Research on Processing and Annealing Techniques of Quartz Glass

This research on the processing and annealing techniques of quartz glass is aimed at fiber optic production and related projects. It seeks to improve the stability of quartz products at high and normal temperatures through practice, ensuring smooth application of products in various scenarios.

Quartz glass is classified by processing methods, uses, and appearance, such as fused transparent quartz glass, fused quartz glass, gas-refined transparent quartz glass, synthetic quartz glass, opaque quartz glass, optical quartz glass, quartz glass for semiconductors, and quartz glass for electric light sources. These are divided into two main categories: transparent and opaque. Based on purity, it is divided into three categories: high purity, ordinary, and doped.

The devitrification of high-temperature-resistant quartz glass is an inherent defect. Quartz glass has higher internal energy than crystalline quartz, making it a thermodynamically unstable metastable state. SiO2 molecules accelerate vibration and form crystals after long-term rearrangement and orientation. Crystallization mainly occurs on the surface, followed by internal defects, as these areas are prone to contamination, leading to localized accumulation of impurity ions. Particularly, alkali ions (such as K, Na, Li, Ca, Mg) reduce viscosity when entering the network, accelerating devitrification.

This paper discusses processed quartz components, covering only transparent synthetic capacitor quartz glass.

When processing quartz glass, a hydrogen-oxygen flame is typically used, with a processing temperature of about 1500-1600°C.

Glass is a poor conductor of heat. When a piece of quartz glass (without pressure) is heated or cooled, the outer layer of the quartz glass is directly heated or starts to cool first, and the internal glass is heated (heat conduction transfers external heat to the inside) or cooled afterward. This creates a temperature difference between the surface and the interior of the quartz glass. When heated, the surface temperature of the directly heated quartz glass is high, and the internal temperature of the quartz glass receiving heat is low, causing the outer layer of the heated quartz glass to expand. The lower temperature interior tries to maintain its original state, hindering the expansion of the outer layer. Thus, expansion and anti-expansion occur within the quartz glass, creating two types of stress due to interaction: compressive stress and tensile stress. The force trying to prevent the outer layer of quartz glass from expanding inward and acting on the outer layer is called compressive stress, while the force exerted by the outer layer of quartz glass expanding inward is known as tensile stress.

Since the compressive strength of quartz glass is much greater than its tensile strength, the inner and outer layers of quartz glass can withstand significant temperature differences during heating. When processing with a lamp, quartz glass can be directly heated in a hydrogen-oxygen flame without breaking. Conversely, when quartz glass heated to 500°C or higher is placed in cooling water, it easily cracks.

The stress distribution generated by lamp processing is roughly as follows:

1. Stress in Rotational Melting The operator’s hands rotate and melt the glass tube in the torch flame. Since the glass tube is heated by rotation rather than in the molten part, stress manifests as circular lines.
2. Stress in Side Melting For openings, side connections, and transverse inner core welding of quartz tubes, the quartz tube does not rotate, resulting in a different stress distribution than mentioned above. At this time, the stress is distributed around the molten part.
3. Stress in Ring Joints Ring joints refer to the welding of the inner core.
4. Stress in Sealed Ends of Jacket Products Quartz instrument jacket products come in various forms but are all sealed. For example, in a standard straight condenser tube, when both ends are sealed, stress is present not only on the outer jacket but also on the inner core, leading to significant stress.

The magnitude of stress varies with the temperature difference and thickness of the quartz glass. The larger the temperature difference and the thicker the glass, the greater the stress. Therefore, stress removal is particularly important.

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

Temporary stress occurs when the temperature change of the glass is below the strain point temperature, resulting in uneven total heat due to poor thermal conductivity, creating certain thermal stress. This thermal stress exists due to the temperature difference and is known as temporary stress.

It should be noted that since the quartz core rods usually processed contain different chemical substances, they are prone to uneven heating. Therefore, after splicing, the flame should be used to evenly heat the rod body, making the overall temperature gradient as smooth as possible, significantly reducing the temporary stress of the quartz core rod.

When glass cools from above the strain point temperature, the thermal stress generated by the temperature difference does not completely disappear after cooling to room temperature, leaving some stress in the glass. The magnitude of permanent stress depends on the cooling rate above the strain point temperature, the viscosity of the quartz glass, the coefficient of thermal expansion, and the thickness of the product.

As mentioned above, the permanent stress generated after processing the quartz rod affects subsequent processing and production. Therefore, permanent stress can only be eliminated through annealing.

Generally, glass products are annealed after processing. Annealing refers to a heat treatment process between the transition temperature and the strain point temperature to eliminate thermal stress generated during the manufacturing process. Typically, the larger the expansion coefficient of the glass, the larger the diameter and the more complex the product state, the more severe the stress. As mentioned earlier, the quartz rod contacted has a large diameter and contains mixed core rods, so strict heat treatment is required to remove stress.

In actual production, it is impossible to completely eliminate the stress within the rod body during annealing of the quartz rod. However, the residual amount is so small that it is not easily detected even under a polariscope.

Theoretically, the highest annealing temperature means that 95% of the stress can be eliminated after 3 minutes; the lowest annealing temperature results in a 5% stress release after 3 minutes. In production practice, the commonly used temperature is 50°C lower than the highest annealing temperature and 100°C higher than the lowest annealing temperature. There are many ways to anneal, but the main method is annealing in a furnace, which is the focus of this discussion.

According to the annealing principle mentioned above, the annealing of quartz glass is divided into four stages: heating stage, constant temperature stage, cooling stage, and natural cooling stage.

1. Heating Stage For quartz glass, this work is based on the annealing requirements of optical products. The entire heating process involves slow heating 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.
2. Constant Temperature Stage When the quartz rod reaches the actual highest annealing temperature, the furnace body is kept at a constant temperature to ensure uniform heating of the product, preparing it for the next cooling step.
3. Cooling Stage To eliminate or produce very little permanent stress during the cooling process of the quartz rod, the temperature should be slowly reduced to prevent a large temperature gradient. The cooling rates are as follows:
   • 1100°C to 950°C: 15°C/hour
   • 950°C to 750°C: 30°C/hour
   • 750°C to 450°C: 60°C/hour
4. Natural Cooling Stage Below 450°C, the power to the annealing furnace is turned off, and the environment is maintained without changing the insulation environment until it naturally cools to below 100°C. Below 100°C, the insulation environment is opened, and it cools to room temperature.

The time and temperature involved in the above steps are based on theoretical and production practice results. Figure 1 shows failed experimental products due to uneven heating caused by too short heating or constant temperature time.

In the process of producing and processing quartz glass, stress exists in the products at any stage, whether temporary or permanent. Methods such as “flame,” “HF acid,” and “annealing furnace” can be used to remove temporary stress or reduce permanent stress. Removing stress is crucial for improving the mechanical stability and optical uniformity of quartz products.

At GlobalQT (Global Quartz Tube), we specialize in high-quality quartz glass products with customizable solutions to meet your specific needs. For more information, visit our website or contact us via email at contact@globalquartztube.com.