JP2009270140A - Aluminum foil for electrolytic capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum foil which can meet a requirement of further increasing the capacitance of the electrolytic capacitor when having been electrolytically etched. <P>SOLUTION: The aluminum foil for the electrolytic capacitor has an aluminum purity of 99.9% or higher, and includes one or more of In, Bi or Sn. In the surface layer from the surface to the depth of 0.1 μm, 10 to 500 ppm of Pb exists and In, Bi or Sn are not concentrated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an aluminum foil for an electrolytic capacitor that is used for an electrode of an electrolytic capacitor and that is not yet etched. In the specification of the present invention,% and ppm are in terms of mass.

  The electrode of the electrolytic capacitor is obtained by subjecting an aluminum foil to an electrolytic etching process to increase the surface area per unit projected area, and further, an anodized film is formed on the surface. As an aluminum foil used for such an application, a foil that has been specially treated and formed a concentrated layer of a trace amount of added elements on the surface of the foil is used.

  The aluminum foil for electrolytic capacitors of the present invention is used as an electrode after being etched by direct current. For example, in Patent Document 1 (Japanese Patent Publication No. 62-42370), Pb, In, and At least one of Bi is imparted in the state of an element or a compound, heat diffusion treatment is performed at a temperature equal to or higher than the melting point of these metals, and annealing is performed according to a conventional method if necessary, and Pb, In, and Bi are added to the surface layer of the foil. A concentrated aluminum foil for electrolytic capacitors is disclosed. In addition, this foil is allowed to contain less than 1 ppm as an impurity element.

  Patent Document 2 (Japanese Patent Laid-Open No. 2000-252170) discloses a foil for manufacturing an anode of an electrolytic capacitor having an Al purity exceeding 99.9%, and the foil is an average of the total of elements Pb + B + In The distribution of these three elements in the surface region having a weight content of 0.1 to 10 ppm and a depth of 0.1 μm is a dispersion ratio Rd = (Imax−Imin) / A thin foil of refined aluminum that exhibits Iaverage has been proposed. In particular, this foil is subjected to intermediate annealing at high and low temperatures with cold rolling in between, and the heating and holding temperature of the final annealing treatment is 580 ° C. × 10 hours under argon gas, so that Pb + B + In is concentrated in the foil surface layer with small variation. It has been disclosed that by etching such a foil, a high capacitance can be provided for a foil having a large variation in the Pb, B, and In concentration of the foil surface layer.

Japanese Patent Publication No.62-42370 JP 2000-252170 A

  However, the conventional technique has a limit in the surface area expansion ratio of the foil by electrolytic etching, and cannot cope with the increase in capacitance. That is, since all of Pb, In, Bi or all of Pb, In, B are concentrated on the surface layer, the number of pits becomes excessive, and bonding between pits and excessive dissolution of the surface layer portion occur, resulting in a capacitance. Does not rise.

  An object of the present invention is to provide an aluminum foil that can cope with an increase in capacitance by electrolytic etching.

  As a result of studying a minute range of elements contained in the surface layer of the aluminum foil, the present inventors have found that an aluminum foil in which Pb is present in the surface layer and In, Bi, Sn is not concentrated in the surface layer is Pb, In, The present invention has been completed by finding that the capacitance is higher than that of the aluminum foil in which Bi and Sn are concentrated in the surface layer.

  That is, the present invention is a foil having an Al purity of 99.9% or more and containing one or more of In, Bi, or Sn, and 10 to 500 ppm of Pb is present in the surface layer from the surface to a depth of 0.1 μm, The aluminum foil for electrolytic capacitors is characterized in that In, Bi or Sn is not concentrated in the surface layer.

  In the present invention, the content of one or more of In, Bi, or Sn is preferably In: 0.1 to 10 ppm, Bi: 0.1 to 10 ppm, and Sn: 1 to 20 ppm.

  Furthermore, it is desirable to contain 5 ppm or less of Pb.

  Since the aluminum foil of the present invention can cope with an increase in the capacitance of the electrolytic capacitor, it has an effect that it is possible to provide a lightweight and small electrolytic capacitor, an electronic device, and the like.

