JP7628057B2 - Method for producing aluminum material for electrolytic capacitor electrodes and method for producing aluminum electrode material for electrolytic capacitors - Google Patents

Description

The present invention relates to a method for manufacturing an aluminum material for electrolytic capacitor electrodes and a method for manufacturing an aluminum electrode material for electrolytic capacitors.

Aluminum foil, which is commonly used as an electrode material for electrolytic capacitors, undergoes an electrochemical or chemical etching process to expand the surface area of the aluminum foil in order to increase its capacitance. In order to expand the surface area, it is necessary to arrange the starting points from which etching begins at equal intervals and create more starting points for etching pits. To achieve this, a method has traditionally been used that controls the distribution of impurities contained in the aluminum. However, this method has limitations in terms of control over the impurity distribution, and it is impossible to precisely control each and every etching starting point. In other words, the conventional method has reached its limit in expanding the surface area through etching.

For this reason, a technology has been proposed in which depressions are formed in a desired pattern on the surface of an aluminum foil or on the surface of an aluminum foil having an oxide film by pressing a master mold having protrusions against the aluminum foil, and the starting point position is controlled by using these depressions as the starting points for etching, thereby producing an electrode foil with high surface area expansion efficiency (see Patent Document 1).
In addition, a technology has been proposed in which a mold having protrusions is pressed against the surface of a smooth aluminum material for electrode foil having a surface roughness (arithmetic mean roughness: Ra) of less than 0.30 to form a desired pattern of indentations, such as recesses that break through the aluminum oxide film or protrusions by indentation transfer, and these are used as starting points for etching, thereby controlling the starting point position and producing an electrode foil with high surface expansion efficiency (see Patent Document 2).

Japanese Patent Application Publication No. 11-74162 JP 2007-042789 A

However, the inventors' research has revealed that even if multiple recesses that serve as the starting points for etching pits are formed by, for example, pressing the aluminum substrate surface against the master mold, it may not be possible to obtain the expected capacitance.

After further research into the cause of this, the inventors discovered that the temperature, humidity, and time of the atmosphere to which the aluminum material is exposed after forming numerous recesses on the surface of the aluminum base material and before being subjected to etching have a significant effect on the occurrence and progression of etching pits.

This invention was made based on this knowledge, and aims to provide a method for manufacturing an aluminum material for electrolytic capacitor electrodes and an aluminum electrode material for electrolytic capacitors, which is an aluminum material having a large number of recesses formed in the aluminum base material that serve as starting points for etching pits, and which has excellent etching characteristics, can improve the surface expansion ratio, and can thereby obtain a large electrostatic capacitance.

The above object can be achieved by the following means.
(1) a step of pressing a master die having a large number of protrusions against an aluminum base having an oxide film on its surface to form a large number of recesses on the surface of the aluminum base, the recesses being sites for forming etching pits;
After forming the recess, starting etching under the condition that the value of d in a×c+a×b×c=d× ¹⁰⁴ satisfies 0 to 202, where a is the temperature (K) of the atmospheric air, b is the humidity (%), and c is the time (hours);
1. A method for producing an aluminum material for electrolytic capacitor electrodes, comprising:
(2) The method for producing an aluminum material for electrolytic capacitor electrodes according to the preceding paragraph (1), wherein the value of d is 0 to 100.
(3) A method for producing an aluminum electrode material for electrolytic capacitors, comprising subjecting an aluminum material produced by the method for producing an aluminum material for electrolytic capacitor electrodes according to the preceding item (1) or (2) to electrolytic etching or chemical etching.

In the method for producing an aluminum material for electrolytic capacitor electrodes according to the present invention, a matrix having a number of protrusions is pressed against an aluminum base having an oxide film on its surface to form a number of recesses on the surface of the aluminum base, which are intended sites for forming etching pits. Then, etching is started under the condition that the value of d in a×c+a×b×c=d× ¹⁰⁴ satisfies 0 to 202, where a is the temperature (K) of the atmospheric atmosphere, b is the humidity (%), and c is the time (hours). This makes it possible to suppress the growth of an oxide film on the inner surface of the recesses after the formation of the recesses and before the start of etching. Therefore, during etching, the oxide film on the inner surface of the recesses becomes the starting point of the etching pits, and the etching proceeds, and thus etching pits are reliably formed at the positions of a number of recesses. As a result, it is possible to produce an aluminum material for electrolytic capacitor electrodes that can improve each surface ratio and realize a large capacitance.

