Devitrification, also known as devitrification, is a natural flaw that occurs in quartz glass. From a thermodynamic perspective, quartz glass has a higher internal energy compared to crystalline cristobalite, making it a metastable state when the temperature exceeds 1000°C. As the temperature rises, the vibration of SiO₂ molecules increases, leading to crystal formation after a long period of molecular reorganization and alignment. The rate at which devitrification occurs depends on the nucleation growth rate. For opaque quartz glass, the maximum crystallization rate is observed at around 1520°C, while for transparent quartz glass, this peak occurs at approximately 1620°C.
The process of crystallization primarily occurs on the surface, followed by internal defects. These areas are more prone to contamination, which can lead to the accumulation of impurity ions—especially alkali ions like K, Na, Li, Ca, and Mg. These ions can enter the glass network, reducing its viscosity and accelerating the devitrification process.
Quartz glass has a thermal expansion coefficient and specific gravity similar to those of β-cristobalite, the crystallized form. When used continuously at high temperatures, the crystallized region expands gradually, but the overall volume change remains minimal, allowing the material to function effectively. This helps reduce plastic deformation and enhances the refractory properties of the glass. However, when the crystallized layer cools down to about 800°C, a fine crack network may develop. Further cooling to between 200°C and 275°C triggers a structural transformation of cristobalite from the high-temperature β-form to the low-temperature α-form. This phase transition causes a volume change, and if the crystallized layer is deep, it can lead to cracking or even rupture of the quartz glass.
Since devitrification often occurs in regions with impurities, the condition of the glass surface before high-temperature use, along with the surrounding refractories and atmosphere, plays a crucial role. Proper handling and environmental control are essential to minimize the risk of devitrification. Additional precautions and best practices for using quartz glass will be discussed in the following sections.
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