JPH0337855B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a method for manufacturing an electrode foil for an aluminum electrolytic capacitor, which prevents deterioration of capacitor performance by forming a high-quality oxide film on an etched aluminum foil during the manufacturing process of the aluminum electrolytic capacitor. . [Technical background of the invention and its problems] In general, the anode side electrode foil used in aluminum electrolytic capacitors is produced by an etching process that roughens the surface of the aluminum foil to increase its surface area, and an anodizing process of the etching foil. It is manufactured through a chemical conversion process that forms an oxide film on the surface. The etching foil produced in the etching process is
It is produced by electrochemically dissolving aluminum foil in an aqueous solution containing chlorides to roughen the surface, then washing with pure water to remove the chlorides, and finally drying. At this time, in order to increase the cleaning effect, cleaning with an aqueous solution such as nitric acid, boric acid, or phosphoric acid may also be used. In this manufacturing process, the surface of the aluminum foil immediately after roughening is highly activated, and during the cleaning and drying processes, it reacts with the cleaning water and oxygen in the air, forming a hydrated film on the aluminum. An air oxide film (hereinafter referred to as natural oxide film) is formed on the surface. This natural oxide film is a whisker-like or porous layer, exhibits weak insulation, and has chemically weak properties. In addition, in the chemical conversion step, the etching foil is anodized in an electrolytic solution using boric acid, phosphoric acid, or an organic acid, so that aluminum oxide (Al ₂ O ₃ ) is formed on the foil surface.
Form a barrier layer consisting of: In this case, the thickness of the oxide film formed by chemical formation corresponds to the voltage obtained by subtracting the withstand voltage of the natural oxide film from the applied chemical formation voltage, and the natural oxide film remains in the upper layer, forming a composite oxide film. Therefore, especially in an oxide film formed at a low voltage, the ratio of the natural oxide film to the entire film becomes larger. By the way, according to the conventional method of manufacturing electrode foil,
Since the natural oxide film formed on the surface of the etching foil had not been subjected to any treatment, the upper layer of the composite oxide film formed through the subsequent chemical conversion process was a natural oxide film that was chemically weak and easily dissolved. It consists of a membrane. Therefore, when such electrode foil is used in a capacitor, it reacts with the driving electrolyte inside the capacitor and dissolves the upper natural oxide film, lowering the withstand voltage and increasing the leakage current of the electrolytic capacitor. It is the cause. In particular, for films chemically formed at low voltage,
Due to the high proportion of the natural oxide film, the leakage current increased significantly. [Object of the Invention] In view of the above-mentioned points, the object of the present invention is to improve the deterioration characteristics of electrolytic capacitors, particularly to reduce the increase in leakage current, by forming a high-quality oxide film on the surface of etched aluminum foil. An object of the present invention is to provide a method for manufacturing an electrode foil for an aluminum electrolytic capacitor that can be made small. [Summary of the Invention] The present invention involves immersing an aluminum foil that has undergone an etching process in an aqueous solution containing phosphoric acid to adsorb phosphoric acid, and then heat-treating the foil to form a thermal oxidation layer containing phosphoric acid on the surface of the foil. Forms a film. Next, by treating with an acid or alkaline solution to dissolve and remove a portion of the easily soluble oxide film, an etched foil having a thermal oxide film containing phosphoric acid that is difficult to dissolve on the surface is obtained. However, by subsequently performing a normal chemical conversion treatment, a composite oxide film having a film portion that is difficult to dissolve is formed in the upper layer. [Embodiments of the Invention] A method for manufacturing an electrode foil for an aluminum electrolytic capacitor according to the present invention will be schematically explained. First, the aluminum foil is roughened in an aqueous solution containing chloride (etching step). Next, chlorides are removed with pure water, etc. (cleaning step) Next, phosphoric acid is applied to the surface of the etching foil by immersing it in a solution containing phosphoric acid, such as phosphoric acid, ammonium phosphate, or potassium phosphate. Adsorb (phosphoric acid treatment step). This phosphoric acid treatment step can also serve as the removal of chlorides at the same time, so if a phosphoric acid aqueous solution is used as the cleaning liquid in the cleaning step, the phosphoric acid treatment step can be used instead of the cleaning step. Thereafter, a thermal oxide film containing phosphoric acid is formed on the surface of the etching foil by passing it through a furnace at 150°C to 600°C (heat treatment step). At this time, on the surface of the etching foil, there are formed a high quality oxide film containing phosphoric acid which is formed by the heat treatment process and which is difficult to dissolve, and a weak film which is easily soluble and which has existed before the heat treatment process. That is, the thermal oxide film containing phosphoric acid has a stable property of not being dissolved in the driving electrolyte of the electrolytic capacitor. However, in this heat treatment, not all of the surface is covered with a high-quality thermal oxide film, and a weak film that is present from the etching process is also included. Next, the etching foil is immersed in an acidic solution such as citric acid, oxalic acid, phosphoric acid, nitric acid, or sulfuric acid, or an alkaline solution such as aqueous ammonia or sodium hydroxide to dissolve and remove the easily soluble film. (hereinafter referred to as the dissolution process). By doing this, only a high-quality film that is difficult to dissolve remains on the foil surface. Wash the solution with pure water. Next, the etching foil, on which a film that is difficult to dissolve is formed on the surface as described above, is anodized in a chemical electrolyte to form an oxide film (chemical conversion step). The oxide film formed at this time becomes a composite oxide film having a film portion that is difficult to dissolve in the upper layer. This composite oxide film has less reaction to the electrolyte of the capacitor, and when the electrode foil is assembled into the capacitor, it is possible to manufacture a capacitor with less deterioration in performance. Table 1 shows examples in which the conditions in the phosphoric acid treatment step, heat treatment step, and dissolution step among the steps described above were changed.

