Laboratory studies on cometary crust formation: The importance of sintering

It is demonstrated by experiments and theoretical considerations that sintering processes, so far used to describe the densification of metal and ceramic powders, are relevant for icy materials and therefore probably also for comets. A theoretical model is presented which describes the evolution of so called sinter necks, the contact zone between ice particles. With this model the strength increase of a porous, loosley packed icy body is calculated in which the sinter necks grow by evaporation and condensation of water vapor at a constant temperature. Experiments with ice powders validate the model qualitatively. An increase in strength up to a factor of four is observed during isothermal sintering. In order to check the relevance of the experimental results and the basic theoretical ideas with respect to real comets, more exact theories and improved experiments taking into account additional mass transport mechanisms are needed

Aerogels: Structure, properties and applications

Aerogels prepared via a sol-gel route and supercritical drying posses a unique microstructure with specifics like a 3D open porous network of nanosized particles, huge specific surface area up to a few thousand square meters per gram, extremely low thermal conductivity, extremely low density, such that applications in a variety of industrial sectors seemed obvious 30 years ago. The structure and properties are essentially independent of the chemical nature of the aerogels, which can be organic, inorganic, a combination of both or composites made from them. Although the large potential initiated intensive research it took almost twenty years before the still costly production process allows making aerogels finding their way into more and more industrial sectors. Aerogels are now utilized as super insolating materials in shoe soles or apparel, daylight illumination systems, pipeline isolation mats, medium temperature isolating materials, tennis rackets, drug delivery systems, foundry core and mould materials, building construction materials and many more are being developed in the last decade. The paper describes briefly typical aerogel structures, properties and then concentrates on a comprehensive presentation of industrial applications today and potential for the future

Enhancing precision radiotherapy: image registration with deep learning and image fusion for treatment planning

Artificial intelligence is advancing in everyday life and supports its user by generating fast results in areas like communication or image recognition. This thesis aims at exploiting the abilities of deep-learning techniques for deformable image registration (DIR) to improve image alignment in medicine. An unsupervised registration and fusion workflow is developed and evaluated for 39 head scans, produced with computed tomography (CT) and magnetic resonance imaging (MRI). The three-part workflow starts by preprocessing the scans to unify the image formats and to perform affine transformation and rigid registration. Then, a deep-learning model trained for DIR is applied to these images. To obtain an appropriate configuration of the model, parameter tuning is required. The evaluation with the mutual-information metric indicates an improvement in image alignment of up to 14 % when using deep-learning-based DIR. Lastly, image fusion combines the registered CT and MRI scans with a wavelet-based method to merge the information of decomposed images. The workflow is designed for unimodal, e.g. T1- and T2-weighted MRI scans, and multimodal, e.g. CT and MRI scans, image pairs. Since medical imaging is an important basis of treatment-planning processes, the registered and fused images obtained from this workflow are expected to enhance precision radiotherapy

Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions

The Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MSL-CETSOL and MICAST) are two investigations which supports research into metallurgical solidification, semiconductor crystal growth (Bridgman and zone melting), and measurement of thermo-physical properties of materials. This is a cooperative investigation with the European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) for accommodation and operation aboard the International Space Station (ISS). Research Summary: Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing (CETSOL) and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MICAST) are two complementary investigations which will examine different growth patterns and evolution of microstructures during crystallization of metallic alloys in microgravity. The aim of these experiments is to deepen the quantitative understanding of the physical principles that govern solidification processes in cast alloys by directional solidification

Sol-Gel Derived Ferroelectric Nanoparticles Investigated by Piezoresponse Force Microscopy

Piezoresponse force microscopy (PFM) was used to investigate the ferroelectric properties of sol-gel derived LiNbO$_3$ nanoparticles. To determine the degree of ferroelectricity we took large-area images and performed statistical image-analysis. The ferroelectric behavior of single nanoparticles was verified by poling experiments using the PFM tip. Finally we carried out simultaneous measurements of the in-plane and the out-of-plane piezoresponse of the nanoparticles, followed by measurements of the same area after rotation of the sample by 90$^{\circ}$ and 180$^{\circ}$. Such measurements basically allow to determine the direction of polarization of every single particle

Flow effects on the dendritic microstructure of AlSi-base alloys

Fluid flow changes heat and mass transport during solidification, thereby affecting the evolution of the microstructure. In order to quantify effects of convection, it is important that fluid flow can be modified experimentally. We performed directional solidification experiments with binary AlSi alloys of different compositions, using a microgravity environment for diffusive solidification and adding rotating magnetic fields to generate flow. Flow velocities up to 10 mm/s and various solidification velocities were realized while maintaining a constant temperature gradient at the solid-liquid interface. The microstructure observed in samples processed on earth and in space is characterized by primary and secondary dendrite arm spacing and the fractal dimension of the dendrites. It is found that fluid flow usually accelerates growth and coarsening of the dendritic structures and leads to new kinetic laws. The branching of dendritic networks, however, is hardly affected by flow