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Precision Quartz Glass Processing & Fabrication

We specialize in processing high-purity fused quartz glass for extreme environments. Our capabilities support semiconductor, optical, and high-temperature applications with superior thermal stability and chemical resistance, from prototype to volume production.
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What Is Quartz Glass (Fused Silica)

Quartz glass, also known as fused silica, is a high-performance material produced by melting high-purity silica (SiO₂) crystals at extremely high temperatures. Unlike conventional glasses (e.g., soda-lime), it contains virtually no additional network modifiers, resulting in a composition of over 99.9% SiO₂. This exceptional purity grants it a unique set of properties, including extreme thermal shock resistance, very low thermal expansion, high optical transparency from UV to IR wavelengths, and outstanding chemical inertness. It is an essential material for demanding applications in semiconductors, precision optics, high-temperature lighting, and laboratory equipment.

Max. Processing Size:

 Sheets over 5m in length, large-format panels

Min. Part Size:

 Precision micro-components down to 10mm.

Cutting Tolerance:

 ±0.1 mm(CNC/Waterjet),±0.05 mm(Laser)

Drilling/Milling Tolerance:

 ±0.1 mmposition &diameter)

Edge Profile Tolerance:

 ±0.1 mm

Standard Thickness Range:

 From 1.5 mm and above.

Ultra-Thin Processing Capability:

 Down to sub-1.0 mm(e.g.,0.3-0.7 mm for specialty applications)

Quartz Glass (Fused Silica): Detailed Technical Specifications & Characteristics

This table details the key performance parameters of high-purity quartz glass (fused silica), highlighting its defining properties of extreme thermal stability, wide optical transmission, and outstanding chemical durability. Refer to the specifications below for exact values on purity, CTE, softening point, and spectral range.
Parameter / Property Typical Value / Range Description & Implication
Primary Chemical Composition - Extreme purity defines its performance.
Silicon Dioxide (SiO₂) > 99.9% (up to 99.999% for high-purity grades) Near-pure amorphous silica. Absence of metallic impurities (like iron) enables superior optical and thermal properties.
Physical & Mechanical Properties - Combines hardness with a degree of brittleness.
Density ~2.2 g/cm³ Lower than most other technical glasses.
Knoop Hardness (HK) ~550 - 750 Very high surface hardness, providing excellent scratch and abrasion resistance.
Young's Modulus (Modulus of Elasticity) ~72 GPa High stiffness, similar to other glasses.
Thermal Properties (Key Advantage) - Outstanding performance under extreme temperature.
Coefficient of Thermal Expansion (CTE) ~0.55 x 10⁻⁶ /K Extremely low, providing unparalleled thermal shock resistance and dimensional stability.
Softening Point ~1680 °C Can operate continuously at temperatures over 1000°C where other glasses would melt or flow.
Maximum Service Temperature (Continuous) ~1100 - 1200 °C Exceptional for high-temperature applications.
Optical Properties (Key Advantage) - Excellent transmission across a wide spectrum.
Spectral Transmission Range Deep UV (~170 nm) to Near IR (~2500 nm) Transmits well in ultraviolet, visible, and infrared wavelengths, unlike most glasses.
Refractive Index (@587.6 nm) ~1.458 Low refractive index and high homogeneity are critical for precision optics.
Electrical & Chemical Properties - Superior insulation and chemical resistance.
Volume Resistivity (350°C) ~10⁷ - 10⁸ Ω·cm Maintains high electrical resistance at elevated temperatures, acting as an excellent insulator.
Chemical Durability Exceptional Highly resistant to water and acids (except hydrofluoric and hot phosphoric). Resists crystallization and devitrification.
Key Design & Fabrication Note High-Temperature Processing Machining (cutting, drilling) requires diamond tools. Final high-temperature flame Machining (cutting, drilling) requires diamond tools. Final high-temperature flame polishing or annealing (>1100°C) is often necessary to achieve optical surfaces and relieve stress.

Surface Finishing & Secondary Processing

We enhance Quartz Glass through specialized surface treatments. Our capabilities include screen printing for branding, coating (sputtering) for functional properties, sandblasting and acid etching for texture, along with precision polishing, CNC engraving, and durable painting/enameling. These processes customize glass for aesthetic, privacy, or performance needs in architectural, industrial, and consumer applications.

