Electricity: electrical systems and devices – Electrolytic systems or devices – Liquid electrolytic capacitor
Reexamination Certificate
2000-08-11
2003-07-29
Ngo, Hung V. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Liquid electrolytic capacitor
C361S509000, C361S306100, C361S321600, C361S311000, C361S523000
Reexamination Certificate
active
06600646
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a niobium powder used for a capacitor having excellent leakage current characteristics; a sintered body produced from the niobium powder; and a capacitor having the sintered body.
(2) Description of the Related Art
Capacitors to be incorporated in electronic apparatuses such as portable phones and personal computers are demanded to have a small size and a high capacitance. Among such capacitors, a tantalum capacitor has been widely used, in view of high capacitance relative to its size, and excellent performance. Generally, in the tantalum capacitor, a sintered body of tantalum powder is used as a positive electrode, and therefore in order to increase the capacitance of the capacitor, the weight of the sintered body must be increased.
When the weight of the sintered body is increased, the capacitor necessarily becomes larger in size and fails to satisfy the demand for a small-sized capacitor. In order to solve this problem, a capacitor containing a sintered body of a powdery material having a dielectric constant higher than that of tantalum has been studied. Niobium and titanium are mentioned as examples of the powdery material having a high dielectric constant.
However, a sintered body of the above-described material has a high specific leakage current index. Since niobium and titanium have high dielectric constants, a capacitor having high capacitance can be produced from these materials, but lower specific leakage current index is required in order to produce a capacitor of high reliability. Specific leakage current index, i.e., leakage current per unit capacitance, can be used to evaluate whether high capacitance can be obtained while maintaining leakage current at a practically permissible low level.
The specific leakage current index is determined as follows. A sintered body having a dielectric layer formed thereon by electrolytic oxidation is prepared, and 70% of formation voltage is continuously applied to the sintered body for three minutes. The leakage current during the application of voltage is divided by the product of formation voltage during electrolytic oxidation and capacitance of the sintered body. Thus, the specific leakage current index is expressed by the following formula:
specific leakage current index=[
LC
/(
C×V
)]
wherein LC: leakage current, C: capacitance and V: formation voltage.
In the case of a sintered body of a tantalum powder, a specific leakage current index is not more than 1,500 [pA/(&mgr;F·V)], as calculated from capacitance and leakage current described in a catalogue entitled “CAPACITOR GRADE TANTALUM” by Showa Cabot Supermetals K.K. In order to guarantee this value, it is generally accepted that the actual measured value of specific leakage current index must be at most ⅓ to ¼ of the value calculated from the catalogue, and a preferred leak current index is not more than 400 [pA/(&mgr;F·V)]. However, a conventional sintered body of niobium or titanium powder has a specific leakage current index much higher than the preferred leak current index, and thus a capacitor containing the sintered body of niobium or titanium has poor reliability and is impractical for use.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a niobium powder suitable for the production of a capacitor having a low specific leakage current index.
Another object of the present invention is to provide a sintered body of niobium powder, used for a capacitor having a low specific leakage current index.
A further object of the present invention is to provide a capacitor with a low specific leakage current index, which has an electrode composed of a sintered body of niobium powder.
In a first aspect of the present invention, there is provided a niobium powder having a degree of nitridation represented by a nitrogen content (hereinafter referred to as “nitrogen content”) of at least about 500 ppm by weight and not more than about 7,000 ppm by weight and having a mean particle diameter of at least about 0.2 &mgr;m and smaller than about 3 &mgr;m.
In a second aspect of the present invention, there is provided a sintered body produced from a niobium powder, which exhibits a specific leakage current index of not more than about 400 [pA/(&mgr;F·V)].
In a third aspect of the present invention, there is provided a sintered body produced from the niobium powder concerned with the first aspect of the present invention.
In a fourth aspect of the present invention, there is provided a capacitor comprising the capacitor concerned with the second or third aspect of the present invention, as one electrode, a dielectric formed on the sintered body, and the other electrode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The niobium powder of the present invention is characterized as having a nitrogen content of at least about 500 ppm by weight and not more than about 7,000 ppm by weight and having a mean particle diameter of at least about 0.2 &mgr;m and smaller than about 3 &mgr;m. A capacitor produced from the niobium powder exhibits a very low specific leakage current index.
The reason for which the capacitor of the niobium powder exhibits a very low specific leakage current index is inferred below.
Generally, capacitance of a capacitor is represented by the following formula:
C
=&egr;×(
S/d
)
wherein C: capacitance, &egr;: dielectric constant, S: specific surface area and d: distance between electrodes.
In the above expression, since d=k×V (k: constant and V: formation voltage), C is represented by the following formula:
C=&egr;×[S
/(
k×V
)], and thus,
C×V
=(&egr;/
k
)×
S.
When specific leakage current index is defined by the following formula as hereinbefore mentioned.
specific leakage current index=[
LC
/(
C×V
)]
(LC: leakage current), the specific leakage current index [LC/(C×V)] can be expressed by the following formula:
specific leakage current index=
LC
/[(&egr;/
k
)×
S].
In consideration of the above formulas, in order to decrease specific leakage current index, there may be selected any measure from among decreasing leakage current (LC), increasing (C×V), increasing &egr;, and increasing S.
In the present invention, the niobium powder of the present invention has a mean particle diameter of smaller than about 3 &mgr;m, the specific surface area of the powder is large. Consequently, the (C×V) value, which is the denominator in the above-described formula providing specific leakage current index, is large. However, when the mean particle diameter of the niobium powder is smaller than about 0.2 &mgr;m, a sintered body produced from the niobium powder has a problem such that permeation of a negative electrode material into the sintered body becomes difficult. As a result, capacitance of the produced capacitor cannot be increased to the desired extent, and the (C×V) value cannot be made large, so that the sintered body is unsuitable for practical use.
Meanwhile, niobium may be bonded more strongly with oxygen than may tantalum, and thus, oxygen atoms in an electrolytic-oxidized film formed on niobium tend to diffuse toward the interior metal, i.e., niobium. In contrast, in the sintered body according to the present invention, a niobium powder is partially bonded with nitrogen, and thus oxygen in an electrolytic-oxidized film formed on niobium is hardly bonded with niobium, preventing diffusion of oxygen atoms toward the niobium. Consequently, the oxidized film can be stabilized and leakage current (LC) may be decreased.
In addition, since the niobium powder according to the present invention comprises a nitrogen content of at least about 500 ppm by weight and not more than about 7,000 ppm by weight, leakage current, serving as the numerator of the above-described formula, becomes especially low. Therefore, specific leakage current index of t
Ha Nguyen
Ngo Hung V.
Showa Denko Kabushiki Kaisha
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