Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2001-02-27
2003-02-11
Lam, Cathy (Department: 1775)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C361S306300
Reexamination Certificate
active
06517745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nickel powder and a conductive paste and more particularly to nickel powder, which is highly dispersible in an organic vehicle during the preparation of a conductive paste because it has low agglomerate properties and it is in an approximately monodispersed condition and which is particularly suitable in the preparation of a conductive paste for use in making a thin internal electrode of a multilayer ceramic capacitor, which is free of any projection, because of it sharp particle size distribution, as well as a conductive paste comprising the nickel powder.
2. Description of the Prior Art
A multilayer ceramic capacitor comprises a plurality of ceramic dielectric layers and a plurality of internal electrode layers, which are alternately put in layers and united. In the preparation of the internal electrode of such a multilayer ceramic capacitor, metal fine powder as a material for the internal electrode is in general converted into a conductive paste, the resulting conductive paste is printed on a green sheet of a ceramic dielectric material, followed by alternately putting, in layers, the green sheets of the ceramic dielectric material and the conductive paste layers, attaching these layers to one another using pressure with heating to thus unify them, and firing the resulting laminate at a high temperature in a reducing atmosphere to thus unite the ceramic dielectric layers and the internal electrode layers.
As materials for such an internal electrode, there have conventionally been used platinum, palladium and silver-palladium, but there have recently been developed various techniques, which make use of base metals such as nickel in place of these precious metals such as platinum, palladium and silver-palladium and which have gradually been advanced.
In general, metal powder, which is prepared by a dry reaction or a wet reaction and which is not subjected to any post-treatment, is in an at least partially agglomerated state and the smaller the particle size thereof, the higher the degree of the agglomeration thereof.
Nickel powder can likewise be prepared by a dry reaction or a wet reaction, but the resulting nickel powder severely suffers from this problem of the agglomeration. In particular, the dispersibility thereof in an organic vehicle is an important factor for the preparation of a conductive paste and therefore, there has been desired for the development of nickel powder, which is less agglomerate or has a dispersed state almost identical to the monodispersed condition. If such less agglomerate nickel powder is used in the preparation of a conductive paste, the denseness of the ultimately obtained conductive layer is considerably improved, the reliability of the product obtained is also increased, in particular, the use thereof permits the inhibition of any deficiencies in electrical characteristic properties such as insulation characteristic properties and dielectric characteristic properties and this leads to an improvement of the yield of the products.
Moreover, the foregoing multilayer ceramic capacitor or the like has recently increasingly been miniaturized, the ceramic dielectric layer and the internal electrode have necessarily been thinner and the number of these layers to be laminated has correspondingly been increased. Accordingly, there have been produced laminated parts, in particular, multilayer ceramic capacitors in which the thickness of the dielectric layer is not more than 2 &mgr;m, that of the internal electrode is not more than 1.5 &mgr;m and the number of multilayer is not less than 100.
If one wishes to obtain a thin internal electrode layer, it would be sufficient to use metal fine particles having a small average particle size corresponding to the reduced thickness of the electrode layer. However, if an internal electrode layer is formed using a conductive paste comprising coarse particle-containing metal powder, such coarse particles form projections on the resulting internal electrode layer and they often break through the thin ceramic dielectric layer to thus form short circuits. To prevent the formation of any such short circuit between these internal electrode layers, it is necessary to use metal powder having an average particle size substantially smaller than the foregoing average particle size corresponding to the reduced thickness of the electrode layer.
For instance, Japanese Un-Examined Patent Publication No. Hei 11-189801 discloses nickel ultra-fine powder having an average particle size ranging from 0.2 to 0.6 &mgr;m and an existing ratio of coarse particles having a particle size of not less than 2.5 times the average particle size of not more than 0.1% as expressed in terms of the number of particles. This patent specification also discloses, in the column 4, lines 21 to 24, as follows: “For instance, if the particle size of these coarse particles is limited to not less than about 1.5 &mgr;m, the average particle size of the nickel ultra-fine powder according to the invention should be limited to 0.6 &mgr;m”. In other words, the formation of a thin internal electrode layer would require the use of metal powder having a considerably small average particle size. However, the smaller the particle size, the higher the viscosity of the conductive paste containing such metal powder. Moreover, such metal powder also suffers from a problem in that heat shrinkage and/or oxidation of the metal powder are promoted upon firing.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide nickel powder in which the agglomeration between nickel particles is inhibited, which is in an almost monodispersed condition, which is accordingly excellent in dispersibility in an organic vehicle in the preparation of a conductive paste and has a sharp particle size distribution, and which accordingly permits the preparation of a thin internal electrode layer, free of any projection, for a multilayer ceramic capacitor, without unnecessarily reducing the average particle size of the nickel powder.
It is a further object of the present invention to provide a conductive paste containing the foregoing nickel powder, which is effectively used in making, for instance, multilayer ceramic capacitor.
The inventors of this invention have conducted various studies to achieve the foregoing objects, have found that a specific deagglomeration treatment of nickel powder permits the reduction of the degree of agglomeration between nickel particles as low as possible, that if the rate of coarse particles is controlled to a relatively low level and particle sizes of most of nickel particles are adjusted so that they fall within a desired range, an internal electrode layer free of any projection can be prepared without unnecessarily reducing the particle size of nickel powder to thus reduce the probability of causing a short circuit between internal electrodes as low as possible, that the oxidation and heat shrinkage of nickel particles are inhibited because of a low rate of ultra-fine particles and that the resulting product does not correspondingly have insufficient electrical characteristic properties such as insulation characteristic properties and dielectric characteristic properties, and thus have completed the present invention based on these findings.
According to an aspect of the present invention, there is provided nickel powder whose average particle size as determined through the observation by a scanning electron microscope (SEM) ranging from 0.1 to 1 &mgr;m and wherein the D
5 0
value as determined by laser diffraction-scattering particle size distribution analysis and the average particle size as determined by SEM observation satisfy the following relation represented by Formula (1):
1≦[(
D
5 0
value)/(average particle size as determined by
SEM
observation)]≦1.8 (1)
According to another aspect of the present invention, there is provided a conductive paste comprising the foregoing nickel powder.
DESCRIPTION OF THE P
Hayashi Takao
Yamaguchi Yasuhide
Lam Cathy
Mitsui Mining and Smelting Co. Ltd.
Young & Thompson
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