Screen Printing

Applies decorative patterns, logos, or functional borders using ink fired onto the glass for permanence and color.

Coating (e.g., Sputtering)
Deposits thin metallic or ceramic layers to create reflective, low-E, or anti-reflective properties for energy control.
Sandblasting
Uses high-pressure air to propel abrasive particles, creating a frosted, matte surface texture for decoration or light diffusion.
Polishing / Grinding
Refines cut edges or surfaces to a clear, smooth finish, removing machining marks and restoring optical clarity.
CNC Engraving
Uses a diamond bit to carve precise, permanent patterns, text, or deep textures into the glass surface.
Painting / Enameling

Applies ceramic-based paint onto the surface, which is then fired at high temperature to fuse into a durable, colored coating.

Quartz Glass: Advantages & Key Considerations

Quartz glass offers unparalleled thermal stability, optical clarity from UV to IR, and exceptional chemical purity. Key considerations include high material cost, processing complexity, and inherent brittleness, which must be evaluated for demanding applications.

Benefits

Extreme Thermal Stability & Shock Resistance: Possesses an exceptionally low coefficient of thermal expansion (CTE ~0.55 x 10⁻⁶/K) and a very high softening point (~1680°C), enabling it to withstand rapid temperature changes and operate continuously in environments above 1000°C where other materials fail.

Superior Optical Clarity Across Broad Spectrum: Offers excellent transmission from deep ultraviolet (UV) to near infrared (IR) wavelengths, making it the material of choice for precision optics, UV lighting, and laser applications where standard glasses are opaque.

Outstanding Chemical Purity & Inertness: With a composition of >99.9% SiO₂ and minimal metallic impurities, it is highly resistant to water and most acids, preventing contamination and ensuring long-term stability in corrosive or ultra-clean environments.

Excellent Electrical Insulation at High Temperatures: Maintains high volume resistivity even at elevated temperatures, making it a reliable insulator for semiconductor processing, high-temperature sensors, and electrical furnaces.

High Dimensional Stability: Its near-zero thermal expansion ensures that components maintain their precise shape and tolerances across wide temperature ranges, critical for metrology, space optics, and precision instruments.

Design Flexibility for Demanding Shapes: Can be fabricated into complex custom components, including transparent parts for viewports, crucibles, and intricate optical elements, though it requires specialized machining and finishing processes.

Low Thermal Conductivity: Provides good thermal insulation in high-temperature applications, helping to protect surrounding components from extreme heat.

Long-Term Reliability & Durability: Resists devitrification (crystallization) and maintains its properties over time under harsh conditions, ensuring consistent performance and reducing failure rates in critical applications.

Key Limitations to Consider

High Material Cost: The raw material and manufacturing process for high-purity quartz glass are significantly more expensive than for borosilicate or soda-lime glass.

Processing Difficulty and Cost: Machining (cutting, drilling) requires specialized diamond tools and is time-consuming. Achieving optical-quality surfaces often necessitates high-temperature flame polishing or annealing (>1100°C), adding complexity and expense.

Brittleness and Handling Sensitivity: Like all glasses, it is inherently brittle and can be prone to chipping or cracking if not handled with extreme care, especially after machining and before final polishing.

Limited Size and Thickness Availability: Producing very large, thick, or complex monolithic pieces is challenging and costly due to material and processing constraints, compared to standard float glass.

Challenging Secondary Processing: Bonding, sealing, or fusing quartz glass to other materials (like metals or different glasses) is difficult due to its extremely low thermal expansion, often requiring specialized techniques and materials.

Potential for Devitrification: If exposed to high temperatures for prolonged periods in certain conditions, it can undergo devitrification (crystallization), forming cristobalite on the surface, which degrades its thermal and optical properties.

Susceptibility to Specific Chemicals: While highly resistant to most acids, it is severely attacked by hydrofluoric acid (HF) and hot phosphoric acid, limiting its use in some chemical processes.

Heavier than Some Alternatives: While its purity is high, its density is not the lowest. For extreme lightweight optical applications, materials like fused silica with engineered voids or other specialty glasses might be considered.

