Electrode for electrolytic capacitor, method for manufacturing same, and electrolytic capacitor

Abstract

The present invention relates to a method for manufacturing an electrode for an electrolytic capacitor, the method comprising a chemical conversion step for forming an oxide coating film on the surface of a metal material containing a valve action metal by flowing a current through the metal material in a chemical conversion solution containing an electrolyte, the chemical conversion liquid contains a nitric acid compound as the electrolyte at a concentration of 0.03 mass% or more, and the concentration of a phosphorus compound in the chemical conversion liquid is less than 0.01 mass%.

Description

[0001] This application is a divisional application of the Chinese patent application with the national application number 202180016881.4 and the invention title "Electrode for electrolytic capacitor, method for manufacturing the same, and electrolytic capacitor", which was filed on February 10, 2021. Technical Field

[0002] The present invention relates to an electrode for an electrolytic capacitor, a method for manufacturing the same, and an electrolytic capacitor. Background Art

[0003] As the anode body of a capacitor element, a metal foil or a porous sintered body containing a valve-acting metal is used. An oxide film is formed on the surface of the metal foil or the porous sintered body by chemical conversion treatment. In the chemical conversion treatment, an aqueous phosphoric acid solution (Patent Document 1, etc.) is usually used.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2011-77257 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] An electrolytic capacitor having an oxide film formed using an aqueous phosphoric acid solution sometimes has a large leakage current.

[0009] Means for Solving the Problems

[0010] A first aspect of the present invention relates to a method for manufacturing an electrode for an electrolytic capacitor, which includes the following chemical conversion step: passing an electric current through a metal material containing a valve-acting metal in a chemical conversion solution containing an electrolyte to form an oxide film on the surface of the metal material, the chemical conversion solution containing a nitrate compound as the electrolyte at a concentration of 0.03% by mass or more, and the concentration of a phosphorus compound in the chemical conversion solution being less than 0.01% by mass.

[0011] A second aspect of the present invention relates to a method for manufacturing an electrode for an electrolytic capacitor, which includes the following chemical conversion step: passing an electric current through a metal material containing a valve-acting metal in a chemical conversion solution containing an electrolyte to form an oxide film on the surface of the metal material, the chemical conversion solution containing a nitrate compound as the electrolyte, the concentration of a phosphorus compound in the chemical conversion solution being less than 0.01% by mass, and the temperature of the chemical conversion solution in the chemical conversion step being 40°C or higher.

[0012] The third aspect of the present invention relates to an electrode for an electrolytic capacitor, which includes a metal material containing a valve metal and an oxide film formed on the surface of the above metal material, and the phosphorus concentration of the above oxide film measured by energy dispersive X-ray spectroscopy is below the detection limit.

[0013] The fourth aspect of the present invention relates to an electrode for an electrolytic capacitor, which includes a metal material containing a valve metal and an oxide film formed on the surface of the above metal material, and the fragment peak intensity of phosphate ions obtained by time-of-flight secondary ion mass spectrometry of the above oxide film is below the detection limit.

[0014] The fifth aspect of the present invention relates to an electrode for an electrolytic capacitor, which includes a metal material containing a valve metal and an oxide film formed on the surface of the above metal material, the above oxide film contains tantalum oxide, and in the spectrum obtained by electron energy loss spectroscopy of the above oxide film, the average intensity I of the first peak observed between 530 eV and 550 eV 1A and the average intensity I of the second peak observed between 560 eV and 570 eV 2A The difference is 10% or less of the average intensity I of the above first peak. 1

[0015] The sixth aspect of the present invention relates to an electrode for an electrolytic capacitor, which includes a metal material containing a valve metal and an oxide film formed on the surface of the above metal material, the above oxide film contains tantalum oxide, and in the spectrum obtained by electron energy loss spectroscopy of the above oxide film, the intensity I of the first peak observed between 530 eV and 550 eV 1 becomes smaller as it approaches the surface of the above metal material.

[0016] The seventh aspect of the present invention relates to an electrode for an electrolytic capacitor, which includes a metal material containing a valve metal and an oxide film formed on the surface of the above metal material, the above oxide film contains tantalum oxide, and in the spectrum obtained by electron energy loss spectroscopy of the above oxide film, the fourth peak adjacent to the high energy side of the third peak attributed to the Ta-N1 end is observed at 570 eV or higher.

[0017] Advantages of the Invention

[0018] According to the present invention, an electrolytic capacitor with suppressed leakage current can be obtained.

[0019] The novel features of the present invention are described in the appended claims, but the present invention relates to both the constitution and the content, and together with other objects and features of the present invention, can be better understood from the following detailed description with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a cross-sectional view schematically showing a capacitor element according to an embodiment of the present invention.

[0021] Figure 2 It is a cross-sectional view schematically showing an electrolytic capacitor according to an embodiment of the present invention. Detailed Embodiments

[0022] In the case of using an aqueous phosphoric acid solution, trace amounts of phosphorus atoms are mixed into the formed oxide film. Due to the presence of phosphorus atoms, conduction paths are formed in the oxide film as an insulator. In addition, by generating impurity levels in the bandgap, electrons are more easily emitted into the oxide film. It is considered that this causes leakage current in the electrolytic capacitor.

