This article presents an in depth look at quartz glass. Read further and learn more about the following:
Quartz is one of the most abundant and widely distributed minerals in nature and is the only stable polymorph of crystalline silica on the Earth‘s surface. It is found in all forms of rocks: igneous, metamorphic and sedimentary. It becomes concentrated in soils, bodies of water and sand when a quartz-bearing rock is weathered or eroded.
The chemical formula of quartz is SiO2. The silicon-oxygen (Si-O) bond is polar and covalent. Elemental silicon contains four valence electrons making the silicon atom bonded to four oxygen atoms. One oxygen atom is bonded to two silicon atoms, making the body-centered tetrahedral crystal system of quartz. The tetrahedral crystal system is composed of four oxygen atoms at the corners and a central silicon atom. In one tetrahedron, the O-Si-O bond makes a 109° angle. In a network of SiO4 tetrahedra, the corner oxygen atoms link the central silicon atom. The Si-O-Si bond makes a 144°. The structure of the networked SiO4 is open with wide spaces, hence giving quartz a hexagonal crystalline form.
Silica sand is a common material used for the production of quartz glass. It is an inert hard mineral that has been broken down over time into sand. High purity silica sand allows for more control of the final product’s strength, clarity, and color. Through the use of consistent chemical processing, each batch of quartz glass is of high quality, uniform, and exceptionally reliable.
Quartz can be manufactured into quartz glass, which is valued for its exceptional purity and serves a wide range of applications. Quartz glass does not contain additives. It is sometimes referred to as fused quartz or fused silica; the difference between the two is that fused quartz is made from pure silicon dioxide (SiO2) while fused silica is made from synthetic precursor.
The majority of quartz comes from silica sand that is used to produce high purity quartz that has extra strength, clarity, and color. Extensive sand processing produces quartz with exceptional purity since the use of product chemistry removes any impurities.
Quartz glass is valued due to its distinct and high value characteristics. Among these are because of its low coefficient of thermal expansion, high gas permeability, and extensive optical transmission.
This chapter presents the steps in transforming the raw quartz into a formed, fused quartz glass.
Dirt, moisture and contaminants present in the natural quartz are removed in the early stages of processing which may affect the quality and performance of the quartz glass to be produced. This is only applicable for mined quartz.
The objective of this step is to reduce the raw quartz into a size suitable for the fusion method and machinery to be utilized. Natural quartz undergoes a series of size reduction steps such as crushing and milling (ball milling or roll milling). Quartz is very brittle in nature, which makes comminution quite easy. Afterwards, the particle size is analyzed and larger grains are separated.
In this stage, thermal energy is used to break the strong silicon-oxygen bond. With increasing temperature, more bonds are broken and result in the less viscous flow of quartz. After shaping and cooling to its final form, the ordered crystalline structure of SiO2 molecules is converted into a vitreous, amorphous structure and metastable form of quartz.
Depending on the desired purity level and end use application, the natural quartz may be homogenized and formed through the following fusion methods:
This method produces an industrially known Type I quartz glass. As drawn the material will contain 100 ppm to 130 ppm OH content. Electric fusion is used if a high level of purity and low hydroxyl (OH) content (> 1 ppm – 30 ppm) is to be obtained. Using the vacuum annealing process, the OH content can be reduced to necessary levels as needed for certain applications. Lower OH levels allow for greater transmission of UV in the IR range (2750nm), which could be critical for use in certain applications. The starting material is natural quartz grains, and may be subject to the following production modes:
In this method, a natural quartz or a synthetic precursor can be a starting material. Natural quartz passes through a chamber with a high temperature hydrogen/oxygen (H2/O2) flame until the starting material is fused. If silicon tetrachloride (SiCl4), a gaseous synthetic precursor, is to be used, it is made to react with the H2/O2 flame.
