Biomedical Engineering Reference
In-Depth Information
However, kinetics occasionally gets the chance to overrule thermody-
namics and will not allow the ordered structure to form if there is not
enough time to arrange the atomic structure and establish the ordered
state: hence the American Society for Testing and Materials (ASTM)
definition of a glass as being a material that has ''cooled from the melt
without crystallizing.'' The logical question would then be: ''How fast
would one have to cool for kinetics to overcome thermodynamics?'' The
speed of this would be dependent upon the composition. Silicate melts
can cool relatively slowly without crystallizing (about 20 degrees per
minute), whereas for a bulk metallic glass it is more like 2 degrees per
second. So, if one has to thermodynamically trick a material into being
a glass, what are the advantages?
The word ''glass'' evolves from the Latin word
glacies
meaning ''ice,''
and by far the most utilized property of glass is its transparency, which
comes as a result of its inherent isotropic nature. Although the atoms are
not organized and are generally quite randomly arranged, the glass as a
whole has a similar structure throughout. While glass can in principle be
made from any mixture of atoms, the majority of commercial glasses are
based upon silicates, and these have a transparency from just into the
ultraviolet to somewhere in the infrared, with a transmission typically
of up to 90%. The clarity of some types of glass used to make optical
fibers is such that the fiber is transparent for miles and miles. A piece
of window glass does not have 100% transparency in the visible mostly
due to reflection loss. The reflection of light from glass depends upon the
refractive index, which is the ratio of the speed the light moves through
a vacuum compared to its speed in the material. Glasses with higher
refractive index reflect more light, and this property is often used for
aesthetic reasons. For example, the ''lead crystal'' that is often used in
fine wine glasses is very sparkly due to the high refractive index of the
lead-containing glass. The name ''lead crystal'' is a little deceiving, as
glasses are certainly not crystalline - they are amorphous in structure.
Reflection loss can be cut down by adding an anti-reflection coating,
which is a coating that is based upon the destructive interference of light
waves reflected from two interfaces. The limits of transparency in the
ultraviolet and the infrared are governed by the electrically insulating
natures of the glass and the type of elements and their bonds, respectively.
Typically, the more electrically insulating a glass, the better ultraviolet
transparency it exhibits. The heavier the elements and the lower the force
constant of the bonds in the glass, the more infrared transparent the glass
is. Another detriment to transparency is the existence of coloring ions.
These are typically either transition metals or rare earths, the transition
