Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
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Dielektrische Festkörper Dienstag<br />
behaviour of the relaxation function obtained in stimulated echo measurements<br />
are explained. Further it will be discussed whether those deviations<br />
are a sign of the so called wing phenomenon that is seen in<br />
dielectric spectra and how far the measurements are influenced by limitations<br />
of the stimulated echo method. Restrictions of the stimulated<br />
echo method are shown by simulations.<br />
DF 4.6 Di 16:00 H23<br />
Ionic transport in alkali-borate glasses — •F. Berkemeier,<br />
Á.W. Imre, S. Voss und H. Mehrer — Institut für Materialphysik,<br />
Wilhelm–Klemm–Str. 10, 48149 Münster, Westfählische–Wilhelms–<br />
Universität Münster, Sonderforschungsbereich 458<br />
We studied the ionic transport in binary and ternary alkali-borate glasses<br />
of the compositions y Na2O·(1 − y) B2O3, y Rb2O ·(1 − y) B2O3 and<br />
0.3 [x Na2O·(1−x) Rb2O]·0.7 B2O3. The total alkali content of the binary<br />
glasses ranges from y = 0.04 up to y = 0.30 while the relative composition<br />
of the mixed glasses varied in between x = 0 and x = 1. First, the ionic<br />
transport was observed by temperature dependent measurements of the<br />
dc conductivity which is connected to the long range transport of the cations.<br />
For each glass composition we observed an Arrhenius behaviour of<br />
the dc conductivity and determined an activation enthalpy ∆H. For the<br />
binary systems we found decreasing values of ∆H with increasing alkali<br />
content while for the mixed glasses we observed a maximum of the activation<br />
enthalpy at the composition range around x ≈ 0.4. Conductivity<br />
measurements only reveal the sum of the contributions of the different<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 />
DF 4.7 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 />
DF 4.8 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 />
DF 4.9 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 86Rb 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 />
DF 4.10 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 />
DF 4.11 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%.