Biomedical Engineering Reference
In-Depth Information
metals being very strong coloring agents. Hence, for applications where
thick optical paths are needed, then highly pure glass is required, because
there are common contaminants such as iron in the silica sand used to
make glass.
The second most utilized property of glass is outstanding chemical
durability in a number of different chemical environments. However, it
should be noted that the chemical durability of glass is not always quite
as outstanding as is sometimes believed. As will be discussed later on,
glass does not always either require or desire high chemical durability.
However, in comparison tomany other materials, the chemical durability
of glass is very good, and its isotropic nature lends itself to both the high
durability and the control of the lower durability.
One of the lesser extolled attributes of glass is its ability to be
engineered to meet need. Unlike a crystalline material, in which phases
tend to have very well-defined and rigorously maintained stoichiometry
(e.g. K
2
O
2SiO
2
). That is not to say that one can make a SiO
3
-based
glass, but that potassium silicate glass can be made over a continuous
range of potassia to silica ratios with properties that will also exhibit
a fairly continuous range of values between those of the end members.
Despite the ubiquitous presence of glass in our daily lives, outside of
the glass community, few people are aware of the staggering array of
compositions there are for glass, from plain old silica to exotic glass
formulations containing many components that might not even contain
oxygen. Even more astounding are the advances that have been made
in metallic glasses over recent years. There is an ever-increasing number
of metallic alloys that can be made in bulk form, with an amorphous
structure that results in extremely interesting mechanical properties and,
in many cases, vastly superior chemical durability.
Although true glasses are amorphous, there are a number of ways
in which a glass can be microstructurally engineered through liquid-
liquid phase separation or devitrification (crystallization). The
pi ece de
resistance
for glass is the plethora of forming modalities, which afford
it flexibility on a par with metals. This chapter introduces some of the
unique features of glass properties, forming structure, and composition
that can be useful to the biotechnologist or health professional.
·
1.2 MAKING GLASS
Most commercial glasses are made by mixing raw materials of the
oxides, carbonates, nitrates, or sulfates and heating to high temperature
(for silicates, usually about 1450
◦
C) for a period of time, creating a melt.
