Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 5
They consist of concentric, cylindrical, graphitic carbon layers capped on
the ends with fullerene-like domes.
The possibility of encapsulating atoms (and molecules) inside the fullerene
cages is of considerable interest, giving rise to materials with highly modified
electronic properties and thus opening the way to novel materials with unique
chemical and physical properties. The Young’s modulus of multiwalled
nanotubes has been measured to be 1.26 TPa and this high strength may be
exploited by incorporating them in composite materials.
The elements can be divided into two classes, electronegative elements
(such as oxygen, sulphur and the halogens) that tend to gain a few electrons
to form negatively charged ions with stable electron shells, and electropositive
elements (such as metals) that easily dissociate into positive ions and free
electrons. Ionic bonding consists of an electrostatic attraction between positive
and negative ions. If free atoms of an electropositive element and an
electronegative element are brought together, positive and negative ions will
be formed which will be pulled together by electrostatic interaction until the
electron clouds of the two ions start to overlap, which gives rise to a repulsive
force. The ions thus adopt an equilibrium spacing at a distance apart where
the attractive and repulsive forces just balance each other.
Figure 1.2 shows a diagram of the structure of a sodium chloride crystal:
here each Na + ion is surrounded by six Cl – ions and each Cl – is surrounded
by six Na + ions. Many of the physical properties of ionic crystals may be
accounted for qualitatively in terms of the characteristics of the ionic bond;
for example they possess low electrical conductivity at low temperatures,
but good ionic conductivity at high temperatures. The important ceramic
Na
Cl
1.2 Crystal structure of sodium chloride.