JP7660843B2 - Chemical conversion solution, chemical conversion treatment method for aluminum substrate, chemically treated substrate, electrode material for aluminum electrolytic capacitor, and capacitor - Google Patents

Description

Applicable under Article 30, Paragraph 2 of the Patent Act Publisher: Surface Finishing Association of Japan Publication name: Abstracts of the 144th Lecture Conference Publication date: September 6, 2021

The present invention relates to a chemical conversion liquid, a method for chemically treating an aluminum-containing substrate using this chemical conversion liquid, a chemically treated substrate obtained by this chemical conversion treatment method, an electrode material for an aluminum electrolytic capacitor that includes this chemically treated substrate, and a capacitor that includes this electrode material for an aluminum electrolytic capacitor.

Aluminum substrates are used for electrodes for capacitors and wiring for various electronically conductive circuits. When used as a capacitor, it is necessary to form an oxide film on the aluminum surface that acts as a dielectric. One known method for this is chemical conversion treatment using anodization. This method forms an oxide film on the surface of a substrate, such as aluminum or an aluminum alloy, by electrochemically oxidizing the surface of the substrate in a chemical conversion solution. The higher the capacitance and withstand voltage of the oxide film, the more useful the device will be in industry. However, for the same electrode area, there is a problem in that increasing the withstand voltage reduces the capacitance.

As a method for forming an oxide film with high electrostatic capacity on an aluminum substrate by chemical conversion treatment, a method of performing chemical conversion treatment using a chemical conversion solution containing boric acid, ammonium borate, branched polypropylene glycol, and water has been disclosed (Patent Document 1). In addition, as a method for suppressing electrolytic corrosion on an aluminum alloy during chemical conversion treatment and thereby forming a good oxide film, a surface conditioner with a pH of 3 to 12 that contains zinc phosphate particles with a D50 of 3 μm or less, a water-soluble organic polymer, and a layered clay mineral and is used before the phosphating of metals, and further contains a divalent or trivalent metal nitrite compound, zinc oxide and/or sodium hydroxide, and a nonionic or anionic surfactant as a function-imparting agent has been disclosed (Patent Document 2).

JP 2019-102595 A International Publication No. 2007/013626

The present invention aims to provide a chemical conversion liquid capable of producing a chemically treated substrate that is suitable as an electrode material for aluminum electrolytic capacitors and that has both a high electrostatic charge and a high withstand voltage. The present invention also aims to provide a method for chemically treating an aluminum substrate using the above-mentioned chemical conversion liquid, and a chemically treated substrate obtained by this chemical conversion method. A further objective of the present invention is to provide an electrode material for aluminum electrolytic capacitors that includes the above-mentioned chemically treated substrate, and a capacitor that includes this electrode material for aluminum electrolytic capacitors.

After extensive research, the inventors discovered that a chemical conversion solution containing smectite, a type of clay mineral, can produce a chemically treated substrate that is suitable as an electrode material for aluminum electrolytic capacitors, has a large capacitance, and maintains its withstand voltage, leading to the invention.

That is, the above-mentioned object of the present invention has been achieved by the following means [1]
at least,
(A) Smectite (B) Chemical conversion solution containing water.
[2]
The chemical conversion solution according to [1], wherein the smectite is a synthetic smectite.
[3]
The chemical conversion solution according to [1] or [2], wherein the content of smectite in the chemical conversion solution is 0.1 to 20 mass%.
[4]
A method for chemically treating an aluminum substrate, the method comprising the steps of: anodizing an aluminum substrate using the chemical conversion solution according to any one of [1] to [3] to obtain a chemically treated substrate having an aluminum oxide film formed on its surface.
[5]
A chemically treated substrate obtained by the chemical conversion treatment method according to [4].
[6]
An electrode material for an aluminum electrolytic capacitor, comprising the chemically converted substrate according to [5].
[7]
A capacitor comprising the electrode material for an aluminum electrolytic capacitor according to [6].

The chemical conversion liquid of the present invention and the method for chemically treating an aluminum substrate using the chemical conversion liquid can provide a chemically treated aluminum substrate with a large electrostatic capacity and excellent voltage resistance. The chemically treated substrate of the present invention has a high electrostatic capacity and a high voltage resistance, and is suitable as an electrode material for an aluminum electrolytic capacitor, and the electrode material for an aluminum electrolytic capacitor is suitable as a capacitor.

The following describes preferred embodiments of the present invention in detail, but the present invention is not limited to these embodiments except as specified in the present invention.

The chemical conversion solution of one embodiment of the present invention is used, for example, to perform a chemical conversion treatment on an aluminum substrate by anodization, to produce a chemically-treated substrate having an aluminum oxide film formed on its surface.

