Microencapsulation of silicon nitride particles with yttria and yttria-alumina precursors
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I. INTRODUCTION
Uniformly coated particles of various shapes can offer some interesting advantages for a variety of powderbased technological applications. In several of these applications microencapsulation of particulates has proven to yield unique products, such as time-release drugs or carbonless copy paper.1 Microencapsulated powders have also been used or proposed in numerous other areas. Sparks1 and Garg and Matijevic2 have reviewed several examples for the synthesis and use of coated powders for diverse applications such as paints, magnetic tapes, carbonless copy paper, insecticides and pesticides, drugs and pharmaceuticals, catalysts, and adhesives. In ceramic technology, uniformly coated powders, while potentially advantageous, have yet to find broad application. One of the difficulties in the manufacture of such powders is that thick coatings, of the order of the core particle radius, are not easily deposited on micron or submicron sized powders. Further, agglomerates formed during the encapsulation process may limit the advantage of the treatment since the agglomerates may lead to an inhomogeneous microstructure. The objectives of microencapsulation of powders in materials processing can be classified in two major categories: A. Homogeneous incorporation of additives
The added substances could be sintering aids, dopants, or components of a nano-composite material. It may be desirable to obtain a homogeneous distribution of such additives within the matrix powders. Obviously, this would require a perfect mixing of the various powders—a condition that is generally difficult to achieve. 136
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Even with random mixing, a homogeneous distribution may not be obtained on a microscale. An excellent approach would be to encapsulate uniformly the powders with the desired materials or their precursors. This would make feasible a homogeneous distribution of the additives throughout the matrix. B. Fabrication of particulate/whisker composite materials
Microencapsulation of the reinforcing materials may be desirable to prevent reaction with the matrix or to manipulate particulate-matrix bonding properties. In the fabrication of fiber and whisker reinforced ceramics, modification of the dispersoid matrix interface by coating the former with materials such as silica, carbon, or boron nitride has been attempted.3 7 Metal-matrix composites are another class of materials where coating of the dispersoids could lead to better properties.8"17 The present work reports on solution-precipitation procedures that have been developed to encapsulate powders and whiskers of silicon nitride with yttria and yttria-alumina precursors. Uniform layers of these were deposited by aging a concentrated dispersion of the silicon nitride particles in aqueous systems containing yttrium and aluminum nitrates, urea, and a polymeric stabilizer. Monodispersed particles of yttrium hydroxycarbonate were synthesized earlier using a similar procedure involving hydrolysis of yttrium salts by urea decomposition.18 II. PREP
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