Manufacturing method for electrodes for electrolytic capacitors

Abstract

Provided is a method for manufacturing an electrode for an electrolytic capacitor, the method comprising a chemical conversion step for applying an electric current to a metal material containing a valve-acting metal in an electrolyte-containing chemical conversion solution to form an oxide film on the surface of the metal material, wherein the chemical conversion solution contains a nitric acid compound as the electrolyte at a concentration of at least 0.03 mass%, and the concentration of a phosphorous compound in the chemical conversion solution is less than 0.01 mass%.

Description

Technical Field

[0001] The present invention relates to an electrode for an electrolytic capacitor, a method for manufacturing the same, and an electrolytic capacitor.

Background Art

[0002] As the anode body of a capacitor element, a metal foil containing a valve action metal or a porous sintered body is used. An oxide film is formed on the surface of the metal foil or the porous sintered body by a forming treatment. Usually, an aqueous phosphoric acid solution is used for the forming treatment (Patent Document 1, etc.).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An electrolytic capacitor having an oxide film formed using an aqueous phosphoric acid solution may have a large leakage current.

Means for Solving the Problems

[0005] A first aspect of the present invention includes a forming step of flowing a current through a metal material containing a valve action metal in a forming solution containing an electrolyte to form an oxide film on the surface of the metal material, the forming solution 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 forming solution is less than 0.01 mass%, and relates to a method for manufacturing an electrode for an electrolytic capacitor.

[0006] A second aspect of the present invention relates to a method for manufacturing electrodes for electrolytic capacitors, comprising a chemical conversion step of 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, wherein the chemical conversion solution contains a nitrate compound as the electrolyte, the concentration of the phosphorus compound in the chemical conversion solution is less than 0.01% by mass, and the temperature of the chemical conversion solution in the chemical conversion step is 40°C or higher.

[0007] A third aspect of the present invention relates to an electrode for an electrolytic capacitor, comprising a metallic material containing a valve-acting metal and an oxide film formed on the surface of the metallic material, wherein the phosphorus concentration of the oxide film, as measured by energy-dispersive X-ray spectroscopy, is below the detection limit.

[0008] A fourth aspect of the present invention relates to an electrode for an electrolytic capacitor, comprising a metallic material containing a valve-acting metal and an oxide film formed on the surface of the metallic material, wherein the fragment peak intensity of phosphate ions obtained by time-of-flight secondary ion mass spectrometry of the oxide film is below the detection limit.

[0009] A fifth aspect of the present invention comprises a metallic material containing a valve metal and an oxide film formed on the surface of the metallic material, wherein the oxide film contains tantalum oxide, and in the spectrum obtained by electron energy loss spectroscopy of the oxide film, the average intensity of a first peak observed between 530 eV and 550 eV is 1A And the average intensity of the second peak observed between 560eV and 570eV 2A The difference is less than or equal to 10% of the average intensity I1 of the first peak, relating to electrodes for electrolytic capacitors.

[0010] A sixth aspect of the present invention relates to an electrode for an electrolytic capacitor, comprising a metallic material containing a valve-acting metal and an oxide film formed on the surface of the metallic material, wherein the oxide film contains tantalum oxide, and in the spectrum obtained by electron energy loss spectroscopy of the oxide film, the intensity I1 of a first peak observed between 530 eV and 550 eV is smaller closer to the surface of the metallic material.

[0011] A seventh aspect of the present invention relates to an electrode for an electrolytic capacitor, comprising a metallic material containing a valve-acting metal and an oxide film formed on the surface of the metallic material, wherein the oxide film contains tantalum oxide, and in the spectrum obtained by electron energy loss spectroscopy of the oxide film, a 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. [Effects of the Invention]

[0012] According to the present invention, an electrolytic capacitor with suppressed leakage current can be obtained. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic cross-sectional view showing a capacitor element according to one embodiment of the present invention. [Figure 2] This is a schematic cross-sectional view showing an electrolytic capacitor according to one embodiment of the present invention. [Modes for carrying out the invention]

[0014] When using an aqueous phosphoric acid solution, trace amounts of phosphorus atoms are mixed into the oxide film that forms. The presence of phosphorus atoms creates conductive paths within the insulating oxide film. Furthermore, impurity levels are generated in the band gap, making it easier for electrons to be released into the oxide film. This is thought to be the cause of leakage current in the electrolytic capacitor.

