Solid electrolytic capacitor and production method thereof

Details Solid electrolytic capacitor and production method thereof Solid electrolytic capacitor and production method thereof

2001-12-26

2003-01-07

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S512000, C361S523000, C361S528000, C361S502000, C029S025030, C252S062200

Reexamination Certificate

active

06504704

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a solid electrolytic capacitor and a method for producing the same. More specifically, the invention relates to a solid electrolytic capacitor with modifications intended for the improvement of the capacitance value and the impedance properties, for the purpose of achieving capacitor downsizing.
BACKGROUND OF THE INVENTION
Following the recent digitalization and high-frequency modification of electronic appliances, a small-type capacitor with a big capacitance and a low impedance in a high-frequency region has been needed in the field of electric source, in particular.
For such need, a solid electrolytic capacitor has been used, which is produced by assembling a capacitor element by winding together a cathode foil comprising a valve metal such as aluminium and an anode foil comprising a valve metal and having an oxide film prepared on the surface thereof through a separator to prepare a solid electrolyte between these cathode and anode foils. Because such winding-type solid electrolytic capacitor has a characteristic profile of a small type and a large capacitance and also has great impedance properties in a high-frequency region, the capacitor is one of the most suitable capacitors for the need. In such type of solid electrolytic capacitor, additionally, tantalum, niob and titanium other than aluminium are used as the anode material, while metals of the same species as those for the anode material are used as the cathode material.
So as to increase the capacitance of electrolytic capacitor, importantly, the capacitance of the cathode material as well as the capacitance of the anode material should be improved. The capacitance of each electrode of electrolytic capacitor is defined by the type and thickness of an insulation film prepared on the electrode surface and the surface area of the electrode. Given that the dielectric constant of insulation film is &egr;; the thickness of insulation film is t; and the surface area of electrode is A, the capacitance C is expressed by the following equation.
C=&egr;
(
A/t
)
So as to raise the capacitance, as apparently shown by the equation, it is effective to enlarge the electrode surface area, select an insulation film material with a higher dielectric constant and prepare a thinner insulation film.
Attempts have been made conventionally to enlarge the electrode surface area so as to increase the capacitance, because the dielectric constant of insulation film is defined by the electrode material, while the thickness of insulation film is defined by voltage resistance among them. However, just simple use of a large-type electrode to get an enlarged surface area is not preferable, because it leads to the scale up of electrolytic capacitor. Thus, the surface of aluminium foil as the substrate of electrode material has been etching processed traditionally to prepare recesses and protrusions thereon to enlarge the substantial surface area.
Additionally, Japanese Patent Laid-open No. Sho-59-167009 (167009/1984) discloses a cathode material with a metal film prepared on the surface of the substrate by utilizing metal deposition technique, as an alternative of the etching process. According to the technique, film preparation conditions should be selected, to prepare fine recesses and protrusions on the film surface to thereby enlarge the surface area, so that a large capacitance can be recovered. Additionally, metals exerting high dielectric constants in the form of oxides thereof, such as Ti, can elevate the dielectric constant of the insulation film prepared on the surface of the cathode material, leading to a larger capacitance.
Furthermore, Japanese Patent Laid-open No. Hei-3-150825 (150825/1991) previously filed by the present applicant discloses a technique for preparing a deposition layer comprising titanium nitride on the surface of high-purity aluminium used as a cathode electrode by cathode arc deposition process, so as to elevate the capacitance value of the cathode side, in light of the finding that the capacitance of electrolytic capacitor is the composite capacitance of the capacitances of the anode side and the cathode side in serial connection.
[Problems to be Solved]
However, solid electrolytic capacitors using the cathode foils prepared by the conventional techniques described above have the following drawbacks. In other words, the surface of aluminium foil as the substrate of the electrode material in conventional solid electrolytic capacitors is etching processed, so as to elevate the capacitance of the electrolytic capacitors. When etching is processed too excessively, the solubilization of the surface of the aluminium foil concurrently progresses, which adversely blocks the elevation of the surface enlargement ratio. The elevation of the capacitance of electrode material by etching technique was limited.
Additionally, the technique for preparing a deposition layer comprising titanium nitride on the surface of cathode foil has also been problematic. More specifically, manganese dioxide prepared by thermal decomposition of manganese nitrate has mainly been used as the solid electrolyte of conventional solid electrolytic capacitors. During the process of preparing manganese dioxide, however, thermal treatment at 200 to 300° C. should be carried out several times. Therefore, oxide film was formed on the surface of the film comprising metal nitride as prepared on the surface of the cathode foil, which caused the reduction of the capacitance of the cathode foil, leading to the reduction of the capacitance of the electrolytic capacitor. Furthermore, ESR reduction was also limited, because manganese dioxide is at a relatively high electric conductivity.
OBJECTS OF THE INVENTION
The present invention has been proposed so as to overcome the problems of the conventional techniques. It is a first object of the invention to provide a solid electrolytic capacitor with an improved capacitance value, and a method for producing the same. It is a second object of the invention to provide a solid electrolytic capacitor not only with an improved capacitance value but also with a low impedance at high frequency, and a method for producing the same.
DISCLOSURE OF THE INVENTION
So as to overcome the problems, the present inventors have made investigations about a solid electrolytic capacitor with an improved capacitance value and an improved impedance property, and a method for producing the same. Consequently, the invention has been achieved.
Specifically, it has been found that the capacitance value of a winding-type solid electrolytic capacitor using an organic semiconductor comprising a TCNQ complex salt can be improved, greatly, by preparing a film comprising a metal nitride on the surface of the cathode foil by deposition process.
It has additionally been found that the capacitance value and impedance property of a winding-type solid electrolytic capacitor using an organic semiconductor comprising a TCNQ complex salt can be improved, greatly, by preparing an oxide film on the surface of the cathode foil and additionally preparing a film comprising a metal nitride thereon by deposition process.
First, the inventors have made various investigations about a winding-type solid electrolytic capacitor using an organic semiconductor having been drawing attention recently. Further, N-n-butylisoquinolinium TCNQ complex salt, N-methyl-3-n-propylimidazol TCNQ complex salt, and N-n-alkylisoquinolinium TCNQ complex salt can be used as the organic semiconductor. Herein, TCNQ means 7,7,8,8-tetracyanoquinodimethane. Additionally, these TCNQ complex salts can be prepared by known methods.
Furthermore, the inventors prepared TiN on the surface of cathode foil by deposition process. Using the resulting cathode foil, the inventors prepared a capacitor under the following conditions, to measure the capacitance of the cathode foil alone. It was shown that the capacitance thereof was infinite. This means that TiN prepared on the cathode foil removed a part of the spontaneous oxide film formed on

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