JPH05198461A - Electrode for electrolytic capacitor - Google Patents

Abstract

PURPOSE:To obtain an electrode for use with an electrolytic capacitor having a large capacitance and a superior mechanical strength. CONSTITUTION:A porous Al layer is formed on the surface of a conductive substrate by electroplating. The thickness and void volume of the porous Al layer are controlled by plating conditions such as the composition and concentration of a plating bath and a current density. When this layer is to be used as a positive electrode, the layer is further subjected to a conversion treatment or anodizing so that an oxide film serving as a dielectric can be formed. Various types of metal material or conductive ceramics are used as the conductive substrate. Hence, the conductive substrate possesses a mechanical strength, and the use of the porous Al layer is intended to increase a surface area, and hence an electrolytic capacitor with superior characteristics and durability is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode capable of obtaining an electrolytic capacitor having an extremely large capacitance.

[0002]

2. Description of the Related Art As electric and electronic devices are used in various fields, the demand for electrolytic capacitors is increasing. In accordance with the trend toward miniaturization and high density of electric devices and electronic devices, electrolytic capacitors are also required to have large electrostatic capacity per unit area and be small in size.

The capacitance C (μF) of the electrolytic capacitor is
It is known that when an Al foil is used as an electrode, it is determined by the following formula. C = εS / d × 8.855 × 10 ⁻⁸ , where S: surface area of Al foil d: thickness of Al ₂ O ₃ -based dielectric film formed on the surface of Al foil, depending on the rated voltage of the capacitor Determined. epsilon: a dielectric constant, Al ₂ O ₃ unique substantially constant value (8
-10)

Therefore, in order to increase the capacitance of the electrolytic capacitor, it is necessary to increase the surface area of the Al foil used as the electrode. As means for increasing the surface area of the Al foil, electrolytic etching, chemical etching, etc. have been conventionally adopted.

A large number of etch pits are formed on the surface of the etched Al foil. Increasing the density of etch pits is effective in increasing the surface area of the Al foil. However, as the density increases, the individual etch pits become shallower, leaving an unetched portion at the center of the Al foil. Therefore, there is a limit in increasing the surface area of the Al foil by increasing the density of the etch pits. In addition, when the etching conditions are changed in multiple steps to deepen the individual etch pits, the mechanical strength of the Al foil is significantly reduced in combination with the high density.

Therefore, Japanese Patent Application Laid-Open No. 2-61041 proposes that etching is performed in at least three stages by changing the addition amounts of sulfuric acid, oxalic acid, phosphoric acid and the like added to hydrochloric acid. As a result, the density of the etch pits increases and the surface area of the Al foil increases, and the unetched portion remains in the center of the Al foil to secure the required mechanical strength.

[0007]

By etching, A
l When increasing the surface area of the foil, Al from which impurity elements were crystallized
In the foil, etching conditions are different between the crystallized substance and the matrix, and defects such as falling out and generation of irregular etch pits are likely to occur. In order to avoid these defects, it is necessary to use high-purity Al foil.

Further, Al eluted from the Al foil by etching is disposed as a waste liquid together with the etching liquid. Therefore, a process of recovering Al produced by consuming a large amount of electric power and extracting a valuable component from the waste liquid is required so as not to adversely affect the environment. In this respect, in the multi-stage etching as introduced in the above-mentioned Japanese Patent Laid-Open No. 2-61041, the production process becomes extremely complicated, and a troublesome operation is required for the management of the etching liquid and the waste liquid treatment. Factors that hinder the productivity of

On the other hand, as an electrolytic capacitor which does not rely on etching, a porous sintered body of an Al--Ti alloy is used as a base of an anode for an electrolytic capacitor.
It is introduced in the first publication. However, since it is a sintering method, there is a limit to thinning the anode, and the mechanical strength is not sufficient.

The present invention has been devised in order to solve such a problem, and a porous Al layer is formed on a substrate having mechanical strength by electroplating, so that the surface area is extremely large. Moreover, it is an object of the present invention to provide an electrode for an electrolytic capacitor having a predetermined strength.

[0011]

In order to achieve the object, the electrolytic capacitor electrode of the present invention is characterized in that a porous Al layer is formed on the surface of a conductive substrate by electroplating. When this electrode is used as an anode, the Al layer is further subjected to chemical conversion treatment or anodization to form an oxide film that functions as a dielectric film.

As the conductive substrate, Cu, Fe, Al,
Various metal materials such as Ni and steel, and conductive ceramic materials are used. Further, a porous conductive material such as a metal foam can also be used as the conductive substrate.

The Al layer is formed on the surface of the conductive substrate by electroplating using a nonaqueous plating bath, for example. As for electric Al plating, JP-A-62-70592 and JP-A-1-272790 introduced by the applicant of the present invention
Various methods are adopted, starting with the publication of the publication.

