Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2002-07-08
2003-12-02
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S244000, C075S245000, C361S528000
Reexamination Certificate
active
06656245
ABSTRACT:
TECHNICAL FIELD
This invention relates to a niobium sintered body for capacitor, which is capable of giving a capacitor having an enhanced capacity per unit weight and exhibiting good leak current (hereinafter abbreviated to “LC”) characteristics. Further, it relates to the capacitor, and a process for producing the capacitor.
BACKGROUND ART
Capacitors used for electronic equipment such as portable telephones and personal computers are desired to be of a small volume. Tantalum electrolytic capacitors are popularly used because they have a large capacity relative to their size and good characteristics. As an anode of the tantalum electrolytic capacitors, a sintered body of a tantalum powder is generally used. To further enhance the capacity of the tantalum electrolytic capacitor, it is necessary to increase the weight of sintered body, or to prepare a sintered body from an ultrafine tantalum powder having an enhanced specific surface area.
The increase of the weight of sintered body inevitably leads to an increase of size of capacitor, and thus a capacitor having a desirably small volume cannot be obtained. When a sintered body is prepared from an ultrafine tantalum powder having an enhanced specific surface area, the tantalum sintered body has pores which have a reduced diameter and part of which are clogged upon sintering, and therefore, the sintered body is difficult to impregnate with a cathode material at an after-treating step. To solve these problems, a proposal has been made wherein a sintered body is made of a powdery material having a dielectric constant larger than that of tantalum, such as niobium or titanium.
However, a conventional capacitor using an electrode made of a sintered body of a powdery material having a large dielectric constant has another problem such that the LC characteristics are not satisfactory and the capacitor is of poor practical use. More specifically, in the case where a sintered body is made from a high-capacity tantalum powder exhibiting a product (CV), i.e., a product of capacity×electrolysis voltage, of 40,000 &mgr;F·V/g, the LC value as measured on a sintered body, which has been subjected to electrolytic oxidation, at a voltage of 70% of the electrolysis voltage when three minutes elapsed from the electrolytic oxidation, is usually approximately 30 &mgr;A/g. In contrast, a sintered body made of a conventional niobium powder in a similar manner has an LC value more than 100 times larger than that of the tantalum powder. Thus, a capacitor manufactured from a niobium sintered body is not satisfactory in LC characteristics and leads to enhancement in electric power consumption of an electrical equipment, and the capacitor has poor reliability.
DISCLOSURE OF THE INVENTION
In view of the foregoing prior art, a primary object of the present invention is to provide a capacitor exhibiting good LC characteristics and having an enhanced capacity per unit weight.
As results of an extensive research, the present inventors succeeded in development of a niobium sintered body giving a capacitor having a reduced LC value, and have completed this invention.
In a first aspect of the invention, there is provided a niobium sintered body for a capacitor, which is made of a niobium powder, and characterized by exhibiting an LC value of not larger than 300 &mgr;A/g as measured after an electrolytic oxide film is formed thereon.
The niobium sintered body for a capacitor preferably exhibits a product (CV) [i.e., a product of capacity (C) with electrolysis voltage (V)] of at least 40,000 &mgr;F·V/g, and further preferably contains at least one kind of niobium compound selected from niobium nitride, niobium carbide and niobium boride.
In a second aspect of the invention, there is provided a process for producing a niobium sintered body for a capacitor characterized by sintering a niobium powder containing at least one kind of niobium compound selected from niobium nitride, niobium carbide and niobium boride.
In a third aspect of the invention, there is provided a capacitor comprising an electrode composed of the above-mentioned niobium sintered body of the invention, a dielectric formed on a surface of the niobium sintered body, and a counter electrode.
BEST MODE FOR CARRYING OUT THE INVENTION
A niobium sintered body of the invention exhibiting a reduced LC value as measured after an electrolytic oxide film is formed thereon is obtained by sintering a niobium powder wherein at least one element selected from nitrogen, carbon and boron is bound to a part of the niobium. The amount of the bound nitrogen, carbon and/or boron, namely, the content of bound nitrogen, bound carbon and/or bound boron in the niobium powder varies depending upon the particular shape of finely divided niobium particles, but, in the case when the niobium powder has a particle diameter of approximately 10 &mgr;m to 30 &mgr;m, the content of each element is usually in the range of 50 to 200,000 ppm by weight. In view of a reduced LC value, said content is preferably in the range of several hundreds to several ten-thousands ppm by weight, and more preferably 500 to 20,000 ppm by weight. In the case when the niobium powder has a particle diameter of at least approximately 3 &mgr;m, but smaller than approximately 10 &mgr;m, said content is usually in the range of 50 to 50,000 ppm by weight. In view of a reduced LC value, said content is preferably in the range of several hundreds to 20,000 ppm by weight, and more preferably 500 to 20,000 ppm by weight. The niobium powder may contain either alone or at least two of the bound nitrogen, bound carbon and bound boron, i.e., niobium nitride, niobium carbide and niobium boride.
The method of nitriding a niobium powder for forming niobium nitride may be any of the conventional methods which include, for example, liquid nitriding, ion nitriding and gas nitriding. However, a gas nitriding carried out in a nitrogen atmosphere is preferable because it is simple and easy. The gas nitriding in a nitrogen atmosphere is effected by allowing a nibium powder to stand in a nitrogen atmosphere. A niobium powder having the objective bound nitrogen content is obtained by carrying out the nitriding at a temperature of not higher than 2,000° C. within tens of hours. In general, the higher the nitriding temperature, the shorter the nitriding time for obtaining the desired bound nitrogen content. Even at room temperature, when a niobium powder is allowed to stand in a nitrogen atmosphere for tens of hours, a niobium powder having a bound nitrogen content of approximately several tens of ppm can be obtained.
The method of carbonizing a niobium powder for forming niobium carbide may also be any of the conventional methods which include, for example, gas carbonization, solid carbonization and liquid carbonization. For example, the carbonization can be effected by allowing a niobium powder to stand together with a carbon source, for example, a carbon material or a carbon-containing organic material such as methane at a temperature of not higher than 2,000° C. under a reduced pressure for from several minutes to tens of hours.
The method of boronizing a niobium powder for forming niobium boride may also be any of the conventional methods which include, for example, gas boronization and solid boronization. For example, niobium boride can be formed by allowing a niobium powder to stand together with a boron source, for example, a boron pellet or a boron halide such as trifluoroboron at a temperature of not higher than 2,000° C. under a reduced pressure for several minutes to tens of hours.
The niobium powder containing at least one of niobium nitride, niobium carbide and niobium boride preferably has an average degree of roundness of at least 0.80. When the niobium powder having an average degree of roundness of at least 0.80 is used, a compact having an appropriate porosity but a high packed density is obtained, and a capacitor with an anode composed of a sintered body made therefrom exhibits an enhanced withstand voltage. Preferably the average degree of round
Naito Kazumi
Shimojima Atsushi
Mai Ngoclan
Showa Denko Kabushiki Kaisha
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