[0015] When nitrate compounds are used as the chemical conversion solution, it has been found that the properties of the oxide film change when nitrogen is mixed in with phosphorus. In particular, leakage current can be further suppressed by controlling the concentration of the nitrate compound or the temperature of the chemical conversion solution.

[0016] In other words, the method for manufacturing electrodes for electrolytic capacitors according to this embodiment comprises a chemical conversion step of 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, wherein the chemical conversion solution contains a nitrate compound as the electrolyte. In the first embodiment, the nitrate compound is contained in the chemical conversion solution at a concentration of 0.03% by mass or more. In the second embodiment, the chemical conversion step is carried out in a chemical conversion solution at a temperature of 45°C or higher.

[0017] The oxide film formed using nitrate compounds has different characteristics from oxide films formed using other chemical conversion solutions. These characteristics become more pronounced by controlling the concentration of the nitrate compound or the temperature of the chemical conversion solution as described above.

[0018] In other words, the electrode for the electrolytic capacitor according to this embodiment comprises a metallic material containing a valve-acting metal and an oxide film formed on the surface of the metallic material. The oxide film is an oxide of the metal containing the valve-acting metal, for example, tantalum pentoxide.

[0019] [Manufacturing method for electrodes for electrolytic capacitors] A-1. First aspect In the chemical conversion process according to this embodiment, the conversion solution containing the nitrate compound contains the nitrate compound as an electrolyte at a concentration of 0.03% by mass or more. This makes it possible to form an oxide film while suppressing the inclusion of phosphorus.

[0020] The concentration of the nitrate compound is preferably 15% by mass or less, as this makes it easier to suppress corrosion of production equipment and to control the thickness of the oxide film. The concentration of the nitrate compound may be 0.04% by mass or more, or 0.08% by mass or more. The concentration of the nitrate compound may be 10% by mass or less, or 5% by mass or less.

[0021] The chemical conversion solution may contain electrolytes other than nitrate compounds. However, their concentrations should be low. In particular, the concentration of phosphorus-containing compounds should be low. The concentration of other electrolytes is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less. Examples of other electrolytes include conventionally known electrolytes used in chemical conversion treatment. Other electrolytes include, 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.

[0022] When compared at the same concentration and temperature, the conductivity of aqueous solutions containing nitrate compounds is greater than that of aqueous solutions containing other electrolytes. Therefore, when using nitrate compounds, chemical conversion treatment can be carried out efficiently.

[0023] In this embodiment, the temperature of the chemical solution during processing is not particularly limited. From the viewpoint of productivity, the temperature of the chemical solution may be 25°C or higher, 40°C or higher, or 45°C or higher. The temperature of the chemical solution may be 75°C or lower, as this suppresses liquid evaporation and thus easily reduces corrosion of production equipment. When the concentration of nitrate compounds is sufficiently low, for example, when the concentration of nitrate compounds is 1% by mass or less, the temperature of the chemical solution may be 70°C or lower. When the concentration of nitrate compounds exceeds 1% by mass, the temperature of the chemical solution may be 55°C or lower.

[0024] A-2. Second aspect In the chemical conversion process according to this embodiment, the temperature during the treatment of the chemical conversion solution containing the nitrate compound is 40°C or higher. This allows for the formation of an oxide film while suppressing the inclusion of phosphorus. It is preferable that the temperature during the treatment of the chemical conversion solution be 75°C or lower, as this suppresses liquid evaporation, thus easily inhibiting corrosion of production equipment, and also makes it easier to control the thickness of the oxide film. When the concentration of the nitrate compound is 1% by mass or less, the temperature of the chemical conversion solution may be 60°C or higher. When the concentration of the nitrate compound exceeds 1% by mass, the temperature during the treatment of the chemical conversion solution may be 43°C or higher, or 45°C or higher. The temperature during the treatment of the chemical conversion solution may be 70°C or lower, or 68°C or lower.

[0025] In this embodiment, the concentration of the nitrate compound is not particularly limited. From the viewpoint of productivity, the concentration of the nitrate compound may be 0.03% by mass or more, or 0.05% by mass or more. In terms of easily suppressing corrosion of production equipment, the concentration of the nitrate compound may be 15% by mass or less, or 10% by mass or less.

[0026] In this embodiment as well, the chemical solution may contain electrolytes other than nitrate compounds. However, its concentration is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less.

