Silicon carbide

Crystal System

SiC takes two types of crystal systems—the cubic structure b-SiC and the hexagonal structure a-SiC. b-SiC takes a zinc blende type structure with a lattice constant of a=0.4349 nm and the spacing between adjacent layers is 0.2512 nm. a-SiC takes many types. 4H and 6H a-SiC have a hexagonal primitive cell with the number of layers of 4 and 6, respectively. 15R, 21R, 33R, and 51R a-SiC have the rhombohedron primitive cell with the number of layers of 15, 21, 33, and 51, respectively.

Uses

Silicon carbide (SiC), nearly as hard as diamonds, is used as an abrasive in grinding wheels and metal-cutting tools, for lining furnaces, and as a refractory in producing nonferrous metals.

Description

Silicon carbide is a hard covalently bonded material predominantly produced by the carbothermal reduction of silica. Silicon carbide is made by heating silica sand and petroleum coke packed around electrodes in an electric resistance furnace to above 2200°C. Depending on the exact reaction conditions the resulting silicon carbide is either a fine powder or a bonded mass that requires crushing and milling to produce a usable feedstock. This material is very resistant to abrasion and to corrosion with a molten slag. It also has excellent resistance to thermal spalling. However as it is a carbide, it will oxidise readily, silicon carbide has a fairly high conductivity.
Several hundred structures of silicon carbide (polytypes) have been identified which have different stacking arrangements for the silicon and carbon atoms. The simplest structure is a diamond structure which is designated /3-SiC. Other structures are either hexagonal or rhombic and are referred to as a-SiC.

Chemical Properties

light grey powder

Chemical Properties

Silicon carbide is a yellow to green to bluishblack, iridescent crystalline substance. Colorless when pure.

Physical properties

the properties of silicon carbide are that it is a refractory material (high melting point), it has excellent thermal conductivity and low thermal expansion, consequently it displays good thermal shock resistance. In addition, the high hardness, corrosion resistance and stiffness lead to a wide range of applications where wear and corrosion resistance are primary performance requirements. Silicon carbide possesses interesting electrical properties due to its semiconductor characteristics, the resistance of different compositions varying by as much as seven orders of magnitude.

Physical properties

Green to bluish black, iridescent crystals. Soluble in fused alkali hydroxides. Abrasives best suited for grinding low-tensilestrength materials such as cast iron, brass, bronze, marble, concrete, stone and glass, optical structural, and wear-resistant components. Corroded by molten metals such as Na, Mg, Al, Zn, Fe, Sn, Rb, and Bi. Resistant to oxidation in air up to 1650°C. Maximum operating temperature of 2000°C in reducing or inert atmosphere.

Physical properties

Semiconductor (Eg=3.03 eV) soluble in fused alkali hydroxides

Characteristics

Silicon carbide is a premium-priced unit which is employed in lining work for its uniformity, abrasion resistance and dimensional stability. It is resistant to most organics, inorganic acids, alkalis and salts in a variety of concentrations except to hydrofluoric acid and acid fluorides. The permeable units have the lowest resistance.

Uses

Manufacture of abrasives and refractories, brake linings, heating elements, and thermistors.

Uses

Silicon carbide is widely used as an abrasive in grinding and cutting glasses; in polishing glass and sharpening stones. It is used in the manufacture of porcelain, refractory brick, furnace linings, and emery paper. The compound also is used in semiconductor technology.

Uses

Production Methods

SiC was synthesized at the end of the 18th century with the name carborundum, and it was used as abrasive. In 1920 in Russia, O.V. Losev discovered two types of SiC from electro-luminescence and named them Type I and Type II. The electro-luminescence of Type I is greenish blue colored and that of Type II changes from orange to violet via yellow and green as the voltage increases from 6 to 28 V. The pure crystal is nominally obtained as 6H (a-II) SiC and the crystals with impurity are obtained as 15R and 4H. a-SiC (6H, 15R) is mainly synthesized using the sublimation method by heating the mixture of coke and silica sands at 2000℃ in the electrical furnace. SiC is segregated by cooling after melting Si in the carbon crucible.

