JPH07180006A - Production of aluminum foil for electrolytic capacitor electrode - Google Patents

Abstract

PURPOSE:To increase the effective area by electrolytic etching and to produce Al foil for the electrode for an electrolytic capacitor high in capacitance by annealing Al foil of a specified grade to remove an oxidized film on the surface, thereafter annealing it again at a specified temp. to coarsen the crystalline grains and furthermore forming an extremely thin oxidized film. CONSTITUTION:Al foil contg., by weight, 0.002 to 0.O05% Fe, 0.002 to 0.005% Si and 0.001 to 0.004% Cu and having the grade of >=99.98% A purity us subjected to primary annealing at 350 to 500 deg.C to regulate the grain size to 55 to 100mum, and on the surface, a relatively thick Al oxide film is formed. Next, it is immersed in an aq. soln. of NaOH to remove an Al oxide film, and after that, it is subjected to secondary annealing at a relatively low temp. of 220 to 300 deg.C in the atmosphere to form an extremely thin oxide film having 25 to 50Angstrom thickness on the surface. This Al foil is subjected to electrolytic etching to increase the effective area of the surface, by which the Al foil as the electrode for an electrolytic capacitor high in capacitance per unit area can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an aluminum foil for an electrolytic capacitor electrode, and more particularly to a method for producing an aluminum foil for an electrolytic capacitor electrode which is used after being subjected to an effective area enlargement treatment by electrolytic etching. Regarding

[0002]

2. Description of the Related Art In order to reduce the size and increase the capacity of electrolytic capacitors, aluminum foil for electrolytic capacitor electrodes is used after its effective area (hereinafter referred to as surface area) is enlarged by electrolytic etching. Here, the expansion of the surface area is intended to improve the electrostatic capacity per unit area, and as a result, to reduce the size and capacity of the electrolytic capacitor.
Therefore, it is desired to increase the surface area.

For the purpose of producing an aluminum foil for electrolytic capacitor electrodes which can withstand the increase in surface area due to such electrolytic etching (no surface dissolution occurs, etching pits grow), adjustment of specific impurities and soaking conditions. , Surface treatment and annealing conditions have been investigated and many results have been reported. For example, JP-A-4-259357 discloses a method by adjusting specific impurities, JP-A-2-200749 and JP-A-4-176847 disclose a soaking method, and JP-A-5-12
Japanese Patent Laid-Open No. 4-311550 describes a method by surface treatment, and Japanese Patent Laid-Open No. 4-311550 discloses a method by annealing conditions.

Among these methods, the method according to the annealing condition is performed in an atmosphere where it is difficult to form an oxide film.
A method of annealing at a temperature of ~ 450 ° C is adopted. That is, in the final annealing step after foil rolling (after rolling to aluminum foil), annealing is performed at a temperature of 350 to 450 ° C. in order to improve the electrolytic etching characteristics of the aluminum foil by forming a texture having many cubic orientations. However, a gas containing oxygen in the atmosphere or almost the same as the atmosphere (oxygen: 17-25 vol%
An oxide film is formed by annealing in a gas containing) and the annealing temperature is higher than 350 ° C. at this time, the thickness of the oxide film formed is significantly increased. When such a thick oxide film is formed by electrolytic etching, the insulating property of the oxide film is high, so that the etching property is deteriorated (a hole is opened and the surface area cannot be increased).
Therefore, a method of annealing at a temperature of 350 to 450 ° C. in an atmosphere where it is difficult to form an oxide film, such as in Dx gas, argon, or vacuum, is adopted.

[0005]

According to the conventional method under the annealing conditions, the electrolytic etching property is relatively good, and it is possible to endure the increase in the surface area by the electrolytic etching. It can be expanded. However, since it is necessary to anneal in an atmosphere in which an oxide film is difficult to form (hereinafter referred to as a special atmosphere) such as in Dx gas, argon, or vacuum, it is necessary to create such a special atmosphere, which causes an increase in manufacturing cost. I don't get. Therefore, it is not a method by annealing in a special atmosphere that causes an increase in the manufacturing cost, but a method by annealing in the atmosphere or a gas containing oxygen approximately equivalent to the atmosphere (hereinafter, these are referred to as atmospheric equivalent gases), That is, it would be convenient to have a technique (annealing condition etc.) that can increase the surface area by improving the electrolytic etching property by annealing in a gas equivalent to the atmosphere, and it is desired to develop the technique.

