Fractal Concepts And Aggregation Of Iron Oxides
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FRACTAL CONCEPTS AND AGGREGATION OF IRON OXIDES 1 2 R. AMALV, J.A.ofRAPER , T.D. WAITE University New South Wales, School nf Chemical Engineering, P.O. Box 1, NSW 2033, Australia. 2 Australian Nuclear Science and Technology Organization (A.N.S.T.O.), Private Mail Bag 1, Menai, NSW, Australia.
ABSTRACT The modelling of the aggregation kinetics of iron oxides has been succesful in predicting the increase in aggregate size as determined by dynamic light scattering measurements. The aggregates were found to exhibit fractal behaviour with fractal dimensions obtained from the scattering exponent in static light scattering studies dependent on the aggregation mechanism and ranging from 2.3 for rapid (diffusion limited) to 2.8 for slow (reaction limited) aggregation. Polydispersity and restructuring of aggregates were found not to affect the relationship between scattering exponent and aggregate fractal dimension. Excellent correspondence over a range of temperatures and ionic strengths has been obtained between results of sizing experiments using dynamic light scattering and sizes predicted using a modified Smoluchowski model incorporating fractal dimensions. INTRODUCTION The structure of solid materials has important implications in many natural and commercial processes. Both physical parameters, such as dimension, surface area and porosity, and chemical parameters, such as solubility and reactivity will be determined by structure. Well structured materials such as crystalline solids can readily be grouped according to their symmetries. However, this procedure is not possible forrandomly formed materials such as aggregates which represent a large proportion of solid material of industrial interest. This lack of apparent symmetry has greatly impeded the progress in understanding random morphologies. However, in the last few years, considerable progress in the study of materials with random structures has been made as a result of the increasing use of fractal concepts '. Of particular interest here are the microscopic structures of oxides - materials that occur widely in nature and form the basis of many ceramic and catalyst preparations. The aggregation kinetics of hematite particles has been studied using dynamic light scattering2 , and experimental results were compared with a model based on theoretical considerations accounting for diffusion, interparticle repulsive forces and the nature of packing within the aggregated material (and referred to below as the "modified Smoluchowski" model). Successful prediction of rapid (diffusion limited) aggregation kinetics was obtained assuming that the resultant aggregates behaved as fractals with a fractal dimension of 2.3. A somewhat higherfractal dimension appeared to be appropriate for aggregation under ionic conditions where a repulsion barrier exists (reaction limited aggregation) . g3 This paper discusses the results of experiments using static light scattering to study the structure of colloidal hematite particles which are induced to aggregate at different rates by alter
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