By using the chemical conversion liquid of this embodiment, a wide range of metal substrates can be anodized. Metals constituting the substrates to be anodized include aluminum and aluminum alloys, which can be made into capacitors by forming an oxide film. There are no particular restrictions on the type of metal that can be combined with aluminum to form an aluminum alloy. By using the chemical conversion liquid of this embodiment, substrates made of aluminum and aluminum alloys that can be used as electrode materials for aluminum electrolytic capacitors can be effectively anodized.

(Aluminum substrate)
The material of the "aluminum substrate" (hereinafter also simply referred to as "substrate") in this specification is not particularly limited as long as it is a metal mainly composed of aluminum. Suitable substrate materials, for example, in the case of aluminum alone, are pure aluminum (aluminum with a purity of 99% or more), more preferably with a purity of 99.9% or more, and most preferably with a purity of 99.99% or more. Aluminum alloys containing aluminum as the main component can also be used. In the case of an aluminum capacitor, the shape of the substrate can be, for example, a foil, a plate, a metal wire, or a molded body using these. The thickness of the aluminum oxide film formed on the surface of the chemically treated substrate is not particularly limited, but is preferably in the range of 1 nm to 2 μm.

(chemical conversion solution)
The chemical conversion solution in this embodiment is a composition containing smectite and water as essential components. The smectite component is presumed to be a component that increases the capacitance while maintaining the withstand voltage of the oxide film formed by chemical conversion. In addition, water is a solvent component. For example, by electrochemically anodizing an aluminum substrate using the chemical conversion solution of this embodiment, an aluminum oxide film, which is an oxide film, is formed on the substrate surface, and a chemically treated substrate with large capacitance that is suitable as an electrode material for aluminum electrolytic capacitors can be produced.

(Smectite)
The type of smectite contained in the chemical conversion solution in this embodiment is not particularly limited and can be appropriately set depending on the purpose. Smectite can be natural or synthetic, but since it is preferable to use a material that contains as few impurities as possible, synthetic smectite is particularly preferable.
The smectite is preferably one or more selected from montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite, and stevensite, and more preferably one or more selected from hectorite, saponite, and stevensite.

There are no particular limitations on the interlayer cation species of the smectite used in the present invention. However, it is particularly preferable for the smectite to contain sodium or lithium, which are cation species that easily swell in water, because it is believed that the smectite also functions as a supporting electrolyte for the electrochemical reaction that causes the chemical conversion treatment by completely swelling in water, which is the solvent for the chemical conversion solution. In addition, the cation exchange capacity (CEC) of the smectite is preferably 20 meq/100 g or more, more preferably 25 meq/100 g, and most preferably 30 meq/100 g, from the viewpoint of easy swelling in water. There are also no particular limitations on the particle size of the smectite, which can be used.

The presumed mechanism by which the capacitance is improved by having smectite coexist in the chemical conversion solution is as follows. The surface of the smectite layer, which has a strong interaction with metal ions and metal compounds, strongly interacts with the surface of the aluminum metal layer, and is incorporated into the chemical conversion film in a planar oriented state as a result of the chemical conversion treatment. At this time, the negatively charged smectite layer acts to incorporate metal cations that are not normally incorporated into the chemical conversion film by anodization, promoting polarization within the oxide film and improving the capacitance.

(water)
The chemical conversion solution of the present invention contains water as an essential component. There is no particular limitation on the water used, but from the viewpoint that it is better that the oxide coating formed during the chemical conversion treatment does not contain any components other than smectite, it is preferable to use water from which ionic components have been removed, such as distilled water or ion-exchanged water. As for the degree of ion removal, the ionic conductivity of the water is preferably 10 μS/m or less, more preferably 5 μS/m or less, and even more preferably 1 μS/m or less.

(pH of chemical conversion solution)
There is no particular restriction on the pH of the chemical conversion solution used in the present invention. When only smectite in which the interlayer metal is sodium or lithium is dispersed in water, the pH is generally in the weak alkaline range of about 10 to 11. The chemical conversion solution may be used as is, or the pH may be adjusted to a neutral to acidic range by adding an acid component, or the pH may be adjusted to a highly alkaline range by adding an alkaline component. However, it is preferable that the pH be in a range in which the smectite can maintain a swollen state in water.

(Concentration of smectite components in chemical conversion solution)
There is no particular restriction on the concentration of the smectite component in the chemical conversion solution used in the present invention. From the viewpoint of obtaining the effect of the chemical conversion treatment and maintaining the fluidity of the chemical product to facilitate the chemical conversion treatment, the content of smectite in the chemical conversion solution is preferably 0.1 to 20 mass%, more preferably 0.5 to 15 mass%, and most preferably 1 to 10 mass%.