The manufacturing method of the aluminum electrode material for electrolytic capacitors according to the present invention makes it possible to manufacture electrode material for electrolytic capacitors with a large electrostatic capacity.

FIG. 2 is a cross-sectional view showing a state in which a large number of recesses are formed on the surface of an aluminum substrate. 1 is a cross-sectional view showing a state before a matrix for forming a large number of recesses is pressed against an aluminum base material. FIG. 11 is a cross-sectional view showing the state in which the mother die is pressed against the aluminum base material.

[Aluminum material for electrolytic capacitor electrodes]
An aluminum material produced by the method for producing an aluminum material for electrolytic capacitor electrodes according to one embodiment of the present invention has a number of recesses, which are intended to be the locations for forming etching pits, formed on the surface by pressing a master die having a number of protrusions against an aluminum base material having an oxide film on its surface.

Figure 1 shows a schematic cross-sectional view of an example of an aluminum material.

The aluminum material is made of an aluminum base material 1 having an oxide film 2 on its surface, on which numerous recesses 3 are formed.

In FIG. 1, each recess 3 has a cone shape, and the shape of a cross section (hereinafter also referred to as a transverse section) of each recess 3 parallel to the plane of the aluminum substrate may be any of a polygon, a circle, and an ellipse. The inner surfaces of these recesses 3 are entirely covered with an oxide film 4.

The surface of oxide film 4 has many irregularities, voids, and cracks, and the average thickness of oxide film 4 is thinner than that of oxide film 2. The average thickness of oxide film 2 is preferably in the range of 1 nm to 50 nm, and more preferably in the range of 2 nm to 20 nm. The average thickness of oxide film 4 is preferably in the range of 0.5 nm to 40 nm, and more preferably in the range of 0.5 nm to 10 nm.

The maximum length of the opening of the recess 3 is preferably in the range of 0.01 μm to 10 μm, and more preferably in the range of 0.1 μm to 2.0 μm. In the example of FIG. 1, the shape of the recess 3 is shown as a cone, but the shape of the recess 3 may be any shape other than a cone, such as a cylinder, a sphere, or an ellipsoid. The depth of the recess 3 is preferably in the range of 0.1 μm to 1.0 μm, and more preferably in the range of 0.2 μm to 0.6 μm.

The distance between the recesses 3 is preferably 1 μm to 10 μm. The distance between the recesses 3 refers to the minimum distance between the nearest recesses. If the distance between the recesses 3 is less than 1 μm, the etching pits formed in the recesses 3 during etching may communicate with each other, hindering the surface-expanding effect of the etching pits. If the distance between the recesses 3 exceeds 10 μm, the number of etching pits decreases, and it may not be possible to obtain a significant surface-expanding effect. A more preferable distance between the recesses 3 is 2 μm to 5 μm.

In the aluminum material for electrolytic capacitor electrodes manufactured by the manufacturing method according to this embodiment, the chemical composition of aluminum constituting the aluminum base material 1 is not limited, and materials used as electrolytic capacitor electrode materials can be used as appropriate. Specifically, in order to regulate the amount of impurities and prevent deterioration of etching characteristics due to over-dissolution, the aluminum purity is preferably 99.9% or more, and particularly preferably 99.99% or more. The cube orientation occupancy rate of the aluminum material, or the occupancy rate of (100), is preferably 90% or more, more preferably 95% or more, and most preferably 99.9% or more. In addition, the thickness of the aluminum material for electrolytic capacitor electrodes is not limited, and includes aluminum plates exceeding 200 μm in thickness in addition to aluminum materials called foils having a thickness of 200 μm or less. The surface roughness (arithmetic mean roughness) of the aluminum base material 1 is preferably less than 0.1 μm, and particularly preferably less than 0.05 μm.
[Method of manufacturing aluminum material for electrolytic capacitor electrodes]
The above-mentioned aluminum material for electrolytic capacitor electrodes can be produced, for example, by the following method.