High Melting Point Limiting Forming Options: Its very high softening point makes it unsuitable for many conventional glass-forming techniques, restricting some design geometr

Quartz Glass Solutions for Extreme Applications

Our quartz glass components serve critical roles in semiconductor fabrication, precision optics, laboratory equipment, and high-temperature lighting. This material is chosen for its unmatched purity, thermal stability, optical clarity, and chemical resistance in demanding environments.

1. Semiconductor & Microelectronics Fabrication
Description: Quartz glass is indispensable in semiconductor manufacturing due to its extreme purity, high-temperature resistance, and excellent dielectric properties. It is used for critical components such as diffusion furnace tubes, wafer carriers (boats), crucibles for silicon melting, and optical components in photolithography equipment, where it prevents contamination and withstands aggressive thermal and chemical processes.

2. High-Precision Optics & Photonics
Description: Its superior optical clarity across a broad spectrum (from deep UV to near IR) and exceptional homogeneity make it the premier material for laser optics, UV and IR lenses, mirrors, windows for space telescopes, and substrates for optical coatings. It enables high-performance optical systems where standard glass would fail.

3. Laboratory & High-Temperature Process Equipment
Description: In scientific and industrial settings, quartz glass is used for laboratoryware (e.g., crucibles, tubes), sight glasses for high-temperature reactors, and thermocouple protection sheaths. Its chemical inertness allows it to handle highly corrosive materials, while its thermal shock resistance is vital for processes involving rapid heating and cooling.

4. High-Intensity & Specialty Lighting
Description: It serves as the envelope material for high-intensity discharge (HID) lamps, UV sterilization lamps, and halogen heater elements. Its ability to transmit UV light efficiently and withstand the extreme operating temperatures of these light sources is unmatched by other glasses.

5. Aerospace, Defense & Sensing
Description: Quartz glass is used in radar and satellite communication windows, UV/IR sensor optics, and thermal protection viewports due to its stability under extreme environmental conditions, low thermal expansion, and ability to protect sensitive instrumentation while maintaining signal clarity.

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Introductory guides

FAQ: Quartz Glass (Fused Silica) for Development & Design

What is quartz glass, and how does it differ from regular borosilicate glass?

Quartz glass (fused silica) is a high-performance material composed of over 99.9% pure silicon dioxide (SiO₂). Its key differences are an extremely low thermal expansion (CTE ~0.55×10⁻⁶/K), a much higher softening point (~1680°C), and superior optical transmission from UV to IR wavelengths, making it suitable for far more demanding applications than borosilicate.

What are the primary advantages that justify its higher cost?

Its cost is justified by unmatched properties: extreme thermal shock resistance for rapid temperature cycling, high optical clarity across a broad spectrum, exceptional chemical purity and inertness, and the ability to operate continuously above 1000°C. These are critical for semiconductor, optical, and high-temperature applications.

Can quartz glass be machined into complex custom shapes? What are the challenges?

Yes, it can be CNC machined into complex components. However, machining is difficult and costly, requiring specialized diamond tools. Achieving optical-quality surfaces or relieving machining stress often requires subsequent high-temperature flame polishing or annealing (>1100°C), adding steps and cost.

Is it compatible with thermal tempering like other glasses?

No. Due to its very low thermal expansion, quartz glass cannot be thermally tempered for strengthening via the standard process used for soda-lime or borosilicate glass. Its strength comes from its inherent material properties and pristine surface finish.

Why might there be color inconsistency (slight yellows or blues) between different batches or panels?

Minor variations in the ultra-low iron and trace element chemistry can cause subtle tonal shifts. For projects requiring absolute color uniformity (like a large facade), source all glass from a single production batch (batch coding) and conduct a visual pre-assembly check.

What are its main limitations or weaknesses in design?

Key limitations include high material and processing cost, inherent brittleness requiring careful handling, difficulty in sealing or bonding to other materials due to CTE mismatch, and susceptibility to devitrification (surface crystallization) if exposed to high temperatures in certain conditions for prolonged periods.

In which applications is quartz glass the mandatory or best choice?

It is essential for: Semiconductor fabrication (furnace tubes, wafer carriers), Deep UV and IR optics (lenses, laser components), High-intensity lighting (UV lamp envelopes), and High-temperature viewports/equipment (sight glasses, reactor linings) where no other material performs adequately.

How does its optical performance compare, especially in UV applications?

