Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2000-03-29
2002-05-21
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S367000, C075S629000
Reexamination Certificate
active
06391084
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a metallic nickel powder suitable for conductive pastes, and in particular, relates to a metallic nickel powder having superior sintering properties and dispersion characteristics, which is specifically suitable for conductive pastes for, and internal electrodes for, 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 been used for electrical parts such as conductive pastes, and have long been specifically used in internal electrodes for multilayer ceramic capacitors. Multilayer ceramic capacitors 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 high dielectric constants, 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 in the metallic layers for forming the internal electrodes. Recently, 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 paste, and the paste is printed on the green sheet by using a screen printing method. This is then subjected to drying, laminating, firmly pressing, and heating to remove the organic component, and is then sintered at about 1300° C. or more. Then, external electrodes are burnt to both ends of the ceramic dielectric layers, and a multilayer ceramic capacitor is thereby manufactured.
In manufacturing process for multilayer ceramic capacitors such as the above, the metallic paste is printed on the dielectric green sheet, and after laminating and firmly pressing, the organic component is vaporized and removed by heating. This heating is usually carried out at 250 to 400 ° C. in the air. Thus, the heating is carried out in an oxidizing atmosphere, and therefore the metallic powder is oxidized, whereby expansion of volume thereof is generated. Furthermore, the powder is further sintered by heating at a higher temperature after the heating to remove the organic component. This sintering is carried out in a reducing atmosphere, such as hydrogen gas atmosphere. Whereby the metallic powder once oxidized is reduced, and contraction of the volume is generated.
Thus, in the manufacturing process for multilayer ceramic capacitors, the volume changes of the metallic powder due to expansion and contraction thereof are generated by the oxidation-reduction reaction. Similarly, the volume of the dielectric body also changes due to the sintering. However the different materials of the dielectric body and the metallic powder are simultaneously sintered, and therefore sintering properties differ from each other due to changes in the volumes by expansion and contraction of these materials during sintering. For this reason, the strain is generated in the metallic paste layer. As a result, the process has problems in that the laminate construction may be broken and crack or peeling called delamination may occurs
Specifically, sintering in a dielectric body comprising barium titanate as a main component initiates 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 initiates at lower temperature than that temperature, for example, normally at a temperature in the range of 400 to 500° C. in the case of metallic 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 difference 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 initiation temperature for sintering be as high as possible and that extreme sintering does not occur.
Various methods for solving the problems of delamination have been proposed. For example, Japanese Unexamined Patent Application Publication No. 246001/96 discloses a metallic nickel powder in which the tap density to specific particle size is higher than the limiting value. In this publication, it has been described that if such metallic nickel powder is used, delamination is not easily occurred when a capacitor is produced by sintering the nickel powder and dielectric body dispersed in a paste.
The above conventional methods can yield some improvement in improving sintering properties. However, these methods are not sufficient to effectively prevent delamination. On the other hand, internal electrodes are required to be formed in thin layers and to have low in electrical resistance in accordance with the trends toward miniaturization and large capacity in capacitors, and therefore metallic powders for internal electrodes are required to be super-fine powders, having not only particle sizes of 1 &mgr;m or less, but also particle sizes 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, further development of metallic powders which can prevent occurrence of delamination in this state and are suitable for conductive pastes, has been required.
DISCLOUSURE OF INVENTION
Therefore, an object of the present invention is to provide a metallic nickel powder 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 preventing occurrence of delamination. More specifically, the present invention provides a metallic nickel powder suitable for conductive pastes, in which the volume changes and the weight changes due to the oxidation-reduction reaction during sintering are small, the initiation temperature for sintering is high compared to that of conventional metallic nickel powders, and is near the sintering initiation temperature of dielectric bodies used in producing multilayer ceramic capacitors, whereby occurrence of delamination can be prevented and dispersion characteristics in solvent are superior.
The inventors have performed intensive research with regard to dispersion characteristics of a metallic nickel powder. As a
Ito Takayuki
Takatori Hideo
Oliff & Berridg,e PLC
Toho Titanium Co., Ltd.
Wilkins, III Harry D.
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