Niobium capacitor and method of manufacture thereof

Details Niobium capacitor and method of manufacture thereof Niobium capacitor and method of manufacture thereof

2001-06-15

2003-03-04

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Solid electrolytic capacitor

C361S311000, C361S525000

Reexamination Certificate

active

06529367

ABSTRACT:

TECHNICAL FIELD
This invention relates to a capacitor having a large capacity per unit weight and good leakage current (hereinafter abbreviated to as “LC”) characteristics.
BACKGROUND ART
By virtue of advancement in the downsizing or higher integration of IC or printed board in recent years, compact and lightweight electronic instruments such as a portable telephone, a laptop personal computer and an electronic memorandum book, have come into wide use. To cope with this tendency, development of capacitors having a small size and a large capacity is being eagerly desired for use in these electronic instruments.
Among the capacitors used in this field, a tantalum electrolytic capacitor is widely used because it has a large capacity for the size and exhibits good performance. In this tantalum electrolytic capacitor, tantalum oxide is used for the dielectric material.
In order to more increase the capacity of a capacitor, development of niobium or titanium capacitors using niobium oxide or titanium oxide having a higher dielectric constant than the tantalum oxide for the dielectric material is being encouraged. However, the capacitors using niobium oxide or titanium oxide for the dielectric material have unsatisfactory LC characteristics and poor practicality, thus, they are still in need of improvements.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a niobium capacitor provided with a niobium oxide dielectric having good dielectric properties, which capacitor has a large capacity per unit weight and good LC characteristics.
Another object of the present invention is to provide a process for producing a niobium capacitor having a large capacity per unit weight and good LC characteristics, and exhibiting uniform LC value.
The present inventors have found that the poor LC characteristics of the niobium capacitor provided with a niobium oxide dielectric are, as one of causes, due to the excessive or deficient amount of oxygen bound to niobium constituting the niobium oxide. The present invention has been accomplished based on this finding.
In accordance with the present invention, there is provided a capacitor comprising two electrodes and a dielectric interposed between the two electrodes, characterized in that the dielectric has a two-layer structure comprising a first layer predominantly comprised of niobium oxide NbO
x
(X=2.5) and a second layer predominantly comprised of a mixture of niobium oxide NbO
x
(X=2.5) and niobium oxide NbO
x
(X=2.0).
In accordance with the present invention, there is further provided a process for producing a capacitor comprising two electrodes, one of which Is comprised of a sintered body of partially nitrided niobium, and a dielectric interposed between the two electrodes, characterized in that a compact of powdery niobium is sintered and then the thus-obtained niobium sintered body is allowed to stand in a nitrogen atmosphere to partially nitride the niobium sintered body.
BEST MODE FOR CARRYING OUT THE INVENTION
In the niobium capacitor of the present invention, the dielectric interposed between two electrodes is a dielectric having a two-layer structure comprising a first layer predominantly comprised of niobium oxide NbO
x
(X=2.5) and a second layer predominantly comprised of a mixture of niobium oxide NbO
x
(X=2.5) and niobium oxide NbO
x
(X=2.0).
In general, if the structure of niobium oxide is expressed by the formula: NbO
x
(x represents a molar ratio of oxygen bonded to niobium), those where x is 0.9, 1.0, 1.1, 2.0 and 2.5 are known. Niobium oxides having such a bonding value in this structure are identified by the X-ray photoelectric spectroscopic analysis. As preferable examples of niobium oxide NbO
x
(x=2.5) and niobium oxide NbO
x
(x=2.0), there can be mentioned Nb
2
O
5
and NbO
2
, respectively.
When the dielectric in a capacitor is constituted by a two layer structure having a first layer predominantly comprised of niobium oxide NbO
x
(X=2.5) and a second layer predominantly comprised of a mixture of niobium oxide NbO
x
(X=2.5) and niobium oxide NbO
x
(X=2.0), among niobium oxides, the capacitor has a very low LC value. The reason therefor is not yet completely elucidated, however, the poor LC characteristics are presumed to result because when a dielectric is predominantly comprised of niobium oxide, which does not have the above-described two-layer structure, oxygen in the dielectric material moves from the dielectric side to the electrode side or internal oxygen adsorbed on the electrode moves from the electrode side to the dielectric side, and due to this moving of oxygen, the characteristics of the dielectric itself become unstable, leading to an increase of the LC value. On the other hand, when a niobium oxide dielectric having the above-descried two-layer structure is used as dielectric, it is considered that the movement of oxygen, even if it occurs, takes place inside the dielectric material and the state is seemingly equilibrated, as a result, characteristics of the dielectric itself can be stabilized.
By the term “predominantly comprised of” used in the niobium oxide dielectric used in the present invention, we mean that niobium oxide NbO
x
(X=2.5) occupies at least 60% by weight of the first layer, and the mixture of niobium oxide NbO
x
(X=2.5) and niobium oxide NbO
x
(X=2.0) occupies at least 60% by weight of the second layer. The LC value of a capacitor is preferably 1 &mgr;A or less. In order to keep the LC value at 1 &mgr;A or less, the content of NbO
x
(x=2.5) in the niobium oxide of the first layer and the content of the mixture of NbO
x
(x=2.5) and NbO
x
(x=2.0) in the second layer each should preferably be at least 90% by weight, more preferably at least 95% by weight.
To produce a capacitor having a more reduced LC value, the ratio of niobium oxide NbO
x
(X=2.5) to niobium oxide NbO
x
(X=2.0), contained in the second layer of the dielectric, is preferably in the range of 1:4 to 4:1 by mole, more preferably from 1:3 to 3:1 by mole; and the content of the first layer in the two-layer structure Is preferably in the range of 0.01% to 10% by volume, especially 0.04% to 3% by volume, based on the volume of the second layer.
For forming the niobium oxide dielectric layer having the above-mentioned two-layer structure, for example, there can be employed a method of depositing a niobium complex such as niobium-containing alkoxy complex or acetyl acetonate complex onto an electrode, and thermally decomposing and/or hydrolyzing the deposited niobium complex; or, in the case of using niobium or partially nitrided niobium for the electrode, which will be described later, a method of electrolytically oxidizing the niobium electrode or the partially nitrided niobium electrode, or a method of depositing a niobium complex, as mentioned above, onto the niobium electrode or the partially nitrided niobium electrode and thermally decomposing and/or hydrolyzing the deposited niobium complex. Depending on the case, these methods may be used in combination.
In the case where the niobium oxide dielectric is made by electrolytically oxidizing the niobium electrode or the partially nitrided niobium electrode, the capacitor of the present invention is an electrolytic capacitor wherein the niobium electrode or the partially nitrided niobium electrode assumes anode. In the case where the niobium oxide dielectric is made by decomposing a niobium complex on the niobium electrode or the partially nitrided niobium electrode, the electrode is theoretically free of polarity and may assume either anode or cathode.
For the electrolytic oxidation of the niobium electrode or the partially nitrided niobium electrode, an aqueous protonic acid solution, for example, a 0.1% aqueous phosphoric acid solution or a 0.1% aqueous sulfuric acid solution is usually used. When the niobium oxide dielectric is made by the method of thermally decomposing and/or hydrolyzing a niobium-cont

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