JPH0365010B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for forming electrode foil used in aluminum electrolytic capacitors.

Conventional structure and its problems There is a strong demand for aluminum electrolytic capacitors to be lighter, thinner, shorter, smaller, and of higher quality. It is inevitable to improve the film properties such as dielectric properties of the film produced in the process.

Conventional electrode foils for this type of aluminum electrolytic capacitor have been obtained by the following method. That is, as a pretreatment before chemical formation, boiling treatment is performed in pure water at 95°C or higher, and then boric acid and boric acid salts, adipic acid and adipic acid salts, or phosphoric acid and phosphate salts, or a single aqueous solution of these, is added. A method of performing chemical conversion using a mixed aqueous solution of boric acid (conventional example 1) or anodic oxidation in the above-mentioned acid or alkali aqueous solution as a pretreatment before chemical formation to form a porous film, and then applying boric acid, Electrode foils for aluminum electrolytic capacitors were obtained by a chemical conversion method (Conventional Example 2) using adipic acid, phosphoric acid, and their salts alone or in a mixed aqueous solution.

These methods had the following problems.

In other words, in the former conventional example 1, as shown in Figures 1 and 2, by increasing the boiling time, the capacitance increases and the leakage current decreases, although there are differences depending on the type of chemical liquid. Although there is a trend,
This is still insufficient. In addition, 1 in the figure is boric acid
8wt% + borax 0.15wt% mixed aqueous solution, 2 is ammonium adipate 0.3wt% aqueous solution, 3 is orthophosphoric acid
In a 0.02wt% aqueous solution, using a constant voltage constant current power supply at a current density of 2.5A/dm ² and a formation voltage of 400V,
After rising to 400V, hold it at a constant voltage for 10 minutes, and after 10 minutes, the voltage will rise to 400V.
The foil was heat treated for 1 minute at °C and then subjected to the same chemical conversion and heat treatment three times in the same solution.

In addition, in the latter conventional example 2, as shown in Figs. 3 and 4, the leakage current can be reduced by increasing the amount of electrolysis generated in the porous film, but at the same time the capacitance also decreases. This method is also insufficient.

Note that Figures 3 and 4 show boric acid as a representative example.
Although the case of a mixed aqueous solution of 8wt% + borax 0.15wt% has been described, the same tendency is observed for both adipic acid type and phosphoric acid type.

Purpose of the Invention It is an object of the present invention to provide a method for chemically forming an electrode foil that can eliminate such conventional drawbacks and realize an aluminum electrolytic capacitor with improved capacitance and reduced leakage current. That is.

Structure of the Invention The present invention comprises a process of anodizing with an acid such as sulfuric acid, oxalic acid, or an alkaline solution of sodium hydroxide to form a porous film on the surface, and a process of boiling the aluminum electrode foil on which the porous film has been formed. This combination is used as a pretreatment, and then the aluminum electrode foil is subjected to a chemical conversion treatment using phosphoric acid and an aqueous phosphate solution at a concentration that allows the formation of a barrier film.

When chemical conversion is performed using this chemical conversion method, a hydrated film (pseudo-boehmite) produced by boiling treatment is uniformly formed as a double layer in the air pores in the porous film obtained by electrode oxidation, and this is coated with phosphoric acid and By performing chemical conversion with an aqueous phosphate solution, phosphorus atoms in the chemical solution are adsorbed and diffused into the hydrated film (pseudo-boehmite) produced by boiling, and the hydrated film (pseudo-boehmite film) is formed by deprotonation. When replacing with α or γ alumina, it remains in the film. The double layer of porous film and hydrated film replaces the hydrated film and phosphorus atoms remain in the film, resulting in a synergistic effect, which makes it possible to improve capacitance and reduce leakage current.

Explanation of Examples Figures 5 and 6 show 1×10 ^-3 , 2×10 ^-3 , 7×10 ^-3 , and 11 in 5 wt% oxalic acid aqueous solution, respectively.
After anodizing with an electricity amount of ×10 ^-3 AH, the temperature is 95℃~
In the figure, 1 is a mixed aqueous solution of 8 wt% boric acid + 0.15 wt% borax, 2 is a 0.3 wt% aqueous solution of ammonium adipate, and 3 is an aluminum electrode foil that has been boiled in boiling water at 100°C for 5 minutes. is orthophosphoric acid 0.02wt%
In an aqueous solution, using a constant voltage current power source at a current density of 2.5 A/dm ² , the formation voltage was increased to 400 V by holding at a constant voltage for 10 minutes after increasing the voltage by 400 V. After 10 minutes, heat treatment was performed at 400°C/min. The leakage current of a foil that had been subjected to the same chemical conversion and heat treatment three times in the same solution was measured in an 8wt% boric acid aqueous solution, and then in an 8wt% ammonium borate aqueous solution using a capacitance meter. be.

According to this example, as shown in FIGS. 5 and 6, after performing a pretreatment with a double layer that produces a porous film and a hydrated film, phosphoric acid and a phosphoric acid aqueous solution are used. By chemical conversion, it is possible to improve capacitance and reduce leakage current due to the synergistic effect of pretreatment with a double layer and adsorption and diffusion of phosphorus atoms into the film.

In addition, in this example, after generating a porous film, a hydrated film (pseudo-boehmite film) was generated,
Although a double layer pretreatment film is used, this double layer pretreatment film may be formed by forming a hydrated film (pseudo-boehmite film) and then forming a porous film.

Effects of the Invention As described above, the present invention provides a two-dimensional structure in which a hydrated film (pseudo-boehmite film) is produced by anodizing in an acid or alkaline solution capable of producing a porous film, and then subjected to boiling treatment. By forming a multi-layer pre-treatment film and performing chemical conversion treatment in phosphoric acid or phosphate aqueous solution in the next step, it is possible to adsorb and diffuse phosphorus atoms into the double-layer pre-treatment film, resulting in high capacitance. This has the effect of making it possible to manufacture an excellent aluminum electrolytic capacitor with low leakage current characteristics.

[Brief explanation of drawings]

Figures 1 and 2 are characteristic diagrams showing the capacitance and leakage current characteristics when using the method according to conventional example 1, respectively, and Figures 3 and 4 are characteristic diagrams showing the characteristics of conventional example 2, respectively.
Figures 5 and 6 are characteristic diagrams showing the characteristics of capacitance and leakage current when using the method according to the present invention, respectively. FIG. 3 is a characteristic diagram showing characteristics.

Claims (1)

[Claims] 1. Anodizing the aluminum electrode foil with an acid such as sulfuric acid, oxalic acid, or an alkaline solution of sodium hydroxide to generate a porous film on the surface;
A pre-treatment is performed by combining the aluminum electrode foil with the porous film formed thereon by boiling it.
A method for chemically forming an electrode foil for an aluminum electrolytic capacitor, which comprises subsequently chemically forming the aluminum electrode foil with phosphoric acid and an aqueous phosphate solution.