26 February 1982 Infrared Transmitting Germanate Glasses
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The infrared cut-off of glasses is primarily determined by the frequency of vibration of the cation-anion bonds. In order to extend infrared transmittance to longer wavelengths cations and anions of larger sizes and lower field strengths should be used; however, physical and chemical properties become poorer. For silicate glasses, this cut-off is about 5 pm. If germanium is used as the glass network former instead of silicon, the cut-oft moves out to about 6 μm. Of all types of glasses, the germanates provide the optimum combination of transmission, physical, and chemical properties. While the size of glass form-ing areas for germanates is smaller than that for silicates, properties can be varied some-what to fit a particular application. Lxpansion coefficients (25°-300°C) can vary from about 50 x 10-11°C to over 100 x 1011°C with mechanical hardness, Young's modulus, and chemical durability generally decreasing with increasing expansion. While the cut-off is due to the germanium-oxygen bond vibration, the shape of the transmission curve approaching zero transmission from about 4.5 pm to 6 pm can be significantly affected by the quantity and type and modifying oxides. An optimum glass with respect to infrared transmission, low thermal expansion, meltability, formability, and cost was selected and designated Code 9/54. Originally, this glass was melted in crucibles and pressed into domes of rather poor quality. However, more zecently a process has been devised to melt Code 9754 glass in an optical tank and then press into various shapes with standard first grade optical quality (i.e., glass contains no visible striae), a total bubble cross section of 5-0.10 mm2 per 100 cm3, and no cracks or checks. The glass has a thermal expansion (25°C-300°C) of 63.6 x 10-7PC, a Young's modulus of 8.58 x 103 kg/mm2, and a refractive index (ND) of 1.660. The minimum uncoated transmission of a 1.35 mm thick sample is 73% at 5 μm and 35% from 4.2 μm down through the visible.
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William H. Dumbaugh, William H. Dumbaugh, } "Infrared Transmitting Germanate Glasses", Proc. SPIE 0297, Emerging Optical Materials, (26 February 1982); doi: 10.1117/12.932488; https://doi.org/10.1117/12.932488

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