Definition

Bluish-black, iridescent crystals. Insoluble in water and alcohol; soluble in fused alkalies and molten iron. Excellent thermal conductivity, electrically conductive, resists oxida- tion at high temperatures. Noncombustible, a nui- sance particulate.

Production Methods

Silicon carbide, also known by the trade name Carborundum, has been manufactured and used as an abrasive material for more than a century. It combines desirable properties of hardness and thermal resistance. It is produced by heating high-grade silica sand with finely ground carbon at 2400°C in an electric furnace. In its powdered or granular form, it has been used as the abrasive material in “paper and wheels.” It is used as an abrasive in sandblasting and engraving. It has been incorporated into ceramics and glass and especially into refractory ceramic materials.

Preparation

Silicon carbide is prepared by fusing a mixture of silica (sand) and carbon (coke) with some salt and saw dust in an electric arc furnace at 3000°C.
SiO2(Sand)+3C(Coke)--(3000℃)--Sic+2CO
Salt and saw dust is added to infuse air into the product so that it can be broken into pieces easily. The product obtained is first washed with strong acid followed by strong base to remove basic and acidic impurities respectively. Finally, it is washed with water.

Reactions

SiC is formed by the reaction with C at high temperature. It is not etched by acid other than the mixture of hydrofluoric acid and nitric acid. It reacts with caustic solution generating H2 to produce alkali silicate. Good alloys can be formed at the composition ratio of 0%–100% with Ge. (Refer to Ge.)

General Description

Yellow to green to bluish-black, iridescent crystals. Sublimes with decomposition at 2700°C. Density 3.21 g cm-3. Insoluble in water. Soluble in molten alkalis (NaOH, KOH) and molten iron.

Reactivity Profile

Silicon carbide is non-combustible. Generally unreactive. Soluble in molten alkalis (NaOH, KOH) and in molten iron.

Hazard

Upper respiratory tract irritant. Probable carcinogen.

Health Hazard

Silicon carbide, in certain forms, may be a cause of pneumoconiosis in exposed workers. Silicon carbide has generally been considered to be an inert dust with little adverse effect on the lungs.

Flammability and Explosibility

Non flammable

Industrial uses

Silicon carbide is one of the very few totally man-made minerals used in refractory work. These are:
Oxide-bonded-(S102, A1201, Si02 or silicate glass), silicon oxynitride (Si2 ON2), silicon nitride (S13N4)
The first three of these four bonding systems result in a permeable product, and when failure occurs in such masonry systems due to chemical degradation, it is usually due to attack on the bond. Thus, permeable units (where the corrodent penetrates the mass) are far more rapidly damaged.
Self-bonded”—(silicon carbide to silicon carbide) impermeable ones, where the attack is limited to the surface.
The self-bonded product can be manufactured by either of two methods: reaction bonded or sintered. Both will produce an impermeable unit, and they have roughly comparable chemical resistances, but they do not have identical physical properties.

Safety Profile

Suspected carcinogen with experimental neoplastigenic data. A nuisance dust.

Potential Exposure

A potential danger to those involved in the manufacture of silicon carbide abrasives, refractories, and semiconductors. Silicon carbide fibers are also produced in fibrous form as reinforcing fibers for composite materials.

Incompatibilities

Dust may form explosive mixture with air. Sublimes with decomposition @ 2700C.

Waste Disposal

Landfill

References

1.https://en.wikipedia.org/wiki/Silicon_carbide#Uses
2.http://www.softschools.com/formulas/chemistry/silicon_carbide_uses_properties_structure_formula/282/
3.https://www.britannica.com/science/silicon-carbide
4.http://accuratus.com/silicar.html
5.https://www.intechopen.com/books/mostdownloaded/silicon-carbide-materials-processing-and-applications-in-electronic-devices
6.https://www.azom.com/article.aspx?ArticleID=3271
7.https://www.chemicalbook.com/productchemicalpropertiescb2431905.htm