JPH0794370A - Electrode foil for electrolytic capacitor - Google Patents

Abstract

PURPOSE:To give a capacitor stable and greater capacitance by providing a metallic foil that comprises a conductive material having microscopic unevenness on its surfaces covered with porous layers of molybdenum or tungsten. CONSTITUTION:Aluminum foil 1 is roughened by electrolytic etching so that unevenness of 0.5-20 microns Rmax, including recesses 2 and projections 3, may be observed on its surface with a scanning electron microscope. A porous film 4 of molybdenum or tungsten of 0.05-0.5 micron in thickness is deposited on each side of the roughened foil in a vacuum by electron beam heating. The foil thus obtained serves to give a capacitor greater capacitance than the conventional foil does.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode foil used as a material for an electrolytic capacitor, and more particularly to an improvement for increasing capacitance per unit area and stability of capacitance. .

[0002]

2. Description of the Related Art An electrolytic capacitor is formed by alternately sandwiching a pair of electrode foils made of aluminum or the like, to which leads are joined, and an electrode paper, wound in a cylindrical shape, impregnated with an electrolytic solution, and sealed in a case. Although manufactured, in recent years, along with the reduction in size and weight of electronic devices, there is an increasing demand for downsizing of electrolytic capacitors, which are electronic components.

In order to reduce the size of the electrolytic capacitor, it is important to increase the capacitance per unit area of the electrode foil. As is clear from the following equation, in order to increase the capacitance C of the electrode foil, it is effective to increase the surface area S of the electrode foil or the dielectric constant ε of the dielectric material or decrease the thickness t of the dielectric material. Is. C = kεS / t (k: constant ε: dielectric constant S: surface area t: dielectric film thickness)

Therefore, conventionally, a method has been generally adopted in which the surface of the aluminum foil is roughened by etching or the like to form fine irregularities on the entire surface to increase the surface area S. However, if the surface is roughened significantly, the electrode foil will become porous and its strength will decrease, and it will easily break during the capacitor manufacturing process. There was a limit to the capacity increase.

As means for overcoming the above-mentioned limit, Japanese Patent Laid-Open No. Sho 5
In Japanese Patent Publication No. 6-83923, a metal foil is roughened and
A structure has been proposed in which aluminum oxide, tantalum oxide, or titanium oxide having a high dielectric constant is vapor-deposited on the rough surface. According to this configuration, the area S can be increased by roughening and the dielectric constant ε can be increased, so that the capacitance of the electrode foil can be increased as compared with the case of only roughening.

Further, in Japanese Patent Laid-Open No. 61-180420, an aluminum foil is roughened and Ti,
Cr, Ag, Sn, Co, Zr, Ta, Si, Cu, F
There is described an electrode foil for an electrolytic capacitor, in which 0.02 to 1.0 μm of metal fine particles made of e or its alloy is coated to form a conductive metal coating having a thickness of 0.02 to 5.0 μm. According to this configuration, the surface area S of the conductive metal coating is
Can be expanded more than when the aluminum foil is subjected to etching treatment, and the capacitance can be increased.

Further, Japanese Patent Application Laid-Open No. 62-58609 describes a structure in which a porous titanium vapor deposition film is formed on a surface-roughened aluminum foil. According to this configuration, the surface area S of the conductive metal coating can be increased to increase the capacitance and corrosion resistance.

[0008]

However, even with the above three types of configurations, there is a limit to the increase in the capacitance of the electrode foil for electrolytic capacitors. Further, in the conventional electrode foil for electrolytic capacitors in which metal such as Ti is deposited on the metal foil, the surface of the electrode foil for electrolytic capacitors is gradually oxidized during storage,
There is an unavoidable increase in the thickness t of the metal oxide, which is a dielectric, with time, and there is a problem that the decrease in capacitance during storage with time cannot be ignored.

In order to solve the above problems, the present inventors have studied in detail the combination of various metal foils and vapor deposition materials, and as a result, formed a Mo or W film on the surface of the porous metal foil. It has been found that when formed, the capacitance can be significantly increased and the capacitance can be stabilized as compared with the electrode foil for electrolytic capacitors coated with Ti or metal oxide that has been conventionally used.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a metal foil made of a conductive material and having microscopic asperities formed on its surface, and a metal foil Mo having a thickness of 0.005 to 0.5 μm provided on the surface
Or a porous coating layer made of W.

A Ti intermediate layer may be formed between the metal foil and the coating layer. In addition, at least a part of the surface of the porous coating layer is oxidized and Mo
Alternatively, it may be an oxide of W.

Further, the metal foil is a roughened aluminum foil having a surface roughness of 0.5 to 20 μmRm.
It may be ax.

[0013]

According to the electrode foil for an electrolytic capacitor of the present invention, the capacitance can be remarkably improved as compared with the conventional electrode foil for an electrolytic capacitor. Further, since an oxide film is unlikely to be formed on the surface, deterioration of the capacitance with time is small, and the quality of the electrolytic capacitor can be improved.

