Direct ink writing of hierarchical porous alumina‐stabilized emulsions: Rheology and printability

Abstract

Bio‐inspired multi‐scaled (hierarchical) porous structures have remarkable strength and stiffness‐to‐density properties. Direct ink writing (DIW) or robocasting, an additive manufacturing (or also commonly known as 3D printing) material extrusion technique is able to create near‐net‐shaped complex geometries. A new approach of combining DIW, colloidal particle‐stabilized emulsion paste inks and partial densification to create tailored architectures of hierarchical porosity on three scales has been demonstrated. The printed and sintered ceramic lattice structures possess relatively high overall porosity of 78.7% (on average), comprising mainly (64.7%) open porosity. The effects of formulation (surfactant and oil concentrations, solids particle size, and mixing speed) on rheology and pore size and morphology have been investigated. The rheological properties (storage modulus, yield stress, and recovery of storage modulus) of the emulsions have been found to delineate the samples with good shape retention from those that slump. Additionally, the internal features of the sintered structures have been analyzed via X‐ray tomography and scanning electron microscope. The role of emulsion stability on printability and the internal structure of the prints has been investigated.

Direct ink writing of hierarchical porous ultra‐high temperature ceramics (ZrB2)

“…The layers are well bonded together and it is not possible to distinguish where one filament melded into the next one. It is quite clear that there are no closed (or mostly closed) spherical pores in the material as were observed when particle‐stabilized foams and emulsions were used for pastes 12,13,24 . Instead, the pores observed (Figure 2B,C) are primarily interconnected and open, and have random or disorganized shape from a few microns to about 20 μm in size.…”

“…These samples clearly show that the void space visible in the center of the filaments observed in Figures 3 and 4 are clearly tube‐like structures. The formation of the tube‐like structures appears to be associated with microwave drying, although the phenomena may also be related to formulations with high surfactant and oil content 24 . The microwave and other attempts to speed drying and oil removal seem to be correlated with a more severe degree of distortion during drying, as apparent particularly in Figure B.…”

“…It is quite clear that there are no closed (or mostly closed) spherical pores in the material as were observed when particle-stabilized foams and emulsions were used for pastes. 12,13,24 Instead, the pores observed (Figure 2B,C) are primarily interconnected and open, and have random or disorganized shape from a few microns to about 20 μm in size. This microstructure is consistent with that produced from partial sintering of capillary suspensions [14][15][16][17] although the amount of oil in our formulations is greater than that typically used in capillary suspensions.…”

“…The storage modulus is slightly lower than those reported in the literature (between 6 × 10 3 and 1 × 10 5 Pa) for pastes which print well, and the yield stress on in the lower range of those reported in the literature (between 100 and 2 × 10 4 Pa) for printable pastes. 4,6,12,23,24 The sunflower oil in the formulation acts as a lubricant and allows for smaller diameter nozzles to be used than when attempts were made to print particle-stabilized foams or dense filaments without oil.…”

“…The volume fraction of open porosity between filaments, the volume fraction of open porosity in filaments, the volume fraction of closed porosity in filaments, and volume fraction of solids are determined by solving four equations simultaneously. The calculations are described in the supplementary materials and in Chan et al 24 The percentages of the total volume of the test bars which are solid, porosity between the filaments, the open porosity within the filaments and closed porosity are presented in Figure 5A. The fraction of solid, open, and closed pores within the printed filaments in the test bars is shown in Figure 5B.…”