[0023] It has been found that: in the case of using a nitric acid compound as the chemical conversion solution, by mixing nitrogen instead of phosphorus into the formed oxide film, the properties of the oxide film change. In particular, by controlling the concentration of the nitric acid compound or the temperature of the chemical conversion solution, leakage current can be further suppressed.

[0024] That is, the method for manufacturing an electrode for an electrolytic capacitor according to this solution includes the following chemical conversion step: passing an electric current through a metal material containing a valve-acting metal in a chemical conversion solution containing an electrolyte, and forming an oxide film on the surface of the metal material, and the chemical conversion solution contains a nitric acid compound as the electrolyte. In the first solution, the nitric acid compound is contained in the chemical conversion solution at a concentration of 0.03% by mass or more. In the second solution, the chemical conversion step is carried out in a chemical conversion solution at a temperature of 45°C or more.

[0025] The oxide film formed using the nitric acid compound has characteristics different from those of the oxide film formed by other chemical conversion solutions. By controlling the concentration of the nitric acid compound or the temperature of the chemical conversion solution as described above, this characteristic is more significantly manifested.

[0026] That is, the electrode for an electrolytic capacitor according to this solution includes a metal material containing a valve-acting metal and an oxide film formed on the surface of the metal material. The oxide film is an oxide of a metal containing a valve-acting metal, such as tantalum pentoxide.

[0027] [Method for Manufacturing Electrode for Electrolytic Capacitor]

[0028] A-1. First Solution

[0029] In the chemical conversion step of this solution, the chemical conversion solution containing the nitric acid compound contains the nitric acid compound as the electrolyte at a concentration of 0.03% by mass or more. Thereby, it is possible to suppress the mixing of phosphorus while forming the oxide film.

[0030] From the aspect of being easy to suppress the corrosion of production equipment and thus easy to control the thickness of the oxide film, the concentration of the nitric acid compound is preferably 15% by mass or less. The concentration of the nitric acid compound is preferably 0.04% by mass or more, and preferably 0.08% by mass or more. The concentration of the nitric acid compound is preferably 10% by mass or less, and preferably 5% by mass or less.

[0031] The chemical conversion liquid preferably contains an electrolyte other than the nitric acid compound. However, its concentration is preferably low. In particular, the concentration of the compound containing phosphorus is preferably low. The concentration of other electrolytes is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less. As other electrolytes, conventionally known electrolytes used in chemical conversion treatment can be cited. Other electrolytes are, for example, inorganic acids such as phosphoric acid and their salts, organic acids such as adipic acid and their salts, and basic substances such as ammonia.

[0032] When comparing at the same concentration and temperature, the conductivity of the aqueous solution containing the nitric acid compound is greater than that of the aqueous solution containing other electrolytes. Therefore, when using the nitric acid compound, the chemical conversion treatment is carried out efficiently.

[0033] In this solution, the temperature of the chemical conversion liquid during the treatment is not particularly limited. From the viewpoint of productivity, the temperature of the chemical conversion liquid is preferably 25 °C or higher, preferably 40 °C or higher, and preferably 45 °C or higher. From the aspect of suppressing liquid evaporation and being easy to suppress the corrosion of production equipment, the temperature of the chemical conversion liquid is preferably 75 °C or lower. When the concentration of the nitric acid compound is sufficiently small, for example, when the concentration of the nitric acid compound is 1% by mass or less, the temperature of the chemical conversion liquid is preferably 70 °C or lower. When the concentration of the nitric acid compound exceeds 1% by mass, the temperature of the chemical conversion liquid during the treatment is preferably 43 °C or higher, and preferably 45 °C or higher. The temperature of the chemical conversion liquid during the treatment is preferably 70 °C or lower, and preferably 68 °C or lower.

[0034] A-2. The second solution

[0035] In the chemical conversion process of this solution, the temperature during the treatment of the chemical conversion liquid containing the nitric acid compound is 40 °C or higher. Thereby, the mixing of phosphorus can be suppressed, and at the same time, an oxide film is formed. From the aspect of suppressing liquid evaporation, being easy to suppress the corrosion of production equipment, and thus easy to control the thickness of the oxide film, the temperature during the treatment of the chemical conversion liquid is preferably 75 °C or lower. When the concentration of the nitric acid compound is 1% by mass or less, the temperature of the chemical conversion liquid is preferably 60 °C or higher. When the concentration of the nitric acid compound exceeds 1% by mass, the temperature during the treatment of the chemical conversion liquid is preferably 43 °C or higher, and preferably 45 °C or higher. The temperature during the treatment of the chemical conversion liquid is preferably 70 °C or lower, and preferably 68 °C or lower.

[0036] In this solution, the concentration of the nitric acid compound is not particularly limited. From the viewpoint of productivity, the concentration of the nitric acid compound is preferably 0.03% by mass or more, and preferably 0.05% by mass or more. From the aspect of easily suppressing the corrosion of production equipment, the concentration of the nitric acid compound is preferably 15% by mass or less, and preferably 10% by mass or less.