  For example, in the case of a medium-high voltage capacitor, an aluminum electrolytic capacitor is typically etched as follows in order to increase the surface area by forming pits. That is, the surface area is increased by drilling pits in a tunnel shape by continuous etching using direct current. This etching process is performed in two stages, a primary etching process and a secondary etching process. In the primary etching process, a large number of initial tunnel-like pits are formed on the foil surface. Deep pits and enlarge the pit diameter. The secondary etching process is divided into multiple stages and processed under different conditions. The number of initial tunnel-like pits on the foil surface formed by the primary etching process is greatly influenced by the properties of the foil surface.

  The present invention is a foil containing one or more of In, Bi or Sn, or further containing 5 ppm or less of Pb, the Pb content in the surface layer between 0.1 μm depth from the foil surface is 10 to 500 ppm, And it is necessary that one or more of In, Bi or Sn is not concentrated in the surface layer. This is the basis of the high-density Pb present in the surface layer of the foil and one or more of In, Bi or Sn in the foil matrix, or the special properties of the foil surface where Pb to be further contained exists. The excessive melting or bonding of the initial tunnel-like pits formed in the primary etching process can be reduced as much as possible, and the starting points of the initial pits can be increased. Although the details of the reason are unclear, it is presumed as follows. Conventionally, a foil is generally 200 μm or less in thickness, but in the present invention, the term “foil” is used, but the thickness is not particularly limited.

  First, the reason why the depth from the foil surface to 0.1 μm is limited to the surface layer is that the initial pit formation during the primary etching process is greatly affected by the depth from the foil surface to the 0.1 μm at most. It is because it is the surface layer. Even if the inner part deeper than 0.1 μm is in the above state, it is sufficient if the initial pits formed by the primary etching process are formed to a depth of 0.1 μm or less, and the pits are deepened by the secondary etching process. This is because it is considered that there is little influence in the process of expanding the diameter.

  In further explanation, the surface layer part of the foil formed in the process from the plate making to the foil making has many defective parts due to the precipitation part, Al and elements other than Al or oxides thereof. If this defective portion has a larger electrochemical potential difference than the surrounding area, it is considered that initial pits can be easily formed in the defective portion or the surrounding area during the primary etching process. Here, it is considered that Pb does not dissolve in Al and is considered to be in a dispersed state. From the microscopic viewpoint, the Pb-existing portion is considered to have a large electrochemical potential difference and a large number of locations where the potential difference is large compared to the surroundings. On the other hand, it is considered that In, Bi, and Sn are not dissolved in Al and are in a dispersed state. Similarly to Pb, In, Bi, and Sn are considered to have a larger electrochemical potential difference than the surroundings when viewed microscopically. Therefore, the number of initial pits formed on the surface layer where Pb is present during the primary etching process and In, Bi, and Sn are not concentrated is not excessively dissolved and there are few bonds between the pits. In the process of deepening the pit and widening the pit diameter in the process, even if there is a further increase in the number of pits due to the presence of In, Bi, Sn, the effect of the initial number of pits is greater, resulting in a larger capacitance. It is thought that foil is obtained. However, according to the recognition of the prior art, not only Pb but also In, Bi, and Sn are concentrated on the surface layer, the number of pits due to Pb, In, Bi, and Sn becomes excessive, and the pits are combined, or the surface layer portion It seems that excessive dissolution occurs and the capacitance does not improve as expected.

  Here, Al purity of 99.9% or more is necessary for drilling pits in a tunnel shape by etching treatment. When the purity decreases, the pits easily branch, and there is no straight tunnel shape without branching. Pit drilling is not possible.

  The average content of Pb in the surface layer from the surface to a depth of 0.1 μm is 10 to 500 ppm, and that one or more elements of In, Bi or Sn are not concentrated in the surface layer is a primary etching process. Required to drill the correct number of initial pits. If the Pb of the surface layer is less than 10 ppm, the number of initial pits is small, and if it exceeds 500 ppm, the initial pits are over-dissolved, bonded or communicated to reduce the number of initial pits, and as a result, a foil having a high electrostatic capacity cannot be obtained. Even if the content of Pb is 10 to 500 ppm, if one or more elements of In, Bi, or Sn contained in the foil are concentrated on the surface layer, the initial pits are over-dissolved, bonded or communicated. The number of pits is small, and as a result, a foil having a high capacitance cannot be obtained.