The present invention has been made in view of such circumstances, and its purpose is not a method of annealing in the special atmosphere but basically a method of annealing in a gas equivalent to the atmosphere. Therefore, an attempt is made to provide a method for producing an aluminum foil for an electrolytic capacitor electrode, which can obtain an aluminum foil for an electrolytic capacitor electrode in which the surface area can be increased by improving the electrolytic etching property, and as a result, the capacitance can be improved. It is a thing.

[0007]

In order to achieve the above object, the present invention provides a method for producing an aluminum foil for electrolytic capacitor electrodes having the following construction. That is, the method for producing an aluminum foil for electrolytic capacitor electrodes according to the present invention has an aluminum purity of 99.98% or more, and Fe
0.002 to 0.005wt%, Si 0.002 to 0.005wt%, Cu 0.001
Aluminum foil containing ~ 0.004 wt% is annealed at 350-500 ° C, then the oxide film formed by the annealing is removed, and then the aluminum foil is heat treated at 220-330 ° C in an oxidizing atmosphere. Grain size of 55-100 μm
And a method for producing an aluminum foil for electrolytic capacitor electrodes, which comprises forming an oxide film having a film thickness of 25 to 50 Å on the surface of the aluminum foil.

[0008]

The function of the present invention is mainly to anneal an aluminum foil in an oxidizing atmosphere such as the atmosphere (hereinafter referred to as an atmosphere-equivalent gas), the composition of the aluminum foil, the foil thickness, the annealing temperature, the additional step after the annealing, etc. The properties and electrolytic etching properties of the obtained aluminum foil were investigated, and the electrostatic capacity after electrolytic etching was also investigated.As a result, (a) an aluminum foil having a specific composition was heated at a relatively high temperature. The finding that the capacitance is improved by annealing and increasing the grain size,
(b) After the oxide film formed by the annealing is removed to improve the etching property, electrolytic etching causes surface dissolution to increase the surface area, but after the oxide film is removed, there is a problem. When heat treatment is performed at a relatively low temperature to form a thin oxide film of a specific thickness, it is possible to carry out electrolytic etching without such a problem, and the knowledge that surface area can be increased can be obtained. It is a thing.

That is, the aluminum purity is 99.98% or more, and 0.002 to 0.005 wt% of Fe and 0.002 to 0.005 wt of Si.
%, Cu aluminum 0.001 ~ 0.004wt% containing aluminum foil,
First, when annealed in a gas equivalent to air,
It is found that the electrostatic capacity increases with the increase of the annealing temperature up to the annealing temperature of 500 ° C. as shown in the mark (uncoated film removal foil). In addition, FIG. 1 shows the annealing temperature of the aluminum foil and the aluminum foil after the annealing (circle mark: non-film removal foil), and
It shows an example of the result of examining the relationship between the aluminum foil after annealing and film removal described later (●: film removal foil) and the electrostatic capacity at the time of forming 20V.

The cause of this increase in capacitance is presumed to be the influence of the oxide film and the influence of the fine structure. Therefore, for the purpose of grasping the cause, the surface of the aluminum foil after the above-mentioned annealing is removed by a method of immersing it in a 2% NaOH aqueous solution at 50 ° C for 30 seconds, etc. to remove the surface oxide film (the oxide film formed by the above-mentioned annealing). After that, the capacitance was measured. As a result, as illustrated in Fig. 1 (● mark: film removal foil), the one with an annealing temperature of 300 ℃ or less has almost the same capacitance as the non-film removal foil, but the one with an annealing temperature of 300 ℃ or more does not. It was found that the film-removing foil has a larger capacitance than the film-removing foil, and the difference in the capacitance between the two films increases with increasing annealing temperature up to the annealing temperature of 450 ° C. Here, the oxide film formed by annealing at 300 ° C. or higher is thicker than the oxide film formed naturally. Therefore, the electrostatic capacity is improved by increasing the crystal grain size, and the electrostatic capacity is decreased by forming a thick oxide film. From these, it was found that an aluminum foil having a large capacitance can be obtained by removing the oxide film formed by the annealing after annealing at 350 to 500 ° C. (hereinafter referred to as primary annealing).

