Investigation of Ionic Diffusion in Alkali Borate Glasses by Impedance Spectroscopy and Atomic-Scale X-Ray Photon Correlation Spectroscopy

Tobias Michael Fritz, Bogdan Sepiol

The main objective of this thesis is to study and characterize the dynamical properties of various alkali borate glasses adopting two different methods. The first part addresses to impedance spectroscopy (IS) investigating macroscopic diffusion phenomena. On an atomic scale, a novel technique called atomic-scale X-ray photon correlation spectroscopy (aXPCS) is applied for obtaining a deeper understanding of atomic diffusion processes within alkali borate glasses, covering the second part of this thesis.

The network of covalent bonded alkali borate glasses is formed by boron trioxide B2O3, where alkali oxide A2O is denoted as network modifier. Alkali metal ions in such glasses xA2O(1−x)B2O3 are positively charged and therefore experience a force in an external electric field. In fact, this is done when performing an IS experiment for characterization of the alkali metal ions. The complex impedance Z∗(ω) of a material system is measured and recorded as a function of the excitation frequency ω and via electronic circuit (EC) models, specified by ohmic resistors as well as ideal and non-ideal capacitors. The dispersive dynamical properties of such a system are probed in this course. The voltage applied within an IS experiment is low, hence structural rearrangements can be excluded. From complex impedance spectra, the ionic conductivity σ∗(ω), the charge diffusion coefficient Dσ, electric modulus M∗(ω) and dielectric relaxation times τ are obtained and compared qualitatively with other techniques like tracer diffusion experiments, quasi-elastic neutron scattering experiments (QENS) or like in this thesis, aXPCS experiments.

Complex impedance and electric modulus both are mathematical convertible concepts. Starting from an impedance spectrum the DC ionic conductivity plateau σDC is obtained directly and therefore the charge diffusion coefficient Dσ is calculated and compared with the collective diffusion coefficient obtained from an aXPCS experiment. However, the electric modulus provides access to relaxation times of the decay of the electric field within the sample. IS measurements in the frequency region are related to a physical description
in the time domain through the electric modulus. The mathematical description is done by a Fourier transform. The major function in this context is the so called Kohlrausch-Williams-Watts (KWW) function which provides information about the relaxation time distribution within the material.

Alkali borate glasses exhibiting ionic conductivity show on the one hand temperature dependence described by an Arrhenius behaviour, on the other hand their ionic conductivity depends on the molar ratio of the alkali oxide to boron trioxide. Furthermore, it becomes apparent that the ionic dynamics is getting slower for increasing atomic numbers. Glasses are amorphous solids, hence unlike crystalline materials, they do not show a certain melting temperature, but rather a temperature range where structural changes from vitreous to viscous behaviour are observed. For the purpose of this thesis, alterations of the ionic dynamics within this temperature range is investigated.

In contrast to dynamical light scattering (DLS) experiments, where the used electromagnetic radiation is in the optical visible range, the underlying principle of aXPCS makes use of highly coherent X-ray radiation from a synchrotron source. As the name implies, consecutively recorded speckle pattern is correlated in time, resulting in an intensity autocorrelation function decaying with a particular time constant τ inversely proportional to the diffusion coefficient D. Quite different from IS experiments, recent experimental results from aXPCS provided a prove of an impact of the high photon flux of X-rays on ionic and atomic mobility. Within the scope of this thesis, results are presented which imply that different dynamical phenomena are observed for both IS and aXPCS measurements. In the case of aXPCS, in addition to the intrinsic dynamics of the alkali metal ions, a beam induced dynamic is observed resulting from the impinging high-energy X-ray beam.

Comparing the results obtained by the mentioned methods it is suggested that the incoming X-ray beam triggers conformational changes around boron and oxygen atoms. Theses atoms migrate then through well-defined atomic jumps across the sample. However, the decisive factor when doing X-ray photon scattering is the content of alkali atoms in the sample. If it is high, the alkali atoms are better visible in an aXPCS experiment. Conversely, if the alkali content is reduced, boron and oxygen atoms become more and more visible.
At the very beginning, a theoretical overview on the structure of alkali borate glasses is given. Subsequently, mathematical derivations and expressions of the used formalisms are presented, which should build the basic framework for the data evaluation afterwards.

Following this, the manufacturing process of various glass samples from powder through to small plates is presented. The final part of the thesis is the presentation and discussion of the obtained experimental results together with possible interpretations and explanations of the beam induced dynamics in alkali borate glasses.

Dynamik Kondensierter Systeme
Anzahl der Seiten
ÖFOS 2012
103008 Experimentalphysik, 103009 Festkörperphysik, 103015 Kondensierte Materie, 103018 Materialphysik
ASJC Scopus Sachgebiete
Condensed Matter Physics
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