[0027] (Nitrate compound) The nitrate compounds are not particularly limited. Examples of nitrate compounds include nitric acid, nitrite, nitrate salts, nitrite salts, nitrate esters, and nitrite esters. Examples of salts of nitrates and nitrites include strontium, magnesium, calcium, barium, aluminum, zirconium, sodium, and lithium. Examples of functional groups of nitrate esters and nitrite esters include methyl, ethyl, and butyl groups. Among these, nitric acid is preferred because it is readily available and inexpensive.

[0028] (metallic material) The metal material includes a porous sintered body or foil (metal foil) containing valve-acting metal. When using metal foil, its main surface may be roughened by electrolytic etching or the like. This increases the capacitance of the electrolytic capacitor. When using a porous sintered body, electrode wires are embedded from one side of the porous sintered body. The electrode wires are used to connect to the lead terminals.

[0029] Examples of valve-acting metals include titanium, tantalum, aluminum, and niobium. The metal material may contain one or more of the above valve-acting metals. The metal material may contain the valve-acting metal in the form of an alloy containing the valve-acting metal or a compound containing the valve-acting metal. In terms of chemical stability, the metal material is preferably a porous sintered body containing tantalum in particular.

[0030] The thickness of the metal material in the metal foil is not particularly limited, for example, 15 μm to 300 μm. The thickness of the metal material in the porous sintered body is not particularly limited, for example, 15 μm to 5 mm.

[0031] (Other chemical conditions) The formation voltage is the maximum voltage applied between the metal material and the counter electrode. The formation voltage affects the thickness of the oxide film and, furthermore, the withstand voltage of the electrolytic capacitor. Therefore, the formation voltage can be set appropriately according to the rated voltage of the electrolytic capacitor and is not particularly limited. The formation voltage may be, for example, 5V or higher. The formation voltage may be, for example, 100V or lower.

[0032] The time for maintaining the above-mentioned conversion voltage (conversion time) is not particularly limited and may be set appropriately considering the thickness of the oxide film, productivity, etc. The conversion time may be, for example, 1 hour or more. The conversion time may be, for example, 20 hours or less.

[0033] The current density flowing through the metal material is not particularly limited and can be set appropriately considering the chemical reaction time, etc. The maximum current density is, for example, 0.001 mA / cm². 2 The above is acceptable. The maximum current density is, for example, 100 mA / cm². 2 The following is acceptable:

[0034] [Electrodes for electrolytic capacitors] The electrode according to 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 oxides of valve-acting metals contained in the metal material.

[0035] The thickness of the oxide film is not particularly limited and is set appropriately considering the rated voltage of the electrolytic capacitor, etc. For example, the thickness of the oxide film is between 10 nm and 300 nm.

[0036] B-1. First aspect In the oxide film according to 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 treated with a chemical conversion solution containing a nitrate compound (hereinafter referred to as nitrate conversion).

[0037] EDX is used in combination with scanning electron microscopes (SEM), transmission electron microscopes (TEM), or scanning transmission electron microscopes (STEM).

[0038] Phosphorus is detected in oxide films formed with phosphoric acid aqueous solutions commonly used in chemical conversion treatments (hereinafter sometimes referred to as phosphorylated oxide films). In other words, phosphorylated oxide films contain a relatively large number of atoms that form conductive paths. On the other hand, nitrogen atoms are hardly detected (below the detection limit). Phosphorus is detected in large quantities near the surface of other oxide films.

[0039] The oxide film according to this embodiment contains almost no phosphorus atoms and only a small amount of nitrogen atoms. Therefore, it has different properties from phosphorylated films, and leakage current of electrolytic capacitors is easily suppressed.

[0040] B-2. Second aspect In the oxide film according to this embodiment, the fragment peak intensity of phosphate ions obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) is below the detection limit. This means that there is little phosphorus contamination in the oxide film. On the other hand, in the oxide film according to this embodiment, a peak presumed to be related to nitrogen ions is observed. From this, the possibility of nitrogen contamination in the oxide film is inferred. The slight inclusion of nitrogen instead of phosphorus changes the properties of the oxide film, suppressing the leakage current of the electrolytic capacitor. Such an oxide film can be formed on nitrate-conjugated metal materials.

[0041] Similar to the EDX analysis results, when other oxide films are analyzed by TOF-SIMS, fragment peaks of phosphate ions are detected. The ion fragment peaks are obtained by evaluating the surface of the oxide film. The oxide film may be etched to evaluate its interior. The evaluation results of the interior also have the same tendency as the evaluation results of the surface.

