# Patent

## Title
Niobium powder, niobium sintered body and capacitor using the sintered body

## Abstract

(1) A niobium powder for a capacitor having a tapping density of 0.5 to 2.5 g/ml, an average particle size of 10 to 1000 μm, angle of repose from 10° to 60°, the BET specific surface area from 0.5 to 40 m2/g and a plurality of pore diameter peak tops in the pore distribution, and a producing method thereof; (2) a niobium sintered body, which is obtained by sintering the above niobium powder and, having a plurality of pore diameter peak tops in a range of 0.01 μm to 500 μm, preferably, the peak tops of two peaks among the plurality of pore diameter peak tops having a highest relative intensity are present in the range of 0.2 to 0.7 μm and in the range of 0.7 to 3 μm, respectively, and a producing method thereof; (3) a capacitor using the above sintered body and a producing method thereof; and (4) an electronic circuit and electronic device using the above capacitor.

## Background
CROSS REFERENCE TO THE RELATED APPLICATIONS
This is a Divisional of application Ser. No. 10/343,912 filed Feb. 6, 2003 now U.S. Pat. No. 6,934,146, which is a 371 of PCT/JP02/04644, filed May 14, 2002, and which claims benefit of U.S. Provisional Appln Nos. 60/291,925 filed May 21, 2001, and 60/331,200 filed Nov. 9, 2001, under the provision of 35 U.S.C. § 111(b), pursuant to 35 U.S.C. § 119(e)(1), the above-noted applications are incorporated herein by reference in their entirety.


TECHNICAL FIELD
The present invention relates to a niobium powder and a sintered body thereof, which can stably produce a capacitor having a large capacitance per unit mass, good leakage current characteristics and excellent moisture resistance, and also relates to a capacitor using the niobium powder and sintered body and production methods of niobium powder, sintered body and the capacitor.
BACKGROUND ART
Capacitors for use in electronic instruments such as portable telephone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance.
Moreover, recent electronic devices are demanded to operate at a low voltage and high frequency with low noise, and also a lower equivalent series resistance (ESR) is required for a solid electrolyte capacitor.
In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. This powder is molded and then sintered, whereby the powder is integrated and works out to an electrode called a sintered body. The inside of this sintered body takes a three-dimensional complicated form resulting from the particles of the powder being electrically and mechanically connected with each other. On the surface of this sintered body including the surface of the inside void, a dielectric film layer is formed and thereinto, a material as a counter electrode is impregnated, whereby a capacitor is fabricated. As long as the dielectric film layer uniformly adheres to the inside or outside surface of the sintered body, the capacitance of the capacitor manufactured greatly depends on, microscopically, the contact state of the counter electrode material with the dielectric film layer.
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 the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the specific surface area, the pore diameter of the tantalum sintered body decreases or closed pores increase at the stage of sintering, as a result, impregnation of the cathode agent at a later step becomes difficult.
For example, assuming that when an aqueous phosphoric acid solution is used as a counter electrode material, the contact state of the solution with a dielectric film layer is complete and the capacitance appearance ratio (also called a cathode agent impregnation ratio) at the time is 100%, a capacitance appearance ratio of 100% can be hardly attained in the case of using a counter electrode material having high viscosity, particularly a solid electrode material. In particular, when the average particle size of tantalum powder is small or the sintered body manufactured from a tantalum powder has a large shape, the difficulty increases and in an extreme case, the capacitance appearance ratio cannot reach even 50%. With such a low capacitance appearance ratio, the capacitor manufactured cannot have a sufficiently high moisture resistance.
In case the tantalum powder to produce a tantalum sintered body has a small pore diameter, it essentially leads to a small pore diameter of the tantalum sintered body and a low capacitance appearance ratio. Consequently, a problem arises that an ESR cannot be lowered.
As one of means for solving these problems, a capacitor using a sintered body as an electrode may be considered, which sintered body is produced using an electrode material providing a dielectric material having a dielectric constant larger than that of tantalum and gives a high capacitance appearance ratio.
As such an electrode material which can be supplied industrially, niobium having a dielectric constant larger than that of tantalum and having a large reserve is known.
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 device for capacitors, where agglomerated valve-acting metal powder having a particle size of 2.0 μm or less is molded under pressure and then sintered, the molded and sintered body is cut into fine pieces, a lead part is joined therewith and these are again sintered. However, the details on the producing method and properties of the niobium capacitor are not described in this patent publication.
U.S. Pat. No. 4,084,965 discloses a capacitor using a sintered body of niobium powder having an average particle size of 5.1 μm obtained by hydrogenating and pulverizing a niobium ingot. However, the capacitor disclosed has a large leakage current (hereinafter sometimes simply referred to as “LC”) value and is of little practical use.
JP-A-10-242004 (WO 98/38660) discloses a technique of partially nitriding a niobium powder and thereby improving the LC value.
The tapping density of a niobium powder for a capacitor is an important factor in molding operation of the niobium powder. The tapping density of conventional niobium powder is 2.5 g/ml or more, specifically about 4 g/ml, and this is insufficient for the molding.
That is, if such a niobium powder is molded and sintered to prepare a sintered body, the niobium powder poorly flows from the hopper of a molding machine to the metal mold and it is difficult to weigh a constant amount of niobium powder and flow it into the metal mold. As a result, it causes problems such that the shape of the molded article is not satisfactorily stabilized, the molded article and the sintered body are deficient in the strength, and a capacitor having bad LC is produced frequently. If a special molding apparatus capable of also handling a powder having bad flowability is used, the molding cost excessively increases and this is not practical.
As such, conventionally known niobium powder for capacitors has a problem that the powder cannot be fully adapted to continuous molding and the productivity of capacitors is low.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide a capacitor having a large capacitance per unit mass, a small leakage current and a high moisture resistance; a sintered body usable as the electrode material of this capacitor and capable of giving a high capacitance appearance ratio; a niobium powder being preferred as the material for this sintered body, exhibiting good flowability in the molding operation, facilitating continuous molding and enabling stable production of capacitors; and the production methods of the capacitor, the sintered body and the niobium powder.
As a result of extensive investigations to solve the above-described problems, the present inventors have found that when a niobium sintered body having a specific pore distribution, preferably having a plurality of pore diameter peak tops in the pore distribution is used for the capacitor electrode, a high capacitance appearance ratio can be obtained and a capacitor having small leakage current and good moisture resistance can be produced. The present inventors also have found that a niobium powder preferably having a tapping density of 0.5 to 2.5 g/ml, more preferably having an average particle size of 10 to 1,000 μm exhibits good flowability, enables continuous molding and is preferred as the material for the above-described sintered body and when this niobium powder is used, a capacitor having small leakage current can be stably produced. The present inventors have found that more preferably, both a high capacitance appearance ratio and low ESR can be achieved with a capacitor wherein the electrodes use a niobium sintered body produced by using a niobium powder having a wide range of pore distribution and a plural pore diameter peak tops all of which are 0.5 μm or more. The present invention has been accomplished based on these findings.
More specifically, the present invention relates to the following niobium powder, niobium sintered body, capacitor using thereof, and production methods of the niobium powder, the niobium sintered body and the capacitor.
(1) A niobium powder for a capacitor having a tapping density of 0.5 to 2.5 g/ml.
(2) The niobium powder as described in 1 above, wherein the average particle size is from 10 to 1,000 μm.
(3) The niobium powder as described in 1 or 2 above, wherein the angle of repose is from 10° to 60°.
(4) The niobium powder as described in any one of 1 to 3 above, wherein the BET specific surface area is from 0.5 to 40 m2/g.
(5) The niobium powder as described in any one of 1 to 4 above, which has a pore distribution having a pore diameter peak top in the range of 0.01 μm to 500 μm.
(6) The niobium powder as described in 5 above, wherein the pore distribution has a plurality of pore diameter peak tops.
(7) The niobium powder as described in 5 or 6 above, wherein any of the pore diameter peak tops is in the range of 0.5 to 100 μm.
(8) The niobium powder as described in any one of 1 to 7 above, wherein at least one element selected from the group consisting of nitrogen, carbon, boron and sulfur elements is contained in an amount of 200,000 mass ppm or less.
(9) A sintered body using the niobium powder described in any one of 1 to 8 above.
(10) The sintered body as described in 9 above, which has a pore distribution having a pore diameter peak top in the range of 0.01 μm to 500 μm.
(11) A niobium sintered body for a capacitor electrode, wherein the pore distribution of the niobium sintered body has a plurality of pore diameter peak tops.
(12) The niobium sintered body as described in 11 above, wherein the pore distribution has two pore diameter peak tops.
(13) The niobium sintered body as described in 11 or 12 above, wherein among the plurality of pore diameter peak tops, the peak tops of the two peaks having a highest relative intensity are present in the range of 0.2 to 0.7 μm and in the range of 0.7 to 3 μm respectively.
(14) The niobium sintered body as described in any one of 11 to 13 above, wherein among the plurality of pore diameter peak tops, the peak top of the peak having a highest relative intensity is present in the larger diameter side than the peak top of the peak having a next highest relative intensity.
(15) The niobium sintered body as described in any one of 9 to 14 above, wherein the sintered body has a volume of 10 mm3 or more including the volume of pore void.
(16) The niobium sintered body as described in any one of 9 to 15 above, wherein the sintered body has a specific surface area of 0.2 to 7 m2/g.
(17) The niobium sintered body as described in any one of 9 to 16 above, wherein a part of the sintered body is nitrided.
(18) The niobium sintered body as described in any one of 12 to 17 above, wherein the sintered body is a sintered body obtained from a niobium molded article which gives a sintered body having a CV value of 40,000 to 200,000 μFV/g when sintered at 1,300° C.
(19) A capacitor comprising one electrode using the niobium sintered body described in any one of 9 to 18 above, a counter electrode and a dielectric material interposed therebetween.
(20) The capacitor as described in 19 above, wherein the dielectric material mainly comprises niobium oxide.
(21) The capacitor as described in 19 above, wherein the counter electrode is at least one material selected from the group consisting of an electrolytic solution, an organic semiconductor and an inorganic semiconductor.
(22) The capacitor as described in 21 above, wherein the counter electrode is an organic semiconductor and the organic semiconductor is at least one material 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 tetracyanoquino-dimethane, and an electrically conducting polymer.
(23) The capacitor as described in 22 above, wherein the electrically conducting polymer is at least one member selected from the group consisting of polypyrrole, polythiophene, polyaniline and substitution derivatives thereof.
(24) The capacitor as described in 22 above, wherein the electrically conducting polymer is an electrically conducting polymer obtained by doping a dopant into a polymer containing a repeating unit represented by the following formula (1) or (2):




(wherein R1 to R4 each independently represents a monovalent group selected from the group consisting of a hydrogen atom, a linear or branched, saturated or unsaturated alkyl, alkoxy or alkylester group having from 1 to 10 carbon atoms, a halogen atom, a nitro group, a cyano group, a primary, secondary or tertiary amino group, a CF3 group, a phenyl group and a substituted phenyl group; each of the pairs of R1 and R2, and R3 and R4 may combine at an arbitrary position to form a divalent chain for forming at least one 3-, 4-, 5-, 6- or 7-membered saturated or unsaturated hydrocarbon cyclic structure together with the carbon atoms substituted by R1 and R2 or by R3 and R4; the cyclic combined chain may contain a bond of carbonyl, ether, ester, amide, sulfide, sulfinyl, sulfonyl or imino at an arbitrary position; X represents an oxygen atom, a sulfur atom or a nitrogen atom; R5 is present only when X is a nitrogen atom, and independently represents a hydrogen atom or a linear or branched, saturated or unsaturated alkyl group having from 1 to 10 carbon atoms).

(25) The capacitor as described in 24 above, wherein the electrically conducting polymer is an electrically conducting polymer containing a repeating unit represented by the following formula (3):




(wherein R6 and R7 each independently represents a hydrogen atom, a linear or branched, saturated or unsaturated alkyl group having from 1 to 6 carbon atoms, or a substituent for forming at least one 5-, 6- or 7-membered saturated hydrocarbon cyclic structure containing two oxygen elements resulting from the alkyl groups combining with each other at an arbitrary position; and the cyclic structure includes a structure having a vinylene bond which may be substituted, and a phenylene structure which may be substituted).

