Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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58 • Chapter 3 / Structures of Metals and Ceramics

deduced from the chemical formulas, the structures for some silicates are among the

most complex of all the inorganic materials.

3.9 CARBON

Carbon is an element that exists in various polymorphic forms, as well as in the

amorphous state. This group of materials does not really fall within any one of the

traditional metal, ceramic, polymer classification schemes. However, we choose to

discuss these materials in this chapter since graphite, one of the polymorphic forms,

is sometimes classified as a ceramic. This treatment of the carbon materials will focus

on the structures and characteristics of graphite, diamond, the fullerenes, and carbon

nanotubes. The characteristics and current and potential uses of these materials are

discussed in Section 13.11.

Diamond

Diamond is a metastable carbon polymorph at room temperature and atmospheric

VMSE

Unit Cells–Diamond pressure. Its crystal structure is a variant of the zinc blende, in which carbon atoms

occupy all positions (both Zn and S), as indicated in the unit cell shown in Figure 3.16.

Thus, each carbon bonds to four other carbons, and these bonds are totally covalent.

This is appropriately called thediamond cubiccrystal structure, which is also found

for other Group IVA elements in the periodic table [e.g., germanium, silicon, and

gray tin, below 13◦C (55◦F)].

Graphite

Graphite has a crystal structure (Figure 3.17) distinctly different from that of diamond

VMSE

Unit Cells–Graphite and is also more stable than diamond at ambient temperature and pressure. The

graphite structure is composed of layers of hexagonally arranged carbon atoms;

within the layers, each carbon atom is bonded to three coplanar neighbor atoms by

strong covalent bonds. The fourth bonding electron participates in a weak van der

Waals type of bond between the layers.

Fullerenes

Another polymorphic form of carbon was discovered in 1985. It exists in discrete

molecular form and consists of a hollow spherical cluster of sixty carbon atoms; a

C

Figure 3.16 A unit cell for the diamond cubic crystal

structure.