The crystallization phenomenon of quartz glass refers to the process in which the SiO _ 2 molecules in quartz glass are rearranged, oriented, and locally crystallized (mainly quartz) under high temperature or specific conditions. During this process, the originally transparent or translucent quartz glass will gradually lose its transparency, become opaque or appear white crystalline spots.

thermodynamic properties

From a thermodynamic point of view, quartz glass has a higher internal energy than crystalline quartz, so it belongs to a thermodynamically unstable metastable state. When used at high temperatures for a long time, quartz glass will tend to thermodynamically stable α-quartz phase transition, which is the phenomenon of crystallization.

temperature factor

Crystallization is closely related to temperature. When the temperature is higher than a certain threshold (e.g. above 1000 ° C), the vibration of SiO2 molecules will accelerate, and after a period of rearrangement and orientation, crystallization may be formed. Especially when the temperature reaches or exceeds 1300 ° C, it has a great impact on the crystallization of quartz glass, because the energy of the glass increases, the viscous flow intensifies, and a certain amount of liquid phase may appear. These conditions are conducive to the crystallization of quartz. On the contrary, when the temperature does not exceed 1100 ° C, the effect on the crystallization of quartz glass is not obvious.

Impurities and induction

Impurities in quartz glass, especially alkali metal ions (such as K, Na, Li) and alkaline earth metal ions (such as Ca, Mg), can greatly reduce the crystallization temperature and accelerate the crystallization process. The presence of these impurity ions in the glass will cause a decrease in viscosity and promote the acceleration of devitrification. In addition, the surface state of quartz glass is also very important. If the surface is contaminated, even a small amount of contamination (such as sweat stains that are not visible to the human eye, alkaline dust, etc.) will play an inducing role, causing the crystallization temperature of quartz glass to drop and crystallize rapidly.

Production process and structural characteristics

The production process of quartz glass also affects its crystallization performance. For example, if there are deficiencies in the production process, such as the melting temperature is not high enough, the time is not long enough, or the cooling speed is not fast enough, etc., there may be unmelted quartz particles or trace crystal nuclei left in the quartz glass, which will become the starting point for crystallization. In addition, the structural characteristics of quartz glass, such as internal defects, bubbles, etc., also easily become a concentration of impurity ions, thereby accelerating the crystallization process.

Effect of crystallization on quartz glass

Crystallization can cause quartz glass to lose its transparency while weakening its strength. However, in some cases, crystallization can also improve the refractoriness or softening point of quartz glass. But in general, crystallization is unfavorable to the performance of quartz glass, especially when used at high temperatures. Crystallization can cause quartz glass to break or mechanical failure.

The crystallization phenomenon of quartz glass is due to its thermodynamic properties, temperature factors, impurities and induction, production process and structural characteristics and other factors. Therefore, when using quartz glass, it is necessary to pay attention to temperature control, avoid impurity pollution, optimize production process and other measures to reduce the occurrence of crystallization phenomenon.

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