Diffusion in glasses studied with X-Ray Photon Correlation Spectroscopy

Glass is being manufactured for thousands of years in a variety of forms. Its area of application is huge, reaching from jewelry to everyday objects and usage under extreme strain to energy storage. Recently, remarkable progress has been achieved in the areas of solid oxide fuel cells, batteries and super conductors, in electrochemical sensors and in functional polymers. However, some fundamental concepts, like the transport of ions in disordered matter, are still poorly understood. The properties of a glass are determined by its constituting atoms. These atoms are in constant motion in the form of diffusion. It is the goal of this research project to study diffusion in glasses on the atomic level. The insight gained in this way can help to improve qualities like the stability and the conductivity of a material. They can also help advancing manufacturing processes. It is important to study a glass in the state where it has its application. Usually glass is utilized in solid form. Therefore analyses must be made at temperatures well below its melting point. With conventional experimental methods for studying atomic jumps this is not possible. At low temperatures the atoms in the glass move very slowly. Their dynamics can therefore not be resolved by common techniques. This is where X-ray Photon Correlation Spectroscopy comes into play. This method follows a new approach, where temporal changes in the scattering images are compared (correlated), allowing to draw conclusions about the movement of the scatterers (atoms). To accomplish this, X-rays with the same properties as laser light are needed. The only radiation source capable of providing X-rays with the appropriate quality is a synchrotron. Only in the last decade did the brilliance, which is the measure of quality for this kind of radiation, get high enough to allow for advancing this new method to the atomic scale. In the frame of a previous project it was possible to show that this technique could be used to study atomic diffusion in crystalline matter. In this project we will use it to study amorphous matter. There is a variety of different theoretical approaches and models to describe diffusion in glasses. However, there is no simple, widely accepted model of transport in the literature. X-ray Photon Correlation Spectroscopy allows for directly comparing atomistic transport models with experimental results. It will therefore possible to make quantitative statements about the quality of different models and thereby improve the understanding of atomic motion in glasses.

Glasses are an active and promising field of research, although glass has been processed in various forms for several thousand years. Understanding disordered solids on a fundamental level and particularly understanding ionic conduction in these materials still presents a funda-mental challenge. The dynamics of basic structural network components play a key role in decoding properties of this form of matter, especially its puzzling ionic conductivity. Considerable technological progress for today's market, such as solid-oxide fuel cells, batteries and supercapacitors, electrochemical sensors and functional polymers, has been achieved recently. There is still no general consensus on a widely accepted model of transport regarding fast ionic conduction. Considering the current strong interest in the field and the plethora of ex-perimental and theoretical works it is our aim to shed light on the motion of ions on the atomic level in ionic conducting boron and silica glass. We are confident that the insights gained can be transferred to many other ionic glasses. The movement of single atoms is a fundamental issue in materials science. The fabrication, specific properties and the stability of materials can be significantly improved with the knowledge about atomic dynamics. Studies of this movement mechanisms also called diffusion, on a scale of single atoms, are despite a number of well-established experimental methods very challenging. Due to the relatively slow atomic motion in glasses, a new, promising technique based on scattering of x-ray photons is our method of choice. Scattering events on atoms, however, induce stationary dynamics in oxide glasses that is proportional to the amount of absorbed photons per atom. Compared with ionic conductivity measurements on the same materials, we could eventually benefit from this acceleration of the dynamics and could infer that the motion of conductive ions is largely decoupled from the glass network and that the arrangement of ionic sites, where ions could move to, remain stable in the same time window as the surrounding network. Hence, there is a discrepancy between the fast diffusion of conductive ions, which in the case of glasses with high ionic concentrations takes place on a configuration of densely populated sites via a vacancy mechanism known from crystalline materials, and the longevity of the glassy network.