Page 82 - Chalcogenide Glasses for Infrared Optics

60 Cha pte r T w o

the glass forming region is dependent upon which elements
are selected and the number of binary compounds they form
with one another.
3. The chalcogenide glasses are characterized by covalent bond-
ing. The molar refraction approach can be used to predict the
refractive index of a glass using the covalent radius of each
constituent atom, its atomic fraction in the glass, and the pre-
dicted density.
4. Glasses containing IVA elements Si and Ge are harder and
stronger than those based more on VA elements P, As, and Sb.
In some areas of the glass forming regions, excess P or As
exists as free molecules which are emitted as such when the
glass is heated. In ternary systems, no vapor molecules con-
taining all three elements were observed.
5. The goal of finding chalcogenide glass compositions with
physical properties comparable to those of oxide optical glasses
was not reached and appears unlikely to be attained using
the IVA-VA-VIA elements.

2.7 Chalcogenide Glasses Containing
Transition Elements 44
At this time, it was concluded that the chalcogenide glasses evaluated
were never going to meet the window requirements of an airborne
infrared optical system. Elements other than those in IVA and VA
groups would have to be used in the glass composition. These must
be elements with lower electronegativities and which form stronger
chemical bonds with chalcogens. These elements can form multibonds,
valences of +3 or +4, and form more than one stoichiometric com-
pound with chalcogens. Titanium (Ti) and zirconium (Zr) from group
IVB and vanadium (V) from group VB were selected. The crystalline
chalcogenide compounds of these elements have melting points in
excess of 1200°C. A high melting point ensures a low-viscosity melt and
thus a homogeneous mixture. The methods previously used never
exceeded 1000°C so a new approach must be developed. Basically,
either an open or a closed system may be used for high-temperature
compounding of materials that contain volatile constituents. In the
open system the reactants are melted so rapidly that a homogeneous
melt is obtained before an appreciable portion of the volatile constitu-
ent is evolved. The open method is simple and rapid. In the closed
system, the reactants are sealed in a vial and slowly raised to a com-
pounding temperature. With this system, the beginning composition
is maintained. However, it is difficult to find materials that can with-
stand the high temperatures and resulting vapor pressure.
The open system was tried, first concentrating on placing the
reactants in recesses in the water-cooled copper plate and using the