Page 31 - Materials Chemistry, Second Edition

18 2 Solid-State Chemistry

shape, because their crystal lattices are riddled with defects. A metallic glass, in
contrast, will spring back to its original shape much more readily.
Amorphous metallic materials may be produced through a variety of procedures.
Pure individual metal powders may be melted at high temperatures in an arc furnace.
Depending on the composition of the melt, supercooling may be possible, resulting
in a vitreous solid rather than a crystalline form. Although facile glass-forming
solids such as B 2 O 3 will form amorphous solids even upon relatively slow cooling
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1
(e.g.,1Ks ), metals generally require very high rates (> 10 Ks ) to prevent
crystal growth. These high rates are accomplished by placing a material with
high thermal conductivity (e.g., copper) in contact with a molten metal or alloy.
This method is referred to as “melt spinning” or “melt extraction,” and results in
metallic ribbons up to 15 cm wide and 30 mm thick.
Another common procedure uses a vapor deposition technique to form amorphous
metallic thin films (see Chapter 4). Upon thermal annealing, the irregularly deposited
atoms in the film have an opportunity to assemble into a crystalline array. A
procedure referred to as ball-milling may also be used to create amorphous metal
alloy powders. This method uses a mixture of crystalline powders that is vigorously
agitated with a stainless steel ball within a round vessel. This results in regions of high
pressure that cause local melting of the crystalline powders, breaking apart metallic
bonds, and facilitating atomic diffusion along preferential crystallite interfaces.

2.2.3. Covalent Network Solids

These solids are characterized by very strong, directional covalent bonds between
their constituent atoms. This bonding array generally leads to high melting points
and bulk hardness. Due to the arrangement of the atoms comprising these solids,
a variety of physical properties may be observed, as evidenced by the very different
properties exhibited by the three allotropes (i.e., discrete structural forms) of carbon.
For instance, diamond is an extremely hard, insulating material that is transparent to
light, whereas graphite is a soft, black solid that is capable of conducting electricity
along the graphitic layers of the extended solid. Buckminsterfullerene (C 60 ) is very
different from either of these carbon forms, being soluble in aromatic solvents, and
thereby capable of undergoing chemical reactions. Other examples of covalent
network solids are quartz (SiO 2 ) x , (BN) x , (ZnS) x , (HgS) x , and the two allotropes of
selenium – grey (Se 1 ) and red (Se 8 ) x . It should be noted that although the discrete
units of the extended solid are covalently bound, there may also be layers that are
held together by weaker intermolecular forces such as van der Waal interactions
(Figure 2.2).

2.2.4. Molecular Solids
This class of solids features discrete molecules that are held together by rather weak
intermolecular forces such as dipole–dipole, London Dispersion, and hydrogen