This invention relates to an ultrafine nickel powder dispersion and, more particularly, to an ultrafine nickel powder dispersion for preparing an electrically conductive paste, for example, such a paste for forming internal electrodes of multilayer ceramic capacitors, to a process for preparing the ultrafine nickel powder dispersion, and also to a process for preparing a highly dispersible electrically conductive paste using the nickel powder dispersion.
Powders of precious metals such as silver, palladium, platinum, and gold, and also of base metals such as nickel, cobalt, iron, molybdenum, and tungsten have hitherto been used in forming electrically conductive paste as electronic material, especially as conductive paste for forming internal electrodes of multilayer ceramic capacitors. In general, a multilayer ceramic capacitor is made up of a plurality of dielectric ceramic layers and a plurality of metal layers that form internal electrodes laminated in an alternate arrangement, and two external electrodes formed at both ends of the two outermost dielectric ceramic layers for connection to the metal layers of the internal electrodes. The material to be used in forming the dielectric members is one consisting mainly of a material of high dielectric constant, such as barium titanate, strontium titanate, or yttrium titanate. For the metal to constitute the internal electrodes, powders of the above-mentioned precious and base metals have usually been used. More recently, weight has been placed on the base metal powders to meet the demand for less expensive electronic materials. In particular, the multilayer ceramic capacitors with electrodes of dielectric ceramic layers of nickel powder films are under active development.
The multilayer ceramic capacitors using electrodes of nickel powder coated films are commonly manufactured in the following way. First, dielectric powder of barium titanate or the like is mixed with an organic binder to form a suspension, and the suspension is spread to a sheet form by the doctor blade method to obtain dielectric green sheets. In the meantime nickel powder to form internal electrodes is mixed and dispersed with organic compounds such as organic solvent, plasticizer, and organic binder to form nickel powder paste. The paste is printed by screen printing onto the green sheets. The printed sheets are then dried, laminated, and compressed with other members, and the organic ingredients are removed by heat treatment. The multilayer structure is sintered at around or above 1300xc2x0 C., and finally external electrodes are baked onto both outermost ends of the laminated dielectric ceramic layers, whereby a multilayer ceramic capacitor is obtained.
The multilayer ceramic capacitors are extensively used in cellular phones, personal computers, and other electronic articles. In recent years rapid progress has been made in the reduction of weight and size of the electronic components and in the improvement and expansion of their functions. To keep pace with the progress there has been growing demand for multilayer ceramic capacitors of smaller sizes and larger capacities. The trend calls for multilayer units of hundreds of layers each, with consequent reduction of the thickness of individual electrode layers and further lowering of resistance. Nowadays each electrode layer is as thin as from 1 to 2 xcexcm, and even thinner layers will certainly be required in the future.
In the manufacture of multilayer ceramic capacitors, nickel powder is mixed with and dispersed in a paste-forming dispersant to form electrodes. At this stage, if coarse nickel particles are present or if the nickel particles during the course of mixing or dispersing aggregate to form secondary nickel particles larger in diameter than the thickness of each electrode layer that ranges from 1 to 2 xcexcm, the resulting electrode layer surfaces will be uneven, with the consequence that the multi-layer ceramic capacitors thus obtained will be too susceptible to shortcircuiting for practical use. Moreover, in the stage of evaporating the organic ingredients from the nickel powder paste and in the subsequent stage of sintering, a volumetric change of the sintered product can take place because of the expansion or shrinkage of the metal powder. The dielectric material too can undergo a similar volumetric change as a result of the sintering. Thus simultaneous sintering of the two different materials, dielectric and nickel powder, causes volumetric changes of the two materials with expansion and shrinkage at different temperatures during the course of sintering. These volumetric changes present a problem of the fracture of laminated mono-lithic structure called delamination, in the form of cracking or separation of the laminated films. The phenomenon of delamination occurs remarkably when there are coarse nickel particles or excessively aggregated nickel particles as noted above.
As means for solving the delamination problem, various methods have heretofore been proposed. For example, Japanese Patent Application Kokai (Public Disclosure) No. 8-246001 teaches that the possibility of delamination is decreased by the use, as the material for electrodes of multilayer ceramic capacitors, of the ultrafine nickel powder produced by the vapor phase hydrogen reduction of nickel chloride vapor and which has a mean particle diameter between 0.1 and 1.0 xcexcm and a tap density that satisfies conditions represented by a specific formula.
The above-mentioned technique of the prior art is effective to some extent in preventing delamination. However, nickel powder is not always fully dispersible when it is mixed and dispersed with a paste-forming dispersant. The nickel particles tend to aggregate, with the result that the increased proportion of coarse particles renders it difficult to decrease the thickness of internal electrodes and makes the electrode surface uneven, leading to shortcircuiting. In extreme cases these factors can combine to cause delamination. Thus there is room for improvement in the dispersibility of nickel particles in a dispersant for forming an electrically conductive paste of nickel powder.
It may be considered possible to provide a nickel powder dispersion prepared by dispersing nickel powder in water beforehand instead of adding nickel powder directly to a paste-forming dispersant. In practice, however, the degree of dispersion of nickel powder at the time of forming a nickel powder paste is far from satisfactory.
It is therefore an object of the present invention to provide a nickel powder which, when used together with a paste-forming dispersant to form an electrically conductive paste, exhibits excellent dispersibility and, when used in a multilayer ceramic capacitor, permits the prevention of shortcircuiting and delamination due to unevenness of electrode surfaces.
It is another object of the invention to provide a process for preparing an electrically conductive paste in which nickel powder is thoroughly and highly dispersed.
After our intensive search for ways of realizing the above objects of the invention, it has now been found that a dispersion obtained by adding an organic solvent to a aqueous dispersion of ultrafine nickel powder exhibits very high dispersibility of the nickel powder when it is added to an electrically conductive paste-forming dispersant, and is suitable for applications as conductive paste, especially as a conductive paste for multilayer ceramic capacitors. The present invention is predicated upon this discovery.
In brief, the invention provides a nickel powder dispersion comprising an aqueous nickel powder dispersion composed of superfine nickel powder having a mean particle diameter of no more than one micrometer and an aqueous solvent, and an organic solvent added to the aqueous dispersion in such form that said organic solvent replaces at least a part of the aqueous solvent. A surface active agent may further be added to the dispersion.
The invention also provides, in a preferred form, a process for preparing a nickel powder dispersion which comprises forming an ultrafine nickel powder with a mean particle diameter of no more than one micrometer by a vapor phase reaction which involves reaction by contacting between nickel chloride gas and a reducing gas, washing the resulting nickel powder with water, adding pure water, thereby forming an aqueous nickel dispersion having an aqueous solvent concentration of no less than one percent by weight, adding a surface active agent as needed, and thereafter adding an organic solvent with at least a partial replacement of the aqueous solvent, whereby a nickel powder dispersion having an organic solvent concentration between 5 and 200 percent by weight is produced.
The invention further provides a process for preparing an electrically conductive paste which comprises adding an organic dispersant for forming an electrically conductive paste to the above nickel powder dispersion, and thereafter mixing and kneading the components.
For the purposes of the invention the expression xe2x80x9chighly dispersiblexe2x80x9d or xe2x80x9cexcellent dispersibilityxe2x80x9d is used to mean that an ultrafine nickel powder undergoes less aggregation of the particles into secondary particles than usual and, when its cumulative size distribution as suspended in a solvent is determined using a laser light scattering particle counter (manufactured by Coulter Co. under the trade designation xe2x80x9cCoulter LS230xe2x80x9d), the mean particle diameter (e.g., D50) and coarse particle diameter (e.g., D90) are smaller than usual.