[0014]

[Operation] When electroplating, the Al layer is uniformly formed on the surface of the conductive substrate with excellent adhesion. For example,
Even when the open cell type porous substrate is electroplated, the Al layer is deposited also on the inner surface of the pore.
The deposited Al is of extremely high purity and has suitable properties as an electrode.

The Al layer grows as dendrite-like crystals in the vertical direction from the surface of the conductive substrate. This crystal growth can be controlled by adjusting the plating conditions. For example, when the current density is reduced to suppress the precipitation of Al from the plating bath, crystal growth is faster than nucleation on the surface of the conductive substrate. As a result, granular or spongy Al is deposited and a porous Al layer is formed.

Since the obtained Al layer has a surface shape controlled at the crystal level, it has a remarkably large surface area as compared with the case where the Al foil is etched. Therefore, when used as an electrolytic capacitor,
Large capacitance is expected. On the other hand, the conductive substrate on which the Al layer is formed has no change in physical properties due to electroplating,
The original mechanical properties are maintained. As a result, an electrolytic capacitor having a large capacitance and excellent mechanical strength can be obtained.

[0017]

Example A 20 mm × 50 mm test piece was cut out from a low-purity Al foil having a thickness of 20 μm manufactured by rolling. After blasting the test piece and activating its surface, it was electroplated under the following conditions while changing the current density.

Conditions for electroaluminum plating Composition of plating bath: AlCl ₃ 906 g / l 1-ethyl-3-methylimidazolium chloride 48
4g / l phenanthroline 1.8g / l Plating bath temperature: 60 ° C

The test piece was pulled up from the plating bath, and the film thickness, adhesion, etc. of the Al layer formed on the surface of the test piece were investigated. In any of the test pieces, the Al layer has a substantially uniform thickness, and even after a bending test with R = 0.5 mm and a bending angle of 150 degrees, peeling of the Al layer from the underlying Al foil is detected. However, it was found that the film had good adhesion.

Next, the surface area of the Al layer was measured by an LCR meter. When the measurement result was expressed in relation to the current density during plating, it was found that the relation shown in Table 1 was established between the two. The surface area of the Al layer in Table 1 is represented by the ratio of the true surface area to the apparent surface area.

[0021]

[Table 1]

As is clear from Table 1, the surface area of the formed Al layer increases as the current density decreases. However, as the current density decreases, the plating time for forming the Al layer having a predetermined thickness becomes longer. Therefore, it is preferable to determine the current density so that the surface area is 30 to 40 times, taking both the surface area and the plating time into consideration.

The test piece of Test No. 3 on which the Al layer was formed was anodized by applying a voltage of 100 V in a phosphoric acid aqueous solution having a concentration of 5%. With respect to the test piece after the treatment, the capacitance, the dielectric loss, and the leakage current when a DC voltage of 200 V was applied were measured. The measurement results are shown in Table 2. In Table 2, a comparative example was prepared by etching an Al foil having a thickness of 100 μm and adjusting the surface area to 30 times the apparent area.

[0024]

[Table 2]

As is clear from this comparison, in the case where the porous Al layer formed by electroplating according to the present invention is used as an electrode, the surface of the Al foil is not affected by any of capacitance, dielectric loss and leakage current. It can be seen that it is superior to the one made porous by etching.

In addition, the same test pieces as those listed in Table 2
A load of 00 g was applied, and bending was performed at a bending angle of 90 degrees with R = 1.0 mm, and the folding strength was examined. The survey results are shown in Table 3. The folding resistance was represented by the number of times of bending until the test piece broke.

[0027]

[Table 3]

As is clear from Table 3, the electrode using the porous Al layer formed by electroplating has remarkably excellent mechanical strength. This is because the Al foil of the base body maintains the initial characteristics without being affected by any physical property during the electric Al plating.

In the above embodiments, the case where the porosity of the porous Al layer is controlled by adjusting the current density has been described. However, the present invention is not limited to this, and the porosity of the porous Al layer can be controlled by the conditions such as the composition, concentration, temperature and stirring of the plating bath.
In any case, as long as the Al layer is made porous, one having excellent characteristics required as an electrode for an electrolytic capacitor can be obtained.

[0030]

As described above, in the present invention, since the surface area is increased by the porous Al layer electroplated on the conductive substrate, the electrostatic capacity is large and the mechanical strength is excellent. An electrode for an electrolytic capacitor is obtained. Then, since the thickness and porosity of the porous Al layer are controlled by the plating conditions, it is possible to provide the characteristics required, and it is possible to provide an electrolytic capacitor that is compatible with higher performance and smaller size. Moreover, it does not cause problems such as multi-step processing and waste solution processing that are observed when etching Al foil, such as complication of processes and installation of incidental equipment. It is possible to manufacture with various equipment.

Claims (2)

[Claims] 1. An electrode for an electrolytic capacitor, wherein a porous Al layer is formed on the surface of a conductive substrate by electroplating. 2. An electrode for an electrolytic capacitor, wherein the conductive substrate according to claim 1 is porous.