Question 15.2: Silicon Carbide Silicon carbide (SiC) provides outstanding o......
Silicon Carbide
Silicon carbide (SiC) provides outstanding oxidation resistance at temperatures even above the melting point of steel. SiC often is used as a coating for metals, carboncarbon composites, and other ceramics to provide protection at extreme temperatures. SiC is used as an abrasive in grinding wheels and as particulate and fibrous reinforcement in both metal matrix and ceramic matrix composites. It is also used to make heating elements for furnaces. SiC carbide exists as two polymorphs: α–SiC, which has the hexagonal zincblende (wurtzite) crystal structure, and β–SiC, which has the cubic zincblende (sphalerite) structure.
How many atoms of each type are there per unit cell in α–SiC? What type of bonding do you expect dominates the behavior and why?
The crystal structure of α–SiC can be described as a hexagonal lattice with a basis of Si (0, 0, 0), Si (2/3, 1/3, 1/2), C (0, 0, 3/8), and C (2/3, 1/3, 7/8). There is one lattice point per unit cell of the hexagonal lattice and two Si atoms in the basis so there are two Si atoms per unit cell.
There is one lattice point per unit cell of the hexagonal lattice and two C atoms in the basis so there are two C atoms per unit cell. The unit cell of SiC is shown in Figure 15-2.
Si and C are both Group IV elements. The electronic structure of Si is [Ne] 3s²3p², and the electronic structure of C is [He] 2s²2p². (In fact, in solid form, each Si and C atom forms four sp³ hybrid orbitals each containing one electron; these electrons occupying sp³ orbitals are then shared covalently between the Si and C atoms so that each atom has four complete orbitals.) We can calculate the fraction of covalent bonds according to Equation 2-1 in Section 2-6, which includes a term for the difference between the electronegativities of the two elements. The electronegativity of Si is 1.8, and the electronegativity of C is 2.5.
Fraction covalent = exp(-0.25ΔE²) (2-1)
Fraction covalent = exp[-0.25(2.5 – 1.8)²] = 0.88
Thus the majority of the bonds are covalent.
Again, we have not considered the specific interstitial sites of hexagonal structures, but the wurtzite structure can be envisioned as a hexagonal close-packed structure of Si atoms with half of the tetrahedral sites occupied by C atoms. The tetrahedral sites are formed, for example, by three adjoining atoms in the A layer of the HCP structure and a fourth atom in the B layer. Therefore, each carbon atom is surrounded by and bonded to four silicon atoms, and each silicon atom is surrounded by and bonded to four carbon atoms as shown in Figure 15-2.