(26) The capacitor as described in 22 above, wherein the electrically conducting polymer is an electrically conducting polymer obtained by doping a dopant into poly(3,4-ethylenedioxythiophene).
(27) The capacitor as described in 19 above, wherein the counter electrode is formed of a material at least partially having a layer structure.
(28) The capacitor as described in 19 above, wherein the material of the counter electrode contains an organic sulfonate anion as a dopant.
(29) A method for producing the niobium powder described in any one of 1 to 8 above, comprising activation-treatment of niobium or a niobium compound.
(30) The method for producing the niobium powder as described in 29 above, wherein the activation treatment of niobium or a niobium compound is performed by at least one step selected from the group consisting of a sintering step and a cracking step.
(31) The method for producing the niobium powder as described in 29 or 30 above, wherein the activation treatment of niobium or a niobium compound is performed using a mixture of niobium or a niobium compound and an activator.
(32) The method for producing the niobium powder as described in any one of 29 to 31 above, wherein the average particle size of the niobium or niobium compound processed with the activation treatment is from 0.01 μm to 10 μm.
(33) The method for producing the niobium powder as described in any one of 29 to 32 above, wherein the niobium or niobium compound contains at least one element selected from the group consisting of nitrogen, carbon, boron and sulfur in an amount of 200,000 ppm or less.
(34) The method for producing the niobium powder as described in any one of 29 to 33 above, wherein the niobium compound is at least one member selected from the group consisting of niobium hydride, niobium alloy and niobium hydride alloy.
(35) The method for producing the niobium powder as described in 34 above, wherein the component other than niobium contained in the niobium alloy or niobium hydride alloy is at least one element selected from the group consisting of elements having an atomic number of 88 or less excluding hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine, niobium, helium, neon, argon, krypton, xenon and radon.
(36) The method for producing the niobium powder as described in 31 above, wherein the mixture containing niobium or a niobium compound and an activator is obtained by mixing these using a solvent.
(37) The method for producing the niobium powder as described in 36 above, wherein the solvent is at least one solvent selected from the group consisting of water, alcohols, ethers, cellosolves, ketones, aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons.
(38) The method for producing the niobium powder as described in 31 above, wherein the activator is used in an amount of 1 to 40 mass % based on the total amount of the niobium or niobium compound.
(39) The method for producing the niobium powder as described in 31 or 38 above, wherein the average particle size of the activator is from 0.01 μm to 500 μm.
(40) The method for producing the niobium powder as described in any one of 31, 38 and 39 above, wherein the activator has a plurality of particle size peak tops.
(41) The method for producing the niobium powder as described in any one of 31 or 38 to 40 above, wherein the activator is a substance which is removed as a gas at 2,000° C. or less.
(42) The method for producing the niobium powder as described in 41 above, wherein the activator is at least one member selected from the group consisting of naphthalene, anthracene, quinone, camphor, polyacrylic acid, polyacrylic acid ester, polyacrylamide, polymethacrylic acid, polymethacrylic acid ester, polymethacrylamide, polyvinyl alcohol, NH4Cl, ZnO, WO2, SnO2 and MnO3.
(43) The method for producing the niobium powder as described in any one of 31 or 38 to 40 above, wherein the activator is at least one member selected from the group consisting of a water-soluble substance, an organic solvent-soluble substance, an acidic solution-soluble substance, an alkaline solution-soluble substance, a substance which forms a complex and becomes a substance soluble in water, organic solvent, acidic solution or alkaline solution, and a substance which becomes a substance soluble in water, organic solvent, acidic solution or alkaline solution at 2,000° C. or less.
(44) The method for producing the niobium powder as described in 43 above, wherein the activator is at least one member selected from the group consisting of compounds of a metal with carbonic acid, sulfuric acid, sulfurous acid, halogen, perhalogen acid, hypohalogen acid, nitric acid, nitrous acid, phosphoric acid, acetic acid, oxalic acid or boric acid, metals, metal hydroxides and metal oxides.
(45) The method for producing the niobium powder as described in 43 above, wherein the activator is at least one member selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, scandium, yttrium, cerium, neodymium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, silver, gold, zinc, cadmium, boron, aluminum, gallium, indium, thallium, silicon, germanium, tin, lead, arsenic, antimony, bismuth, selenium, tellurium, polonium and compounds thereof.
(46) The method for producing the niobium powder as described in any one of 29 to 40 and 43 to 45 above, wherein the activation treatment is a treatment of removing the activator by heating and/or under reduced pressure before or during the sintering step.
(47) The method for producing the niobium powder as described in any one of 29 to 40 and 43 to 45 above, wherein the activation treatment is a treatment of removing the activator component by contacting a solvent with the sintered or cracked product after the sintering step and during or after the cracking step.
(48) The method for producing the niobium powder as described in 47 above, wherein the solvent is at least one member selected from the group consisting of water, an organic solvent, an acidic solution, an alkaline solution and a solution containing a ligand of forming a soluble complex.
(49) The method for producing the niobium powder as described in 48 above, wherein the acidic solution is a solution of at least one member selected from the group consisting of nitric acid, sulfuric acid, hydrofluoric acid and hydrochloric acid.
(50) The method for producing the niobium powder as described in 48 above, wherein the alkaline solution contains at least one member selected from the group consisting of an alkali metal hydroxide and ammonia.
(51) The method for producing the niobium powder as described in 48 above, wherein the ligand is at least one member selected from the group consisting of ammonia, glycine and ethylenediaminetetraacetic acid.
(52) The method for producing the niobium powder as described in 48 above, wherein the organic solvent is methyl isobutyl ketone.
(53) A method for producing a nitrogen-containing niobium powder, comprising treating the niobium powder described in any one of 1 to 7 above by at least one method selected from the group consisting of liquid nitridation, ion nitridation and gas nitridation.
(54) A method for producing a carbon-containing niobium powder, comprising treating the niobium powder described in any one of 1 to 7 above by at least one method selected from the group consisting of solid phase carbonization and liquid carbonization.
(55) A method for producing a boron-containing niobium powder, comprising treating the niobium powder described in any one of 1 to 7 above by at least one method selected from the group consisting of gas boronization and solid phase boronization.
(56) A method for producing a sulfur-containing niobium powder, comprising treating the niobium powder described in 1 or 7 above by at least one method selected from the group consisting of gas sulfudization, ion sulfudization and solid phase sulfudization.
(57) A niobium powder obtained by the production method described in any one of 29 to 56 above.
(58) A method for producing a niobium sintered body, using the niobium powder described in any one of 1 to 8 and 57 above.
(59) A method for producing a capacitor comprising one electrode using a niobium sintered body, a dielectric material formed on the surface of the sintered body and a counter electrode provided on the dielectric material, wherein the niobium sintered body is obtained by sintering the niobium powder described in any one of 1 to 8 and 57 above.
(60) The method for producing a capacitor as described in 59 above, wherein the dielectric material is formed by electrolytic oxidation.
(61) A method for producing a capacitor comprising one electrode using a niobium sintered body, a dielectric material formed on the surface of the sintered body and a counter electrode provided on the dielectric material, wherein the niobium sintered body is the niobium sintered body described in any one of 9 to 18 above.
(62) An electronic circuit using the capacitor described in any one of 19 to 28 above.
(63) An electronic instrument using the capacitor described in any one of 19 to 28 above.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is an enlarged view schematically illustrating the niobium powder of the present invention which has two or more pore diameter peaks.
FIG. 2 is an outline of a pore distribution of the niobium powder measured by a method of mercury penetration.



