SILICON CARBIDE: CHEMICAL STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The silicon carbide is a solid covalent consisting of carbon and silicon. It is very hard with a value of 9.0 to 10 on the Mohs scale, and its chemical formula is SiC, which may suggest that carbon is bound to silicon by a covalent triple bond, with a positive charge (+) on Si and a negative charge (-) on carbon (⁺ Si≡C ^-).

Actually, the bonds in this compound are totally different. It was discovered in 1824 by the Swedish chemist Jön Jacob Berzelius, while trying to synthesize diamonds. In 1893 the French scientist Henry Moissani discovered a mineral whose composition contained silicon carbide.

This discovery was made while examining rock samples from the crater of a meteorite in Devil's Canyon, USA He named this mineral moissanite. On the other hand, Edward Goodrich Acheson (1894) created a method to synthesize silicon carbide, reacting high-purity sand or quartz with petroleum coke.

Goodrich called the product obtained carborundum (or carborundium) and founded a company to produce abrasives.

Chemical structure

The top image illustrates the cubic and crystalline structure of silicon carbide. This arrangement is the same as that of diamond, despite the differences in atomic radii between C and Si.

All bonds are strongly covalent and directional, unlike ionic solids and their electrostatic interactions.

SiC forms molecular tetrahedra; that is, all the atoms are linked to four others. These tetrahedral units are joined together by covalent bonds, adopting layered crystalline structures.

Also, these layers have their own crystal arrangements, which are of three types: A, B and C.

In other words, a layer A is different from layer B, and the latter from C. Thus, the SiC crystal consists of the stacking of a sequence of layers, the phenomenon known as polytypism occurring.

For example, the cubic polytype (similar to diamond) consists of a stack of ABC layers and therefore has a 3C crystal structure.

Other stacks of these layers also generate other structures, between these rhombohedral and hexagonal polytypes. In fact, the crystalline structures of SiC end up being a "crystalline disorder".

The simplest hexagonal structure for SiC, the 2H (upper image), is formed as a result of the stacking of the layers with the ABABA sequence… After every two layers the sequence repeats, and that's where the number 2 comes from.

Properties

General properties

Molar mass

40.11 g / mol

Appearance

It varies with the method of obtaining and the materials used. It can be: yellow, green, blackish blue or iridescent crystals.

Density

3.16 g / cm3

Melting point

2830 ° C.

Refractive index

2.55.

Crystals

There are polymorphisms: αSiC hexagonal crystals and βSiC cubic crystals.

Hardness

9 to 10 on the Mohs scale.

Resistance to chemical agents

It is resistant to the action of strong acids and alkalis. Also, silicon carbide is chemically inert .

Thermal properties

- High thermal conductivity.

- Withstands high temperatures.

- High thermal conductivity.

- Low linear thermal expansion coefficient, so it supports high temperatures with low expansion.

- Resistant to thermal shock.

Mechanical properties

- High resistance to compression.

- Resistant to abrasion and corrosion.

- It is a light material of great strength and resistance.

- Maintains its elastic resistance at high temperatures.

Properties

It is a semiconductor that can fulfill its functions at high temperatures and extreme voltages, with little dissipation of its power to the electric field.

Applications

As an abrasive

- Silicon carbide is a semiconductor capable of withstanding high temperatures, high voltage or electric field gradients 8 times more than silicon can. For this reason, it is useful in the construction of diodes, transitors, suppressors and high-energy microwave devices.

- With the compound, light-emitting diodes (LEDs) and detectors of the first radios (1907) are manufactured. Currently, silicon carbide has been replaced in the manufacture of LED bulbs by gallium nitride that emits a light that is 10 to 100 times brighter.

- In electrical systems, silicon carbide is used as a lightning rod in electrical power systems, since they can regulate its resistance by regulating the voltage across it.

In the form of structured ceramics

- In a process known as sintering, the silicon carbide particles -as well as those of the companions- are heated to a temperature lower than the melting temperature of this mixture. Thus, it increases the resistance and strength of the ceramic object, by forming strong bonds between the particles.

- Silicon carbide structural ceramics have had a vast range of applications. They are used in disc brakes and motor vehicle clutches, in diesel particulate filters, and as an additive in oils to reduce friction.

- The uses of silicon carbide structural ceramics have become widespread in parts exposed to high temperatures. For example, this is the case of the throat of the rocket injectors and the rollers of the furnaces.

- The combination of high thermal conductivity, hardness and high temperature stability makes the components of the tubes of heat exchangers manufactured with silicon carbide.

- Structural ceramic is used in sandblast injectors, automotive water pump seals, bearings and extrusion dies. It is also the material for crucibles, used in metal smelting.

- It is part of the heating elements used in the smelting of glass and non-ferrous metals, as well as in the heat treatment of metals.

Other uses

- It can be used in the measurement of the temperature of gases. In a technique known as pyrometry, a silicon carbide filament is heated and emits radiation that correlates with temperature in a range of 800-2500ºK.

- It is used in nuclear plants to prevent the leakage of material produced by fission.

- In the production of steel it is used as fuel.

References

1. Nicholas G. Wright, Alton B. Horsfall. Silicon Carbide: The Return of an Old Friend. Material Matters Volume 4 Article 2. Retrieved on May 05, 2018, from: sigmaaldrich.com
2. John Faithfull. (February 2010). Carborundum crystals. Retrieved on May 05, 2018, from: commons.wikimedia.org
3. Charles & Colvard. Polytypism and Moissanite. Retrieved on May 5, 2018, from: moissaniteitalia.com
4. Materialscientist. (2014). SiC2HstructureA.. Retrieved on May 05, 2018, from: commons.wikimedia.org
5. Wikipedia. (2018). Silicon carbide. Retrieved on May 05, 2018, from: en.wikipedia.org
6. Navarro SiC. (2018). Silicon carbide. Retrieved on May 5, 2018, from: navarrosic.com
7. University of Barcelona. Silicon Carbide, SiC. Retrieved on May 05, 2018, from: ub.edu
8. CarboSystem. (2018). Silicium carbide. Retrieved on May 05, 2018, from: carbosystem.com