Most current research on infrared glasses has concentrated on non-oxide systems which transmit to beyond 7 μm. There are still applications in the 2 to 6 μm range which can best he filled by oxide glasses which generally have better physical and chemical properties than the oxide-free materials. Of all glass systems, silicates have the largest compositional area which provides very stable glasses with good physical and chemical properties. Unfortunately, the IR absorption edge extends only to between 4 and 9 μm. This absorption edge can be optimized by minimizing silica content by dilution with oxides whose cation-oxygen bond vibrates with lower frequency. Transmission can be extended even farther by replacing Si+4 as the network former with other network forming cations with weaker bonding, such as Ge+4, Sb+3, or Te+4. Properties and stability to crystallization of these glasses, as a rule, become poorer as IR transmission improves. By far, the best transmission of any oxide glass belongs to a relatively new class of glasses based on networks formed by bismuth and/or lead oxides. They transmit out to R to q pm, have expansion coefficients (29°-200°C) from 83x10-7/°C to 112x10-7/°C, and refractive indices as high as 2.2 at 4 μm. They have sufficient stability to be cast into 8cm x Rcm x 1.9cm slabs.
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