Biomedical Engineering Reference
In-Depth Information
The first attempts to produce crystals in glasses were made a long
time ago. However, the final product always ended up having worse
properties than the original material. The uncontrolled way in which
crystals formed in the base glass caused forces (known as uncontrolled
residual stresses) in the material that then made it break easily. In 1959,
Stookey, a ceramics specialist, discovered a way of controlling the way
in which crystals form. In the process, he learned that the controlled
formation of crystals in a base glass goes hand in hand with the controlled
formation of extremely small phases, which we call nuclei. According to
his discovery, the controlled formation of crystal phases is connected to
the controlled formation of nuclei (the start of the crystallization process
called nucleation). In other words, nucleation and crystallization are so
closely linked that one cannot take place without the other.
Because controlled nucleation produced crystallites (very small crys-
tals) in specific areas of the base glass, glass-ceramics with very different
and special characteristics could be produced. As a result of this finding,
the characteristics that had previously been known only for glasses or
ceramics could be combined in one material. Later on, we will take
a closer at look at these special properties and examine glass-ceramic
materials in more detail. In addition, we will find out about glass-
ceramics that transmit light almost as well as glass and do not change
size with changing temperature. This is important for esthetic dental
restorations. We will look at moldable glass-ceramics, which allow us to
create a wide variety of shapes. Moldable glass-ceramics are also used
in dentistry, where biocompatible materials are also very important. We
will also talk about very strong glass-ceramics that are either shaped
with computer-assisted tools or molded. In order to understand the
special characteristics and ways of using these materials, we will take
a close look at the methods of controlling crystallization in the glasses
used in the manufacture of glass-ceramics.
7.2 METHODS USED FOR THE CONTROLLED
CRYSTALLIZATION OF GLASSES
As you now know, a specific crystallization method needs to be followed
if the finished glass-ceramic is to contain glass components as well as
crystals. One method of controlled crystallization involves controlling
the nucleation and crystallization processes within the base glass (also
called internal processes or bulk processes), while in the other method,
the crystallization process is controlled on the surface of the glass.
