Page 52 - Chalcogenide Glasses for Infrared Optics

30 Cha pte r T w o

100 percent A on the left to 100 percent B on the right. Percentages
may be either weight percent or atomic percent with the latter pre-
ferred. The results presented represent equilibrium results, i.e., the
reactants at a specific concentration held at a specific temperature for
periods of time reaching days or even weeks. The purpose is to iden-
tify the crystalline compounds formed under the conditions of the
experiment. If there is no AB compound formed, the diagram may be
essentially two lines from the melting point of A to the melting point
of B describing solid solutions of A and B. An example is the diagram
33
for Ge-Si. A diagram might show different solid compounds in
equilibrium with a liquid phase. Above all the areas outlined will be
a line representing a temperature above which A and B mixtures are
liquids. The liquidus curve for an AB mixture identifies a melt tem-
perature at each indicated composition point. The same description
fits the AC and the BC diagrams. If a compound is found, a straight
line will be drawn down from the temperature point to the composi-
tion point. Later, we will see that the size of the glass forming area in
a diagram is affected by the number of compounds that may form
from the specific combination of elements. From the diagrams one
may find the temperature required to produce a homogeneous melt
to test the ability of the composition to form a single-phase glass
when quenched.
After all these years, most of the IVA-VA-VIA elements have been
extensively investigated in various combinations. The investigator
may wish to find a unique combination of elements with a chalcogen
or even more than one chalcogen. There may be a desire to revisit
previous investigations of a particular combination of elements. This
discussion is only meant to serve as a guide for the start of the inves-
tigation. Obviously, the first step is to go to the literature to find any
previous work that may be pertinent.
The qualitative investigation of a glass forming combination of
elements requires making many 25- to 50-g small samples. Quartz
vials are usually used because of their purity and high-temperature
softening points. High-purity reactants are used and weighed accu-
rately with an analytical balance.
The tube is evacuated to remove the atmosphere and sealed with
a hydrogen-oxygen torch. Samples are then placed in some type of a
rocking furnace in a hood and heated to a temperature about the
same as the boiling point of the most volatile constituent element.
The rocking action is begun to start the reaction of the elements. After
some time, the furnace is raised in temperature to produce a homoge-
neous melt of the reactants. The estimated temperature is based on
the binary compound diagrams. Mixing continues for some time
(hours) above the melt temperature. Rocking is stopped with the vial
in a vertical position. The melt is allowed to cool at the melt tempera-
ture. The vial is removed from the furnace, held in a vertical position,