The ability to pattern materials in three dimensions is critical for several technological applications, including composites, microfluidics, photonics, and tissue engineering. Direct-write assembly allows one to design and rapidly fabricate materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in direct ink writing are reviewed with an emphasis on the push towards finer feature sizes. Opportunities and challenges associated with direct ink writing are also highlighted.

/pdf/direct-ink-writing-of-3d-functional-materials-3agrqrwm1s.pdf

Direct ink writing of 3D functional materials

Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate-derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.

/pdf/colloidal-processing-of-ceramics-1f3f618kn7.pdf

Colloidal Processing of Ceramics

1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

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 droplet- and 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.

/pdf/direct-ink-writing-of-three-dimensional-ceramic-structures-f2k162jg49.pdf

Direct Ink Writing of Three‐Dimensional Ceramic Structures

The recent interest in the properties of ceramics with grain sizes less than 100 nm has created a need for processing routes with which to manufacture such ceramics. This article briefly reviews the variety of techniques currently used to manufacture ultrafine starting powders, compact the powders, and sinter them into bulk nanocrystalline ceramics. The unique challenges associated with processing such fine structures are discussed together with each technique. Major obstacles have included the difficulty (now largely overcome) of producing sufficient quantities of ultrafine powders; the strong tendency of nanocrystalline powders to agglomerate; the difficulty of manufacturing homogeneous, stress free compacts from ultrafine powders; and the ever present obstacle of unwanted grain growth during sintering. Despite numerous technical difficulties, researchers have managed to develop techniques for processing ultrafine powders into nanocrystalline ceramics which are both fully dense (or near fully de...

Processing of nanocrystalline ceramics from ultrafine particles

The compressive rheological responses of suspensions containing flocculated kaolin, alumina (average particle sizes of 0.2 and 0.5 {micro}m), and hydrous zirconia (average particle sizes of 8, 57, and 139 nm) particles have been measured using three different techniques: pressure filtration, volume fraction profile during centrifugation, and sediment height during centrifugation at multiple spinning speeds. While the volume fraction profile technique appears to be experimentally most robust, equivalent responses are found using the different techniques, indicating that the compressive yield stress is a material property of a given suspension. The compressive yield stress of each suspension increases rapidly with volume fraction but cannot be generally described using simple power-law or exponential fits. The compressive yield stress also increases with the inverse square of particle size. The packing behavior of the suspensions undergoing osmotic consolidation is compared with the mechanical compressive yield response. Some suspensions exhibited the same packing behavior as in the mechanical techniques, while others consistently packed to higher densities during osmotic consolidation. Although equivalent osmotic and mechanical loads do not always result in the same volume fractions, the similar increases in volume fraction with applied driving force suggest that both the osmotic and mechanical techniques are controlled by themore » force needed to rearrange the particle network.« less

Comparison of the Compressive Yield Response of Aggregated Suspensions: Pressure Filtration, Centrifugation, and Osmotic Consolidation

The effects of microstructure on the compressie properties of aggregated alumina suspensions are determined by intentionally introducing heterogeneities into the suspen- sion. Suspensions are prepared at a higholume fraction and diluted with low shear hand mixing to a series of initial concentrations. As the initial concentration is in- creased, larger heterogeneities are introduced, and the suspension becomes more com- pressible relatie to the compressie yield stress of the uniform suspension. A simple model is proposed in which the heterogeneous suspensions compress by rearrangement ( of the dense aggregates until a critical concentration f , which coincides with the c ) ¤olume fraction prior to dilution is reached. Aboe f , the suspensions consolidate c ( identically to the uniform suspension. With a single fitting parameter the size of the ) heterogeneities , the model shows semiquantitatie agreement with the experimental data.

/pdf/effects-of-microstructure-on-the-compressive-yield-stress-hqvmt5uqqr.pdf

Effects of microstructure on the compressive yield stress

An osmotic method for the consolidation of suspensions of ceramic particles is demonstrated. Concentrated solutions of poly(ethylene oxide) are separated from a suspension of ceramic particles by a semipermeable membrane, creating a gradient in solvent chemical potential. Solvent passes from the suspension into the polymer solution, lowering its free energy and consolidating the suspension. Dispersions of stable 8-nm hydrous zirconia particles were consolidated to over 47% by volume. Suspensions of α–alumina in three states of aggregation (dispersed, weakly flocculated, and strongly flocculated) were consolidated to densities greater than or equal to those produced in conventional pressure filtration. Moreover, the as–consolidated alumina bodies were partially drained of fluid during the osmotic consolidation process, producing cohesive partially dried bodies with improved handling characteristics.

Osmotic consolidation of suspensions and gels

Compressive yield stresses have been measured for pastes (0.35 ≤ w/c ≤ 0.50) of portland cement, calcium aluminate cement, and weakly and strongly flocculated alumina (Φ0 = 0.20) using the centrifuge sediment height technique. Equilibrium sediment heights are reached quickly, allowing all measurements to be taken during the cement's induction period. The compressive behavior showed little dependence on the compressive history. Compressive yield stress was, however, dependent upon initial volume fraction, decreasing as the initial volume fraction increases. This behavior was observed in both the cements and alumina suspensions, implying that strong dependencies on initial structure may be a general property of the compressive behavior of flocculated suspensions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400422037

Compressive Yield Stress of Cement Paste

The mechanical behavior of suspensions containing particles in the nanoscale (or sub-100 nm) size region is important in many processing operations. In this chapter, we discuss the similarities and differences between the properties of suspensions containing these particles and those composed of larger particles. While the same underlying interparticle forces govern the microstructure and flow behavior of suspensions of nanoscale and conventionally sized particles, the magnitudes of these forces, which include thermal, hydrodynamic, van der Waals, electrostatic, steric, and solvation/structural forces, have different dependencies upon particle size. As a result, suspensions of nanoscale particles may have properties which are unexpected when one's intuition is based upon suspensions of larger particles. Here the basic concepts governing suspension mechanics are described, and examples of the trends which are seen as the particle size shrinks toward the molecular level are provided.

Kelly T. Miller

Papers

Comparison of the Compressive Yield Response of Aggregated Suspensions: Pressure Filtration, Centrifugation, and Osmotic Consolidation

Effects of microstructure on the compressive yield stress

Osmotic consolidation of suspensions and gels

Compressive Yield Stress of Cement Paste

The mechanics of nanoscale suspensions