[0014]

FIG. 1 is a schematic enlarged sectional view showing a first embodiment of an electrode foil for electrolytic capacitors according to the present invention. This electrode foil for electrolytic capacitors is made of a conductive material, and has an aluminum foil (metal foil) 1 on both sides of which microscopic asperities (recesses 2) are formed, and a thickness provided on both sides of this aluminum foil 1. The porous coating layer 4 is made of Mo or W and has a thickness of 0.005 to 0.5 μm.

In order to prepare the above-mentioned roughened aluminum foil, chemical etching or electrolytic etching is suitable, and the degree of roughening is 0.5 to 20 μm.
About mRmax is preferable. If it is less than 0.5 μmRmax, the unevenness of the surface of the porous coating layer 4 is lowered, and the capacity increasing effect is reduced. On the contrary, when it is larger than 20 μmRmax, the strength of the aluminum foil 1 is lowered, which is not preferable.

If the thickness of the porous coating layer 4 made of Mo or W is less than 0.005 μm, the effect of increasing the electrostatic capacity is poor, and if it is more than 0.5 angstrom, the manufacturing cost is increased and the electrostatic capacity is not increased. The increase is almost saturated. Part of the surface of the porous coating layer 4 may be oxidized to generate an oxide of Mo or W.

As a method for forming the porous coating layer 4, known methods such as a vacuum vapor deposition method, a sputtering method and an ion plating method can be used. A gas such as an inert gas may be introduced during film formation to form fine particles, or an oxidizing gas such as oxygen may be introduced to accelerate the oxidation of a part of Mo or W.

Next, FIG. 2 shows a second embodiment of the present invention.
This example is characterized in that an intermediate layer 5 made of Ti is formed between the aluminum foil 1 and each porous coating layer 4. A thin oxide layer (TiO ₂ ) may be formed on the surface of the intermediate layer 5, but it is desirable that the intermediate layer 5 and the porous coating layer 4 are electrically connected.

The electrode foil for electrolytic capacitors according to the present invention is suitable for use as a cathode material for capacitors. In that case, as the anode material to be combined, a metal foil formed with an oxide insulating film such as Al ₂ O ₃ is suitable in terms of withstand voltage. This is because the anode of the capacitor is required to have a higher withstand voltage than the cathode.

[0020]

[Experimental Example] Next, the effect of the present invention will be demonstrated with reference to an experimental example. An aluminum foil having a thickness of 50 μm and a purity of 99.8% was subjected to electrolytic etching treatment under the following conditions to roughen both surfaces thereof. Etching treatment liquid: 2.5 wt% hydrochloric acid aqueous treatment treatment condition / temperature: 60 ° C. time: 200 seconds Electrolytic current: 20 A / 50 cm ²

Observation of the surface of the obtained aluminum foil with a scanning electron microscope revealed that microscopic irregularities of about 0.5 μm Rmax were formed over the entire surface. The electrostatic capacity of this roughened aluminum foil was measured and found to be 0.290 mF / cm ² . The capacitance was measured under the following conditions. Measuring instrument: LCR meter 4326A (trade name) manufactured by HP Co., Ltd. Applicable condition: 120 Hz, 50 mV Electrolyte solution: ammonium borate 10 wt% solution (measurement value after 3 minutes of immersion was recorded) Specimen size: 10 mm x 50 mm

On both surfaces of the roughened aluminum foil, coating layers made of Mo or W having various thicknesses were formed by electron beam heating vapor deposition in a vacuum having a vacuum degree of 5 × 10 ⁻³ Pa. The result of measuring the capacitance of the obtained electrode foil for electrolytic capacitors is shown in FIG. The capacitance was measured under the same conditions as above.

On the other hand, 0.75 μm (7500 angstroms) was formed on both surfaces of the roughened aluminum foil by electron beam heating vapor deposition in a vacuum with a vacuum degree of 5 × 10 ⁻³ Pa.
After Ti was vapor-deposited, a coating layer made of Mo or W having various thicknesses was formed under the same vacuum degree. The results of measuring the capacitance of the obtained electrode foil for electrolytic capacitors are also shown in FIG. 3 (plotted with the thickness of Mo or W). The capacitance was measured under the same conditions as above.

Further, as comparative examples, Ti layers having various thicknesses were formed on both surfaces of the roughened aluminum foil by electron beam heating vapor deposition in a vacuum having a vacuum degree of 5 × 10 ⁻³ Pa. The results of measuring the capacitance of the obtained electrode foil for electrolytic capacitors are also shown in FIG. The capacitance was measured under the same conditions as above.

As is apparent from FIG. 3, when Mo or W is vapor-deposited, the capacitance is large relative to the thickness of Ti vapor-deposited. For example, when the film thickness is 2500 angstroms. Comparing with, Mo is about 4 of Ti
A double value and a double value for W were obtained. That is, the product standard of the capacitance of a normal electrode foil for electrolytic capacitors is, for example, 1
In the case of mF / cm ² , in order to obtain this capacitance, the Ti film needs to have 5100 angstroms, whereas Mo has 1500 angstroms and W has 2,000 angstroms.
Angstrom is enough. Therefore, the speed of the film forming line can be increased and the productivity can be improved. Further, even when the Ti layer was present as the intermediate layer, the effect of increasing the electrostatic capacitance, which is a characteristic of Mo and W, was obtained by coating with Mo and W.