It is the benchmark material for UV transmission. Unlike most glasses that absorb UV light, high-purity fused quartz offers excellent transparency down to 170 nm, making it irreplaceable for UV lithography, sterilization, and spectroscopic applications.

What drives the lead time and cost for custom quartz components?

Cost is driven by material grade (optical vs. technical), part complexity, and required surface finish (e.g., optical polish). Lead time is influenced by machining complexity and the need for post-process annealing or polishing. Prototypes are feasible, but full production requires detailed planning.

Can it be joined or fused to other materials?

Joining is challenging. It can be fusion-welded to itself in a high-temperature flame. However, creating durable seals with metals or different glasses requires specialized techniques using graded seals or compatible adhesives due to its vastly different thermal expansion.

What quality control measures are critical for quartz parts?
Key QC includes verifying dimensional accuracy post-annealing (to account for potential slight distortion), optical homogeneity and transmission testing, inspection for micro-cracks (especially after machining), and ensuring the surface is free from devitrification or contamination that could impair performance in clean or high-temperature environments.

What are the advantages of quartz glass compared to other glasses?

Quartz glass has several distinct advantages over other types of glass. First, the extremely high purity of quartz glass (typically over 99.9% SiO₂) means it is more chemically stable and resistant to harsh environments than soda-lime or borosilicate glass. Quartz glass is in a class of its own when it comes to high-temperature resistance, withstanding sustained temperatures of up to 1200°C, and even short exposures to 1700°C. You may be wondering – what about rapid temperature changes? Quartz glass has a very low thermal expansion coefficient and is almost unaffected by thermal shock. This is why it is not prone to cracking even under sudden heating or cooling.

What is the anti-crystallization performance of quartz glass?

The high-temperature service life of quartz glass depends on the anti-crystallization property of quartz glass. When the crystallization layer of quartz glass reaches a certain thickness, it will burst and be damaged when cooled. Therefore, reducing the high-temperature crystallization rate of quartz glass can increase its service life. Theoretically, the stable silicon dioxide (SiO₂) structure at (1730-1470℃) is cristobalite. When the temperature is higher than 1730℃, cristobalite melts into quartz glass. At high temperatures, cristobalite is an equiaxed crystal system, called β-cristobalite. When the temperature drops to 200-300℃, it rapidly transforms into α-type cristobalite and a tetragonal crystal system. Its axis is slightly larger than the former, the crystal symmetry changes, the density also changes, and the volume suddenly shrinks by 2.8%, causing bursting and collapse. The cracks will extend deep into the glass body, causing the quartz vessel to be damaged and scrapped.
Transparent quartz glass has a high purity and no crystal nucleus in the glass body, so it generally does not crystallize. Crystallization often occurs on the surface. The factors that affect crystallization are as follows.
1. The higher the temperature, the faster the crystallization. There is no crystallization below 1000℃. At 1280℃ (in a clean electric furnace), the crystallization rate of the quartz glass surface is 0.01μm/h, and 1400℃ is 9~12μm/h. The crystallization rate of quartz glass reaches its maximum at 1630℃.
2. Surface contamination seriously affects crystallization. For example, impure hydrofluoric acid, insufficient water purity, hand sweat, watermarks, and dust will aggravate crystallization when touched directly by hand. Due to surface contamination, the crystallization temperature can be reduced by 200~300℃, and crystallization begins at 800℃.
3. When using quartz glass, water vapor, chlorine, fluoride, and phosphate in the furnace atmosphere will promote crystallization.
4. The furnace refractory materials using quartz glass contain a large amount of alkali metals and alkaline earth metal components, which will volatilize at high temperatures and cause serious crystallization. The crystallization rate at 1280℃ can be as high as 10μm/h, which is 1000 times higher than normal. In particular, sodium ions have the most serious impact. The author has done such an experiment. The quartz glass sheet is placed in a quartz glass box, and there will be no crystallization at 1400℃. Therefore, the quartz glass annealing furnace uses quartz wool as the furnace refractory material (also a heat preservation material) to maintain the quality of the quartz glass surface.
5. The best fuel gas for the hot processing of quartz glass is hydrogen and oxygen, and it should be filtered and purified.
6. If there is a large amount of hydroxyl in quartz glass, it will reduce the viscosity of quartz glass and aggravate crystallization. Synthetic quartz glass contains 1200×10-6 hydroxyl, its temperature resistance drops by 70℃, and the crystallization performance is poor, but crystallization only occurs on the surface.
7. The bubble surface, white spots, black spots, raw material particles, etc. in the quartz glass will become crystal nuclei, around which crystallization will first occur and accelerate the growth of cristobalite particles.