  In the present invention, the fact that one or more elements of In, Bi, or Sn are not concentrated in the surface layer means that the concentration of In, Bi, or Sn in the surface layer is the concentration of In, Bi, or Sn in the average composition of the foil (each of the foil It means not to exceed twice the element content. The average composition corresponds to the composition of the melt. Although the degree of concentration of In, Bi, and Sn is preferably as small as possible, these elements are easily concentrated on the surface layer in the process of manufacturing the foil, and therefore the basic upper limit is set to twice the content in the average composition. The concentration in the surface layer is preferably 1.5 times or less, more preferably 1.3 times or less of the content in the average composition.

  If the content exceeds twice the content in the average composition, pits are over-dissolved or combined as compared with the foil that is not concentrated, and the number of initial pits is small, and as a result, a foil having a high capacitance cannot be obtained.

  It is necessary for the foil to contain one or more of In, Bi, or Sn in order to make it easy to drill or expand the initial pit during the etching process of the foil. Preferable contents are In: 0.1 to 10 ppm, Bi: 0.1 to 10 ppm, Sn: 1 to 20 ppm, and more preferably In + Bi ≦ 10 ppm and In + Bi + Sn ≦ 20 ppm. Below the lower limit, the effect is small, and when the upper limit is exceeded, initial pits tend to be over-dissolved or combined in the primary etching process, reducing the number of initial pits and increasing the number of tunnel-like pits in the secondary etching process. As a result, it becomes difficult to obtain a foil having a high capacitance.

  More preferably, when Pb is contained in the foil in an amount of 5 ppm or less, the same effect as In, Bi or Sn can be expected. When the upper limit is exceeded, initial pits tend to be over-dissolved or bonded during the primary etching process, and the number of tunnel-like pits decreases during the secondary etching process, resulting in difficulty in obtaining a foil with high capacitance. Become.

  Although the definition of Al purity has been described above, the content of elements other than those specified in the present invention can be appropriately determined within a range not impeding the effects of the present invention. For example, as preferable ranges, the content of Fe and Si is 5 to 100 ppm each, the content of Cu is 20 to 100 ppm, Zn, Ga, and B are each 20 ppm, preferably 10 ppm or less, more preferably 5 ppm or less, Zn, The total of Ga and B is preferably 20 ppm or less. Elements other than the above are each 5 ppm or less, preferably 3 ppm or less, more preferably 1 ppm or less, and 10 ppm or less in total.

<Method for producing the aluminum foil of the present invention>
The aluminum foil of the present invention can be produced by various methods. An example is as follows. First, on the premise that the Al content is 99.9% by mass or more (that is, Al purity is 99.9% by mass or more), for example, In: 0.1 to 10 ppm, Bi: 0.1 to 10 ppm, or Sn: 0.5 to The composition is adjusted and melted in the range of a composition containing at least 20 ppm or further containing 5 ppm or less of Pb. The additive element is added and contained as a base metal, a return material, a metal or a master alloy, etc. during melting.

  In melting, degassing, degassing, and if necessary, passing through a filter and casting into a sheet ingot. Samples for analysis were collected according to JIS H 2111 and analyzed with a solid state emission spectrometer (ARL3460 manufactured by ARL (*) (*: former company name. Present: Thermo Fisher Scientific)) to obtain a foil composition. . The ingot is subjected to homogenization heat treatment, hot rolled, cold rolled, and intermediate annealed as necessary to obtain a foil. Next, an acidic solution in which Pb is dissolved in various concentrations is applied to the foil surface and, for example, Pb is present in the surface layer at a high concentration by heating and holding at a temperature of 100 ° C. to 250 ° C., and In, Bi, and Sn are surface layers. It is possible to obtain a foil that has not been concentrated. That is, the heating conditions described above can be appropriately selected by combining conditions of temperature and time in which Pb applied on the surface diffuses in the surface layer, but In, Bi, and Sn contained therein are not substantially diffused. Well, it is not necessary to limit in particular.

