Biomedical Engineering Reference
In-Depth Information
The melt is then poured and cooled to create a solid glass. The reasons
that the rawmaterials are not all oxides are numerous: the melting points
of some of the alternatives are lower and they do in fact decompose
to the oxides, releasing gases, which sweep other gases out of the melt
and ultimately help to stir the melt to mix it. The latter is called fining
and helps to ensure that the material is homogeneous. Most commercial
glasses are melted in large gas furnaces in huge tanks made from
refractory materials, except for some very special glasses that require
ultra-high purities. For example, highly reactive (bioactive) glasses that
will be made into medical devices have to be melted in platinum crucibles,
which puts their manufacturing costs up significantly. Silica cannot be
made by simply melting, since the temperatures required are too high,
so silica will be addressed separately. The real difference in glass comes
in the forming of the glasses.
How the glass is poured determines the shape of the glass obtained.
A required shape can be made by pouring into a mould, often made of
preheated graphite. Glass powders can be made by pouring the melt into
water, creating a frit (see Figure 1 in colour section), which can then
be ground. Fibers can be produced by drawing strands from the melt,
which can be done at a controlled rate using a rotating drum.
Flat (window) glass for the construction and automotive industry is
made using the float glass process, where the melt floats on molten
tin that is kept in a reducing atmosphere to ensure no oxidation. The
glass comes out almost perfectly flat under those circumstances. Not all
glasses can be floated on molten tin owing to the viscosity-temperature
relations, so some may be drawn up by sheet from the melt or they may
be fed into an overflowing trough, which results in a highly pristine and
thin flat glass; the process is known as a fusion draw.
Regardless of the forming processes used for the glasses, there is always
a danger due to the nature of glass cooling, which dictates that different
cooling rates will yield different structures. The bonding together as a
solid of any two structures that do not have the same volume is going to
result in stress; such stress can ultimately destroy the glass. The problem
is that, if the thermal conductivity of the glass were good, then the
cooling would spread more evenly. Unfortunately, this is not the case,
and frozen-in stresses are commonplace. Hence the glasses are typically
annealed; that is, they are either cooled or heated to a point where the
said stresses have enough kinetic energy to relax, and then the glass is
subsequently cooled slowly so as not to reintroduce further stresses. If
the glasses are not annealed, then the competing stresses will likely lead
to crack growth, which results in catastrophic failure. The mechanical
