Portal de Periódicos da CAPES

Mixed-alkali effect in Na–Rb borate glasses: A tracer diffusion and electrical conductivity study

2004; Royal Society of Chemistry; Volume: 6; Issue: 13 Linguagem: Inglês

10.1039/b315772e

1463-9084

S. Voß, Árpád W. Imre, H. Mehrer,

Nuclear materials and radiation effects

The ionic transport in a series of 0.2 [X Na2O·(1 − X)Rb2O]·0.8B2O3 glasses has been studied by measuring both the electrical conductivity of these glasses and the tracer diffusivities of 22Na and 86Rb as functions of composition and temperature. At constant temperature (380 °C) the tracer diffusivities of 22Na and 86Rb decrease monotonically with decreasing Na- or Rb-content, respectively. The tracer diffusivities as functions of the composition parameter X intersect near X = 0.2 ('crossover composition'). This behaviour gives rise to a minimum in the conductivity which appears near X = 0.4. At the crossover composition X = 0.2 the diffusivities of 22Na and 86Rb are practically the same within the measured temperature range resulting in similar Arrhenius parameters. For each other investigated composition the Arrhenius parameters for 22Na and 86Rb diffusion differ significantly. Comparing the 22Na-diffusivity in the pure Na-borate glass with the corresponding conductivity diffusion coefficient calculated via the Nernst–Einstein equation from the measured conductivity yields a temperature-independent Haven ratio. In contrast, a temperature-dependent Haven ratio was found in the pure Rb-borate glass. In order to compare the tracer diffusivities in mixed-alkali glasses with their ionic conductivity we introduce a 'common Haven ratio' for the mixed-alkali glasses which coincides with the conventional definition of the Haven ratio for single-alkali glasses. On the Rb-rich side the common Haven ratio decreases with temperature for Na–Rb borate glasses (X = 0.2 and X = 0.4) similar to the Haven ratio in the pure Rb borate glass.