MODE FOR CARRYING OUT THE INVENTION
The capacitor having good leakage current characteristics and excellent moisture resistance, the niobium sintered body which enables these properties and giving a high capacitance appearance ratio, the niobium powder preferred as a material for the sintered body, having good flowability and capable of continuous molding, and the production methods of the capacitor, the niobium sintered body and the niobium powder are described below.
In the present invention, a niobium powder for capacitors (sometimes simply referred to as a. “niobium powder”) having a tapping density of 0.5 to 2.5 g/ml is used as the niobium powder which satisfies the above-described properties of a capacitor and improves the productivity in the production of capacitors.
The niobium powder for capacitors as used herein means a niobium powder mainly comprising niobium and capable of being a material for producing a capacitor. This niobium powder may contain, for example, a component capable of forming an alloy with niobium, namely, a component other than niobium such as nitrogen and/or oxygen.
A capacitor can be produced by molding and sintering the niobium powder for capacitors using a method as follows to obtain a niobium sintered body for capacitors (sometimes simply referred to as a niobium sintered body) followed by forming a dielectric layer and a counter electrode thereon.
The niobium powder for capacitors is added to a solution obtained by dissolving a binder which is described later, in an organic solvent such as toluene or methanol, and thoroughly mixed using a shaking mixer or a V-type mixer. Then, the organic solvent is distilled off under reduced pressure using a drier such as conical drier to prepare a niobium mixed powder containing the binder. This mixed powder is charged into the hopper of an automatic molding machine, weighed while flowing the niobium powder through an inlet tube from the hopper to the metal mold of the molding machine to automatically cause spontaneous falling in the metal mold, and molded together with a lead wire. After removing the binder, this molded article is sintered at 500 to 2,000° C. under reduced pressure to manufacture a niobium sintered body.
The niobium sintered body is electrochemically formed, for example, in a electrolyte solvent such as phosphoric acid and adipic acid of a concentration of 0.1% by mass and 30° C. to 90° C. at a voltage from 20 to 60 V for 1 to 30 hours to form a dielectric layer mainly comprising a niobium oxide. A solid electrolyte layer such as manganese dioxide, lead dioxide and conductive polymer and a graphite layer and a silver paste layer are formed on the dielectric layer. After a cathode terminal is connected thereon by soldering, these are sealed with a resin to fabricate a solid electrolyte capacitor.
In the case of a mixed powder having no appropriate flowability or angle of repose in molding, the powder does not smoothly flow from the hopper to the metal mold and the molding cannot be stably performed. In particular, since the mixed powder is transported from the hopper using a method such as vibration, too large or too small tapping density or average particle size of the mixed powder leads to large dispersion in the mass of the molded article or in the strength or shape of the sintered body and in some cases, to the generation of chipping or cracking, resulting in a bad leakage current value. Thus, the tapping density, the average particle size, the flowability and the angle of repose of the mixed powder are important factors for producing a good sintered body and a good capacitor.
These physical properties of the mixed powder scarcely change between before and after the mixing with a binder but are determined by the physical properties of the niobium powder for capacitors used. Accordingly, important are the tapping density, the average particle size, the flowability and the angle of repose of the niobium powder used. The flowability and the angle of repose of the niobium powder are greatly affected by the tapping density or the average particle size and therefore, the tapping density and the average particle size become important factors.
In the present invention, for increasing the productivity and the strength of the sintered body accompanying the improvement of the flowability or the angle of repose and thereby obtaining an effect of reducing the leakage current, the tapping density is preferably from 0.5 to 2.5 g/ml, more preferably from 0.8 to 1.9 g/ml. The average particle size of the niobium powder of the present invention is preferably from 10 to 1,000 μm, more preferably from 50 to 200 μm.
For allowing the niobium powder to spontaneously fall from the hopper to the metal mold of a molding machine, the angle of repose of the niobium powder of the present invention is preferably from 10° to 60°, more preferably from 100 to 50°.
The niobium powder having the above-described physical properties can be produced starting from a mixture (hereinafter referred to as “a starting material mixture”) containing a niobium powder or a niobium compound powder (hereinafter these are called “a starting material niobium powder”) and an activator (also called “pore forming material”, and hereinafter sometimes referred to as “an additive”) through at least a sintering step and a cracking step in sequence.
The activator is removed at either the sintering step or the cracking step during the production of the niobium powder of the present invention from the starting material mixture. The removal of the activator may also be performed independently from the sintering step or the cracking step.
Various methods for removing the activator can be arbitrarily employed according to the chemical properties of the activator. One of the methods capable of easily removing the activator may be used or a plurality of these methods may be used in combination.
Examples of the method for removing the activator include a method of evaporating, sublimating or thermally decomposing the activator and removing it as a gas, and a method of removing the activator by dissolving it in a solvent.
In the case of removing the activator as a gas, the removal may be performed at the sintering step, or a step of removing the activator under heating and/or reduced pressure may be provided before the sintering.
In the case of removing the activator by dissolving it in a solvent after the sintering of the starting material mixture or during or after the cracking, a solvent which is described later is contacted with the sintered product or a cracked product thereby dissolving and removing the activator.
A step of nitriding, boronizing, carbonizing or sulfudizing a part of niobium powder may be provided at any stage in the process of producing the niobium powder of the present invention from the starting material mixture.
The method for producing the niobium powder of the present invention is described in detail below.
The starting material niobium powder may be at least one powder selected from niobium, niobium hydride, niobium alloy and niobium hydride alloy. A part of the powder may be nitrided, sulfudized, carbonized or boronized. The “alloy” as used in the present invention includes a solid solution with other alloy component.
The average particle size of the starting material niobium powder is preferably from 0.01 to 10 μm, more preferably 0.02 to 5 μm, still more preferably from 0.05 to 2 μm.
Examples of the method for obtaining niobium used as the starting material niobium powder include a method of hydrogenating, pulverizing and dehydrogenating a niobium ingot, a niobium pellet or a niobium powder, a method of reducing potassium fluoroniobate with sodium or the like and pulverizing the reduction product, a method of reducing niobium oxide using at least one of hydrogen, carbon, magnesium and aluminum and pulverizing the reduction product, and a method of hydrogen-reducing niobium halide.
Examples of the method for obtaining niobium hydride used as the starting material niobium powder include a method of hydrogenating and pulverizing a niobium ingot, a niobium pellet or a niobium powder.
Examples of the method for obtaining a niobium hydride alloy used as the starting material niobium powder include a method of pulverizing a hydride of niobium alloy ingot, niobium alloy pellet or niobium alloy powder. Examples of the method for obtaining a niobium alloy used as the starting material niobium powder include a method of dehydrogenating a niobium hydride alloy obtained as above.
The niobium alloy or niobium hydride alloy contains, as an alloy component other than niobium, at least one element selected from the group consisting of the elements having an atomic number of 88 or less excluding hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine, niobium, helium, neon, krypton, argon, xenon and radon.
The activator is a substance which can be removed at any step during the production of the niobium powder of the present invention from the starting material mixture. In the niobium powder of the present invention, a pore is usually formed in the portion where the activator is removed.
The particle size of the activator affects the pore diameter of the niobium powder of the present invention, the pore diameter of the niobium powder affects the pore diameter of the niobium sintered body, and the pore diameter of the niobium sintered body affects the capacitance of a capacitor and the impregnating ability of a cathode agent in the production step of a capacitor.
The impregnating ability of a cathode agent has a great influence in producing a capacitor with a high capacitance as well as a low ESR. As a niobium sintered body is produced by molding a niobium powder under pressure, the pore diameter of the niobium sintered body is naturally smaller than that of the niobium powder. Considering the difficulty to impregnate a cathode agent to a niobium sintered body produced from a niobium powder having a small pore diameter, the average pore diameter of a niobium powder is preferably 0.5 μm or more, and more preferably 1 μm or more.
The average pore diameter of the niobium powder is preferably from 0.01 to 500 μm, more preferably from 0.03 to 300 μm, still more preferably from 0.1 to 200 μm. For having a pore diameter in this range, the average particle size of the activator is preferably from 0.01 to 500 μm, more preferably from 0.03 to 300 μm, still more preferably from 0.1 to 200 μm.
The average pore diameter of the niobium powder is most preferably from 0.5 μm to 100 μm, and the average particle size of the activator is most preferably from 0.5 to 100 μm.
The pore diameter may be reduced by using an activator having a small particle size and the pore diameter may be increased by using an activator having a large particle size.
The pore diameter distribution can be adjusted by adjusting the particle size distribution of the activator.
In order to obtain a capacitor having a sufficiently large capacitance and free from problems involved in the impregnating ability of a cathode agent, it is preferred to appropriately provide pores small enough to give a desired capacitance and pores large enough to ensure satisfactory impregnation of a cathode agent, in the niobium sintered body according to the physical properties of the cathode agent.
For adjusting the pore diameter distribution of the niobium powder or niobium sintered body, for example, the niobium powder can be made to have a pore diameter distribution having two or more peak tops by using an activator (powder) with a particle size distribution with two or more peak tops. By sintering this niobium powder, a niobium sintered body having two or more peak tops of equal pore diameter in the pore diameter distribution can be obtained. In this case, the pore diameter peak top is preferably present in the range of 0.01 to 500 μm, more preferably from 0.03 to 300 μm, still more preferably from 0.1 to 200 μm, particularly preferably from 0.1 to 30 μm and most preferably from 0.2 to 3 μm.
The niobium powder to produce the niobium sintered body in above has a two or more peak tops in the particle size distribution. Any of the two or more peak tops is preferably 0.5 μm or more. For example, to produce a niobium sintered body having two peak tops in the particle size distribution of 0.7 and 3 μm, the two peak tops of the niobium powder may be adjusted to about 1.5 and about 25 μm. To obtain a niobium powder having a small pore diameter of 1.5 μm and having a large pore diameter of 25 μm, the activator having an average pore diameter of 1.5 μm and 25 μm is to be used respectively. Generally, when a pore of a small diameter and that of a large diameter are present in the niobium powder, a large diameter becomes small during molding under pressure. Therefore, a peak top of a large diameter is preferably 20 μm or more. Also, preferably 30% by volume or more of the entire pore volume has a pore diameter of 20 μm or more, and more preferably 40% by volume or more.
The above example is herein described with details based on drawings. FIG. 1 is an enlarged view schematically illustrating the niobium powder of the present invention. The niobium powder in FIG. 1 is a granulated powder comprising a primary powder which has a specific pore diameter formed by an activator. Pores (A) and (B) are formed by using an activator having an average diameter of about 1.5 μm and about 25 μm respectively. Thus, it is possible to coagulate a primary powder to each other efficiently. FIG. 2 is an outline of a pore distribution of the niobium powder measured by a method of mercury penetration. Peak (A) is the peak corresponding to pore (A) formed by the activator having an average diameter of about 1.5 μm and peak (B) is the peak corresponding to pore (B) formed by the activator having an average diameter of about 25 μm respectively. Peak (B) is higher than peak (A) and 44% of the entire pore volume has a pore diameter of 20 μm or more.
The activator having two or more peak tops in the particle size distribution can be obtained, for example, by mixing two or more activators having a different peak top in the particle size distribution.
Examples of the substance as the activator include a substance which becomes a gas at the sintering temperature or lower, and a substance which is soluble in a solvent at least after the sintering.
Examples of the substance which becomes a gas at the sintering temperature or lower include a substance which becomes a gas through evaporation, sublimation or thermal decomposition. An inexpensive substance capable of easily becoming a gas even at a low temperature without leaving a residue is preferred. Examples thereof include aromatic compounds such as naphthalene, anthracene and quinone, camphor, NH4Cl, ZnO, WO2, SnO2, MnO3 and organic polymers.
Examples of the organic polymer include polyacrylic acid, polyacrylic acid ester, polyacrylamide, polymethacrylic acid, polymethacrylic acid ester, polymethacrylamide and polyvinyl alcohol.
The substance which is soluble at least after the sintering is a substance such that the residue of the activator or a thermally decomposed product thereof is soluble in a solvent. A substance capable of easily dissolving in a solvent which is described later, after the sintering, or during or after the cracking is particularly preferred. Such a substance can be selected from many substances according to the combination with the solvent.
Examples thereof include compounds of a metal with carbonic acid, sulfuric acid, sulfurous acid, halogen, perhalogen acid, hypohalogen acid, nitric acid, nitrous acid, phosphoric acid, acetic acid, oxalic acid or boric acid, metal oxides, metal hydroxides and metals.
Among these, preferred are compounds having a large solubility in a solvent such as acid, alkali or ammonium salt solution which are described later. Examples thereof include compounds containing at least one member selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, scandium, yttrium, cerium, neodymium, erbium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, silver, gold, zinc, cadmium, aluminum, gallium, indium, thallium, germanium, tin, lead, antimony, bismuth, selenium, tellurium, polonium, boron, silicon and arsenic. Among these, preferred are metal salts and more preferred are, for example, barium oxide, manganese(II) nitrate and calcium carbonate.
These activators may be used individually or in combination of two or more thereof.
To form a specific pore diameter efficiently, a substance which exists as a solid at a sintering temperature is preferable as the material of the activator.
The reason is that the activator being present in a solid state at a sintering temperature blocks excessive agglomeration of the primary niobium powder and allows the fusion of the niobium powder only at a contact point to each other. If the activator exists as a liquid or gas at a sintering temperature, it has a small effect to block such agglomeration and may form a smaller pore than desired. Consequently, a pore diameter becomes more stable with an activator comprising a substance of a higher melting point such as barium oxide, calcium carbonate, aluminum oxide and magnesium oxide than an activator comprising a substance of a lower melting point such as an aluminum metal, magnesium metal, magnesium hydride and calcium metal.
