Page 42 - Chalcogenide Glasses for Infrared Optics

Chalcogenide Glasses 21

Hughes, Martin Marietta, and Westinghouse in several programs.
Amtir 1 is still the major FLIR glass produced by AMI. Standard pro-
duction is 9-kg plates 8-in diameter. AMI has produced 35 tons of
Amtir 1 glass from 1978 to 2007.
Later, in 1991 after the TI 1173 patent expired, the Night Vision
Laboratory provided a letter contract to qualify AMI as a second
source of the glass which AMI renamed Amtir 3. It is interesting to
note that in the United States during the period from 1950 to the pres-
ent, only three glass compositions have been produced in ton quanti-
ties: arsenic trisulfide, TI 1173 (Amtir 3), and TI 20 (Amtir 1). No other
widely used new compositions have emerged. The reason in part is
due to the considerable effort required to identify, produce, and char-
acterize a new glass composition to the state that optical designers,
system designers, and corporate management are willing to use it in
a new system. Even if a new, better glass emerged, there would be
great reluctance to redesign a system once it is in production.

2.2 The Periodic Table and Glass Formation
Previously, it was pointed out that efforts to find and develop chalco-
genide glasses for infrared systems were most successful using ele-
ments from the IVA, VA, and VIA groups of the periodic table. This
statement is to point out that the periodic table is not an inexhaustible
supply of elemental combinations that should be investigated. The
three groups named have fueled investigations of many systems:
binary, ternary, or those containing even more elements. A review of
other materials transparent in the infrared but from different elemen-
tal families may help to explain why some elements are favored more
than others. Figure 2.1 presents a shortened version of the periodic
table of the elements. Outlined are the families of the elements from
which infrared optical materials are formed.
As indicated in the chart, the alkali halides form from the IA alkali
metal elements Li, Na, K, Rb, and Cs in combination with the group
VIIA halogens F, Cl, Br, and I. The alkaline earth halides form from
the IIA alkaline earth metal elements Be, Mg, Ca, Sr, and Ba in combi-
nation with the VIIA halogen elements F, Cl, Br, and I. Notice also in
Fig. 2.1 that a change in Pauling electronegativity is indicated as one
27
moves up or down in the chart or from left to right. On the left, the
value decreases going from lighter alkali and alkaline earth elements
to heavier. Thus, Cs and Ba have the lowest values for the two fami-
lies. Conversely, Pauling electronegativity is indicated to be increas-
ing with atomic number going across the chart from IA elements to
VIIA elements. At the same time on the right of the chart, Pauling
electronegativity increases going up the chart from the heavier halo-
gens to the lighter elements. Fluorine in the top right corner has the
highest value, 4.0, one unit higher than its next row element Cl.