Alumina Suspensions (alumina + suspension)

Distribution by Scientific Domains

Selected Abstracts

Reaction-limited aggregation in presence of short-range structural forces

AICHE JOURNAL, Issue 4 2005
Venkataramana Runkana
Abstract A geometrically discretized sectional population balance model for reaction-limited aggregation of colloidal suspensions is presented. The two important model parameters are collision frequency factor and collision efficiency factor. The collision frequency factor is derived from physically realistic arguments proposed for collision of fractal aggregates. The collision efficiency factor is computed as a function of total interaction energy between particles, including short-range structural repulsion forces. The irregular and open structure of aggregates is taken into account by incorporating their mass fractal dimension. The characteristic time constant of reaction-limited aggregation, derived from dynamic scaling of mean aggregate size-aggregation time data, is found to correlate with electrolyte concentration. The population balance model is tested with published experimental data for aggregation of ,-alumina suspensions in the presence of different electrolytes. It is shown that the slow kinetics of aggregation under certain conditions of pH and electrolyte concentration require inclusion of short-range structural repulsion forces along with van der Waals attraction and electrical double layer repulsion forces in an extended DLVO theory. The model predictions are in good agreement with experimental data for time evolution of mean aggregate diameter in the reaction-limited aggregation regime. © 2005 American Institute of Chemical Engineers AIChE J, 2005 [source]

Effects of microstructure on the compressive yield stress

AICHE JOURNAL, Issue 1 2000
Glenn M. Channell
The effects of microstructure on the compressive properties of aggregated alumina suspensions are determined by intentionally introducing heterogeneities into the suspension. Suspensions are prepared at a high volume fraction and diluted with low shear hand mixing to a series of initial concentrations. As the initial concentration is increased, larger heterogeneities are introduced, and the suspension becomes more compressible relative to the compressive 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 (,c, which coincides with the volume fraction prior to dilution) is reached. Above ,c, the suspensions consolidate identically to the uniform suspension. With a single fitting parameter (the size of the heterogeneities), the model shows semiquantitative agreement with the experimental data. [source]

Cross-linked Polyvinyl Alcohol as a Binder for Gelcasting and Green Machining

France Chabert
Submicrometer alumina suspensions, dispersed in aqueous acidic solutions of polyvinyl alcohol (PVA) and 2,5-dimethoxy-2,5-dihydrofuran, have been evaluated for suitability as a cross-linkable binder system for casting complex-shaped ceramic components. Suspensions of up to 50 vol% solids have rheological behavior, which is suitable for pouring and filling molds. Complex-shaped green bodies are then formed by heating the suspension in the mold for a period of time (typically 15,60 min) at moderate temperature (60°,80°C) to gel the suspension. High green densities (58%,62% of full density) can be obtained. The dried green bodies have strength in excess of 1 MPa and may be readily machined. No more than 1,3 wt% PVA per weight of alumina is necessary, ensuring burnout that minimizes generation of flaws. The ceramic components can be fired to >96% of full density when fired for 2 h at 1400°,1450°C. Cross-linkable PVA may receive more widespread acceptance in ceramic processing than previous gelcasting formulations because PVA is already a common processing additive. [source]

Gelling of Alumina Suspensions Using Alginic Acid Salt and Hydroxyaluminum Diacetate

Andre R. Studart
This paper proposes a novel direct casting method of alumina suspensions using alginic acid salt and the coagulation agent hydroxyaluminum diacetate (HADA). These two compounds allowed the consolidation of alumina suspensions through a simultaneous time-delayed physical and chemical gelation process. The physical gel was formed by the gradual release of aluminum and acetate ions from the HADA in water, while the chemical gel originated from the cross-linking of alginate molecules by the polyvalent aluminum ions. Wet alumina green bodies displayed enhanced mechanical properties with the addition of minimal contents of organic material (<0.1 wt%). [source]

Formulation of aqueous concentrated alumina suspensions.

Influence of a disaccharide: trehalose
Abstract Trehalose, a disaccharide, is very well known to protect living cells efficiently from dehydration and has been tested in the formulation of aqueous alumina suspensions. It has been added into slurries stabilized with Tiron®, (HO)2C6H2(SO3Na)2, which permits a high state of dispersion by creating repulsive potential between particles. Trehalose added into such suspensions acts as a lubricant and enhances flowing properties of suspensions because of its strong interaction with water which breaks the hydrogen-bonded network of the solvent. Trehalose addition is beneficial for carrying out shaping methods of alumina components by coagulation as it increases solid concentration in the suspension, which facilitates pouring the suspension into a mould. Unfortunately this addition hinders coagulation of particles. © 2003 Society of Chemical Industry [source]