1. 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 present in all forms of rocks: igneous, metamorphic and sedimentary. When quartz-bearing rock weathers or erodes, it collects in soil, water and sand.
The chemical formula of quartz is SiO2. Silicon-Oxygen (Si-O) bonds are polar and covalent bonds. Elemental silicon contains four valence electrons that bond silicon atoms to four oxygen atoms. One oxygen atom combines with two silicon atoms to form the body-centered tetrahedral crystal system of quartz. The tetrahedral crystal system consists of four corner oxygen atoms and one central silicon atom. In a tetrahedron, the O-Si-O bonds form a 109° angle. In the SiO4 tetrahedral network, angular oxygen atoms connect the central silicon atom. Si-O-Si bonds form 144°. The structure of networked SiO4 is open with wide spaces, thus giving quartz a hexagonal crystal shape.
Quartz can be manufactured into quartz glass, which is valued for its exceptional purity and used in a wide range of applications. Quartz glass contains no additives. It is sometimes called fused silica or fused silica; the difference between the two is that fused silica is made from pure silica (SiO2), while fused silica is made from synthetic precursors. Natural quartz is rarely used in industry because it may contain a variety of impurities; the most commonly used raw material is "cultured quartz," i.e. quartz crystals grown under controlled conditions.
Quartz glass is valued for its unique and high-value properties. One of the reasons is its low coefficient of thermal expansion, high air permeability, and wide light transmission range.
2. Production of Quartz Glass
This chapter describes the steps for converting pristine quartz into shaped fused silica glass.
washing and drying
Dirt, moisture and contaminants present in natural quartz are removed in the early stages of processing, which can affect the quality and performance of the quartz glass to be produced. This only applies to mined quartz.
The purpose of this step is to reduce the pristine quartz to a size suitable for the fusion method and machine to be used. Natural quartz goes through a series of comminution steps, such as crushing and grinding (ball or roller milling). Quartz is very brittle in nature, which makes crushing very easy. Then, the particle size is analyzed and the larger particles are isolated.
At this stage, thermal energy is used to break the strong silicon-oxygen bond. As the temperature increases, more bonds are broken, causing the quartz to become less viscous. After shaping and cooling to its final form, the ordered crystalline structure of the SiO2 molecules transforms into glassy, amorphous, and metastable quartz.
Depending on the desired level of purity and end use, natural quartz can be homogenized and formed by the following fusion methods:
This method produces the type I quartz glass known to the industry. Electrofusion is used if high purity and low hydroxyl (OH) content (> 1 ppm – 30 ppm) are to be obtained. The quartz glass with low OH content produced by this method has high infrared transmittance, but there are beautiful bubbles and pull lines on the glass surface. The raw material is natural quartz particles, which can be divided into the following production methods:
Continuous Mode: Quartz sand is continuously fed into the top of a refractory metal crucible column containing an electrical heating device. The inner cavity of the crucible is maintained in a dry and vacuum-tight atmosphere to prevent the molten quartz from reacting with the refractory material. After passing through the hot crucible column, the molten quartz is collected in an orifice located at the bottom of the column, where it is shaped and cut into plates, tubes and rods. This method is suitable for high volume manufacturing.
Batch or Boule Mode: A large quantity of quartz is placed in a refractory-lined vacuum chamber that also contains an electrical heating device. After the quartz is melted, the viscous melt is collected and shaped into its final form. This method is used to make quartz glass with more complex shapes and details. flame fusion
In this method, natural quartz or synthetic precursors can be the starting material. Natural quartz is passed through a chamber with a high temperature hydrogen/oxygen (H2/O2) flame until the raw material melts. If silicon tetrachloride (SiCl4) is used, a gaseous synthesis precursor, it reacts with the H2/O2 flame. The viscous melt is deposited in a refractory-lined vacuum chamber, slowly collected by a mold at the bottom of the vessel, and formed into its final shape. Due to the direct contact with the H2/O2 flame, the method produces quartz glass with an OH content of 150-200 ppm, derived from natural quartz, while synthetic silica has an OH content of up to 1000 ppm.
Glass made from crystalline quartz by flame melting is type II, while glass made from synthetic precursors is type III. Type III synthetic quartz glass is the product of a chemical reaction. The combustion of silicon tetrachloride produces synthetic quartz and leaves behind environmentally toxic by-products, chlorine and hydrochloric acid.
The process is similar to flame fusion, using a plasma flame without water vapor as the heat source. Plasma fused silica glass has high purity, low OH content, low bubble content, and no wire drawing.
Natural quartz or synthetic precursors may be the starting material for this method. The quartz glass produced by the combustion of synthetic precursors in a plasma flame is called Type IV.
Quartz sand is melted in an electric arc furnace. The resulting glass ingot is crushed and shaped; the shaped part is dried and sintered. The quartz glass produced by this method is white and opaque and generally does not belong to any type of quartz glass. However, it is comparable to clear quartz in terms of purity levels.
Forming and Finishing Processes
Manufacturers can process quartz glass like any other kind of glass.
Due to its hardness, the shaping and shaping of quartz glass may require diamond cutting tools. Furthermore, such operating parameters must be optimized because quartz glass is also brittle and has limited forces that can be applied before cracking or fracture occurs. Some mechanical processes include:
Cutting: Use band and wire saws, chop saws, CO2 lasers and water jet cutters to cut quartz glass. Using a laser cutter leaves a smooth cut, while using a saw to cut quartz glass leaves a rough cut. Thick quartz glass sheets may require multiple consecutive cuts if a single cut is not enough. Annealing may be required to relieve thermal stress and prevent it from shattering.
Drilling: Drilling can be done with a diamond drill as described in the fused silica glass product. Laser drilling machines can be used to cut thin and small sheets. Proper cooling must be ensured to prevent premature tool wear.
Grinding: The quartz glass surface may become smooth and its thickness may be reduced, depending on the end use.
The high melting point and steep viscosity of quartz glass make thermoforming very complex, so it can be formed in a very narrow temperature range. If the temperature is too low, the glass is solid; if the temperature is too high, the viscosity and volatility of the glass will decrease, causing the part to evaporate. In addition to this, single or multiple annealing steps are required to relieve thermal stress and prevent breakage caused by thermoforming. Here are some thermoforming methods that manufacturers can use to strengthen glass products:
Welding: Two components of quartz glass are joined together by welding. The end of each component is heated and a piece of quartz glass is melted to fill the seam or gap at the seam. To avoid thermal stress, it is critical to maintain a sufficiently high temperature.
Folding: During this process, the quartz glass rod is reduced to a smaller diameter. The metal tube is heated to the softening temperature of quartz glass and pressure is applied under the tube to push the glass rod.
Elongation and Compression: Positive or negative radial force is applied to elongate or compress the quartz glass rod to its final diameter. This is done at the softening temperature of quartz glass and optimum force must be applied to prevent cracking and cracking.
Glassblowing: A piece of fused silica glass is expanded with the help of a blowpipe to obtain a hollow shape.