<title>Infrared Transmitting Germanate Glasses</title>

Abstract

AbstractThe infrared cut -off of glasses is primarily determined by the frequency of vibration ofthe cation -anion bonds. In order to extend infrared transmittance to longer wavelengthscations and anions of larger sizes and lower field strengths should be used; however, physi-cal 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 -oftmoves out to about 6 pm. Of all types of glasses, the germanates provide the optimum com-bination 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. Expansion coefficients (25 °- 300 °C) can vary fromabout 50 x 10- / /°C to over 100 x 10 -/ /C with mechanical hardness, Young's modulus, andchemical durability generally decreasing with increasing expansion. While the cut -off isdue to the germanium- oxygen bond vibration, the shape of the transmission curve approachingzero transmission from about 4.5 pm to 6 pm can be significantly affected by the quantityand type and modifying oxides. An optimum glass with respect to infrared transmission, lowthermal 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 recently a process has been devised to melt Code 9754 glass in an optical tankand then press into various shapes with standard first grade optical quality (i.e., glasscontains no visible striae), a total bubble cross section of 50.1O mm2 per 100 cm3, and nocracks or checks. The glass has a thermal expansion (25 °C- 300 °C) of 63.6 x 10-7 /C, aYoung's modulus of 8.58 x 103 kg/mm2, and a refractive index (ND) of 1.660. The minimumuncoated transmission of a 1.35 mm thick sample is 73% at 5 pm and 35;o from 4.2 pm downthrough the visible.Comparison of germanates with other IR- transmitting oxide glassesThe infrared cut -off of glasses is determined by the frequency of vibration of cation -anion bonds. This frequency is dependent on the mass and structural arrangement of constit-uent atoms and the interatomic forces within the structure. Larger cations and anions withlower field strengths will move the infrared transmittance to longer wavelengths, however,physical properties and chemical durability become poorer. As can be seen in Figure 1, theinfrared cut -off tor silicate glasses is about 5 micrometers. As cations of lower fieldstrength are used as glass network formers, the cut -oft moves out to 6 micrometers forgermanates and calcium aluminates, 6.5micrometers for tellurites, and 8.75 microm-eters for the lead bismuthates. Table 1