According to the JC/T597-95 industry standard, the average thickness of the crystallization layer of a quartz tube should be less than 100μm when it is kept at a constant temperature of (1400±5)℃ for 6h to be qualified. The results of the crystallization rate measurement of various quartz glasses at 1400℃ are shown in Table 16-13. The samples are mechanically polished and pickled, and the measurement accuracy is 0.1μm.

Type Inner Wall - Max Inner Wall - Min Inner Wall - Avg Inner Wall - Max Inner Wall - Min Inner Wall - Avg
Gas-refined tube 49 25 37 64 34 49
Gas-refined material 58 30 44 73 37 55
Japanese tube 58 51 54.5 64 46 55
British tube 41.8 41.8 41.8 39.5 39.5 39.5
Synthetic quartz 70.8 70.8 70.8 86.1 86.1 86.1

Why is quartz glass more expensive than soda-lime glass?

High Purity Raw Materials

First off, quartz glass demands extremely high-purity silicon dioxide (SiO₂), usually over 99.9% pure. Obtaining and refining raw materials to this level of purity is neither easy nor cheap, which naturally drives up the cost. On the other hand, the materials of soda-lime glass are silicon dioxide (sand), sodium carbonate (soda), and calcium oxide (lime), which are very cheap, making quartz glass more expensive than soda-lime glass.

Complex Manufacturing Process

The production of quartz glass is a completely different world compared to soda-lime glass. Quartz has a very high melting point – around 1700°C – which means you need specialized high-temperature furnaces and a lot of energy to melt it. In contrast, soda-lime glass melts at a lower temperature of around 1500°C, and you only need a basic furnace and minimal energy to melt it. So quartz glass is more expensive in the manufacturing process.

Difficult Machining and Shaping

Quartz glass is hard and brittle, so it is difficult to shape and cut. Specialized tools and techniques are required to process quartz glass, which adds a lot of cost and time. Soda-lime glass is easy to shape and process, resulting in low cost. Therefore, quartz glass is more expensive.

