Capillary suspension based additive manufacturing of highly porous, cellular ceramic structures with unprecedented mechanical strength/density ratio
Final Report Abstract
In line with the projects purpose, we extended the capillary suspension route for porous ceramics and added direct ink writing as additive manufacturing route. Capillary suspensions are ideally suited for direct ink writing and can be debinded and sintered without cracking after printing. Combination of these two methods (ceramic capillary suspensions with direct ink writing) was used to fabricate lightweight structures with outstanding mechanical properties as well as functional material with good electroceramic properties. Capillary suspension processing, including these capillary nanosuspension pastes, is a low cost and, due to absence of conventional volume displacer, an environmentally-friendly processing route. The fully open, tunable porous structure and applicability to a wide range of sinterable materials provides a pathway to meet the requirements for targeted lightweight materials used at high temperature or in chemically harsh environment, as well as construction and/or thermal insulation materials. We successfully combined an additive manufacturing route with the capillary suspension route for porous ceramics. We developed a suitable paste for direct ink writing as well as a non-destructive debinding and sintering method. Having this, we printed honeycomb structured and tested their lightweight properties. Additionally, we introduced the versatile capillary nanosuspension concept, that uses a nanoparticle-laden liquid as secondary phase in capillary suspensions, thus depositing nanoparticles exclusively at the coarse particles contact regions. By this approach, trace amounts of silica nanoparticles (< 5 vol% of total solids) are introduced to an alumina based capillary suspensions. Even though silica is a significantly weaker material than alumina, we increased the mechanical strength by a factor of 5 and even produced samples with porosities of 75 % that offer a mechanical strength of 2 MPa. In combination with a 3D-printing (DIW) process, we produced cellular honeycomb structures with low density and high specific strength. These structures offer the highest compressive strength to weight ratios presented for lightweight open porous ceramics, so far. It doubled typical values at a relative density of 0.3, offering a compressive strength of 60 MPa at this relative density. The compressive strength is dominated by the solid bridges and ceramics created from weaker aluminosilicate coarse particles resulted in nearly identical strength as alumina. This capillary nanosuspension approach is versatile in terms of the coarse particle’s chemical composition. The nanoparticles can function as sintering aid to enhance the sintering process of the coarse particles or as bonding agent between coarse particles by a low-temperature sintering process that only sinters the nanoparticles. This approach is so intriguing and versatile, that we decided not to follow up on the reinforcement concept with added zirconia as outlined in WP3c of the grant proposal. Furthermore, we demonstrated how AM can be used to improve the properties of functional ceramics when combined with tailored ink formulation and sintering concepts. We were approached by specialists for functional ceramics to do this in collaboration. The rich, highly valuable opportunities offered by this concept convinced us to work on this topic instead of building and testing a more straight forward cross-filtration demonstrator described in WP5. We utilize an ink formulation concept based on the capillary suspension phenomenon in combination with direct ink writing and skillful sintering to tailor and substantially increase dielectric and electromechanical properties of the functional ceramic barium titanate. With this approach, we achieve remanent piezoelectric coefficient values similar as for dense BT at ~ 60% porosity and an energy harvesting figure of merit that is four times higher than any documented data for this particular material. This dramatic increase in functionality is on one hand to due to the distinct particle network structure in CapS, which is preserved in the 3D printed and sintered structures with 3-3 connectivity, yielding a high openporosity and pore sizes adjustable in the µm range within the printed struts. On the other hand, the specific flow properties of CapS enable DIW of highly filigree structures with unprecedented low diameter values (≈70 µm) and strut to gap ratios (1:6). In this way, extremely high overall porosities can be achieved while maintaining sufficient mechanical strength after extremely short debinding times and almost no shrinkage during sintering. The self-organized particle network leads to an extraordinary electromechanical coupling. The combination of this coupling and the high porosity results in the dramatic increase of the FOM33.The presented concept may be transferred to a broad range of other materials and applications, such as electromechanical energy harvesting, electrode materials for batteries or fuel cells, thermoelectrics or bone tissue engineering with piezoelectrically stimulated cell growth. Our method is thus a major advance for the fabrication of high-performance materials, which is technically easy to implement. Furthermore, it is a versatile basic concept for the design of complex hierarchical ceramics. Meanwhile, two ZIM projects have been started together with four SME companies emerging from the research done and the results obtained within the framework of this DFG grant aiming at the fabrication of ceramic filters using DIW-type additive manufacturing of CapS based ceramic pastes.
Publications
- (2017). 3D printing of open-porous cellular ceramics with high specific strength. Journal of the European Ceramic Society, 37(15), 4833-4842
Maurath J., Willenbacher N.
(See online at https://doi.org/10.1016/j.jeurceramsoc.2017.06.001) - (2020). 3D-Printed lightweight ceramics using capillary suspensions with incorporated nanoparticles. Journal of the European Ceramic Society, 40(08), 3140-3147
Weiß M., Sälzler P., Willenbacher N., Koos, E.
(See online at https://doi.org/10.1016/j.jeurceramsoc.2020.02.055)
