Phase separation and the mixed alkali effect

The mixed alkali effect in lithium sodium silicate glasses is strongly affe cted by the morphology of the sample. Properties studied include electrical conductivity, thermal expansion coefficient, glass transformation and dila tometric softening temperature and transformation range viscosity. A sharp transition region caused by phase separation exists between properties of g lasses which exhibit the traditional mixed alkali effect and those that do not. The mixed alkali effect in electrical conductivity does not result in a minimum in conductivity unless the alkali rich phase exhibits a connected morphology. Under some circumstances mixed alkali glasses have a maximum i n conductivity. The mixed alkali effect is also controlled by the morpholog y for phase separated glasses in which the alkali rich phase exhibits a con nected structure. Results of this study emphasise the interaction which can occur between effects of atomistic and morphologic structures in controlli ng the properties of glasses.