This article presents an in depth look at quartz glass. Read further and learn more about the following:
Quartz is one of the most prevalent and extensively distributed minerals on Earth and is the only stable form of crystalline silica found at the Earth's surface. It appears in all rock types—igneous, metamorphic, and sedimentary. Quartz accumulates in soils, water bodies, and sand as quartz-bearing rocks undergo weathering and erosion.
Quartz has the chemical formula SiO2. The bonds between silicon and oxygen in quartz are both polar and covalent. Silicon, with its four valence electrons, forms bonds with four oxygen atoms. Each oxygen atom bonds with two silicon atoms, creating a body-centered tetrahedral crystal structure. This structure consists of four oxygen atoms at the vertices and a central silicon atom. Within each tetrahedron, the O-Si-O bond forms an angle of 109°. In a network of SiO4 tetrahedra, the oxygen atoms at the corners connect to the central silicon atoms, forming Si-O-Si bonds with a 144° angle. The open network structure of SiO4 creates wide spaces, resulting in quartz's hexagonal crystal form.
Silica sand, a prevalent material in the production of quartz glass, is an inert and durable mineral that has naturally disintegrated into sand over time. High-purity silica sand enables precise control over the strength, clarity, and color of the final product. Consistent chemical processing ensures that each batch of quartz glass is high-quality, uniform, and reliable.
Quartz can be transformed into quartz glass, known for its high purity and versatility across various applications. This type of glass is free from additives and is often referred to as fused quartz or fused silica. The distinction between the two lies in their production: fused quartz is made from pure silicon dioxide (SiO2), whereas fused silica is derived from synthetic precursors.
Most quartz is sourced from silica sand, which is refined to produce high-purity quartz with enhanced strength, clarity, and color. The extensive processing of sand ensures exceptional purity by removing impurities through meticulous chemical treatment.
Quartz glass is highly prized for its unique and valuable properties, including its low coefficient of thermal expansion, excellent gas permeability, and broad optical transmission range.
This chapter outlines the process of converting raw quartz into finished, fused quartz glass.
In the initial stages of processing, dirt, moisture, and contaminants are removed from natural quartz to ensure the quality and performance of the resulting quartz glass. This step is specifically relevant for quartz that is mined.
The goal of this stage is to reduce the raw quartz to a size that is suitable for the fusion process and machinery. Natural quartz undergoes several size reduction procedures, including crushing and milling (either ball milling or roll milling). Due to its brittle nature, quartz is relatively easy to comminute. Following this, the particle size is assessed, and larger grains are separated.
In this stage, thermal energy is applied to disrupt the strong silicon-oxygen bonds. As the temperature rises, more bonds are broken, leading to a less viscous flow of quartz. After shaping and cooling, the structured crystalline arrangement of SiO2 molecules transforms into a vitreous, amorphous structure, resulting in a metastable form of quartz.
Depending on the required purity and intended application, natural quartz may be homogenized and processed using the following fusion methods:
This method yields Type I quartz glass, which typically contains 100 to 130 ppm OH content as initially produced. For achieving high purity and lower hydroxyl (OH) content (ranging from >1 ppm to 30 ppm), electric fusion is employed. The vacuum annealing process can further reduce the OH content to the required levels for specific applications. Lower OH concentrations enhance UV transmission in the IR range (2750 nm), which is crucial for certain uses. The starting material for this process is natural quartz grains, which may undergo the following production methods:
This method can utilize either natural quartz or a synthetic precursor as the starting material. Natural quartz is exposed to a high-temperature hydrogen/oxygen (H2/O2) flame until it melts. For synthetic precursors like silicon tetrachloride (SiCl4), the gaseous precursor reacts with the H2/O2 flame to produce the desired material.
The molten quartz is then deposited into a refractory-lined vacuum chamber, where it is slowly collected through a die at the bottom and shaped into its final form. This method produces quartz glass with an OH content of 150-200 ppm from natural quartz and up to 1000 ppm from synthetic silica. The resulting material has a high, stable OH content that cannot be further reduced through vacuum annealing, and exhibits a lower softening point and operating temperature.
Quartz glass produced by this flame fusion method is characterized by its higher and more stable OH content, which remains even after vacuum annealing. This results in a lower softening point and operating temperature for the material.
Glass made from crystal quartz through flame fusion is classified as Type II, while glass from synthetic precursors is classified as Type III. Type III synthetic silica glass is produced through a chemical reaction involving the combustion of silicon tetrachloride, which generates synthetic quartz and releases environmentally harmful byproducts such as chlorine and hydrochloric acid.
This process resembles flame fusion but utilizes a water-vapor-free plasma flame for heat. Plasma-fused quartz glass is known for its high purity, low OH content, minimal bubble formation, and absence of drawing lines.
For this method, either natural quartz or a synthetic precursor can be used as the starting material. Quartz glass made from the combustion of a synthetic precursor in a plasma flame is classified as Type IV.
Quartz sand is melted in an electric arc furnace, producing a vitreous material with gaseous micro-bubbles that cause light diffraction and impart opacity to the glass. The resulting glass ingots are crushed, molded into parts, then dried and sintered. This type of quartz glass is typically white and opaque, not classified under standard types of quartz glass, and contains an OH content ranging from 100 to 130 ppm.
