Mesoscale periodic structures have been fabricated via directed assembly of colloidal inks. Concentrated colloidal gels with tailored viscoelastic properties were designed to form self-supporting features. The inks were deposited in a layer-by-layer sequence to directly write the desired 3-D pattern. Periodic structures with spanning features that vary between ∼100 µm and 1 mm were assembled. Shear rate profiles were calculated on the basis of the measured rheological properties of the inks under slip and no-slip boundary conditions during flow through a cylindrical deposition nozzle. Deflection measurements of spanning elements were used to probe the relationship between gel strength, deposition speed, and shear rate profiles in the nozzle. These observations revealed that the ink adopted a rigid (gel) core-fluid shell architecture during assembly, which simultaneously facilitated bonding and shape retention of the deposited elements.

The ability to pattern ceramic materials in three dimensions (3D) is critical for structural, functional, and biomedical applications. One facile approach is direct ink writing (DIW), in which 3D structures are built layer-by-layer through the deposition of colloidal-or polymer-based inks. This approach allows one to design and rapidly fabricate ceramic materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. In this feature article, we present both dropletand filament-based DIW techniques. We focus on the various ink designs and their corresponding rheological behavior, ink deposition mechanics, potential shapes and the toolpaths required, and representative examples of 3D ceramic structures assembled by each technique. The opportunities and challenges associated with DIW are also highlighted.

Stability of aqueous a-A1,O3 suspensions with Na+ salt of poly(methacry1ic acid) (PMAA-Na) polyelectrolyte was studied as a function of pH. At a given pH, the transition from the flocculated to the dispersed state corresponded to the adsorption saturation limit of the powders by the PMAA. As the pH was decreased, the adsorption saturation limit increased until insolubility and charge neutralizatioin of the PMAA was approached. The critical amount of PMAA required to achieve stability is outlined in a stability map.

Colloidal inks, comprised of silica microspheres with tailored attractive interactions between them, have been directly assembled via a robotically controlled deposition technique. 3D periodic lattices, whose periodicity far exceeds the dimensions of the colloidal building blocks (see Figure for a cross‐sectional image), were created through layer‐ by‐layer patterning of parallel rods.

Stability and rheology of aqueous aY-A1203 suspensions with poly(methacry1ic acid) and poly(acry1ic acid) polyelectrolytes were studied as a function of pH, solids loading, and molecular weight. Past work has found polyelectrolyte-stabilized suspensions to be fairly pH independent at low (e.g., 20 ~0 1 % ) solids loadings. However, we now show that the effective pH range to provide dispersed and fluid suspensions narrows as the concentration of solids increases as related to interparticle forces. At high solids levels, the presence of excess polymer in solution has detrimental effects on stability, which increases as the molecular weight increases. Finally, with knowledge of these concepts, highly concentrated fluid suspensions of 60 vol% A1,03 (>85 wt%) with submicrometer-size particles have been processed. Higher consolidated densities and reduced sintering temperatures are attained when compared with more conventional processing techniques.

Robocasting is a freeform fabrication technique for dense ceramics and composites that is based on layer-wise deposition of highly loaded colloidal slurries. The process is essentially binderless with less than 1% organics and parts can be fabricated, dried, and completely sintered in less than 24 hours. This review will highlight materials developments for structural applications and modelling of slurry flow. Fabrication of preforms for alumina / metal composites will be discussed as well as techniques for multimaterial deposition in both graded structures and discrete placement of fugitive materials.

A freeform fabrication technique for dense ceramics and composites has been developed. The technique requires less than 2 volume percent of organic additives and relies on the principle of layerwise deposition of highly loaded colloidal slurries. Components can be manufactured into complex geometries with thick solid sections as well as with thin-walled sections with high aspect ratios. Process feasibility and quality is dependent on the processing parameters of solids loading, slurry rheology, deposition rate, and drying rate. These interrelated parameters must be controlled so that sintering defects are prevented and shape tolerance is maintained. A review of this freeform fabrication technique, called robocasting, will be discussed for fabrication of aluminum oxide parts. Recent developments for a finite element analysis technique for modelling the drying process will also be presented.

Enhanced catalytic combustion of methane gas was achieved using novel ceramic support structures. These support structures consist of a three-dimensional lattice of rods, created by a direct fabrication technique (robocasting). The rods (725-1000 µm in diameter) are oriented in such a way as to create no line-of-sight pathways, to have high surface-to-volume ratios (15-43 cm 2 /cm 3 ), and to generate highly turbulent flow, promoting increased mass transfer without sacrificing low pressure drops when compared to conventional extruded-honeycomb supports. The flow behaviors of both robocast and honeycomb supports were characterized, and the catalytic activities for the combustion of methane gas were compared. Under otherwise equivalent conditions, robocast lattices converted approximately 6 times more methane at 600 °C than extruded-honeycomb supports.