Page 49 - Chalcogenide Glasses for Infrared Optics

Chalcogenide Glasses 27

attributed to nucleation of very small crystallites. The second region,
well above the reference line, indicates rapid growth of the crystal-
lites, an exothermic process. After the peak is past, the curve begins to
fall, indicating the crystallites are now dissolving in the surrounding
low-viscosity glass.
AMI C1 is an arsenic-selenium-tellurium glass which has a ten-
dency to crystallize. Figure 2.4 shows microscope photographs of crys-
tals forming on the surface of C1 glass after heating for an extended
period. Figure 2.4a shows nucleation at 215°C for 8 h. Figure 2.4b
shows much larger crystals grown at 238°C for 15 h. When producing
the glass, one must quench (rapidly cool) the glass in a timely manner
beginning at a temperature above the crystallization range down to a
temperature below the nucleation range. These factors must be taken

(a)

(b)
FIGURE 2.4 Crystallite formation in heated AMI C1 glass. (a) C1 glass
heated 8 h @ 215°C; (b) C1 glass heated 15 h @ 238°C.