3D-printed porous mullite lattice structures by hybrid direct ink writing of silicone suspension-emulsions

Abstract

Silicones added with nano-sized alumina particles are already known as starting materials for phase pure mullite ceramics, synthesized at quite low temperatures. The present paper deals with a fundamental upgrade, based on a novel suspension-emulsion concept, for the easy fabrication of highly porous lattice structures. An aqueous suspension of γ-Al₂O₃ nanoparticles in water was first distributed as emulsion within an “oily phase,” consisting of a silicone/acrylates blend, with the help of a surfactant. The mixture was later employed to fabricate highly porous structures (∼80% open porosity), by direct ink writing, that is, an extrusion-based 3D printing technology requiring specific rheological behavior of the feedstock ink. Finally, the structures were rapidly stabilized through a photo-polymerization step (configuring a form of “hybrid” direct ink writing). The presence of water also allowed the application of a freeze-curing procedure, for a second series of samples. The abundant water vapor release from the starting mixtures, upon firing (up to 1300°C), led to structures with enhanced pore interconnectivity. The freeze-curing protocol proved beneficial to the homogeneity of pore distribution and to the achievement of high strength-to-density ratios.