Devitrification, also referred to as devitrification, is a natural defect that occurs in quartz glass. From a thermodynamic perspective, the internal energy of quartz glass is higher than that of crystalline cristobalite, making it a metastable state when temperatures exceed 1000°C. At these high temperatures, the SiO₂ molecules vibrate more rapidly, leading to crystal formation after prolonged rearrangement and orientation. The rate at which nuclei form determines the extent of devitrification. For opaque quartz glass, the crystallization rate peaks at around 1520°C, while for transparent quartz glass, this maximum occurs at approximately 1620°C.
The process of crystallization typically begins on the surface and then progresses into internal defects. These areas are more prone to contamination, which can lead to the accumulation of impurity ions—especially alkali ions like potassium (K), sodium (Na), lithium (Li), calcium (Ca), and magnesium (Mg). These ions can disrupt the glass network, reducing 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. As a result, when used continuously at high temperatures, the crystallized region expands gradually without significant volume changes, allowing the material to remain functional. This helps reduce plastic deformation and improves its refractory properties. However, when the material cools below 800°C, a fine crack network may develop. Further cooling between 200°C and 275°C causes a structural transformation of cristobalite from the high-temperature β-form to the low-temperature α-form. This phase change is accompanied by 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 in preventing unwanted crystallization. Proper handling and environmental control are essential to maintaining the integrity of quartz glass in high-temperature applications. Additional guidelines on safe usage will be discussed in the following sections.
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