What are the characteristics of quartz glass and what is its principle？

High Purity and Chemical Stability

Quartz glass is made of more than 99.9% pure silicon dioxide (SiO2), which means that there are almost no impurities in the material, so many unnecessary chemical reactions can be avoided. Quartz glass has good chemical stability. At room temperature, various strong acids (hydrochloric acid, nitric acid, sulfuric acid, nonyl water, organic acids) have no corrosive effect on it, and alkaline solutions (NaOH, KOH, NH(OH), Na2COs, etc.) have no corrosive effect at room temperature.
See the table for the amount of corrosion caused by various acids
Acid Name Relative Density Temperature(℃） Treatment Time（h） Weight Loss（g/㎡）- Weight Loss（g/㎡）-
Sulfuric acid 1.84 20 240 0.016 0.046
Sulfuric acid 1.84 205 24 0.06 0.13
Nitric acid 1.4 20 240 0.06 0.092
Nitric acid 1.4 115 24 0.11 0.15
Hydrochloric acid 1.19 20 240 0.18 0.33
Hydrochloric acid 1.19 66 24 0.14 0.33
Acetic acid(70%） 70%（content） 108 24 0 0.03
Oxalic acid（30%） 30%（content） 108 24 0.05 0.15
See the table for the amount of corrosion caused by alkaline solutions
Alkali Solution Name Concentration(%) Reaction Temperature(℃） Treatment Time（h） Weight Loss（mg/c㎡）-
NH₄OH 10 20 100 0.019
NaOH 1 20 100 0.031
NaOH 10 18 100 0.0095
KOH 1 20 100 0.019
KOH 30 18 100 0.027
Na₂CO₃ 5 18 100 0.0015
NaOH 5 100 10 1.5
NaOH 8 100 10 1.21
KOH 10.2 100 10 1.13
Na₂CO₃ 10 100 10 0.37
Excellent Thermal Resistance
The high temperature viscosity of quartz glass ensures its high temperature resistance; quartz glass, like other glasses, is non-crystalline, amorphous, has no melting point, only a softening range, is solid (supercooled liquid) at low temperatures, and has a very high viscosity at high temperatures. The viscosity at 1100℃ is 1013.9Pa·s, and this viscosity will not deform due to its own weight; at 1300℃, it is 1010.8Pa·s, and it will deform slightly after long-term use. Therefore, the long-term use temperature of quartz glass is determined to be 1100℃, and the short-term use temperature is 1300℃.
Quartz glass is the strain point at 10¹³-5Pa·s, and the temperature of the electric melting tube at this point is 1122℃. This temperature is kept for 4h, and most of the stress is eliminated; 1012.2Pa·s is the annealing point, and the temperature of the electric melting tube at this point is 1215℃. Maintaining this temperature for 15min will eliminate most of the stress. Quartz glass hot-processed products often have stress, which can be annealed (stress eliminated) at this temperature. The cooling rate of annealing is: products with a thickness of less than 25mm drop by 100℃ per hour.
Temperature(℃） lg viscosity/Pa·s
510 Crucible(Arc Method) 214 Type(Standard) 219 Type(Ozone-free) 254 Type(UV Tube)
1100 13.7 13.88 13.75 13.11
1200 12 12.22 12.28 11.66
1300 10.5 10.76 10.38
Temperature(℃） lg viscosity/Pa·s
510 Crucible （Arc Method）（Points） 214 Type(Standard) 219 Type(Ozone-free)（Points） 254 Type(UV Tube)
Annealing Point（℃） 1200 1215 1204 1163
Strain Point（℃） 1100 1122 1106 1066
TLow thermal expansion coefficient and thermal shock resistance
The average thermal expansion coefficient of transparent quartz glass (20～320℃) is 5.5×10-7/℃, which is 1/34 of copper, 1/7 of borosilicate glass, and 1/20 of ordinary glass. The average linear expansion coefficient at each temperature is shown in the table.
Temperature range(℃） Linear expansion coefficient/（×10-7/℃） Tenperature range/℃ Linear expansion coefficient/（×10-7/℃）
-50～0 2.7 0～300 5.9
0～100 5.1 0～600 5.4
0～200 5.8 0～900 4.8
Transparent quartz glass resistance to thermal shock (sudden cooling and heating): Due to the low expansion of quartz glass, it has strong resistance to thermal shock and will not break.

Hydroxyl Treatment

The presence of hydroxyl groups in quartz glass can significantly affect its properties, especially in optical applications, as they alter the glass structure and affect optical performance. The removal of hydroxyl groups is necessary in certain applications, such as in fiber optics and semiconductor materials.

（1）Hydroxyl Formation: Hydroxyl groups form in quartz glass during the manufacturing process, depending on the raw material and heating conditions. For example, quartz glass made from SiCl₄ using hydrogen-oxygen flames can contain hydroxyl groups in the range of 1000 – 1400 × 10^-6.

（2）Mechanism of Hydroxyl Removal: Hydroxyl removal occurs through a reversible reaction involving high-temperature water vapor or hydrogen, under conditions of low H₂ and water vapor pressure. When treated under vacuum or dry nitrogen at temperatures between 950°C and 1100°C, hydroxyl groups can be effectively removed.

（3）Hydroxyl Removal Process: The time and conditions for removing hydroxyl groups depend on the type of quartz glass, the thickness, and the processing atmosphere. In reducing atmospheres, the removal process is faster and more efficient.

By following this detailed process, quartz glass can be homogenized, annealed, and treated to meet the requirements of specific applications, ensuring improved performance in various industrial uses.

What are the specific properties of quartz glass?

The outstanding advantage of quartz glass is that it has unique comprehensive properties that other materials do not have, namely very low thermal conductivity, excellent thermal shock resistance, high deformation temperature and softening temperature, very low thermal conductivity, and very Low dielectric losses and optical transmission over an extremely broad spectral range from ultraviolet to infrared.

Mechanical properties

Quartz glass is a brittle and hard material, with very low tensile strength but very high compressive strength, the latter being 20 times greater than the former. Table 1-2 shows the mechanical properties of quartz glass. It should be noted that the performance testing of quartz glass is closely related to the internal quality and surface condition of the sample. Certain defects and surface roughness can significantly affect the test results. Figure 1-1 illustrates the variation of the elastic modulus of quartz glass with temperature.

