Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-10-16
2002-05-14
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S232000, C361S303000, C361S321500
Reexamination Certificate
active
06387150
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a powdered niobium for a capacitor having a large capacity per unit weight and good specific leakage current property, a sintered body of the powdered niobium, a capacitor using the sintered body and production method of the capacitor.
DESCRIPTION OF RELATED ART
Capacitors used for electronic instruments such as portable telephone and personal computer are demanded to be compact and have a larger capacity. Among these capacitors, a tantalum capacitor is preferably used because it has a large capacity for the size and exhibits good performance. In this tantalum capacitor, a sintered body of powdered tantalum is generally used for the anode moiety. In order to increase the capacity of the tantalum capacitor, it is necessary to increase the weight of sintered body or to use a sintered body increased in the surface area by pulverizing the powdered tantalum.
The former method of increasing the weight of sintered body is naturally accompanied by enlargement of the capacitor size and the requirement for downsizing cannot be satisfied. On the other hand, in the latter method of pulverizing the powdered tantalum to increase the surface area, the pore size of tantalum sintered body is reduced or closed pores are increased at the stage of sintering, therefore, a cathode agent can be difficultly impregnated in the after process. As a means for solving these problems, a capacitor using a sintered body of a powdered material having a dielectric constant larger than the tantalum is being studied. Examples of such a material having a larger dielectric constant include niobium and titanium.
However, conventional capacitors using a sintered body of such a material are disadvantageous in that the specific leakage current property is greatly dispersed and not satisfactory by any means. There is no capacitor but meets the criterion that the specific leakage current value in actual measurement is 10 nA/&mgr;F·V or less if the capacitor is produced by preparing a sintered body using the tantalum powder, oxidizing the sintered body electrolytically, and then combining with the counter electrode. However, in capacitors using conventional powderd niobium and titanium, the specific leakage current values are greatly dispersed and there are many cases which exceed this value.
Furthermore, conventional capacitors using a sintered body of such a material are deficient in the high-temperature property and are not put into practical use. Because, when a sintered body is electrolytically oxidized and then combined with counter electrode to manufacture a capacitor, capacity property at high temperature usually falls within ±20% in the case of a sintered body using powdered tantalum, however, in some sintered bodies using conventional powdered niobium, capacity property at high temperature does not fall within ±20%.
Therefore, capacitors using a niobium sintered body and a titanium sintered body must be estimated to have low reliability also at room temperature and are duly judged deficient in the service life, thus cannot be used in practice.
SUMMARY OF THE INVENTION
The present inventors have made an intensive study on a capacitor using a sintered body of niobium. As a result, the present inventors have developed a powdered niobium with a lower content of impurity elements which is capable of providing a capacitor having a small dispersion in the specific leakage current value. Furthermore, the present inventors have found that a capacitor having good high temperature property is obtained when a crystal of a given niobium compound is comprised in a niobium sintered body, and then accomplished the present invention based on these findings.
Namely, the present invention relates to the following powdered niobium for capacitor, sintered body thereof, capacitor using the same and production method of the capacitor.
1) A powdered niobium for a capacitor, containing elements such as iron, nickel, cobalt, silicon, sodium, potassium and magnesium, wherein an amount of each element is 100 ppm by weight or less.
2) A powdered niobium for a capacitor, containing elements such as iron, nickel, cobalt, silicon, sodium, potassium and magnesium, wherein the total amount of the elements is 350 ppm by weight or less.
3) A powdered niobium for a capacitor, containing elements such as iron, nickel, cobalt, silicon, sodium, potassium and magnesium, wherein an amount of each element is 100 ppm by weight or less and the total amount of the elements is 350 ppm by weight or less.
4) The powdered niobium for a capacitor described in any one of the above 1) to 3), which contains at least one of niobium nitride, niobium carbide and niobium boride.
5) A sintered body for a capacitor using a powdered niobium described in any one of the above 1) to 4).
6) A niobium sintered body for a capacitor, comprising at least one of niobium monoxide crystal and a diniobium mononitride crystal.
7) The niobium sintered body for a capacitor according to the above 6), wherein the content of niobium monoxide crystal is from 0.1 wt % to 20 wt %.
8) The niobium sintered body for a capacitor according to the above 6), wherein the content of diniobium mononitride crystal is from 0.1 wt % to 20 wt %.
9) A capacitor comprising one party electrode assigned to the niobium sintered body described in any one of the above 5) to 8), the other party electrode and a dielectric material interposed between the two electrodes.
10) The capacitor according to the above 9), wherein the dielectric material is tantalum oxide, niobium oxide, polymer material, or ceramics compound.
11) The capacitor according to the above 10), wherein the dielectric material is niobium oxide formed by chemical forming on a niobium sintered body.
12) A process for producing a capacitor, comprising preparing the second electrode opposing on the dielectric material, after forming the dielectric material on the niobium sintered body (first electrode) described in any one of the above 5) to 8).
13) The process for producing a capacitor according to the above 12), wherein the dielectric material is tantalum oxide, niobium oxide, polymer material, or ceramics compound.
14) The process for producing a capacitor according to the above 13), wherein the dielectric material is niobium oxide formed on a niobium sintered body by chemical forming.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have found that a capacitor having a small dispersion in the specific leakage current value can be obtained provided that a powdered niobium for a capacitor contains impurity elements such as iron, nickel, cobalt, silicon, sodium, potassium and magnesium each in an amount of about 100 ppm by weight or less, and these elements in a total amount of 350 ppm by weight or less.
The reason for obtaining such a result is not clear in detail, but it is assumed that when the impurity elements as impurities such as iron, nickel, cobalt, silicon, sodium, potassium and magnesium present more than some content in a powdered niobium, the impurity elements enter the dielectric layer when a capacitor is manufactured using the powdered niobium containing the elements and cause abnormal concentration of charges when a voltage is applied, as a result, the specific leakage current value of the capacitor is dispersed.
Furthermore when the niobium sintered body comprises a given niobium compound crystal, the high temperature property of a capacitor is improved. The reason is supposed as below.
Namely, the niobium-sintered body is inferior in the stability of the oxide dielectric film as compared with the tantalum-sintered body. Many reasons may be considered for this, but in one thinking, heat strain developed at a high temperature due to difference between the composition of the oxide dielectric film and the composition of the niobium sintered body seems to accelerate the deterioration of the oxide dielectric film. However, this heat strain seems to mitigate when a niobium monoxide crystal and/or a diniobium mononitride crystal is contained in a niobium sintered bod
Mai Ngoclan
Showa Denko K.K.
Sughrue & Mion, PLLC
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