Next, the above electrode foils for electrolytic capacitors were left in the atmosphere at room temperature under the same conditions, and the change in capacitance with time was examined. Fig. 4 shows the result of comparison between Ti and Mo.
5 shows the result of comparison between Ti and Mo +, and FIG. 6 shows the result of comparison between Ti and Ti + W. When only Ti is vapor-deposited, the electrostatic capacity is greatly reduced in the first few days, whereas when Mo is vapor-deposited to 2000 angstroms or less, the capacitance almost disappears as shown in FIG. I couldn't do it. In the case of depositing 2500 Å of Mo, it was confirmed that the capacitance increased conversely with time. This is considered to indicate that the surface of the Mo coating becomes more porous as Mo is oxidized.

Further, from the results of FIGS. 5 and 6, when Ti is deposited at 7500 angstroms and then Mo or W is deposited, the change in capacitance with time is reduced as compared with the case where only Ti is deposited. However, it was confirmed that the capacitance can be stabilized over time.

Next, FIG. 7 shows the result of measuring the element distribution in the thickness direction from the Mo vapor deposition side of the electrode foil for electrolytic capacitors in which Mo was vapor-deposited by 1000 Å, using the ESCA apparatus. For the measurement, elemental analysis was performed by irradiating the foil surface with Ar ions and irradiating with X-rays while sputtering at a constant rate. X-ray intensity is 8.0 kV x 30 mA, Ar ⁺
The ion acceleration conditions were 2.0 kV × 20 mA. As is clear from FIG. 7, the Mo vapor deposition film formed on the foil surface contains oxygen atoms of approximately 20 atom% almost uniformly.

Next, with respect to the electrode foil on which the Ti intermediate layer was formed, the capacitances of the deposited film of Mo or W and the deposited film of another metal were compared with each other.
After 0.75 μm of Ti was vapor-deposited on the roughened aluminum foil, the substances shown in Table 1 were vapor-deposited on the Ti film by 300 angstroms each, and the capacitance of the obtained electrode foil was measured by the method described above. The film forming conditions and the like are the same as in the above experiment. The results are shown in Table 1.

[0030]

[Table 1]

As is clear from the results shown in Table 1, the effect of increasing the capacitance is obtained only when Mo and W are vapor-deposited, and when another metal is vapor-deposited on Ti, T
The capacity was the same as that of the i-deposited film alone or decreased.

[0032]

As described above, according to the electrode foil for an electrolytic capacitor of the present invention, as compared with the conventional electrode foil for an electrolytic capacitor in which Ti or the like is deposited on the roughened metal foil, the electrostatic foil The capacitance can be significantly improved, and if the same capacitance is obtained, the vapor deposition thickness can be reduced to improve the productivity. Further, since an oxide film is unlikely to be formed on the surface, deterioration of the capacitance with time is small, and the quality of the electrolytic capacitor can be improved.

[Brief description of drawings]

FIG. 1 is an enlarged cross-sectional view showing a first embodiment of an electrode foil for an electrolytic capacitor according to the present invention.

FIG. 2 is an enlarged sectional view showing a second embodiment of the electrode foil for an electrolytic capacitor according to the present invention.

FIG. 3 is a graph showing the relationship between the coating layer thickness and the capacitance of the electrode foil for electrolytic capacitors of the present invention.

FIG. 4 is a graph showing changes with time of electrostatic capacitance of the electrode foil for electrolytic capacitors of the present invention.

FIG. 5 is a graph showing changes with time of electrostatic capacitance of the electrode foil for electrolytic capacitors of the present invention.

FIG. 6 is a graph showing changes with time of electrostatic capacitance of the electrode foil for electrolytic capacitors of the present invention.

FIG. 7 is a graph showing element distribution on the surface of the electrode foil of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal foil 2 Recessed portion 3 Convex portion 4 Porous coating layer 5 Ti intermediate layer

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[Procedure amendment]

[Submission date] November 10, 1993

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Claims (4)

[Claims] 1. A metal foil which is made of a conductive material and has microscopic asperities formed on its surface, and Mo or W having a thickness of 0.005 to 0.5 μm provided on the surface of the metal foil. An electrode foil for an electrolytic capacitor, comprising: 2. A Ti layer is provided between the metal foil and the coating layer.
The electrode foil for an electrolytic capacitor according to claim 1, wherein an intermediate layer is formed. 3. The electrode foil for an electrolytic capacitor according to claim 1, wherein at least a part of the surface of the porous coating layer is oxidized to be an oxide of Mo or W. 4. The electrolytic capacitor according to claim 1, wherein the metal foil is a roughened aluminum foil, and the surface roughness is 0.5 to 20 μmRmax. Electrode foil.