Page 241 - Chalcogenide Glasses for Infrared Optics

AMI Infrar ed Crystalline Materials 217

The solid line represents the liquidus curve for CdTe solutions.
Complete discussion of the growth and properties may be found in the
extensive work published by D. de Nobel. The vapor growth approach
6
is discussed in detail by Shiozawa and colleagues of Gould Labora-
7
tories in Ohio. The AMI approach was confined to modifying our
glass production methods. AMI had considerable experience with
forming glass plates in round 6-, 8-, and even 10-in quartz chambers.
Recalling the AMI glass production process, the elements are placed
in the compounding chamber, melted, mixed, and compounded.
Then the compounded material is physically distilled through a
porous quartz filter to remove particulate matter and is condensed in
the round casting chamber. The transfer process also leaves behind
most all the impurities found in the beginning reactants. It was natu-
ral for us to try first taking precompounded CdTe and transferring it
as vapor through a filter to form a plate in the round chamber. How-
ever, because of the high melting point of the material, the filters con-
stantly became plugged, thwarting our efforts. We decided it would
be better to separate the cadmium and tellurium. We added a third
chamber to our standard glass unit, as shown in Fig. 9.5. The reader
may recall that the same conclusion was reached in the preparation of
arsenic trisulfide glass. The decision regarding CdTe predates the
decision on the glass.
Referring to the diagram, the cadmium is slowly transferred to
the central chamber over a period of 4 h at a temperature of 750°C.
Vapor pressure is about 1 atm. Then tellurium is transferred at 800°C
at a pressure of about 150 mm over a period of 8 to 10 h. During the
transfer and filtration of the Cd and Te, particulate material, mostly
carbon, and other impurities are left behind, improving the final
purity of the CdTe plate. After the material is transferred, the tem-
perature of the central chamber is slowly raised above the melting
point, 1097°C. The two side chambers are left at 800°C. The center

T c

T c
Filter Filter
Tellurium Cadmium

T 1 T 3
Transfer T 2 Compound
T 1 800°C T c T 800°C
1
T 400°C T 1100°C
2
2
T 750°C T 800°C
3
3
FIGURE 9.5 AMI three-chamber method for producing plates of CdTe.