If the activator is added in a small amount, the tapping density and the angle of repose become large, whereas if added in a large amount, the tapping density becomes small and closed pores increase at the stage of sintering. For obtaining a repose angle of 600 or less and a tapping density of 0.5 to 2.5 g/ml without the problem of closed pores at the sintering stage, the amount of the activator added is generally from 1 to 40 mass % or less (unless otherwise indicated, mass % is hereinafter simply referred to as %), preferably from 5 to 25%, more preferably from 10 to 20%, based on the starting material niobium, though this varies depending on the average particle size of the activator.
The starting material mixture may be obtained by mixing the activator and the niobium starting material each in the powder form using no solvent or by mixing the activator and the niobium starting material using an appropriate solvent and drying the mixture.
Examples of the solvent which can be used include water, alcohols, ethers, cellosolves, ketones, aliphatic hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons.
The mixing may be performed using a mixer. As for the mixer, a normal apparatus such as shaking mixer, V-type mixer and Nauter mixer may be used without any problem. The temperature at the mixing is limited by the boiling point and freezing point of the solvent but is generally from −50° C. to 120° C., preferably from −50° C. to 50° C., more preferably from −10° C. to 30° C. The time spent for the mixing is not particularly limited insofar as it is 10 minutes or more, however, the mixing is generally performed for 1 to 6 hours preferably in an oxygen-free atmosphere using an inert gas such as nitrogen or argon.
In the case of using a solvent, the mixture obtained is dried at lower than 80° C., preferably lower than 50° C., using a conical drier or a compartment drier. If the mixture is dried at a temperature of 80° C. or higher, the oxygen contained in the niobium or niobium hydride powder disadvantageously increases.
In the case where the activator becomes a gas at the sintering temperature or lower, the activator may be removed at the sintering but a step of forming the activator into a gas and removing it before the sintering may be independently provided by setting the conditions such as temperature, pressure and time period to those of facilitating the removal according to the chemical properties of the activator. In this case, the activator is vaporized, for example, at 100° C. to 800° C. under reduced pressure within a few hours.
In the case of using a niobium hydride or a niobium hydride alloy as the starting material niobium, the dehydrogenation can be attained irrespective of the kind of the activator by performing the above-described step.
The sintering step is performed at 500° C. to 2,000° C., preferably from 800° C. to 1,500° C., more preferably from 1,000° C. to 1,300° C., under reduced pressure or in a reducing atmosphere such as argon. After the completion of sintering, the sintered product is preferably cooled until the niobium temperature (sometimes simply referred to as a “product temperature”) becomes 30° C. or less, an inert gas such as nitrogen or argon containing from 0.01 vol % to 10 vol %, preferably from 0.1 vol % to 1 vol % of oxygen is gradually added such that the product temperature does not exceed 30° C., and after standing for 8 hours or more, the sintered product is taken out to obtain a sintered lump.
In the cracking step, the sintered lump is cracked to an appropriate particle size using a cracking machine such as a roll granulator.
In the case where the activator is soluble in a solvent at least after the sintering step, an appropriate solvent is contacted with the sintered lump or the cracked powder after the sintering and before, during or after the cracking or at a plurality of these steps and thereby, the activator component is dissolved and removed. In view of easiness of the removal, the activator component is preferably dissolved and removed from the cracked powder after cracking.
The solvent used here is a solvent in which the activator to be dissolved has a sufficiently high solubility. A solvent which is inexpensive and hardly remains is preferred. For example, in the case of a water-soluble activator, water may be used; in the case of an organic solvent-soluble activator, an organic solvent such as methyl isobutyl ketone, ethanol or dimethyl sulfoxide (DMSO) may be used; in the case of an acid-soluble activator, an acid solution such as nitric acid, sulfuric acid, phosphoric acid, boric acid, carbonic acid, hydrofluoric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid or organic acid may be used; in the case of an alkali-soluble activator, an alkali solution such as hydroxide of alkali metal, hydroxide of alkaline earth metal or ammonia may be used; and in the case of an activator which forms a soluble complex, a solution of an amine such as ammonia or ethylenediamine, an amino acid such as glycine, a polyphosphoric acid such as sodium tripolyphosphate, a crown ether, a thiosulfate such as sodium thiosulfate, or a chelating agent such as ethylenediaminetetraacetic acid, which becomes a ligand of the complex, may be used.
A solution of an ammonium salt such as ammonium chloride, ammonium nitrate and ammonium sulfate, cation exchange resin and anion exchange resin may also be used suitably. It is preferable to dissolve and remove the activator at a lower temperature. If the activator is dissolved and removed at a high temperature, the surface of a niobium becomes oxidized because a niobium has a high affinity with oxygen. Consequently, a temperature of dissolving and removing is preferably 50° C. or lower, more preferably from −10° C. to 40° C. and still more preferably from 0° C. to 30° C. According to the above reason, it is preferable to select a method generating little heat in dissolving and removing. For example, when using a metal oxide or metal as an activator, dissolving and removing by an acid generally yields a high heat of neutralization. Therefore, it is preferable to select a method which generates little heat such as dissolving in water and an organic solvent, forming a soluble complex such by using a solvent of ammonium nitrate salt, and ethylenediaminetetraacetate and dissolving in a solvent containing ion exchange resin.
Specific examples of the combination of an activator and a solvent include barium oxide and water, calcium oxalate and hydrochloric acid, aluminum oxide and aqueous sodium hydroxide solution, hafnium oxide and methyl isobutyl ketone, and magnesium carbonate and aqueous tetrasodium ethylenediaminetetraacetate solution.
After dissolving and removing the activator, the residue is thoroughly washed and dried. For example, in the case where barium oxide is removed with water, the residue is thoroughly washed using an ion-exchanged solution until the electric conductivity of the washing solution is reduced to 5 μS/cm or less. Subsequently, the product is dried at a product temperature of 50° C. or less under reduced pressure. Here, the amount of the remaining activator or solvent component is usually 100 ppm or less, though this varies depending on the washing conditions.
In order to improve the LC value, the thus-obtained niobium powder, sintered lump and niobium starting material powder may be subjected to nitridation, boronization, carbonization or sulfudization of a part of the niobium powder, or to a plurality of these treatments.
The niobium powder of the present invention may contain the obtained niobium nitride, niobium boride, niobium carbide or niobium sulfide or a plurality of these species. The total content of respective elements of nitrogen, boron, carbon and sulfur varies depending on the shape of the niobium powder, however, it is from 0 to 200,000 ppm, preferably 50 to 100,000 ppm, more preferably 200 to 20,000 ppm. If the total content exceeds 200,000 ppm, the capacitor produced is deteriorated in the capacitance characteristics and not suitable as a capacitor.
The nitridation of the niobium powder can be performed by any one of liquid nitridation, ion nitridation and gas nitridation or by a combination thereof. Among these, gas nitridation in a nitrogen gas atmosphere is preferred because the apparatus therefor is simple and the operation is easy. For example, the gas nitridation in a nitrogen gas atmosphere can be attained by allowing the above-described niobium powder to stand in a nitrogen gas atmosphere. With a nitridation atmosphere temperature of 2,000° C. or less and a standing time of 100 hours or less, a niobium powder having an objective nitrided amount can be obtained. The treatment time can be shortened by performing the treatment at a higher temperature.
The boronization of the niobium powder may be either gas boronization or solid phase boronization. For example, the niobium powder may be boronized by allowing it to stand together with a boron source such as boron pellet or boron halide (e.g., trifluoroboron), at 2,000° C. or less for from 1 minute to 100 hours under reduced pressure.
The carbonization of the niobium powder may be any one of gas carbonization, solid phase carbonization and liquid carbonization. For example, the niobium powder may be carbonized by allowing it to stand together with a carbon source such as carbon material or organic material having carbon (e.g., methane), at 2,000° C. or less for from 1 minute to 100 hours under reduced pressure.
The sulfudization of the niobium powder may be any one of gas sulfudization, ion sulfudization and solid phase sulfudization. For example, the gas sulfudization in a sulfur gas atmosphere can be attained by allowing the niobium powder to stand in a sulfur atmosphere. With a sulfudization atmosphere temperature of 2,000° C. or less and a standing time of 100 hours or less, a niobium powder having an objective sulfudized amount can be obtained. The treatment time can be shortened by performing the treatment at a higher temperature.
The BET specific surface area of the thus-obtained niobium powder of the present invention is usually 0.5 to 40 m2/g, preferably 0.7 to 10 m2/g, more preferably 0.9 to 2 m2/g.
The niobium powder of the present invention may be a mixture of niobium powders different in the tapping density, the particle size, the angle of repose, the BET specific surface area, the pore diameter distribution and the treatment by nitridation, boronization, carbonization or sulfudization.
The sintered body of the present invention, which can be used as an electrode for capacitors, is preferably produced, for example, by sintering the above-described niobium powder of the present invention. For example, the sintered body can be obtained by press-molding the niobium powder into a predetermined shape and then heating it at from 500° C. to 2,000° C., preferably from 800° C. to 1,500° C., more preferably from 1,000° C. to 1,300° C., for 1 minute to 10 hours under a pressure of 10−5 to 102 Pa.
The pore size distribution of the sintered body obtained from the niobium powder of the present invention usually has a pore diameter peak top in the range of 0.01 μm to 500 μm.
By adjusting the applied pressure at the molding to a specific pressure value, the sintered body can have a larger number of pore diameter peak tops than that of the niobium powder. This applied pressure value varies depending on the press-molding conditions such as physical properties of niobium powder, shape of a molded article and molding machine, but is in the range from a pressure capable of press-molding to a pressure where pores of the sintered body are not closed. The preferred pressure value can be determined by a preliminary experiment according to the conditions such as the physical properties of the niobium powder to be molded so as to have a plurality of pore size peak tops. The applied pressure value can be controlled, for example, by controlling the load of the molding machine applied on the molded article.
The pore size distribution of the sintered body preferably has at least two pore size peak tops so as to contain pores small enough to obtain a desired capacitance and pores large enough to ensure satisfactory impregnation of a cathode agent according to the physical properties of the cathode agent. From such a sintered body having a plurality of peak tops in the pore diameter distribution, a capacitor having excellent impregnating ability of a counter electrode and a high capacitance appearance ratio can be produced.
When among a plurality of pore diameter distribution peak tops, the peak tops of two peaks having a highest relative intensity are present in the range from 0.2 to 0.7 μm and in the range from 0.7 μm to 3 μm, respectively, preferably from 0.2 μm to 0.7 μm and from 0.9 μm to 3 μm, respectively, the capacitor produced from this sintered body can have good moisture resistance. Among a plurality of the pore diameter distribution peak tops, the peak top of the peak having a highest relative intensity is preferably present in the larger diameter side than the peak top of the peak having a next highest relative intensity, because the capacitor can have more excellent moisture resistance.
The specific surface area of the thus-produced sintered body is generally from 0.2 m2/g to 7 m2/g.
Usually, as the shape of the sintered body is larger, the impregnation of a counter electrode is more difficult. For example, in the case where the sintered body has a size of 10 mm3 or more, the sintered body of the present invention having a plurality of peak tops in the pore diameter distribution can be particularly effectively used.
The sintered body of the present invention may be partially nitrided. As for the nitridation method, the method and reaction conditions described above with respect to the niobium powder can be employed. It is also possible to previously nitride a part of the niobium powder for use of producing a sintered body and nitride a part of the sintered body produced from this niobium powder.
Such a sintered body usually contains from 500 to 70,000 mass ppm of oxygen, because natural oxidation oxygen contained in the niobium powder before the sintering and oxygen added by the natural oxidation after the sintering are present. In the sintered body of the present invention, the content of elements except for niobium, alloy-forming element, oxygen and nitrogen is usually 400 mass ppm or less.
As one example, when the sintered body of the present invention is sintered at 1,300° C., the CV value (the product of the electrochemical forming voltage in the electrochemical forming at 80° C. for 120 minutes in an aqueous 0.1 mass % phosphoric acid solution and the capacitance at 120 Hz) is 40,000 to 200,000 μFV/g.
The production of a capacitor device is described below.
For example, a lead wire comprising a valve-acting metal such as niobium or tantalum and having appropriate shape and length is prepared and this lead wire is integrally molded at the press-molding of the niobium powder such that a part of the lead wire is inserted into the inside of the molded article, whereby the lead wire can work out to a leading line of the sintered body. Or, the niobium powder is molded and sintered without using a lead wire and then, a lead wire separately prepared is connected thereto by welding or the like.
Using this sintered body as one electrode, a capacitor can be produced by interposing a dielectric material between this electrode and a counter electrode. For example, a capacitor is fabricated by using a niobium sintered body as one electrode, forming a dielectric material on the surface of the sintered body (including the inside surface of pore) and providing a counter electrode on the dielectric material.
The dielectric material used here for the capacitor is preferably a dielectric material mainly comprising niobium oxide, more preferably a dielectric material mainly comprising niobium pentaoxide. The dielectric material mainly comprising niobium pentaoxide can be obtained, for example, by electrolytically oxidizing the niobium sintered body as one electrode. For electrolytically oxidizing the niobium electrode in an electrolytic solution, an aqueous protonic acid solution is generally used, such as aqueous 0.1% phosphoric acid solution, aqueous sulfuric acid solution, aqueous 1% acetic acid solution or aqueous adipic acid solution. In the case of obtaining a niobium oxide dielectric material by electrochemically forming the niobium electrode in an electrolytic solution as such, the capacitor of the present invention is an electrolytic capacitor and the niobium electrode serves as an anode.
In the capacitor of the present invention, the counter electrode to the niobium sintered body is not particularly limited and for example, at least one material (compound) selected from electrolytic solutions, organic semiconductors and inorganic semiconductors known in the art of aluminum electrolytic capacitor, may be used.
Specific examples of the electrolytic solution include a dimethylformamide-ethylene glycol mixed solution having dissolved therein 5 mass % of an isobutyltripropylammonium borotetrafluoride electrolyte, and a propylene carbonate-ethylene glycol mixed solution having dissolved therein 7 mass % of tetraethylammonium borotetrafluoride.
Specific examples of the organic semiconductor include an organic semiconductor comprising benzenepyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane, and an electrically conducting polymer containing a repeating unit represented by the following formula (1) or (2):




