Page 53 - Chalcogenide Glasses for Infrared Optics

Chalcogenide Glasses 31

and quenched in air. Caution must be exercised. The vial is held by
metal tongs. A face mask and protective gloves are worn. The danger
of an explosion for some combinations should not be ignored. As
mentioned above, the rate of cooling affects the extent of the compo-
sition range which is judged to be glass forming.
After cooling, the vial is opened and the sample is removed for
examination. Usually, a sample can be identified as glass or crystal-
line by visual examination. Sometimes use of a microscope sensitive
to some degree in the near infrared is useful. A glass sample can have
two phases, glass and crystal or even two immiscible glasses. If x-ray
diffraction is used, a completely blank film demonstrates a single-
phase amorphous glass. The glass samples are cylinders with round
bottoms. Slices are carefully sawed out, some with different thickness,
2 to 5 mm. The disks are polished on both sides using an optical abra-
sive such as aluminum oxide. The last sample with the round bottom
is saved for reflection measurements. The one flat side is polished in
the same manner as the disks.
The infrared transmission T is measured using a standard spec-
trophotometer covering the wavelength range of interest, generally
1 to 20 µm. A reflection attachment, if available, is used to measure
the infrared reflectivity R from the flat side of the bottom sample. A
qualitative measure of the refractive index n and the bulk absorption
coefficient α can be obtained from solving the simple equation

2 –αx
T = (1 – R) e
2
2
where x is thickness in centimeters and R ~ (n − 1) / (n + 1) . In the
absence of a direct measurement of R, n and α can be calculated from
the measured T for the two samples of different thickness.
Another number very important to the glass evaluation is an
approximate glass softening point. An ASTM method or a dilato-
metric measurement is impractical for small survey samples. A sim-
ple apparatus much like the one shown in Fig. 2.6 may be devised
and easily built. One of the polished disk samples is placed in the
bottom of the chamber. A thermocouple is placed inside next to the
sample. Inert gas may be slowly circulated if needed. As the disk
sample is heated and begins to soften, the glass rod in the center
drops down, moving the indicator arm and signifying the softening
point has been reached. The size of the weight, the weight of the
rod, and the thickness of the sample are important variables. Cali-
bration with known glasses should be carried out to support the
results.
Determining the glass forming compositions for a binary sys-
tem is of course the easiest since you have only two elements. The
results are then plotted on a composition percent straight line with
glass or crystalline marked at each composition point on the line.