​Why Does Quartz Require Annealing?​

Quartz glass is classified by ​​processing method​​, ​​application​​, and ​​appearance​​ into categories such as fused transparent quartz glass, gas-refined transparent quartz glass, synthetic quartz glass, opaque quartz glass, optical quartz glass, semiconductor-grade quartz glass, and photoelectric quartz glass. Based on purity, it is further categorized into ​​high-purity​​, ​​standard​​, and ​​doped​​ types.

​​Devitrification​​ (crystallization) is an inherent defect in quartz glass. Due to its ​​metastable state​​ with higher internal energy than crystalline cristobalite, SiO₂ molecules vibrate and gradually rearrange into crystalline structures over time. This process accelerates in regions with impurities (e.g., alkali ions like K, Na, Li, Ca, Mg), which reduce viscosity and promote nucleation. Crystallization typically initiates at surfaces and propagates inward, forming defects.

· Thermal Stress Formation​​

As a poor thermal conductor, quartz glass develops ​​thermal gradients​​ during heating/cooling. For example:

• ​​Heating​​: Surface layers expand faster than the cooler interior, generating ​​compressive stress​​ (surface) and ​​tensile stress​​ (interior).
• ​​Cooling​​: Rapid cooling induces ​​tensile stress​​ at the surface and ​​compressive stress​​ internally.

Quartz’s high compressive strength tolerates thermal shocks during flame processing (e.g., hydrogen-oxygen flame welding). However, abrupt cooling (e.g., quenching in water at >500°C) causes fractures due to excessive tensile stress.

· Stress Types​​

1. ​Temporary Stress​​: Generated below the ​​strain point​​ (where viscosity limits stress relief). Resolved by temperature equalization.
2. ​​Permanent Stress​​: Remains after cooling to room temperature, caused by thermal gradients during cooling above the strain point. Affects subsequent processing and requires ​​annealing​​ to eliminate.

· Annealing Process

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Annealing involves four stages to redistribute internal stress:

1. Heating​​: Gradually raise temperature to 1100°C at a rate of ​​4.5/R² °C/min​​ (R = radius of the quartz product) to avoid thermal shock.

​​2. Soaking​​: Maintain peak temperature (1100–1150°C) to homogenize thermal gradients and reduce stress.

3. ​​Cooling​​:

• ​​1100–950°C​​: 15°C/h
• ​​950–750°C​​: 30°C/h
• ​​750–450°C​​: 60°C/h

Slow cooling minimizes residual stress.

​​4. Natural Cooling​​: Below 450°C, disconnect furnace power and allow gradual cooling to <100°C, then ambient temperature.

· Key Considerations​​

• ​​Virtual Temperature​​: Rapid cooling creates a metastable state (virtual temperature > annealing temperature), causing post-annealing dimensional shifts. Pre-annealing minimizes this effect.

• ​​Optical Applications​​: Critical for stress-free optical components; residual stress alters refractive index uniformity.

By systematically addressing thermal gradients and stress mechanisms, annealing ensures quartz glass’s mechanical stability, optical clarity, and long-term performance in high-temperature or precision applications.

ZMSH specializes in the R&D and production of advanced annealing processes for quartz glass, offering comprehensive solutions from process design and custom equipment development to rigorous quality testing. Our services cater to diverse quartz products, including semiconductor-grade, optical-grade, medical-grade, and industrial-grade quartz components, addressing critical annealing requirements across industries.