1. Field of the Invention
The present invention relates to an electrolytic capacitor comprising an anode of a valve metal such as aluminum or tantalum, and a cathode of a conductive polymer layer, and also relates to a method of producing such an electrolytic capacitor.
2. Prior Art
A conventional electrolytic capacitor have been composed of an anode formed of the valve metal, a dielectric layer formed of a film of the valve metal oxide, and a cathode formed of a liquid electrolyte or inorganic solid electrolyte, with leads connected to the anode and the cathode, respectively, being packaged to make a product.
The anode is made of aluminum or tantalum as the valve metal, whose oxide metal is used for the dielectric layer. In the case of aluminum electrolytic capacitor, the surface of an aluminum foil is coarsened and anodized to form the oxide film, thereby obtaining an aluminum anode element. In the case of a tantalum electrolytic capacitor, a compact made of powdered tantalum is fired in a oxidizing atmosphere, to obtain a tantalum anode element.
Conventionally, as a cathode, the organic solvent including an organic acid or the like have been used in the case of the aluminum electrolytic capacitor. The solid electrolyte was, for example, manganese dioxide in the case of a tantalum electrolytic capacitor.
In these days, electronic components have been requested to have excellent high-frequency response property, particularly, to be adaptable to digital circuits, so that electrolytic capacitors are also required to have excellent high-frequency response and lower inner resistance. In order to meet these market needs, some conductive polymers having a high conductivity have been examined and developed for use in the solid electrolytic cathode of such electrolytic capacitors.
In the electrolytic capacitor wherein the solid electrolyte is formed of a conductive polymer, the conductive polymer layer is formed on the anode element to make direct contact to the entire dielectric layer of the oxide film on the valve metal.
The electrolytic capacitor using such a conductive polymer layer is known, for example, in the Japanese Patent Publication JP-A 6-168855.
The chemical polymerization technique have been known, which forms the conductive polymer on an anode elements into a cathode. In this technique, the anode element is immersed in a monomer solution, and the monomer is polymerized at the presence of an oxidizing agent as a polymerizing initiator. As a result, a conductive polymer layer is formed on the oxide film on the surface of the anode.
The electrolytic polymerization technique have also been known for polymerizing, wherein polymerization is performed by anodizing a conductive monomer on the surface of a solid film to have polymerized as the electrolyzing electrode. According to this technique, a first conductive layer, having slight conductivity, must be formed on a dielectric oxide film. This first conductive layer on the anode element is used a start point for further polymerizing electrolytically. For this first conductive layer is used a manganese dioxide film previously attached to the oxide film, or a very thin conductive polymer layer previously formed by the above chemical polymerization technique.
However, the formation of the conductive polymer layer by the conventional techniques caused some problems. It was difficult to grow a conductive polymer layer having a uniform and sufficient thickness on the entire surface of the dielectric layer of the oxide film. For this reason, the electrolytic capacitor obtained as the final product could not show capacitance high enough to be estimated from the anode film design. Furthermore, in the prior art chemical polymerization technique, the polymerizing operations had to be repeated several tens of times to increase the thickness of the grown polymer, in order to obtain the sufficiently high capacitance.
In the case of using the conventional electrolytic polymerization technique, the first conductive layer is formed of a manganese dioxide or a very thin conductive polymer layer formed by the chemical polymerization technique. Therefore, electric current supplied for electrolysis flows through the first conductive layer having a relatively high electric resistance. For this reason, a relatively long polymerization period of time was required to form the conductive polymer layer on the entire surface of the dielectric layer.
Conventionally, to electrolytically form a conductive polymer layer on both sides of an anode valve metal foil, polymerization had carried out, starting from a part of one side of the anode valve metal foil, and then advancing to the opposite side through roundabout routes. In this conventional method, when a conductive polymer layer is formed by electrolytic polymerization on both sides of an anode foil having a relatively large area, it takes a long time for polymerization. In addition, the thickness of the conductive polymer layer at the starting point for polymerizing differs from that at the ending point. This difference in thickness of the conductive polymer layer is caused by the total polymerizing period of time different at the start point and the end point. Therefore, to obtain a conductive polymer layer having a more relatively uniform thickness, the speed of polymerization is required to be made slower, then reducing the difference in polymerization time. This results in extending the operating time for polymerization.
For these reasons, the above technique, not to have formed a conductive polymer layer uniformly on a wide anode foil, was limited to only on a relatively narrow anode foil which is suited for the shape of the final product, just as in the example of the Japanese Patent Publication JP-A 63-239917.
In order to form a necessary and sufficient conductive polymer layer for the final product, the processes for capacitor production have been complicated and taken a long time, then, requiring high production cost.