Westfälische
Wilhelms-Universität Münster
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Institut für Materialphysik Wilhelm-Klemm-Str. 10 48149 Münster Geschäftsführender Direktor: Prof. Dr. Helmut Mehrer |
Tel. (0251) 83-33571
Fax: (0251) 83-38346 e-mail: mehrer@nwz.uni-muenster.de www: http://www.uni-muenster.de/Physik/MP/ |
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Forschungsschwerpunkte 2001 - 2002 Fachbereich 11 - Physik
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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 and
133Ba in alkali alkaline-earth silicate glasses of compositions
A2O2CaO4SiO2 with
A=Li, K, Cs and K2O2BaO4SiO2 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 calcium and
barium 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 -21
m2s -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. The diffusion study strongly supports recent
results from mechanical loss spectroscopy. A model is proposed which qualitatively explains the
correlation of the alkaline-earth mobility with the radius ratio of the cations.
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