Capillary suspension based additive manufacturing of highly porous, cellular ceramic structures with unprecedented mechanical strength/density ratio

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Porous ceramics are used in a broad range of applications including filtration membranes, catalyst carriers, bone scaffolds or composite materials for lightweight construction elements. Recently, we have developed a new processing route for highly porous sintering materials based on well-established unit operations. This innovative concept uses so-called capillary suspensions (three-phase systems solid/fluid/fluid) as pre-cursor to fabricate highly porous sintering materials. Thus open porosities between 50% and 80% and pore sizes ranging from 0.5 µm to 50 µm can be obtained easily. Here we want to advance this technology to fabricate highly porous ceramic structures with so far unmatched mechanical strength. Two strategies will be developed and evaluated in parallel:1. An additive manufacturing technique, the so-called Direct Ink Writing (DIW) method, shall be used to create cellular structures with highly porous ceramic struts. This is supposed to result in ceramic bodies with porosities > 90% and high strength/density ratio close to Balsa wood. Pastes optimized for this printing technique will be developed, and capillary suspensions are especially suited due to their high yield stress and pronounced shear-thinning behavior. Preliminary experiments suggest that carefully prepared capillary suspensions enable to create fine feature sizes (~ 50 µm) previously not accessible with DIW. Cellular structures will be fabricated using DIW and tested with respect to their mechanical strength.2. Capillary suspensions will be utilized to obtain ceramic materials with outstanding mechanical strength at a given porosity. The basic idea of this approach is to use the secondary liquid of the three-phase capillary suspension as a carrier for targeted deposition of small, sinter-active particles at the contact zones of the large particles. This should result in higher pore roundness and reinforced sintering necks finally resulting in an increased mechanical strength of the porous ceramic bodies. In preliminary experiments e.g. the compressive strength of porous Al2O3 ceramics could be tripled via addition of a fine ZrO2 fraction. Chemical composition and size of the fine particles will be varied systematically and the resulting structures will be carefully analyzed for a targeted further enhancement of mechanical strength. Finally, the insight from both subprojects shall be combined using DIW and appropriate material combinations for manufacturing porous ceramic structures with unprecedented specific mechanical strength. The fully open porous struts of capillary suspension based cellular structures makes the materials fabricated according to our new approach promising candidates for filtration and separation applications. Accordingly, DIW will be used to print a filter demonstrator and its separation properties will be evaluated.