Page 101 - Chalcogenide Glasses for Infrared Optics

Glass Pr oduction 79

85 percent. With the new casting method, the MTF score was raised to
94 percent, equal to that of germanium. However, for TI 1173, the
bottom glass plug was found to form a large amount of crystallites
grown during the glass mixing process prior to casting. When casting
occurred, the crystallites flowed out and ruined a sizable portion of
the cast plate. The yield of striae free large diameter lens blanks was
very low.
The author was asked to assume control of the TI glass produc-
tion in the fall of 1974. The first change made was to implement the
element purification process for compounding all the TI 1173 in the
production process. It was not necessary to do the same for TI 20
glass since it was no longer in production. In working with the glass
blowers, a process was devised so that double-chamber quartz tubes
could be fabricated on the glass lathe. The chambers were separated
by a porous quartz filter called a frit. One chamber end, the one to
hold the finished glass, was left rounded and closed. The other had a
wide mouth so the reactants could be easily loaded. The charge in the
tube was enough to make 7 kg of glass. About 10 ppm of pure alumi-
num wire was added to the reactants to serve as a getter for the 5 ppm
of oxides anticipated. After loading the cap was sealed, the tube
evacuated, heated to melt the chalcogen, and finally sealed off for
compounding. After the reactions were believed complete, the tem-
perature was raised and the entire 7 kg distilled through the frit into
the glass chamber, where it was rocked and mixed prior to quenching
using blown air. The goal was high-purity glass free of particles and
oxides.
The second major change made was to eliminate the particles
caused by the glass plug used to control the flow of glass into the
mold. Again, in working with the glass blowers, a procedure was
developed by which a ground glass joint was placed in the bottom of
the crucible to control the flow of glass into the mold. Thus, the entire
cast plate would be useful material. The ball joint was controlled by a
long glass tube that reached outside the top of the chamber. After
much effort, the standard production became striae-free, high–purity,
12 in × 12 in, 7-kg plates. Part of the TI success was due to vapor pres-
sure and viscosity information supplied by Larry Swink. Figure 3.8 is
a diagram of a sealed pressure measuring device produced at that
time by TI. In the diagram, TI 1173 or TI 20 glass was sealed in the
bottom chamber. When the glass was heated, the resultant vapor
pressure produced a twist of the Bourdon tube that deflected the
attached mirror. The vapor pressure was measured by the amount of
nitrogen pressure applied in the top chamber to return the mirror to
its original position. Vapor pressure results of the measurements are
shown in Fig. 3.9 covering the elements S, Se, As, and Sb. Notice Sb
is only showing 10 mm vapor pressure at 1000°C. Pressures for both
TI 1173 and TI 20 are about 20 to 50 mm at 600°C near their casting
temperatures.