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DF: Dielektrische Festkörper
DF 4: Gl
äser II (gemeinsam FV DF/DY)
DF 4.9: Talk
Tuesday, March 9, 2004, 16:45–17:00, H23
Mixed-alkali effect in sodium rubidium borate glasses: ionic conductivity and diffusivity — •Árpád W. Imre, Stephan Voss, and Helmut Mehrer — Institut für Materialphysik, Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany and Sonderforschungsbereich 458
Measurements of the tracer diffusion coefficient and ionic conductivity have been made in a series of 0.2[X Na2O·(1−X)Rb2O]·0.8 B2O3 glasses where X=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 in order to investigate the mixed-alkali effect. The glasses were characterised with differential scanning calorimetry, density measurements and X-ray diffraction experiments. Both 22Na and 86Rb tracer diffusion coefficients were determined as a function of temperature and composition. For each composition the temperature dependence of the tracer diffusion coefficients and dc conductivity times temperature σdc × T follow the Arrhenius law. The manifestations of the mixed-alkali effect were observed: i) the ionic conductivity shows a minimum with composition near X=0.4, ii) the activation enthalpy of σdc × T has a maximum near X=0.4, iii) the glass-transition temperature has a minimum near X=0.2, iv) the diffusivities of 22Na and 86Rb crossover near X=0.2. The diffusivity crossover is found to be temperature independent. The composition dependence of the tracer diffusivities has an asymmetric character. The tracer diffusion coefficients are compared with ionic conductivity and the corresponding Haven ratios obtained. In contrast to 0.2Na2O · 0.8 B2O3 glass, the Haven ratio in the 0.2Rb2O · 0.8 B2O3 depends on temperature.