Material of the month – silicon carbide

Materials World magazine
,
5 Jan 2016

It occurs in the extremely rare mineral moissanite, and has been used in various applications from LEDS and composite armour to automotive parts. This month, Anna Ploszajski explores silicon carbide. 

At the end of the 18th Century, it was discovered that diamond was an allotrope of carbon. Chemists argued that it must be possible to create diamonds from cheap sources of carbon if the conditions of natural diamond formation in the Earth could be mimicked in the laboratory. The following decades saw many attempts, none of which succeeded in synthesising diamond from carbon, although many tried to claim success. The American Edward Goodrich Acheson was one frustrated chemist, whose attempts at synthetic diamond were under the direction of Thomas Edison for use in his electric lightbulbs in the 1880s. In his research, Acheson heated a mixture of clay  (aluminium silicate), and powdered coke (carbon) in an iron bowl with a carbon arc, and afterwards found shiny hexagonal crystals attached to the carbon electrode. This wasn’t diamond, but it was a compound, which he named carborundum. Acheson would eventually patent this method for producing powdered silicon carbide (SiC), a compound of silicon and carbon, in 1893. It is still the most popular processing route today.

The mineral form of silicon carbide is called moissanite and gets its name from Dr Ferdinand Henry Moissan, who first discovered it in the Canyon Diablo Crater in Arizona in 1904, while studying rock samples from the site of a meteorite impact. The next year he won the Nobel Prize for Chemistry for his work isolating fluorine from its compounds.

Silicon carbide is unusual because it was discovered synthetically before its natural form was unearthed. This is partly because, as minerals go, moissanite is extremely rare, generally brought in on meteorites from space, or found as inclusions in diamond or rocks, such as kimberlite. Moissan’s discovery was disputed by naysayers who claimed that the sample may have been contaminated by synthetic silicon carbide used in saw blades to prepare rock samples.

Sharp scale

These cutting-edge blades and tools were the first application that Acheson found for his new shiny black crystals, and he began to mass-produce them in 1895. Sitting at an impressive 9–9.5 on the Mohs hardness scale, silicon carbide proved to be a much more powerful abrasive than those based on emery, corundum and garnet that came before. Alongside synthetic alumina, silicon carbide abrasives reigned supreme until 1955, when Howard Tracy Hall at the General Electric Company (GE) finally succeeded in producing synthetic diamonds. More durable, wear-resistant, efficient, and with a longer lifespan, synthetic diamond-based abrasives and cutting tools now dominate the high-end markets. GE went on to earn a fortune from Hall’s invention, yet he was only rewarded with a US$10 savings bond in addition to his salary. After GE, Hall became Professor of Chemistry and Director of Research at Brigham Young University before leaving academia to become a missionary. 

To the naked eye, pure moissanite gems look just like diamonds, and their very similar thermal conductivity means that they are often mistaken for one another. But, unlike diamonds, silicon carbide crystals can be strongly birefringent, meaning the crystals exhibit different refractive indices down different axes. For this reason, moissanite jewels are cut along the optic axis to mitigate these effects. To identify counterfit diamonds, jewellers have developed special testing devices that exploit the difference in electrical conductivity between the two otherwise extremely similar stones.

Whereas, SiC powder production involves the Acheson resistance furnace, these synthetic moissanite gems are produced by the Lely Process. This method produces large single crystals by sublimating silicon carbide powder to form a high-temperature species called silicon dicarbide (SiC2) and disilicon carbide (Si2C). This is done under argon at 2,500°C, and singe crystals are deposited on a slightly colder substrate. These crystals can then be cut and shaped into diamond-like gems.

Silicon success

All petrol-heads will certainly have heard of ceramic break pads. These are, in fact, based on silicon carbide. A carbon-fibre reinforced graphite composite disc has silicon infiltrated into it, which reacts with the graphite matrix to form carbon-fibre reinforced silicon carbide. This has the benefits of increasing the hardness, wear resistance and thermal management of the discs for more efficient and higher performance, thanks to the material’s high thermal conductivity, durability, and resistance to corrosive environments compared to conventional iron-based discs.

In February 2015, I was lucky enough to take a work trip to some labs based at the NASA Kennedy Space Centre in Florida. One weekend I took a look round the Visitor Complex and my favourite exhibit was the Space Shuttle Atlantis. The entire spacecraft has been mounted inside an enormous indoor exhibition and visitors can observe it from almost every angle. What struck me most were the impressive scorch marks along the black-tiled bottom edge of Atlantis, testament to the heat of re-entry into the Earth’s atmosphere, which reached temperatures of 1,648°C. That the crew survived re-entry is thanks to silicon carbide. The structural components of the hottest parts of the spacecraft, the nose cap and leading edges of the wings, were made from reinforced carbon-carbon composite impregnated with silicon to form a silicon carbide coating to protect the carbon substrate from oxidation at such elevated temperatures, and bring the crew safely back to Earth.

The astronomical applications of silicon carbide didn’t stop with the ending of the Space Shuttle Programme. The Herschel Space Observatory was launched in 2009 by the European Space Agency. Its aim was to monitor the coldest and dustiest corners of space and observe the formation of new stars and galaxies to trace the path where potentially life-giving molecules, such as water might form. In order to do this, it had to travel 1,500,000km from Earth, and look out into space with an eye capable of seeing far infrared and submillimetre light. This eye was a mirror made from a single piece of silicon carbide, polished to a roughness of less than 30 millionths of a millimetre and then coated in nickel-chromium and highly reflective aluminium. Lighter weight than metal or glass, silicon carbide was used due to its extremely low thermal expansion coefficient, high hardness, rigidity and thermal conductivity. This mirror, at 3.5m across, is the largest silicon carbide structure ever made, and the largest single-component telescope reflector ever sent into space. Thanks to Herschel, we know a lot more about the formation of stars and the transport of water by comets, which may represent the origin of water on Earth. Sadly, the Observatory’s lifetime was limited by the amount of coolant onboard, which the instruments required to function, and the Observatory closed in 2013 .

Short lived 

Silicon carbide is a semiconductor and, like silicon, can be doped with trace amounts of other elements to form diodes, junctions and transistors. Semiconducting silicon carbide first found application as a detector in early radios at the beginning of the 20th Century. In 1907, one of radio’s early pioneers, Captain Henry Joseph Round, observed light coming from a diode that he was investigating for radio detectors. This diode was made from silicon carbide, and his work led to the light emitting diode (LED).

Although silicon carbide was experimented with to make early LEDs, it was soon replaced by gallium nitride (GaN), which gave much brighter light thanks to its direct bandgap, compared to SiC’s less efficient indirect bandgap. However, silicon carbide is still a popular substrate for making GaN-based devices, and it also comes out top in applications that require performance in high temperatures, harsh environments, high voltage and high power. A silicon carbide-based LED can withstand temperatures over 600°C, compared to silicon’s limit of 150°C