JP3352798B2 - Aluminum electrolytic capacitor anode foil - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a foil for an anode of an aluminum electrolytic capacitor which is used for an electrode for an electrolytic capacitor and is particularly suitable for a medium and high voltage capacitor.

[0002]

2. Description of the Related Art Normally, aluminum foil having a purity of 99.9% or more is used for an anode foil of a capacitor for medium and high pressures. Is given. When manufacturing aluminum foil for electrolytic capacitors, hot rolling and cold rolling are performed by a conventional method, and in that case, an intermediate heat treatment is performed on the way as needed, and finally, for example, about,
The foil is rolled to a thickness of about 100 μm. After that, the aluminum foil is subjected to final annealing at a temperature of 500 to 600 ° C., and the above-described surface roughening treatment is performed.

[0003] This surface roughening treatment is performed by flowing a direct current in a solution mainly composed of hydrochloric acid and electrolytically corroding the surface of the aluminum foil. When observing the surface of the foil after the surface roughening treatment, countless circular corrosion holes having a diameter of 1 to 3 μm are formed, and each of the corrosion holes extends perpendicularly to the foil surface, that is, a columnar corrosion hole is formed. Corrosion holes are formed. These pits result in an increase in surface area and therefore a high capacitance.

As described above, it has been clarified that the capacitance greatly depends on the number and size of the corrosion holes formed by the surface roughening treatment. Efforts to increase the number of corrosion holes by this surface roughening process have been continuously performed,
Studies have been made on the etching conditions and the surface properties of aluminum foil. One of the reasons is that the foil for medium and high pressure has a good oxide film (film thickness: 5 ± 0.5) when treated in a final annealing step at a high temperature and in a non-oxidizing atmosphere.
It has been found that the capacitance is improved due to the formation of about (nm), but the detailed reason is unknown.

[0005]

However, when the surface of the material subjected to the final annealing is roughened, the number of corroded holes often becomes non-uniform in an actual product. Failure due to insufficient electric capacity has occurred. Possible causes include variations in the amount of rolling oil deposited during cold rolling and variations in the amount of oxygen in the atmosphere during final annealing.However, it cannot be strictly controlled in the mass production process, and the There is lot-to-lot fluctuation, which is a problem in quality control.

[0006]

The present inventors have conducted various studies on the cause of the difference in the surface roughening rate, that is, the capacitance, depending on the annealing and rolling lots. As a method, it was confirmed that the surface oxide film was thinned. However, improvement of the capacitance is not sufficient by itself, and further studies have revealed that a high capacitance can be stably obtained in the aluminum foil of the present invention. That is, the present invention is characterized in that the thickness of the oxide film formed on the surface of the aluminum foil is 4.5 nm or less, and that the oxide film contains fluorine atoms as hetero atoms. The second invention is characterized in that, in the above invention, the atomic weight of fluorine is 1 to 70 atomic%.

[0007]

According to the present invention, during the surface roughening treatment, a uniform and sufficiently large number of corrosion holes are formed on the surface of the aluminum foil, and a high capacitance can be obtained. The reason why the thickness of the surface oxide film of the aluminum foil is set to 4.5 nm or less in the present invention is that the capacitance is improved by making the thickness smaller. That is, if the thickness of the surface oxide film exceeds 4.5 nm, it is difficult to obtain a sufficient capacitance. Further, for the same reason, it is desirable that the thickness be 2 nm or less. However, in order to form pits uniformly during the surface roughening treatment, the thickness is desirably 1 nm or more.

However, simply reducing the thickness of the oxide film does not have a significant effect of improving the capacitance. In addition to this, by including fluorine atoms as foreign atoms in the crystal of the oxide film, the capacitance can be reduced. Improve dramatically. However, if the atomic weight of fluorine is less than 1 atomic% in the oxide film, uniform pits cannot be sufficiently generated. Further, it is desirably at least 20 atomic weight%. On the other hand, if the number of fluorine atoms is increased too much, the improvement in capacitance not only gradually decreases, but also the productivity decreases due to an increase in working time, etc., so that the fluorine content is desirably 70 atomic% or less. More desirably, the content is 50 atomic% or less.

