1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

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 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

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

Grain growth kinetics in alumina-rich, zirconia-alumina composites have been examined as a function of volume fraction of zirconia. Both the microstructural scale and character alter during grain growth. Grain growth exponents between n = 3 and 4 were observed and have been rationalized in terms of current models for grain growth and particle coarsening. Grain growth constants were measured and compared with available data. Coupled grain growth was observed in that the ratio of the alumina-to-zirconia grain size was constant, for a given zirconia content. This behavior is discussed in terms of the topological constraint present during growth and the observed microstructural evolution. The measured activation energies were consistent with boundary-diffusion-controlled grain growth.

Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M[sub s] for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO[sub 2] content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M[sub s] can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO[sub 2] grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M[sub s] grain size, when T>M[sub s]. In alumina containing 20 vol% ZrO[sub 2] (12 mol% CeO[sub 2]), a toughness of [approximately]10 MPa [center dot] [radical]m can be achieved for a ZrO[sub 2] grain size of [approximately]2 [mu]m (M[sub s] [approximately]225K). However, at a grain size of [approximately]2[mu]m, the alumina-40 vol% ZrO[sub 2] (12 mol% CeO[sub 2]) has a toughness of only 8.5more » MPa [center dot] [radical]m (M[sub s] [approximately] 150K) but reaches 12.3 MPa [center dot] [radical]m (M[sub s] [approximately] 260K) at a grain size of [approximately]3 [mu]m. These findings show that composition (and matrix properties) play critical roles in determining the ZrO[sub 2] grain size to optimize the transformation toughening in ZrO[sub 2]-toughened ceramics.« less

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

The aqueous processing of binary suspensions that contain 0.50-μm alumina and either 0.20- or 0.71-μm zirconia was studied from pH 3 to 11, emphasizing colloidally stable systems. Differences in particle size and density can induce differential sedimentation and lead to a nonuniform oxide distribution in pellets prepared by pressure filtration. Uniformity improves when the zeta potentials of the unlike particles are of the same sign but differ greatly in magnitude, i.e., in the pH range 5 to 6, where the oxides experience a weak, mutual association. However, when they are more similar electrostatically, they do not mutually associate and uniformity suffers. This nonuniformity occurs with the adjustment of pH to ≪5 (pos ive particles) and the adsorption of soluble species that strongly alter the zeta potential, e.g., after the preexposure of zirconia to aluminum chloride (positive particles) and the addition of polyacrylic acid (negative particles). In these cases, either high viscosity or rapid consolidation are required to ensure a uniform distribution. The disparity in the particle number fraction, derived from the size differences, and the thermodynamic interparticle energies control the prefired microstructure and are central to the prediction of the proper processing conditions.

Effects of pH and Particle Size on the Processing of and the Development of Microstructure in Alumina–Zirconia Composites

Near net-shaped parts of zirconia-alumina composites have been successfully formed by gelcasting, a technique which utilizes in situ polymerization of acrylamide monomers. The high solids loading required for gelcasting ({approximately}50 vol %) was obtained by controlling the pH-dependent stability of the aqueous zirconia-alumina suspensions. A strong correspondence was found among the surface charges on the particles, colloidal stability, and the maximum solids loading. 14 refs., 3 figs., 2 tabs.

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Alan Bleier

Papers

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

Grain Growth Kinetics in Alumina–Zirconia (CeZTA) Composites

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Effects of pH and Particle Size on the Processing of and the Development of Microstructure in Alumina–Zirconia Composites

Gelcast Zirconia-Alumina Composites