[0042] B-3. Third Aspect The oxide film according to this aspect contains tantalum oxide. In the oxide film according to this aspect, 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, 100×|I 1A -I 1A -I 2A | / I 1A ≦10 (%) is satisfied. Such an oxide film can be formed on a nitrided metal material.

[0043] The first peak is attributed to the O-K edge (O-K edge. Excitation process by K-shell electrons of oxygen). The second peak is attributed to the Ta-N1 edge (Ta-N1 edge. Excitation process by N1-shell electrons of tantalum). The relationship between the first peak and the second peak indicates the oxidation state of tantalum atoms.

[0044] When using a chemical conversion solution containing electrolytes other than the conventionally used nitric acid compounds, such as inorganic acids and their salts such as phosphoric acid, organic acids and their salts such as adipic acid, and basic substances such as ammonia, the relationship between the first peak and the second peak in the formed oxide film (hereinafter referred to as other oxide films) is 100×|I 1A -I 2A | / I 1AThe value does not satisfy ≤10(%). In other words, the oxidation state of tantalum atoms differs between the oxide film formed by nitration and other oxide films. The reason is not yet clear, but it is thought that this difference affects the electronic structure of the oxide film and is effective in suppressing the leakage current of the capacitor.

[0045] I 1A >I 2A It is also fine, I 1A 2A It is also fine, I 1A =I 2A That's fine.

[0046] Average intensity of the first peak I 1A The intensity of the first peak is calculated as follows: At a total of six points, including an arbitrary point on the surface of the oxide film, four points that divide the thickness of the oxide film into five equal parts on a straight line drawn from this point toward the metal material, and the intersection of the straight line and the surface of the metal material, the intensity of the peak observed between 530 eV and 550 eV is measured. Furthermore, the intensity of the peak observed between 530 eV and 550 eV is measured at a total of six points at different depths in the same manner for four other arbitrary points. 1A This is the average of these 30 points.

[0047] Average intensity I of the second peak 2A This is the average value of the peak intensities observed between 560 eV and 570 eV at the same 30 points where the intensity of the first peak was measured. If there are multiple peaks observed between 530 eV and 550 eV, the peak on the lower energy side should be used. If there are multiple peaks observed between 560 eV and 570 eV, the peak on the lower energy side should be used.

[0048] EELS is used in combination with scanning electron microscopes (SEM), transmission electron microscopes (TEM), or scanning transmission electron microscopes (STEM).

[0049] B-4. Fourth aspect ​The oxide film according to this embodiment contains tantalum oxide. In the oxide film according to this embodiment, the intensity I1 of the first peak observed between 530 eV and 550 eV in the spectrum obtained by EELS is smaller the closer it is to the surface of the metallic material. In other words, it can be said that the electronic structure of the oxide film changes in the same way in the thickness direction. Such an oxide film can be formed on a metallic material that has been nitrated.

[0050] In other oxide films, it is not the case that the intensity I1 of the first peak decreases 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 may be greater than the intensity of the first peak inside the material. In other words, in other oxide films, the bonding state of oxygen changes randomly in the thickness direction. The reason is not yet clear, but it is thought that this difference affects the electronic structure of the oxide film and is effective in suppressing leakage current in capacitors.

[0051] The intensity of the first peak I1 is measured at a total of six points, including, for example, an arbitrary point on the surface of the oxide film (depth zero), four points (depths 1 to 4) that divide the thickness of the oxide film into five equal parts on a straight line drawn from this point toward the metal material, and the intersection of this straight line with the surface of the metal material (depth 5). Furthermore, the intensity of the first peak is measured similarly at four other arbitrary locations, for a total of six points at different depths. The intensities of the five points measured at the same depth but at different locations are averaged to obtain the intensity of the first peak at that depth. If there are multiple peaks observed between 530 eV and 550 eV, the peak on the lowest energy side should be used.

[0052] The intensity I1 of the first peak should, as an overall trend, decrease as it approaches the surface of the metal material. For example, among the six points from depth zero to depth five mentioned above, the intensity of the shallower point may be greater than or equal to the intensity of the deeper point. However, the intensity I1 at depth zero 10 This is the intensity I at depth 5. 15 Larger.

