Silicon carbide: Variety History Physical and Chemical Properties

What is silicon carbide

Silicon carbide is an inorganic substance with a chemical formula of SiC, which is smelted in a resistance furnace with raw materials such as quartz sand, petroleum coke (or coal coke), wood chips (salt is required to produce green silicon carbide), and the like. Silicon carbide also exists in nature as a rare mineral, moissanite. Among the non-oxide high-tech refractory raw materials such as C, N, B, etc., silicon carbide is the most widely used and economical one, which can be called corundum or refractory sand. The silicon carbide industrially produced in China is divided into black silicon carbide and green silicon carbide, both of which are hexagonal crystals, with a specific gravity of 3.20-3.25 and a microhardness of 2840-3320kg/mm2.

Figure 1: Physical map of silicon carbide

Substance Variety

Silicon carbide has two commonly used basic varieties, black silicon carbide and green silicon carbide, both of which belong to α-SiC.

1. Black silicon carbide contains about 95% SiC, and its toughness is higher than that of green silicon carbide. It is mostly used for processing materials with low tensile strength, such as glass, ceramics, stone, refractory materials, cast iron and non-ferrous metals.

2. Green silicon carbide contains more than 97% SiC and has good self-sharpening properties. It is mostly used for processing hard alloys, titanium alloys and optical glass, and is also used for honing cylinder liners and fine grinding high-speed steel tools. In addition, there is cubic silicon carbide, which is a yellow-green crystal produced by a special process. The abrasive tools used to make it are suitable for superfinishing of bearings. The surface roughness can be processed from Ra32-0.16 micron to Ra0.04- 0.02 microns.

Development History

Silicon carbide is a kind of carbide accidentally discovered in the laboratory by American Acheson in 1891 during the electric fusion diamond experiment. At that time, it was mistaken for a mixture of diamonds, so it was named emery. It was researched by Acheson in 1893. The method of industrial smelting silicon carbide, which is often called the Acheson furnace, has been used until now. The resistance furnace with carbonaceous material as the furnace core body heats the mixture of quartz SiO2 and carbon to form silicon carbide.

Several events about silicon carbide

In 1905, silicon carbide was first found in meteorites.

In 1907, the first silicon carbide crystal light-emitting diode was born.

1955 A major breakthrough in theory and technology, LELY proposed the concept of growing high-quality carbonization, and since then SiC has been regarded as an important electronic material.

In 1958, the first world silicon carbide conference was held in Boston for academic exchanges.

In 1978, silicon carbide was mainly researched by the former Soviet Union in the 1960s and 1970s. By 1978, the grain purification and growth method of "LELY improvement technology" was adopted for the first time.

From 1987 to now, a silicon carbide production line was established based on CREE's research results, and suppliers began to provide commercial silicon carbide bases.

Physical and chemical properties

Material Properties

Due to its stable chemical properties, high thermal conductivity, small thermal expansion coefficient, and good wear resistance, silicon carbide has many other uses besides being used as an abrasive. The inner wall can improve its wear resistance and prolong the service life by 1 to 2 times; the high-grade refractory material used to make it is heat-shock resistant, small in size, light in weight and high in strength, and has a good energy-saving effect. Low-grade silicon carbide (containing about 85% SiC) is an excellent deoxidizer, which can speed up steelmaking and facilitate the control of chemical composition to improve the quality of steel. In addition, silicon carbide is also widely used to make silicon carbide rods for electric heating elements.

Silicon carbide is very hard, with a Mohs hardness of 9.5, second only to the hardest diamond in the world (grade 10). It has excellent thermal conductivity and is a semiconductor that can resist oxidation at high temperatures.

Silicon carbide has at least 70 crystal forms. α-Silicon carbide is the most common allomorph, formed at high temperatures above 2000 °C, and has a hexagonal crystal structure (like wurtzite). β-silicon carbide, cubic crystal structure, similar to diamond, is formed below 2000 °C, and the structure is shown in the attached picture on the page. Although in the application of heterogeneous catalyst supports, it attracts attention because it has a higher unit surface area than the α-type, and another type of silicon carbide, μ-silicon carbide, is the most stable and has a more pleasant sound when colliding. But until now, these two types have not been used commercially.

Because of its specific gravity of 3.2g/cm3 and high sublimation temperature (about 2700 °C), silicon carbide is very suitable as a raw material for bearings or high-temperature furnaces. It does not melt at any attainable pressure and has relatively low chemical reactivity. Due to its high thermal conductivity, high breakdown electric field strength and high maximum current density, many people try to use it to replace silicon in the application of semiconductor high-power components. In addition, it has a strong coupling effect with microwave radiation, and all its high sublimation points make it practical for heating metals.

Pure silicon carbide is colorless, while industrial production is brown to black due to impurities containing iron. The rainbow-like luster on the crystal is due to the protective layer of silica produced on its surface.

Substance Structure

Pure silicon carbide is a colorless and transparent crystal. Due to the different types and contents of impurities, industrial silicon carbide is light yellow, green, blue or even black, and its transparency varies with its purity. The crystal structure of silicon carbide is divided into hexagonal or rhombohedral α-SiC and cubic β-SiC (called cubic silicon carbide). α-SiC has many different variants due to the different stacking sequences of carbon and silicon atoms in its crystal structure, and more than 70 species have been discovered. β-SiC transforms into α-SiC above 2100°C. The industrial method of silicon carbide is to use high-quality quartz sand and petroleum coke to refine it in a resistance furnace. The smelted silicon carbide blocks are made into products of various particle sizes through crushing, acid-base washing, magnetic separation and sieving or water separation.