Conventionally known methods for the manufacture of copper powders include an electrolytic method, an atomization method, mechanical pulverization, etc., and such copper powders produced by those methods are used mainly in powder metallurgy and the like. Although those methods which generally produce powders having relatively a large particle diameter have come to produce finer powders of copper by controlling the production conditions or by seiving, the production efficiency is low and the fineness attainable by those methods is limited.
For use in purposes such as coating compositions, pastes, and resins, on the other hand, copper powders are required to be composed of powder particles which are finer, i.e., 10 .mu.m or less, and uniform in shape from the standpoints of uniform dispersion and uniform coating. For use in electronic parts, copper powders containing only a slight amount of alkali metals such as Na or K, sulfur, and halogens such as Cl are preferred mainly from the standpoint of preventing corrosion and electrical property deterioration due to moisture.
Copper fine powders for use in the above purposes are manufactured, for example, by the liquid-phase reduction precipitation of a copper compound, evaporation under vacuum or in an inert gas, the gas-phase reduction of a copper salt, and the solid-phase reduction of an oxide.
However, the liquid-phase reduction precipitation method is defective in performance and cost because the particle diameter distribution is wide, the reducing agent is expensive, and the process must be practiced batchwise. The evaporation under vacuum or in an inert gas is defective in that although copper powders which are extremely fine and have a large specific surface area can be obtained, the oxidation inhibition and handling of the copper powders are difficult, production facilities are costly and the mass productivity is poor. The gas-phase reduction of a copper salt, particularly a copper halide, which is carried out at high reaction temperatures, has problems such as the corrosion of the equipment by a halogen generated by the decomposition of the halide, troublesome collection of the powder produced, etc., and is also defective in that the halogen remains in a large amount in the copper powder produced. In practicing the solid-phase reduction of an oxide, it is essential that the starting material should be finely powdered and purified before use since the shape and purity of the copper powder to be produced depend on the starting material, and the particles should be prevented from agglomerating and growing due to their sufficient contact with the reducing gas and also due to heat generation accompanying the reduction. Thus, the solid-phase reduction method has been defective in that the production efficiency is low and control of the production conditions is difficult.