[0053] From the viewpoint of uniformity of oxide film quality, the intensity at zero depth is I 10 And, intensity I at depth 5 15 The difference should not be excessively large. Intensity I 10 and intensity I 15 The difference between (=(I 10 -I 15 )) is intensity I 10 It is preferable that it be 30% or less of that. That is, 100 × (I 10 -I 15 ) / I 10 It is preferable to satisfy ≤30(%). 100 × (I 10 -I 15 ) / I 10 It is more preferable that the value be ≤20%.

[0054] From a similar perspective, the intensity I at depth 1 11 This is the intensity I at zero depth. 10 Smaller is preferable, strength I 10 and intensity I 11 The difference should be sufficiently large. Intensity I 10 and intensity I 11 The difference between (=I 10 -I 11 ) is intensity I 10 It is preferable that it be between 3% and 20%. That is, 3(%) ≤ 100 × (I 10 -I 11 ) / I 10 It is preferable to satisfy ≤20(%). 5(%) ≤ 100 × (I 10 -I 11 ) / I 10 It is more preferable that the value be ≤20%.

[0055] B-5. Fifth aspect The oxide film according to this embodiment contains tantalum oxide. In the oxide film according to 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 edge is observed at 570 eV or higher. Such an oxide film can be formed on a nitrate-conjugated metal material.

[0056] The third peak is attributed to the Ta-N1 edge (excitation process by tantalum's N1 shell electrons). The position of the fourth peak indicates the state of the distance between oxygen atoms. A shift of the fourth peak to the higher energy side means that the distance between oxygen atoms is decreasing. In other words, it can be inferred that the density of the oxide film is improving. The third peak coincides with the second peak in the third embodiment.

[0057] The fourth peak in other oxide films is observed at energies lower than 570 eV. This suggests that the oxidation state of tantalum atoms differs between oxide films formed by nitration and other oxide films. While the reason is not yet clear, this difference is thought to affect the electronic structure of the oxide film and contribute to suppressing capacitor leakage current.

[0058] The third and fourth peaks are identified as follows: An EELS spectrum is obtained at a point within 10 nm from the surface of the oxide film toward the metal material (e.g., at a depth of 5 nm). Next, the third peak, which is attributed to the Ta-N1 edge, is identified. The third peak usually appears between 563 eV and 567 eV. Then, the fourth peak adjacent to this third peak is identified. The position of the fourth peak is preferably confirmed by further EELS evaluation of nine other arbitrary points located within 10 nm of the oxide film depth. If the fourth peak is observed at 570 eV or higher at eight of the ten arbitrary points, this oxide film may be considered to satisfy the fifth aspect.

[0059] (others) In the third to fifth embodiments, it is desirable that the following conditions be met. a) In the spectrum obtained by EELS of the oxide film according to this embodiment, the average intensity of the fifth peak observed between 1770 eV and 1790 eV is I 5A This is the average intensity of the fifth peak in other oxide films. 5R Lower.

[0060] In particular, average intensity I 5Aand average intensity I 5R The difference between (=I 5R -I 5A ) but average intensity I 5R It is preferable that it be 10% or more of the total. That is, (I 5R -I 5A ) / I 5R It is preferable that the value be ≥ 0.1.

[0061] The fifth peak is attributed to the Ta-M5 edge (excitation process by tantalum's M5 shell electrons).

[0062] b) In the spectrum obtained by EELS of the oxide film according to this embodiment, the average intensity of the sixth peak observed between 1830 eV and 1850 eV 6A This is the average intensity of the sixth peak in other oxide films. 6R Lower.

[0063] In particular, average intensity I 6A and average intensity I 6R The difference between (=I 6R -I 6A ) but average intensity I 6R It is preferable that it be 5% or more of the total. That is, (I 6R -I 6A ) / I 6R It is preferable that the value be ≥ 0.05.

[0064] The sixth peak is attributed to the Ta-M4 edge (excitation process by tantalum's M4 shell electrons).

[0065] average intensity I 5A and average intensity I 6A is average intensity I 1A It can be calculated in the same way. Average strength I of the oxide film being compared 5R and average intensity I 6R is average intensity I 1A It can be calculated in the same way.

[0066] In the first to fourth embodiments, it is desirable that the following conditions be met. c) The current (leakage current) flowing through the electrode having an oxide film according to this embodiment is 10% or more less than the leakage current value of other electrodes having an oxide film. This further suppresses the leakage current of the electrolytic capacitor.

