​​Annealing Process of Quartz Glass​​

Quartz glass generates stress when subjected to non-uniform temperatures. At any specific temperature, quartz glass has a corresponding atomic structure, where the spatial arrangement of atoms is "optimal." Atomic gaps change with temperature, a phenomenon termed thermal expansion. When quartz glass is heated unevenly, it causes differential expansion. Stress arises when heated regions expand outward but are constrained by cooler surroundings—a compressive stress that typically does not damage the product. However, if the temperature drops too rapidly, the viscosity increases too quickly, preventing atomic structures from adjusting to the lower temperature, resulting in tensile stress that can cause fractures. Stress accumulates as temperature decreases, reaching critical levels upon cooling completion. The temperature at which quartz glass viscosity exceeds 10¹⁴.⁶ poise is termed the ​​strain point​​; at this stage, stress cannot be relieved due to high viscosity.

Annealing Parameters​​

To mitigate stress, quartz glass must be heated to a temperature allowing atomic rearrangement (typically ≤1 hour). The ​​annealing point​​, defined as the temperature 15 minutes after heating begins, is slightly above the strain point (~10–100 poise viscosity). For low-water-soluble fused quartz, strain and annealing points are approximately 1050°C and 1080°C, respectively. Annealing does not require reaching the annealing point; it can occur at any temperature above the strain point, with lower temperatures requiring longer durations. However, excessively high temperatures risk opacity and increased residual stress.

Optimally, annealing at 1150–1180°C for 20–30 minutes balances stress relief and minimizes opacity. Key principles include using the ​​lowest peak temperature​​ feasible and maintaining uniform temperature gradients. Post-annealing cooling must be controlled:

• ​​First 200°C cooling phase​​: ≤100°C/min to avoid thermal shock.
• Subsequent cooling: Accelerate as permitted by thermal expansion coefficients.

Residual Stress and Virtual Temperature​​

Even after stress relief, residual stresses persist due to cooling-induced thermal gradients. These stresses are minimized by slowing cooling during initial stages. Residual stress impacts non-optical applications minimally but may cause detectable dimensional changes in thick components.

Virtual Temperature Phenomenon​​

Rapid cooling creates a ​​virtual temperature​​—a metastable state where atomic structures mimic high-temperature configurations. If virtual temperature exceeds the annealing temperature, density changes occur. For example, a quartz rod annealed at 1150–1180°C (vs. virtual temperature of 1250°C) shrinks by 0.01–0.03 mm, risking dimensional nonconformity.

Practical Considerations​​

• ​​Long-bar components​​: Pre-anneal raw materials to reduce virtual temperature and post-processing warpage.
• ​​Optical applications​​: Strict cooling rates ensure refractive index uniformity.
• ​​Stress detection​​: Polarized light analysis identifies residual stress in thick sections.

Conclusion​​

Precise temperature control and gradient management during annealing are critical to preserving quartz glass integrity. By adhering to strain point principles and virtual temperature dynamics, manufacturers achieve optimal stress relief while maintaining dimensional and optical stability.

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