Aerogels have been gaining a huge interest in the foreground of material science and emerging technology due to their extraordinary properties like high porosity, open pore structure or low density. Additionally, the preparation of aerogels using natural and biodegradable materials, such proteins or polysaccharides, provides new options for life science and food applications. For instance, aerogels are used as matrix material for active substances, in particular for pharmaceuticals or food products. However, the encapsulated compound and aerogel matrix are sensitive to variation of environmental conditions which leads to irreversible changes of the formulation. Therefore, to create an environmental barrier, the surface of aerogels is modified with a secondary protective coating layer.
Nevertheless, the exposition of the particles to different process conditions during coating e.g. in a spouted bed (Figure 1), cause irreversible structural changes too. This is due to the thermal or mechanical stresses and high capillary forces caused by pore-penetrating coating material. Therefore, the aim of this research project is to develop a strategy for coating of these open-pore structured, nanoporous materials. For the coating two kinds of coating materials are applied – solutions and melts. They affect the surface and structure of aerogels in a different way due to their various properties and required process conditions. During experimental investigations the question, which type of coating material (solution or melt) and under which process conditions is the most suitable for coating of aerogels with the goal of keeping the 3D structure unaffected, is answered.
Due to low density and small size aerogel particles are coated in a prismatic spouted bed (Figure 1). This process is excellent for coating with thin, uniform and dense layers. Afterwards, the coating layer thickness of coated and cross-sectioned particles is measured. The cross-sectioning of single coated particles is performed using focused ion beam (FIB) technique combined with scanning electron microscope (SEM) imaging and is schematically shown in Figure 2. The feature of this method is the possibility of cross-sectioning of a sample at a desired position with high precision. Combination of different ion beam properties and milling tools subsequently controlled with SEM result in cross-sectioned particles shown in Fig. 3.
According to SEM-images in Figure 2, these light-weighting and fine aerogel particles are successfully coated in a prismatic spouted bed. The proposed FIB-cross sectioning technique enables measurement of coating layer thickness and has the advantage over existing techniques that it enables very precise measurement of the layer thickness of particles in micrometre size range. The layer thickness of the particle shown in Figure 3 is about 1 µm.
Additionally, the project applies a modelling approach to optimize the coating procedure by means of the coupled CFD (Computational Fluid Dynamics) - DEM (Discrete Element Method) approach. Additionally, the structure of the particles is modelled with DEM.
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• Institute of Thermal Separation Processes, Hamburg University of Technology (I. Smirnova)