wherein R1 to R4 each independently represents a monovalent group selected from the group consisting of a hydrogen atom, a linear or branched, saturated or unsaturated alkyl, alkoxy or alkylester group having from 1 to 10 carbon atoms, a halogen atom, a nitro group, a cyano group, a primary, secondary or tertiary amino group, a CF3 group, a phenyl group and a substituted phenyl group; each of the pairs of R1 and R2, and R3 and R4 may combine at an arbitrary position to form a divalent chain for forming at least one 3-, 4-, 5-, 6- or 7-membered saturated or unsaturated hydrocarbon cyclic structure together with the carbon atoms substituted by R1 and R2 or by R3 and R4; the cyclic combined chain may contain a bond of carbonyl, ether, ester, amide, sulfide, sulfinyl, sulfonyl or imino at an arbitrary position; X represents an oxygen atom, a sulfur atom or a nitrogen atom; R5 is present only when X is a nitrogen atom, and independently represents a hydrogen atom or a linear or branched, saturated or unsaturated alkyl group having from 1 to 10 carbon atoms.

In the present invention, R1 to R4 in formula (1) or (2) each independently preferably represents a hydrogen atom or a linear or branched, saturated or unsaturated alkyl or alkoxy group having from 1 to 6 carbon atoms, and each of the pairs of R1 and R2, and R3 and R4 may combine to form a ring.
In the present invention, the electrically conducting polymer containing a repeating unit represented by formula (1) is preferably an electrically conducting polymer containing a structure unit represented by the following formula (3) as a repeating unit:




wherein R6 and R7 each independently represents a hydrogen atom, a linear or branched, saturated or unsaturated alkyl group having from 1 to 6 carbon atoms, or a substituent for forming at least one 5-, 6- or 7-membered saturated hydrocarbon cyclic structure containing two oxygen elements resulting from the alkyl groups combining with each other at an arbitrary position; and the cyclic structure includes a structure having a vinylene bond which may be substituted, and a phenylene structure which may be substituted.

