Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
2001-04-17
2003-03-11
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S362000, C075S369000
Reexamination Certificate
active
06530972
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for preparing a metal powder suited for use in electronics, and more particularly to a method for preparing a highly crystallized metal powder that is useful as a conductive powder for use in a conductor paste.
2. Description of the Related Art
A conductive metal powder used in a conductor paste for forming an electronic circuit should contain minimum impurities, be a fine powder (with an average particle size from no more than 0.1 &mgr;m to about 10 &mgr;m), have a uniform particle shape and size, and consist of monodispersed particles with no aggregation. This powder is also required to disperse well in a paste, and have good enough crystallinity that it will not sinter unevenly. In particular, when the powder is used to form an internal or external conductor for a multilayer ceramic electronic part, such as a multilayer capacitor or multilayer inductor, it is required to be a fine spherical, low activity, high crystallinity or single-crystal metal powder which is insusceptible to expansion or shrinking due to oxidation and reduction during firing and high sintering commencement temperature, and which consists of submicron particles having a finely uniform shape and particle size, in order to prevent delamination, cracks and other structural defects and enable the reduction of the film thickness of a conductor.
Specifically, a multilayer ceramic electronic part is generally prepared by alternately laminating a plurality of unfired ceramic green sheet layers of a dielectric material, a magnetic material or the like, and internal conductor paste layers comprising as a conductive component a powder of a noble metal such as palladium or silver-palladium, or a powder of a base metal such as nickel or copper, and co-firing the laminated layers at a high temperature. When a base metal that is susceptible to oxidation is used for the internal conductor, various problems are encountered. For instance, in the case of using nickel powder as the conductive component in the internal conductor paste, the laminated body is heated in an oxidizing atmosphere up to the binder removal step that is usually carried out at a temperature of about 300 to 600° C., so that the organic vehicle in the paste and the ceramic green sheets has been completely burned off. During this organic removal step, nickel is slightly oxidized. Then, firing is performed in an inert atmosphere or a reducing atmosphere, and a reduction treatment is performed if needed. However, since completely reducing nickel powder that has been oxidized in the binder removal step is very difficult, deterioration in electrical characteristics, such as an increase in resistance results. Also, the oxidation and reduction are accompanied by volumetric expansion and shrinkage of the electrodes. Because these volumetric changes do not coincide with the sintering shrinkage behavior of the ceramic layer, delamination, cracking, and other such structural defects are apt to occur. Furthermore, a nickel powder quickly sinters in a non-oxidizing atmosphere, and provides a discontinuous film of internal conductor due to its oversintering, thereby causing problems such as an increased resistivity or internal disruption and resulting in an increased conductor thickness, which is contrary to the need for a reduction in thickness of internal conductor layers made along with an increase in the number of laminated layers in recent years. The above oxidation and oversintering have also caused similar problems in the case where an external conductor is formed by co-firing using a nickel paste. Therefore, there is a need for a highly crystallized nickel powder that is resistant to oxidation, at least during binder removal, and has a high sintering commencement temperature.
Meanwhile, palladium, which is a noble metal, has a property that it is oxidized at a relatively low temperature during firing, and is reduced when further heated to a higher temperature, and this leads to structural defects caused due to disagreement in sintering and shrinking behavior between the conductor layer and the ceramic layer. Therefore, oxidation resistance is desirable with palladium and palladium alloys as well, and in this respect a spherical, highly crystallized powder is superior, with a single-crystal powder being particularly good.
Spray pyrolysis has heretofore been known as a conventional method for preparing a well-crystallized metal powder such as this. As discussed in Japanese Patent Publication 63-31522 and elsewhere, spray pyrolysis is a method in which a solution containing one or more metal compounds, or a suspension in which these compounds have been dispersed, is sprayed in the form of fine droplets, and these droplets are heated at a temperature higher than the decomposition temperature of the metal compounds, and preferably a temperature near or above the melting points of the metals contained in the metal compounds, so that the metal compounds are pyrolyzed and metal or alloy powder is precipitated. This method yields a highly crystallized or single-crystal, high-density, highly dispersible, true-spherical metal powder or alloy powder. Unlike a wet-type reducing method, preparation is easy because there is no need for solid-liquid separation, and since no additives or solvents that would affect the purity of the product are used, an advantage is that a high-purity powder containing no impurities is obtained. Furthermore, it is easy to control the particle size, and since the composition of the produced particles is basically the same as the composition of the starting metal compounds in the solution, it is also easy to control the composition of the produced particles.
A problem encountered with this method, however, is that because water or an organic solvent such as alcohol, acetone, or ether is used as a dispersion medium or solvent for making droplets out of the metal compound raw material, there is considerable energy loss during pyrolysis, and the cost is high. Specifically, in this method, pyrolysis of the metal compound is carried out simultaneously with the evaporation of the solvent by heating, or pyrolysis of the metal compound is carried out after the evaporation of the solvent, but in either case a tremendous amount of energy is needed to evaporate the solvent. Also, the droplets are subjected to aggregation or disruption inside the reaction vessel, which results in a broad particle size distribution in the resulting powder. This makes it difficult to set the spray velocity, the droplet concentration in the carrier gas, the dwelling time in the reaction vessel, and the reaction conditions. Particularly, with a powder of a base metal such as nickel, iron, cobalt, or copper, the pyrolysis must be conducted in a carefully controlled reducing or weakly reducing atmosphere in order to prevent oxidation. Furthermore, when water is used as a solvent, oxidation tends to occur a at high temperature because of the oxidizing gas produced by the decomposition of the water, and this makes it difficult to obtain a powder with good crystallinity.
A method for preparing metal ultra fine particles by a vapor phase process is also well known. For instance, to obtain a nickel powder, nickel chloride is vaporized and then reduced by a reducing gas at a high temperature. However, a powder obtained by a precipitation reaction from the vapor phase is prone to aggregation, and furthermore it is difficult to control its particle size. It is also impossible to produce an alloy of metals with different vaporization pressures in an accurately controlled composition.
U.S. Pat. No. 5,976,217 discloses a method for reducing a metal compound powder, such as tungsten oxide powder, by a solid-gas reaction using a reducing agent. This method comprises introducing a metal compound powder material along with a reducing gas and, optionally, a carrier gas into a heated reaction chamber, and passing them through the chamber along predetermined paths, wherein the material is subjected to
Akimoto Yuji
Maekawa Masayuki
Nagashima Kazuro
Takushima Hirotaka
Yoshida Hiroshi
Flynn ,Thiel, Boutell & Tanis, P.C.
Shoei Chemical Inc.
Wyszomierski George
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