Submicrometer zinc oxide particles: Elaboration in polyol medium and morphological characteristics
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A novel and easy route for preparing submicrometer particles of zinc oxide, involving hydrolysis of zinc salt in a polyol medium, is proposed. Zinc acetate dihydrate and diethyleneglycol appear to be the best candidates for obtaining a high yield of particles with well-defined morphological characteristics. Monodisperse spherical particles in the submicrometer range (0.2-0.4 /u,m) have been obtained for a salt concentration less than 0.1 mol I"1. The particle size depends mainly on the heating rate. The particles are microporous (surface area: 80 m 2 g"1) and are formed by aggregation of small crystallites (10 nm). Calcination at moderate temperature drastically reduces this porosity without significant interparticle sintering. At higher concentration, no aggregation occurs and tiny single crystallite particles are obtained.
I. INTRODUCTION Powders with particles of uniform shape and narrow size distribution lying in the submicrometer range have been shown to possess interesting properties. For instance, these powders sinter to dense bodies at a lower temperature than do classical coarser ones due to their smaller particle size. Furthermore, their microstructures appear uniform after sintering. Zinc oxide finds various ceramic applications such as semiconductors and varistors. Accordingly, the preparation of monodisperse zinc oxide particles with submicrometer size has attracted much attention. Spherical particles have been obtained by spray pyrolysis,1 aqueous precipitation process,2 thermal decomposition of zinc acetate,3 evaporative decomposition of solutions,4 and hydrolysis of alkoxide.5 Rod-like ones have been prepared by hydrolysis of zinc salts in the presence of hexamethylenetetramine,6 thermal decomposition of organometallic zinc complexes,7 calcination of basic zinc carbonate samples obtained by aging zinc salts solutions in the presence of urea,8 and hydrolysis of bis(acetylacetonato)zinc (n) in aqueous solution.9 In order to prepare inorganic compounds via precipitation from a liquid phase, solvents as polyols offer two interesting properties: owing to their high dielectric constants, they act as solvents that can dissolve inorganic compounds and, owing to their relatively high boiling points, they offer a wide operating-temperature range (from 25 °C to boiling point) for preparing inorganic compounds at relatively low temperature. This range may permit hydrous reactions that cannot be conducted in aqueous solution under atmospheric pressure. These properties have been exploited previously to prepare, by reduction, several powdery metals composed J. Mater. Res., Vol. 10, No. 1, Jan 1995
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of monodisperse spherical particles in the submicrometer range.10"12 It should be pointed out that a route called glycothermal treatment has been developed for preparing double oxides from acetate, acetylacetonate, or alkoxides.13 This route involves the use of ethyleneglycol instead of water for the hydrothermal method. The reaction is carried out under the auto
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