The electrically conducting polymer containing such a chemical structure is doped with a dopant and for the dopant, known dopants can be used without limitation.
Specific examples of the inorganic semiconductor include an inorganic semiconductor mainly comprising lead dioxide or manganese dioxide, and an inorganic semiconductor comprising triiron tetraoxide. These semiconductors may be used individually or in combination of two or more thereof.
Examples of the polymer containing a repeating unit represented by formula (1) or (2) include polyaniline, polyoxyphenylene, polyphenylene sulfide, polythiophene, polyfuran, polypyrrole, polymethylpyrrole, and substitution derivatives and copolymers thereof. Among these, preferred are polypyrrole, polythiophene and substitution derivatives thereof (e.g., poly(3,4-ethylenedioxythiophene)).
When the organic or inorganic semiconductor used has an electrical conductivity of 10−2 S/cm to 103 S/cm, the capacitor produced can have a smaller impedance value and can be increased in the capacitance at a high frequency.
The electrically conducting polymer layer is produced, for example, by a method of polymerizing a polymerizable compound such as aniline, thiophene, furan, pyrrole, methylpyrrole or a substitution derivative thereof under the action of an oxidizing agent capable of satisfactorily undergoing an oxidation reaction of dehydrogenative two-electron oxidation. Examples of the polymerization reaction from the polymerizable compound (monomer) include vapor phase polymerization and solution polymerization. The electrically conducting polymer layer is formed on the surface of the niobium sintered body having thereon a dielectric material. In the case where the electrically conducting polymer is an organic solvent-soluble polymer capable of solution coating, a method of coating the polymer on the surface of the sintered body to form an electrically conducting polymer layer is used.
One preferred example of the production method using the solution polymerization is a method of dipping the niobium sintered body having formed thereon a dielectric layer in a solution containing an oxidizing agent (Solution 1) and subsequently dipping the sintered body in a solution containing a monomer and a dopant (Solution 2), thereby performing the polymerization to form an electrically conducting polymer layer on the surface of the sintered body. Also, the sintered body may be dipped in Solution 1 after it is dipped in Solution 2. Solution 2 used in the above-described method may be a monomer solution not containing a dopant. In the case of using a dopant, the dopant may be allowed to be present together in the solution containing an oxidizing agent.
The operation of performing these polymerization steps is repeated once or more, preferably from 3 to 20 times, per the niobium sintered body having thereon a dielectric material, whereby a dense and stratified electrically conducting polymer layer can be easily formed.
In the production method of a capacitor of the present invention, any oxidizing agent may be used insofar as it does not adversely affect the capacitor performance and the reductant of the oxidizing agent can work out to a dopant and elevate the electrically conductivity of the electrically conducting polymer. An industrially inexpensive compound easy to handle at the production is preferred.
Specific examples of the oxidizing agent include Fe(III)-base compounds such as FeCl3, FeClO4 and Fe (organic acid anion) salt; anhydrous aluminum chloride/cupurous chloride; alkali metal persulfates; ammonium persulfates; peroxides; manganeses such as potassium permanganate; quinines such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachloro-1,4-benzoquinone and tetracyano-1,4-benzoquinone; halogens such as iodine and bromine; peracid; sulfonic acid such as sulfuric acid, fuming sulfuric acid, sulfur trioxide, chlorosulfuric acid, fluorosulfuric acid and amidosulfuric acid; ozone; and a mixture of a plurality of these oxidizing agents.
Examples of the fundamental compound of the organic acid anion for forming the above-described Fe (organic acid anion) salt include organic sulfonic acid, organic carboxylic acid, organic phosphoric acid and organic boric acid. Specific examples of the organic sulfonic acid include benzenesulfonic acid, p-toluenesulfonic acid, methanesulfonic acid, ethanesulfonic acid, α-sulfo-naphthalene, β-sulfonaphthalene, naphthalenedisulfonic acid and alkylnaphthalenesulfonic acid (examples of the alkyl group include butyl, triisopropyl and di-tert-butyl).
Specific examples of the organic carboxylic acid include acetic acid, propionic acid, benzoic acid and oxalic acid. Furthermore, polymer electrolyte anions such as polyacrylic acid, polymethacrylic acid, polystyrene-sulfonic acid, polyvinylsulfonic acid, poly-α-methylsulfonic acid, polyvinylsulfuric acid and polyethylenesulfonic acid and polyphosphoric acid may also be used in the present invention. These organic sulfuric acids and organic carboxylic acids are mere examples and the present invention is not limited thereto. Examples of the counter cation to the above-described anion include alkali metal ions such as H+, Na+ and K+, and ammonium ions substituted by a hydrogen atom, a tetramethyl group, a tetraethyl group, a tetrabutyl group or a tetraphenyl group, however, the present invention is not limited thereto. Among these oxidizing agents, preferred are oxidizing agents containing a trivalent Fe-base compound, cuprous chloride, an alkali persulfate, an ammonium persulfate or a quinone.
For the anion having a dopant ability which is allowed 12 to be present together, if desired, in the production of a polymer composition for the electrically conducting polymer (anion other than the reductant anion of the oxidizing agent), an electrolyte anion having as a. counter anion an oxidizing agent anion (a reductant of oxidizing agent) produced from the above-described oxidizing agent, or other electrolyte anion may be used. Specific examples thereof include protonic acid anions including halide anion of Group 5B elements such as PF6−, SbF6− and AsF6−; halide anion of Group 3B elements such as BF4−; halogen anion such as I−(I3−), Br− and Cl−; perhalogenate anion such as ClO4−; Lewis acid anion such as AlCl4−, FeCl4− and SnCl5−; inorganic acid anion such as NO3− and SO42−; sulfonate anion such as p-toluenesulfonic acid, naphthalenesulfonic acid and alkyl-substituted naphthalenesulfonic acid having from 1 to 5 carbon atoms (hereinafter simply referred to as “C1-5”); organic sulfonate anion such as CF3SO3− and CH3SO3−; and carboxylate anion such as CH3COO− and C6H5COO−.
Other examples include polymer electrolyte anions such as polyacrylic acid, polymethacrylic acid, polystyrene-sulfonic acid, polyvinylsulfonic acid, polyvinylsulfuric acid, poly-α-methylsulfonic acid, polyethylenesulfonic acid and polyphosphoric acid. However, the present invention is not limited thereto. Among these anions, preferred is a high molecular or low molecular organic sulfonic acid compound or polyphosphoric acid compound. Preferably, an aromatic sulfonic acid compound (e.g., sodium dodecylbenzenesulfonate, sodium naphthalenesulfonate) is used as the anion-donating compound.
Among the organic sulfonate anions, more effective dopants are a sulfoquinone compound having one or more sulfo-anion group (—SO3−) within the molecule and having a quinone structure, and an anthracene sulfonate anion.
Examples of the fundamental skeleton for the sulfoquinone anion of the above-described sulfoquinone compound include p-benzoquinone, o-benzoquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, 2,6-naphthoquinone, 9,10-anthraquinone, 1,4-anthraquinone, 1,2-anthraquinone, 1,4-chrysenequinone, 5,6-chrysenequinone, 6,12-chrysenequinone, acenaphthoquinone, acenaphthenequinone, camphorquinone, 2,3-bornanedione, 9,10-phenanthrenequinone and 2,7-pyrenequinone.
In the case where the counter electrode is solid, an electrically conducting layer may be provided thereon so as to attain good electrical contact with an exterior leading line (for example, lead frame) which is used, if desired.
The electrically conducting layer can be formed, for example, by the solidification of an electrically conducting paste, the plating, the metallization or the formation of a heat-resistant electrically conducting resin film. Preferred examples of the electrically conducting paste include silver paste, copper paste, aluminum paste, carbon paste and nickel paste; and these may be used individually or in combination of two or more thereof. In the case of using two or more kinds of pastes, the pastes may be mixed or may be superposed one on another as separate layers. The electrically conducting paste applied is then solidified by allowing it to stand in air or under heating. Examples of the plating include nickel plating, copper plating, silver plating and aluminum plating. Examples of the metal vapor-deposited include aluminum, nickel, copper and silver.
More specifically, for example, carbon paste and silver paste are stacked in this order on the second electrode and these are molded with a material such as epoxy resin, thereby fabricating a capacitor. This capacitor may have a niobium or tantalum lead which is sintered and molded integrally with the niobium sintered body or welded afterward.
The thus-fabricated capacitor of the present invention is jacketed using, for example., resin mold, resin case, metallic jacket case, dipping of resin or laminate film, and then used as a capacitor product for various uses.
In the case where the counter electrode is liquid, the capacitor fabricated comprising the above-described two electrodes and a dielectric material is housed, for example, in a can electrically connected to the counter electrode to complete the capacitor. In this case, the electrode side of the niobium sintered body is guided outside through a niobium or tantalum lead described above and at the same time, insulated from the can using an insulating rubber or the like.
By producing a sintered body for capacitors using the niobium powder produced according to the embodiment of the present invention described in the foregoing pages and producing a capacitor from the sintered body, a capacitor having a small leakage current and good reliability can be obtained.
The capacitor of the present invention has a larger electrostatic capacitance for the volume than the conventional tantalum capacitors and therefore, a more compact capacitor product can be obtained.
The capacitor of the present invention having such properties can be applied to uses, for example, as a by-pass capacitor or a coupling capacitor which are frequently used in an analogue circuit and a digital circuit, and also to uses of conventional tantalum capacitors.
In general, such a capacitor is frequently used in an electronic circuit and when the capacitor of the present invention is used, the limitation in the arrangement of electronic parts or the discharge of heat can be relieved, as a result, an electronic circuit having high reliability can be disposed in a narrower space than that necessary for conventional electronic circuits.
Furthermore, when the capacitor of the preseft invention is used, an electronic instrument having smaller size and higher reliability than conventional ones can be obtained, such as a computer, computer peripheral equipment (e.g., a PC card), mobile equipment (e.g., a portable telephone), home appliance, equipment mounted on vehicles, artificial satellite and communication equipment.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is described in detail below by referring to Examples and Comparative Examples, however, the present invention is not limited to these Examples.
In each Example, the tapping density, the angle of repose, the particle size and the pore diameter of the niobium powder and the capacitance, leakage current, capacitance appearance ratio, moisture resistance of the capacitor and ESR value of the capacitor were measured by the following methods.
(1) Measurement of Tapping Density
The tapping density was measured in accordance with the Method by Tapping Apparatus and the Measuring Instrument in the Apparent Specific Gravity Measuring Method of Industrial Sodium Carbonate specified in JIS (Japanese Industrial Standard, Edition of 2000) K1201-1.
(2) Measurement of Angle of Repose
The angle of repose was measured using the flowability measuring instrument and the sample amount specified in JIS (Japanese Industrial Standard, Edition of 2000) Z2504. More specifically, niobium powder was dropped on a horizontal plane from the hopper whose lower end was at a height of 6 cm above the horizontal plane and the angle of the slant face from the apex of the circular cone generated to the horizontal plane was designated as the angle of repose.
(3) Measurement of Particle Size
Using an apparatus manufactured by Microtrack (HRA 9320-X100), the particle size distribution was measured by the laser diffraction scattering method. A particle size value (D50; μm) when the accumulated volume % corresponded to 50 volume % was designated as the average particle size.
(4) Measurement of Pore Diameter
Using Poresizer 9320 manufactured by Micromeritics, the pore size distribution was measured by the mercury porosimetry method.
In the present invention, the maximal value was determined from the rate of change in the press-fitted amount and by defining the pore diameter shown by the maximal value as the peak top, the maximal value was used as the size of relative intensity of the peak to which this peak top belongs.
(5) Measurement of Capacitance of Capacitor
The LCR meter manufactured by Hewlett-Packard was connected between terminals of the produced chip at room temperature and the measured capacitance at 120 Hz was defined as the capacitance of the capacitor processed into a chip.
(6) Measurement of Leakage Current of Capacitor
The current value measured after a d.c. voltage of 6.3 V was continuously applied between terminals of the produced chip for 1 minute at room temperature was defined as the leakage current value of the capacitor processed into a chip.
(7) Capacitance Appearance Ratio of Capacitor
Assuming that the capacitance when a sintered body electrochemically formed in an aqueous 0.1% phosphoric acid solution for 1,000 minutes under the conditions of 80° C. and 20 V was measured in 30% sulfuric acid was 100%, the capacitance appearance ratio was expressed by the ratio to the capacitance after a capacitor was produced.
(8) Moisture Resistance Value of Capacitor
The moisture resistance value was expressed by the number of units where the capacitance after the produced capacitor was left standing at 60° C. and 95% RH for 500 hours was less than 110% or less than 120% of the initial value. As the number of units of less than 110% is larger, the moisture resistance value was judged better.
(9) Measurement of ESR Value of Capacitor
The LCR meter manufactured by Hewlett-Packard was connected between terminals of the produced chip at room temperature and the measured ESR value at 100 kHz, 1.5 VDC and 0.5 Vrms was defined as the ESR value of the capacitor.
EXAMPLE 1
Into a nickel-made crucible, 5,000 g of potassium fluoroniobate thoroughly vacuum-dried at 80° C. and sodium in a molar amount of 10 times the potassium fluoroniobate were charged and allowed to undertake a reduction reaction at 1,000° C. for 20 hours under an argon atmosphere. After the completion of reaction, the reduction product was cooled, washed with water, washed with 95% sulfuric acid and then with water in sequence, vacuum-dried and pulverized for 40 hours using a ball mill of alumina pot containing silica alumina balls. The pulverized product was dipped and stirred in a 3:2 (by mass) mixed solution of 50% nitric acid and 10% aqueous hydrogen peroxide. Thereafter, the pulverized product was thoroughly washed with water until the pH reached 7 to remove impurities, and then vacuum-dried. The starting material niobium powder had an average particle size of 1.2 μm.
In a niobium-made pot, 500 g of this starting material niobium powder was placed and thereto, 50 g of butyl polymethylmethacrylate and 1 liter of toluene were added. Furthermore, zirconia balls were added and the contents were mixed for 1 hour using a shaking mixer. After removing zirconia balls, the mixture was placed in a conical drier and vacuum-dried under the conditions of 1×102 Pa and 80° C.
Subsequently, this niobium powder was heated under 1×10−2 Pa at 250 to 400° C. for 12 hours to decompose and remove butyl polymethylmethacrylate, and then sintered under reduced pressure of 4×10−3 Pa at 1,150° C. for 2 hours. The niobium sintered lump was cooled until the product temperature was lowered to 30° C. or lower, and then cracked using a roll granulator to obtain a niobium cracked powder having an average particle size of 100 μm.
This niobium cracked powder was subjected to a nitridation treatment at 300° C. for 2 hours by passing nitrogen therethrough under pressure and about 450 g of niobium powder was obtained. The nitrogen content was 0.22%.
The physical properties of this niobium powder, such as tapping density, average particle size, angle of repose, BET specific surface area and pore diameter peak top are shown in Table 1.
The thus-obtained niobium powder (about 0.1 g) was charged into the hopper of a tantalum device automatic molding machine (TAP-2R, manufactured by Seiken) and automatically molded together with a 0.3 mmφ niobium wire to manufacture a molded article having a size of approximately 0.3 cm×0.18 cm×0.45 cm. The outer appearance and the dispersion in the mass of the molded article are shown in Table 1.
This molded article was left standing in a vacuum of 4×10−3 Pa at 1,250° C. for 30 minutes to obtain a sintered body. 100 Units of this sintered body were prepared and each was electrochemically formed using an aqueous 0.1% phosphoric acid solution at a voltage of 20 V for 200 minutes to form an oxide dielectric film on the surface.
Subsequently, an operation of dipping the sintered body in an aqueous 60% manganese nitrate solution and then heating it at 220° C. for 30 minutes was repeated to form a manganese dioxide layer as the counter electrode layer on the oxide dielectric film. On this counter electrode layer, a carbon layer and a silver paste layer were stacked in this order. After mounting a lead frame thereon, the device as a whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio of the capacitor, and the average capacitance and the average leakage current (hereinafter simply referred to as “LC”) of the chip-type capacitors (n=100 units) are shown in Table 1. The LC value is a value measured at room temperature by applying a voltage of 6.3 V for 1 minute.
EXAMPLE 2
In an SUS 304-made reactor, 1,000 g of a niobium ingot was placed and thereinto, hydrogen was continuously introduced at 400° C. for 10 hours. After cooling, the hydrogenated niobium lump was placed in an SUS 304-made pot containing zirconia balls and pulverized for 10 hours. Thereafter, this hydride was formed into a 20 vol % slurry with water, charged together with zirconia balls into a spike mill and wet-pulverized at 40° C. or lower for 7 hours to obtain a pulverized slurry of niobium hydride. This starting material niobium hydride powder had an average particle size of 0.9 μm.
In an SUS-made pot, this slurry (slurry concentration of 98%) was charged and thereto, 200 g of barium oxide having an average particle size of 1 μm was added. Furthermore, zirconia balls were added and the contents were mixed for 1 hour using a shaking mixer. After removing zirconia balls, the mixture was placed in a niobium-made vat and vacuum-dried under the conditions of 1×102 Pa and 50° C.
Subsequently, the obtained mixture was heated under 1×10−2 Pa at 400° C. for 4 hours to dehydrogenate the niobium hydride, and then sintered under reduced pressure of 4×10−3 Pa at 1,100° C. for 2 hours. The resulting barium oxide-mixed niobium sintered lump was cooled until the product temperature was lowered to 30° C. or lower, and then cracked using a roll granulator to obtain a barium oxide-mixed niobium cracked powder having an average particle size of 95 μm.
