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Plenarvorträge - DPG-Tagungen

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Dynamik und Statistische Physik Dienstag<br />

cations. Because of this we measured additionally the diffusivities of 22 Na<br />

and 86 Rb in the mixed glasses at 380 ◦ C using the radiotracer technique.<br />

We observed an exponential decrease of the Rb-diffusivity with decreasing<br />

Rb-content, while the Na-diffusivity also decreases with decreasing<br />

Na-content on the Na-rich side, but remains nearly constant on the Rbrich<br />

side. Both diffusivities intersect at the composition range around<br />

x ≈ 0.2. A similar behaviour was reported from our laboratory for mixed<br />

glasses with a total alkali content of y = 0.2.<br />

DY 29.7 Di 16:15 H23<br />

Diffusion von Na und Ca und ionische Leitfähigkeit in Standard-<br />

Kalk-Natron-Gläsern — •Eugène Molière, Tanguep Njiokep<br />

und Helmut Mehrer — Institut für Materialphysik, Westfälische-<br />

Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster<br />

Als Standardsubstanzen für die wichtigsten physikalischen und chemischen<br />

Prüfmethoden in der Glastechnologie hat die Deutsche Glastechnologie<br />

Gesellschaft (DGG) zwei Flachgläser (Standard-Kalk-Natron-<br />

Gläser) von hoher Reinheit und Homogenität herstellen lassen. Bei der<br />

Physikalisch-Technischen-Bundesanstalt (PTB) wurden das Viskositäts-<br />

Temperatur-Verhalten und die Bruchbarkeit dieser Gläser als Viskosität-<br />

Standard untersucht. Diese Gläser weisen keine Störung durch Entglasung<br />

und Entmischung auf. Neben der praktischen Bedeutung der Standards<br />

liegen hier gut definierte Gläser vor, über deren verschiedensten<br />

Eigenschaften nun umfangreiche Daten existieren. Dagegen gibt es noch<br />

keine Informationen über die Beweglichkeit der im SiO2-Netzwerk eingebauten<br />

Alkali- und Erdalkaliionen. Um Informationen über die Transportprozesse<br />

der Netzwerkwandler zu gewinnen, wurden die ionische<br />

Leitfähigkeit der Proben im Frequenzbereich zwischen 5Hz und 1.28MHz<br />

mit der Impedanz-Spektroskopie und die Diffusion von 22 Na und 45 Ca<br />

mit der Radiotracermethode im Temperaturbereich zwischen 490K und<br />

759K untersucht. Die Tracerdiffusions- und Leitfähigkeitsmessungen werden<br />

verglichen und diskutiert.<br />

DY 29.8 Di 16:30 H23<br />

Ion dynamics in single and mixed alkali silicate glasses studied<br />

by molecular dynamics simulations — •Heiko Lammert and<br />

Andreas Heuer — Westfälische Wilhelms–Universität, Institut für<br />

Physikalische Chemie, Corrensstraße 30, D-48149 Münster<br />

We use molecular dynamics (MD) simulations to study the ion dynamics<br />

in alkali silicate glasses with the general composition<br />

x K2O · (1 − x) Li2O · 2 SiO2, for x = 0, 0.5, 1.<br />

All individual alkali sites provided by the frozen network can be identified<br />

by a statistical analysis of MD trajectories, for both the single<br />

and the mixed alkali systems. Sites are located by a cluster search on a<br />

discretised alkali population density.<br />

The ion dynamics can be analyzed in terms of fast hops between these<br />

sites, also yielding average residence times to characterize the sites. As<br />

the number of free sites is low compared to the number of ions, correlated<br />

back jumps are found to be probable, especially for ions on fast sites. The<br />

mobility of ions can be correlated to the local network structure around<br />

their site, particularly to the occurence of bridging oxygens.<br />

DY 29.9 Di 16:45 H23<br />

Mixed-alkali effect in sodium rubidium borate glasses: ionic conductivity<br />

and diffusivity — •Árpád W. Imre, Stephan Voss, and<br />

Helmut Mehrer — Institut für Materialphysik, Universität Münster,<br />

Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany and Sonderforschungsbereich<br />

458<br />

Measurements of the tracer diffusion coefficient and ionic conductivity<br />

have been made in a series of 0.2[XNa2O · (1 − X)Rb2O] · 0.8B2O3<br />

glasses where X=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 in order to investigate<br />

the mixed-alkali effect. The glasses were characterised with differential<br />

scanning calorimetry, density measurements and X-ray diffraction experiments.<br />

