Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Dienstag
cations. Because of this we measured additionally the diffusivities of 22 Na
and 86 Rb in the mixed glasses at 380 ◦ C using the radiotracer technique.
We observed an exponential decrease of the Rb-diffusivity with decreasing
Rb-content, while the Na-diffusivity also decreases with decreasing
Na-content on the Na-rich side, but remains nearly constant on the Rbrich
side. Both diffusivities intersect at the composition range around
x ≈ 0.2. A similar behaviour was reported from our laboratory for mixed
glasses with a total alkali content of y = 0.2.
DY 29.7 Di 16:15 H23
Diffusion von Na und Ca und ionische Leitfähigkeit in Standard-
Kalk-Natron-Gläsern — •Eugène Molière, Tanguep Njiokep
und Helmut Mehrer — Institut für Materialphysik, Westfälische-
Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster
Als Standardsubstanzen für die wichtigsten physikalischen und chemischen
Prüfmethoden in der Glastechnologie hat die Deutsche Glastechnologie
Gesellschaft (DGG) zwei Flachgläser (Standard-Kalk-Natron-
Gläser) von hoher Reinheit und Homogenität herstellen lassen. Bei der
Physikalisch-Technischen-Bundesanstalt (PTB) wurden das Viskositäts-
Temperatur-Verhalten und die Bruchbarkeit dieser Gläser als Viskosität-
Standard untersucht. Diese Gläser weisen keine Störung durch Entglasung
und Entmischung auf. Neben der praktischen Bedeutung der Standards
liegen hier gut definierte Gläser vor, über deren verschiedensten
Eigenschaften nun umfangreiche Daten existieren. Dagegen gibt es noch
keine Informationen über die Beweglichkeit der im SiO2-Netzwerk eingebauten
Alkali- und Erdalkaliionen. Um Informationen über die Transportprozesse
der Netzwerkwandler zu gewinnen, wurden die ionische
Leitfähigkeit der Proben im Frequenzbereich zwischen 5Hz und 1.28MHz
mit der Impedanz-Spektroskopie und die Diffusion von 22 Na und 45 Ca
mit der Radiotracermethode im Temperaturbereich zwischen 490K und
759K untersucht. Die Tracerdiffusions- und Leitfähigkeitsmessungen werden
verglichen und diskutiert.
DY 29.8 Di 16:30 H23
Ion dynamics in single and mixed alkali silicate glasses studied
by molecular dynamics simulations — •Heiko Lammert and
Andreas Heuer — Westfälische Wilhelms–Universität, Institut für
Physikalische Chemie, Corrensstraße 30, D-48149 Münster
We use molecular dynamics (MD) simulations to study the ion dynamics
in alkali silicate glasses with the general composition
x K2O · (1 − x) Li2O · 2 SiO2, for x = 0, 0.5, 1.
All individual alkali sites provided by the frozen network can be identified
by a statistical analysis of MD trajectories, for both the single
and the mixed alkali systems. Sites are located by a cluster search on a
discretised alkali population density.
The ion dynamics can be analyzed in terms of fast hops between these
sites, also yielding average residence times to characterize the sites. As
the number of free sites is low compared to the number of ions, correlated
back jumps are found to be probable, especially for ions on fast sites. The
mobility of ions can be correlated to the local network structure around
their site, particularly to the occurence of bridging oxygens.
DY 29.9 Di 16:45 H23
Mixed-alkali effect in sodium rubidium borate glasses: ionic conductivity
and diffusivity — •Árpád W. Imre, Stephan Voss, and
Helmut Mehrer — Institut für Materialphysik, Universität Münster,
Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany and Sonderforschungsbereich
458
Measurements of the tracer diffusion coefficient and ionic conductivity
have been made in a series of 0.2[XNa2O · (1 − X)Rb2O] · 0.8B2O3
glasses where X=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 in order to investigate
the mixed-alkali effect. The glasses were characterised with differential
scanning calorimetry, density measurements and X-ray diffraction experiments.
Both 22Na and 86Rb tracer diffusion coefficients were determined
as a function of temperature and composition. For each composition the
temperature dependence of the tracer diffusion coefficients and dc conductivity
times temperature σdc × T follow the Arrhenius law. The manifestations
of the mixed-alkali effect were observed: i) the ionic conductivity
shows a minimum with composition near X =0.4, ii) the activation
enthalpy of σdc × T has a maximum near X =0.4, iii) the glass-transition
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
temperature independent. The composition dependence of the tracer diffusivities
has an asymmetric character. The tracer diffusion coefficients
are compared with ionic conductivity and the corresponding Haven ratios
obtained. In contrast to 0.2Na2O · 0.8B2O3 glass, the Haven ratio in the
0.2Rb2O · 0.8B2O3 depends on temperature.