[0037] In this solution, it is preferable that the chemical conversion liquid also contains electrolytes other than the nitric acid compound. However, its concentration is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less.

[0038] (Nitric acid compound)

[0039] The nitric acid compound is not particularly limited. Examples of the nitric acid compound include nitric acid, nitrous acid, nitrates, nitrites, nitrate esters, and nitrite esters. Examples of the salts of nitrates and nitrites include strontium, magnesium, calcium, barium, aluminum, zirconium, sodium, and lithium. Examples of the functional groups of nitrate esters and nitrite esters include methyl, ethyl, and butyl. Among them, nitric acid is preferred from the aspects of easy availability and low cost.

[0040] (Metal material)

[0041] The metal material includes a porous sintered body or foil (metal foil) containing valve-acting metal. In the case of using a metal foil, its main surface can also be roughened by electrolytic etching or the like. Thereby, the electrostatic capacitance of the electrolytic capacitor becomes larger. In the case of using a porous sintered body, electrode wires are erected from one side of the porous sintered body. The electrode wires are used for connection with lead terminals.

[0042] Examples of the valve-acting metal include titanium, tantalum, aluminum, and niobium. The metal material may also contain one or two or more of the above valve-acting metals. The metal material may also contain the valve-acting metal in the form of an alloy containing the valve-acting metal or a compound containing the valve-acting metal. From the aspect of chemical stability, the metal material is particularly preferably a porous sintered body containing tantalum.

[0043] The thickness of the metal material of the metal foil is not particularly limited, for example, it is 15 μm to 300 μm. The thickness of the metal material of the porous sintered body is not particularly limited, for example, it is 15 μm to 5 mm.

[0044] (Other chemical conversion conditions)

[0045] The chemical conversion voltage is the maximum value of the voltage applied between the metal material and the counter electrode. The chemical conversion voltage affects the thickness of the oxide film and, in turn, the withstand voltage of the electrolytic capacitor. Therefore, the chemical conversion voltage can be appropriately set according to the rated voltage of the electrolytic capacitor and is not particularly limited. For example, the chemical conversion voltage is preferably 5V or more. For example, the chemical conversion voltage is preferably 100V or less.

[0046] The time for maintaining the above chemical conversion voltage (chemical conversion time) is not particularly limited and can be appropriately set considering the thickness of the oxide film, productivity, etc. For example, the chemical conversion time is preferably 1 hour or more. For example, the chemical conversion time is preferably 20 hours or less.

[0047] The current density flowing in the metal material is not particularly limited and can be appropriately set considering the chemical conversion time, etc. The maximum current density is, for example, 0.001 mA / cm 2 or more. The maximum current density is, for example, 100 mA / cm 2 or less.

[0048] [Electrode for electrolytic capacitor]

[0049] The electrode of this embodiment has an oxide film on its surface. The oxide film is formed by oxidizing the surface of the metal material. Therefore, the oxide film contains the oxide of the valve-acting metal contained in the metal material.

[0050] The thickness of the oxide film is not particularly limited and is appropriately set considering the rated voltage of the electrolytic capacitor, etc. The thickness of the oxide film is, for example, 10 nm to 300 nm.

[0051] B-1. First embodiment

[0052] In the oxide film of this embodiment, the phosphorus concentration measured by energy dispersive X-ray spectroscopy (EDX) is below the detection limit. Such an oxide film can be formed on a metal material that has been chemically converted (hereinafter referred to as nitric acid chemical conversion) using a chemical conversion solution containing a nitric acid compound.

[0053] EDX can be used in combination with a scanning electron microscope (SEM), a transmission electron microscope (TEM), or a scanning transmission electron microscope (STEM).

[0054] Phosphorus is detected in the oxide film formed from a phosphoric acid aqueous solution generally used for chemical conversion treatment (hereinafter sometimes referred to as a phosphoric acid chemical conversion film). That is, in the phosphoric acid chemical conversion film, atoms such as those forming a conduction path are mixed in relatively large amounts. On the other hand, nitrogen atoms are basically not detected (below the detection limit). Phosphorus is detected more in the vicinity of the surface of other oxide films.

[0055] In the oxide film of this solution, phosphorus atoms are hardly confirmed, and very few nitrogen atoms are mixed in. Therefore, it has properties different from those of the phosphoric acid chemical conversion film, and it is easy to suppress the leakage current of the electrolytic capacitor.

[0056] B-2. The Second Solution

[0057] In the oxide film of this solution, the fragment peak intensity of phosphate ions obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) is below the detection limit. That is, it means that the incorporation of phosphorus into the oxide film is small. On the other hand, in the oxide film of this solution, a peak presumed to be a fragment of nitrogen ions is confirmed. From this, the possibility of nitrogen being incorporated into the oxide film is speculated. By mixing a small amount of nitrogen instead of phosphorus, the properties of the oxide film change, and the leakage current of the electrolytic capacitor can be suppressed. Such an oxide film can be formed on a metal material that has been chemically converted with nitric acid.

