Biomedical Engineering Reference
In-Depth Information
In covalent solids, bond angles and bond lengths, as well as a
number of the nearest neighbors, are all part of the appropriate
bonding scheme. Thus, due the nature of chemical bonding, even the
truly amorphous materials have some structural SRO and, perhaps,
some MRO. For example, MRO regions of ~15 Å in dimensions
and comprising about 100 atoms have been directly observed in
amorphous carbon [90]. Some order in two-dimensional projections
of thin amorphous three-dimensional structures was found [91].
Besides, covalent amorphous solids were found to exhibit a MRO at
length scales up to 20 Å or so [88]. Such MRO clusters are called
paracrystals [86]. These paracrystals have a crystalline topology
but the atomic positions are highly distorted from those of a perfect
crystal. However, in solids there is a serious problem of very small
particles. Specifically, if the crystal sizes are extremely small, it
is difficult to make a distinction between the truly amorphous
and crystalline solids. Namely, if a powder consists of tiny perfect
crystals with dimensions of 2 nm × 2 nm × 2 nm (8 nm
3
) or less,
both this powder and any bulk materials prepared from this powder
(e.g., by compaction) will be amorphous, just due to the case their
sizes are below the minimal value of LRO. Additionally, in very small
crystals a large fraction of the atoms are located at/or near surface.
Relaxation of the surface and various interfacial effects distort the
atomic positions, decreasing the structural order. Thus, even the
most advanced structural characterization techniques, such as X-ray,
neutron and electron diffraction, as well as transmission electron
microscopy (TEM), have difficulties in distinguishing between the
amorphous and crystalline structures on these length scales [83,
92].
Many studies revealed that the majority of solids could be found
or prepared in an amorphous state. For example, cooling strongly
reduces atomic and/or molecular mobility. Thus, in principle, given
a sufficiently high cooling rate, any liquid can be transformed into
an amorphous solid. As cooling is performed, the material changes
from a super-cooled liquid, with properties one would expect from
a liquid state material, to a solid. The temperature at which this
transition occurs is called glass transition temperature. If a cooling
rate is faster than the rate at which atoms and/or molecules can be
organized into a more thermodynamically favorable crystalline state,
then an amorphous solid will be formed. In contrast, if atoms and/
or molecules have a sufficient time to be organized into structures
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