How to Make Silicon Carbide

Silicon carbide is a hard, ceramic material with great heat-resistance under high temperatures that is used in abrasives and provides outstanding heat resistance properties.

SiC is an incredible semiconductor that can be doped with nitrogen and phosphorus to form n-type SiC; aluminum, boron or gallium may be added for p-type SiC production. So how does one go about creating such an indispensable material?

Synthesis

Silicon carbide (SiC) is an ultrahard material composed of silicon and carbon. Although naturally found as moissanite gems, more frequently produced as powder or crystal form for use as abrasives or bulletproof vest ceramic plates, SiC ranks amongst one of the world’s hardest materials, requiring controlled environments half that of our Sun’s temperature to produce on an industrial scale – thus becoming expensive and difficult.

SiC manufacturing begins by creating its raw material by reacting silicon dioxide with carbon at high heat and pressure, producing either black or green solid that can take on various shapes and sizes depending on the raw materials used. SiC is typically produced using resistance furnaces in which carbon source was heated along with silica or quartz sand until chemical reaction took place resulting in an ingot that is then cut up into finished products; this method requires considerable labor intensive work that could yield inferior products depending on raw material quality.

Edward Acheson from Pennsylvania came up with a cheaper and faster method for producing silicon carbide in 1891 and patented his process as Carborundum. Acheson reduced production costs using cheaper natural gas and recycled industrial silicon waste as sources for silicon; coke provided carbon as carbon source while high temperatures proved essential to ensure rapid reactions.

Today’s silicon carbide synthesis involves various reactions and processes that produce raw material or crude for crushing into finished grains or powders for sale to customers. Washington Mills employs various crushing, milling, and classifying equipment to ensure their products comply with ANSI, FEPA, and JIS standards.

Raw material is combined with non-oxide sintering aids known as binder, including organosilicon binder. Binders can then be added to coarse, fine and nanosized silica particles for compacting either by extrusion or cold isostatic pressing – this step being important as its effects depend on crystal morphology and grain size of the final product.

Reactions

Acheson process is the go-to solution for creating pure silicon carbide. It involves mixing silica with petroleum coke and heating it at high temperatures until chemical reactions take place to produce silicon carbide. Once produced, this green-colored silicon carbide material has excellent thermal and mechanical properties that make it suitable for use in both abrasive and refractory applications.

Silicon carbide production can be controlled by adjusting the concentrations of two precursor compounds: an aqueous sucrose solution and silicon tetrachloride. Silicon tetrachloride must be added in sufficient quantity to provide the required ratio of three carbon atoms for every silicon atom, creating a homogenous mixture of silica and carbon that becomes highly reactive when heated – this material is known as reaction-bonded silicon carbide (RBSC).

Washington Mills utilizes an impressive array of crushing, milling and classifying equipment that meets all North American, European and Japanese standards as well as custom solutions tailored specifically for each industry. Once formed, the RBSC is processed further into grains and powders for distribution across a range of markets worldwide.

Silicon carbide possesses an interlocked crystal structure held together covalently by covalent bonds, consisting of two primary coordination tetrahedra of four silicon and four carbon atoms that form Polytypes that link through their corners to form polar structures called Polytypes. Silicon carbide is insoluble in water, alcohol and acid environments as well as having interesting semiconductor characteristics; furthermore it exhibits resistance against corrosion and abrasion at temperatures and environments beyond water insoluble inversion. It offers great toughness as a hard material resistant against both corrosion and abrasion resistance; resistance from corrosion as well as resistance from corrosion/abrasion in any conditions/environments/climates/ environments/situations/environments/environments/situations/situations/situations etc.

Reaction sintering takes place in an oxygen-free atmosphere to avoid carbon monoxide emissions and other impurities from forming, while silicon metal acts as a catalyst. Aluminum has the capacity to slow the reaction while molybdenum and boron offer intermediate effects.

After reaction sintering, ingots are cooled and graded by experienced workers to meet different application needs. Grading of ingots is critical, as its determination affects their chemistry and crystallography – an aspect which ultimately determines its performance in its intended application.

By-products

Silicon carbide’s byproducts are invaluable resources in manufacturing an assortment of products, with silicon tetrachloride used to make chlorine and vinyl chloride which have many uses in industries like automobile manufacturing. Silicon monoxide produced during processing can also be used to produce kerosene and paraffin wax; and calcium carbide and sodium carbonate may also be produced as useful byproducts.

Silicon carbide production generates considerable waste material. Most of this consists of unreacted sand or coke that does not react fully with pure silicon to form silicon carbide products; typically this material is collected and sent to landfills; however, an innovative new process for reusing byproducts from silicon carbide production could significantly decrease waste generation.

RECOSiC(c)’s proprietary process uses inert heating of leftover raw materials like quartz sand and petroleum coke in order to allow their full reaction into silicon carbide production. No open air furnaces are needed – meaning it can take place inside a factory building with reduced pollution levels and contaminants, plus recycled carbon dioxide can provide energy costs savings while eliminating natural gas use entirely.

Once silicon carbide has been produced, it must be processed into finished grains and powders using classification equipment from Washington Mills. This process determines the chemistry and crystallography of its final form – while Washington Mills provides customers with access to various crushing, grinding and classification machines for optimal product results.

Solid ingots produced can then be cut into blocks or shaped to meet various requirements, from making abrasives and advanced refractories, to serving as raw materials for metallurgy. Doping of these solid ingots with nitrogen or phosphorus creates n-type semiconductors; doping with beryllium, boron aluminum gallium creates p-type semiconductors.

Workers exposed to silicon carbide byproducts such as carborundum can increase their chances of diffuse interstitial pulmonary fibrosis – which may cause serious lung damage and even lead to death – through prolonged contact. Furthermore, this material has also been reported as the cause of dermatitis for some individuals.

Purification

Silicon carbide (SiC) is an industrial material with many uses. It can be doped with various metals to alter its characteristics for particular tasks – doping with nitrogen or phosphorus can turn SiC into a semiconductor while aluminium, boron or gallium doping turns it into conductor material. Whatever its application, SiC must always be purified to ensure high quality performance as any impurities in it could significantly compromise performance.

Purifying SiC begins with a gel of precursor compounds that is dehydrated to form powder. Next, this powder is combined with non-oxide sintering aid and mixed with an appropriate binder (organosilicon binders can also be used), before being compacted and shaped via extrusion or cold isostatic pressing. Finally, after completion of shaping and compacting processes have taken place, the product is then exposed to an aqueous stripping or etching compound to remove any oxide layer formation on structures – helping reduce levels of nitrogen impurities significantly.

Once stripping is completed, the product can then be exposed to hydrogen gas at temperatures exceeding 1200 deg C for at least four times, in order to cause silicon carbide to produce an organic layer called silicon dioxide on its surface. Its thickness depends on temperature and exposure time; typically this step occurs one or four times.

This technique has been in use for more than 100 years and is very successful at extracting nitrogen from SiC. It is more reliable than carbothermal reduction and produces purer material; however, the technique is expensive and complex; raw material costs present an obstacle for wider adoption of this technology; however, costs may be reduced by scaling back reactor size or using cheaper fuels such as coal or petroleum coke as fuel sources.