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Quartz glass manufacturing methods can be divided into two main categories.

High-Purity Quartz Glass Preparation Method Classifications

1. Natural Quartz Glass
The first category involves melting natural quartz crystals or silica to produce natural quartz glass (Natural Silica Glass). Depending on the heat source and process, this method can be further classified into:

Electric melting

Gas-flame fusion

Plasma melting

Electric Melting Method:

Quartz powder is melted in a crucible using electric heating (resistance or electromagnetic induction).

During melting, quartz undergoes phase transitions: from β-quartz → α-quartz → α-cristobalite (with amorphous phase formation), finally forming a quartz melt at ~1723°C.

The process is conducted under high vacuum (0.1–10 Pa) to remove gases and reduce bubble content.

Purity Limitation: Metal impurities in raw quartz are hard to eliminate, so electrically fused quartz glass typically has higher metal impurity levels.

Low Hydroxyl Content: Since moisture can be removed by drying, this method yields quartz glass with low OH⁻ content.

High-purity quartz sand (Image source: SIBICO)

2. Synthetic Quartz Glass
The second category involves synthesizing quartz glass from silicon-containing compounds (e.g., halides, hydrides, or organosilicon) via chemical reactions. Based on reaction mechanisms, this includes:

Thermal Oxidation in Anhydrous Environments

Example: Plasma Chemical Vapor Deposition (PCVD)

Hydrolysis in Hydrogen-Oxygen Flame (Flame Hydrolysis Deposition, FHD)

The most widely used industrial method, including:

Direct Synthesis (e.g., Chemical Vapor Deposition, CVD)

High-temperature hydrolysis produces molten SiO₂, directly deposited and cooled into quartz glass.

Indirect Synthesis (e.g., Modified CVD, Vapor Axial Deposition/VAD, Outside Vapor Deposition/OVD)

Low-temperature hydrolysis generates soot-like SiO₂, forming a porous body that undergoes dehydroxylation, doping, sintering, and cooling.

Applications:

CVD & Modified CVD: Large quartz glass ingots.

VAD & OVD: Optical fiber preforms.

Process comparison diagram of quartz glass ingot preparation by electric fusion and two-step CVD methods

3. Thermal Forming
Industrial applications often require specific shapes (e.g., tubes, rods for semiconductors). Thermal forming is the primary method:

Quartz glass is heated until soft, then shaped via sagging, drawing, etc.

Induction Heating: Alternating current in coils generates an electromagnetic field, heating the quartz.

Precision Control: Temperature and drawing speed determine the diameter of rods/tubes. Uniform heating is critical for quality.

Source: China Nonferrous Metals Industry Association Silicon Branch. Information provided is for reference only. Contact us for copyright concerns.