Page 137 - Chalcogenide Glasses for Infrared Optics

114 Cha pte r F o u r

demonstrates an electronic bandgap >1 eV. The bandgap indicates an
average bond energy for the solid. Most all selenium-based glasses
are semiconductors with bandgaps of 1 eV or less. Just like crystalline
semiconductors, they are electronic conductors with electrons and
holes but with extremely low carrier mobility. Their resistance is clas-
sified as semi-insulating. When they are heated, free carrier absorp-
tion in the infrared, as in crystalline semiconductors, if it occurs at all, is
too weak to be observed. Glasses containing or based on tellurium may
show much higher conductance than selenium-based glasses. Greater
metallic character is due to the presence of tellurium. Radiation dam-
ages the lattice of crystalline semiconductor materials, knocking atoms
out of place and creating defects. For visible light-transmitting solids,
color centers may result as impurity metallic elements become acti-
vated. For crystalline semiconductors, the conductivity may change
due to defects decreasing the carrier mobility. Chalcogenide glasses are
already disordered solids. A large amount of radiation would be required
to make the solid much more disordered than it is already. Also, most
do not transmit visible light, so color centers would not be observed.
Carrier mobility is already very low.

4.4 Resistance to Chemical Attack
Glasses generally are less chemically active than crystal materials.
Chalcogenide glasses are chemically inert to most common substances
with the exception of strong alkaline solutions. Chalcogenide glasses
are inert to most common organic liquids such as acetone and alcohol.
Exposure for weeks to nonoxidizing acids such as hydrochloric, hydro-
fluoric, and sulfuric acids at 5N concentration has no effect. Amtir 1
was tested by the Navy in San Diego Bay with flowing seawater with
no effect after 3 months. Still, it is always best to test. Glasses based on
different elements may show different chemical reactivity. For example,
Si-Se glasses are very reactive with water, evolving H Se gas, while
2
Ge-Se glasses are stable to water. One test method to follow is to cut
and polish a number of small disks, with about 1 in diameter and 0.2 in
thick. Weigh each one, identify them, and measure the transmission of
each. Expose each one to a chemical to be tested, varying the condi-
tions such as time and temperature. At the conclusion, weigh and run
the transmission. Comparison before and after will indicate the resis-
tance to each chemical tested.

4.5 Final Production Procedure
Once the chalcogenide glass composition has been selected, the related
physical properties and optical properties are measured accurately,
and the decision is made to produce the glass, the process begins to
produce the glass in quantity and verified quality. In the AMI process,
the reactants are weighed out accurately to 0.1 g using an electronic