An aluminum ingot of a specified chemical composition is subjected to a homogenization treatment, and then hot rolling, cold rolling, and final annealing are performed in sequence to form an oxide film, producing an aluminum base material having an oxide film 2 on its surface. If the surface is smoothed by chemical polishing or electrolytic polishing after the final annealing, the aluminum base material is then exposed to the atmosphere to form a natural oxide film, resulting in an aluminum base material. The oxide film 2 may not be a natural oxide film, but may be a chemical oxide film formed by chemical conversion treatment.

Next, a large number of recesses 3 are formed in the aluminum base material 1. Although not limited to a method for forming the recesses 3, an example is a method in which a master mold 100 having a large number of protrusions 101 as shown in FIG. 2 is pressed against the aluminum base material 1 as shown in FIG. 3 to form the recesses 3 as indentations corresponding to the protrusions 101. By aligning the large number of protrusions 101 in the master mold 100 to the arrangement of the recesses 3 described above, the recesses 3 can be formed in the desired pattern at the desired positions on the aluminum base material 1.

The master mold 100 can be produced by any method capable of producing a fine shape that can be subjected to the desired patterning, but is preferably produced using mold processing or lithography techniques. The material may be any material that is harder than the aluminum base material 1, regardless of whether it is electrically conductive or not. The shape of the protrusions 101 may be any of a cone, cylinder, sphere, or ellipsoid, and the outer shape of the protrusions becomes the inner shape of the recesses 3.

When pressing the master mold 100 against the aluminum substrate 1, the protrusions 101 and the aluminum substrate 1 are in contact perpendicularly. The pressing pressure must be increased according to the surface roughness of the aluminum substrate 1. In other words, if the aluminum substrate 1 is flat, uniform recesses 3 can be formed even with a low pressing pressure. The pressing method can be either a flat surface or a roll.

In the process of forming the recesses 3 by pressing the master mold 100 in the above-described manner, stress is applied to the inner surface of the recesses 3, causing irregularities, voids, cracks, etc. in the oxide film 4. If the master mold 100 is pressed deeper, the oxide film 4 becomes more easily stretched as the angle increases, and the average thickness decreases.

The aluminum material thus formed with numerous recesses 3 is stored or transported in the atmosphere until it is subjected to etching. During storage or transport, the oxide film 4 on the surface of the recesses 3 is exposed to the atmosphere, causing oxygen and water to react with the aluminum, forming amorphous aluminum oxide and hydrates of aluminum oxide. These are particularly likely to form in uneven, void, and cracked areas, which increases the average thickness of the oxide film 4. The oxide film 2 also increases in thickness, but since it is sufficiently thicker than the oxide film 4 due to being exposed to the atmosphere for a sufficiently long time before the formation of the recesses 3, its growth rate is much slower than that of the oxide film 4, and the growth of the oxide film 2 may be ignored.
The inventors of the present application have discovered that the amount of growth of the oxide film 4 depends on the temperature, humidity, and exposure time in the atmosphere, and have found that these affect the surface expansion efficiency after etching and, in turn, the electrostatic capacitance.
The oxide film 4 grows through a process in which aluminum ions and oxygen ions diffuse through the oxide film. The amount of diffusion depends on temperature and time; the higher the temperature and the longer the time, the greater the amount of diffusion. Furthermore, the formation of aluminum oxide hydrate depends on the amount of moisture. The greater the amount of moisture, i.e., the higher the humidity, the greater the amount of growth. In other words, the greater the atmospheric temperature, humidity, and exposure time, the greater the amount of growth of the oxide film 4, which does not become the starting point of etching pits and results in a loss of selectivity during etching.

Specifically, when the temperature (K) in the atmospheric air is a, the humidity (%) is b, and the time (hours) is c, the value of d in a×c+a×b×c=d× ¹⁰⁴ must be between 0 and 202. If the value of d in a×c+a×b×c=d× ¹⁰⁴ exceeds 202, the amount of growth of the oxide film 4 increases and selectivity during etching is lost. It is particularly desirable for the value of d to be between 0 and 100. Here, "atmosphere" refers to a state of 1 atmosphere containing nitrogen, oxygen, argon, carbon dioxide, etc., which are the components of air in the troposphere.