<Other ingredients>
The chemical conversion liquid of the present invention may contain other components within a range that does not impair the effects of the present invention. For example, in order to facilitate the progress of the chemical conversion treatment, it may contain ammonium borate or ammonium adipate, which is a supporting electrolyte of an electrolyte used in a known aluminum chemical conversion treatment. It may also contain other salts that function as supporting electrolytes. Other additives may include stabilizers, defoamers, pH adjusters, specific gravity adjusters, viscosity adjusters, wettability improvers, chelating agents, oxidizing agents, reducing agents, organic solvents, and surfactants. The other additives may be used alone or in combination of two or more. The content of the additives in the chemical conversion liquid is preferably 0.01 to 10 mass% per additive so as not to interfere with the effect of the smectite on the chemical conversion treatment.

The method for chemically treating an aluminum substrate according to one embodiment of the present invention (hereinafter, also simply referred to as the "chemically treating method") includes a step of chemically treating an aluminum substrate by anodizing using the above-mentioned chemical conversion solution to obtain a chemically treated substrate having an aluminum oxide film formed on its surface.

The conditions for anodizing an aluminum substrate using the chemical conversion solution of the present invention are not particularly limited, and normal conditions can be applied. The temperature during anodization is preferably set to a temperature at which the chemical conversion solution is stable as a liquid. Specifically, the temperature is preferably set to 0 to 95°C, more preferably 10 to 95°C, and even more preferably 20 to 90°C. The current and voltage during anodization may be appropriately controlled so as to provide conditions for forming an oxide film (aluminum oxide film) on the surface of the substrate. Usually, the chemical conversion is performed at a constant current until a predetermined chemical conversion voltage (Vf) is reached, and then the chemical conversion voltage (Vf) is maintained for a certain period of time for anodization. The current density is preferably set to a range of 0.1 to 100 mA/ ^cm2 , and even more preferably to a range of 0.5 to 50 mA/ ^cm2 . The chemical conversion voltage (Vf) is usually set to a range of 2 to 800 V, preferably a range of 5 to 700 V, and even more preferably a range of 8 to 650 V. Alternatively, the formation may be performed with AC having a constant peak current value until the formation voltage (Vf) is reached, and then switched to DC when the formation voltage (Vf) is reached and maintained for a certain period of time.

In the chemical conversion treatment method of this embodiment, the entire substrate may be anodized, or only a portion of the substrate may be anodized. When forming an aluminum oxide film on only a portion of the substrate surface, the portion to be anodized may be selected in advance using a method such as photoresist.

The chemically treated substrate produced by the above-mentioned chemical treatment method can be used to produce an electrode material for an aluminum electrolytic capacitor. That is, an electrode material for an aluminum electrolytic capacitor according to one embodiment of the present invention includes the above-mentioned chemically treated substrate.

Furthermore, a capacitor can be manufactured using the above-mentioned electrode material for aluminum electrolytic capacitors. That is, a capacitor according to one embodiment of the present invention is equipped with the above-mentioned electrode material for aluminum electrolytic capacitors. The above-mentioned capacitor equipped with the electrode material for aluminum electrolytic capacitors can be manufactured by a known method. In addition to the electrode material (anode material and cathode material) of the present invention, the capacitor further comprises an electrolyte, a separator, electrolytic paper, etc.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited thereto.

<Chemical conversion solution>
(Chemical conversion solution used in Examples 1 to 4)
As shown in Table 1, various smectites were dispersed in distilled water (electrical conductivity 0.2 μS/cm or less) to obtain 1 to 5 mass % viscous, nearly transparent liquids. The chemical conversion solutions of Examples 1, 2, and 4 exhibited an alkaline pH of 10 to 11. In Example 3, 11 g of 0.2 M sulfuric acid aqueous solution was added dropwise to 100 g of a 2 mass % Stevensite aqueous solution while stirring with a stirrer, thereby obtaining a chemical conversion solution in which the clay dispersion was adjusted to pH 7.

(Chemical conversion solution used in Comparative Examples 1 and 2)
As shown in Table 1, a chemical conversion solution not containing smectite was obtained by dissolving a predetermined supporting electrolyte in distilled water (electrical conductivity 0.2 μS/cm or less).
The pH of the chemical conversion solution obtained for each of the above examples and comparative examples was measured using a pH meter (manufactured by Horiba, Ltd., model number: LAQUAtwin-PH-33B).

(Aluminum substrate)
A 4N high purity aluminum foil was used as the aluminum substrate for chemical conversion. A flag-shaped electrode was prepared by connecting the aluminum foil without cutting it to supply power to the electrode with an electrode area of 1 cm ² (1×0.5 cm on one side), and the aluminum substrate for chemical conversion was used. In addition, in order to clean the electrode surface before the chemical conversion treatment described below, the oil on the surface was removed by alkaline degreasing with a 1M aqueous sodium hydroxide solution. After that, the electrode was washed three times in pure water, neutralized in a 1M aqueous nitric acid solution, and washed again three times in pure water, and then the following chemical conversion treatment was performed immediately.