Both 22Na and 86Rb tracer diffusion coefficients were determined<br />

as a function of temperature and composition. For each composition the<br />

temperature dependence of the tracer diffusion coefficients and dc conductivity<br />

times temperature σdc × T follow the Arrhenius law. The manifestations<br />

of the mixed-alkali effect were observed: i) the ionic conductivity<br />

shows a minimum with composition near X =0.4, ii) the activation<br />

enthalpy of σdc × T has a maximum near X =0.4, iii) the glass-transition<br />

temperature has a minimum near X=0.2, iv) the diffusivities of 22Na and 86 Rb crossover near X =0.2. The diffusivity crossover is found to be<br />

temperature independent. The composition dependence of the tracer diffusivities<br />

has an asymmetric character. The tracer diffusion coefficients<br />

are compared with ionic conductivity and the corresponding Haven ratios<br />

obtained. In contrast to 0.2Na2O · 0.8B2O3 glass, the Haven ratio in the<br />

0.2Rb2O · 0.8B2O3 depends on temperature.<br />

DY 29.10 Di 17:00 H23<br />

From binary Na-silicate to ternary Na-alumino-silicate melts:<br />

the evolution of Na dynamics and intermediate range order<br />

— •Florian Kargl 1 , Andreas Meyer 1 , and Helmut Schober 2<br />

— 1 Physikdepartment E13, TU München, 85748 Garching — 2 Institut<br />

Laue-Langevin, 38042 Grenoble, France<br />

Neutron scattering investigations of binary alkali silicate melts revealed<br />

correlations on intermediate lengthscales of 6-8 ˚A. Within MDsimulations<br />

on Na-silicates evidence was found that these correlations<br />

arise from Na-rich channels percolating in the remaining SiO-network.<br />

Fast Na-diffusion takes place via these channels [1]. Substituting the<br />

network forming component SiO2 by GeO2 leads to a change in structure<br />

and in the microscopic transport mechanism, respectively, as verified<br />

by inelastic neutron scattering. We present our recent investigations on<br />

sodium alumino-silicate melts containing besides SiO2 a second network<br />

former, Al2O3. Our main aim was to study the interplay between<br />

microscopic dynamics and structure and its influence on macroscopic<br />

properties. From quasielastic neutron scattering experiments we<br />

conclude that the channel structure is disrupted by adding Al2O3 to the<br />

Na-silicates resulting in an increase of viscosity as well as a decrease of<br />

Na-relaxation times [2].<br />

[1] A. Meyer, J. Horbach, W. Kob, F. Kargl, H. Schober (submitted)<br />

[2] F. Kargl, A. Meyer, Chem. Geol. (submitted)<br />

DY 29.11 Di 17:15 H23<br />

Fragilität niedermolekularer organischer Glasbildner unter<br />

Druck — •Andreas Reiser, Gernot Kasper und Siegfried<br />

Hunklinger — Kirchhoff-Institut für Physik, Universität Heidelberg,<br />

Im Neuenheimer Feld 227, 69120 Heidelberg<br />

Die Fragilität einer glasbildenden Flüssigkeit ist definiert durch die<br />

Steigung bei der Glasübergangstemperatur Tg im Angell-Plot. Sie erlaubt<br />

eine Klassifikation in starke bzw. fragile Glasbildner. Stark bedeutet ein<br />

Arrheniusverhalten der Relaxationszeit über den gesamten Temperaturbereich<br />

T ≥ Tg. Die Relaxationszeit fragiler Glasbildner dagegen zeigt<br />

eine stärkere Temperaturabhängigkeit. Wir haben den Einfluss von hohem<br />

hydrostatischen Druck auf die Fragilität von typischen organischen<br />

Glasbildnern mit dielektrischer Spektroskopie gemessen: Glycerin und<br />

meta-Fluoranilin sind Vertreter wasserstoffgebrückter Glasbildner. Glycerin<br />

rangiert im Zwischengebiet zwischen stark und fragil, wohingegen<br />

meta-Fluoranilin und der Van-der-Waals-Glasbildner Propylencarbonat<br />

vom fragilen Typ sind. Im untersuchten Druckbereich (0 −600 MPa) zeigen<br />

alle drei Glasbildner keine Druckabhängigkeit der Fragilität im Rahmen<br />

der Fehlergrenzen von ±5%.<br />

DY 29.12 Di 17:30 H23<br />

A binary colloidal mixture in 2D: An ideal model system for the<br />

glass transition — •Hans König — Fachbereich Physik, Universität<br />

Konstanz, 78457 Konstanz<br />

Investigating a binary 2D colloidal glass former with a repulsive (1/r 3 )<br />

potential we have measured particle positions as a function of time by<br />

video-microscopy. The local packing of the big and small particles as well<br />

as the heterogeneous dynamics were analysed depending on the system<br />

temperature. Finally, we could describe the amorphous 2D short-range<br />

order by local density-optimized triangles. However, there is one triangle<br />

for each 3-particle combination of big and small colloids. Regions with<br />

structural frustration occur between clusters of optimally packed triangles.<br />

For decreasing system temperature the number of densely packed<br />

triangles increased and they began to form clusters. For low system temperatures<br />

such local density-optimized regions moved cooperatively resulting<br />

in heterogeneous dynamics. By structural and dynamical investigations<br />

we identify the 2D glass transition as a percolation of local<br />

density-optimized triangles, which form a stable frame through the whole<br />

glass former. At last, we call this glass description the concept of local<br />

density-optimized crystallite clusters.

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