Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2002-02-14
2004-06-08
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C106S001050
Reexamination Certificate
active
06746628
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aqueous nickel slurry, a method for preparing the same and a conductive paste. More specifically, the present invention relates to an aqueous nickel slurry in which nickel fine particles are stably dispersed in a high concentration without again causing any cohesion and which can be used as a conductive paste for firing, in particular, a conductive paste for forming a multilayer ceramic capacitor as well as a method for preparing the aqueous nickel slurry and a conductive paste.
2. Description of the Prior Art
A multilayer ceramic capacitor comprises a plurality of alternately laminated ceramic dielectric layers and internal electrode layers, which are integrated together. Such a multilayer ceramic capacitor can, for instance, be prepared according to the following method. A ceramic dielectric material is first dispersed in a medium to give a slurry. On the other hand, metal fine powder as a material for internal electrodes is converted into a paste to thus give a conductive paste. Then a green sheet is prepared from the ceramic dielectric slurry, the conductive paste is printed on the green sheet, another green sheet is put on top of the printed green sheet and then the foregoing steps are repeated over a desired time to thus give a laminate, which comprises a desired number of the green sheets of the ceramic dielectric and a desired number of the printed conductive paste layers, alternately laminated. Alternatively, the ceramic dielectric slurry and the conductive paste are alternately subjected to screen printing to give a laminate comprising a plurality of printed ceramic dielectric layers and conductive paste layers. Then the resulting laminated layers are attached to one another using pressure with heating and then the resulting laminate is fired in a reducing atmosphere at a high temperature to thus unify the ceramic dielectric layers and the internal electrode layers.
As such materials for internal electrodes, there have been used, for instance, platinum, palladium and silver-palladium, but there have recently been developed and advanced techniques for employing base metals such as nickel in place of precious metals such as platinum, palladium and silver-palladium in order to save the production cost. Moreover, the conductive paste used for the production of internal electrodes for multilayer ceramic capacitors is in general prepared by if necessary adding an inorganic substance such as a vitreous material and other additives to a vehicle comprising, for instance, an organic binder and an organic solvent, in addition to nickel powder for imparting conductivity to the resulting paste and then uniformly mixing these components to give a uniform dispersion.
Incidentally, the size of the foregoing multilayer ceramic capacitor or the like has recently been increasingly reduced and this correspondingly leads to the gradual reduction of the thickness of ceramic dielectric layers and internal electrode layers constituting each capacitor and to the increase in the number of laminated layers. In fact, there have been produced such a laminated part, in particular, a multilayer ceramic capacitor, which has a thickness of the dielectric layer of not more than 2 &mgr;m, a thickness of the internal electrode film of not more than 1.5 &mgr;m and a number of the laminated layers of not less than 400.
Recently, there have been proposed a variety of techniques for further reducing the thickness of the internal electrode layer to thus give a chip laminated to a higher extent. Among these techniques, there has been known a method, which makes use of an aqueous conductive paste instead of the conventional organic solvent-containing conductive paste. The use of an aqueous conductive paste has attracted special interest recently from the viewpoint of the environmental hygiene.
In general, the metal powder produced by a dry or wet reaction is in a cohered state in itself although the degree of cohesion may vary depending on the kinds thereof. Moreover, the smaller the primary particle size of the metal powder, the higher the degree of such cohesion.
The nickel powder may likewise be prepared by either a dry or wet reaction method. In case of the nickel powder, however, it is a matter of course that such cohesion becomes a serious problem. Moreover, even if the cohered nickel powder is de-agglomerated by a disintegration treatment, nickel particles again undergo agglomeration with time in water.
There have been conducted various studies, in which nickel powder is disintegrated in the presence of a variety of dispersants and/or surfactants, to solve the problem of re-agglomeration and to obtain an aqueous nickel slurry having a high concentration, but the nickel concentration of the slurry thus prepared is in general on the order of about 10% by mass and these techniques never permitted the preparation of a nickel slurry having a nickel concentration of higher than 20% by mass. Although an aqueous nickel slurry having a high concentration could temporarily be obtained, nickel particles present therein necessarily undergo re-agglomeration. For this reason, it has been impossible to maintain nickel particles in their stably dispersed condition in the slurry. Therefore, the foregoing techniques never allow the preparation of a stable aqueous nickel slurry having a high nickel concentration.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an aqueous nickel slurry in which nickel fine particles are stably dispersed in a high concentration without again causing any cohesion (or causing re-agglomeration) and which can be used as a conductive paste for firing, in particular, a conductive paste for use in making a multilayer ceramic capacitor.
It is another object of the present invention to provide a method for preparing the foregoing aqueous nickel slurry.
It is a further object of the present invention to provide a conductive paste prepared from the foregoing aqueous nickel slurry.
The inventors of this invention have conducted various investigations according to a rule of trial and error on the basis of a variety of hypotheses, have fortunately found that nickel fine particles may stably be dispersed in a high concentration in an aqueous slurry without causing any re-agglomeration if a specific substance is adhered to the surface of individual nickel particles and a specific compound is dissolved in an aqueous medium in advance, have further conducted various investigations and have thus completed the present invention.
According to a first aspect of the present invention, there is provided an aqueous nickel slurry, which comprises:
water;
nickel fine powder provided thereon with an insoluble inorganic oxide adhered to the surface of the individual nickel fine particles constituting the fine powder;
polyacrylic acid or an ester or salt thereof; and
at least one member selected from the group consisting of ammonium hydroxides substituted with organic substituents, and hydroxyl group-containing amine compounds.
According to a second aspect of the present invention, there is provided a method for preparing an aqueous nickel slurry, which comprises the steps of dispersing, in water, nickel fine powder provided thereon with an insoluble inorganic oxide adhered to the surface of the individual nickel fine particles constituting the fine powder, adding: polyacrylic acid or an ester or salt thereof; and at least one member selected from the group consisting of ammonium hydroxides substituted with organic substituents and hydroxyl group-containing amine compounds to the resulting aqueous dispersion and then stirring the resulting mixture.
According to a third aspect of the present invention, there is provided a conductive paste, in particular, a conductive paste for use in forming a multilayer ceramic capacitor, which comprises the foregoing aqueous nickel slurry and a binder.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the aqueous nickel slurry of the present invention, the nic
Kamikoriyama Yoichi
Ogata Sumikazu
Kopec Mark
Mitsui Mining and Smelting Co. Ltd.
Young & Thompson
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