Making of Fused Quartz and Fused Silica
As the title implies, there are two distinctive ways to making of fused quartz and fused silica. One that starts with high purity quartz, or other Silicon dioxide containing minerals that are fused using various heat sources. This Material class is termed fused quartz. The other is called fused silica and starts with gaseous silicon containing chemicals (e.g. SiCl4) that are burned in the presence of oxygen to form silicon dioxide.
Fused Quartz
Electric Fusion
The electric fusion is the most commonly used melting process for manufacturing quartz glass. Two methods of electric fusion can be used:
- Continuous fusion:
In the continuous method, quartz sand is poured into the top of a vertical melter that consists of a refractory metal crucible surrounded by electric heating elements. The interior is maintained in a neutral or slightly reducing atmosphere that keeps the silica from reacting with the refractory metal. The melted material exists the bottom orifice of the crucible, which is shaped to produce rods, tubes or plates or other products of various dimensions. - Batch or boule fusion:
In the batch fusion method, a large quantity of raw material is placed inside a refractory lined vacuum chamber which also contains heating elements. Although this method has historically been used to produce large single boules of material, it can also be adapted to produce much smaller, near-net shapes.Purity is mostly determined by the extent of refinement of the raw material and the process used. Heraeus uses only very highly refined quartz sand coupled with rigorous quality control to make its products.
Flame Fusion
Fused Silica
In this process, the silicon containing precursors (e.g. Silicon Tetra Chloride; STC) are burned in the presence of oxygen to form nano-particles of silicon dioxide also called soot. Because the precursors are specifically produced and refined, they are available in exceptionally high purity and the resulting fused silica has a very low metallic impurity content.
As the production process involves vapors of chemicals (silicon containing precursors), it is called chemical vapor deposition (CVD). There are two sets of process families, one where the deposited nano-particles are directly melted to a condensed fused silica layer, and one, where the soot is accumulated and in a secondary process step condensed to transparent fused silica (this process is called vitrification).
The electric fusion is the most commonly used melting process for manufacturing quartz glass. Two methods of electric fusion can be used:
- Continuous fusion:
In the continuous method, quartz sand is poured into the top of a vertical melter that consists of a refractory metal crucible surrounded by electric heating elements. The interior is maintained in a neutral or slightly reducing atmosphere that keeps the silica from reacting with the refractory metal. The melted material exists the bottom orifice of the crucible, which is shaped to produce rods, tubes or plates or other products of various dimensions. - Batch or boule fusion:
In the batch fusion method, a large quantity of raw material is placed inside a refractory lined vacuum chamber which also contains heating elements. Although this method has historically been used to produce large single boules of material, it can also be adapted to produce much smaller, near-net shapes.Purity is mostly determined by the extent of refinement of the raw material and the process used. Heraeus uses only very highly refined quartz sand coupled with rigorous quality control to make its products.
Historically, the first method of producing fused quartz was by small-scale fusion of quartz crystals in a flame. Heraeus chemist Richard Küch first began fusing quartz rock crystal in a hydrogen/oxygen (H2/O2) flame more than 100 years ago. Heraeus has been producing quartz glass on an industrial scale with this process ever since.
Today flame fused quartz is manufactured on a large scale by a continuous process in which highly refined quartz sand is fed through a high temperature flame and deposited on the surface of a melt contained in a tank lined with refractory material. The viscous melt is withdrawn slowly through a die in the bottom of this tank, and it solidifies in a shape determined by the die. In this way it is possible to produce an ingot of transparent fused quartz of the required cross-section (round, rectangular or hollow), which is cut off at intervals, and removed for further processing.
Fused Silica
In this process, the silicon containing precursors (e.g. Silicon Tetra Chloride; STC) are burned in the presence of oxygen to form nano-particles of silicon dioxide also called soot. Because the precursors are specifically produced and refined, they are available in exceptionally high purity and the resulting fused silica has a very low metallic impurity content.
As the production process involves vapors of chemicals (silicon containing precursors), it is called chemical vapor deposition (CVD). There are two sets of process families, one where the deposited nano-particles are directly melted to a condensed fused silica layer, and one, where the soot is accumulated and in a secondary process step condensed to transparent fused silica (this process is called vitrification).
One Step Fused Silica Production