Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2001-04-24
2003-12-30
Zacharia, Ramsey (Department: 1773)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C428S411100, C428S403000, C420S425000, C075S252000, C361S323000, C361S321500
Reexamination Certificate
active
06671164
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to niobium powder employed for producing a capacitor exhibiting excellent reliability; a sintered body formed from the powder; and a capacitor comprising the sintered body.
BACKGROUND OF THE INVENTION
Capacitors employed in electronic apparatuses such as cellular phones and personal computers desirably have small size and high capacitance. Among such capacitors, a tantalum capacitor has been widely employed, since the tantalum capacitor has high capacitance relative to its size and exhibits excellent performance. Generally, in the tantalum capacitor, a sintered body of tantalum powder is employed as a positive electrode. Therefore, in order to increase the capacitance of the capacitor, the mass of the sintered body must be increased.
However, when the mass of the sintered body is increased, the capacitor unavoidably becomes large, and therefore fails to satisfy the demand for small size. In order to solve such a problem, a capacitor comprising a sintered body of a powder material having a dielectric constant higher than that of tantalum has been studied. Niobium is an example of such a material having a high dielectric constant.
Japanese Patent Application Laid-Open (kokai) No. 55-157226 discloses a production process for a sintered element employed in a capacitor, in which niobium agglomeration and niobium powder having a particle size of 2.0 &mgr;m or less are press-shaped and sintered. The resultant sintered body is cut into small pieces, a lead portion is connected to each piece, and the resultant piece is sintered again. However, the publication does not disclose capacitor characteristics in detail.
U.S. Pat. No. 4,084,965 discloses a capacitor formed from niobium powder having a particle size of 5.1 &mgr;m, which is obtained through pulverization of a hydrogenated niobium ingot. However, a niobium sintered body has a higher leakage current (LC) than LC of a tantalum sintered body, and thus may fail to be practically useful.
Although niobium exhibits poor leakage current characteristics, the present inventors have proposed that the leakage current of niobium can be reduced when the niobium is subjected to, for example, partial nitridation, (see Japanese Patent Application Laid-Open (kokai) No. 10-242004, U.S. Pat. No. 6,115,235). In order to further reduce the leakage current of niobium when a sintered body of niobium is produced, the sintering temperature is elevated. However, when the sintering temperature is elevated, the product (hereinafter the product will be referred to as “CV”) of capacitance per unit mass of the produced sintered body and formation voltage during production of a dielectric on the sintered body decreases. Therefore, obtaining a well-balanced niobium sintered body having high CV and low LC is difficult. Meanwhile, when a capacitor is produced from a niobium sintered body obtained to attain only a high CV, the capacitor encounters the problem in that LC is especially high.
Since the affinity of niobium to oxygen is higher than that of tantalum to oxygen, niobium contains a large amount of portions which are oxidized at room temperature. As described below, when niobium is sintered and the resultant sintered body is employed as one electrode of a capacitor, the aforementioned oxidized portions adversely affect a dielectric of the capacitor, possibly resulting in poor reliability of the capacitor. For example, when voltage is applied to the capacitor at room temperature after the capacitor is subjected to a high-temperature load test, which is an acceleration test for evaluating reliability of the capacitor, spike noise may be generated. In the case in which the capacitor is provided on a circuit substrate, generated noise may adversely affect other electronic parts on the circuit board when the capacitor is used for a prolonged period of time. Therefore, the capacitor involves some problems when it is used.
In view of the foregoing, an object of the present invention is to provide a niobium powder for producing a capacitor exhibiting excellent reliability; a sintered body formed from the powder; and a capacitor comprising the sintered body, even when niobium exhibiting high affinity to oxygen is employed, by regulating the amount of oxygen contained in the powder.
SUMMARY OF THE INVENTION
The present inventors have performed extensive studies on oxygen contained in niobium powder, and have developed a niobium powder capable of forming a sintered body for producing a capacitor exhibiting excellent reliability. The present invention has been accomplished on the basis of this development. Accordingly, the present invention provides the following embodiments:
(1) niobium powder comprising niobium, nitrogen and oxygen, which has undergone partial oxidation and partial nitridation, wherein the mass ratio of the content of nitrogen in the powder to that of oxygen in the powder is about 1/45 or more;
(2) niobium powder according to (1) above, wherein the nitrogen content is about 300 mass ppm or more and about 9,000 mass ppm or less;
(3) niobium powder according to (1) or (2) above, wherein the mean particle size of the powder is about 0.05 &mgr;m or more and about 3 &mgr;m or less;
(4) granulated niobium powder obtained through granulation of niobium powder as described in (3) above;
(5) a sintered body formed from niobium powder as described in any one of (1) through (4) above;
(6) a capacitor comprising a sintered body as described in (5) above serving as a first electrode, a dielectric formed thereon, and a second electrode;
(7) a capacitor according to (6) above, wherein the dielectric is niobium oxide;
(8) a capacitor according to (7) above, wherein the niobium oxide is formed through electrolytic oxidation;
(9) a capacitor according to (6) above, wherein the second electrode is formed from at least one material (compound) selected from among an electrolyte, an organic semiconductor, and an inorganic semiconductor;
(10) a capacitor according to (6), wherein the second electrode is formed from an organic semiconductor, and the organic semiconductor is at least one semiconductor selected from the group comprising of an organic semiconductor comprising benzopyrroline tetramer and chloranyl, an organic semiconductor predominantly comprising tetrathiotetracene, an organic semiconductor predominantly comprising tetracyanoquinodimethane, and an organic semiconductor predominantly comprising a conductive polymer in which a dopant is doped in a polymer including two or more repeating units represented by the following formula (1) or (2):
wherein each of R
1
through R
4
, which may be identical to or different from one another, represents hydrogen, a C1-C6 alkyl group, or a C1-C6 alkoxy group; X represents an oxygen, sulfur, or nitrogen atom; R
5
, which is present only when X is a nitrogen atom, represents hydrogen or a C1-C6 alkyl group; and R
1
and R
2
, and R
3
and R
4
may be bonded to each other to form a cyclic structure; and
(11) a capacitor according to (9) above, wherein the organic semiconductor is at least one species selected from among polypyrrole, polythiophene, and a substituted derivative thereof.
DESCRIPTION OF THE PRESENT INVENTION
As described above, in the niobium powder of the present invention, the mass ratio of the nitrogen content to the oxygen content must be about 1/45 or more, preferably about 1/40 or more, more preferably about 1/30 or more. When the mass ratio falls outside the above range, a capacitor produced from the niobium powder fails to exhibit excellent reliability. For example, when voltage is applied to the capacitor at room temperature after the capacitor is subjected to a high-temperature load test, which is an acceleration test for evaluating reliability of the capacitor, spike noise may be generated (see Comparative Examples in Table 2). Generation of spike noise is not preferable in view of the reliability of the capacitor.
Therefore, in the present invention, it is important to regulate the mass ratio of the nitrogen content to the oxygen content to about 1/45
Nagato Nobuyuki
Naito Kazumi
Showa Denko Kabushiki Kaisha
Sughrue & Mion, PLLC
Zacharia Ramsey
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