[0058] If TOF-SIMS analysis is performed on other oxide films in the same way as the EDX analysis results, the fragment peak of phosphate ions is detected. The fragment peak of the ions is obtained by evaluating the surface of the oxide film. It is also possible to etch the oxide film and evaluate its interior. The evaluation results of the interior also have the same tendency as the evaluation results of the surface.

[0059] B-3. The Third Solution

[0060] The oxide film of this solution contains tantalum oxide.

[0061] In the oxide film of this solution, the average intensity I of the first peak observed between 530 eV and 550 eV in the spectrum obtained by electron energy loss spectroscopy (EELS) 1A and the average intensity I of the second peak observed between 560 eV and 570 eV 2A The difference (=|I 1A -I 2A |) is 10% or less of the average intensity I of the first peak. That is, it satisfies 100×|I 1A -I 1A | / I 2A ≤10 (%). Such an oxide film can be formed on a metal material that has been chemically converted with nitric acid. 1A ≤10 (%). Such an oxide film can be formed on a metal material that has been chemically converted with nitric acid.

[0062] The first peak belongs to the O-K edge (O-K side. The excitation process caused by the K-shell electrons of oxygen). The second peak belongs to the Ta-N1 edge (Ta-N1 side. The excitation process caused by the N1-shell electrons of tantalum). The relationship between the first peak and the second peak indicates the oxidation state of tantalum atoms.

[0063] When using a chemical conversion solution containing an electrolyte other than the nitric acid compounds used in the past, such as inorganic acids and their salts like phosphoric acid, organic acids and their salts like adipic acid, and basic substances like ammonia, the relationship between the first peak and the second peak in the formed oxide film (hereinafter referred to as other oxide films) does not satisfy 100×|I 1A -I 2A | / I 1A ≤10 (%). That is, it can be said that the oxidation states of tantalum atoms are different in the oxide film formed by nitric acid chemical conversion and other oxide films. Although the reason is not clear at present, it is considered that this difference affects the electronic structure of the oxide film and is effective in suppressing the leakage current of the capacitor.

[0064] It can also be I 1A >I 2A , it can also be I 1A <I 2A , it can also be I 1A =I 2A .

[0065] The average intensity I 1A of the first peak is calculated by operating as follows. At any point on the surface of the oxide film, at a total of 6 points including 4 points that divide the thickness of the oxide film on the straight line drawn from this point toward the metal material into 5 equal parts and the intersection of this straight line and the surface of the metal material, measure the intensity of the peak observed between 530 eV and 550 eV. Furthermore, for any other 4 points, also measure the intensity of the peak observed between 530 eV and 550 eV at a total of 6 points with different depths in the same way. The average intensity I 1A of the first peak is the average value of these 30 points.

[0066] The average intensity I 2A of the second peak is the average value of the intensities of the peaks observed between 560 eV and 570 eV at the same 30 points where the intensity of the first peak is measured. When there are multiple peaks observed between 530 eV and 550 eV, only the peak on the lowest energy side needs to be used. When there are multiple peaks observed between 560 eV and 570 eV, only the peak on the lowest energy side needs to be used.

[0067] EELS is used in combination with a scanning electron microscope (SEM), a transmission electron microscope (TEM), or a scanning transmission electron microscope (STEM).

[0068] B-4. The Fourth Solution

[0069] The oxide film of this solution contains tantalum oxide.

[0070] In the oxide film of this solution, the intensity I of the first peak observed in the spectrum obtained by EELS between 530 eV and 550 eV 1 becomes smaller as it approaches the surface of the metal material. That is, it can be said that the electronic structure in the oxide film changes in the same tendency in the thickness direction. Such an oxide film can be formed on a metal material subjected to nitric acid chemical conversion.

[0071] In other oxide films, it cannot be said that the intensity I of the first peak 1 becomes smaller as it approaches the surface of the metal material. For example, in other oxide films, the intensity of the first peak on the surface of the metal material is sometimes greater than the intensity of the first peak inside. That is, in other oxide films, it can be said that the bonding state of oxygen changes randomly in the thickness direction. Although the reason is not clear at present, it is considered that this difference affects the electronic structure of the oxide film and is effective in suppressing the leakage current of the capacitor.

[0072] The intensity I of the first peak 1 For example, it is measured at a total of six points including any point (depth zero) on the surface of the oxide film, four points (depths 1 to 4) that divide the thickness of the oxide film on the straight line drawn from this point toward the metal material into five equal parts, and the intersection point (depth 5) of this straight line and the surface of the metal material. Furthermore, for any other four places, the intensity of the first peak is also measured at a total of six points with different depths in the same way. The intensities of the five points measured at the same depth in different parts are averaged and set as the intensity of the first peak at that depth. When there are multiple peaks observed between 530 eV and 550 eV, only the peak on the lowest energy side needs to be used.

[0073] The intensity I of the first peak 1 As long as the overall tendency is that it becomes smaller as it approaches the surface of the metal material. For example, at two adjacent points among the six points from the above-mentioned depth zero to depth 5, the intensity of the shallower point is preferably greater than or the same as the intensity of the deeper point. However, the intensity I at depth zero 10 is greater than the intensity I at depth 5 15 .

