1. Field of the Invention
The present invention relates to a method for manufacturing a highly-crystallized double oxide powder comprising two or more metal elements and/or semi-metal elements, and more particularly relates to a method for manufacturing a highly-crystallized double oxide powder having a uniform particle size and a high purity and composed of a single crystal phase, which is useful as industrial materials in a variety of fields, including various functional materials used in electronics, such as phosphor materials, dielectric materials, magnetic materials, conductor materials, semiconductor materials, superconductor materials, piezo-electric materials, magnetic recording materials, secondary cell-use positive electrode materials, electromagnetic wave absorption materials, catalyst materials, etc.
2. Description of the Prior Art
Mechanical pulverization has been used in the past to manufacture a double oxide powder. This process involves mixing raw material powders, putting this mixture in a crucible or other such firing vessel and heating at a high temperature for an extended time to bring about a solid phase reaction, and then pulverizing this product in a ball mill or the like. The double oxide powder manufactured by this method, however, is an agglomerate of particles with an irregular particle shape and a broad particle size distribution, and a considerable amount of impurities come from the crucible. Furthermore, the treatment is inefficient because it has to be carried out at a high temperature for a long time in order to raise the homogeneity of the composition. In addition, the particles are often modified on their surface by the mechanical impact and chemical reactions to which they are subjected during the pulverization process, which means that many defects develop at the surface and in the interior of the powder, thereby leading to the lowering of crystallinity and the deterioration of the physical properties inherent in the double oxide.
A sol-gel process, hydrothermal process, co-precipitation, spray pyrolysis, and so forth are known methods for preventing the generation of a surface modification layer and obtaining a double oxide powder with high crystallinity. Still, a sol-gel process entails high raw material cost because it requires a high-purity raw material, while a hydrothermal process and co-precipitation both take a long time and have low yield per unit of time, so the manufacturing cost is high.
Spray pyrolysis is a known method for manufacturing a fine oxide powder, in which one or more metal compounds or semi-metal compounds are uniformly dissolved or dispersed in water or an organic solvent, this mixed solution is atomized in the form of fine droplets, these droplets are heated at a temperature higher than the decomposition temperature of the metal compounds and under conditions that will cause precipitation of metal oxides, and the metal compounds are pyrolyzed to produce metal oxide powder. This method makes it easy to obtain a fine spherical powder of uniform particle size, with high crystallinity and no agglomeration, by properly selecting the pyrolysis conditions. Also, since a solution of metal or semi-metal compounds is used as a raw material, advantages are that various metal components can be mixed in the desired ratio and uniformly at the ion level, inclusion of impurities is suppressed, and the particle size of the produced powder can be easily controlled by process control of the spray conditions and so forth. This method is therefore considered a good way to manufacture a double oxide powder, and has been used in the manufacture of a fine double oxide powder for oxide phosphors, oxide dielectrics, oxide magnetic bodies, oxides used for secondary cell positive electrodes, electromagnetic wave absorption materials, catalyst materials, etc.
Nevertheless, when a double oxide powder is manufactured by spray pyrolysis, the retention time of the atomized droplets in a reaction vessel is extremely short, generally lasting only 0.1 to 10 seconds, so in the manufacture of a double oxide with a high melting point, a double oxide with a high sintering reaction temperature, or a double oxide composed of metal elements with low sintering reactivity, the produced particles are not a single phase, and multiple crystal phases tend to be produced, or unreacted material tends to remain. Particularly when many metal compounds are used for the raw material, it is believed that the compounding reaction is impeded because the various components have different pyrolysis temperatures or sintering temperatures. This causes the powder that is produced to have a low crystallinity, or the particle surface may be porous or the particle may be cracked, and it is difficult to fully realize the physical properties inherent to the double oxide, among other problems. One of the problems encountered if the heating is carried out at a higher temperature in an effort to promote pyrolysis and compounding is that the desired crystal phase will not be obtained
Meanwhile, complex polymerization is a known method for manufacturing a double oxide powder composed of a single crystal phase. With this method, a raw material compound including two or more metal elements that constitute the double oxide, a polyamino chelating agent or a hydroxycarboxylic acid such as citric acid, and a polyol such as ethylene glycol are mixed to produce an aqueous solution, which is heated at a specific temperature of at least 110° C. to bring about an esterification reaction and form a composite polymer of a metal complex in the solution, thereby forming a homogeneous precursor having a uniform element arrangement on the metal ion level. After this, as discussed in Japanese Patent Publication Nos. 6-115934A, 10-330118A, and 10-99694A, for example, powder of the composite polymer of the metal complex is separated, calcined and then fired at a high temperature to obtain a double oxide powder, or as discussed in Japanese Patent Publication No. 10-182161A, this composite polymer is dissolved in water and atomized into a high-temperature atmosphere and pyrolyzed to obtain a double oxide powder. This method, however, is complicated in that it requires a composite polymer of a metal complex to be synthesized prior to the pyrolysis step, and for this polymer to be separated or redissolved. It also requires a large amount of ethylene glycol or other co-precipitant or complexing agent, and the polycondensation reaction takes an extremely long time with a low yield, thereby resulting in a higher manufacturing cost. Furthermore, the homogeneity of the complex ions of the solution may not always be maintained in the removal of the solvent from the produced complex. Particularly when the polycondensation reaction is conducted by gradually removing the solvent over an extended period of time, any gel that is produced may redissolve and form salts with anions present in the solution, and these salts may precipitate and affect the composition, which means that homogeneity is lost.
In yet another known method, a double alkoxide, double salt, or the like of the metals that constitute the double oxide is synthesized ahead of time, and this is pyrolyzed, but this method also entails a complex procedure, the metals that can be used must be selected from a narrow range, and it is difficult to achieve homogeneity because of differences in the hydrolysis rates of the respective metal compounds, for example, and therefore a single phase of sufficient crystallinity is not obtained.