Biomedical Engineering Reference
In-Depth Information
covalent bonds) and a particular atomic arrangement or
crystal structure. As with ionic solids, localization of the
valence electrons in the covalent bond renders these ma-
terials poor electrical conductors.
closely packed [i.e., have themaximumpossible number of
near (contacting) neighbors] so that the number of primary
bonds is maximized and the energy of the aggregate is
minimized.
Crystal structures are often represented by repeating
elements or subdivisions of the crystal called unit cells
(
Fig. 3.1.2-1
). Unit cells have all the geometric properties
of the whole crystal. A model of the whole crystal can be
generated by simply stacking up unit cells like blocks or
hexagonal tiles. Note that the representations of the unit
cells in Fig. 3.1.2-1 are idealized in that atoms are shown as
small circles located at the atomic centers. This is done so
that the background of the structure can be understood. In
fact, all nearest neighbors are in contact, as shown in
Fig. 3.1.2-1B
(
Newey and Weaver, 1990
).
Metallic bonding
The third, the least understood of the strong bonds, is the
metallic bond. Metal atoms, being strong electron donors,
do not bond by either ionic or covalent processes. Nev-
ertheless, many metals are very strong (e.g., cobalt) and
have high melting points (e.g., tungsten), suggesting that
very strong interatomic bonds are at work here, too. The
model that accounts for this bonding envisions the atoms
arranged in an orderly, repeating, three-dimensional (3D)
pattern, with the valence electrons migrating between the
atoms like a gas.
It is helpful to imagine a metal crystal composed of
positive ion cores, atoms without their valence electrons,
about which the negative electrons circulate. On the av-
erage, all the electrical charges are neutralized throughout
the crystal and bonding arises because the negative elec-
trons act like a glue between the positive ion cores. This
construct is called the free electron model of metallic
bonding. Obviously, the bond strength increases as the ion
cores and electron ''gas'' become more tightly packed
(until the inner electron orbits of the ions begin to over-
lap). This leads to a condition of lowest energy when the
ion cores are as close together as possible.
Once again, the bonding leads to a closely packed
(atomic) crystal structure and a unique electronic configu-
ration. In particular, the nonlocalized bonds within metal
crystals permit plastic deformation (which strictly speaking
does not occur in any nonmetals), and the electron gas ac-
counts for the chemical reactivity and high electrical and
thermal conductivity of metallic systems (
Hummel, 1997
).
Materials
The technical materials used to build most structures are
divided into three classes, metals, ceramics (including
glasses), andpolymers. These classesmay be identifiedonly
roughly with the three types of interatomic bonding.
Metals
Materials that exhibit metallic bonding in the solid state
are metals. Mixtures or solutions of different metals are
alloys.
About 85% of all metals have one of the crystal struc-
tures shown in
Fig. 3.1.2-1
. In both face-centered cubic
(FCC) and hexagonal close-packed (HCP) structures,
every atom or ion is surrounded by twelve touching
neighbors, which is the closest packing possible for
spheres of uniform size. In any enclosure filled with close-
packed spheres, 74% of the volume will be occupied by
the spheres. In the body-centered cubic (BCC) structure,
each atom or ion has eight touching neighbors or eightfold
coordination. Surprisingly, the density of packing is only
reduced to 68% so that the BCC structure is nearly as
densely packed as
Weak bonding
In addition to the three strong bonds, there are several
weak secondary bonds that significantly influence the
properties of some solids, especially polymers. The most
important of these are van derWaals bonding and hydrogen
bonding, which have strengths 3-10% that of the primary
C-C covalent bond.
the FCC and HCP structures
(
Hummel, 1997
).
Ceramics
Ceramic materials are usually solid inorganic compounds
with various combinations of ionic and covalent bonding.
They also have tightly packed structures, but with special
requirements for bonding such as fourfold coordination
for covalent solids and charge neutrality for ionic solids
(i.e., each unit cell must be electrically neutral). As might
be expected, these additional requirements lead to more
open and complex crystal structures. Aluminum oxide or
alumina (Al
2
O
3
) is an example of a ceramic that has
Atomic structure
The 3Darrangement of atoms or ions in a solid is one of the
most important structural features that derives from the
nature of the solid-state bond. In themajority of solids, this
arrangement constitutes a crystal. A crystal is a solid whose
atoms or ions are arranged in an orderly repeating pattern in
three dimensions. These patterns allow the atoms to be
