What is quartz glass?

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

Quartz is the only stable crystalline silicon dioxide polycrystalline form on the Earth's surface. It exists in all forms of rock: igneous rock, metamorphic rock and sedimentary rock. When quartz containing rocks are weathered or eroded, they will concentrate in soil, water, and sand.

The chemical formula of quartz is SiO2. Silicon oxygen (Si-O) bonds are polar and covalent bond bonds. The element silicon contains four valence electron, which makes silicon atoms bond with four oxygen atoms. One oxygen atom combines with two silicon atoms to form a body centered tetrahedral crystal system of quartz. The tetrahedral crystal system consists of oxygen atoms at four corners and silicon atoms at one center. In a tetrahedron, the O-Si-O bond forms a 109 ° angle. In the SiO4 tetrahedral network, the corner oxygen atoms connect to the central silicon atoms. The formation of Si O-Si bonds is 144 °. The structure of network like SiO4 is open and has a wide space, thus giving quartz a hexagonal crystal shape.

Quartz can be manufactured into quartz glass, which is highly valued for its excellent purity and is widely used in various applications. Quartz glass does not contain additives. It is sometimes called fused silica or fused silica; The difference between the two is that fused silica is made of pure silicon dioxide (SiO2), while fused silica is made of synthetic precursors. Natural quartz is rarely used in industry because it may contain various impurities; The most commonly used raw material is "cultured quartz", which is a quartz crystal 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 permeability and wide light transmission range.

Production of quartz glass

This chapter describes the steps of transforming the original quartz into molded fused silica glass.

Washing and drying

The dirt, moisture, and pollutants present in 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 only applies to mined quartz.

smash

The purpose of this step is to reduce the original quartz to a size suitable for the fusion method and machine to be used. Natural quartz undergoes a series of crushing steps, such as crushing and grinding (ball or roller milling). Quartz is inherently very brittle, which makes crushing very easy. Then, analyze the particle size and separate larger particles.

fuse

At this stage, thermal energy is used to break the powerful silicon oxygen bonds. As the temperature increases, more bonds are broken, resulting in a decrease in the viscosity of quartz. After forming and cooling to the final form, the ordered crystal structure of SiO2 molecules transforms into glassy, amorphous, and metastable quartz.

According to the required purity level and end use, natural quartz can be homogenized and formed through the following fusion methods:

Electrofusion

This method produces type I quartz glass known in industry. If high purity and low hydroxyl (OH) content (>1 ppm – 30 ppm) are to be obtained, the electrofusion method is used. The quartz glass with low OH content produced by this method has high infrared transmittance, but there are beautiful bubbles and wires on the surface of the glass. The raw material is natural quartz particles, which can be divided into the following production methods:

• Continuous mode: quartz sand is continuously fed to the top of refractory metal crucible column containing electric heating device. The inner chamber of the crucible should be kept dry and vacuum sealed to prevent the reaction between melted quartz and refractory materials. After passing through the hot crucible column, the melted quartz is collected in an orifice located at the bottom of the column, where it is formed and cut into plates, tubes, and rods. This method is suitable for mass production.

Batch or Boule mode: Place a large amount of quartz in a vacuum chamber lined with refractory material, which also includes an electric heating device. After the quartz melts, the viscous melt is collected and formed into its final form. This method is used to manufacture 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 passes through a chamber with a high-temperature hydrogen/oxygen (H2/O2) flame until the raw material melts. If silicon tetrachloride (SiCl4), a gaseous synthesis precursor, is used, it will react with the H2/O2 flame. The viscous melt is deposited in a vacuum chamber lined with refractory material, slowly collected by a mold at the bottom of the container, and formed into the final shape. Due to direct contact with the H2/O2 flame, the OH content of the quartz glass produced by this method is 150-200 ppm, derived from natural quartz, while the OH content of synthetic silica is as high as 1000 ppm.

Glass made from crystal quartz by flame melting belongs to type II, while glass made from synthetic precursors belongs to type III. Type III synthetic quartz glass is a product of chemical reactions. The combustion of silicon tetrachloride produces synthetic quartz and leaves behind toxic by-products such as chlorine and hydrochloric acid that are toxic to the environment.

Plasma fusion

This process is similar to flame fusion, using a plasma flame with anhydrous steam 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 materials for this method. The quartz glass produced by the combustion of synthetic precursors in a plasma flame is called type IV.

Arc fusion

Quartz sand is melted in an electric arc furnace. Crush the obtained glass ingot into shape; The formed parts are 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, in terms of purity level, it is comparable to transparent quartz.

Forming and finishing processes

Manufacturers can process quartz glass just like any other type of glass.

Mechanical forming

Due to its hardness, the formation and shaping of quartz glass may require diamond cutting tools. In addition, it is necessary to optimize such operating parameters, as quartz glass is also very brittle and the force that can be applied before rupture or fracture occurs is limited. Some mechanical processes include:

Cutting: Use band and wire saws, splitting saws, CO2 lasers, and water jet cutting machines to cut quartz glass. Using a laser cutting machine can leave a smooth cut, while using a saw to cut quartz glass can leave a rough cut. If a single cut is not enough, thick quartz glass panels may require multiple consecutive cuts. Annealing may be necessary to reduce thermal stress and prevent its fragmentation.

Drilling: As described in fused silica glass products, a diamond drill can be used to drill holes. Laser drilling machines can be used to cut thin and small plates. Proper cooling must be ensured to prevent premature tool wear.

Grinding: The surface of quartz glass may become smooth, and its thickness may decrease depending on the end use.

Thermoforming

The high melting point and steep viscosity of quartz glass make its thermal forming very complex, so it can be formed within a very narrow temperature range. If the temperature is too low, glass is solid; If the temperature is too high, the viscosity and volatility of the glass will decrease, leading to the evaporation of the parts. In addition, single or multiple annealing steps are required to reduce thermal stress and prevent fracture caused by hot forming. The following are some hot forming methods that manufacturers can use to enhance glass products:

Welding: The two components of quartz glass are connected together by welding. The ends of each component are heated, and a piece of quartz glass is melted to fill the gaps or seams. It is crucial to maintain a sufficiently high temperature in order to avoid thermal stress.

Folding: In this process, the quartz glass rod is reduced to a smaller diameter. Heat the metal tube to the softening temperature of quartz glass, and apply pressure under the tube to push the glass rod.

Elongation and compression: Apply positive or negative radial force to extend or compress the quartz glass rod to its final diameter. This is done at the softening temperature of quartz glass, and the best force must be applied to prevent cracking and cracking.

Glass blowing: A piece of fused silica glass expands with the help of a blowpipe to obtain a hollow shape.