Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2002-07-26
2003-12-16
Mai, Ngoclan (Department: 2831)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S252000, C361S528000
Reexamination Certificate
active
06663687
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a niobium powder for capacitors, from which a capacitor having a large capacitance per unit weight and good high-temperature characteristics can be produced, and also relates to a sintered body using the niobium powder and a capacitor using the sintered body.
BACKGROUND ART
Capacitors for use in electronic instruments such as potable 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. In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the weight of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The former method of increasing the weight of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the latter method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering and therefore, impregnation of the cathode agent in the later process becomes difficult. As one of means for solving these problems, a capacitor using a sintered body of powder of a material having a dielectric constant larger than that of tantalum is being studied. The materials known to have a larger dielectric constant include niobium.
However, capacitors using a sintered body manufactured from these materials suffer from insufficient high-temperature characteristics and cannot be used in practice. The ratio between the initial capacitance C
0
at room temperature and the capacitance C after the capacitor is left standing in an atmosphere of 105° C. for 2,000 hours while applying a voltage and then returned to room temperature, that is, (C−C
0
)/C
0
is defined as a high-temperature property. When a sintered body is electrolytically oxidized and then combined with another part electrode to produce a capacitor, insofar as tantalum powder is used, the high-temperature property usually falls within ±20%, but if a conventional niobium powder is used, some capacitors cannot have a high-temperature property falling within ±20%.
As such, capacitors using a niobium powder must be estimated low in the reliability at room temperature and because of this, these capacitors are judged defective in the service life and not used in practice.
As for the technology related to the present invention, a capacitor using partially reduced niobium oxide obtained by heat-treating diniobium pentoxide (Nb
2
O
5
) in the presence of hydrogen, which has a large capacitance and excellent leakage current characteristics, has been proposed (see, WO 00/15555). However, when a capacitor is manufactured using niobium oxide obtained in a test by the present inventors and subjected to an accelerated test at a high temperature, the capacitor performance is deteriorated and not satisfied.
DISCLOSURE OF INVENTION
As a result of extensive investigations, the present inventors have found that when a niobium powder containing niobium monoxide crystal and hexaniobium monoxide crystal are used as the starting material niobium powder of a niobium sintered body, a capacitor having good high-temperature characteristics can be obtained. The present invention has been accomplished based on this finding.
More specifically, the object of the present invention is to provide the following niobium powder for capacitors, a sintered body thereof and a capacitor using the sintered body.
1. A niobium powder for capacitors, containing niobium monoxide crystal and hexaniobium monoxide crystal.
2. The niobium powder for capacitors as described in 1 above, wherein the content of the niobium monoxide crystal is from 0.05 to 20% by mass.
3. The niobium powder for capacitors as described in 1 above, wherein the content of the hexaniobium monoxide crystal is from 0.05 to 20% by mass.
4. The niobium powder for capacitors as described in any one of 1 to 3 above, which contains a partially nitrided niobium powder.
5. The niobium powder for capacitors as described in 4 above, wherein the nitrided amount is 10~100,000 ppm by mass.
6. A sintered body using the niobium powder for capacitors described in any one of 1 to 5 above.
7. A capacitor fabricated from the sintered body described in 6 above as one part electrode, a dielectric material formed on the surface of the sintered body, and another part electrode provided on the dielectric material.
8. The capacitor as described in 7 above, wherein the dielectric material is mainly composed of niobium oxide.
9. The capacitor as described in 8 above, wherein the niobium oxide is formed by electrolytic oxidation.
10. The capacitor as described in any one of 7 to 9 above, wherein the another part electrode is at least one material selected from an electrolytic solution, an organic semiconductor or an inorganic semiconductor.
11. The capacitor as described in 10 above, wherein the another part electrode is composed of an organic semiconductor and the organic semiconductor is at least one organic semiconductor selected from the group consisting of an organic semiconductor comprising a benzopyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquinodimethane and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant into a polymer comprising two or more repeating units represented by the following formula (1) or (2):
(wherein R
1
to R
4
, which may be the same or different, each represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms or an alkoxy group having from 1 to 6 carbon atoms, X represents an oxygen atom, a sulfur atom or a nitrogen atom, R
5
is present only when X is a nitrogen atom and represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms, and each of the pairs R
1
and R
2
, and R
3
and R
4
may combine with each other to form a ring).
12. The capacitor as described in 11 above, wherein the organic semiconductor is at least one member selected from polypyrrole, polythiophene or substitution derivatives thereof.
DETAILED DESCRIPTION OF INVENTION
The niobium powder of the present invention is characterized by containing niobium monoxide crystal and hexaniobium-monoxide crystal.
The niobium powder is inferior to tantalum powder in the stability of oxide dielectric film formed on the sintered body and this difference is particularly remarkable at high temperatures. Many reasons may be considered therefor but as one of those reasons, the inferior stability is considered to come out because the composition of oxide dielectric film is different from the composition of niobium and due to thermal strain at high temperatures, the deterioration of oxide dielectric film is accelerated.
On the other hand, when niobium monoxide crystal and hexaniobium monoxide crystal are incorporated into the niobium powder, it is presumed, the thermal strain at high temperatures is relaxed and therefore, the capacitor manufactured from the niobium powder is improved in the heat stability.
One embodiment for obtaining the niobium powder of the present invention is described below.
The niobium powder for capacitors of the present invention is obtained by granulating a niobium primary particle powder (hereinafter simply referred to as a “primary powder”) into an appropriate size. The primary powder of niobium can be granulated by a conventionally known method. Examples thereof include a method where a primary powder is left standing at a high temperature of 500 to 2,000° C. in a vacuum and then wet or dry cracked, and a method where a primary powder is mixed with an appropriate binder such as acrylic resin or polyvinyl alcohol and then crack
Nagato Nobuyuki
Naito Kazumi
Mai Ngoclan
Showa Denko K.K.
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