The production and shaping of quartz glass differ significantly from those used for typical glass. Higher temperatures are required because quartz glass does not flow; instead, it softens and becomes viscous at elevated temperatures.
Shaping and forming quartz glass often necessitate the use of diamond cutting tools because of its hardness. Additionally, operating parameters must be carefully optimized, as quartz glass is also brittle and can crack or fracture under excessive force. Mechanical processes involved include:
Thermoforming quartz glass is challenging due to its high melting point and narrow viscosity range, which means it can only be shaped within a specific temperature range. If the temperature is too low, the glass remains solid; if it's too high, the glass becomes too viscous and volatile, leading to evaporation of the material. Additionally, one or more annealing steps are necessary to relieve thermal stress and prevent fractures during hot forming. The following hot forming methods can be employed by manufacturers to improve the quality of the glass product:
A variety of machines are available for producing quartz glass items, which are crucial in today’s society due to their use in optical and lighting devices, as well as chemical apparatuses. These machines are essential for the precise shaping, fabrication, and processing of quartz glass components. Below is an overview of some notable manufacturers that provide machinery and equipment for quartz glass processing.
Haas CNC machining centers are highly precise and capable of executing various operations on quartz glass, such as cutting, drilling, milling, and grinding. These machines utilize computer-controlled movements to achieve high accuracy and consistency.
Meyer Burger's diamond wire saws are known for their precision in cutting quartz glass. These saws use a thin wire embedded with industrial diamonds to slice through the material with minimal damage and chipping. They are particularly effective for cutting complex shapes and detailed profiles.
Schiatti Angelo's glass lathes are specialized machines designed for precise turning and shaping of quartz glass. They are particularly adept at creating cylindrical forms, threads, and other complex geometries. These lathes are typically equipped with diamond or carbide tools for accurate cutting and shaping of the glass.
BENTELER's glass grinding machines are designed for precise grinding of quartz glass. They utilize abrasive belts or wheels to remove material and achieve the desired shape, smoothness, and surface finish of the glass.
The DMG Mori DMU 50 is a versatile CNC machining center ideal for working with quartz glass. It provides high precision and is capable of performing various operations, including cutting, drilling, milling, and grinding. The machine incorporates advanced technologies to ensure accuracy and efficiency in processing quartz glass.
Please note that specific models, features, or components may vary. For the most current information on quartz glass processing equipment available in the United States and Canada, it is recommended to consult the respective manufacturers or industry resources.
This chapter highlights the key properties and characteristics of quartz glass.
Purity is crucial in the manufacturing of quartz glass. Even trace amounts of contaminants can affect the thermal, electrical, and optical properties of the final product and its performance in applications. To maintain high purity, strict handling precautions are necessary throughout all stages of production, starting from the raw material. Common impurities include metal oxides (such as Al2O3, Fe2O3, MgO), water, and chlorine.
Water appears in quartz glass as hydroxyl (OH) groups. The OH content can vary based on thermal treatment and moisture exposure at elevated temperatures. OH affects infrared transmission, viscosity, and attenuation. High OH levels can decrease infrared transmission and lower thermal stability, making the quartz glass unsuitable for high-temperature applications. An annealing step can help reduce OH content, particularly in electric fused quartz glass.
Quartz glass is chemically inert to most substances, including water, salt, and acids, making it a valuable material in chemical laboratories and industries. It is virtually impermeable to gases. The only agents that can etch or disintegrate quartz glass at ambient temperatures are hydrofluoric acid and phosphoric acid. However, alkali and alkali earth compounds can attack the surface, leading to accelerated devitrification. Even small amounts, such as 0.1 mg of alkali per square centimeter, can significantly transform semi-stable molecules. Additionally, even fingerprints, which contain traces of alkali, can trigger devitrification.
Quartz glass is renowned for its exceptionally low coefficient of thermal expansion (CTE), which measures the fractional change in size of a material in response to temperature variations. Unlike many materials, where CTE increases with temperature changes, quartz glass maintains stability. Additionally, it boasts excellent thermal shock resistance, allowing it to endure sudden and extreme temperature fluctuations. Quartz glass also exhibits low thermal conductivity.
Quartz glass begins to soften at temperatures around 1630°C and behaves like a viscous liquid at elevated temperatures, similar to other types of glass. Its viscosity decreases with increasing temperature, and the presence of impurities can further increase viscosity.
Quartz glass shares many mechanical properties with other types of glass, including high compressive strength. However, it also exhibits significant brittleness, and surface defects can impact its overall strength. Machine-polished surfaces are generally weaker compared to fire-polished ones. Additionally, the glass's reliability can be influenced by its age and environmental exposure.
Quartz glass is extensively studied for its exceptional optical transmission properties across ultraviolet, visible, and infrared wavelengths. These properties can be further enhanced by adding doping materials. The transmission characteristics of quartz glass are influenced by its purity and OH content. Increased metallic impurities and OH molecular vibrational and rotational excitations can lead to light absorption, thereby affecting the overall transmission.