[0074] From the viewpoint of the qualitative uniformity of the oxide film, the intensity I at depth zero 10 and the intensity I at depth 5 15 The difference is preferably not overly large. The difference between the intensity I 10 and the intensity I 15 (=(I 10 - I 15 )) is preferably 30% or less of the intensity I 10 . That is, it is preferably satisfied that 100×(I 10 - I 15 ) / I10 ≤ 30 (%)。More preferably, it satisfies 100×(I 10 - I 15 ) / I 10 ≤ 20 (%).

[0075] From the same perspective, the intensity I at depth 1 11 is preferably less than the intensity I at depth zero 10 , and the difference between the intensity I 10 and the intensity I 11 is preferably sufficiently large. The difference between the intensity I 10 and the intensity I 11 (= I 10 - I 11 ) is preferably 3% to 20% of the intensity I 10 . That is, it preferably satisfies 3 (%) ≤ 100×(I 10 - I 11 ) / I 10 ≤ 20 (%). More preferably, it satisfies 5 (%) ≤ 100×(I 10 - I 11 ) / I 10 ≤ 20 (%).

[0076] B-5. The fifth embodiment

[0077] The oxide film of this embodiment contains tantalum oxide.

[0078] In the oxide film of this embodiment, in the spectrum obtained by EELS, the fourth peak adjacent to the high-energy side of the third peak attributed to the Ta-N1 end is observed at 570 eV or higher. Such an oxide film can be formed on a metal material chemically converted with nitric acid.

[0079] The third peak is attributed to the Ta-N1 end (excitation process caused by the N1 shell electrons of tantalum). The position of the fourth peak indicates the state of the distance between oxygen atoms. The displacement of the fourth peak to the high-energy side means a decrease in the distance between oxygen atoms. That is, it is speculated that the denseness of the oxide film is improved. The third peak is consistent with the second peak in the third embodiment.

[0080] The fourth peak in other oxide films is observed on the low-energy side of 570 eV. That is, it can be said that the oxidation state of tantalum atoms is different between the oxide film formed by chemical conversion with nitric acid and other oxide films. Although the reason is not clear at present, it is considered that this difference affects the electronic structure of the oxide film and is effective in suppressing the leakage current of the capacitor.

[0081] The third peak and the fourth peak are specified as follows. At a point within 10 nm from the surface of the oxide film toward the metal material (for example, a depth of 5 nm), a spectrum is obtained by EELS. Then, the third peak attributed to the Ta-N1 end is specified. The third peak generally appears at 563 eV to 567 eV. Further, the fourth peak adjacent to the third peak is specified. The position of the fourth peak is preferably confirmed by further evaluating any other nine points within 10 nm of the depth of the oxide film by EELS. When the fourth peak is observed at 570 eV or higher at eight out of any ten points, it is considered that the oxide film satisfies the fifth aspect.

[0082] (Other)

[0083] In the third to fifth aspects, it is preferable to satisfy the following.

[0084] a) In the spectrum obtained by EELS of the oxide film of this aspect, the average intensity I of the fifth peak observed between 1770 eV and 1790 eV 5A is lower than the average intensity I of the fifth peak in other oxide films 5R .

[0085] In particular, the difference between the average intensity I 5A and the average intensity I 5R (=I 5R - I 5A ) is preferably 10% or more of the average intensity I 5R . That is, it is preferable to satisfy (I 5R - I 5A ) / I 5R ≥0.1.

[0086] The fifth peak is attributed to the Ta-M5 end (excitation process caused by M5 shell electrons of tantalum).

[0087] b) In the spectrum obtained by EELS of the oxide film of this aspect, the average intensity I of the sixth peak observed between 1830 eV and 1850 eV 6A is lower than the average intensity I of the sixth peak in other oxide films 6R .

[0088] In particular, the difference between the average intensity I 6A and the average intensity I 6R (=I 6R - I 6A ) is preferably 5% or more of the average intensity I 6R . That is, it is preferable to satisfy (I 6R - I 6A ) / I 6R ≥0.05.

[0089] The sixth peak belongs to the Ta-M4 end (excitation process caused by the M4 shell electrons of tantalum).

[0090] Average intensity I 5A and average intensity I 6A can be calculated in the same way as the average intensity I 1A Similarly, the average intensity I of the oxide film of the comparison object 5R and average intensity I 6R can be calculated in the same way as the average intensity I 1A Similarly.

[0091] In the first to fourth embodiments, it is preferable to satisfy the following.

[0092] c) The value of the current (leakage current) flowing through the electrode with the oxide film of this embodiment is at least 10% less than the leakage current value of the electrode with other oxide films. Thus, the leakage current of the electrolytic capacitor can be further suppressed.

[0093] The leakage current value of the electrode of this embodiment is preferably at least 15% less, and preferably at least 30% less, than the leakage current value of the electrode with other oxide films.

[0094] The leakage current of the electrode is the current value when the electrode and the counter electrode are immersed in an aqueous electrolyte solution and a voltage of 70% of the chemical conversion voltage is applied.

