Page 180 - Chalcogenide Glasses for Infrared Optics

156 Cha pte r Se v e n

In 1982 prior to this program, AMI suddenly had a serious prob-
lem in producing Amtir 1 glass due to excess absorption. We found
out our KBI source selenium was really produced by Asarco in Amarillo.
A visit there revealed that they had shut down their high-purity unit
2 years earlier. Asarco had selected a certain fraction of selenium in
the distillation process they had learned produced excellent results
for our application. Asarco continued supplying AMI out of their
inventory. When that fraction was exhausted, the material supplied
to us contained too much iron which lowered the overall transmis-
sion of the glass. We switched to the Belgium source marketed in the
United States by Indussa.
To verify that selenium was the problem, Glen Whaley of AMI
constructed the fractional distillation column, from high-purity quartz,
shown in Fig. 7.1. In the drawing, the column is shown in a vertical
position inside a two-zone furnace. To begin, Asarco selenium powder
was placed inside through the bottom chamber. The chamber was
sealed and evacuated. The unit was then placed inside in the vertical
position and heated to melt all the selenium. The two-zone furnace
was adjusted to produce a vertical thermal gradient with the maximum
temperature at the first chamber, number 1. Each chamber had its
own thermocouple for temperature monitoring. The column was left
for a long period to allow the vapors and reflux to come to equilibrium.

Reflux
4

3

Furnace 1

2

1
Furnace 2

Bottom

FIGURE 7.1 Fractional distillation column used on Asarco selenium.