Evolution of Surface area During the Controlled Growth of Silica Spheres
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EVOLUTION OF SURFACE AREA DURING THE CONTROLLED GROWTH OF SILICA SPHERES GREGORY H. BOGUSH,* C. J. BRINKER,* P. D. MAJORS" AND D. M. SMITH** * Sandia National Laboratories, Albuquerque, NM 87185 USA ** UNM/NSF Center for Micro-Engineered Ceramics, University of New Mexico, Albuquerque, NM 87131 USA ABSTRACT Several mechanisms have been proposed for the growth of silica spheres by the controlled hydrolysis of silicon alkoxides, the limiting cases of which are the conventional and aggregative growth models. The evolution of surface area predicted from the two models is substantially different at early times into the reaction. In order to probe the change in surface area during growth, 1H NMR measurements of the solvent were made during the growth process. It has been demonstrated that the spin-spin relaxation time.(T2) for an absorbed phase is less than that of the bulk solvent. Using this principal, the change in surface area can be followed in situ during the reaction. The experimental results were compared to the predictions of the conventional model and found not to be in agreement. INTRODUCTION According to the conventional model for the nucleation and growth of monodisperse particles, proposed by LaMer and Dinegar [1], a single burst of nucleation is followed by growth by monomer addition to the particle surface. Since all of the particles are formed at nearly the same time, they retain a narrow size distribution at the end of the reaction. More recently, evidence has been shown, for several systems, that aggregation during the growth process may be important [2,3]. The change in surface area predicted from these two models is quite different. For the conventional model, the change in surface area should be directly related to the change in particle size during growth, but if aggregation is occurring, a more complicated dependence might be expected. Davis, et al., [4] have demonstrated that the rapid determination of surface area is possible with the use of IH NMR relaxation measurements. This method is based on the observation that the relaxation time for an absorbed phase is less than that of the bulk, therefore, the surface area can be related to the deviation of the measured relaxation time from the bulk relaxation time. In this paper, the results of 1H NMR spin-spin relaxation measurements made during the growth of silica particles from alkoxides are presented. These results are analyzed in an attempt to distinguish between the two growth mechanisms. EXPERIMENTAL To start the precipitation reaction, 2 ml of tetraethyl orthosilicate (TEOS) (Petrarch Systems, g9%), 2.3 ml of ammonium hydroxide (Fisher Scientific, Reagent Grade), 2 ml of deuterium oxide (Chemical Dynamics, 99.95 atom% D) and 40 ml of ethanol (Aaper Alcohol and Chemical, Absolute) were mixed together. An aliquot of the reaction mixture was then transferred to a 5 mm NMR tube. Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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Temporally resolved 1H NMR T2 relaxation measurements were performed at a Larmor frequency of
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