Electricity: electrical systems and devices – Electrolytic systems or devices – Liquid electrolytic capacitor
Reexamination Certificate
2001-04-27
2003-11-04
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Liquid electrolytic capacitor
C361S502000, C361S512000, C361S516000, C361S509000
Reexamination Certificate
active
06643120
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a niobium powder for capacitors, having a large capacitance per unit mass and good specific leakage current properties. The present invention also relates to a sintered body using the powder and a capacitor using the sintered body.
BACKGROUND OF THE INVENTION
There is a demand for capacitors for use in electronic instruments, such as portable telephones and personal computers, to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for its size and good performance. In the tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body with an increased the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body unavoidably causes enlargement of the capacitor shape, and thus, cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering. Therefore, impregnation of a cathode agent in a later process becomes difficult. As means for solving these problems, a capacitor using a sintered body of powder of a material having a dielectric constant larger than that of tantalum is being studied. Materials having a larger dielectric constant include niobium and titanium.
However, the sintered body produced from these materials is not satisfactory because of its large “specific leakage current value”. Elemental niobium or titanium has a large dielectric constant, and therefore, a capacitor having a large capacitance may be obtained. However, a small “specific leakage current value” is a key point for obtaining a capacitor having good reliability. By evaluating the leakage current value per capacitance, namely, “specific leakage current value”, it can be estimated whether a large capacitance can be obtained in a state where leakage current value is reduced to a practically usable value or less.
The “specific leakage current value” as used herein is defined as a value obtained when a dielectric layer is formed on the surface of a sintered body by electrolytic oxidation, and a leakage current value when a voltage corresponding to 70% of the chemical forming voltage is continuously applied at room temperature for 3 minutes is divided by the product of the chemical forming voltage during electrolytic oxidation and the capacitance. That is,
Specific leakage current value=(LC/(C×V)) (LC: leakage current value, C: capacitance, and V: forming voltage).
In the case of a sintered body using a tantalum powder, the specific leakage current value obtained from the capacitance and the leakage current value described in the catalogue “CAPACITOR GRADE TANTALUM” of Showa Cabot Super Metal is 1,500 pA/(&mgr;F·V) or less. In general, the measured specific leakage current value for guaranteeing this specific leakage current value is said to be from ⅓ to ¼ of the value in the catalogue and is preferably 400 pA/(&mgr;F·V) or less.
However, in conventional sintered body capacitors where a niobium powder using elemental niobium or a titanium powder is used, the specific leakage current value is large and exceeds the above-described value. Accordingly, these capacitors are lacking in reliability as a capacitor and are not used in practice.
JP-A-55-157226 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a sintered element for capacitors, where agglomerated powder is molded under pressure into a niobium fine powder having a particle size of 2.0 &mgr;m or less, the fine powder is sintered, the molded and sintered body is cut into fine pieces, a lead part is joined thereto, and then these pieces are again sintered. However, this patent publication discloses neither a niobium powder containing antimony nor properties of the capacitor manufactured using this powder.
U.S. Pat. No. 4,084,965 discloses a capacitor manufactured using a niobium powder of 5.1 &mgr;m obtained by hydrogenating a niobium ingot and pulverizing it. However, U.S. Pat. No. 4,084,965 discloses neither a niobium powder containing antimony nor properties of the capacitor manufactured using this powder.
JP-A-10-242004 discloses a technique of partially nitriding niobium, thereby improving the leakage current value. However, JP-A-10-242004 discloses neither a niobium powder containing antimony nor properties of the capacitor manufactured using this powder.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a niobium powder for capacitors, which can yield a capacitor having a large capacitance per unit mass and good specific leakage current properties; a sintered body using the powder; and a capacitor using the sintered body.
As a result of extensive investigations, the present inventors have accomplished the present invention, which comprises the following embodiments.
(1) A niobium powder for capacitors, comprising niobium and antimony.
(2) The niobium powder for capacitors as described in 1 above, wherein the content of antimony is from about 0.1 to about 10 mol %.
(3) The niobium powder for capacitors as described in 1 or 2 above, wherein the average particle size of the powder is from about 0.2 &mgr;m to less than about 5 &mgr;m.
(4) The niobium powder for capacitors as described in any one of 1 to 3 above, wherein the niobium powder comprises at least one member selected from the group consisting of niobium nitride, niobium carbide, niobium boride and niobium sulfide.
(5) A sintered body using the niobium powder described in any one of 1 to 4 above.
(6) The sintered body as described in 5 above, which has a specific leakage current value of about 400 pA/(&mgr;F·V) or less.
(7) A capacitor comprising the sintered body described in 6 above as one electrode, a dielectric material formed on the surface thereof, and a second electrode.
(8) The capacitor as described in 7 above, wherein the dielectric material comprises niobium oxide.
(9) The capacitor as described in 8 above, wherein the niobium oxide is formed by electrolytic oxidation.
(10) The capacitor as described in 7 above, wherein the second electrode is at least one material (compound) selected from an electrolytic solution, an organic semiconductor and an inorganic semiconductor.
(11) The capacitor as described in 7 above, wherein the another part electrode is formed of an organic semiconductor and the organic semiconductor is at least one organic semiconductor selected from the group consisting of an organic semiconductor comprising a benzopyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane, and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant into a polymer comprising two or more repeating units represented by formula (1) or (2):
wherein R
1
to R
4
, which may be the same or different, each represents hydrogen, an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R
5
is present only when X is a nitrogen atom and represents hydrogen or an alkyl group having from 1 to 6 carbon atoms, and R
1
and R
2
, or R
3
and R
4
may be combined with each other to form a ring.
(12) The capacitor as described in 10 above, wherein the organic semiconductor is at least one member selected from polypyrrole, polythiophene and substitution derivatives thereof.
DESCRIPTION OF THE PRESENT INVENTION
In the present invention, the capacitance of a capacitor is generally represented by the following formula:
Capacitance
C
=∈×(
S/d
)
(C: capacitance, ∈: d
Naito Kazumi
Omori Kazuhiro
Nguyen Ha
Reichard Dean A.
Showa Denko Kabushiki Kaisha
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