A study of selected physical properties of alkali germanate glasses over wide ranges of composition

The compositional ranges for various alkali germanate glasses have been explored, with some reaching as high as J=3 (or 75 molar percent alkali oxide), where J is the molar ratio of alkali oxide to germanium oxide. The Tg (glass transition temperature) and carbon dioxide retention were found for each sample, while the density was found for the more stable samples. These data were then compared to the previous work that had been done on the alkali silicate and alkali borate families by this lab. It was found that the Tg trend in the alkali germanates was similar to that found in the alkali borates due to a change in coordination in each system. Also, the density and Tg could be related and explained with an understanding of the coordination of the units present in the glasses. Volumes of the structural units were obtained in the alkali germanate systems and compared with the alkali borates and silicates. In addition, packing fractions of the units were obtained in all three systems. It was found that the silicate units did not have an anomonously large packing fraction of any of its structural units at low alkali concentrations, whereas the germanate octahedral units (N6) and the borate tetrahedral units (f2) did. At high alkali contents the packing fractions were similar in all systems indicating a common effect due to both the formation of non-bridging oxygens and the alkali ions filling space in a random manner.