(chemical conversion treatment)
A stainless steel container (SUS304) with an inner diameter of 30 mm was filled with the chemical conversion solution prepared for each Example and Comparative Example, and only the electrode portion of the aluminum substrate was immersed in the solution. The aluminum substrate and the stainless steel container corresponding to the counter electrode were connected to a high voltage power source for chemical conversion treatment (Takasago Manufacturing Co., Ltd., HV-1.5-03) and a digital multi-tester for measuring current and voltage (Sanwa Electric Instruments Manufacturing Co., Ltd., model number: RD701), and chemical conversion treatment was performed under conditions of 25°C and a current density of 1 mA/ ^cm2 until the voltage reached 10 V.

(Evaluation of Capacitance of Chemical Conversion Film)
The electrostatic capacitance of the aluminum substrate having an oxide film formed by the above-mentioned chemical conversion treatment was evaluated using the composition of the chemical conversion solution of each Example and Comparative Example. Using a 15% by mass aqueous solution of ammonium adipate prepared in advance as an electrolyte, the electrostatic capacitance C per 1 cm2 of each chemical conversion film was measured using an LCR meter (manufactured by NF Corporation, model number: ZM2355) based on the electrostatic capacitance test method for low-voltage chemical conversion foils under the conditions of a frequency of 120 Hz, a measurement voltage of 0.5 ^Vrms , and a bias voltage of 1.5 V. The measurement results are shown in the capacitor evaluation column in the lower part of Table 1.

(Evaluation of the dielectric strength of chemical conversion films)
The chemical conversion films obtained using the chemical conversion solutions and devices used in the Examples and Comparative Examples were subjected to re-chemical conversion treatment under the same operation and conditions. The voltage value immediately after the start of the re-chemical conversion treatment in this treatment step was defined as the film withstand voltage/V, which was used as an index of the withstand voltage of the capacitor.

(Evaluation of the capacity of the chemical conversion film)
The CV product was calculated from the capacitor evaluation results for the composition of the chemical conversion solution used in each Example and Comparative Example. The CV product was calculated by multiplying the evaluation results of the capacitance of the chemical conversion film by the evaluation results of the film withstand voltage of the chemical conversion film. This value was used as an index of the capacitor's charge storage capacity.

Chemical conversion solution used (smectite)
Mineral name: Hectorite; Product name: Sumecton-SWN; Cation exchange capacity: 43 meq/100g
Mineral name: Saponite; Product name: Sumecton-SA; Cation exchange capacity: 66 meq/100 g
Mineral name: Stevensite; Product name: Sumecton-ST; Cation exchange capacity: 25 meq/100 g
In both cases, the correlated ion is sodium ion, manufactured by Kunimine Kogyo Co., Ltd. (others)
Ammonium adipate, sodium hydroxide, sodium bicarbonate, nitric acid 1.38: all Fujifilm Wako Pure Chemical reagents, special grade.

As shown in the lower part of Table 1, the electrolytic capacitors (Examples 1 to 4) having an aluminum oxide film obtained by performing a chemical conversion treatment by anodization in a dispersion liquid composed of smectite and water, which is the chemical conversion liquid of the present invention, showed the same film withstand voltage value as the capacitors made from the two types of electrolytes not containing smectite in Comparative Examples 1 and 2. In addition, the aluminum electrolytic capacitors obtained in Examples 1 to 4 showed a 25 to 50% increase in capacitance compared to the capacitors in Comparative Examples 1 and 2, and were able to be capacitors with high capacitance.

Claims (6)

at least,
(A) smectite (B) water ;
A chemical conversion solution for anodizing an aluminum substrate, having a pH of 7 to 11 . The chemical conversion solution according to claim 1 , wherein the smectite is a synthetic smectite. The chemical conversion solution according to claim 1 or 2 , wherein the content of smectite in the chemical conversion solution is 0.1 to 20 mass %. A method for chemically treating an aluminum substrate, comprising the steps of: anodizing an aluminum substrate in a chemical conversion solution for anodizing an aluminum substrate, the chemical conversion solution containing at least smectite and water , to obtain a chemically treated aluminum substrate having an aluminum oxide film formed on its surface. A method for producing a chemically treated aluminum base material, comprising a step of chemically treating an aluminum base material by anodizing in a chemical conversion solution for anodizing an aluminum base material, the chemical conversion solution containing at least smectite and water , to obtain a chemically treated aluminum base material having an aluminum oxide film formed on its surface. A method for producing a capacitor, comprising the step of incorporating an electrode material for an aluminum electrolytic capacitor, the electrode material including a chemically treated aluminum base material obtained by the method for producing a chemically treated aluminum base material according to claim 5 , into a capacitor.