How does stress develop in quartz materials?

How does stress develop in quartz materials?

1. Thermal Stress During Cooling (Primary Cause)

Quartz glass develops internal stress when exposed to non-uniform temperatures. At any given temperature, quartz glass exhibits a specific atomic structure that is most "suitable" or stable under those thermal conditions. The spacing between atoms changes with temperature—this is known as thermal expansion. When quartz glass experiences uneven heating or cooling, differential expansion occurs.

Stress typically arises when hotter regions attempt to expand but are constrained by surrounding cooler areas. This results in compressive stress, which usually does not damage the product. If the temperature is high enough to soften the quartz glass, the stress may be relieved. However, if the cooling process is too rapid, the viscosity of the material increases too quickly, and the atomic structure cannot adjust in time to accommodate the temperature drop. This leads to the formation of tensile stress, which is more likely to cause structural damage.

Stress increases progressively as the temperature drops and can reach high levels after cooling ends. In fact, when the viscosity of quartz glass exceeds 10^4.6 poise, the temperature is referred to as the strain point—at this stage, the viscosity is too high for stress relaxation to occur.

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2. Stress from Phase Transition and Structural Relaxation

• Metastable Structural Relaxation: In the molten state, quartz exhibits a highly disordered atomic arrangement. During cooling, atoms attempt to transition toward a more stable configuration. However, due to the high viscosity of the glassy state, atomic movement is limited, leaving the structure in a metastable state. This generates relaxation stress, which may be slowly released over time (as observed in the aging phenomenon in glasses).

• Microscopic Crystallization Tendency: If molten quartz is held at specific temperature ranges (e.g., near the devitrification temperature), microscopic crystallization may occur (e.g., precipitation of cristobalite microcrystals). The volume mismatch between crystalline and amorphous phases can induce phase transition stress.

3. External Loads and Mechanical Actions

1) Stress Induced During Machining

Mechanical processing such as cutting, grinding, and polishing can introduce surface lattice distortion, resulting in machining stress. For example, cutting with a grinding wheel generates localized heat and mechanical pressure at the edge, leading to stress concentration. Improper techniques during drilling or slotting can create notches that act as crack initiation sites.

2) Load Stress in Service Environments

When used as a structural material, fused quartz may bear mechanical loads such as pressure or bending, generating macroscopic stress. For instance, quartz containers holding heavy substances develop bending stress.

4. Thermal Shock and Sudden Temperature Changes

1) Instantaneous Stress from Rapid Heating or Cooling

Although fused quartz has an extremely low coefficient of thermal expansion (~0.5×10⁻⁶/°C), rapid temperature changes (e.g., heating from room temperature to high temperatures or immersion in ice water) can result in localized thermal expansion or contraction, causing instantaneous thermal stress. Laboratory glassware made of quartz may fracture under such thermal shocks.

2) Cyclic Temperature Fluctuations

Under long-term cyclic thermal environments (e.g., furnace linings or high-temperature optical windows), repeated thermal expansion and contraction can accumulate fatigue stress, accelerating material aging and cracking.

5. Chemical Effects and Stress Coupling

1) Corrosion and Dissolution Stress

When fused quartz comes into contact with strong alkaline solutions (e.g., NaOH) or high-temperature acidic gases (e.g., HF), its surface may undergo chemical corrosion or dissolution, disrupting structural uniformity and causing chemical stress. Alkaline attack can cause surface volume changes or form microcracks.

2) CVD-Induced Stress

In chemical vapor deposition (CVD) processes, coating quartz with materials like SiC may introduce interfacial stress due to mismatches in thermal expansion coefficients or elastic moduli between the film and the substrate. Upon cooling, such stress may cause film delamination or substrate cracking.

6. Internal Defects and Impurities

1) Bubbles and Embedded Impurities

During melting, residual gas bubbles or impurities (e.g., metal ions or unmelted particles) may become trapped in fused quartz. The difference in physical properties (e.g., thermal expansion coefficient or modulus) between these inclusions and the surrounding glass can lead to localized stress concentration, increasing the risk of crack formation around bubbles under load.

2) Microcracks and Structural Defects

Impurities in raw materials or melting defects can lead to microcracks in the quartz. When subjected to external loads or temperature fluctuations, stress concentration at crack tips can intensify, accelerating crack propagation and ultimately compromising the material's integrity.