Silicon Carbide (SiC): Part One

Originally produced using a high temperature electro-chemical reaction of sand and carbon, silicon carbide is recognized as one of the most promising structural materials due to its excellent high temperature strength, good oxidation, and thermal shock resistance to name just a few advantages.
The secret of these excellent attributes lies in the composition of the material which is composed of tetrahedra of carbon and silicone atoms which inherently display extremely strong bonds within the crystal lattice.

Silicon carbide (SiC) has been recognized as one of the most promising structural materials for many mechanical or thermomechanical applications because of its excellent high-temperature strength, good oxidation and thermal shock resistance, high hardness, and low specific weight. It is the only chemical compound of carbon and silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon.

Silicon carbide is an excellent abrasive and has been produced and made into grinding wheels and other abrasive products for over one hundred years. Today the material has been developed into a high quality technical grade ceramic with very good mechanical properties. It is used in abrasives, refractories, ceramics, and numerous high-performance applications. The material can also be made an electrical conductor and has applications in resistance heating, flame igniters and electronic components. Structural and wear applications are constantly developing.

As mentioned above, the monolithic SiC ceramics and SiC-based ceramic composites are promising structural materials because of their excellent heat resistance and mechanical properties. Moreover, silicon carbide fibre-reinforced composites are of specific interest for future thermonuclear fusion reactor applications as they exhibit a low radioactivity by neutron transmutations, a high stability after the reactor shut-down, good high-temperature properties, a low plasma contamination and a low specific weight.

Silicon Carbide Production

Silicon Carbide is derived from powder or grain, produced from carbon reduction of silica. It is produced as either fine powder or a large bonded mass, which is then crushed. To purify (remove silica) it is washed with hydrofluoric acid.

There are three main ways to fabricate the commercial product. The first method is to mix silicon carbide powder with another material such as glass or metal, this is then treated to allow the second phase to bond.

Another method is to mix the powder with carbon or silicon metal powder, which is then reaction bonded.

Finally silicon carbide powder can be densified and sintered through the addition of boron carbide or other sintering aid. It should be noted that each method is suited to different applications.

Key Silicon Carbide Properties

– Low density
– High strength
– Low thermal expansion coefficient
– High thermal conductivity
– Extreme hardness
– High elastic modulus
– Excellent thermal shock resistance
– Superior chemical inertness
– Semiconductor
– Refractive index greater than a diamond

Silicon Carbide Typical Uses

– Fixed and moving turbine components
– Suction box covers
– Seals, bearings
– Ball valve parts
– Hot gas flow liners
– Heat exchangers
– Semiconductor process equipment

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Table 1: Common values of the Silicon Carbide properties

Silicon carbide is composed of tetrahedra of carbon and silicon atoms with strong bonds in the crystal lattice. This produces a very hard and strong material. Silicon carbide is not attacked by any acids or alkalis or molten salts up to 800°C. In air, SiC forms a protective silicon oxide coating at 1200°C and is able to be used up to 1600°C. The high thermal conductivity coupled with low thermal expansion and high strength gives this material exceptional thermal shock resistant qualities. Silicon carbide ceramics with little or no grain boundary impurities maintain their strength to very high temperatures, approaching 1600°C with no strength loss.

Chemical purity, resistance to chemical attack at temperature, and strength retention at high temperatures has made this material very popular as wafer tray supports and paddles in semiconductor furnaces. The electrical conduction of the material has lead to its use in resistance heating elements for electric furnaces, and as a key component in thermistors (temperature variable resistors) and in varistors (voltage variable resistors).

Figure 1: Silicon Carbide products

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