Density Change of Some Borosilicate Glasses

Abstract

The density changeof borosilicate glasses, air drawn or those given various theymal histories, by heating at higher temperatures ranging from 475° to 650°C, was measured by the suspension method using bromoform-benzene solution.The characteristic density curves at constant temperatures were interpreted as the superposition of several transition mechanisms with different time constants. The effects of the heat treatment at comparatively low temperatures disappeared rather rapidly by heating at a higher temperature. On the other hand, the effects of pretreatment at higher temperatures remained partly throughout the heating period of about 8 hours at a lower temperature.By an interferometric dilatometer the thermal expansion of the specimens was measured after treating at different temperatures. Bringing the curves to the same plot by shifting each origin with an amount corresponding to the difference of the densities at ordinary temperature the general trend of the curves could be understood as the same phenomena discussed by Ritland paying special attension to the separation of borosilicate glass. The density change due to thermal after-effect was also investigated.In order to analyze the above results the author put forward the energy levels L1, L2, L3, and L4 each representing the minimum free energies assigned to the cubes dividing a mass of glass into equal volume so that the state of a glass may be represented by the sum of the number of cubes being in different levels. The levels L1, …L4 represent, respectively, the chilled state, the compacted state, and the segregated states by heating at comparatively low and high temperatures. Furthermore, the subdivided structure of each level was also taken into account. Considering the activation energy for the transition from one to other state the author was able to interpret the observed density change. The conclusions were compared with the experimental results and discussed.