Quartz glass is an excellent electrical insulator, maintaining high resistivity even at elevated temperatures. It features high dielectric strength, attributable to the absence of charged mobile ions within its molecular lattice and the strong silicon-oxygen bonds, which impart very low polarizability to the material.
The table below summarizes some of the key property coefficients relevant to quartz glass, as discussed in this article:
| Property | Value |
|---|---|
| Density | 2.2 x 10-3 kg/m3 |
| Hardness | 5.5 – 6.5 in Mohs‘ scale |
| Design tensile strength | 7,000 psi |
| Design compressive strength | 160,000 psi |
| Rigidity Modulus | 4.5x106 psi |
| Coefficient of Thermal Expansion | 0.55 x 10-6/°C |
| Thermal Conductivity (20°C) | 1.4 W/m-°C |
| Specific Heat (20°C) | 670 J/kg-°C |
| Softening Point | 1683°C |
| Annealing Point | 1215°C |
| Strain Point | 1120°C |
| Electrical Resistivity (350°C) | 7x10-7/° ohm-cm |
| Dielectric Properties (20°C, 1 MHz) · Constant · Strength |
3.75 5x10-7 V/m |
| Index of refraction | 1.4585 |
| Constringence (Nu value) | 67.56 |
| Velocity of Sound-Shear Wave | 3.75 x 10-3/° m/s |
| Velocity of Sound/Compressional Wave | 5.90 x 10-3° m/s |
| Sonic Attentuation | < 11dB/m MHz |
| Permeability Constants (700°C) | |
| Helium | 2.1 x 10-8 mm/cm°2 sec. cm Hg |
| Hydrogen | 2.1 x 10-9 mm/cm°2 sec. cm Hg |
| Deutrium | 1.7 x 10-9 mm/cm°2 sec. cm Hg |
| Neon | 9.5 x 10-10 mm/cm°2 sec. cm Hg |
Here are some common applications of quartz glass:
Quartz glass is widely used for its optical properties, thanks to its broad transparency range and excellent light transmittance across ultraviolet to infrared wavelengths. It is resistant to damage from ultraviolet and high-energy radiation, allowing light to pass through with minimal distortion. Common optical applications of quartz glass include prisms, lenses, beam splitters, polarizers, mirrors, and windows.
High-purity quartz glass is employed in various lamps and lighting systems, including mercury lamps, halogen lamps, xenon lamps, ultraviolet lamps, and arc and filament lamps, all of which operate at high temperatures. These lamps are used across several industries, such as sterilization and cleaning equipment in the food and medical fields, as well as exposure devices in the semiconductor industry.
Quartz glass is a valuable, though costly, alternative to other types of glass for high-temperature applications due to its chemical inertness. It is commonly used in applications such as glassware, plates, and tubes, where its ability to withstand elevated temperatures is essential.
Fused silica offers exceptional wave transmission in the ultraviolet spectrum, making it ideal for manufacturing ultraviolet windows, lenses, and optics.
This section outlines recommended practices for handling and using quartz glass products to maintain their valuable properties and extend their service life:
Quartz glass can have a long service life if kept clean before and after use. Even minor impurities can lead to gradual devitrification. It is advisable to use clean, lint-free, powder-free, or cotton gloves when handling quartz glass to prevent contamination and maintain its integrity.
To clean quartz glass, immerse it in a solution of >7% ammonium bifluoride for no more than ten minutes, or a >10% hydrofluoric acid solution for no more than five minutes. After cleaning, thoroughly rinse the glass with deionized or distilled water and allow it to dry completely.
When not in use, quartz glass should be stored in an enclosed container to protect it from surface damage and moisture, which could impact its quality and performance. Ideally, the glass should be wrapped, and if it is in tube form, the end openings should be covered.
Quartz glass offers superior resistance to extreme heat and thermal shock compared to other types of glass. However, its resistance decreases with thickness. Additionally, thick and opaque quartz glass products may develop cracks if exposed to rapid temperature changes.
Before annealing, quartz glass reaches a distortion point, also known as the strain point. If quartz glass is cooled too quickly after reaching this distortion temperature (approximately 1100°C), it may develop distortions.
Quartz glass has a relatively low thermal expansion coefficient. Fused quartz may crack if it is attached to, fastened, or clamped with another material that has a significantly higher coefficient of thermal expansion.
Due to its low thermal conductivity, quartz glass can develop surface cracks if it is locally heated or exposed to a flame at temperatures above its distortion point. Additionally, quartz glass becomes less viscous as the temperature increases. It is important to consider these factors when using quartz glass as a finished product or as a component in other equipment or devices.
Devitrification can shorten the service life of quartz glass, and drastically remove all the desirable characteristics of quartz. Devitrification is the conversion of the metastable quartz glass into a stable, crystalline cristobalite. This occurs when quartz is heated at high temperatures into an extended period of time, or when it is heated with impurities attached to its surface, even in small amounts. With no impurities present, devitrification normally starts at 1200°C, and hastens with increased temperature. Impurities lower the devitrification threshold.
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