B10 - HD Dr. H. Bracht
Mixed cation effects in alkali alkaline-earth silicate glasses
Compared to the investigations conducted on mixed-alkali glasses which until today stimulate the discussion about the underlying microscopic jump mechanisms, little
is known about the mobility of alkaline-earth ions in silicate glasses. This is surprising since many silicate glasses used for technical applications, such as the float glass
produced by Pilkington plc. with 72 mol% SiO2, 13 mol% Na2O,9 mol% CaO, and 4 mol% MgO, contain alkali and alkaline-earth oxides.
Diffusion
of the radioactive isotopes 45Ca, 133Ba, and 85Sr in alkali alkaline-earth silicate glasses of compositions
A2O·2MO·4SiO2 with A=Li, Na, K, Cs and M=Mg, Ca, Sr, Ba was investigated at temperatures
below the respective glass-transition temperatures. The diffusion profiles of the radiotracers were recorded by means of an ion-beam sputtering technique in conjunction
with activity measurements. Analysis of the radiotracer profiles is based on the thin-film solution of the diffusion equation and a contribution which reflects the
instrumental broadening of the profiles caused by sputtering effects. The diffusion coefficients of the alkaline-earth ions were determined down to values of as low as
10-21m2s-1. Our diffusion data enables us to determine accurate values for the activation enthalpy of diffusion and the
pre-exponential factor. The activation enthalpies reveal a distinct correlation with the radius ratio of the alkaline-earth ions to the alkali ions and show that the
alkaline-earth ions are most mobile when the radii of the alkaline-earth and alkali ions are similar.Our results provide strong evidence that the faster alkali ions play an
important role for the dynamics and the diffusion of the slower alkaline-earth ions.
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