However, since the aluminum foil is in a state in which the oxide film is removed as it is, it is considered that there is a problem that the surface area cannot be increased due to surface dissolution during electrolytic etching. So, further,
It was thought that this problem could be solved if a slight oxide film was formed by heat treatment (hereinafter referred to as secondary annealing) in a gas equivalent to the atmosphere, and the effect of this secondary annealing was investigated by experiments. As a result, in the state where the oxide film is removed (film removal foil), the surface dissolution cannot occur at the time of electrolytic etching to increase the surface area, and the capacitance is lowered, which is unsatisfactory. When the removal foil is heat-treated (secondary annealing) in a gas equivalent to the atmosphere to form an oxide film, surface dissolution does not occur during electrolytic etching, the surface area can be increased, and the electrostatic capacity is improved. For electrolytic capacitor electrodes that have a large degree of increase in electrostatic capacity and have a sufficient electrostatic capacity when a treatment (secondary annealing) is performed at 220 to 330 ° C to form an oxide film with a film thickness of 25 to 50Å It was found that an aluminum foil was obtained.

FIG. 2 shows an example of an experimental result concerning the influence of the secondary annealing. This is a heat treatment (secondary annealing) in the atmosphere (oxygen: 21 vol%) of the aluminum foil after primary annealing at 450 ° C or 400 ° C and removal of the oxide film with 2% NaOH aqueous solution at 50 ° C. Then, after performing electrolytic etching, the electrostatic capacity at the time of forming 20 V was measured, and it was created based on these. From FIG. 2 as well, the electrostatic capacity of the aluminum foil after electrolytic etching is improved by the secondary annealing, and the degree of this electrostatic capacity improvement is that the secondary annealing temperature is 220 to
It can be seen that it is particularly remarkable at 330 ° C. Incidentally, FIG.
In the case of, the capacitance shows a maximum value when the secondary annealing temperature is 275 ° C.

Based on the above findings, the method for producing an aluminum foil for electrolytic capacitor electrodes according to the present invention has been completed, the aluminum purity is 99.98% or more, and Fe is 0.002 to 0.005 wt%, Si 0.002-0.005wt
3% aluminum foil containing 0.001 to 0.004 wt% Cu
After annealing at 50 to 500 ° C (primary annealing), the oxide film formed by the annealing is removed, and then heat treatment at 220 to 330 ° C in an oxidizing atmosphere (in the atmosphere equivalent gas) (secondary) Annealing) to reduce the grain size of aluminum foil to 55-100
μm and film thickness on aluminum foil surface: 25-50
The Å oxide film is formed. Therefore, according to the method for producing an aluminum foil for an electrolytic capacitor electrode according to the present invention, unlike the conventional method by annealing in a special atmosphere, it is basically a method by annealing in an atmosphere-equivalent gas, It is possible to obtain an aluminum foil for electrolytic capacitor electrodes in which the surface area can be increased by improving the etching property, and as a result, the capacitance can be improved. That is, the annealing can be performed in a gas equivalent to the air, for example, in the air atmosphere, not in a special atmosphere, which is simplified.

The reason for limiting the numerical values in the present invention will be described below. With respect to the composition of the aluminum foil, the aluminum purity is set to 99.98% or more because when it is less than 99.98%, many crystallized substances are formed and coarse pores are formed during electrolytic etching, resulting in a decrease in capacitance.

The Fe content is set to 0.002 to 0.005 wt% because when it is less than 0.002 wt%, the precipitates which are the starting points of the formation of etching pits are remarkably reduced and the capacitance is lowered.
This is because if it exceeds 5 wt%, many crystallized substances are formed and coarse pores are formed during electrolytic etching, resulting in a decrease in capacitance. The Si content is set to 0.002 to 0.005 wt% because when it is less than 0.002 wt%, the precipitates that are the starting points of etching pits are significantly reduced, leading to a decrease in electrostatic capacity, and when it exceeds 0.005 wt%, crystallized substances are formed. This is because a large number of holes are formed and coarse pores are formed during electrolytic etching, resulting in a decrease in electrostatic capacity. Also, the Cu content is 0.001 to 0.
004 wt%, if less than 0.001 wt%, the precipitates that form the starting point of etching pits are significantly reduced, leading to a decrease in capacitance, and if it exceeds 0.004 wt%, many crystallized substances are formed, and during electrolytic etching. This is because the holes become coarse and the capacitance is lowered.

The primary annealing temperature is set in the range of 350 to 500 ° C. When the temperature is less than 350 ° C., the crystal grain size does not become sufficiently large, so that the electrostatic capacity cannot be improved. This is because the effect of improving the electric capacity is saturated, the heating energy is wasted, and the oxide film thickness exceeds 150 Å, which makes it impossible to remove the oxide film in a short time.