  The aluminum foil for medium- and high-voltage capacitors in the present invention, that is, the aluminum foil that is subjected to chemical conversion treatment at a chemical conversion voltage of 100 V or more, is typically etched as follows in order to increase the area expansion ratio due to pit formation. For example, the area expansion rate is increased by drilling tunnel-like pits by direct etching using direct current. This etching process is divided into two stages, a primary etching process and a secondary etching process. In the primary etching process, a large number of initial pits are formed on the foil surface, and then in the secondary etching process, the processing conditions are changed to change the initial pits to the foil thickness. Advancing in the direction and expanding the pit diameter.

The electrolytic solution used in the primary etching process may be a known mixed acid aqueous solution containing one or more of hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid and the like, and is not particularly limited. When the temperature of the electrolytic solution is high, the reaction is promoted, which is preferable. However, when the temperature is too high, the reaction is too fast and the foil surface is so melted that it becomes difficult to form uniform initial pits. A preferable liquid temperature is 60 to 95 ° C. Moreover, a preferable processing time is about 2 to 4 minutes. The amount of electricity is preferably 15 to 30 K / cm ² , and the current density is preferably 100 to 300 mA / cm ² .

  The initial pit diameter increasing process by the secondary etching process increases the surface area by increasing the diameter of the initial pit formed by the primary etching process by using a direct current electrolytic process, a chemical process, or a combination of both.

The conditions for the secondary etching treatment need not be particularly limited. For example, the electrolyte is preferably a mixed acid aqueous solution obtained by adding a small amount of sulfuric acid, phosphoric acid, oxalic acid or the like to hydrochloric acid, or a mixed acid aqueous solution obtained by adding nitric acid. Alternatively, chemical treatment may be performed using the same kind of primary etching treatment liquid. The temperature of the liquid is preferably 60 to 95 ° C., and when electrolysis is performed, the current density is preferably 60 to ²⁰⁰ mA / cm ² . The treatment time is typically about 2 to 20 minutes, although it depends on the diameter of the tunnel pit. The length (depth) of the tunnel-like pit varies depending on the foil thickness and the usage of the etched foil, but is generally 10 to 50 μm. The aluminum foil whose surface area is increased by forming pits by etching treatment is subjected to chemical conversion treatment using this foil as an anode. The chemical conversion treatment may be performed under known conditions. For example, as an electrolytic solution, a buffer solution such as ammonium borate, ammonium phosphate, or organic acid ammonium is used, and a voltage of about 100 to 200 V or more is applied in one or more stages depending on the use of the capacitor. A chemical conversion film, that is, a dielectric film is formed by application. Prior to the etching process, the surface of the foil may be treated with an acid or an alkali solution for deoiling and surface adjustment. In this treatment, for example, 0.05 to 1 mol / liter of nitric acid or a caustic soda aqueous solution may be used as a treatment solution, and the treatment may be performed at a temperature of 40 to 60 ° C.

  The aluminum foil of this invention was manufactured in the following procedure, and the characteristic was evaluated.

[Sample preparation method]
<Casting>
Each of Si and Fe contents 10-40 ppm, Cu content 50-70 ppm, one or more contents of In, Bi or Sn are set to various values, or further Pb is added, and the composition is adjusted within the above range and dissolved. The aluminum alloy melt shown in Table 1 or Table 2 having an Al purity of 99.9% by mass or more was made into an ingot for a sheet having a thickness of 560 mm by DC casting, and the ingot was held at a temperature of 600 ° C. for 10 hours. The surfaces were homogenized and each side was shaved at 15 mm at room temperature.

<Rolling>
It was reheated and hot rolling was started at an ingot temperature of 520 ° C. to obtain a hot rolled plate having a thickness of 6 mm. The end temperature of hot rolling was 210 ° C. The obtained hot-rolled sheet was subjected to intermediate annealing and cold-rolled to 0.25 mm to obtain a foil.