Furthermore, taking into consideration the operating environment of the etching equipment, from the press equipment for pressing the matrix 100 in an indoor environment, it is preferable that each numerical value is within the following range.
[1] a: 288-353K
[2] c: 0.0008~ hour (3sec~)
It is desirable that the temperature a be within the controllable temperature range of the etching solution. Also, considering the method of transporting the workpiece from the press equipment to the etching equipment, it is practically difficult to transport the workpiece within 3 seconds for the time c.
When the temperature of the aluminum base 1 is lower than the temperature a of the atmosphere, moisture in the air condenses on the aluminum base, increasing the amount of growth of the oxide film. Therefore, it is desirable that the aluminum base 1 is always at least as hot as the atmosphere.

The aluminum material for electrolytic capacitor electrodes thus produced is then etched to improve the surface area ratio. In the irregularities, voids, and cracks in the oxide film 4 on the inner surface of the recesses 3, pits are preferentially formed because they are highly reactive during etching. Therefore, the recesses 3 become the starting points of etching pits, and erosion progresses parallel or perpendicular to the (100) plane of the crystal grains to form tunnel pits. By generating etching pits in the desired recesses 3 that are regularly arranged, it is possible to reduce the reduction in the effective area due to the combination of etching pits and the void regions of the pits. In other words, an improvement in the pit generation rate in the desired recesses 3 leads to an improvement in the effective area.
The pit occurrence rate for the recess 3 at the desired position can be judged at the time of forming the initial pit before the formation of the tunnel pit. The initial pit here is a facet-type pit with a pit diameter and depth of 0.1 μm to 2 μm.
The etching conditions are not limited, and may be electrochemical etching or chemical etching. As an example, the conditions for forming the initial pits are shown below. As a treatment liquid for electrochemical etching, an aqueous hydrochloric acid solution, or a solution in which sulfuric acid, nitric acid, or phosphoric acid has been added to an aqueous hydrochloric acid solution, may be exemplified. The treatment liquid temperature is preferably 15°C to 80°C. In addition, a platinum electrode or a carbon electrode having an area sufficiently larger than that of the aluminum material is used as the counter electrode of the aluminum material, and the preferred current value is 100mA/ ^cm2 to 3000mA/ ^cm2 , and the preferred current application time is 0.01s to 30.0s.

The following are examples of the present invention and comparative examples.

A recess 3 was formed in the aluminum substrate 1 using a protrusion-equipped master mold 100 to produce an aluminum material for electrolytic capacitor electrodes.

The aluminum base material 1 is an aluminum foil having a thickness of 120 μm and a composition of Si: 22 ppm, Fe: 16 ppm, Cu: 59 ppm, and Al purity of 99.99%, and an oxide film 2 having a thickness of 0.003 μm is formed on the surface. The arithmetic mean roughness Ra of the surface of the aluminum base material 1 is 0.05.