Into a polytetrafluoroethylene container, 500 g of this barium oxide-mixed niobium cracked powder and 1,000 g of ion-exchanged water were charged and cooled to 15° C. or lower. Separately, an aqueous solution obtained by mixing 600 g of 60% nitric acid, 150 g of 30% hydrogen peroxide and 750 g of ion-exchanged water and cooled to 15° C. or lower was prepared. Then, 500 g of this aqueous solution was added dropwise with stirring to an aqueous solution having suspended therein the barium oxide-mixed niobium cracked powder while taking care not to allow the water temperature to exceed 20° C. After the completion of dropwise addition, the solution was continuously stirred for another 1 hour, left standing for 30 minutes and then decanted. Thereto, 2,000 g of ion-exchanged water was added and the resulting solution was stirred for 30 minutes, left standing for 30 minutes and then decanted. This operation was repeated 5 times. Thereafter, the niobium cracked powder was charged into a Teflon-made column and washed with water for 4 hours while flowing ion-exchanged water. At this time, the electrical conductivity of the washing water was 0.9 μS/cm.
After the completion of water washing, the niobium cracked powder was dried at 50° C. under reduced pressure and subjected to a nitridation treatment at 300° C. for 3 hours by passing nitrogen therethrough under pressure, as a result, about 350 g of niobium powder was obtained. The nitrogen content was 0.28%.
The physical properties of this niobium powder, such as tapping density, average particle size, angle of repose, BET specific surface area and average pore diameter, are shown in Table 1.
The thus-obtained niobium powder (about 0.1 g) was charged into the hopper of a tantalum device automatic molding machine (TAP-2R, manufactured by Seiken) and automatically molded together with a 0.3 mmφ niobium wire to manufacture a molded article having a size of approximately 0.3 cm×0.18 cm×0.45 cm. The outer appearance and the dispersion in the mass of the molded article are shown in Table 1.
This molded article was left standing under reduced pressure of 4×10−3 Pa at 1,250° C. for 30 minutes to obtain a sintered body. 100 Units of this sintered body were prepared and each was electrochemically formed using an aqueous 0.1% phosphoric acid solution at a voltage of 20 V for 200 minutes to form an oxide dielectric film on the surface.
Subsequently, an operation of contacting the oxide dielectric film with an equivalent mixed solution of an aqueous 10% ammonium persulfate solution and an aqueous 0.5% anthraquinonesulfonic acid solution and then with pyrrole vapor was repeated at least 5 times to form a counter electrode comprising polypyrrole on the oxide dielectric film.
On this counter electrode, a carbon layer and a silver paste layer were stacked in this order. After mounting a lead frame thereon, the device as a whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio of this capacitor, and the average capacitance and the average LC value of the chip-type capacitors (n=100 units) are shown in Table 1. The LC value is a value measured at room temperature by applying a voltage of 6.3 V for 1 minute.
EXAMPLES 3 TO 10
Niobium powders, molded articles thereof, sintered bodies and capacitors were produced in the same manner as in Example 1 except for changing the average particle size and the amount added of the butyl polymethylmethacrylate, or in the same manner as in Example 2 except for changing the average particle size and the amount added of the barium oxide. The physical properties of niobium powder and the outer appearance and the dispersion in the mass of molded article, and the capacitance and the LC of capacitor are shown in Table 1.
EXAMPLES 11 TO 22
Niobium powders, molded articles and sintered bodies of Examples 11 to 14 and 16 to 18 were produced in the same manner as in Example 1 and niobium powders, molded articles and sintered bodies of Examples 15 and 19 to 22 were produced in the same manner as in Examples 2, each except for using the activator shown in Table 1 in place of the butyl polymethylmethacrylate or barium oxide. The physical properties of niobium powder, and the outer appearance and the dispersion in the mass of molded article are shown in Table 1.
These molded articles were then left standing under reduced pressure of 4×10−3 Pa at 1,250° C. for 30 minutes to obtain sintered bodies. 100 Units of each sintered body were prepared and electrochemically formed using an aqueous 0.1% phosphoric acid solution at a voltage of 20 V for 200 minutes to form an oxide dielectric film on the surface.
Subsequently, each sintered body having formed thereon a dielectric material was dipped in an aqueous solution containing 25 mass % of ammonium persulfate (Solution 1), pulled up, dried at 80° C. for 30 minutes, dipped in an isopropanol solution containing 18 mass % of 3,4-ethylenedioxythiophene (Solution 2), pulled up and then left standing in an atmosphere of 60° C. for 10 minutes, thereby performing the oxidation polymerization. This sintered body was again dipped in Solution 1 and then treated in the same manner as above. The operation from the dipping in Solution 1 until the oxidation polymerization was repeated 8 times. Then, the sintered body was washed with warm water at 50° C. for 10 minutes and dried at 100° C. for 30 minutes, whereby a counter electrode comprising electrically conducting poly(3,4-ethylenedioxythiophene) was formed.
On this counter electrode, a carbon layer and a silver paste layer were stacked in this order. After mounting a lead frame thereon, the device as a whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio of this capacitor, and the average capacitance and the average LC value of the chip-type capacitors (n=100 units) are shown in Table 1. The LC value is a value measured at room temperature by applying a voltage of 6.3 V for 1 minute.
EXAMPLES 23 TO 25
Niobium powders, sintered bodies and capacitors were produced in the same manner as in Example 2 except for using, as the starting material, a niobium-tin alloy powder in Example 23, a niobium hydride-rhenium alloy powder in Example 24, and a niobium hydride-yttrium-boron alloy powder in Example 25. The physical properties, the capacitance and the LC of the capacitors are shown in Table 1.
COMPARATIVE EXAMPLES 1 TO 3
Into a nickel-made crucible, 2,000 g of potassium fluoroniobate thoroughly vacuum-dried at 80° C. and sodium in a molar amount of 10 times the potassium fluoroniobate were charged and allowed to undertake a reduction reaction at 1,000° C. for 20 hours in an argon atmosphere. After the completion of reaction, the reduction product was cooled, washed with water, washed with 95% sulfuric acid and then with water in sequence, vacuum-dried and pulverized using a ball mill of alumina pot containing silica alumina balls by varying the pulverization time period. The pulverized products each was dipped and stirred in a 3:2 (by mass) mixed solution of 50% nitric acid and 10% aqueous hydrogen peroxide. Thereafter, each pulverized product was thoroughly washed with water until the pH reached 7 to remove impurities, and then vacuum-dried. The produced niobium powder had an average particle size of 1.3 to 10 μm.
Subsequently, 50 g of the obtained niobium powder was placed in an SUS 304-made reactor and thereinto, nitrogen was continuously introduced at 300° C. for 2 to 4 hours to obtain a niobium nitride.
The physical properties of each niobium powder, such as tapping density, average particle size, angle of repose, BET specific surface area and average pore diameter, are shown in Table 1.
The thus-obtained niobium powder (about 0.1 g) was charged into the hopper of a tantalum device automatic molding machine (TAP-2R, manufactured by Seiken) and the automatic molding together with a 0.3 mmφ niobium wire was attempted. The results obtained are shown in Table 1.
COMPARATIVE EXAMPLES 4 TO 9
Niobium powders having a tapping density of 0.2 to 0.4 g/ml or 2.6 to 3.3 g/ml were obtained in the same manner as in Example 2 except for changing the amount added of the barium oxide having an average particle size of 1 μm. The physical properties thereof are shown in Table 1.
Each of the thus-obtained niobium powders (about 0.1 g) was charged into the hopper of a tantalum device automatic molding machine (TAP-2R, manufactured by Seiken) and automatically molded together with a 0.3 mmφ niobium wire to manufacture molded articles having a size of approximately 0.3 cm×0.18 cm×0.45 cm. The outer appearance and the dispersion in the mass of these molded articles are shown in Table 1.
These molded articles were left standing in a vacuum of 4×10−3 Pa at 1,250° C. for 30 minutes to obtain sintered bodies. 100 Units of each sintered body were prepared and electrochemically formed using an aqueous 0.1% phosphoric acid solution at a voltage of 20 V for 200 minutes to form an oxide dielectric film on the surface.
Subsequently, an operation of contacting the oxide dielectric film with an equivalent mixed solution of an aqueous 10% ammonium persulfate solution and an aqueous 0.5% anthraquinonesulfonic acid solution and then with pyrrole vapor was repeated at least 5 times, whereby a counter electrode comprising polypyrrole was formed on the oxide dielectric film.
On this counter electrode, a carbon layer and a silver paste layer were stacked in this order. After mounting a lead frame thereon, the device as a whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio of the capacitor and the average capacitance and the average LC value of the chip-type capacitors (n=100 units) are shown in Table 1. The LC value is a value measured at room temperature by applying a voltage of 6.3 V for 1 minute.
EXAMPLES 26 TO 31
A hydride of niobium ingot was pulverized and dehydrogenated to obtain primary particles having an average particle size of 0.8 μm. The obtained primary particles were sintered and pulverized to obtain a niobium granulated, powder. Then, 0.1 g of this granulated powder was charged in a metal mold (4.0 mm×3.5 mm×1.8 mm) together with a separately prepared niobium wire having a length of 10 mm and a thickness of 0.3 mm and a load was applied thereto as shown in Table 2 using a tantalum device automatic molding machine (TAP-2R, manufactured by Seiken) to produce molded articles. The molded articles each was then sintered at 1,300° C. for 30 minutes to obtain an objective sintered body. By controlling the load applied by the molding machine, sintered bodies having a pore diameter distribution shown in Table 2 were produced. The size, the specific surface area and the CV value of the sintered body of Example 26 were 24.7 mm3, 1.1 m2/g and 85,000 μFV/g, respectively. In other Examples, each value was within ±2% of Example 26.
EXAMPLES 32 TO 34
Sintered bodies were obtained in the same manner as in Examples 26 to 28 except for changing the average particle size of the primary particles to 0.5 μm by classifying the primary particles. The size, the specific surface area and the CV value of the sintered body of Example 32 were 24.9 mm3, 1.5 m2/g and 125,000 μFV/g, respectively. In other Examples, each value was within ±1% of Example 32. The pore diameter distribution of each sintered body produced is shown in Table 2.
EXAMPLE 35
A sintered body was obtained in the same manner as in Example 31 except for using a niobium powder obtained in the same manner as in Example 4 in place of the granulated powder. The size, the specific surface area and the CV value of the sintered body of Example 35 were 24.8 mm3, 1.2 m2/g and 78,000 μFV/g, respectively. The pore diameter distribution of the sintered body produced is shown in Table 2.
COMPARATIVE EXAMPLES 10 TO 12
Sintered bodies were produced in the same manner as in Examples 26 to 28 except that a niobium powder obtained by heat-treating at 1,100° C. a niobium powder prepared by reducing niobium chloride with magnesium was used in place of the niobium granulated powder used in Examples 26 to 28. The size, the specific surface area and the CV value of the sintered body of Comparative Example 10 were 24.3 mm3, 0.8 m2/g and 84,000 μFV/g, respectively. In other Examples, each value was within ±2% of Comparative Example 10. The pore diameter distribution of each sintered body produced is shown in Table 2.
EXAMPLE 36
60 Units of respective sintered bodies produced in the same manner as in Example 21 and Examples 26 to 35 each was electrochemically formed in an aqueous 0.1% phosphoric acid solution at 80° C. and 20 V for 1,000 minutes to form an oxide dielectric film on the surface of the sintered body. These sintered bodies after the electrochemical forming were divided into groups each consisting of 30 units. 30 Units of the sintered body in each group were impregnated with two kinds of cathode agents A and B shown in Table 3. Thereon, carbon paste and silver paste were stacked in this order and the device was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio and the moisture resistance value of each capacitor manufactured are shown in Table 4.
COMPARATIVE EXAMPLE 13
60 Units of respective sintered bodies produced in the same manner as in Comparative Examples 9 to 12 each was electrochemically formed in an aqueous 0.1% phosphoric acid solution at 80° C. and 20 V for 1,000 minutes to form an oxide dielectric film on the surface of the sintered body. These sintered bodies after the electrochemical forming were divided into groups each consisting of 30 units. 30 Units of the sintered body in each group were impregnated with cathode agent A shown in Table 3. Thereon, carbon pate and silver paste were stacked in this order and the device was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio and the moisture resistance value of each capacitor manufactured are shown in Table 4.
EXAMPLE 37
Slurry was obtained by pulverizing the starting material niobium hydride powder in the same manner as in Example 2. The niobium hydride powder had an average particle size of 0.6 μm. After centrifugal sedimentation of the slurry, the supernatant was removed by decantation. Thereto, acetone anhydrous was added so to make a concentration of the slurry 40% by mass and well suspended. After centrifugal sedimentation of the resulting solution, the supernatant was removed by decantation. This operation was repeated three times. Thereto, acetone anhydrous was added so to make a concentration of the slurry 60% by mass and well suspended. This slurry was charged in an SUS-made pot, and thereto, barium oxide having an average particle size of 1.4 μm and 23 μm were added in an amount of 15% by mass and 10% by mass of the niobium respectively. Furthermore, zirconia balls were added and the contents were mixed for 1 hour using a shaking mixer. After removing zirconia balls, the mixture was placed in a niobium-made vat and vacuum-dried under the conditions of 1×102 Pa and 50° C.
The barium oxide-mixed niobium sintered lump and a niobium cracked powder were obtained in the same manner as in Example 2.
500 g of this barium oxide-mixed niobium cracked powder was added to 1,000 g of ion-exchanged water cooled to 15° C. or lower with stirring while taking care not to allow the water temperature to exceed 20° C. After the completion of addition, the slurry was continuously stirred for another 1 hour, left standing for 30 minutes and then decanted. Thereto, 2,000 g of ion-exchanged water was added and the resulting slurry was stirred for 30 minutes, left standing for 30 minutes and then decanted. This operation was repeated 5 times. Thereafter, the niobium cracked powder was charged into a Teflon-made column and washed with water for 4 hours while flowing ion-exchanged water. At this time, the electrical conductivity of the washing water was 0.5 μS/cm.
After the completion of water washing, the niobium cracked powder was dried at 50° C. under reduced pressure and subjected to a nitridation treatment at 300° C. for 3 hours by passing nitrogen therethrough under pressure, as a result, about 350 g of niobium powder was obtained. The nitrogen content was 0.30%.
The physical properties of this niobium powder, such as tapping density, average particle size, angle of repose, BET specific surface area and average pore diameter, are shown in Table 5.
A molded article was manufactured in the same manner as in Example 2. The outer appearance and the dispersion in the mass of the molded article are shown in Table 5.
Next a dielectric film was formed on the surface of a sintered body of the molded article in the same manner as in Example 2. Then a counter electrode was formed and a carbon layer and a silver paste layer were stacked thereon in this order. After mounting a lead frame thereon, the device as a whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio of this capacitor, and the average capacitance and the average LC value of the chip-type capacitors (n=100 units) are shown in Table 5.
EXAMPLES 38 TO 44
A niobium cracked powder mixed with an activator was obtained in the same manner as in Example 37 by varying the kind of the activator to be added, the average pore diameter and amount of the two kinds of niobium powder to be mixed. The solvent to dissolve the activator was selected from water, acid, alkali, solvent containing an ion exchange resin, solvent of ammonium nitrate and solution containing ethylenediaminetetraacetate. The activator was dissolved in the same manner as in Example 37 to obtain a niobium powder. The physical properties of this niobium powder are shown in Table 5.
A molded article and a sintered body were manufactured in the same manner as in Example 37 to fabricate a chip-type capacitor. The outer appearance and the dispersion in the mass of the molded article and the capacitance and the average LC value of the capacitor are shown in Table 5.
EXAMPLES 45 TO 47
A niobium alloy powder was obtained in the same manner as in Example 37 using as a starting material a niobium-neodymium alloy powder in Example 45, a niobium-tungsten alloy powder in Example 46 and a niobium-tantalum alloy powder in Example 47 respectively. The physical properties of the niobium alloy powder are shown in Table 5.
A molded article and a sintered body were manufactured in the same manner as in Example 37 to fabricate a chip-type capacitor. The outer appearance and the dispersion in the mass of the molded article and the capacitance and the average LC value of the capacitors are shown in Table 5.
EXAMPLES 48 TO 58
A niobium sintered body was produced in the same manner as in Example 2 using the niobium powder obtained in Example 37 to 47. The pore diameter distribution of the sintered body is shown in Table 6.
EXAMPLES 59 TO 69
100 units of each sintered body obtained in Examples 48 to 58 were prepared and electrochemically formed using an aqueous 0.1 mass % phosphoric acid solution at a voltage of 20 V and 80° C. for 1,000 minutes to form an oxide dielectric film on the surface. These sintered bodies were impregnated with the cathode agents shown in Table 3. Thereon, a carbon layer and a silver paste layer were stacked in this order and the whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio and ESR of the manufactured capacitors are shown in Table 7.
COMPARATIVE EXAMPLES 14 TO 17
100 units of each sintered body obtained in Comparative Examples 9 to 12 were prepared and electrochemically formed using an aqueous 0.1 mass % phosphoric acid solution at a voltage of 20 V and 80° C. for 1,000 minutes to form an oxide dielectric film on the surface. These sintered bodies were impregnated with the cathode agents shown in Table 3. Thereon, a carbon layer and a silver paste layer were stacked in this order and the whole was molded with an epoxy resin to manufacture a chip-type capacitor. The capacitance appearance ratio and ESR of the manufactured capacitors are shown in Table 7.









