Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2000-03-24
2002-09-10
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S359000, C075S369000
Reexamination Certificate
active
06447571
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a metallic powder suitable for various uses, such as in electrical materials such as conductive pastes, bonding materials for titanium, and for catalysts, and in particular, the invention relates to a metallic powder having superior sintering properties and dispersion characteristics, which is specifically suitable for conductive pastes and internal electrodes in multilayer ceramic capacitors.
BACKGROUND ART
Noble metals such as silver, palladium, platinum, and gold, and base metals such as nickel, cobalt, iron, molybdenum, and tungsten have long been used for electrical materials such as conductive pastes, and have specifically been used for internal electrodes in multilayer ceramic capacitors. Multilayer ceramic capacitors generally have a construction such that ceramic dielectric layers and metallic layers used for internal electrodes are alternately laminated, and external electrodes, which are connected to the metallic layers, are connected to both ends of the ceramic dielectric layers. Materials having a high dielectric constant such as barium titanate, strontium titanate, and yttrium oxide, are used as primary components for forming the ceramic dielectric layers. Powders of the above noble metals and the base metals are used as metals forming internal electrodes. Less expensive electrical parts are recently required, and therefore research on multilayer ceramic capacitors using the latter base metallic powders has been aggressively pursued, and nickel powder is typical among these metals.
Multilayer ceramic capacitors are generally manufactured by the following method. That is, a dielectric powder, such as barium titanate, is mixed and suspended in an organic binder, and this is then formed into a sheet by using a doctor blade method so as to produce a dielectric green sheet. On the other hand, a metallic powder for internal electrodes is mixed with an organic compound such as an organic solvent, a plasticizer, or an organic binder so as to produce a metallic powder paste, and the paste is printed on the green sheet by using a screen printing method. This is then subjected to drying, laminating, firm pressing, and heating to remove the organic component, and is then sintered at about 130° C. or more. Then, external electrodes are burned to both ends of the ceramic dielectric layers, and a multilayer ceramic capacitor is thereby manufactured.
In manufacturing processes for multilayer ceramic capacitors such as the above, the volume of the metallic powder changes due to expansion and contraction thereof during the processing to vaporize and remove the organic component from the metallic paste and the subsequent process for sintering. Similarly, the volume of the dielectric body changes due to the sintering. That is, the different materials of the dielectric body and the metallic powder are simultaneously sintered, and it is therefore inevitable that sintering properties will differ due to changes in volumes due to expansion and contraction of these materials during sintering. As a result, the process has problems in that the laminate construction may be broken and crack or peeling called delamination may occur.
Specifically, sintering in a dielectric body comprising barium titanate as a primary component is initiated at 1000° C. or more, typically at a temperature in the range of 1200 to 1300° C. However, sintering in a metallic powder for internal electrodes is initiated at a lower temperature than this temperature, for example, normally at a temperature in the range of 400 to 500° C. in the case of nickel powder, and as a result, the volume changes due to extreme contraction, and the portion between the internal electrode and the dielectric sheet is strained. Thus, the difference between the initiation temperatures for sintering results in differences between sintering properties of the internal electrode and the dielectric body, and this is therefore a primary cause of delamination. Moreover, when sintering is suddenly initiated at low temperatures, volume change in the final period of the sintering is large, so that delamination readily occurs. Therefore, in metallic powders used for internal electrodes, it is desirable that the initiating temperature for sintering be as high as possible and that extreme sintering does not occur.
Heretofore, various methods for solving the problems of delamination have been proposed. For example, Japanese Patent Application, First Publication, No. 157903/96 discloses a method in which a spherical palladium powder having a predetermined diameter is heated at a temperature in the range of 100 to 200° C. in air for 24 hours or more, and a paste is then produced by using the palladium powder. Japanese Patent Application, First Publication, No. 176602/96 discloses a method in which a palladium powder is kneaded with an acid soluble salt, such as alkaline salts, and an organic solvent; then the organic solvent is vaporized and removed, and the mixture of the palladium powder and the acid soluble salt is heated to 300° C. or more, and is then cooled in air; and finally, the compound is dissolved so as to yield a palladium powder
The above conventional methods can yield some improvements in improving sintering properties. However, these methods complicate processing and operations, and are not sufficient to effectively prevent delamination even though the methods consume large amounts of energy. On the other hand, internal electrodes are required to be formed in thin layers and to have low electrical resistance in accordance with the trends toward miniaturization and large capacity in capacitors, and therefore powders for internal electrodes are required to be super-fine powders, having not only diameters of 1&mgr;m or less, but also diameters of 0.5&mgr;m or less. When such a powder consisting of super-fine particles is mixed with an organic solvent, the dispersion characteristics of the powder is deteriorated and the metallic particles agglomerate with each other. As a result, thin layers in internal electrodes cannot be easily formed due to an increase in the number of coarse particles, and bumps and depressions formed on a surface of electrodes may cause short circuiting and also may result in delamination. Therefore, further improvements in dispersion characteristics of metallic powders in organic solvents to form conductive pastes are desired.
Furthermore, as mentioned above, multilayer ceramic capacitors with internal electrodes made from base metallic powders, typified by nickel, are researched according to the requirements for inexpensive electrical parts. However, at present, such metallic powders, which can inhibit delamination and are suitable for conductive pastes, have not yet been produced.
DISCLOSURE OF THE INVENTION
Therefore, an object of the invention is to provide a metallic powder, typified by inexpensive base metals such as nickel, in which superior sintering properties are exhibited during production processes for multilayer ceramic capacitors, and superior dispersion characteristics are exhibited in forming conductive pastes, thereby avoiding delamination. More specifically, the invention provides a metallic powder in which the initiation temperature for sintering is high compared to that of conventional metallic powders, and is near the sintering initiation temperature of dielectric bodies used in producing multilayer ceramic capacitors, so that delamination is inhibited.
The inventors have performed intensive research to achieve the above objects. As a result, they have made the invention based on knowledge that the desired powder can be produced by treating surfaces of metallic powders with a surfactant. That is, the invention provides a metallic powder in which the surface was treated with a surfactant. The metallic powders according to the invention are metals suitable for conductive pastes, and noble metals such as silver, palladium, platinum, and gold and the like, and base metals such as nickel, cobalt, iron molybdenum, and tungsten and the like can be applied t
Ito Takayuki
Takatori Hideo
King Roy
Oliff & Berridg,e PLC
Toho Titanium Co., Ltd.
Wilkins, III Harry D.
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