Page 95 - Chalcogenide Glasses for Infrared Optics

Glass Pr oduction 73

not have been measured. The metallic impurities are often covered by
routine emission spectrographic analysis. Negative impurities are
more difficult and are often neglected. Arc-source mass spectroscopy
will measure negative impurities such as chlorine or sulfur. Often, the
only way to find out for sure if a reactant meets the need of the glass
is to make a batch, perhaps a kilogram of the glass, and evaluate the
outcome, usually by measuring infrared transmission through a sample
1 to 2 cm thick. The test is expensive but often necessary.

3.2 Compounding Methods
The simple method of compounding a glass is illustrated in Fig. 3.2.
Reactants are accurately weighed and placed in a glass tube, preferably
a pure silica tube. Semiconductor-grade quartz is extremely pure and
can be used without fear up to temperature of 1000°C under vacuum.
A cap is provided for the end of the tube so that it can be evacuated
after the reactants are in place. The loaded tube is placed in a rocking
furnace and heated while pumping so that any moisture can be
removed from the chamber. The temperature is raised enough to melt
the chalcogen being used to remove dissolved gases. One must keep in
mind sulfur melts at 119°C and boils at 445°C, selenium melts at 217°C
and boils at 685°C, while tellurium melts at 450°C and boils at 990°C.
After a period of time, the pump tube is sealed and removed so
that the rocking can begin to form the glass into a homogeneous melt.
Temperatures are kept below the boiling point of the volatile chalcogen,
allowing time for the metallic elements to react which lowers the
volatility of the melt. Sulfur-based glasses require special care because
of sulfur’s low boiling point and because reaction with arsenic, as an
example, is very exothermic. The heat generated may increase the
internal pressure due to the presence of sulfur that has not reacted.
Some germanium-containing glasses are reacted for hours to ensure

FIGURE 3.2 Reactants sealed in a tube placed in a rocking furnace.