Activation volumes and ion transport mechanisms in glasses and polymers
Ion transport in glasses and polymer electrolytes is a major scientific subject, mainly because it affects the properties and applications of these materials. The new approach of this project is to study the effect of pressure, in addition to the effect of temperature on the ionic conductivity of glasses and polymers with Variable Pressure-Variable Temperature (VPVT) apparatus. This allows the determination of two independent activation parameters: namely, the activation energy and the activation volume. The majority of this work considers the systematic study of the variation of these two activation parameters for single and mixed cation aluminoborate glasses, with respect to both temperature and glass composition. The following glasses are studied: single sodium aluminoborate glasses of the formula: xNa2O – (1-x) (0.87 B2O3: 0.13 A12O3), and mixed cation aluminoborate glasses of the formula: 0.45 [(1-y) Na2O : y M2O] – 0.55 (0.87 B2O3: 0.13 A12O3), where M is Li, K and Cs. The aim is to extract new information about ion transport and to shed more light onto the Mixed Alkali Effect (MAE). This systematic study has allowed us to consider existing models and mechanisms. The recently proposed leader-follower mechanism is subjected to detail consideration, and appears to provide a valid description for ion transport processes in both single and mixed alkali glasses. The brief study of the two liquid crystalline polymer complexes using the same VPVT measurements, shows elevated activation volumes, typical of the ‘coupled’ ion transport mechanism in polymer electrolytes, where the ion movements depend on the movement of the polymer segments. These comparative experiments allow us to highlight the main differences between ion transport in glasses and polymers.