Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2002-01-29
2004-02-03
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S404000, C428S210000, C361S321400, C361S306300, C252S512000, C252S513000
Reexamination Certificate
active
06686045
ABSTRACT:
This invention relates to composite fine particles comprising a nickel base core and an oxide coating, an electrically conductive paste using the particles, and an electrically conductive film obtained therefrom.
BACKGROUND OF THE INVENTION
In the current trend toward the size reduction of portable equipment and digital electric appliances, it is under study to reduce the size and increase the capacitance of multilayer ceramic capacitors. For the size reduction and capacitance increase of multilayer ceramic capacitors, it is most effective to reduce the thickness of dielectric layers and internal electrode layers in the multilayer structure. To reduce the thickness of internal electrode layers, the conductive paste from which they are made must contain finer metal particles. Among the currently available internal electrode layers, the thinnest layers have a thickness of the order of 1 micron and are made of a conductive paste containing metal particles of a single metal element such as nickel, silver or palladium with a mean particle size of about 1 micron. There is a good possibility of reducing the thickness of internal electrode layers to about 0.3 micron. To form such thin layers, presumably the metal particles in the conductive paste must have a mean particle size of less than about 0.2 micron.
However, mainly three problems arise in reducing the size of metal particles. A first problem is that finer metal particles suffer greater thermal shrinkage. During the firing of multilayer ceramic capacitors, cracks and other defects occur due to the differential thermal shrinkage between dielectric layers and internal electrode layers. A second problem arises from the greater surface area of finer metal particles, which permits metal oxidation during binder burnout from ceramic material. A third problem is due to the ferromagnetism of nickel. As nickel particles are finely divided, they become single domain particles having strong coercivity. Due to magnetic interaction, such particles tend to agglomerate together. A number of solutions have been proposed to overcome these three problems.
The method of preparing fine metal particles is generally divided into three types.
(1) Metal particles are prepared from an aqueous solution of a soluble or insoluble metal compound by reducing with a reducing agent such as hydrazine. Because of the solution reaction, this process is often designated wet process.
(2) Metal particles are prepared from an aqueous solution of a soluble or insoluble metal compound by spraying and heating at elevated temperatures in hydrogen gas for direct reduction. Because hydrogen gas is used, this process is often designated gas phase process.
(3) Metal particles are prepared by pyrolysis of carbonyl and similar compounds at elevated temperatures.
The wet process can produce metal particles with a sharp particle size distribution. However, these metal particles have a great shrinkage factor because they have not been heat treated. To eliminate such inconvenience, JP-A 11-172306 proposes to add magnesium and/or calcium to metal particles, and Japanese Patent No. 2992270 and JP-A 2000-282102 disclose to coat part of the particle surface with an oxide. Likewise, the metal particles produced by the gas phase process have a similar shrinkage factor although the shrinkage start is retarded because they are produced at a higher temperature than in the wet process. Similar countermeasures are taken as disclosed in JP-A 2000-63901. Although the shrinkage of metal particles can be improved by the above modifications, all these processes are unsatisfactory in that the diameter of metal particles is as large as about 1.0 &mgr;m. Especially, particles of a single metal element such as nickel inevitably generate a magnetic field due to their own magnetism.
SUMMARY OF THE INVENTION
An object of the invention is to provide composite fine particles capable of overcoming the problems of shrinkage during firing and metal oxidation. Another object of the invention is to provide an electrically conductive paste using the particles, and an electrically conductive film obtained therefrom.
It has been found that a useful electrode material is constructed by composite fine particles each comprising a core of the general formula (1):
Ni
(1-a-b)
Z
a
Z′
b
(1)
wherein Z is Ag, Au, Co, Cu, Pd or a mixture of any, Z′ is Li, K, Na, B, P or a mixture of any, “a” and “b” representative of a weight proportion of Z and Z′ are 0≦a≦0.4 and 0≦b≦0.1, respectively, and a+b>0, which is at least partially surface coated with an oxide of Ag, Ba, Co, Cu, Ni, Sn, Ti, Zr, rare earth elements including Y, Pd or a mixture of any. When these composite fine particles are used as an internal electrode material for multilayer ceramic capacitors or an electronic ceramic material, the shrinkage during the firing of multilayer ceramic capacitors is minimized and the metal oxidation during binder burnout from electronic ceramic materials is suppressed. The invention is predicated on this finding.
Therefore, the invention provides the composite fine particles defined above; an electrically conductive paste comprising the composite fine particles and an organic vehicle; and an electrically conductive film obtained by firing the conductive paste and having a surface resistivity of up to 100 m&OHgr;.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The composite fine particles of the invention is defined as comprising a core of a nickel base metal material whose surface is at least in part coated with an oxide. The metal material has the general formula (1):
Ni
(1-a-b)
Z
a
Z′
b
(1)
wherein Z is at least one element selected from the group consisting of Ag, Au, Co, Cu, and Pd, Z′ is at least one element selected from the group consisting of Li, K, Na, B, and P, “a” representative of a weight proportion of Z is 0 to 0.4, “b” representative of a weight proportion of Z′ is 0 to 0.1, and “a” and “b” are not zero at the same time (i.e., “a”+“b”>0). The oxide is an oxide of at least one element selected from the group consisting of Ag, Ba, Co, Cu, Ni, Sn, Ti, Zr, rare earth elements including Y, and Pd, preferably Ag, Ba, Co, Cu, Ni, Sn, and Pd.
Metal particles serving as the core are nickel based and preferably produced by the wet process. When fine metal particles are produced by reductive reaction in solution, it is crucial how to generate many nuclei. The reaction must be instantaneous. Then, if there is present an element which is reduced, prior to nickel, to form nuclei, finer particles can be produced. For this reason, one or more of Ag, Au, Co, Cu, and Pd are used as the nucleating element Z. Of these, Ag and Cu are electrically conductive and preferred. Cu is also preferred because of low cost.
For restraining shrinkage, the content of element Z is 0 (preferably more than 0%) to 40% by weight of the metal material, that is, “a” (weight proportion) in formula (1) is 0≦a≦0.4. With a Z content in excess of 40% by weight, the melting point of metal material lowers below the practically acceptable level when Z is Cu or Pd, and the metal material becomes expensive when Z is Ag, Au or Co. The lower limit of the Z content is preferably 1% by weight or more. The preferred Z content is in the range of 3 to 10% by weight when it is desired to produce particles with a mean particle size of up to 0.2 &mgr;m.
For further improvement in shrinkage, it is effective to add to the metal particles another element Z′ selected from Li, K, Na, B, P and a mixture of any. Of these, B and P form such compounds as Ni
3
B and Ni
3
P which are harder than nickel and serve to suppress the shrinkage of nickel. Li, K and Na form with nickel a solid solution which contributes to a further suppression of shrinkage.
The content of element Z′ is 0 (preferably more than 0%) to 10% by weight of the metal material, that is, “b” (weight proportion) in formula (1) is 0<b≦0.1. With a Z′ content in excess of 10% by weight, a eutectic point appears and
Kubota Iwao
Takai Yasushi
Kiliman Leszek
Shin-Etsu Chemical Co. , Ltd.
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