What is Quartz Glass?


Quartz is one of the most abundant and widely distributed minerals in nature. Quartz 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.


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. Natural quartz is rarely used in the industry since it may contain several impurities; the most commonly used raw material is "cultured quartz", which is quartz crystals that are grown in controlled conditions.


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.


Production of Quartz Glass


This chapter presents the steps in transforming the raw quartz into a formed, fused quartz glass.


Washing and Drying


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 banger.

Comminution


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.

Fusion


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:

Electric fusion


This method produces an industrially known Type I quartz glass. Electric fusion method is used if a high level of purity and low hydroxyl (OH) content (> 1 ppm – 30 ppm) is to be obtained. The quartz glass with low OH content produced from this method has high infrared transmission, but aesthetically pleasing bubbles and drawing lines are present in the glass surface. The starting material is natural quartz grains, and may be subject to the following production modes:

Continuous Mode: The quartz sand is continuously fed on top of a refractory metal crucible column which contains an electric heating device. The internal chamber of the crucible is maintained at a dry and vacuum-sealed atmosphere to keep the melted quartz from reacting with the refractory material. After passing through the hot crucible column, melted quartz is collected in an orifice located at the bottom of the column in which it is shaped and cut into plates, tubes and rods. This method is suitable for high volume manufacturing.


Batch or Boule Mode: Large quantity of quartz is placed inside a refractory-lined vacuum chamber which also contains an electric heating device. After the quartz is fused, the viscous melt is collected and shaped into its final form. This method is used to create quartz glass with more sophisticated shapes and details.


Flame Fusion


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.


Glass produced from crystal quartz through flame fusion is classified as Type II, and from synthetic precursor 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.


Plasma Fusion


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.


Electric Arc Fusion


The quartz sand is melted in an electric arc furnace. The resulting glass ingots are crushed and molded; the formed parts are dried and sintered. In this method, the quartz flask glass produced is white and opaque and does not generally belong to any types of quartz glass. However, it is comparable to transparent quartz in terms of purity level.

Shaping and Finishing Processes


A manufacturer can process quartz glass just like any other kinds of glass.


Mechanical forming


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:


Cutting: Band and wire saws, chop saws, CO2 lasers, and water jet cutters are used to cut the quartz glass. Using a laser cutter can leave a glazed and smooth cut, while those quartz glass which used saw cutting can leave a rough cut. Thick quartz glass sheet slabs may require multiple consecutive cuts if a single cut would not suffice. Annealing may be required to relieve the thermally-induced stress and to keep it from shattering.


Drilling: As detailed in the fused quartz glass product, holes may be produced using a diamond driller. A laser driller may be used to cut thin, small plates. Proper cooling must be ensured in order to prevent the tools from premature worn-out.


Grinding: The quartz glass surface may be smoothened and its thickness may be reduced, depending on the end-use application.


Hot Forming


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:


Welding: Two components of quartz glass are joined together through a weld. The ends of each component are heated, and a piece of quartz glass is melted to fill the gap in the seam or joint. It is critical to keep the temperature just high enough in order to avoid thermal stress.


Collapsing: In this process, quartz glass rods are reduced to a smaller diameter. A metal tube is heated to the softening temperature of the quartz glass and pressure is applied under the tube to push the glass rods.


Elongation and Compression: A positive or negative radial force is applied to elongate or compress the quartz glass rod to its final diameter. This is performed at the softening temperature of the quartz glass, and an optimal force must be applied in order to prevent fracture and cracking.


Glass Blowing: A piece of molten quartz glass is inflated with the aid of a blowpipe in order to acquire a hollow shape.


Properties of Quartz Glass


This chapter presents the notable properties and characteristics of quartz glass.


Chemical Purity


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 tube 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.

Chemical Behavior


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.


Thermal Properties


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 instrument also has low thermal conductivity.


Quartz glass is softened starting at 16300C 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.


Mechanical Properties


Quartz glass has almost similar mechanical properties compared to other glass types. Quartz glass rod 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.

Optical Properties


Quartz crucible 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.

Electrical Properties


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.