Mechanisms of Silica and Titania Colloidal Particle Formation from Metal Alkoxides
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MECHANISMS OF SILICA AND TITANIA COLLOIDAL PARTICLE FORMATION FROM METAL ALKOXIDES JOSEPH K. BAILEY AND MARTHA L. MECARTNEY Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 ABSTRACT Control of the preparation of monodisperse particles for green body formation can be achieved by understanding the growth mechanism. Growth processes for colloidal silica and titania were followed by cryo-TEM, which allows direct observation of the particles in the liquid state. Structural development was tracked by quenching samples at successive reaction times. The preparation of silica particles involved the formation of ramified species which subsequently collapsed after reaching a certain size. In the titania system, initially small dense nuclei are observed which eventually form uniformly textured homogeneous particles. INTRODUCTION High quality advanced ceramic materials can be prepared by using unagglomerated monodisperse powders; therefore, there is incentive to understand methods of producing monodisperse unagglomerated. Stober et al. [1] first showed that monodisperse silica particles can be derived from silicon alkoxides, and Bogush et al. [2] demonstrated that control over particle size and mass fraction can be obtained. In these processes, a solution of silicon alkoxide monomers undergoes hydrolysis and condensation to form the particles. Barringer and Bowen [4] demonstrated that an analogous preparation of titania particles was feasible. In order to understand the growth mechanism that produces monodisperse silica particles, Bogush and Zukoski performed kinetic and structural investigations on the silica system [ 3]. They inferred that nucleation of particles was occurring throughout the reaction, and discredited the idea of a traditional nucleation and growth mechanism. They proposed a nucleation and aggregation mechanism whereby small particles nucleate and then aggregate until reaching a colloidally stable size. These larger, colloidally stable particles diffuse through the solution aggregating with small particles which are continuing to nucleate. The rate at which the particles add onto the colloids is sufficient to prevent any of the smaller particles from growing to a colloidally stable size, thus ensuring monodispersity. A similarly intensive study has not yet been performed on the titania, however, microscopic investigations on the final dried titania particles indicate that they appear to be comprised of many small particles [5]. In order to understand the influence of the processing conditions on the morphology of the particles, the growth mechanism needs to be understood. This paper describes an comparative investigation of the structural development in silica and titania colloids by cryo-TEM. Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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EXPERIMENTAL Silica colloids were prepared from a solution of 0.17M tetraethoxysilane, 1.0-1.3M NHi3 , and i.OM to 3.8M H 2 0 in n-propanol solvent [1,3]. Titania colloids were prepa
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