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

GTBL042-03 GTBL042-Callister-v2 September 6, 2007 15:33

3.9 Carbon • 59

Carbon

atom

Figure 3.17 The structure of graphite.

single molecule is denoted by C 60. Each molecule is composed of groups of carbon

atoms that are bonded to one another to form both hexagon (six-carbon atom) and

pentagon (five-carbon atom) geometrical configurations. One such molecule, shown

in Figure 3.18, is found to consist of 20 hexagons and 12 pentagons, which are arrayed

such that no two pentagons share a common side; the molecular surface thus exhibits

the symmetry of a soccer ball. The material composed of C 60 molecules is known as

buckminsterfullerene, named in honor of R. Buckminster Fuller, who invented the

geodesic dome; each C 60 is simply a molecular replica of such a dome, which is often

referred to as “buckyball” for short. The termfullereneis used to denote the class of

materials that are composed of this type of molecule.

Diamond and graphite are what may be termednetwork solids,in that all of the

carbon atoms form primary bonds with adjacent atoms throughout the entirety of the

solid. By way of contrast, the carbon atoms in buckminsterfullerene bond together so

as to form these spherical molecules. In the solid state, the C 60 units form a crystalline

structure and pack together in a face-centered cubic array.

As a pure crystalline solid, this material is electrically insulating. However,

with proper impurity additions, it can be made highly conductive and semiconduc-

tive.

Figure 3.18 The structure of a C 60 molecule.