Structure of Molten Oxides. I. Viscosity of GeO2, and Binary Germanates Containing Li2O, Na2O, K2O, and Rb2O

An improved counterbalanced sphere viscometer was used to determine the viscosity of pure molten GeO2 at temperatures from 1470° to 1720°C. This data agrees with the Kurkjian and Douglas data obtained below 1500°C. Activation energy and entropy calculations suggest a gradual thermal breakdown of molten GeO2 compared to SiO2. Viscosity information was obtained at temperatures between 1000° and 1500°C for binary alkali germanate melts containing up to 32 mole % Li2O, Na2O, K2O, and Rb2O. Viscosity isotherms, activation energies, and activation entropies suggest gross similarities between the viscous flow mechanisms of binary alkali germanate and silicate melts. However, differences of detail imply divergent modes of network alteration with the addition of a given amount of alkali oxide.