Figure 1-1 Relationship between elastic modulus of quartz glass and temperature

Property Transparent Quartz Glass Synthetic Quartz Glass Opaque Quartz Glass
Density (g/cm³) 2.203 2.201 2.07 - 2.12
Elastic Modulus (MPa) 7.25×10⁴ 7×10⁴ Approx. 500
Poisson's Ratio 0.17 0.17 Approx. 40
Compressive Strength (MPa) 1150 1150 Approx. 60
Tensile Strength (MPa) 50 50 Approx. 67
Flexural Strength (MPa) 67 67 Approx. 67
Mohs Hardness 5.5 - 6.5 5.5 - 6.5 8000 - 9000

Table 1-2 Mechanical properties of quartz glass

Figure 1-2 Relationship between line shadow coefficient and temperature of quartz glass

Due to its extremely low coefficient of thermal expansion, quartz glass exhibits excellent thermal shock resistance. Transparent quartz glass samples, when heated to 1000°C and then immersed in 20°C water, do not crack. Thermal shock resistance is typically measured by the number of times the material can be heated and cooled before breaking.
Quartz glass is a poor conductor of heat. Tables 1-3 and 1-4 provide the relationship between the thermal conductivity of quartz glass and quartz glass wool with temperature changes.

Thermal Properties

The coefficient of linear expansion of quartz glass is lower than that of all other materials. In fact, doped quartz glass can even achieve zero expansion. The relationship between the expansion coefficient and temperature is shown in Figure 1-2. Additionally, the coefficient of linear expansion of transparent quartz glass and opaque quartz glass differs very little, allowing them to be welded together even when producing large-scale products.
Thermal Conductivity of Quartz Glass
Temperature (°C) Transparent Quartz Glass (W/°C·m) Opaque Quartz Glass (W/°C·m)
-253 0.10 -
-223 0.50 -
20 1.38 1.1
100 1.47 -
200 1.55 1.5
300 1.67 -
400 1.84 1.8
900 2.68 2.3
0-100 - 1.4

Table 1-3 Relationship between thermal conductivity of quartz glass and temperature

Thermal Conductivity of Quartz Glass Wool
Temperature (°C) Quartz Wool Thermal Conductivity (Density 0.04g/cm³) [W/(°C·m)] Quartz Wool Thermal Conductivity (Density 0.1g/cm³) [W/(°C·m)]
50 0.044 0.047
100 0.057 0.056
200 0.091 0.074
300 0.135 0.098
400 0.188 0.125
500 0.247 0.156
600 0.316 0.189
700 0.397 0.227
800 0.488 0.273

Table 1-4 Relationship between thermal conductivity of quartz glass wool and temperature

Figure 1-3 illustrates the variation of the average specific heat capacity of quartz glass with temperature.
Viscosity is one of the most important properties of quartz glass. Quartz melts into glass at 1723°C and remains an amorphous material with no fixed melting point. Viscosity is used to characterize its thermal processing and application performance. The viscosity of molten quartz glass is 10⁵–10⁶ dPa·s (or “poise”), much higher than that of common glass, which is only around 10² dPa·s. The viscosity-temperature relationship for normal and synthetic quartz glasses is shown in Figure 1-4.
When heated, quartz glass begins to deform at a viscosity of 10¹³–10¹⁴ dPa·s (approximately 1200°C). It is generally required that quartz glass products operate below 1100°C. At temperatures between 1400–1500°C, its viscosity reaches 10⁹–10¹⁰ dPa·s, and deformation accelerates. Further heating to 1700°C results in a viscosity of 10⁶ dPa·s, making it suitable for processes like blowing and bending. However, the thermal processing of quartz glass is significantly more difficult than other materials, as it hardens quickly once removed from heat.

The factors that influence the viscosity of quartz glass include temperature, thermal history, hydroxyl content, and impurity levels.