The viscous melt is deposited in a refractory lined vacuum chamber, collected slowly by a die at the bottom of the container, and shaped to its final form. Due to its direct contact with H2/O2 flame, this method produces quartz glass with 150-200 ppm OH content from natural quartz and up to 1000 ppm for synthetic silica. Material produced in this way has a high stable OH content that cannot be vacuum annealed. The quartz produced will have a lower temperature softening point and lower operating temperature.
Quartz glass made using this method has a higher and more stable OH content that cannot be vacuum annealed out. The quartz material will have a lower temperature softening point and lower operating temperature.
Glass produced from crystal quartz through flame fusion is classified as Type II, and from synthetic precursors as Type III. Type III synthetic silica glass is a product of a chemical reaction. The combustion of silicon tetrachloride gives synthetic quartz and leaves environmentally toxic byproducts, chlorine, and hydrochloric acid.
This process is similar to flame fusion with water-vapor free plasma flame being used as a source of heat. Plasma fused quartz glass has high purity level, low OH content, minimal bubble content and no drawing lines.
Natural quartz or a synthetic precursor may be the starting material for this method. Quartz glass produced from the combustion of a synthetic precursor in plasma flame is known as Type IV.
The quartz sand is melted in an electric arc furnace. The heating of the sand produces a vitreous material with gaseous micro-bubbles that diffract light giving the material its opacity. The resulting glass ingots are crushed and molded into parts that are dried and sintered. The quartz glass that is produced is white and opaque and does not generally belong to any types of quartz glass and contains a 100 ppm to 130 ppm OH content.
The manufacture and shaping of quartz glass is unlike processes that are used to manufacture typical glass. The higher temperatures are necessary due to the fact that quartz glass does not flow but softens and becomes viscous.
Shaping and forming of quartz glass may require diamond cutting tools due to its hardness. Also, such operating parameters must be optimized since the quartz glass is also brittle and there is a limited force that can be applied before cracking or fracture occurs. Some of the mechanical processes include:
The quartz glass is quite complex to thermoform due to its high melting point and steep viscosity, allowing it to be formed on a very narrow temperature range. If the temperature is too low, the glass is solid; if the temperature is too high, the glass is less viscous and volatile resulting in evaporation of the parts. In addition to this, single or multiple annealing steps are required to relieve the thermal stress and prevent fracture induced by hot forming. The following are some hot forming methods which a manufacturer can use in order to enhance the glass product:
This chapter presents the notable properties and characteristics of quartz glass.
Purity is one of the most important aspects in quartz glass manufacturing. Contaminants, even in very low levels, influence the thermal, electrical and optical properties of the resulting quartz glass and material in contact in their final application. Strict handling precautions must be taken at the starting material source and all stages of production to ensure high purity. The most common impurities are metal oxides (Al2O3, Fe2O3, MgO, etc.), water, and chlorine.
Water is present in quartz glass as hydroxyl (OH) groups. The OH content can change depending on the thermal treatment and amount of moisture to which the quartz glass is exposed at an elevated temperature. OH influences infrared transmission, viscosity and attenuation. High levels of OH reduces infrared transmission. OH also lowers thermal stability; higher OH content means that the quartz glass is not suitable for high temperature end applications. An annealing step may reduce the OH content of the quartz glass in electric fused quartz glass.
Quartz glass is chemically inert to most chemical compounds: water, salt and acids, making it an advantageous material in chemical laboratories and industries. It is essentially impermeable to gases. Hydrofluoric acid and phosphoric acid are the only agents that can etch and disintegrate quartz glass at ambient temperatures. However, alkali and alkali earth agents attack the surface, causing accelerated devitrification. 0.1 mg of alkali per square centimeter of alkali compounds can amplify to transform all of the semi-stable molecules. Even fingerprints, which contains traces of alkali, can trigger devitrification.
Quartz glass is known for its very low coefficient of thermal expansion (CTE). Thermal expansion refers to the fractional change in size of an object in response to the change of its temperature. For most materials, CTE is directly proportional to temperature change. Quartz glass also has excellent thermal shock resistance, which can withstand sudden and extreme changes in temperature. Quartz glass also has low thermal conductivity.