[Table] In Examples 1 to 5, aluminum foil that had been etched and washed with pure water was treated with phosphoric acid under the phosphoric acid treatment conditions shown in Table 1, and then heat treated under the heat treatment conditions shown in Table 1. After performing the dissolution treatment under the dissolution conditions shown in 1, the solution was washed with pure water to remove the solution, and then dried. In addition, as a comparative example, a case in which one of the three processes mentioned above is excluded, and as a conventional example, three
The cases in which none of the two treatments are performed are shown. Next, each foil treated under the conditions in Table 1 was
In both cases, chemical conversion treatment was performed in a 0.1% aqueous ammonium dihydrogen phosphate solution at a formation voltage of 15V. Next, these electrode foils were wound together with a cathode foil with a separator paper interposed between them, impregnated with an electrolytic solution, and then sealed in an aluminum case using a packing material. Aluminum electrolytic capacitors with a rated voltage of 10 V and a rated capacity of 470 μF corresponding to Examples 1 to 3 and the conventional example were manufactured. The leakage current of the capacitor fabricated as described above was measured, and the leakage current after fabrication was taken as the initial value, and the leakage current values after being left at 85°C for 500 hours in an unloaded state were compared and shown in Table 2. Indicated.

[Table] As shown in Table 2, each of Examples 1 to 5 in which a phosphoric acid treatment step, a heat treatment step, and a dissolution step according to the present invention were added was more effective at 85°C than in the conventional example and Comparative Examples 1 to 3. , the leakage current after being left for 500 hours is kept small. Therefore, it can be said that by performing the above-mentioned phosphoric acid treatment step, heat treatment step, and melting step, a high-quality oxide film that can suppress leakage current deterioration can be formed. Next, the results of testing the relationship between melting time and leakage current in the melting step after the heat treatment step are shown in the figure. In this test, the etching foil was immersed in a 0.5% phosphoric acid aqueous solution and then heat-treated at 500℃ for 1 minute.
Subsequently, it was immersed in a 1% aqueous sodium hydroxide solution at 25°C for dissolution treatment. Thereafter, the lysate was washed away. In this case, samples were prepared in which the dissolution treatment was not performed and the samples were subjected to the dissolution treatment for each time of 0.5, 1, 2, 4, and 6 minutes. Then, after performing chemical conversion on each sample in a 0.1% ammonium dihydrogen phosphate aqueous solution at a voltage of 15V,
Rated voltage 10V, rated capacity 470μF using each electrode foil
We manufacture aluminum electrolytic capacitors at 85℃,
Leakage current was measured after being left unloaded for 500 hours. As can be seen from the figure, leakage current deterioration is large in samples that have not been subjected to dissolution treatment, and the dissolution time is 1.
At ~3 minutes, the weak film portion was dissolved and removed, and the deterioration was reduced. However, if the dissolution time is further increased, the oxide film that is difficult to dissolve will also dissolve and disappear, resulting in increased deterioration. Generally, the optimum dissolution time is determined by the amount of thermal oxide film produced in the heat treatment process, and the type, concentration, and temperature of the dissolving solution. It should be noted that the phosphoric acid treatment step in which the etching foil is treated with phosphoric acid, the heat treatment step in which a thermal oxide film is formed by heat treatment, and the dissolution step in which a part of the film is melted are separated in time between the etching step and the chemical conversion step. The same effect can be obtained even if these steps are performed as consecutive steps, or even if each step or one step is performed separately in time. [Effects of the Invention] As described above, according to the present invention, a high-quality oxide film can be formed on the surface of an etched aluminum foil, and an aluminum electrolytic capacitor with less deterioration due to leakage current can be realized.

[Brief explanation of drawings]

The figure shows 85 aluminum electrolytic capacitors manufactured by changing the melting time of the melting process in the present invention.
3 is a graph showing the results of measuring leakage current after being left at ℃ for 500 hours.

Claims (1)

[Claims] 1. In a method for manufacturing an electrode foil for an aluminum electrolytic capacitor, which comprises an etching process to roughen the surface of the aluminum foil, and a chemical conversion process to form an oxide film on the surface of the etched foil, the etching foil is rinsed before the chemical conversion process. A step of treating in an aqueous solution containing an acid, then a step of treating at high temperature to form a thermal oxide film on the foil surface, and then a step of treating with an acid or alkaline solution to form a part of the thermal oxide film. 1. A method for manufacturing an electrode foil for an aluminum electrolytic capacitor, comprising a step of leaving a residue.