Biomedical Engineering Reference
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of vacancies, interstitial atoms, impurities, dislocations, grain
boundaries, surfaces, and other interfaces disrupt the periodicity
of otherwise “perfect” crystals and in many cases determine their
physical properties. By contrast, highly disordered solids are those
solids that are so irregular that the concept of a reference crystal
lattice must be abandoned. Such highly disordered materials
are called amorphous materials [82]. As Wikipedia, the free
encyclopedia, has it: an
(from the Greek term αμορφος,
which means “shapeless” or “without form”) solid is a solid, in which
there is no translational and orientational long-range order (LRO)
of the atomic positions [83]. Early researchers categorized solids
as amorphous or crystalline materials based on the macroscopic
properties such as their external shapes, fracture mechanisms, and
optical properties long before X-ray diffraction techniques and other
methods became available to reveal their atomic structures. Only
in the past century, an understanding of the microscopic nature of
amorphous materials has become possible [82]. However, there is
still much debate concerning the exact nature of these materials.
For example, in a recent article, Sheng et al., [84] have mentioned:
“the atomic arrangements in amorphous alloys remain mysterious
at present.”
An amorphous structure is distinctly different from a densely
packed assembly of microcrystals and is closely related to the
structure of a liquid phase. Ideally, an amorphous solid should
be described by the model of a perfectly random structure [85];
however, this is the boundary condition. As such, the structure of
amorphous solids is normally described in terms of statistical
distributions. Nevertheless, prior a further description, one must
specify the existing atomic length scales. The shortest length scale
usually used to describe the structure of a material consists of an
atom and its nearest neighbors, out to perhaps two or three atoms
distant. All solids and liquids have some structure on this scale,
which is called a short-range order (SRO). For crystalline solids,
structural order persists over much longer distances (at least, tens
or hundreds of atomic distances), such that the atoms occupy sites in
a periodic three-dimensional array. Such materials are said to have
a LRO and include most metals and many covalently bonded solids.
Non-crystalline solids, including glasses, lack a LRO, and are said to
be amorphous even though they can have a SRO that is quite well
defined [86].
amorphous
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