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.

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Colloidal Processing of Ceramics

Issues concerning powder consolidation methods compatible with the colloidal approach and issues associated with other powder processing steps, viz., densification and microstructural control, are presented with regard to research directions leading to more reliable ceramics.

Powder Processing Science and Technology for Increased Reliability

Porcelain represents the foundation of the ceramics discipline and one of the most complex ceramic materials. Composed primarily of clay, feldspar, and quartz, porcelains are heat-treated to form a mixture of glass and crystalline phases. This review focuses on raw materials, processing, heat treatment, and mechanical behavior. Because of the complexities of the porcelain system and despite the substantial amount of research already conducted within the field, there remain significant opportunities for research and study, particularly in the areas of raw material understanding, processing science, and phase and microstructure evolution.

Porcelain—Raw Materials, Processing, Phase Evolution, and Mechanical Behavior

Stability of aqueous α-Al2O3 suspensions with Na+ salt of poly(methacrylic 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 neutralization of the PMAA was approached. The critical amount of PMAA required to achieve stability is outlined in a stability map.

Stability of Aqueous α-Al2O3 Suspensions with Poly(methacrylic acid) Polyelectrolyte

This paper describes studies on a new processing route for fabricating highly porous ceramics. The method is based on the generation of a foam from an aqueous suspension of ceramic powder and the subsequent stabilisation of the structure by in situ polymerisation of organic monomers. The influence of the slip viscosity on the foam volume and stability was determined using concentrated alumina suspensions containing dispersing agents and two commercial foaming agents. The in situ polymerisation of organic monomers led to fast solidification, resulting in strong, porous bodies which could withstand machining. The resulting ceramic foams consisted of a highly interconnected network of spherical cells with densities as low as 6% of theoretical. The distribution of cell size was dependent both on the density of the specimen produced and on the time for polymerisation onset. The size ranged from approximately 30 to 600 μm. Enlargement of cell size to achieve materials of higher permeability was possible through expansion of the foam via pressure reduction before polymerisation. The creation of highly densified struts between the cells led to flexural strengths as high as 26 MPa. ©

Processing of cellular ceramics by foaming and in situ polymerisation of organic monomers

Stability and rheology of aqueous α-Al2O3 suspensions with poly(methacrylic acid) and poly(acrylic 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 vol%) 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% Al2O3 (>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.

Processing of Highly Concentrated Aqueous α‐Alumina Suspensions Stabilized with Polyelectrolytes

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Stability of ,Aqueous a =Al203 Suspensions with Poly(methacry1ic acid) Polyelectrolyte

Colloidal stability of aqueous α-Al2O3 suspensions with polymethacrylic acid (PMAA) was studied in the pH range of 3 to 10. Stability is related to the adsorption of PMAA on α-Al2O3 as controlled by the chemistries of both the α-Al2O3 surface and the PMAA. The adsorption behavior of PMAA on α-Al2O3 is basically “high affinity” type near and below the zero point of charge of α-Al2O3 (pH 8.7). A stability map was determined which outlines the critical amount of adsorbed PMAA required to achieve dispersion.

Joseph Cesarano

Papers

Stability of Aqueous α-Al2O3 Suspensions with Poly(methacrylic acid) Polyelectrolyte

Processing of Highly Concentrated Aqueous α‐Alumina Suspensions Stabilized with Polyelectrolytes

Stability of ,Aqueous a =Al203 Suspensions with Poly(methacry1ic acid) Polyelectrolyte

Interactions between Polyelectrolytes and Oxides in Aqueous Suspensions