JPH025009B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for etching aluminum foil for anodes of electrolytic capacitors, particularly for medium and high voltages (100 W.V. or higher). Conventional configurations and their problems Conventional aluminum electrolytic capacitors of this type either have to increase their capacitance, or are smaller or smaller.
In order to reduce the price and improve other characteristics, the aluminum electrode foil is etched to increase its surface area. Various methods have been considered and implemented in the past to expand the surface area of electrode foils for aluminum electrolytic capacitors, and the most common method is electrolytic etching of aluminum anodically in an aqueous chloride solution such as hydrochloric acid or common salt. A method is being taken. By the way, the manufacturing conditions for aluminum electrolytic capacitors, especially as etched foils for medium and high voltages, are that a very thick anodic oxide film is formed by chemical conversion treatment after etching, and very fine etching pits (due to etching) are formed on the aluminum foil surface. The setting is made so that the generated irregularities on the aluminum surface and inside will not be buried and become unable to contribute to the etching magnification. In other words, the ideal etching pit shape for medium-high voltage electrode foil is
It is generally considered ideal that the etching pit diameter is 1.3 μm (when used at a formation voltage of up to 600 V) and that the etching pit is a tunnel pit (referring to the state in which the etching pit penetrates from the surface to the back of the aluminum foil). As a result of various studies aimed at creating an ideal etching pit state, the present inventors added a film-forming substance such as oxalic acid, sulfuric acid, phosphoric acid, or boric acid to an aqueous solution of hydrochloric acid as the first stage of etching, and etched with a direct current. After that, as a second stage of etching, direct current etching is performed using an aqueous solution of sodium chloride, which is a metal chloride that has a greater tendency to ionize than aluminum, or potassium chloride or ammonium chloride. Electrode foils have been manufactured through a process of forming an oxide film two to six times. However, in conventional etching methods, surface dissolution often progresses as etching progresses, and the etching pit diameter is too small relative to the formation voltage, making it impossible to obtain a sufficient surface enlargement magnification (hereinafter referred to as magnification), or the magnification is too small. Even when enlarged, the shape of the etching pits is not controlled, and the etching pits propagate too much branchingly in directions other than the depth direction inside the foil, significantly reducing the strength of the electrode foil. This has been a major obstacle in achieving miniaturization of aluminum electrolytic capacitors by improving the Here, an example of the conventional etching method will be explained in more detail as follows. That is, when conventionally the first stage was etched with a direct current in a hydrochloric acid solution and the second stage was etched with a direct current in a hydrochloric acid solution (see Example 1 in Figures 1 and 2), or the second stage was etched with a hydrochloric acid solution. In the case of chemical etching (see example 3 in Figures 1 and 2),
The advantages are that the etching pit of the electrode foil can be tunneled and the mechanical strength of the electrode foil is excellent, but the disadvantage is that even if the melt loss is increased, the capacity is saturated at an early stage and high magnification cannot be obtained. It was hot. This is because when hydrochloric acid is used as an etching solution, even if a film-forming substance is added to the surface of the electrode foil, the activation of the aluminum surface progresses as the amount of dissolution of aluminum increases due to etching, and the etching progresses due to surface dissolution. This is because the etching pits on the foil surface are destroyed as the etching progresses, and pits that branch in the lateral direction other than the tunnel direction are hardly generated. Next, conventionally, the first stage was etched with a direct current in a hydrochloric acid solution, and the second stage was etched with a direct current in a neutral etching solution consisting of ammonium chloride or a chloride of a metal that has a greater tendency to ionize than aluminum (Fig. 1). , Example 2) in Figure 2 has the advantage of obtaining a high magnification, but the disadvantage is that the mechanical strength of the foil rapidly decreases as the melt loss of aluminum increases, so the melt loss is only 8 mg/ ^cm2. This was the limit, and any further increase in magnification was restricted. This is because the oxide film formed on the foil surface in the neutral etching solution is thick, resulting in non-uniformity in surface activity over a relatively wide area on the foil surface due to the hydration film on the surface. The etching pits formed in the foil grow in the direction of the tunnel at the same time as the existing etching pits grow in the tunnel direction, and new etching pits are generated from active points caused by non-uniform surface activity. This is because the etching pits develop into branched etching pits in the vertical and lateral directions and grow excessively, causing the etching pits to connect with each other, making it impossible to maintain the mechanical strength of the electrode foil.
In addition, neutral etching solutions tend to cause non-uniformity in surface activity over a relatively wide range, so compared to hydrochloric acid etching solutions, the overall dissolution function for adjusting the pit diameter inside the etching pit is lower. Because the dissolution tends to concentrate at the tip of the etching pit, the diameter of the etching pit becomes narrower.
When an anodic oxide film was later formed, many etching pits were generated that did not contribute to the magnification, and this became a problem as a factor that hindered the improvement of the magnification. OBJECT OF THE INVENTION The present invention solves these conventional problems, and while suppressing surface dissolution during etching, the etching pits are aligned in the depth direction perpendicular to the foil.
