Why does quartz need to be annealed

According to processing method, use, and appearance, quartz glass is classified into two categories: transparent and opaque. The transparent category includes types such as fused transparent quartz glass, fused quartz glass, gas-refined transparent quartz glass, and synthetic quartz glass. The opaque category consists of opaque quartz glass, optical quartz glass, quartz glass for semiconductors, and quartz glass for electric light sources. Additionally, quartz glass is divided into three categories based on purity: high purity, ordinary, and doped.

Devitrification is an inherent defect in high-temperature resistant quartz glass. The internal energy of quartz glass is higher than that of crystalline quartz, placing it in a thermodynamically unstable metastable state. As temperature increases, the vibration of SiO2 molecules accelerates, and over time, this leads to rearrangement and crystallization. The growth of crystallization primarily occurs on the surface, followed by internal defects. This is because these areas are more susceptible to contamination, resulting in the local accumulation of impurity ions. Alkali ions such as K, Na, Li, Ca, and Mg can lower the viscosity of the glass, thereby accelerating devitrification.

It is important to note that glass is a poor conductor of heat. When a piece of quartz glass (when not under pressure) is heated or cooled, the outer layer of the glass experiences a temperature change first. The outside heats up or cools down before the heat is conducted to the inside of the glass, creating a temperature difference between the surface and the interior. When heated, the outer layer of the quartz glass expands due to higher temperatures, while the cooler interior resists this expansion, maintaining its original state. This interaction produces two types of internal stress: "compressive stress," which acts on the outer layer to resist expansion, and "tensile stress," which is the force exerted by the expanding outer layer on the inner layer. Collectively, these forces are referred to as stress in the quartz glass.

Since the compressive strength of quartz glass is significantly greater than its tensile strength, both the inner and outer layers can withstand large temperature differences when heated. During lamp processing, quartz glass can be directly heated in a hydrogen-oxygen flame without breaking. However, if quartz glass heated to temperatures of 500°C or higher is suddenly placed in cooling water, it is likely to shatter.

Thermal stress in quartz glass products can be divided into temporary stress and permanent stress.

Temporary stress:

When the temperature change of glass is lower than the strain point temperature, the thermal conductivity is poor and the total heat is uneven, thus generating certain thermal stress. This thermal stress has a temperature difference. This thermal stress is called temporary stress. It should be noted that since the core layer of the quartz core rods produced and processed in normal times is mixed with different chemical substances, it is very easy to produce uneven heating. Therefore, after the splicing is completed, the temperature of the rod body is uniformed by a flame to make the overall temperature gradient as gentle as possible, thereby greatly eliminating the temporary stress of the quartz core rod.

Permanent stress:

When the glass is cooled from above the strain point temperature, the thermal stress generated by the temperature difference will not completely disappear after the glass is cooled to room temperature and the temperature of the inner and outer layers is equal. There is still a certain amount of stress in the glass. The size of the permanent stress depends on the cooling rate of the product above the strain point temperature, the viscosity of the quartz glass, the thermal expansion coefficient and the thickness of the product. After processing, the permanent stress generated has affected the subsequent processing and production. Therefore, the permanent stress can only be eliminated by annealing.

The annealing of quartz glass is divided into four stages: heating stage, constant temperature stage, cooling stage, and natural cooling stage.

Heating stage: For the requirements of quartz glass, this work is based on the annealing requirements of optical products. The entire heating process is slowly heated to 1100°C. According to experience, the temperature rise is 4.5/R2°C/min, where R is the radius of the quartz glass product.

Constant temperature stage: When the quartz rod reaches the actual maximum annealing temperature, the furnace body is subjected to constant temperature treatment to slow down the thermal gradient of the product and heat evenly at all positions. Prepare for the next cooling.

Cooling stage: In order to eliminate or generate very small permanent stress during the cooling process of the quartz rod, the temperature should be slowly reduced in this stage to prevent excessive temperature gradients. The cooling rate from 1100°C to 950°C is 15°C/hour. The cooling rate from 950°C to 750°C is 30°C/hour. The cooling temperature from 750°C to 450°C is 60°C/hour.

Natural cooling stage: below 450°C, cut off the annealing furnace power supply without changing the insulation environment to allow it to cool naturally to below 100°C. Below 100°C, open the insulation environment to allow it to cool to room temperature.

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