<Foil rolling>
Next, after cold rolling to a thickness of 130 μm, intermediate annealing at 260 ° C. × 50 minutes was performed to roll to a thickness of 110 μm to obtain a foil. This foil was finally annealed at 530 ° C. for 30 minutes to obtain a foil.

<Process to make Pb exist on foil surface layer>
Next, a hydrochloric acid solution in which Pb was dissolved in various concentrations was prepared, applied to the foil surface as a hydrochloric acid aqueous solution of about 0.3 to 0.5 N, and kept at a temperature of 160 ° C. for 10 minutes to prepare a sample.

[Evaluation]
<Surface composition>
The composition of the surface layer portion of the obtained sample was analyzed by the following method. The results are shown in Table 1 or Table 2.

<< Surface Layer Composition Analysis Method: Pb, In, Bi, Sn >>
Dissolve the surface layer from the foil surface to 0.1 μm in 20% (5 mol / liter) caustic soda solution, acidify with nitric acid, and then add flameless atomic absorption method (SpectAA220P manufactured by Varian Inc. Analytical Methods for Graphite Tube Atomizers , Method according to Varian ”).

<Capacitance>
The obtained sample was subjected to electrolytic etching under the conditions shown below, then formed into 250V with a boric acid aqueous solution according to JEITA RC-2364A, and the capacitance was measured.

<Etching conditions>
[Preprocessing]
Etching solution: 0.1 mol sodium hydroxide / liter aqueous solution Temperature: 50 ° C
Immersion time: 60 seconds
[Primary etching process]
Electrolyte: (1 mol hydrochloric acid + 3 mol sulfuric acid) / liter aqueous solution Temperature: 85 ° C.
Current density: 200 mA / cm ²
Electrolysis time: 120 seconds
[Secondary etching treatment (immersion treatment)]
Etching solution: (1 mol hydrochloric acid + 3 mol sulfuric acid) / liter aqueous solution Temperature: 85 ° C.
Immersion time: 900 seconds The electrostatic capacity after the secondary etching treatment was measured. The results are shown in Table 1. In the table, the relative value of the capacitance of each sample is shown with the capacitance of sample number 2-1 (Table 2) of the comparative example being 100 (reference).

  From the results in Table 1, it can be seen that the inventive examples in which In, Bi, and Sn are not concentrated on the surface layer have a higher capacitance than the concentrated comparative examples.

[Comparative example]
Using the molten metal having the composition of the example and the molten metal to which Pb was further added, casting, rolling and foil rolling were performed in the same procedure as in the example, and the final annealing treatment was performed.

This foil was heated and held at 580 ° C. for 20 hours in an argon gas atmosphere.
Next, about the sample numbers 2-1 to 2-4 of Table 2, the process described in the <process which makes Pb exist in foil surface layer> of an Example was given, and Pb was made to exist in foil surface layer.

  Table 2 shows the analytical values of each element and the capacitance after etching. In the table, the relative value of the capacitance of each sample is shown with the capacitance of sample number 2-1 being 100 (reference).

  From the results in Table 2, it can be seen that the comparative example in which In, Bi, and Sn are concentrated together with Pb on the surface layer has a lower capacitance than the inventive example in which the surface layer does not have In, Bi, and Sn concentrated.

  ADVANTAGE OF THE INVENTION According to this invention, the aluminum foil which can respond to the increase in electrostatic capacity by an electrolytic etching process is provided.

Claims (3)

  A foil having an Al purity of 99.9% or more and containing one or more of In, Bi or Sn, wherein Pb is present in the surface layer from the surface to a depth of 0.1 μm, and In, Bi or An aluminum foil for electrolytic capacitors, characterized in that Sn is not concentrated in the surface layer.   The content of one or more of In, Bi or Sn is In: 0.1 to 10 ppm, Bi: 0.1 to 10 ppm, Sn: 1 to 20 ppm, for an electrolytic capacitor according to claim 1, Aluminum foil.   The aluminum foil for electrolytic capacitors according to claim 1 or 2, further comprising 5 ppm or less of Pb.