By controlling the growth in the lateral direction perpendicular to this and adjusting the etching pit shape according to the voltage used, we are able to achieve higher magnification and strength of the electrode foil, which is required for the miniaturization of aluminum electrolytic capacitors. The purpose is to simultaneously satisfy the above contradictory characteristics. Structure of the Invention To achieve this object, the present invention provides an etching solution in which a direct current is applied to an aqueous solution of hydrochloric acid or an etching solution in which at least one film-forming substance selected from oxalic acid, sulfuric acid, phosphoric acid, boric acid, or a salt thereof is added to the aqueous solution. It has a first stage etching process in which etching is carried out by applying electricity, and a second stage etching process in which etching is carried out by passing a direct current in an etching solution consisting of a metal chloride or ammonium chloride, which has a greater tendency to ionize than aluminum. In the third stage etching, an aqueous solution of hydrochloric acid, a solution prepared by adding at least one film-forming substance selected from oxalic acid, sulfuric acid, phosphoric acid, boric acid or their salts to the aqueous solution, or an aqueous nitric acid solution is used as the etching solution. He invented an effective method of chemical etching. Here, in particular, the second stage etching conventionally consisted of direct current etching alone using a hydrochloric acid etching solution, chemical etching alone using a hydrochloric acid etching solution, or direct current etching alone using a neutral etching solution. A combination of current etching and chemical etching using a hydrochloric acid etching solution was used to take advantage of the advantages of each etching method while separating the etching functions to increase effectiveness. DESCRIPTION OF EMBODIMENTS Examples of the present invention, particularly basic methods, will be described below. First, as a second step, we will develop the basic structure of the etching pit shape (tunneling in the depth direction of the etching pit and the number of branching pits in the vertical direction) to improve the magnification by direct current etching using a neutral etching solution. This is followed by a new third stage of chemical etching using a hydrochloric acid etching solution, which increases the dissolution loss compared to the conventional method without significantly reducing the mechanical strength of the foil, and increases the formation voltage. It has been discovered that it is possible to effectively improve the capacity and the tunneling performance of a tunnel pit by increasing the pit diameter of the etching pit according to the requirements. Describing in more detail the overall structure of etching for the purpose of fully and effectively performing this etching function, the etching method consists of three steps as shown below. That is, in the first stage, the oxidizing additive forms on the foil surface under the condition that a current is applied at a high current density higher than the pitting corrosion potential in high-temperature hydrochloric acid containing the oxidizing additive. By utilizing the unevenness of the porous oxide film, etching pits are formed at high density and uniformly, and then oxidation is performed in the same aqueous solution under conditions where a current is applied at a low current density below the pitting potential. The oxide film formed on the foil surface by the anti-corrosion additive serves as an anti-corrosion film, and the etching pit generation point is made more uneven on the foil surface, and the pore dissolution rate is higher than the surface dissolution rate. The etching pit is advanced rapidly in the depth direction under prevailing conditions to form a tunnel in the etching pit. In the second stage, a current is applied at low current density in a high-temperature aqueous solution of a neutral salt containing chloride ions, and the oxide film formed on the foil surface by the solution is used as an anticorrosive film to form the inside of the etching pit. Under the dominant etching conditions, where the pore dissolution rate is faster than the surface dissolution rate, tunneling in the depth direction of the etching pit and Corrosion progresses in the vertical direction. Here, we create a situation that makes it easier to suppress surface corrosion than when using a hydrochloric acid solution, and while preventing surface dissolution, new slight effects are created not only in the depth direction of the etching pit but also in the direction perpendicular to the depth direction. It functions to generate, grow, and simultaneously expand the diameter of the etching pit. In the third stage, under conditions of immersion treatment in high-temperature hydrochloric acid containing an oxidizing additive, the oxide film formed on the foil surface by the oxidizing additive is used as an anticorrosive film, and the inner wall surface of the etching pit and the foil are coated. By making the surface condition more uneven and making the dissolution rate inside the etching pit faster and more dominant than the surface dissolution,
Etching progresses in the radial direction of the etching pit. Here, etching of the inner wall of the tunnel pit can be promoted and progressed more uniformly than in the case of electroetching in a chloride solution, and the generation and growth of new etching pits in directions other than the direction of expanding the etching pit diameter can be suppressed. By doing so, it is possible to expand and adjust the etching pit diameter in accordance with the intended voltage used for all etching pits. Moreover, this makes it possible to maintain the penetrability of the etching tunnel pit after chemical formation, making it possible to obtain high capacity and low tan δ. FIG. 4 (Relationship between etching pit diameter and number of pits by etching method) shows a comparison of the differences in the etching pit shapes formed by the conventional etching method (Example 2) and the etching method according to the present invention (Example 4). It can be seen that the etching pit diameter is enlarged by the etching method of the present invention. Specific examples based on the present invention will be shown below. Example 1 An aluminum annealed foil with a purity of 99.99% and a thickness of 100 μm was used as a sample, and the first stage etching was performed using an aqueous solution of 5% hydrochloric acid and 0.03% oxalic acid at a temperature of 80°C at a current density of 150.
Etching foil was etched by applying a direct current of mA/cm ² for 80 seconds and then applying a direct current of 40 mA/cm ² for 10 minutes in the same aqueous solution.
After 370V chemical formation, etching reduction, capacitance,
Table 1 shows the results of measuring strength.

