Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-02-02
2002-06-18
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C252S513000
Reexamination Certificate
active
06406513
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to nickel powder and a conductive paste containing the same and more particularly to nickel powder and a conductive paste containing the same, which can control heat shrinkage while inhibiting the occurrence of any rapid oxidation and which permit the production of a thin, uniform internal electrode for a multilayer ceramic capacitor without being accompanied by any crack formation and delamination during firing.
(b) Description of the Prior Art
A multilayer ceramic capacitor comprises a plurality of ceramic dielectric layers and internal electrode layers, which are alternately laminated and united. In the production of the internal electrode for such a multilayer ceramic capacitor, it is common that metal fine powder as a material for internal electrodes is first formed into a paste thereof to give a conductive paste, followed by printing a green sheet of a ceramic dielectric with the resulting conductive paste, alternately putting, in layers, a plurality of these green sheets of the ceramic dielectric and the conductive paste layers, attaching the latter to the former using pressure to thus unify them, and then firing the resulting laminated assembly in a reducing atmosphere at a high temperature to thus firmly unify the ceramic dielectric layers and the internal electrodes.
As materials for such an internal electrode, there have conventionally been used, for instance, precious metals such as platinum, palladium and silver-palladium. However, there have recently been developed techniques in which base metals such as nickel are substituted for these precious metals and such techniques have gradually been advanced, for the purpose of reducing the production cost. When forming a conductive paste layer using a paste containing nickel powder and then firing the resulting paste layer to produce a thin and uniform internal electrode, however, problems such as crack formation and delamination arise. The occurrence of such crack formation and delamination would be caused due to, for instance, heat shrinkage of the conductive paste layer during firing.
Accordingly, there have conventionally been proposed a variety of powdery nickel products, which can solve the problem of heat shrinkage during firing.
SUMMARY OF THE INVENTION
Similarly, it is an object of the present invention to provide nickel powder as well as a conductive paste containing the nickel powder, which can control heat shrinkage while inhibiting the occurrence of any rapid oxidation and which accordingly, permit the production of a thin, uniform internal electrode for a multilayer ceramic capacitor without being accompanied by any crack formation and delamination during firing.
The inventors of this invention have conducted various studies to solve the foregoing problems associated with the conventional techniques, have found that the foregoing problems of rapid increase in the oxidation of the conductive paste layer and the heat shrinkage thereof during firing are solved by increasing the average particle density of the nickel powder to a level greater than a desired value and reducing the average diameter of the crystallites present in the particles to a level smaller than a predetermined value and that the resulting conductive paste permits gentle sintering, can ensure a uniform sintering speed and accordingly, permits the formation of a thin and uniform internal electrode for a multilayer ceramic capacitor without being accompanied by any crack formation and delamination, and have thus completed the present invention based on the foregoing findings.
According to an aspect of the present invention, there is provided nickel powder having an average particle size, as determined by the observation under a scanning electron microscope (SEM), of not more than 1 &mgr;m, a particle density of not less than 8.0 g/cm
3
, and an average diameter of crystallites present in the nickel particles of not more than 550 Å.
According to a second aspect of the present invention, there is provided a conductive paste, which comprises nickel powder having characteristic properties defined above.
DETAILED DESCRIPTION OF THE INVENTION
The nickel powder of the present invention is particularly suitable for use in making an internal electrode for a multilayer ceramic capacitor or in the preparation of a conductive paste for a multilayer ceramic capacitor. Accordingly, the average particle size of the nickel powder of the present invention, as determined by the observation thereof with an SEM with a magnification of about ×10000, is limited to a level of not more than 1 &mgr;m, preferably 0.1 to 1 &mgr;m and more preferably 0.2 to 0.8 &mgr;m, while taking into consideration the foregoing applications of the nickel powder.
The meaning of the term “particle density”, used in this specification, is identical to that defined in JIS R 1600 (1993). More specifically, the term “particle density” is the density of particles, in which the closed spaces included in the particles are likewise regarded as parts of particles. If the particle density is reduced, the amount of void spaces present within particles correspondingly increases, while if the particle density increases, the amount of void spaces present within particles are correspondingly reduced. These void spaces expand when exposed to a high temperature during the high temperature firing step in the production of a multilayer ceramic capacitor and at least part thereof is destroyed to cause shrinkage of the particle. Thus, the presence of such void spaces would greatly affect the thermal properties, in particular, heat shrinkage of the conductive paste layer containing nickel powder. In the present invention, the particle density of the nickel powder is controlled to not less than 8.0 g/cm
3
, preferably not less than 8.3 g/cm
3
and more preferably not less than 8.5 g/cm
3
. Increasing the particle density of the nickel powder can thus eliminate the problems of any crack formation and delamination.
Moreover, the average diameter of crystallites present in the nickel particles of nickel powder of the present invention is controlled to a level of not more than 550 Å, preferably not more than 500 Å and more preferably not more than 300 Å. A product obtained using a paste containing such nickel powder, for instance, a multilayer ceramic capacitor, is composed of a tidy sintered film and the sintering of the paste proceeds gently because the average diameter of the crystallites present in the nickel particles is small. In other words, the sintering of such a paste rapidly proceeds if the crystallites present in the nickel particles have a large diameter, but if the average diameter thereof is small, the crystallites present in the nickel particles are first sintered and then the sintering between the nickel particles gradually proceeds. Accordingly, the nickel powder-containing paste is gradually sintered at a uniform speed to thus form a tidy film and the paste hardly causes any crack formation and delamination.
As has been discussed above in detail, the nickel powder of the present invention has an average particle size as determined by the SEM observation thereof of not more than 1 &mgr;m, a particle density of not less than 8.0 g/cm
3
, and an average diameter of crystallites present in the nickel particle of not more than 550 Å. Accordingly, the conductive paste obtained using such nickel powder would permit the formation of a thin and uniform internal electrode for a product such as a multilayer ceramic capacitor, without causing any crack formation and delamination during firing.
The nickel powder of the present invention, which satisfies various requirements discussed above, is suitably used for preparing a conductive paste, in particular, a conductive paste for making a multilayer ceramic capacitor.
Therefore, the conductive paste according to the present invention comprises the foregoing nickel powder having excellent characteristic properties described above and is thu
Hayashi Takao
Yamaguchi Yasuhide
Mai Ngoclan
Mitsui Mining and Smelting Co. Ltd.
Young & Thompson
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