Biomedical Engineering Reference
In-Depth Information
found some use as an orthopedic implant material.
(
Kingery, 1976
).
Carbon is often included with ceramics because of its
many ceramic-like properties, even though it is not
a compound and conducts electrons in its graphitic form.
Carbon is an interesting material since it occurs with two
different crystal structures. In the diamond form, the
four valence electrons of carbon lead to four nearest
neighbors in tetrahedral coordination. This gives rise to
the diamond cubic structure (
Fig. 3.1.2-2A
). An in-
teresting variant on this structure occurs when the tet-
rahedral arrangement is distorted into a nearly flat sheet.
The carbon atoms in the sheet have a hexagonal ar-
rangement, and stacking of the sheets (
Fig. 3.1.2-2B
)
gives rise to the graphite form of carbon. The (covalent)
bonding within the sheets is much stronger than the
bonding between sheets.
The existence of an element with two different crystal
structures provides a striking opportunity to see how
physical properties depend on atomic and electronic
structure (
Table 3.1.2-1
).
usually carbon and are joined in a linear chainlike struc-
ture by covalent bonds. The bonds within the chain re-
quire two of the valence electrons of each atom, leaving
the other two bonds available for adding a great variety of
atoms (e.g., hydrogen), molecules, functional groups, etc.
Based on the organization of these chains, there are two
classes of polymers. In the first, the basic chains are all
straight with little or no branching. Such ''straight'' chain
or linear polymers can be melted and remelted without
a basic change in structure (an advantage in fabrication)
and are called thermoplastic polymers. If side chains are
present and actually form(covalent) links between chains,
a 3D network structure is formed. Such structures are
often strong, but once formed by heating will not melt
uniformly on reheating. These
are
thermosetting
polymers.
Usually both thermoplastic and thermosetting poly-
mers have intertwined chains so that the resulting
structures are quite random and are also said to be
amorphous like glass, although only the thermoset poly-
mers have sufficient cross linking to form a 3D network
with covalent bonds. In amorphous thermoplastic poly-
mers, many atoms in a chain are in close proximity to the
atoms of adjacent chains, and van der Waals and hydrogen
bonding holds the chains together. It is these interchain
bonds together with chain entanglement that are re-
sponsible for binding the substance together as a solid.
Since these bonds are relatively weak, the resulting solid
is relatively weak. Thermoplastic polymers generally
have lower strengths and melting points than thermo-
setting polymers (
Billmeyer, 1984
).
Inorganic glasses
Some ceramic materials can be melted and upon cooling
do not develop a crystal structure. The individual atoms
have nearly the ideal number of nearest neighbors, but an
orderly repeating arrangement is not maintained over
long distances throughout the 3D aggregates of atoms.
Such noncrystals are called glasses or, more accurately,
inorganic glasses and are said to be in the amorphous state.
Silicates and phosphates, the two most common glass
forming materials, have random 3D network structures.
Microstructure
Polymers
Structure in solids occurs in a hierarchy of sizes. The in-
ternal or electronic structures of atoms occur at the finest
scale, less than 10
4
m
m (which is beyond the resolving
power of the most powerful direct observational tech-
niques), and are responsible for the interatomic bonds. At
the next higher size level, around 10
-4
m
m (which is
detectable by X-ray diffraction, field ion microscopy,
scanning tunneling microscopy (STM), etc.), the long-
range, 3D arrangement of atoms in crystals and glasses can
be observed.
At even larger sizes, 10
3
to 10
2
m
m(detectable by light
and electron microscopy), another important type of
structural organization exists.When the atoms of amolten
sample are incorporated into crystals during freezing,
many small crystals are formed initially and then grow
until they impinge on each other and all the liquid is used
up. At that point the sample is completely solid. Thus,
most crystalline solids (metals and ceramics) are com-
posed of many small crystals or crystallites called grains
The third category of solid materials includes all the
polymers. The constituent atoms of classic polymers are
Table 3.1.2-1 Relative physical properties of diamond and graphite
a
Property
Diamond
Graphite
Hardness
Highest known
Very low
Color
Colorless
Black
Electrical conductivity
Low
High
Density (g/cm
3
)
3.51
2.25
Specific heat (cal/gm atm/deg.C)
1.44
1.98
a
Adapted from D. L. Cocke and A. Clearfield, eds., Design of New Materials,
Plenum Publ., New York, 1987, with permission.
