Diffusion and ionic conductivity in borate glasses

Diffusion of ²²Na was studied in X Na₂O·(1-X)B₂O₃ glasses with X = 0.2 and X = 0.3 using the radiotracer method. A slight decrease of the diffusion coefficients with increasing diffusion time was observed and is attributed to a structural relaxation of the glass network. The temperature dependence of diffusion was measured below and also in a small temperature range above the glass transition temperature (T_G). Below T_G the diffusion coefficients obey Arrhenius relations. Diffusion in the glass with higher Na content is considerably faster. The results are compared with dc conductivity data.

The pressure dependence of ²²Na diffusion in 0.3Na₂O·0.7B₂O₃ yields an activation volume of 0.52 molar volumes. This relatively high value indicates a significant relaxation of the glass network associated with a Na jump.

In sodium-rubidium borate glasses with a total alkali content of 30 mol percent and Na/(Na+Rb) ratios between 0.4 and 1.0 diffusion of ²²Na and ⁸⁶Rb was investigated at 673 K. Diffusion of alkali ions in mixed-alkali borate glasses shows a concentration dependence which is different from published mixed-alkali effects for diffusion in silicate glasses. Sodium diffusion is always faster than rubidium diffusion. Na diffusion shows a minimum and Rb diffusion a maximum for a relative concentration of about 60 % sodium. The ionic conductivity deduced from the diffusivities displays a minimum as a function of the relative composition of sodium like in other mixed-alkali oxide glasses.

For mixed sodium-rubidium borate glasses with compositions 0.2[XNa₂O·(1-X)Rb₂O]·0.8B₂O₃ (X = 1.0; 0.8; 0.6; 0.4; 0.2; 0.0)
measurements of the electrical conductivity using an ac complex impedance technique have been performed between 5 Hz and 1.3 MHz over the entire composition range. The conductivity diffusion coefficient of the mobile alkali ions was deduced from the electrical dc conductivity via the Nernst-Einstein relation. The diffusivity has a minimum and the activation enthalpy a maximum at X»0.4 indicating the mixed-alkali effect. Supposing an Arrhenius-type temperature dependence the activation parameters and pre-exponential factors were determined. The deviations from the Arrhenius function were also studied and found to be most pronounced around X = 0.2 to 0.4.

Drittmittelgeber:

Deutsche Forschungsgemeinschaft (SFB 458)

Beteiligte Wissenschaftler:

Dr. C. Cramer (Institut für Physikalische Chemie, WWU), Dr. A. Imre (seit November 2000), Prof. Dr. H. Mehrer (Leiter), Dr. E. Ratai (Institut für Physikalische Chemie, WWU), Dr. U. Schoo (bis Juli 2000)

Veröffentlichungen:

Schoo, U., H. Mehrer: Diffusion of ²²Na in sodium-borate glasses, Solid State Ionics 130, 243-258 (2000)

Schoo, U., C. Cramer, H. Mehrer: Tracer diffusion in sodium-rubidium-borate glasses - an unconventional mixed alkali effekt?, Solid State Ionics 138, 105-114 (2000)

Schoo, U., C. Cramer, E. Ratai, H. Mehrer: Diffusion in single and mixed alkali borate glasses, Defect and Diffusion Forum 194-199, 925-933 (2001)

Imre, A.W., S. Voss, H. Mehrer: Ionic transport in 0.2[XNa₂O·(1-X)Rb₂O]·0.8B₂O₃ mixed-alkali glasses, PCCP, in print