Nanocomposite powders, a new concept for their synthesis

• Published in: Scripta materialia 2000-08, Vol.44 (8-9), p.2197-2201
• Main Authors: Hofmann, H, Bowen, P, Jongen, N, Lemaitre, J
• Format: Article
• Language: English
• ISSN: 1359-6462

The synthesis of inorganic materials of specific size and morphology is a key aspect in the development of new materials in fields as diverse as catalysis, medicine, electronics, ceramics, pigments, and cosmetics. The morphology control has been very difficult to achieve by means of classical colloidal chemistry although micrometer-sized particles with various shapes, such as spheres, needles, cubes, can be obtained by the use of controlled precipitation conditions in combination with additives. Particles can precipitate as single crystals or in the form of polycrystals made up of a large number of nanocrystallites. The formation of single crystals is often well described by classic nucleation and growth theory for many systems but the nucleation/diffusional growth mechanism alone cannot explain many experimental observations. Most polycrystalline particles are made up of random oriented nanocrystallites and show therefore a spherical shape, while few polycrystalline particles are made up of well-organized subunits. Most of the studies published about the self-organization of nanocrystallites into particles during precipitation is very recent. The materials showing such behaviour are (NH sub 4 ) sub 3 PW sub 12 O sub 40 , alpha -Fe sub 2 O sub 3 and, barium sulfate, barium chromate and fluoroapatite in the presence of additives which seem contribute greatly to the self-assembly. The formation of particles by precipitation often seems to be the result of two combined growth processes after the initial nucleation: growth of crystallites and aggregation of these primary crystallites to particles. Both crystallites and aggregates vary in size and shape depending on the experimental conditions during precipitation. If the crystallite size and shape is very homogeneous, the agglomeration of these crystallites to dense particles with a controlled shape is possible.