Figure 1-3 Relationship between the average specific heat capacity of quartz glass and temperature

Figure 1-4 Relationship between viscosity and temperature of quartz glass

Figure 1-5 Viscosity of quartz glass made by different processes

1-infrared quartz glass (hydroxyl content about 3 × 10-): 2-gas quartz glass (hydroxyl content about 270 × 10-6):
3-gas quartz glass (hydroxyl content about 400 × 10-), 4-gas phase deposition synthesis quartz glass
(hydroxyl content about 1200 × 10-6)

Annealing Treatment

Although quartz glass has a low expansion coefficient, it can develop thermal stress due to temperature differences during high-temperature melting or thermal processing. Thermal stress can significantly reduce the optical uniformity, mechanical strength, and structural stability of quartz glass. Annealing is performed to eliminate or reduce this thermal stress.

Mechanism of Thermal Stress Formation

Thermal stress can be categorized into:

  • Temporary Stress: This type of stress occurs when the temperature gradient causes a difference in cooling rates between the inner and outer layers of the glass. The outer layer cools faster, creating tensile stress, while the inner layer resists, creating compressive stress. This stress disappears when the temperature gradient is eliminated.
  • Permanent Stress: When the temperature gradient disappears but stress remains in the glass, it becomes permanent stress, also known as internal stress. This occurs if the cooling rate is too fast and does not allow stress relaxation.

Annealing Process

To relieve the internal stress and improve optical uniformity, quartz glass is heated to a temperature range between the strain point and the annealing point (around 10^13 dPa·s viscosity). The glass is then held at this temperature for a sufficient period, allowing the internal stresses to relax, and finally cooled slowly to avoid the formation of new stresses.

Homogenization Process

The basic process of glass homogenization involves the dissolution of inhomogeneous phases (such as SiO₂ particles and microbubbles) and the diffusion of impurities (such as metal ions and hydroxyl groups). Since diffusion occurs at a slower rate than dissolution, the speed of homogenization in glass is governed by the diffusion rate. The diffusion of ions in glass follows Fick’s second law of diffusion.

The diffusion rate of ions in glass depends on the diffusion coefficient (D), the contact area between the diffusing phases (ds), and the concentration difference (ΔC). The diffusion coefficient (D) and the contact area (ds) should be increased to improve the degree of homogenization in glass. The diffusion coefficient (D) is a function of temperature (T) and activation energy (Q).

To increase the diffusion coefficient, the glass temperature must be raised to lower viscosity. However, since quartz glass has a high melting temperature, conventional glass stirring methods cannot be used to enhance the contact area between the phases. Instead, high temperatures and pressure are applied to promote the displacement of particles, increasing the contact area.

Determining Process Parameters

Temperature: Higher temperatures reduce the viscosity of the glass, which increases the diffusion coefficient. However, excessive heating can lead to volatilization of the glass, increasing vapor pressure and generating bubbles. This would lead to secondary bubbles that are difficult to eliminate through homogenization. Additionally, volatilized substances can diffuse into the glass, raising impurity content. Therefore, the temperature must be controlled, typically around the melting temperature of quartz glass (~1800°C), with a glass viscosity of approximately 10^7.6 dPa·s.
Pressure: Applying pressure helps the movement of glass particles, which is beneficial for homogenization. However, excessive pressure may lead to the diffusion of external impurities into the glass, negatively affecting its purity. It is typically recommended to maintain pressure between 2.5 and 4 MPa.
Homogenization Time: A longer homogenization time can improve the process, but it is not ideal if too long. This is because prolonged exposure can result in increased volatilization and intensified reactions between the glass and container, causing contamination. The optimal time to maintain high temperature and pressure is between 30 and 60 minutes after the glass has softened and flattened.

Homogenization Process Flow

Preparation: Place the quartz glass ingot in a graphite crucible coated with tungsten powder. Seal the crucible in a vacuum furnace.
Heating: Begin heating under vacuum (160 Pa) with a gradual increase in temperature. Rapid temperature increases at low temperatures (below 350°C) are avoided to prevent stress and cracking caused by temperature differences between the inner and outer parts of the quartz glass ingot.
Pressurization: After uniform temperature distribution is achieved, increase the temperature to the desired level. Then, pressurize the system by introducing nitrogen (N₂) at about 4 MPa. Hold for 40 to 60 minutes, depending on the homogenization degree.
Cooling: After homogenization, the furnace is slowly cooled to avoid excessive stress that could cause the glass to crack.

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