[0067] The leakage current value of the electrode according to this embodiment is preferably 15% or more less than the leakage current value of other electrodes having an oxide film, and preferably 30% or more less.

[0068] The electrode leakage current is the current value obtained when the electrode and counter electrode are immersed in an electrolyte solution and a voltage of 70% of the conversion voltage is applied.

[0069] The comparative oxide film is formed using, for example, a conversion solution containing 0.1% by mass of phosphoric acid. The conversion conditions, other than the composition of the conversion solution, are the same as those for the oxide film according to this embodiment. The conversion conditions are, for example, a conversion voltage of 15V, a temperature of 60°C, and a processing time of 10 hours.

[0070] [Electrolytic capacitor] The electrodes obtained by chemical treatment of metal foil as described above are used in capacitor elements. The capacitor element comprises a first electrode and a second electrode. The second electrode comprises, for example, a solid electrolyte layer and a cathode extraction layer. The leakage current of the electrolytic capacitor according to this embodiment is 30% or more less than the leakage current of an electrolytic capacitor having electrodes with other oxide films.

[0071] An electrolytic capacitor comprises, for example, one or more capacitor elements, an outer casing that encloses the capacitor elements, and first lead terminals and second lead terminals. At least a portion of each lead terminal is exposed from the outer casing. Such capacitor elements are, for example, in the form of a sheet or a flat plate.

[0072] (1st electrode) The first electrode is a metallic material having an oxide film formed as described above. The first electrode is, for example, an anode.

[0073] (2nd electrode) The second electrode comprises a solid electrolyte layer and an electrode extraction layer. The second electrode is, for example, a cathode.

[0074] (solid electrolyte layer) The solid electrolyte layer is formed to cover at least a portion of the oxide film. The solid electrolyte layer may also be formed to cover the entire surface of the oxide film. The thickness of the solid electrolyte layer is not particularly limited.

[0075] The solid electrolyte layer comprises one or more solid electrolyte layers. The solid electrolyte layer is formed, for example, from a manganese compound or a conductive polymer. As the conductive polymer, polypyrrole, polyaniline, polythiophene, polyacetylene, and their derivatives can be used. The solid electrolyte layer containing the conductive polymer can be formed, for example, by chemical polymerization and / or electrolytic polymerization of raw material monomers on an oxide film. Alternatively, it can be formed by applying a solution in which the conductive polymer is dissolved, or a dispersion in which the conductive polymer is dispersed, to an oxide film.

[0076] (Cathode extraction layer) The cathode extraction layer may be formed to cover at least a portion of the solid electrolyte layer, or it may be formed to cover the entire surface of the solid electrolyte layer.

[0077] The cathode extraction layer includes, 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, for example, a composition containing silver particles and resin. However, the configuration of the cathode extraction layer is not limited to these, and any configuration that has a current collection function is acceptable.

[0078] (Lead terminals) The material of the first and second lead terminals is not particularly limited as long as it is electrochemically and chemically stable and conductive, and may be metal or nonmetal. Their shapes are also not particularly limited.

[0079] 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 made, for example, by welding them together. The electrical connection between the second electrode and the second lead terminal is made, for example, by bonding the second electrode and the second lead terminal together via a conductive adhesive layer.

[0080] (Exterior) The casing covers the capacitor element and a portion of the lead terminals. This electrically insulates the first and second lead terminals and protects the capacitor element. The casing is made of an insulating material (casing material). The casing material includes, for example, a cured thermosetting resin or an engineering plastic.

[0081] Figure 1 is a schematic cross-sectional view showing a capacitor element according to this embodiment. The capacitor element 10 comprises a first electrode 11 and a second electrode 13. The first electrode 11 comprises a porous sintered body 111, electrode wires 112 embedded in the porous sintered body 111, and an oxide film 113 covering at least a portion of the porous sintered body 111. The second electrode 13 comprises 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 extraction layers. Such a capacitor element 10 is generally cubic in shape.

[0082] Figure 2 is a schematic cross-sectional view showing the structure of an electrolytic capacitor according to this embodiment. The electrolytic capacitor 100 comprises a capacitor element, an outer casing 20 that encloses the capacitor element, and first lead terminals 30 and second lead terminals 40, each at least partially exposed to the outside of the outer casing 20.

[0083] 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 via an adhesive layer 50 formed, for example, by a conductive adhesive (such as a mixture of thermosetting resin and carbon particles or metal particles).