[0095] For the oxide film of the comparison object, for example, a chemical conversion solution containing 0.1% by mass of phosphoric acid is used to form it. The chemical conversion conditions other than the composition of the chemical conversion solution are the same as those of the oxide film of this embodiment. The chemical conversion conditions are, for example, a chemical conversion voltage of 15 V, a temperature of 60 °C, and a treatment time of 10 hours.

[0096] [Electrolytic capacitor]

[0097] The electrode obtained by chemically converting the metal foil as described above is used for the capacitor element. The capacitor element includes the above-mentioned electrode, i.e., the first electrode and the second electrode. The second electrode includes, for example, a solid electrolyte layer and a cathode lead-out layer. The leakage current of the electrolytic capacitor of this embodiment is at least 30% less than the leakage current of the electrolytic capacitor with an electrode having other oxide films.

[0098] The electrolytic capacitor includes, for example, one or more of the above-mentioned capacitor elements, an outer package for sealing the capacitor element, and a first lead terminal and a second lead terminal. At least a part of each lead terminal protrudes from the outer package. Such a capacitor element is, for example, in the shape of a sheet or a flat plate.

[0099] (First electrode)

[0100] The first electrode is a metal material having an oxide film formed as described above. The first electrode is, for example, an anode.

[0101] (The second electrode)

[0102] The second electrode includes a solid electrolyte layer and an electrode lead-out layer. The second electrode is, for example, a cathode.

[0103] (Solid electrolyte layer)

[0104] The solid electrolyte layer is formed so as to cover at least a part of the oxide film. The solid electrolyte layer may also be formed so as to cover the entire surface of the oxide film. The thickness of the solid electrolyte layer is not particularly limited.

[0105] The solid electrolyte layer includes one or two or more solid electrolyte layers. The solid electrolyte layer is formed of, for example, a manganese compound or a conductive polymer. As the conductive polymer, polypyrrole, polyaniline, polythiophene, polyacetylene, their derivatives, etc. can be used. The solid electrolyte layer containing a conductive polymer can be formed, for example, by chemically polymerizing and / or electrolytically polymerizing a raw material monomer on the oxide film. Alternatively, it can be formed by coating a solution in which a conductive polymer is dissolved or a dispersion in which a conductive polymer is dispersed on the oxide film.

[0106] (Cathode lead-out layer)

[0107] The cathode lead-out layer only needs to be formed so as to cover at least a part of the solid electrolyte layer, and may also be formed so as to cover the entire surface of the solid electrolyte layer.

[0108] The cathode lead-out layer has, for example, a carbon layer and a metal paste layer formed on the surface of the carbon layer. The carbon layer is composed of a composition containing a conductive carbon material such as graphite. The metal paste layer is composed of a composition containing silver particles and a resin, for example. It should be noted that the configuration of the cathode lead-out layer is not limited to this, as long as it has a current collecting function.

[0109] (Lead terminal)

[0110] The materials of the first lead terminal and the second lead terminal are not particularly limited as long as they are electrochemically and chemically stable and have conductivity, and can be metals or non-metals. Their shapes are also not particularly limited.

[0111] The first lead terminal is connected to the first electrode, and the second lead terminal is connected to the second electrode. The electrical connection between the first electrode and the first lead terminal is performed, for example, by welding them. The electrical connection between the second electrode and the second lead terminal is performed, for example, by bonding the second electrode and the second lead terminal via a conductive adhesive layer.

[0112] (Outer package)

[0113] The outer package covers a part of the capacitor element and the lead terminals. Thereby, the first lead terminal and the second lead terminal are electrically insulated, and the capacitor element is protected. The outer package is made of an insulating material (outer package material). The outer package material includes, for example, a cured product of a thermosetting resin and an engineering plastic.

[0114] Figure 1 It is a cross-sectional view schematically showing the capacitor element of this solution.

[0115] The capacitor element 10 includes a first electrode 11 and a second electrode 13. The first electrode 11 includes a porous sintered body 111, an electrode wire 112 standing upright from the porous sintered body 111, and an oxide film 113 covering at least a part of the porous sintered body 111. The second electrode 13 includes a solid electrolyte layer 131, a carbon layer 132, and a metal paste layer 133. The carbon layer 132 and the metal paste layer 133 function as cathode lead-out layers. Such a capacitor element 10 is generally in a cubic shape.

[0116] Figure 2 It is a cross-sectional view schematically showing the structure of the electrolytic capacitor of this solution.

[0117] The electrolytic capacitor 100 includes a capacitor element, an outer package 20 that seals the capacitor element, and a first lead terminal 30 and a second lead terminal 40 at least a part of which are respectively exposed to the outside of the outer package 20.

[0118] The electrode wire 112 and the first lead terminal 30 are electrically connected, for example, by welding. The metal paste layer 133 and the second lead terminal 40 are electrically connected, for example, through an adhesive layer 50 formed by a conductive adhesive (a mixture of a thermosetting resin and carbon particles or metal particles, etc.).

