6cm/sec at 630°C. Crystallization rate of Li₂O⋅2SiO₂ and Li₂O⋅1.75SiO₂ glasses were 2.5-3.0 and 3.5-4.0 times larger respectively compared with the values described above at all over the temperature range examined.
Activation energies in the range 61-74kcal/mole were obtained.
Electron-microscopic observations suggested that Li₂O⋅(1.5-2.0)SiO₂ glasses had homogeneous structure, but (2.5-3.5) SiO₂ glasses heterogeneous even immediatly after the normal air quenching of melts.
Consequently, it is conceivable that the rate of crystallization is more a consequence of microstructure in glass than of their bulk compositions as far as (2.5-3.5) SiO₂ glasses concern.
X-ray showed that Li₂O⋅2SiO₂ crystals nucleated at glass surface grew inward along c-axis.
Crystallization of heterogeneous structure glasses proceeded only at the matrix part just as if it proceeded in single phase glass, i.e. independently to the existence of dispersed particles.
Chemical composition of matrix glasses is believable to be almost exactly same for (2.5-3.5) SiO₂ glasses and to be close to Li₂O⋅2SiO₂. Determination of crystallization rate, however, suggested that it is not identical to Li₂O⋅2SiO₂ but must be somehow rich in SiO₂, i.e. some very limited composition between Li₂O⋅2SiO₂ and Li₂O⋅2.5SiO₂. Excess SiO₂ in matrix glass is conceivable to form solid solution with Li₂O⋅2SiO₂.