Page 57 - Chalcogenide Glasses for Infrared Optics

Chalcogenide Glasses 35

regarding glass forming tendency in the IVA-VA-VIA ternaries
were

S > Se > Te
As > P > Sb
Si > Ge > Sn

The reversal between As and P may be due to the fact that As forms
bonds with Si and Ge while P does not. Efforts to form glasses with
33
Sn were unsuccessful except in very low Sn concentrations.

2.4 Qualitative Evaluation of Compositions
for Development
For the Si-P-Te system, only one-third to one-half produced glasses of
good quality, and those had softening points below 200°C. Disks were
cut from the glass sample, and polished and evaluated relative to
infrared transmission as a function of wavelength, and a qualitative
estimate of the refractive index of each was determined. Figure 2.8
shows the results of this effort. Note that at this time, measurements
were crude in comparison to today and could only be considered
qualitative.
Following this procedure developed in the exploratory program,
seven ternary IVA-VA-VIA systems were evaluated during the three-
year period from 1962 to 1965. Results of the evaluation are given in
Table 2.2. Each system was evaluated with respect to softening point,
approximate refractive index, and infrared absorption in the two

60 (29) Si 3 P 3 Te 14 N = 3.30

Absorption coefficient (cm –1 ) 40 (41) Si P 2 Te 15 N = 3.40
3
(42)
N = 3.45
3
Si PTe 14
20

0
0 4 8 12 16 20
Wavelength (µm)
FIGURE 2.8 Measured infrared transmission of some Si-P-Te glasses.