Silicon carbide (SiC) boasts low thermal expansion rates, making it an excellent material choice for mirrors at the focal points of astronomical telescopes. Furthermore, SiC offers excellent rigidity and strength as well as chemical corrosion resistance.
Silicon carbide production typically follows the Acheson Process. This involves mixing silica sand with carbon in an electric resistance furnace before passing an electric current – this method of production is known by this name.
Acheson Process
Modern industrial silicon carbide production relies largely on the Acheson Process created by Edward Goodrich Acheson (1856-1931). This carbothermal reduction process uses petroleum coke and quartz as raw materials, creating SiC through high temperature reactions of over 2000 degC within a heat resistance furnace.
The Acheson process is not energy efficient and emits large volumes of reduced gases (CO, NH3, N2 and H2). To mitigate this effect, a gas collection system can be added; however, this would increase overall energy requirements while adding complexity to engineering designs of furnaces.
Modelling of the Acheson process is essential to understanding its formation at high temperatures and optimizing efficiency of this method of producing SiC. Impurity behaviour must be predicted over the full temperature range (1500-2500 degC) that SiC formation occurs (1500-2500 degC), particularly important when applied to raw materials containing Al, Fe, Ca, Mg and alkaline metals that make up this raw material mixture used in Acheson processes like SiC production.
An innovative approach has been taken to modeling the Acheson process. A mathematical model incorporates conduction, convection and radiation into its heat balance equations using Finite Volume Method – this model was then validated against experimental data to provide an excellent comparison between calculated results and experimental ones.
Lely Process
The Lely process is an all-in-one solution for growing large diameter silicon carbide crystals. The first stage, known as Seed Growth, involves heating small seed crystals of sufficient quality until their size increases through sublimation into an isothermal cavity containing crystalline sources of silicon carbide; heating occurs at temperatures lower than those used to sublimate grains, creating a temperature gradient, providing growth surfaces within this isothermal space for these larger seeds to grow on.
In this preferred embodiment, a system 10 includes a closed-end carbon susceptor or crucible 12 with carbon lining (preferably graphite) at its sealed end and an empty center space defined by it. An induction heater 24 is typically employed as the primary heating source to initiate sublimation of silicon carbide from its source material.
Carbon lining in crucibles must have a high Si/C ratio in order to react with free Si vapor produced during sublimation of SiC and reduce its removal from the process. Hydrocarbon reactions could clog installations and interfere with operation of induction heaters; to protect installations and operation of induction heaters from this potential threat, protective gas such as argon is often flowed through closed ends of crucibles to create an atmosphere conducive for heating precursor material safely.
Chemical Vapor Deposition
Chemical Vapor Deposition, or CVD, is a manufacturing technique used to form films by introducing precursor compounds into a chamber where they react together and deposit on surfaces of substrates. From creating silvery coatings on potato chip bags to complex electronic manufacturing equipment today’s electronics require it, CVD remains a valuable manufacturing technique and continues to expand and be refined.
Silicon carbide manufacturing processes like CVD allow for the creation of products with superior durability that withstand high temperatures, radiation exposure and chemical erosion resistance. Furthermore, design engineers often prefer it due to its lower electrical resistance of about one ohm cm.
Contrary to thermal oxide processes used to produce silicon dioxide, which consume part of what it coats, CVD processes for silicon carbide can be performed on any crystalline semiconductor material. They combine decomposed methyltrichlorosilane (MTS) with hydrogen at elevated temperature and reduced pressure at an elevated temperature to produce pure SiC crystals with theoretical density exceeding 90%.
TevTech HTCVD systems combine an MTS vapor delivery system with precise temperature, pressure and substrate positioning controls to produce unparalleled quality and uniformity in CVD SiC films. This enables you to maximize their use for your application.
Thermal Sintering
Sintering is a versatile process used to transform materials such as ceramics and cemented carbides, metals and polymers into shape. Sintering can also produce sintered silicon carbide which has numerous applications; for instance, flat structures like electronic substrates or porcelain statuary (tape casting), three dimensional pieces such as mechanical seal faces or watch cases or one-of-a-kind prototypes can all be produced using this technique.
Prior to sintering, silicon carbide powder must first be formed into its desired form using one of several processes. Die compaction is often the preferred technique for simpler forms; injection molding should be considered for complex ones. Other technologies exist for shaping long, thin structures like electronic circuit boards or refills for mechanical pencils.
Gel-casting processes involve mixing the powdered material with water or non-aqueous solvent, dispersant, and dispersion aid to form a ceramic slurry. A partial vacuum is then applied to remove air bubbles that could create flaws in the final product, before adding chemical polymerization initiators that cause monomers to link together into rubbery polymer-water gel.
Slurry is cast into molds made of metal, glass, plastic or wax for casting into molds for heating at sintering temperatures to create a molten liquid state – usually comprised of fluorite but sometimes other compositions capable of melting at this temperature and offering electrical conductivity as well.
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