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Company USA
The company produces nanocrystalline metal oxide powders via Physical-Vapour Synthesis (PVS). The process involves vaporizing a metallic or metal-oxide composite precursor in a plasma, followed by rapid quenching to initiate condensation and formation of extremely small metal oxide crystallites. The size of the crystalline particles can be tightly controlled through variation in the condensation rate and the particle concentration in the downstream quench zone.
The discrete metal oxide nanocrystalline particles form loose aggregates that allow them to be collected in a dry powder form. These loose aggregates can then be redispersed in solution to provide stable suspensions of the individual nanocrystalline particles. Because the PVS process uses metals or metal oxide composites as the only starting materials, any product contamination that may result from solvent or solvated precursors material is avoided.
As a result, purity of the metal oxides nanoparticles, in both the bulk phase and on the surface, can be maintained at a very high level. In addition, the PVS process can be scaled up to provide a production rate on the order of tons/year. Examples of nanocrystalline oxides currently produced on a bulk scale include alumina, ceria, titania, zinc oxide, iron oxide, anti-mony/tin oxide, and indium/tin oxide. Numerous other pure oxides and mixed metal oxides are also accessible with the PVS process, and can be produced upon market demand.
Metal oxides prepared by the PVS process are comprised of crystalline, equiaxed, nonporous, discrete particles exhibiting mean diameters in the 10-50 nm range, with surface areas of 15-90 m2/g. The high surface area and small particle size provides a substanti-al surface/bulk atomic ratio, and results in a highly strained surface with numerous reactive sites.
Surface Treatment
Though nanocrystalline metal oxide powders have found use in a variety of coating applications, in most cases the powders require surface treatment prior to their incorporation into a product. Several proprietary surface treatment processes have been developed for metal oxide nanoparticles, designed to provide one or more of the following properties.
· Dispersability in liquids (aqueous, alcohol and hydrocarbons);
· Prevention of particle agglomeration;
· Compatibility with resin matrix;
· Functionalization of oxide surface with reactive groups;
· Refractive index matching;
· Passivation of the oxide surface chemistry.
For coatings applications, the nanocrystalline metal oxides require dispersion into a liquid medium, such as a solvent, or blended directly into the resin system. As produced, the metal oxide powders disperse well in aqueous environments wherein hydrogen bonding is sufficiently strong to disrupt the loose agglomerates and provide stable dispersions of the primary crystalline particles. The affinity of nanocrystalline powders for aqueous environments is often sufficient to allow the powders to be used in many waterborne coating formulations. However, because the powders do not disperse well in non-aqueous media, several specialized surface treatments have been developed that reduces particle agglomerates and yield stable dispersions in hydrocarbon solvents. Such treatments also prevent reagglomeration and thus enable the oxides to be used in a variety of solvent borne coating applications.
The surface treatment process is also designed to enable compatibility of the particles with the resin film matrix. In certain cases, the surface treatment process is used to incorporate functional groups on the oxide particles, allowing for direct interaction with the resin polymers. For certain systems, refractive index matching is necessary to yield transparent coatings, and surface treatment chemistry can be used to minimize the particle/matrix refractive index difference. Finally, as mentioned earlier, the nanocrystalline oxide surface is very reactive, and in some coating systems this necessitates a surface treatment process to passivate this reactivity so as not to interfere with the film curing process.
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