Electricity: electrical systems and devices – Electrolytic systems or devices – Solid electrolytic capacitor
Reexamination Certificate
2002-07-10
2004-05-04
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Solid electrolytic capacitor
C252S062200
Reexamination Certificate
active
06731497
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a solid electrolytic capacitor and a method for manufacturing a solid electrolytic capacitor and, particularly, to a solid electrolytic capacitor having an excellent leakage current characteristic and a method for manufacturing a solid electrolytic capacitor which can produce a solid high molecular polymer electrolyte layer comprising polyethylene dioxythiophene under a normal temperature and in a short time without damaging a valve metal substrate constituting an electrode and produce a solid electrolytic capacitor whose leakage current is low.
DESCRIPTION OF THE PRIOR ART
An electrolytic capacitor is conventionally formed by employing a so-called valve metal capable of forming an insulating oxide film such as aluminum, titanium, brass, nickel, tantalum or the like as an anode, anodizing the surface of the valve metal to form an insulating oxide film thereon, forming an electrolyte layer substantially serving as a cathode, and forming a conductive layer of graphite, silver or the like as a cathode.
For example, an aluminum electrolytic capacitor is formed by employing as an anode a porous aluminum foil whose specific surface area is increased by etching, and providing a separation paper impregnated with an electrolytic solution between an aluminum oxide layer formed on the surface of the anode and a cathode foil.
In general, although an electrolytic capacitor using an electrolytic solution for an electrolyte layer between an insulating oxide film and a cathode has the disadvantage that its lifetime is determined by liquid leakage, evaporation of the electrolytic solution and the like, a solid electrolytic capacitor using a solid electrolyte containing metal oxide or organic compounds does not have such a disadvantage and is preferable.
Manganese dioxide is a typical metal oxide usable for the solid electrolyte of a solid electrolytic capacitor. On the other hand, as an organic compound usable for the solid electrolyte of a solid electrolytic capacitor, 7,7,8,8-tetracyanoxydimenthane (TCNQ) complex salt disclosed in Japanese Patent Application Laid Open No. 58-191414 can be cited.
Recently, as the frequency of power circuits of electronic devices has become higher, corresponding performance is required of capacitors used therewith. However, a solid electrolytic capacitor using a solid electrolyte layer containing manganese dioxide or TCNQ complex salt has the following disadvantages.
Although a solid electrolyte layer containing manganese dioxide is generally formed by repeating thermal decomposition of manganese nitrate, an insulating oxide film is damaged or degraded by heat applied during thermal decomposition or oxidative effect of NOx gas generated during thermal decomposition. Therefore, in the case where a solid electrolyte layer is formed using manganese dioxide, leakage current becomes high, for example, and various characteristics of the capacitor finally obtained tend to be lowered.
Further, in the case where a solid electrolyte layer is formed using manganese dioxide, the solid electrolytic capacitor has the disadvantage that impedance thereof becomes higher in the high frequency region.
On the other hand, a solid electrolytic capacitor using TCNQ complex salt does not adequately satisfy the requirement for low impedance of current solid electrolytic capacitors, since the electric conductivity of TCNQ complex salt is about 1 S/cm or lower.
It has been further pointed out that the reliability of a solid electrolytic capacitor using TCNQ complex salt as a solid electrolyte is not sufficient for the reason that adhesive strength between TCNQ complex salt and an insulating oxide film is low and the thermal stability of TCNQ complex salt during soldering and with lapse of time is low, and the like. In addition, TCNQ complex salt is expensive and, therefore, the cost of a solid electrolytic capacitor using TCNQ complex salt as a solid electrolyte is high.
For solving these problems occurring when manganese dioxide or TCNQ complex salt is used as a solid electrolyte and obtaining a solid electrolytic capacitor having better characteristics, it has been proposed to use as a solid electrolyte a high molecular compound having high electric conductivity whose manufacturing cost is relatively low, whose adhesive strength to an insulating oxide film is relatively good and whose thermal stability is excellent.
For example, Japanese Patent No. 2,725,553 discloses a solid electrolytic capacitor in which polyaniline formed on an insulating oxide film on the surface of an anode by the chemical oxidation polymerization process.
Further, Japanese Patent Publication No. 8-31400 proposes a solid electrolytic capacitor in which a thin film of metal or manganese dioxide is formed on an insulating oxide film and a conductive polymer film of polypyrrole, polythiophene, polyaniline, polyfuran or the like is formed on the thin film of metal or manganese dioxide by the electrolysis polymerization process, for the reason that it is difficult to form a conductive polymer film having high strength on an insulating oxide film on the surface of an anode only by the chemical oxidation polymerization process and that it is impossible or extremely difficult to directly form an electrolysis polymerization film on an insulating oxide film on the surface of an anode by the electrolysis polymerization process because the insulating oxide film on the surface of an anode is a non-conductor.
Furthermore, Japanese Patent Publication No. 4-74853 proposes a solid electrolytic capacitor in which a conductive polymer film of polypyrrole, polythiophene, polyaniline, polyfuran or the like is formed on an insulating oxide film by the chemical oxidation polymerization process.
At present, polypyrrole and polyethylene dioxythiophene are widely used as polymers for forming a solid high molecular polymer electrolyte layer of a solid electrolytic capacitor. Among these, although polyethylene dioxythiophene has high thermal stability and preferable characteristics as a polymer for forming a solid high molecular polymer electrolyte layer, since there are few merits when producing it by the electrolysis polymerization process, polyethylene dioxythiophene is normally synthesized using the chemical oxidation polymerization process. However, when producing polyethylene dioxythiophene using the chemical oxidation polymerization process, since polyethylene dioxythiophene is hard to react under a normal temperature and the rate of polymerization thereof is low, it is necessary to heat the reaction system up to about 50 to 70 degrees (centigrade) to facilitate the reaction.
Moreover, since an oxidizing agent which is a strong acid is used and the polymerization reaction system including an electrode has to be heated, there is a high risk of materially damaging the electrode during the polymerization process. This lowers yield in an aging process and degrades the leakage current characteristic of the electrolytic capacitor.
Japanese Patent Application Laid Open No. 11-238648 proposes a method for forming a solid high molecular polymer electrolyte layer by alternately immersing a valve metal substrate formed with an insulating oxide film in a monomer solution for forming a conductive polymer compound and a solution of an aqueous solvent system oxidizing agent such as peroxodisulphuric acid, sodium salt thereof, potassium salt thereof, ammonium salt thereof; cerium (IV) nitrate, cerium (IV) nitrate ammonium; ferric sulfate, ferric chloride; or the like or a solution of an organic solvent system oxidizing agent such as ferric dodecyl benzene sulfonate, ferric p-toluene sulfonate or the like one to twenty times, preferably, three to ten times, each for five minutes to five hours, preferably, fifteen minutes to three hours.
Further, Japanese Patent Application Laid Open No. 11-238649 proposes a method for forming a solid high molecular polymer electrolyte layer by separately immersing a valve metal substrate formed with an insulating oxide film in a monomer solution for formi
Kobayashi Masa-aki
Yokouchi Yumiko
Reichard Dean A.
Seed IP Law Group PLLC
TDK Corporation
Thomas Eric
LandOfFree
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