Crystallization of the Li
2O⋅(1.5-3.5) SiO
2 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 Li
2O⋅1.5SiO
2, with zero induction period, and grew at constant rate with time at the temperatures 550°-640°C. Li
2O⋅1.5SiO
2 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 1.75SiO
2 glasses, crystalline phases were Li
2O⋅SiO
2 and Li
2O⋅2SiO
2, and for (2.0-3.5) SiO
2 glasses, Li
2O⋅2SiO
2 was the only detectable crystalline phase.
DTA suggested that exothermic peak due to crystallization appeared at higher temperature with increasing SiO
2 content, but (2.5-3.5) SiO
2 glasses had their peaks at exactly same temperature.
Rate of crystallization was largest for Li
2O⋅1.75SiO
2 glass and decreased with increasing of SiO
2 content. However, for (2.5-3.5) SiO
2, 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 Li
2O⋅2SiO
2 and Li
2O⋅1.75SiO
2 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 Li
2O⋅(1.5-2.0)SiO
2 glasses had homogeneous structure, but (2.5-3.5) SiO
2 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) SiO
2 glasses concern.
X-ray showed that Li
2O⋅2SiO
2 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) SiO
2 glasses and to be close to Li
2O⋅2SiO
2. Determination of crystallization rate, however, suggested that it is not identical to Li
2O⋅2SiO
2 but must be somehow rich in SiO
2, i.e. some very limited composition between Li
2O⋅2SiO
2 and Li
2O⋅2.5SiO
2. Excess SiO
2 in matrix glass is conceivable to form solid solution with Li
2O⋅2SiO
2.
View full abstract