Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-12-19
2004-02-10
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S255000
Reexamination Certificate
active
06689185
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a niobium powder for a capacitor free of reduction in the capacitance, 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 potable telephone and personal computer to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferably used because of its large capacitance per unit volume and good performance. In a 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 increased in surface area by pulverizing the tantalum powder. The former method of increasing the mass of the sintered body necessarily incurs enlargement of the capacitor shape and cannot satisfy the requirement for downsizing.
On the other hand, in the latter method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body is reduced or closed pores increase at the stage of sintering. Therefore, impregnation of the cathode agent in the later process becomes difficult.
As means for solving these problems, a capacitor using a sintered body of a material having a dielectric constant larger than that of tantalum is being studied. The material having a larger dielectric constant includes niobium. 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 an alloy mainly comprising tantalum, titanium, niobium, aluminum or the like is used as a valve-acting powder and the agglomerated powder of the alloy 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 those pieces are again sintered. However, JP-A-55-157226 neither discloses nor suggests the tantalum content or the amount of niobium powder and moreover, the properties of the capacitor manufactured using this powder are not disclosed at all.
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 neither discloses nor suggests the tantalum content and the amount of niobium powder.
Although niobium is deficient in the leakage current (hereinafter simply referred to as “LC value”), the present inventors have previously proposed that the LC value can be improved by nitriding a part of niobium (see, JP-A-10-142004). Increased reduction in the LC value can be attained, for example, by elevating the sintering temperature at the time of manufacturing the above-described niobium sintered body. However, if the sintering temperature is elevated, there arises a problem that the product of the capacitance (simply “C”) per mass of the sintered body manufactured and the chemical forming voltage (simply “V”) at the time of forming a dielectric material on the surface of the sintered body (hereinafter the product is simply referred to as “CV value”) becomes small.
The CV value is considered to be proportional to the surface area of the sintered body and, the surface area of the sintered body is estimated to depend on the specific surface area of the niobium powder with the same production conditions and molding conditions of niobium powder and with same sintering conditions in obtaining a sintered body. However, even if a niobium sintered body is manufactured from niobium powder using the same conditions in respective stages, the CV value of the manufactured niobium sintered body is not always the same but disadvantageously decreases.
SUMMARY OF THE INVENTION
By taking into account these problems, the present invention provides a niobium sintered body free of reduction in the CV value, a niobium powder for use in the manufacture of this niobium sintered body, and a capacitor using this niobium sintered body.
As a result of extensive investigations, the present inventors have developed a niobium powder for capacitors, which has a tantalum content (hereinafter “ppm by mass” is simply referred to as “ppm”) reduced to a predetermined value or less thereby enabling the manufacture of a niobium sintered body free of reduction in the CV value. Based on this finding, the present invention has been accomplished. More specifically, the present invention includes the following embodiments.
(1) A niobium powder comprising niobium and tantalum, wherein the tantalum is present in an amount of at most about 700 ppm by mass.
(2) The niobium powder as described in 1 above, which is partially nitrided.
(3) The niobium powder as described in 2 above, wherein the amount nitrided is from about 10 to about 100,000 ppm by mass.
(4) A sintered body comprising the niobium powder described in 1 to 3 above.
(5) A method for producing a niobium sintered body, comprising sintering a niobium powder compact at a high temperature, wherein the niobium powder is the niobium powder described in any one of 1 to 3 above and heating the niobium powder under reduced pressure at about 500 to about 2,000° C. for about 1 minute to about 10 hours.
(6) The method for producing a niobium sintered body as described in 5 above, wherein the niobium powder is obtained by granulating a niobium powder having an average primary particle size of about 1 &mgr;m or less.
(7) A capacitor comprising a pair of electrodes having interposed therebetween a dielectric material, one of the electrodes being the niobium sintered body described in 4 above.
(8) The capacitor as described in 7 above, which has a dielectric material comprising niobium oxide formed by electrolytic oxidation.
(9) The capacitor as described in 7 above, wherein the other electrode is at least one material (compound) selected from the group consisting of an electrolytic solution, an organic semiconductor and an inorganic semiconductor.
(10) The capacitor as described in 7 above, wherein the other electrode is formed of at least one organic semiconductor selected from the group consisting of an organic semiconductor comprising 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 containing 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.
(11) The capacitor as described in 7 above, wherein the organic semiconductor is at least one selected from the group consisting of polypyrrole, polythiophene and substitution derivatives thereof.
(12) An electronic circuit using the capacitor as described in 7 above.
(13) Electronic equipment using the capacitor as described in 7 above.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment for obtaining the sintered body of the present invention is described below.
The raw material compound used for the niobium powder may be a generally available material. For example, a niobium powder obtained by the reduction of niobium halide with magnesium or sodium, by the sodium reduction of potassium fluoroniobate, by the molten-salt (NaCl+KCl) electrolysis of potassium fluoroniobate onto a nickel anode, or by the introduction of hydrogen into a metal niobium ingot and then pulveriz
Naito Kazumi
Omori Kazuhiro
Mai Ngoclan
Showa Denko Kabushiki Kaisha
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