Silicon carbide

Silicon carbide is the result of combining carbon and silicon. It is sometimes abbreviated as SiC. It shares many physical characteristics with diamonds, such as appearance, but it is not related to diamonds. While it can occur naturally, it is more often manufactured artificially. Originally, it was used for grinding wheels because of its abrasive characteristics. Since then, many other properties have been identified that make silicon carbide ideal for use in the manufacture of a number of parts for mechanical objects. Its ability to conduct electrical energy and resist heat has made it valuable for use in ceramic parts such as semiconductors and the linings of equipment that carries hot fuels. It shows promise for many future uses, such as inclusion in medical and dental devices.

Background

Silicon carbide does exist in nature, but it is rare. When it does form spontaneously in nature, it is known as moissanite, named after French chemist Henri Moissan. In 1893, Moissan was studying the debris from a meteor that crashed in Arizona's Diablo Canyon more than fifty thousand years ago when he found fragments of a brilliant crystal formation. Moissan initially identified the crystal as diamond, but by 1904, he had amended that statement to correctly describe it as a combination of silicon and carbon. It was named moissanite in his honor by George Kunz, a gemologist with the famed Tiffany & Company jewelry retailer.

The misidentification was not the only issue concerning the discovery of moissanite. Because it was found in a meteor impact crater, some claimed it was not from Earth but was a product of outer space. Others claimed that the combination of carbon and silicon occurred because of the use of the tools used to cut the sample. Eventually, it was accepted that moissanite was not the result of the cutting tools. During the 1950s, very small amounts were found that supported the idea that the substance developed on Earth and not in outer space. It would take nearly one hundred years, however, before the idea of moissanite having extraterrestrial origins was put to rest. That finally occurred in 2003 when enough other samples of moissanite were found in areas that experts were confident contained no outer space debris.

Meanwhile, another scientist, American Edward G. Acheson, had discovered a method for creating artificial silicon carbide. Acheson worked for a time for famed inventor Thomas A. Edison before striking out to be an inventor in his own right in 1884. While trying to combine clay with carbon under high heat generated by electricity, Acheson created some small fragments of a hard, shiny crystal. He called the crystals carborundum and identified them as a good abrasive. By 1894, Acheson's Carborundum Company in southwestern Pennsylvania was incorporating his discovery—silicon carbide—into knife sharpening stones, grinding stones, and powdered abrasive materials. Other inventors also discovered ways to manufacture these crystals, but Acheson is generally credited with the find because he also developed commercial industrial uses for silicon carbide.

Throughout the twentieth century, other properties besides abrasiveness were identified in silicon carbide. In contemporary times, it is found in such varied uses as seals and bearings, heat exchangers, semiconductors, and turbine parts. It is also used in some brake parts for automobiles and in the ceramic plates that make up some bulletproof vests. In addition, it continues to be used as an abrasive in many forms, including sandblasting material and grinding devices.

In addition, moissanite—in both natural and synthetic forms—has become a desirable gemstone for use in jewelry. Its resemblance to a diamond's brilliance was acknowledged when Moissan discovered it, but it would take until the 1980s before a process was developed to make single crystals large enough to be used like diamonds as a setting stone in jewelry. In 1995, the Moissanite Company and others began creating jewelry set with moissanite crystals.

Overview

Silicon carbide in its untreated form is an iridescent crystal that ranges in color from yellow-green to bluish-black. In addition to resembling a diamond in appearance, silicon carbide has a number of physical characteristics and properties that make it appropriate for many uses. It is extremely dense, meaning it weighs a lot for its size. It is very strong and hard, and resistant to breakage. It is not adversely affected by heat; although it conducts heat very well, it does not expand much when exposed to heat, and it is resistant to the effects of sudden heat changes. Ceramics made with silicon carbide resist any change in shape or consistency up to temperatures of 2900 degrees Fahrenheit (1600 degrees Celsius). It is also relatively unaffected by most other chemicals, including acids, alkalis, and salts.

Silicon carbide's resistance to both chemical and physical alterations makes it very valuable for many industrial uses. It can also be produced in different formats, based on how it is created. Sintered silicon carbide is produced by mixing fine silicon carbide powder with a material that will help it reach high temperatures of more than 3600 degrees Fahrenheit (2000 degrees Celsius), without becoming a liquid. This mixture is then heated, often under pressure, in a process known as sintering. The resulting form of silicon carbide is second only to diamonds in hardness and is extremely resistant to both liquids and corrosives. Another way to form silicon carbide is through reaction bonding. This is done by mixing silicon carbide with additional carbon and liquid silicon. This process forms more particles of silicon carbide that bond tightly with the original particles. Producing silicon carbide in this manner enhances the many natural properties of the chemical compound.

In addition to its many uses in manufacturing of industrial goods and its attractiveness to jewelry making, researchers continue to look for other applications for this very versatile chemical compound. One area of study that some believe shows great promise is in bioceramics, or ceramic products that are used for medical purposes. It is thought that silicon carbide can become a viable material for use in such items as tooth implants, joint replacements, and other medical devices.

Bibliography

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Di Pierro, Simonpietro, et al. "Rock-Forming Moissanite." American Mineralogist, vol. 88, 2003, pp.1817–21, www.krist.unibe.ch/pdf/2003/SiC.pdf. Accessed 10 Oct. 2017.

"Edward Goodrich Acheson." Chemical Heritage Foundation, 22 Oct. 2015, www.chemheritage.org/historical-profile/edward-goodrich-acheson. Accessed 10 Oct. 2017.

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