DY 29.10 Di 17:00 H23
From binary Na-silicate to ternary Na-alumino-silicate melts:
the evolution of Na dynamics and intermediate range order
— •Florian Kargl 1 , Andreas Meyer 1 , and Helmut Schober 2
— 1 Physikdepartment E13, TU München, 85748 Garching — 2 Institut
Laue-Langevin, 38042 Grenoble, France
Neutron scattering investigations of binary alkali silicate melts revealed
correlations on intermediate lengthscales of 6-8 ˚A. Within MDsimulations
on Na-silicates evidence was found that these correlations
arise from Na-rich channels percolating in the remaining SiO-network.
Fast Na-diffusion takes place via these channels [1]. Substituting the
network forming component SiO2 by GeO2 leads to a change in structure
and in the microscopic transport mechanism, respectively, as verified
by inelastic neutron scattering. We present our recent investigations on
sodium alumino-silicate melts containing besides SiO2 a second network
former, Al2O3. Our main aim was to study the interplay between
microscopic dynamics and structure and its influence on macroscopic
properties. From quasielastic neutron scattering experiments we
conclude that the channel structure is disrupted by adding Al2O3 to the
Na-silicates resulting in an increase of viscosity as well as a decrease of
Na-relaxation times [2].
[1] A. Meyer, J. Horbach, W. Kob, F. Kargl, H. Schober (submitted)
[2] F. Kargl, A. Meyer, Chem. Geol. (submitted)
DY 29.11 Di 17:15 H23
Fragilität niedermolekularer organischer Glasbildner unter
Druck — •Andreas Reiser, Gernot Kasper und Siegfried
Hunklinger — Kirchhoff-Institut für Physik, Universität Heidelberg,
Im Neuenheimer Feld 227, 69120 Heidelberg
Die Fragilität einer glasbildenden Flüssigkeit ist definiert durch die
Steigung bei der Glasübergangstemperatur Tg im Angell-Plot. Sie erlaubt
eine Klassifikation in starke bzw. fragile Glasbildner. Stark bedeutet ein
Arrheniusverhalten der Relaxationszeit über den gesamten Temperaturbereich
T ≥ Tg. Die Relaxationszeit fragiler Glasbildner dagegen zeigt
eine stärkere Temperaturabhängigkeit. Wir haben den Einfluss von hohem
hydrostatischen Druck auf die Fragilität von typischen organischen
Glasbildnern mit dielektrischer Spektroskopie gemessen: Glycerin und
meta-Fluoranilin sind Vertreter wasserstoffgebrückter Glasbildner. Glycerin
rangiert im Zwischengebiet zwischen stark und fragil, wohingegen
meta-Fluoranilin und der Van-der-Waals-Glasbildner Propylencarbonat
vom fragilen Typ sind. Im untersuchten Druckbereich (0 −600 MPa) zeigen
alle drei Glasbildner keine Druckabhängigkeit der Fragilität im Rahmen
der Fehlergrenzen von ±5%.
DY 29.12 Di 17:30 H23
A binary colloidal mixture in 2D: An ideal model system for the
glass transition — •Hans König — Fachbereich Physik, Universität
Konstanz, 78457 Konstanz
Investigating a binary 2D colloidal glass former with a repulsive (1/r 3 )
potential we have measured particle positions as a function of time by
video-microscopy. The local packing of the big and small particles as well
as the heterogeneous dynamics were analysed depending on the system
temperature. Finally, we could describe the amorphous 2D short-range
order by local density-optimized triangles. However, there is one triangle
for each 3-particle combination of big and small colloids. Regions with
structural frustration occur between clusters of optimally packed triangles.
For decreasing system temperature the number of densely packed
triangles increased and they began to form clusters. For low system temperatures
such local density-optimized regions moved cooperatively resulting
in heterogeneous dynamics. By structural and dynamical investigations
we identify the 2D glass transition as a percolation of local
density-optimized triangles, which form a stable frame through the whole
glass former. At last, we call this glass description the concept of local
density-optimized crystallite clusters.