TABLE 1









Physical Properties of Niobium Powder












Activator

BET
Pore

















Amount
Average
Tapping
Average
Angle of
Specific
Diameter




Added
Particle
Density
Particle
Repose
Surface
Peak Top



Kind
(mass %)
Size (μm)
(g/ml)
Size (μm)
(°)
Area (m2/g)
(μm)





Example 1
Butyl polymethylmethacrylate
10
1.0
0.7
120
42
1.4
1.2


Example 2
BaO
17
1.0
1.1
95
45
1.6
1.1


Example 3
Butyl polymethylmethacrylate
5
1.0
1.2
120
40
1.4
1.0


Example 4
Butyl polymethylmethacrylate
1
1.0
1.9
120
37
1.2
0.8


Example 5
BaO
35
1.0
0.5
95
53
1.8
0.9


Example 6
BaO
25
1.0
0.8
95
48
1.7
1.0


Example 7
BaO
17
3.0
1.1
95
45
1.6
2.8


Example 8
BaO
17
5.0
1.2
95
45
1.4
5.3


Example 9
BaO
17
9.0
1.5
95
40
1.2
7.5


Example 10
BaO
17
21
1.7
95
35
0.9
22


Example 11
camphor
40
100
1.9
180
32
1.3
78


Example 12
butyl polyacrylate
8
10
1.7
250
30
1.4
7.5


Example 13
polyvinyl alcohol
3
3.0
1.2
80
49
1.4
3.2


Example 14
ZnO
15
3.0
1.1
85
44
1.7
2.4


Example 15
Re2O7
10
4.0
1.0
110
42
1.8
3.1


Example 16
WO2
7
3.0
1.3
100
45
1.6
3.1


Example 17
SnO2
10
0.8
1.2
85
48
1.6
0.8


Example 18
MgO
20
3.0
1.1
90
45
1.8
2.6


Example 19
Mn(NO3)2
10
2.0
1.5
100
43
1.5
2.3


Example 20
CaCO3
10
1.0
1.0
85
42
1.7
0.8




5
5.0




5.8


Example 21
CaCO3
9
0.5
1.1
88
44
1.7
0.6



B2O3
9
1.0




1.4


Example 22
Y2O3
3
0.7
1.1
90
46
1.6
0.9



B2O3
15
1.0




1.4


Example 23
BaO
17
1.0
1.0
95
46
1.4
1.2


Example 24
BaO
17
1.0
1.1
90
45
1.5
0.9


Example 25
Mn(NO3)2
17
1.0
1.1
85
49
1.3
1.3


Comp. Ex. 1
—
0
—
2.6
1.3
75
3.1
0.7


Comp. Ex. 2
—
0
—
2.9
2.6
72
1.5
1.9


Comp. Ex. 3
—
0
—
3.0
10
69
0.49
9.0


Comp. Ex. 4
BaO
41
1.0
0.4
95
49
1.8
0.8


Comp. Ex. 5
BaO
45
1.0
0.3
90
56
1.9
0.8


Comp. Ex. 6
BaO
50
1.0
0.2
65
59
2.2
0.9


Comp. Ex. 7
BaO
0.8
1.0
2.6
85
48
1.2
0.7


Comp. Ex. 8
BaO
0.6
1.0
3.0
90
48
1.0
0.6


Comp. Ex. 9
BaO
0.4
1.0
3.3
100
44
0.9
0.6















Sintered Body

Electrical Properties

















Outer Appearance:

Capacitance







chipping, cracking,
Dispersion in Mass
Appearance Ratio
Capacitance




distortion
(g/sintered body)
(%)
(μF)
LC (μA)







Example 1
none
0.1 ± 0.002
92
409
13



Example 2
none
0.1 ± 0.002
91
460
19



Example 3
none
0.1 ± 0.002
92
409
17



Example 4
none
0.1 ± 0.002
89
387
15



Example 5
none
0.1 ± 0.002
90
472
21



Example 6
none
0.1 ± 0.002
92
460
22



Example 7
none
0.1 ± 0.002
88
563
26



Example 8
none
0.1 ± 0.002
91
523
18



Example 9
none
0.1 ± 0.002
93
419
13



Example 10
none
0.1 ± 0.002
91
263
 5



Example 11
none
0.1 ± 0.002
90
392
19



Example 12
none
0.1 ± 0.002
90
387
12



Example 13
none
0.1 ± 0.002
93
539
25



Example 14
none
0.1 ± 0.002
89
516
22



Example 15
none
0.1 ± 0.002
92
501
14



Example 16
none
0.1 ± 0.002
89
512
17



Example 17
none
0.1 ± 0.002
92
408
11



Example 18
none
0.1 ± 0.002
95
608
23



Example 19
none
0.1 ± 0.002
91
382
17



Example 20
none
0.1 ± 0.002
94
588
29



Example 21
none
0.1 ± 0.002
97
590
23



Example 22
none
0.1 ± 0.002
90
446
21



Example 23
none
0.1 ± 0.002
91
450
13



Example 24
none
0.1 ± 0.002
93
558
15



Example 25
none
0.1 ± 0.002
91
446
13



Comp. Ex. 1
could not be molded

—
—
—



Comp. Ex. 2
could not be molded

—
—
—



Comp. Ex. 3
could not be molded

—
—
—



Comp. Ex. 4
present
0.1 ± 0.015
91
396
18



Comp. Ex. 5
present
0.1 ± 0.026
92
409
18



Comp. Ex. 6
present
0.1 ± 0.046
95
432
17



Comp. Ex. 7
none
0.1 ± 0.002
42
143
 9



Comp. Ex. 8
none
0.1 ± 0.002
37
115
 7



Comp. Ex. 9
none
0.1 ± 0.002
22
 64
 5



















TABLE 2







Examples

Pore Distribution











and
Molding
Pore
Pore
Peak having


Comparative
Load
Diameter of
Diameter of
Larger Relative


Examples
Applied, N
Peak 1, μm
Peak 2, μm
Intensity














Example 26
392
0.63
1.0
Peak 2


Example 27
686
0.42
1.3
Peak 2


Example 28
981
0.28
0.77
Peak 2


Example 29
490
0.35
2.3
Peak 2


Example 30
785
0.49
0.96
Peak 2


Example 31
294
0.52
2.8
Peak 2


Example 32
392
0.60
2.1
Peak 2


Example 33
686
0.44
2.8
Peak 2


Example 34
981
0.34
1.1
Peak 1


Example 35
294
0.61
2.7
Peak 2


Comparative
392
0.65
none
—


Example 10


Comparative
686
0.42
none
—


Example 11


Comparative
981
0.25
none
—


Example 12


















TABLE 3







Method for Impregnating Cathode


Method
Cathode Agent
Agent







A
Polypyrrole
Vapor phase polymerization with




pyrrole vapor on the sintered body




having attached thereto ammonium




persulfate and anthraquinonesulfonic




acid, was repeated.


B
mixture of lead
Dipping of sintered body in a mixed



dioxide and lead
solution of lead acetate and



sulfate (lead
ammonium persulfate was repeated.



dioxide: 98 mass %)


















TABLE 4









Moisture Resistance



Value














Number of
Number of





units
units





having
having



Method of

capacitance
capacitance


Manufacturing
Impregnating
Capacitance
of 100% to
of 110% to


Method of
Cathode
Appearance
less than
less than


Sintered Body
Agent
Ratio, %
110%
120%





Example 21
A
97
30/30
 0/30


Example 26
A
82
30/30
 0/30



B
88
30/30
 0/30


Example 27
A
84
30/30
 0/30



B
87
30/30
 0/30


Example 28
A
79
27/30
 3/30


Example 29
A
83
30/30
 0/30


Example 30
A
80
30/30
 0/30


Example 31
A
81
30/30
 0/30


Example 32
A
85
30/30
 0/30


Example 33
A
81
30/30
 0/30


Example 34
A
82
28/30
 2/30


Example 35
A
95
30/30
 0/30


Comparative
A
22
 4/30
26/30


Example 9


Comparative
A
71
15/30
15/30


Example 10


Comparative
A
73
17/30
13/30


Example 11


Comparative
A
68
19/30
11/30


Example 12


















TABLE 5









Physical Properties of Niobium Powder











Activator

Pore

















Amount
Average
Tapping
Average
Angle of
BET Specific
Diameter




Added
Par ticle
Density
Particle
Repose
Surface Area
Peak Top



Kind
(mass %)
Size (μm)
(g/ml)
Size (μm)
(°)
(m2/g)
(μm)





Example 37
BaO
15
1.4
0.9
 98
49
1.7
1.5




10
23




25


Example 38
BaO
17
2.0
1.0
130
47
1.6
2.2




15
30




34


Example 39
CaCO3
20
3.0
1.2
202
44
1.5
3.1




10
15




18


Example 40
MgO
18
1.5
1.0
121
45
1.7
1.6



B2O3
 8
10




12


Example 41
Al2O3
20
2.0
0.9
140
49
1.7
2.1




20
25




24


Example 42
MgO
15
1.5
0.9
138
47
1.8
1.8



Al2O3
15
20




22


Example 43
Y2O3
15
2.0
1.1
 98
47
1.6
1.9



Al2O3
10
25




24


Example 44
MgO
15
1.5
0.9
135
46
1.9
1.9




20
25




22


Example 45
BaO
15
1.4
0.9
150
45
1.9
1.3




10
23




22


Example 46
BaO
15
1.4
0.9
111
49
1.7
1.7




10
23




26


Example 47
BaO
15
1.4
0.9
130
46
1.7
1.4




10
23




21













Sintered Body













Outer Appearance:

Capacitance
Electrical Properties













chipping, cracking,
Dispersion in Mass
Appearance Ratio
Capacitance




distortion
(g/sintered body)
(%)
(μF)
LC (μA)





Example 37
none
0.1 ± 0.002
98
592
23


Example 38
none
0.1 ± 0.002
97
587
23


Example 39
none
0.1 ± 0.002
93
557
22


Example 40
none
0.1 ± 0.002
93
556
20


Example 41
none
0.1 ± 0.002
98
585
24


Example 42
none
0.1 ± 0.002
98
589
20


Example 43
none
0.1 ± 0.002
97
581
22


Example 44
none
0.1 ± 0.002
98
590
20


Example 45
none
0.1 ± 0.002
98
592
18


Example 46
none
0.1 ± 0.002
98
587
19


Example 47
none
0.1 ± 0.002
98
587
24



















TABLE 6









Method to
Pore Distribution












obtain a
Pore
Pore
Peak having



niobium
Diameter of
Diameter of
Larger Relative


Examples
powder
Peak 1, μm
Peak 2, μm
Intensity





Example 48
Example 37
0.73
3.1
Peak 2


Example 49
Example 38
0.89
5.1
Peak 2


Example 50
Example 39
1.05
2.4
Peak 2


Example 51
Example 40
0.71
2.1
Peak 2


Example 52
Example 41
0.91
2.9
Peak 2


Example 53
Example 42
0.69
2.7
Peak 2


Example 54
Example 43
0.88
2.9
Peak 2


Example 55
Example 44
0.72
3.2
Peak 2


Example 56
Example 45
0.71
3.0
Peak 2


Example 57
Example 46
0.72
3.3
Peak 2


Example 58
Example 47
0.71
3.1
Peak 2




















TABLE 7





Examples






and
Method to
Capacitance


Comparative
obtain a
appearance


Examples
sintered body
Ratio, %
Capacitance
ESR Ω



















Example 59
Example 48
98
592
0.024


Example 60
Example 49
97
587
0.023


Example 61
Example 50
93
557
0.022


Example 62
Example 51
93
556
0.022


Example 63
Example 52
98
585
0.023


Example 64
Example 53
98
589
0.023


Example 65
Example 54
97
581
0.024


Example 66
Example 55
98
590
0.022


Example 67
Example 56
98
592
0.024


Example 68
Example 57
98
587
0.023


Example 69
Example 58
98
587
0.025


Comparative
Comparative
22
64
0.158


Example 14
Example 9


Comparative
Comparative
71
305
0.083


Example 15
Example 10


Comparative
Comparative
73
314
0.081


Example 16
Example 11


Comparative
Comparative
68
291
0.090


Example 17
Example 12









Industrial Applicability

A niobium powder of the present invention having a tapping density of 0.5 to 2.5 g/ml, an average particle size of 10 to 1000 μm, angle of repose from 10 to 60° and the BET specific surface area from 0.5 to 40 m2/g is excellent in flowability and capable of continuous molding. By using a niobium sintered body, which is obtained by sintering the niobium powder and has a pore diameter peak top in a range of 0.01 μm to 500 μm, preferably having a plurality of pore diameter peak tops in the pore distribution, for the capacitor electrode, a high capacitance appearance ratio can be obtained and a capacitor having low leakage current and excellent moisture resistance can be produced.

## Claims

1. A niobium sintered body for a capacitor electrode, wherein the pore distribution of the niobium sintered body has a plurality of pore diameter peak tops;
wherein among the plurality of pore diameter peak tops, the peak top of the peak having a highest relative intensity is present in the larger diameter size than the peak top of the peak having a next highest relative intensity; and
wherein among the plurality of pore diameter peak tops, the peak tops of the two peaks having a highest relative intensity are present in the range of 0.2 to 0.7 μm and in the range of 0.7 to 3 μm respectively.

2. The niobium sintered body as claimed in claim 1, wherein the pore distribution has two pore diameter peak tops.

3. The niobium sintered body as claimed in claim 1, wherein the sintered body has a volume of 10 mm3 or more including the volume of pore void.

4. The niobium sintered body as claimed in claim 1, wherein the sintered body has a specific surface area of 0.2 to 7 m2/g.

5. The niobium sintered body as claimed in claim 1, wherein a part of the sintered body is nitrided.

6. A capacitor comprising one electrode using the niobium sintered body claimed in claim 1, a counter electrode and a dielectric material interposed therebetween.

7. A method for producing a capacitor comprising one electrode using a niobium sintered body, a dielectric material formed on the surface of the sintered body and a counter electrode provided on the dielectric material, wherein the niobium sintered body is the niobium sintered body claimed in claim 1.