The secondary annealing temperature is set to 220 to 330 ° C. The reason why the oxide film formed is too thin when the temperature is less than 220 ° C. and the effect of improving the capacitance due to the oxide film formation in the secondary annealing is small. This is because the oxide film is insufficient, and if the temperature exceeds 330 ° C., the oxide film formed becomes too thick and the insulating property of the oxide film becomes high, so that the etching property deteriorates, and as a result, the capacitance decreases.

The crystal grain size of the aluminum foil is 55 to 100 μm
The reason for this is that if it is less than 55 μm, the crystal grain size is too small to improve the capacitance, and if it exceeds 100 μm, the mechanical strength is reduced and cutting etc. occurs in the manufacturing process, which is not practical. is there. The above crystal grain size: 55 to 100 μ
m is the grain size of the aluminum foil after the secondary annealing, and the grain size is adjusted mainly by the primary annealing as described above. That is, the secondary annealing has an annealing temperature of 2
Since the temperature is low at 20 to 330 ° C, the crystal grains are not coarsened or refined, the crystal grain size is hardly changed by the secondary annealing, and the crystal grain size is almost determined by the primary annealing.
It will be adjusted by the primary annealing.

The thickness of the oxide film on the surface of the aluminum foil after the secondary annealing is set to 25 to 50 Å because if it is less than 25 Å, the surface is dissolved during electrolytic etching and the surface area cannot be increased, resulting in an unsatisfactory capacitance. If it exceeds 50Å, the insulating property of the oxide film will be high, and the etching property will decrease.
As a result, the capacitance decreases.

The oxide film removed after the primary annealing is
It can be carried out by a method of immersing in an aqueous solution of NaOH, a method of immersing in a mixed solution of phosphoric acid-chromic acid, a method of immersing in hydrochloric acid, a method of immersing in dilute sulfuric acid, and the like.

In the case of applying the method of immersing in an aqueous NaOH solution, the higher the temperature of the aqueous NaOH solution and the higher the NaOH concentration is 1%, the higher the reactivity and the advantage that the oxide film removal treatment can be carried out in a short time. , Temperature of NaOH aqueous solution: When the temperature exceeds 55 ° C, the aluminum foil surface becomes rough, which promotes chemical dissolution during electrolytic etching, which tends to lower the electrostatic capacity. Also, when the NaOH concentration exceeds 10%, the same tendency occurs. There is. Na
There is a similar tendency when the immersion time in the OH aqueous solution: more than 60 seconds. If the temperature of the aqueous solution of NaOH is less than 30 ° C., the concentration of NaOH is less than 1%, or the immersion time is less than 5 seconds, the oxide film removal treatment tends to be difficult. Therefore, the temperature of NaOH aqueous solution: 30 ~
It is desirable that the temperature is 55 ° C, the NaOH concentration is 1 to 10%, and the immersion time is 5 to 60 seconds. When applying the method of immersing in a phosphoric acid-chromic acid mixed solution, the temperature of the mixed solution: 30 to 80 ° C, the immersion time: 1 to
5 minutes is recommended.

[0022]

Example An aluminum foil having the composition shown in Table 1 (coil state after foil rolling) was subjected to primary annealing in an air atmosphere using a batch furnace. Here, the temperature and time of the primary annealing were changed as shown in Table 7 (primary annealing / columns of Examples 1 to 5). The annealing time is set to be relatively long in order to form an oxide film uniformly in the width direction of the coil.

After the above-mentioned primary annealing, in order to remove the oxide film formed by the primary annealing, a 30% 5% NaOH aqueous solution at 50 ° C. is used.
Soaked for 2 seconds. Then, secondary annealing was performed in the air atmosphere. Here, the secondary annealing temperature and time were changed as shown in Table 7 (secondary annealing / columns of Examples 1 to 5).

A test piece was taken from the aluminum foil after the secondary annealing, and the oxide film thickness on the surface of the aluminum foil was measured using this test piece, and the crystal grain size was also measured. Here, the oxide film thickness is measured by the capacitance method, and the measurement conditions are as shown in Table 6. The crystal grain size was measured by the line segment method, and the average value of the sample number n = 5 was used. As a result, it was confirmed that the oxide film thickness was 39 to 40Å as shown in Table 7 (the oxide film thickness after secondary annealing / columns of Examples 1 to 5). Therefore, the aluminum foil after the secondary annealing is an aluminum foil for electrolytic capacitor electrodes obtained by the method for manufacturing an aluminum foil for electrolytic capacitor electrodes according to the examples of the present invention (hereinafter, referred to as Examples 1 to 5). Called aluminum foil).

