Materials for engineering, 3rd Edition - (Malestrom)

4
Materials for engineering
molecule, Fig.1.1(a). In graphite, only three of the four electrons form covalent
bonds, so a layer structure forms, Fig. 1.1(b), and the fourth electron is free,
which gives some metallic properties to this form of carbon. Graphite crystals
are flat and plate-like, and they are so soft that graphite is used as a lubricant.
It is clear from Fig. 1.1 that the different dispositions of the covalent bonds
in space have a profound influence on the atomic arrangements and hence
upon properties of the material.
For many years diamond and graphite were the only known forms of
carbon, but, in 1985, a new form of carbon (buckminsterfullerene), C 60, was
identified, Fig. 1.1(c), as a soccer-ball-like cage of 60 carbon atoms with a
diameter of 0.71 nm. This was the only allotrope of any element to have been
discovered in the twentieth century. Other, larger, fullerene ‘buckeyballs’
have subsequently been discovered and, in 1991, multiwalled carbon nanotubes
were discovered. Two years later, single-walled carbon nanotubes were
discovered with diameters generally varying between 1.3 and 1.6 nm. Figure
1.1(d) is an electron micrograph showing a series of fullerene buckeyballs
within a carbon nanotube. Carbon nanotubes can be synthesized by a number
of techniques, including carbon arcs, laser vaporization and ion bombardment.
(a)
(b)
(c)
5 nm
(d)
1.1 Crystal structure of (a) diamond; (b) graphite; (c) buckminsterfullerene,
C 60 ; and (d) electron micrograph of a series of buckeyballs within a
carbon nanotube: a diagram of the structure is shown underneath.
(Courtesy Dr Andrei Khlobystov.)