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.
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. 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.
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.