Quartz glass is softened starting at 1630°C and acts like a viscous liquid at high temperatures like most glass types. This state occurs at a wide range of temperature, and viscosity decreases with increasing temperature. Viscosity is also increased by the presence of impurities.
Quartz glass has almost similar mechanical properties compared to other glass types. Quartz glass has high compressive strength, but also exhibits high brittleness. Surface defects can also affect the overall strength of this material. Machine-polished parts tend to be weaker than fire-polished ones. Also, the age of the glass also affects reliability due to exposure to the environment.
Quartz glass has been a subject of research due to its extensive optical transmission properties, covering the ultra-violet regions, visible and infrared wavelengths. It can be further enhanced through addition of doping materials. Transmission is influenced by the quartz glass‘ purity and OH content. The increase in metallic impurities and OH-molecular vibrational and rotational excitations can lead to light absorption and hence affect the consequent transmission.
Quartz glass is an excellent electrical insulator, retaining high resistivity at elevated temperatures. It has a high dielectric strength. This is due to the absence of charged mobile ions in the molecular lattice and the strong silicon-oxygen bond which imparts very low polarizability to the structure.
The table below summarizes the some of the important property coefficients absolute to quartz glass, which are 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 |
The following are the common applications of quartz glass:
A majority of the applications of quartz glass utilize its optical properties due to its wide transparency range and superior light transmittance, ranging from the ultraviolet to infrared regions. Quartz glass is not easily damaged by ultra-violet and high energy radiation. Light can pass through a quartz glass in a functionalized optical path with minimal distortions. Examples of products with optical applications are: prisms, lenses, beam splitters, polarizers, mirrors and windows.
High purity quartz glass is used in various lamps and lighting systems, such as mercury lamps, halogen lamps, xenon lamps, ultra-violet lamps and arc and filament lamps which provide light source at high temperatures. These lamps are utilized in several industries, among which are sterilization and cleaning apparatuses in the food and medical industries and exposure devices in the semiconductor industry.
Quartz glass material is a good but expensive alternative, since it is chemically inert, to other glass types which cannot withstand high temperature application for a specific use. Common applications are glasswares, plates and tubes.
Fused silica has highly efficient wave transmission in the ultraviolet spectrum and is commonly used to make ultraviolet windows, lenses, and optics.
This section gives the recommended practices when handling or using quartz glass products, to preserve its valued characteristics and maximize its service life:
Quartz glass may be used for a long time if they are kept clean before and after it is used. Even small amounts of impurities can promote gradual devitrification. It is recommended to use clean and lint- free, powder free, or cotton gloves when handling the quartz glass to prevent further contamination.
Quartz glass may be cleaned by immersing it in a >7% Ammonium Bifluoride solution for no longer than ten minutes or a >10% by volume Hydrofluoric Acid solution for no longer than five minutes. After cleaning, it must be thoroughly rinsed by deionized or distilled water and dried.
Quartz glass must be stored in an enclosed container when not in use to protect it from surface flaws and moisture that could affect the quality and performance of the quartz glass. Ideally, the glass must be wrapped. In case of a tube, the end openings must be covered.
Quartz glass can resist extreme heat and thermal shock better than other glass types. However, heat and thermal shock resistance is lower when the quartz glass is thicker. Also, thick and opaque glass products can develop cracks with rapid temperature change.
Before it is annealed, quartz reaches a distortion point, or the strain point. When a quartz glass is cooled very rapidly after the distortion temperature (approximately 1100°C), distortions may again be developed.
Quartz glass has a relatively low thermal expansion coefficient. The fused quartz may crack if another material of significantly higher coefficient is attached, fastened or clamped into it.
Due to its low thermal conductivity, cracks may develop on the surface of the glass when it is heated locally or when it comes in contact with a flame, at a temperature above the distortion point. Also, quartz glass becomes less viscous with increasing temperature. It is advised to take this into consideration when utilizing quartz glass as a finished product or as a component of another equipment or device.
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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