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Manufacture of natural and synthetic quartz glass

As the title suggests, there are two different ways to produce quartz glass. In one of these, high-purity quartz or other minerals that contain silicon dioxide are melted by various heat sources. This class of materials is called natural quartz glass. The other way that leads to synthetic fused silica uses gaseous silicon chemicals (e.g. SiCl4) which, in the presence of oxygen, are burned to form silicon dioxide.

Electric melting is the most widely used method of manufacturing quartz glass. A distinction is made between two methods:

  • Continuous melting:
    In the continuous process, quartz sand is continuously poured into a vertical melting furnace, which essentially consists of a heat-resistant crucible surrounded by electrical heating elements. A neutral or slightly reducing atmosphere is maintained inside the crucible, which prevents the silicon dioxide from reacting with the crucible. The molten glass emerges from an opening at the bottom of the crucible, with the geometry of the products being set in the form of rods, tubes or plates.
  • Block or batch melting:
    In the batch process, a larger amount of the quartz raw material is placed in a heat-resistant vacuum chamber which is surrounded by heating elements. Although this process was traditionally used to manufacture large individual quartz glass blocks, the process can also be used to manufacture smaller, shape-optimized bodies made of quartz glass.

The purity of the quartz glass is essentially determined by the degree of preparation of the raw materials and the process used. Heraeus only uses highly processed quartz sand in connection with very strict quality control in the manufacture of its products.

Historically, the first method of making natural quartz glass was the small-scale melting of quartz crystals in a flame. The Heraeus chemist Richard Küch began melting rock crystal in an oxyhydrogen (H2 / O2) flame for the first time more than 100 years ago. Since then, quartz glass has been manufactured at Heraeus on an industrial scale using this process.

Today, flame-melted natural quartz glass is produced on a large scale by a continuous process in which high-purity quartz sand is dosed into a high-temperature hydrogen / oxygen flame and deposited in a container made of refractory materials. The viscous melt is slowly drawn through a nozzle in the bottom of this container and solidifies in a shape determined by the geometry of the nozzle. In this way it is possible to produce a block of transparent, natural quartz glass with the desired cross-section (round, rectangular or hollow), which is cut off at intervals and removed for further processing.

In this process, the silicon-containing raw materials (e.g. silicon tetrachloride, STC) are burned in the presence of oxygen to form nanoparticles from silicon dioxide, which are also known as "soot particles". Since the raw materials are specifically manufactured and processed, they are available in an exceptionally high degree of purity; therefore, the resulting synthetic quartz glass has a very low content of metallic impurities.

Since chemical vapors (silicon-containing raw materials) are used in the manufacturing process, it is known as chemical vapor deposition (CVD). Two process families should be mentioned: In one case, the deposited nanoparticles are melted directly onto a compacted quartz glass layer; in the other, the soot particles collect and are compressed into transparent quartz glass in a secondary process step (this process is known as vitrification).

Single-stage production of quartz glass

Chemical Vapor Deposition (CVD)
In the manufacture of optical fiber core rods, quartz glass is coated with a defined refractive index inside quartz glass tubes. The chemicals are introduced into the pipe by a carrier gas. The reaction to form soot is triggered by a heat source. CVD processes are differentiated according to the type of heat source used. Either a flame (MCVD), an oven (FCVD) or a plasma (PCVD) is used as the heat source. All gases that did not react are treated in a gas scrubber.
More information on the manufacture of optical fibers
More information on quartz glass tubes

Plasma Outside Deposition (POD)
In this process, a plasma heat source is used to "burn" chemical raw materials and apply a thin layer of glass to a rotating target body. This target can be a tube or a solid rod that does not necessarily have to have a round shape. Due to the high temperature of the plasma, this process is best suited for the production of fluorine-doped quartz glass. The maximum achievable content of fluorine in silica is a function of the coating temperature. The maximum fluorine content is limited because quartz glass is also etched away by fluorine. The higher the fluorine content in the gases, the slower the separation takes place.

The POD process is generally used to manufacture highly fluorine-doped quartz glass, which has a lower refractive index than non-doped quartz glass. This different index of refraction is required for optical fibers. The products available are heavily fluorine-doped tubes and rods, but Heraeus also offers this process as a service.
Learn more about fiber optic products

Two-stage production of quartz glass

In this process, soot particles are deposited on a rotating support rod (the so-called "bait rod") (Outside Vapor Deposition, OVD) or at the end of a rod that is pulled upwards (Vapor Axial Deposition, VAD; axial vapor deposition) , deposited. The soot particles accumulate and form a porous body with a density that is less than 25% of the density of quartz glass. This porous soot body is then consolidated into a transparent quartz glass body.

Due to its large surface, the porous soot body can easily be doped. In the fiber optic industry, hydrogen is replaced by chlorine in a dehydration step before the soot body is vitrified.