On the other hand, for comparison, an aluminum foil having the composition shown in Table 1 (coil state after foil rolling) was subjected to primary annealing in a Dx gas atmosphere using a batch furnace. Here, the temperature and time of the primary annealing were changed as shown in Table 7 (columns of primary annealing / Comparative Examples 1 to 4). This one
When the oxide film thickness and the crystal grain size of the aluminum foil after the subsequent annealing were measured, the oxide film thickness was as shown in Table 7.
52 to 67Å (columns of Comparative Examples 1 to 4), and the crystal grain size is
It was 55 to 88 μm. This oxide film thickness is that after the primary annealing, but in Table 7, for convenience, it is described in the column of the oxide film thickness after the secondary annealing. The aluminum foil after the primary annealing is the aluminum foil for electrolytic capacitor electrodes obtained by the method according to the comparative example (hereinafter, comparative examples 1 to 1).
4 called aluminum foil).

The aluminum foil according to Examples 1 to 5,
The aluminum foils according to Comparative Examples 1 to 4 were subjected to electrolytic etching under the electrolysis conditions shown in Table 3 using the electrolytic solution shown in Table 2, and then subjected to 20 V chemical conversion using the chemical conversion treatment solution shown in Table 4. Then, the capacitance was measured by the counter electrode method using the electrolytic solutions shown in Table 5. The results are shown in Table 7. As can be seen from Table 7, the aluminum foils according to Examples 1 to 5 are higher than the aluminum foils according to Comparative Examples 1 to 4,
Capacitance is generally high. More specifically, when viewed at the same primary annealing temperature and time (comparing those having the same primary annealing temperature and time), the aluminum foil according to the example is considerably higher than the aluminum foil according to the comparative example. High capacitance. For example, the aluminum foil according to Comparative Example 3 (primary annealing temperature: 400 ° C., time: 24 hr) has a capacitance of 42.3 μF / cm.
² , while the aluminum foil (1
Next annealing temperature: 400 ℃, time: 24hr) Capacitance: 49.6μ
F / cm ^2, which is extremely high. The reason why it is extremely high is because the difference between them: 7.3 μF / cm ² is extremely large in the field of electrolytic capacitor electrodes.

Among the aluminum foils according to Examples 1 to 5, those according to Examples 2 to 5 have a relatively high electrostatic capacity, and among them, those according to Examples 2 to 3 have a primary annealing time and Since the secondary annealing time is relatively short, the conditions (primary annealing time: 24 hr, according to Examples 2 to 3 among the Examples 1 to 5).
It can be said that it is desirable to adopt the secondary annealing time: 6 hours in view of performance and economy.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

[0033]

[Table 6]

[0034]

[Table 7]

[0035]

The method for producing an aluminum foil for an electrolytic capacitor electrode according to the present invention has a conventional special atmosphere (Dx gas,
Unlike the method of annealing in an atmosphere where it is difficult to form an oxide film such as argon or vacuum), a special atmosphere is not required for annealing, and the surface area can be increased by improving the electrolytic etching property. It is possible to obtain an aluminum foil for electrolytic capacitor electrodes, which can improve the battery capacity, and therefore, it is possible to easily and economically obtain an aluminum foil for electrolytic capacitor electrodes that can improve the electrostatic capacitance.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an annealing temperature of an aluminum foil and a capacitance.

FIG. 2 is a diagram showing a relationship between a secondary annealing temperature of an aluminum foil and a capacitance.

Claims (1)

[Claims] 1. An aluminum purity of 99.98% or more, 0.002 to 0.005 wt% of Fe, and 0.002 to 0.005 wt of Si.
3% aluminum foil containing 0.001 to 0.004 wt% Cu
After annealing at 50 to 500 ° C, the oxide film formed by the annealing is removed, and then in an oxidizing atmosphere.
Heat treatment at 20-330 ℃ to reduce the grain size of aluminum foil.
A method for producing an aluminum foil for electrolytic capacitor electrodes, which comprises forming an oxide film having a thickness of 55 to 100 μm and a film thickness of 25 to 50 Å on the surface of the aluminum foil.