[0119] In this solution, an electrolytic capacitor using a solid electrolyte as the electrolyte and having the capacitor element sealed by an outer package is exemplified, but it is not limited thereto. The electrodes of this solution can be applied, for example, to an electrolytic capacitor having a capacitor element formed by winding a first electrode and a second electrode with a separator and an electrolytic solution. In this case, the electrodes of this solution are used for at least one of the first electrode and the second electrode.

[0120] [Examples]

[0121] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0122] <<Example 1>>

[0123] Twenty Figure 2 of the electrolytic capacitors shown in are manufactured and their characteristics are evaluated according to the following procedure.

[0124] (i) Fabrication of capacitor element

[0125] (i-i) Preparation of the first electrode

[0126] As the valve-acting metal, tantalum metal particles were used. The tantalum metal particles were formed into a rectangular parallelepiped in such a way that one end of the electrode wire made of tantalum was embedded in the tantalum metal particles, and then the formed body was sintered in vacuum. Thus, a porous sintered body containing tantalum and a precursor of the first electrode, which has one end embedded in the porous sintered body and the other end standing upright from one surface of the porous sintered body, were obtained.

[0127] (i-ii) Formation of the oxide film

[0128] An aqueous solution of 0.06 mass% nitric acid was prepared as the chemical conversion solution. The chemical conversion tank was filled with this chemical conversion solution, and the porous sintered body and a part of the electrode wire were immersed. The temperature of the chemical conversion solution was 60 °C. The other end of the electrode wire was connected to the counter electrode, and anodic oxidation was carried out at a chemical conversion voltage of 15 V for 10 hours. By such operation, a uniform oxide film (thickness of about 30 nm) of tantalum oxide (Ta 2 O 5 ) was formed on the surface of the porous sintered body and a part of the surface of the electrode wire, and 20 first electrodes X1 were obtained.

[0129] (i-iii) Formation of the solid electrolyte layer

[0130] After infiltrating a dispersion containing polypyrrole into the porous sintered body on which the oxide film was formed for 5 minutes, it was dried at 150 °C for 30 minutes, and a solid electrolyte layer was formed on the oxide film.

[0131] (i-iv) Formation of the carbon layer

[0132] After coating a dispersion (carbon paste) obtained by dispersing carbon particles in water on the solid electrolyte layer, a carbon layer was formed on the surface of the solid electrolyte layer by heating at 200 °C.

[0133] (i-v) Formation of the metal paste layer

[0134] A metal paste containing silver particles, a binder resin, and a solvent was coated on the surface of the carbon layer. Then, it was heated at 200 °C to form a metal paste layer, and a capacitor element was obtained.

[0135] (ii) Fabrication of the electrolytic capacitor

[0136] A conductive adhesive was coated on the metal paste layer, and the second lead terminal was joined to the metal paste layer. The electrode wire was joined to the first lead terminal by resistance welding. Subsequently, the capacitor element and the material of the outer package body (uncured thermosetting resin and filler) to which each lead terminal was joined were accommodated in a mold, and the capacitor element was sealed by transfer molding to fabricate an electrolytic capacitor.

[0137] 《Example 2》

[0138] The concentration of nitric acid in the chemical conversion solution was set to 10% by mass, and the temperature of the chemical conversion solution was set to 45°C. Except for this, 20 first electrodes X2 were fabricated in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0139] 《Comparative Example 1》

[0140] A chemical conversion solution containing phosphoric acid (concentration: 0.1% by mass) was used instead of nitric acid. Except for this, 20 first electrodes Y1 were fabricated in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0141] 《Comparative Example 2》

[0142] A chemical conversion solution containing diammonium adipate (concentration: 0.2% by mass) was used instead of nitric acid. Except for this, 20 first electrodes Y2 were fabricated in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0143] 《Comparative Example 3》

[0144] A chemical conversion solution containing ammonia (concentration: 2.5% by mass) was used instead of nitric acid. Except for this, 20 first electrodes Y3 were fabricated in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0145] [Evaluation]

[0146] (1) Analysis of the oxide film

[0147] After the formation of the oxide film (i - ii), the first electrodes X1, Y1 - Y3 were analyzed.

[0148] (1 - 1) EELS analysis

[0149] Spectral analysis was performed using a TEM - EELS apparatus. The results are shown in Table 1.