[Table] The foil strength is based on the bending strength, 1.0R,
With a load of 200g and a bending angle of 90°, one reciprocation was made once. Example 2 Using the same sample as in Example 1, the first stage etching was carried out using an aqueous solution of 5% HCl and 0.03% oxalic acid at a temperature of 80°C, and applying direct current at a current density of 150 mA/cm ² for 80 seconds. After this, a second stage of etching is performed using a 2% ammonium chloride aqueous solution at a temperature of 90°C at a current density of 80 m.
Table 2 shows the results of measurements of etching dissolution loss, capacitance, and strength after etching the etched foils by applying a direct current of A/cm ² for 4 minutes at 370 V in a boric acid solution.

[Table] Example 3 Using the same sample as in Example 1, the first stage etching was performed using an aqueous solution of 5% hydrochloric acid and 0.03% oxalic acid at a temperature of 80°C, and a direct current at a current density of 150 mA/cm ² for 80 seconds. After this, the etching foil was chemically dissolved by immersion treatment for 6 minutes in an aqueous solution of 5% hydrochloric acid and 0.03% oxalic acid at a temperature of 90°C, and then chemically etched at 370V in a boric acid solution. Table 3 shows the results of measurements of etching dissolution loss, capacitance, and strength.

[Table] Example 4 Using the same sample as in Example 1, the first stage etching was performed using an aqueous solution of 5% hydrochloric acid and 0.03 oxalic acid at a temperature of 80°C, and applying direct current at a current density of 150 mA/cm ² for 70 seconds. After that, the second stage etching was performed using a 2% ammonium chloride aqueous solution at a temperature of 90°C at a current density of 80 mA/cm ²
After applying a direct current for 3 minutes and 30 seconds, the third stage of etching was performed using a solution of 5% hydrochloric acid and 0.03% oxalic acid at a temperature of 90°C.
Chemically etched in an aqueous solution at ℃ (see Figure 3 for the relationship between chemical etching temperature, weight loss, and capacity).
Table 4 shows the results of measurements of etching dissolution loss, capacitance, and strength after 370V chemical formation.

[Table] As shown in Table 4, by using the etching method of the present invention, even if the dissolution loss is increased, the capacity can be increased proportionally without becoming saturated, and at the same time, the strength can be maintained. It became possible to do so. (See Example 4 in Figures 1 and 2) Effects of the Invention As described above, compared to the case where the electrode foil for aluminum electrolytic capacitors is etched by the method of the present invention,
It is possible to increase the capacitance value by 8 to 25%, making it possible to downsize aluminum electrolytic capacitors, and at the same time, when used as a capacitor product, tan δ can be lowered by 20% compared to conventional products. Furthermore, running costs can be reduced by reducing the cost of incidental equipment required for production and the power consumption of the etching method.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the relationship between dissolution loss and capacitance by each etching method listed in Examples 1 to 4, and Figure 2 is a characteristic diagram showing the relationship between dissolution loss and capacitance by each etching method listed in Examples 1 to 4. FIG. 3 is a characteristic diagram showing the relationship between the etching foil strength ratio, FIG. FIG. 4 is a characteristic diagram showing the relationship between the etching pit diameter and the number of pits in the etching method of Example 2) and the etching method of the present invention (Example 4).

Claims (1)

[Claims] 1. A first step in which etching is carried out by applying a direct current in an aqueous hydrochloric acid solution or an etching solution in which at least one film-forming substance selected from oxalic acid, sulfuric acid, phosphoric acid, boric acid, or a salt thereof is added to the aqueous solution.
It has a step etching step, a second step etching step in which etching is carried out by passing a direct current in an etching solution made of metal chloride or ammonium chloride, which has a greater tendency to ionize than aluminum, and a third step etching step. Hydrochloric acid aqueous solution,
Alternatively, chemical etching is performed using a solution obtained by adding at least one film-forming substance selected from oxalic acid, sulfur residue, phosphoric acid, boric acid, or their salts to the aqueous solution, or an aqueous nitric acid solution as an etching liquid. A method for manufacturing electrode foil for aluminum electrolytic capacitors.