Chemical Synthesis of Fine Powders
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s or inorganic anions are commonly used to improve the process. This article emphasizes the rôle of chemistry in the synthesis of oxide powders. Several examples are described for both inorganic and alkoxides routes to point out the main chemical parameters that allow a better control of nucleation reactions via hydrolysis and condensation of molecuiar precursors. Another discussion takes into account phenomena such as Ostwald ripening or aggregation which are also involved in the précipitation of powders from solutions.
Nucleation and Growth The growth of colloidal particles from nuclei proceeds by diffusion of soluté species onto the existing nuclei. Différent mechanisms can be involved. The rate of particle growth might be controlled either by diffusion or by surface reactions.4-5 The size distribution of particles sometimes narrows with time as their size increases. However, in gênerai, the formation of monodispersed particles is favored when nuclei are small and of a narrow size distribution. This obviously requires a careful control of the génération of soluté species. The most common qualitative expianation for the formation of solid particles from a solution was proposed by LaMer.6 Monodispersed particles can be obtained when the critical supersaturation of the soluté precursors is reached so that only one burst of nuclei occurs. Many nuclei are gênerated. They grow rapidly, lowering the molecuiar species concentration to a point below the nucleation concentration but high enough to allow particles to grow without secondary nucleation. • Supersaturation can be monitored if a kind of chemical réservoir is incorporated in the solution so that the release of soluté species can be governed by
such parameters as température, pH, or complexation. • Nucleation and particle growth usually take place simultaneously. It would be better to sépara te them so that no secondary nucleation occurs while particles are growing. This can be obtained when the nucleation rate remains rather slow while the growth rate is fast. Two main parameters are used to describe nucleation and growth:4 (1) The supersaturation ratio S = C/C$ governs the size of critical nuclei. C is the concentration of soluté precursors and Cs, the solubility of the solid phase having a molecuiar volume V. For oxides typical values are for Cs = 10'5 moldnr3 and V = 30 Â3, (2) The interfacial energy y whose typical value is y = 0.1 Jm2. Nucleation and growth rates can be taken into account with two probability terms, g = exp(-AG„/RT)ands' = exp(-AG'„/RT), respectively. Activation free énergies AG correspond to ail chemical effects that are not included in S or y. The value of g mainly dépends on complexation of soluté precursors (typically AG0 = 50 kj.mol1), while g' dépends on the condensation process responsible for the growth of thèse species. Two condensation reactions are involved in sol-gel chemistry: (1) Formation of OH bridges via dation M—OH + M—XOH-> M—OH — M + XOH, (X = H or alkyl group) (2) Formation of oxygen bridges via oxolation M—OH + M —OX -
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