The matrix 100 used was made of nickel and had a large number of pyramidal protrusions 101 with a height of 1.8 μm formed on its surface at intervals of 2.1 μm. The large number of pyramidal protrusions 101 were arranged with a square consisting of them as a unit lattice.
Example 1
The matrix 100 was pressed against the aluminum base 1 with a surface pressure of 30 MPa in an atmospheric air atmosphere at a temperature of 293 K and a humidity of 40%, to form recesses 3 corresponding to the pyramidal protrusions 101 on the surface of the aluminum base 1 .
Example 2
After Example 1, the aluminum base 1 was allowed to stand for 24 hours in an air atmosphere at a temperature of 293 K and a humidity of 0.8%, to obtain an aluminum material for electrolytic capacitor electrodes.
Example 3
After Example 1, the aluminum base 1 was allowed to stand for 72 hours in an air atmosphere at a temperature of 293K and a humidity of 40%, to obtain an aluminum material for electrolytic capacitor electrodes.
Example 4
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 293K and a humidity of 40% for 168 hours to obtain an aluminum material for electrolytic capacitor electrodes.
Example 5
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 293K and a humidity of 5% for 960 hours to obtain an aluminum material for electrolytic capacitor electrodes.
Example 6
After Example 1, the aluminum base 1 was allowed to stand for 24 hours in an air atmosphere at a temperature of 308 K and a humidity of 85%, to obtain an aluminum material for electrolytic capacitor electrodes.
(Example 7)
After Example 1, the aluminum base 1 was allowed to stand for 24 hours in an air atmosphere at a temperature of 333K and a humidity of 85%, to obtain an aluminum material for electrolytic capacitor electrodes.
(Comparative Example 1)
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 308 and a humidity of 85% for 168 hours to obtain an aluminum material for electrolytic capacitor electrodes.
(Comparative Example 2)
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 333K and a humidity of 85% for 80 hours to obtain an aluminum material for electrolytic capacitor electrodes.
(Comparative Example 3)
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 293K and a humidity of 40% for 960 hours to obtain an aluminum material for electrolytic capacitor electrodes.
(Comparative Example 4)
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 293K and a humidity of 5% for 4,800 hours to obtain an aluminum material for electrolytic capacitor electrodes.
(Comparative Example 5)
After Example 1, the aluminum base 1 was allowed to stand in an air atmosphere at a temperature of 293 K and a humidity of 0.8% for 4,800 hours to obtain an aluminum material for electrolytic capacitor electrodes.
The 10 types of aluminum materials for electrolytic capacitor electrodes obtained as described above were immersed in a 6 mol/L aqueous hydrochloric acid solution at a liquid temperature of 35°C, and subjected to an electrolytic etching treatment under conditions of a current of 800 mA/ ^cm2 for 0.5 seconds, and then washed with pure water and dried.

Each of the etched aluminum materials for electrolytic capacitor electrodes was observed with an SEM to observe the occurrence of etching pits and to confirm the rate of pit occurrence relative to the recessed portion 3. The results, as well as each temperature, humidity, time, and the value of d in a×c+a×b×c=d× ^104, are shown in Table 1.

In Example 1, etching started immediately (actually 0.00083 hours later) after the recess 3 was formed, so d was set to zero.

As shown in Table 1, in the aluminum materials for electrolytic capacitor electrodes according to Examples 1 to 7, etching pits that occurred from the recesses 3 accounted for 75% or more of the total. For this reason, it was expected that a large surface expansion ratio and high capacitance would be obtained.

In contrast, in Comparative Examples 1 to 5, in which the value of d in a×c+a×b×c=d× ¹⁰⁴ is outside the range of the present invention, the etching pits generated from the recesses 3 were few, at 60% or less, and the surface expansion ratio was smaller than in the Examples, and it was expected that the capacitance would be inferior.

The aluminum material produced by the method of the present invention for producing aluminum material for electrolytic capacitor electrodes can be etched to obtain a high surface area expansion ratio, and when used as an electrode material, it is useful for improving the capacitance of electrolytic capacitors.

1 Aluminum substrate 2 Oxide film 3 Recess 4 Oxide film 100 Mother die 101 Protrusion

Claims (3)

a step of pressing a master die having a large number of protrusions against an aluminum base having an oxide film on its surface, thereby forming a large number of recesses on the surface of the aluminum base, the recesses being sites for forming etching pits;
After forming the recess, starting etching under the condition that the value of d in a×c+a×b×c=d× ¹⁰⁴ satisfies 0 to 202, where a is the temperature (K) of the atmospheric air, b is the humidity (%), and c is the time (hours);
2. A method for producing an aluminum material for electrolytic capacitor electrodes, comprising: The method for manufacturing an aluminum material for electrolytic capacitor electrodes according to claim 1, wherein the value of d is 0 to 100. 3. A method for producing an aluminum electrode material for electrolytic capacitors, comprising the steps of: subjecting an aluminum material produced by the method for producing an aluminum material for electrolytic capacitor electrodes according to claim 1 to electrolytic etching or chemical etching.

Images