[0150] Table 1

[0151] First electrode X1 Y1 Y2 Y3 <![CDATA[Average intensity I 1A > 6.4E+06 6.7E+06 6.3E+06 5.3E+06 <![CDATA[Average intensity I 2A > 6.3E+06 5.5E+06 5-4E+06 5-4E+06 <![CDATA[Average intensity I 3A > 6-4E+06 5.6E+06 5-5E+06 5-5E+06 <![CDATA[Average intensity I 4A > 6.5E+06 5-6E+06 5-6E+06 5-5E+06 <![CDATA[Intensity I 10 > 7.2E+06 7.1E+06 6.5E+06 4.6E+06 <![CDATA[Intensity I 11 > 6.4E+06 6.4E+06 6.1E+06 5.9E+06 <![CDATA[Intensity I 12 > 6.2E+06 6.7E+06 6.1E+06 6.1E+06 <![CDATA[Intensity I 13 > 6.2E+06 7.0E+06 6.6E+06 6.1E+06 <![CDATA[Intensity I 14 > 6.1E+06 6.5E+06 6.1E+06 3.6E+06 <![CDATA[Intensity I 15 > 6.1E+06 6.5E+06 6.4E+06 - <![CDATA[(I 1A -I 2A ) / I 1A > 0.010 0.179 0.143 -0.027 <![CDATA[(I 10 -I 15 ) / I 10 > 0.153 0.085 0.015 1.000 <![CDATA[(I 10 -I 11 ) / I 10 > 0.111 0.099 0.062 -0.283 Position of the third peak 566 eV 564 eV 566 eV 566 eV Position of the fourth peak 572 eV 569 eV 571 eV 572 eV <![CDATA[(I 5R -I 5A ) / I 5R > 0.174 - 0.022 0.065 <![CDATA[(I 6R -I 6A ) / I 6R > 0.059 - -0.059 0.000

[0152] (1 - 2) EDX analysis

[0153] Using a TEM-EDX apparatus, elemental analysis of the surface of the oxide film in the first electrodes X1 and Y1 was performed. The results are shown in Table 2.

[0154] Table 2

[0155] First electrode X1 Y1 P concentration (atomic %) (Below the detection limit) 0.9-1.1 N concentration (atomic %) 1.5 (Below the detection limit)

[0156] (1-3) TOF-SIMS analysis

[0157] Analysis of the surface and the inside (depth: 1 nm to 10 nm) of the oxide film was performed using a TOF-SIMS apparatus. The oxide film was etched using an Ar gas cluster ion beam.

[0158] Regarding the first electrodes X1 and X2, phosphate ions were not detected (below the detection limit) in either the surface or the inside of the oxide film. Regarding the first electrodes Y1 to Y3, phosphate ions were detected in either the surface or the inside of the oxide film.

[0159] (2) Leakage current

[0160] After the formation of the film (i-ii), the leakage current values of the first electrodes X1, Y2, and Y3 were measured.

[0161] The fabricated first electrode and the counter electrode (SUS316L) were immersed in 0.1 wt% phosphoric acid. A voltage of 70% of the chemical conversion voltage was applied between the electrodes, the current value flowing through the first electrode was measured, and its average value was obtained. Taking the average current value of the first electrode Y1 as 100%, the average current values (leakage current values) of the respective first electrodes were obtained. The results are shown in Table 3. In Table 3, for reference, the average current value of the first electrode X2 is also shown.

[0162] Table 3

[0163] First electrode X1 X2 Y1 Y2 Y3 Leakage current (%) 64 85 100 75 71

[0164] Industrial applicability

[0165] Since the electrode manufactured by the method of the present invention suppresses leakage current, it can be used in electrolytic capacitors for various applications.

[0166] The present invention has been described with respect to the presently preferred embodiments, but the disclosure is not to be construed in a limiting sense. Various modifications and changes will be apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, it should be construed that the appended claims cover all modifications and changes without departing from the true spirit and scope of the present invention.

[0167] Explanation of symbols

[0168] 100: Electrolytic capacitor

[0169] 10: Capacitor element

[0170] 11: First electrode

[0171] 111: Metal material (porous sintered body)

[0172] 112: Electrode wire

[0173] 113: Oxide film

[0174] 13: Second electrode

[0175] 131: Solid electrolyte layer

[0176] 132: Carbon layer

[0177] 133: Metal paste layer

[0178] 20: Outer package

[0179] 30: First lead terminal

[0180] 40: Second lead terminal

[0181] 50: Adhesive layer

Claims

1. An electrolytic capacitor, comprising: a metallic material containing a valve-acting metal; an oxide film formed on the surface of the metallic material; and a solid electrolyte layer covering at least a part of the oxide film; the oxide film contains tantalum oxide, In the spectrum of the oxide film obtained by electron energy loss spectroscopy, the average intensity I of the first peak observed between 530 eV and 550 eV 1A and the average intensity I of the second peak observed between 560 eV and 570 eV 2A The difference is 10% or less of the average intensity I of the first peak 1A ​ The average intensity I of the first peak 1A is greater than the average intensity I of the second peak 2A .

2. The electrolytic capacitor according to claim 1, wherein in the spectrum of the oxide film obtained by electron energy loss spectroscopy, the third peak attributed to the Ta-N1 end is observed at 566 eV or higher, and the fourth peak adjacent to the high energy side of the third peak is observed at 570 eV or higher.

3. The electrolytic capacitor according to claim 1 or 2, wherein In the spectrum obtained by electron energy loss spectroscopy of the oxide film, the intensity I of the first peak 1 is smaller closer to the surface of the metal material.

4. The electrolytic capacitor according to claim 3, wherein the phosphorus concentration of the oxide film measured by energy dispersive X-ray spectroscopy is below the detection limit.

5. The electrolytic capacitor according to claim 4, wherein the fragment peak intensity of phosphate ions of the oxide film obtained by time-of-flight secondary ion mass spectrometry is below the detection limit.

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