Methods for manufacturing capacitors and electrodes using ultrafine nickel powder

#1. Field of the Invention

#The present invention relates to an ultrafine nickel powder which may be used for an internal electrode of a laminated ceramic capacitor, an electrode of a secondary cell, a fuel cell or the like.

#2. Description of the Related Art

#FIG. 1 is a microscopic photography of an ultrafine nickel powder for a laminated ceramic capacitor in Example 1. Magnification: 40000×.

#FIG. 2 is a microscopic photography of an ultrafine nickel powder for a laminated ceramic capacitor in Example 2. Magnification: 40000×.

#FIG. 3 is a microscopic photography of an ultrafine nickel powder for a laminated ceramic capacitor in Comparative Example 1. Magnification: 40000×.

#The ultrafine nickel powder is made into paste, and the paste is printed on a dielectric layer film to form an internal electrode. In order to make the internal electrode thin and dense, the average particle size of the ultrafine nickel powder is suitably from 0.1 to 1.0 μm.

#If the average particle size is less than 0.1 μm, the nickel layer shrunk excessively when the laminated ceramic capacitor is sintered, so that the internal electrode will become porous. The ultrafine nickel powder having an average particle size of less than 0.1 μm is not preferable because the internal electrode will have high resistivity, or delamination and/or cracks will be caused. On the contrary, if the average particle size is more than 1.0 μm, it will be difficult to make the internal electrode of the laminated ceramic capacitor into a thin layer. The ultrafine nickel powder having an average particle size of more than 1.0 μm is not preferable because the surface of the internal electrode layer will have large irregularities, resulting in generation of cracks. The more preferable average particle size is from 0.2 to 0.6 μm. The average particle size is defined by the 50% particle size (d50) in particle size distribution on the basis of the number of particles, obtained from image-analyzing an electron microscopic photograph of the particles.

#For an ultrafine nickel powder for a laminated ceramic capacitor, it is important that the particle shape is spherical, and the size is proper. In the process for producing a laminated ceramic capacitor, spherical particles exhibit ideal characteristics that make it possible to form a thin layer internal electrode having a high density of an ultrafine nickel powder and permit no cracks nor delamination to be generated. The inventors have found that the sulfur content therein has a decisive effect in order to make a shperical ultrafine nickel powder. Unless the sulfur content is within the range of 0.02 to 1.0 wt %, angular particles such as hexahedral or octahedral particles appear. These partcicles are not desirable to make a laminated ceramic capacitor. When the sulfur content is from 0.02 to 1.0% by weight, ultrafine nickel powder particles are sufficiently spherical. Therefore, the sulfur content should be controlled within this range to produce spherical ultrafine nickel powder particles.