Coating the surface of a powder with another substance has conventionally been conducted in order to obtain a powder having a special property or to modify the surface properties of the powder. Since it is difficult to evenly coat the surface of a powder having a small particle diameter, various methods have been investigated as powder-coating means.
Unexamined Published Japanese Patent Application No. 1-119062 discloses a technique for coating a powder with silver for the purpose of heightening the electroconductivity of the powder. Methods for forming a metal coating film on the surface of a metal or metal oxide powder are described, e.g., in Unexamined Published Japanese Patent Applications Nos. 3-271376 and 3-274278.
With respect to methods for forming a metal oxide coating film on the surface of a metal powder, a film of an oxide of the same metal can be obtained also by a method comprising placing the powder in an oxidizing atmosphere. However, this method is not applicable to the formation of a coating film of an oxide of another kind of metal. It is a matter of course that a completely different method should be used when the powder is made of a metal compound or a plastic.
A method usable for forming a coating film of a metal oxide on a powder comprises placing the powder in an aqueous solution of a metal salt, reacting the metal salt to cause precipitation and thus form a metal compound deposit layer on the powder particles, and drying or heating the covered particles to form a metal oxide layer to thereby obtain a coated powder. However, a dense oxide film is difficult to obtain by this method. If the above method is carried out so as to yield a dense oxide film, it is not easy to evenly form a dense film in a desired thickness, for example, because a large film thickness is difficult to obtain.
The present inventors previously invented a method for forming a film of a metal oxide by dispersing a metal powder or a metal oxide powder into a metal alkoxide solution and hydrolyzing the metal alkoxide, and made an application for patent (Unexamined Published Japanese Patent Application No. 6-228604).
The present inventors further developed highly functional powders by alternately forming a metal film and a metal oxide film on the surface of a metal powder or a metal oxide powder, and made an application for patent (Unexamined Published Japanese Patent Application No. 7-90310). For example, the inventor succeeded in obtaining a magnetic powder of a sufficiently white color by forming a coating film of a metal oxide on the surface of a magnetic material powder, such as ferrite or chromium oxide, and forming a coating film of cobalt metal or silver metal thereon, and also in obtaining an insulating powder having satisfactory thermal conductivity by forming a metal oxide film on a powder of a metal having satisfactory thermal conductivity, such as silver metal or copper metal. Furthermore, the present inventors made an application for patent concerning a process which comprises similarly forming a multilayered metal oxide film on the surface of a metal powder or metal compound powder and heating the powder coated with the multilayered metal oxide film to produce a powder having a multilayered denser and more stable metal oxide film (WO96/28269).
As described above, the present inventors have made efforts with a view to developing a highly functional metal or metal compound powder by forming one or more films of a metal oxide or metal on the surface of a metal powder or metal compound powder to thereby impart a property other than those possessed by the metal or metal compound powder serving as core particles.
However, the above-described method in which a metal alkoxide is used as a starting material for forming a film of a metal oxide on the surface of a metal powder or metal compound powder has a drawback that the product developed with the alkoxide has very limited uses because the metal alkoxide is an expensive compound and, hence, the product is, of course, costly.
Although the method in which a film of a metal oxide is formed by precipitation from an aqueous metal salt solution has the problem that an even metal oxide film is difficult to obtain, it has a more serious drawback. Namely, since an acid or alkali is used in reacting the aqueous metal salt solution to cause precipitation or since the reaction yields an acid or alkali as a by-product, the powder particles serving as cores are attacked by the acid or alkali used as a reactant or yielded as a by-product and are thus dissolved, especially when the particles are made of a metal. Because of this drawback, the above method cannot be applied.
An object of the present invention is to eliminate the drawbacks of the above-described prior art techniques and to provide: a highly functional metal or metal compound powder inexpensively; and a technique with which a metal film or a metal oxide film can be formed on an organic powder or the like , and which is applicable to a wide range of powders besides metal or metal compound powders, e.g., organic powders.