Sonderforschungsbereiche
Sonderforschungsbereich 458 - Ionenbewegung in Materialien mit ungeordneten Strukturen -
vom Elementarschritt zum makroskopischen Transport -
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-earthions 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 A2O-2CaO-4SiO₂ with A=Li, K, Cs and K₂O-2BaO-4SiO₂ 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^-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 ionsare 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.

Drittmittelgeber:

Deutsche Forschungsgemeinschaft

Beteiligte Wissenschaftler:

HDoz. Dr. H. Bracht (Institut für Materialphysik, Universität Münster), Prof. Dr. M.D. Ingram (Department of Chemistry, University of Aberdeen), Dipl. Chem. C. Martiny (Physikalische Chemie, Universität Münster), Dr. S. Murugavel (Physikalische Chemie, Universität Münster), Dipl. Phys. F. Natrup (Institut für Materialphysik, Universität Münster), PD. Dr. B. Roling (Physikalische Chemie, Universität Münster)

Veröffentlichungen:

F. Natrup, H. Bracht, C. Martiny, B. Roling: Diffusion beweglicher Erdalkaliionen in Misch-Alkali-Erdalkali-Silikatgläsern, Verhandl. DPG (VI) 36 (2001) 72.

C. Martiny, S. Murugavel, B. Roling, F. Natrup, H. Bracht, and M. D. Ingram: Mobilities of divalent ions in glass, Conference Proceedings of the International Congress on Glass (Edinburgh, 2001), Glass Technol. 43C (2002) 309 - 314.

F. Natrup, H. Bracht, C. Martiny, S. Murugavel, and B. Roling: Diffusion of calcium and barium in alkali alkaline-eath silicate glasses, Physical Chemistry Chemical Physics 4 (2002)  3225 - 3231.

F. Natrup, H. Bracht, C. Martiny, B. Roling: Einfluss der Kationenradien auf die Diffusion der Erdalkaliionen in Misch-Alkali-Erdalkali-Silikatgläsern, Verhandl. DPG (VI) 37 (2002) 67.