Hierarchically Structured Porous Bodies from Capillary Suspensions

The combination of ceramic’s intrinsic properties such as chemical resistance, temperature stability, and mechanical properties, with the low density and high permeability of porous materials has been of interest in many industrial applications such as in biomedical, filtration, energy storage, heat exchange, gas adsorption, insulation, as lightweight construction materials or as catalyst monoliths or catalyst supports. The ceramics need to offer tailored porosity, pore size, chemical composition and are often shaped for the specific applications. At present, there are four commonly applied processing routes for macroporous ceramics: direct foaming, replica templating, sacrificial templating, and partial sintering. However, high porosities (> 50 %) with pore sizes 1 - 10 µm are not easily accessible with these methods. This could be achieved by the capillary suspensions approach.

Capillary suspensions are three-phase systems with a small amount (< 5 vol%) of an immiscible secondary phase added to a suspension. The secondary phase creates a strong particle network due to capillary forces, which changes the rheological properties from fluid-like or weakly elastic to strong gel-like behavior. The particle network connected through capillary bridges is much stronger than the corresponding network created only by attractive van der Waals forces; it does not collapse during debinding or sintering, and therefore can serve as precursor for sintered materials with high open-porosity. It was previously shown that ceramic capillary suspensions can be used to create sintered bodies from various materials with tailored microstructure and can be even applied to a 3D printing process. However, the achievable maximum porosity is currently limited to a maximum of 65 % and the achievable mechanical strength is surpassed by the direct foaming and replica methods. Moreover, the direct applicability of this processing method in an industrially relevant environment has not yet been shown.

In this thesis, I examined the increased mechanical and thermal stability of the extruded green bodies by adding a secondary phase. Shrinkage during debinding was determined to be only 20 - 30 % for ceramic capillary suspensions, remarkably lower than for the other methods. This low shrinkage also results in a high dimensional accuracy, which enables shaping complex forms with undercuts and overhangs. I succeeded in forming the capillary suspensions through a suitable twin-screw apparatus from the raw components without any difference from premixed capillary suspensions. The effect on pore size distribution and pore size is preserved through the extrusion process and is similar to mold casting. Capillary suspensions are applicable to a 3D-printing and a continuous extrusion process, which makes them suitable for prototyping as well as mass production. The low shrinkage and high shape accuracy enables reliable processing.

In order to improve mechanical properties and extend the achievable porosity, the secondary fluid is used to deposit ceramic nanoparticles specifically in the contact regions of the microparticles. Thereby, I have advanced control over the resulting microstructure while harnessing the nanoparticles as sintering aids. Based on this concept, the mechanical strength is increased by up to 5 times and the limit for the maximum obtainable porosity is pushed to 75 % while still preserving a high level of mechanical strength. Thus, I demonstrate state-of-the-art mechanical properties without sacrificing versatility and tunability. The combination with a 3D-printing (direct ink writing) process yields cellular structures with specific strength close to that of balsa wood. For a relative density of 0.3, I achieved a compressive strength of 60 MPa, doubling typical values for cellular ceramics at this relative density. I reached these values for both investigated raw materials, alumina and aluminosilicate, strongly differing in original strength, indicating the method’s invariance in the used chemical compositions and highlighting its versatility.

Furthermore, I can use the nanoparticle-laden secondary liquids as a temperature stable “bonding agent” for catalytically active particles, e.g. zeolites, to generate an additional hierarchical level. The sintering activated neck formation of the ceramic nanoparticles “glues” the coarse catalytic particles together and provides mechanical stability while preserving the inherent porosity in the zeolite particles. The resulting monoliths have a fully open macroporosity of more than 50 % while preserving at least 85 % of raw powder’s BET surface area. I confirmed the zeolite’s functionality by catalytic methanol to olefins reactions where the monolith shows similar selectivity as the initial powder. Therefore, I can manufacture hierarchically structured porous monoliths with high specific surface area for high-temperature catalytic applications.

In summary, I show a versatile processing route for porous bodies. The fully open, tunable porous structure and applicability to a wide range of sintering materials offers the possibility to meet the requirements for targeted lightweight high temperature processes, e.g. in biomedical, filtration, energy storage, heat exchange, gas adsorption, insulation or catalytic applications. Due to the high specific strength, independent of the material, and the precise shaping, this route has the potential to find widespread use in industrial processes.