The method for forming the oxide film having the above-described structure can be adjusted by, for example, a method called dry etching. This method is a method of ionizing fluorine by, for example, bringing a CF4 gas into a plasma state, accelerating the ionized fluorine, and applying the ionized fluorine to the aluminum surface. In this method, fluorine ions can be implanted while the oxide film is thinned. The implanted fluorine atoms remain as substitutional or interstitial heteroatoms in the crystal of the oxide film.

The reason why a higher capacitance can be obtained when fluorine ions are implanted in addition to the thinner oxide film is considered as follows. First, it can easily be inferred that the thinner the oxide film, the lower the corrosion resistance. Therefore, the film is relatively easily destroyed during electrolytic etching, and the number of corrosion holes due to electrolytic etching increases. However, this alone did not eliminate the occurrence of non-uniform corrosion holes, but it is clear from this study that etching a sample in which F ions were implanted into an oxide film could produce fairly uniform and high-density corrosion holes. Became. The reason is that the cl-ions contained in the hydrochloric acid of the electrolytic etching solution under roughening conditions and the F ions in the oxide film are replaced during the electrolytic etching, and the oxide film is destroyed while the process continues. Therefore, it is considered that a large number of corroded holes are formed uniformly and the capacitance is stably and increased.

[0011]

An embodiment of the present invention will be described below. A roll-up aluminum foil (thickness: 100 μm) having a purity of 99.99% was produced by an ordinary method, and the aluminum foil was heated at 530 ° C. for 6 hours in an inert gas as final annealing. Next, a high-frequency plasma was generated by introducing a CF 4 gas in a vacuum vessel, and a process was performed by applying F ions to a sample placed at a positive potential. By changing the time of this treatment, the thickness of the oxide film and the implantation amount of F ions were changed.

For comparison, the comparative material No. 1 in which F ions were implanted without reducing the oxide film by reducing the acceleration voltage of F ions was used. 11 facilitated. Furthermore, after annealing, a sample (N
o. 10) and a sample in which the oxide film thickness was reduced by cleaning with a caustic soda solution without F ion implantation (No. 1)
And 2) were separately prepared as comparative materials. For these samples, the F ion content and oxide film thickness
The measurement was performed according to A, and the surface roughening treatment was performed under the following conditions, and the capacitance was measured. The results are shown in Table I.

(Roughening treatment) First stage of etching-hydrochloric acid 1 mol / l + sulfuric acid 2.5 mol / l, 7
1. Etching for 30 seconds at a current density of 0.6 A / cm ^{2 in} a solution at 5 ° C. Second stage of etching-Etching for 3 minutes in a solution of nitric acid 10 liter + water 10 liter, 80 ° C

[0014]

[Table 1]

As a result, it is clear that the inventive material has much less unetched portions than the conventional material, and as shown in Table 1, the capacitance is greatly improved. On the other hand, in the case of the conventional material, the result was that there were many unetched portions and the capacitance was small. From these results,
It has been found that a combination of the reduction in the thickness of the oxide film and the content of fluorine atoms significantly improves the capacitance during the surface roughening treatment.

[0016]

As described above, in the foil for an anode of an aluminum electrolytic capacitor of the present invention, the thickness of the oxide film formed on the surface of the aluminum foil is 4.5 nm or less, and different types of oxide film are contained in the oxide film. Since fluorine atoms are contained as atoms, a large number of pits are formed uniformly and at high density on the surface of the aluminum foil during the surface roughening treatment, and the capacitance is dramatically improved.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01G 9/04

Claims (2)

(57) [Claims] 1. An anode foil for an aluminum electrolytic capacitor, characterized in that the oxide film formed on the surface of the aluminum foil has a thickness of 4.5 nm or less, and that the oxide film contains fluorine atoms as foreign atoms. 2. The foil for an anode of an aluminum electrolytic capacitor according to claim 1, wherein the atomic weight of fluorine is 1 to 70 atomic%.