Agglomeration of silica spheres under ultrasonication
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Agglomeration of silica spheres under ultrasonication Naoya Enomoto, Shingo Maruyama, and Zenbe-e Nakagawa Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226, Japan (Received 28 September 1996; accepted 26 November 1996)
Power ultrasound of 20 kHz was applied to the synthesis of silica spheres via the controlled hydrolysis of tetraethoxysilane (TEOS). Silica spheres of about 0.3 mm were agglomerated to form tolerably uniform, dense particles of about 2 mm through 90 min sonication. This agglomeration behavior was examined by laser diffraction particle size analysis and transmission electron microscopy. It was found that the agglomeration process involves (I) an incubation period in which no agglomeration occurs, (II) rapid formation of ramified particles, and (III) their densification. It was inferred that sonication enhances collision among silica spheres.
I. INTRODUCTION
Incorporation of a power ultrasound into ceramic powder preparation via precipitation from solution, what we call sonochemical powder processing, has been studied recently.1–5 It has been found so far that an ultrasound fastened or altered reactions, nucleation, and growth in a variety of precipitation systems. One of the most interesting powder preparation methods is the controlled hydrolysis of metal alkoxides that yields monosized and spherical particles of oxide (hydrate).6 We previously found in the sonochemical processing of silica spheres that a power ultrasound at low frequency (20 kHz) induces hard agglomeration among spheres.3 This phenomenon made us curious to learn how an ultrasound acts upon the motion of particles in solution. It is well-known in practice that an ultrasonication brings about dispersion of agglomerated powders in solution, but it is less known that it also causes agglomeration. Aoki et al.7 attempted to formulate the ultrasonic dispersion and coagulation of very fine (,100 nm) SiC powders by considering the interaction among the SiC particles and the ultrasonic-induced cavities. Suslick et al.8 reported that a strong sonication induced agglomeration among metal powders such as Zn, Cu, Ni, Fe, and Cr, due to intensive interparticle collisions. In the former case, supposedly, the bonding among SiC particles originates from an electric double layer and an adsorption layer on the particle surface; while in the latter case, they considered that the metal particles partially fused together. On the contrary in our study, silica spheres, whose surface is occupied by silanol groups, are inferred to be combined by the following chemical reaction during agglomeration: Si - OH 1 HO - Si °! Si - O - Si 1 H2 O . Since this reaction is the same as that of polycondensation in the sol-gel process of silicon alkoxides, 1410
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 5, May 1997
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the agglomeration phenomenon of silica spheres may be related to the “polymerization” which can be visibly pursued
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