Introduction
Silicon carbide, SiC, is a very hard and strong non-oxide ceramic that possesses unique thermal and electronic properties. With strengths ranging from 15 GPa in polycrystalline bodies up to 27 GPa in SiC single crystals and its excellent creep resistance, silicon carbide also lends itself to many high-temperature mechanical applications. Although it oxidizes in air at above 1600°C, silicon carbide's upper limit of stability is around 2500°C and has a melting temperature of around 2830°C, and its peculiarly good thermal conductivity (comparable to that of copper) make it a very useful material for use as heating elements in furnaces.
In addition, SiC has a narrow band gap (2.2 eV in α-SiC and 3.3 in β-SiC) which makes it valuable as a low-temperature semiconductor. In fact, silicon carbide so readily surrenders electrons at elevated temperatures, it shows behavior consistent with metallic conductors after a certain temperature limit.
Structure and Phases
Silicon carbide, due to the close proximity of silicon and carbide on the periodic table, is a highly covalent material that forms tetrahedra that are centered around either carbon or silicon atoms. These tetrahedra form a close-packed structure with half of the tetrahedral sites filled, and occur primarily in the α-SiC and β-SiC phases.
β-SiC takes the diamond cubic structure with half of the carbons being replaced with silicon; this is a very stable structure that is conducive to phononic heat conduction. For example, diamond, beryllia, and aluminum nitride all take this structure and have good to excellent thermal conductivity. The similarity in atomic radii also contribute to this good conductivity, as disproportionate atoms in a lattice tend to cause phonons to scatter more readily.
α-SiC is the more common form of silicon nitride and occurs in a hexagonal close-packed structure with the SiC tetrahedra half-filled. This is the form of SiC that is used in polishing paper and grinding media and is produced by the β-α phase transformation that occurs between 1500°C to 1600°C. However, the α-SiC is also highly irregular in that it possesses crystallographic faults every four to six lattice spacings.
Synthesis
The Acheson process is the most prevalent method of producing silicon carbide in massive quantities and involves running huge currents through enormous tanks of silica sand and carbon logs (coke). The silicon and carbon react at high temperature to produce α-SiC and carbon monoxide according to the very simple reaction
SiO2 + 3C = SiC + 2CO
This method is very energy intensive and generally is powered by hydroelectric sources. Other methods of synthesizing silicon carbide include gas-phase synthesis or vapor deposition processes.
Densification
Unfortunately, this silicon carbide powder is difficult to sinter due to the low diffusion coefficients inherent in the material. Regular sintering conditions usually result in large amounts of neck growth without densification, a process exacerbated by the volatization of SiO2 to SiO + O2 which condenses at the neck region between particles. Thus, several methods of densifying SiC have been developed.
Probably the cheapest dense silicon carbide is clay-bonded SiC which is simply SiC powder combined with 10-50% clay and fired. The clay inhibits evaporation/condensation of SiO2, and it turns out that this combination produces a refractory with very good thermal shock resistance.
Another method is by reaction bonding silicon carbide; this process entails mixing SiC powder with a binder which is carburized when heated in a reducing furnace. This SiC-carbon body is then infiltrated with liquid silicon which reacts and produces a dense mass of silicon carbide. The only downfall to this process is the presence of excess silicon that may be left unreacted.
Nitride-bonded silicon carbide is made by dispersing fine silicon powder in a compact of larger silicon carbide particles and sintering in a nitrogen furnace; this produces a predominantly silicon carbide structure with a large percentage of its porosity filled with silicon nitride. Of course, the addition of sintering aids works as well too, with 0.5% carbon and 0.5% boron being a popular combination. This enhances densification significantly because the carbon reacts with surficial SiO2 to prevent the SiO2 to SiO condensation/evaporation while the boron is theorized to eliminate surface diffusion and modify grain boundary energy to make the dihedral angle conducive to pore shrinkage.
Applications
Silicon carbide is used as a semiconductor replacement for silicon in many high-powered applications because of its high temperature capabilities, high frequency abilities, and good switching speed. However, SiC also has found use in ballistic armoring in the form of fiber reinforcers or wet/dry-milled silicon carbide combined with aluminum nitride.