[0084] In this embodiment, an electrolytic capacitor is described as using a solid electrolyte and having a capacitor element sealed by an outer casing, but the embodiment is not limited thereto. The electrode according to this embodiment can be applied, for example, to an electrolytic capacitor comprising a capacitor element in which a first electrode and a second electrode are wound with a separator in between, and an electrolyte. In this case, the electrode according to this embodiment is used for at least one of the first electrode and the second electrode.

[0085] [Examples] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0086] Example 1 Twenty electrolytic capacitors, as shown in Figure 2, were fabricated according to the following procedure, and their characteristics were evaluated.

[0087] (i) Fabrication of capacitor elements (ii) Preparation of the first electrode Tantalum metal particles were used as the valve metal. The tantalum metal particles were formed into a rectangular parallelepiped so that one end of an electrode wire made of tantalum was embedded in the tantalum metal particles, and then the formed body was sintered in a vacuum. This yielded a precursor for a first electrode, which included a porous sintered body of tantalum and an electrode wire, one end of which was embedded in the porous sintered body and the other end of which was planted from one surface of the porous sintered body.

[0088] (i-ii) Formation of oxide film A 0.06 mass% aqueous solution of nitric acid was prepared as the chemical conversion solution. The chemical conversion tank was filled with this solution, and the porous sintered body and a portion of the electrode wire were immersed in it. 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 performed at a conversion voltage of 15V for 10 hours. In this way, a uniform oxide film of tantalum oxide (Ta2O5) (thickness approximately 30 nm) was formed on the surface of the porous sintered body and the surface of a portion of the electrode wire, and 20 first electrodes X1 were obtained.

[0089] (i-iii) Formation of the solid electrolyte layer A dispersion containing polypyrrole was impregnated into a porous sintered body with an oxide film for 5 minutes, and then dried at 150°C for 30 minutes to form a solid electrolyte layer on the oxide film.

[0090] (i-iv) Formation of the carbon layer A carbon layer was formed on the surface of the solid electrolyte layer by applying a dispersion of carbon particles in water (carbon paste) to the solid electrolyte layer and then heating it at 200°C.

[0091] (iv) Formation of the metal paste layer A metal paste containing silver particles, a binder resin, and a solvent was applied to the surface of a carbon layer. Then, it was heated at 200°C to form a metal paste layer, thereby obtaining a capacitor element.

[0092] (ii) Fabrication of electrolytic capacitors A conductive adhesive was applied to the metal paste layer, and the second lead terminal and the metal paste layer were joined. The electrode wire and the first lead terminal were joined by resistance welding. Next, the capacitor element and the outer casing material (uncured thermosetting resin and filler) with each lead terminal joined were placed in a mold, and the capacitor element was sealed by transfer molding to produce an electrolytic capacitor.

[0093] Example 2 Except for setting the concentration of nitric acid in the chemical solution to 10% by mass and setting the temperature of the chemical solution to 45°C, 20 first electrodes X2 were prepared in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0094] Comparative Example 1 Except for using a chemical conversion solution containing phosphoric acid (0.1% by mass) instead of nitric acid, 20 first electrodes Y1 were prepared in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0095] Comparative Example 2 Twenty first electrodes Y2 were fabricated in the same manner as in Example 1, except that a chemical conversion solution containing diammonium adipate (0.2% by mass) was used instead of nitric acid, and an electrolytic capacitor was constructed.

[0096] Comparative Example 3 Except for using a chemical conversion solution containing ammonia (2.5% by mass) instead of nitric acid, 20 first electrodes Y3 were prepared in the same manner as in Example 1, and an electrolytic capacitor was fabricated.

[0097] [evaluation] (1) Analysis of oxide film After the formation of the oxide film (i-ii), the first electrodes X1 and Y1-Y3 were analyzed. (1-1)EELS analysis Spectral analysis was performed using a TEM-EELS instrument. The results are shown in Table 1.

[0098] [Table 1]

[0099] (1-2)EDX analysis Elemental analysis of the oxide film surface on the first electrode X1 and Y1 was performed using a TEM-EDX instrument. The results are shown in Table 2.

[0100] [Table 2]

[0101] (1-3)TOF-SIMS analysis The surface and interior (depths from 1 nm to 10 nm) of the oxide film were analyzed using a TOF-SIMS system. The oxide film was etched using an Ar gas cluster ion beam.

[0102] No phosphate ions were detected (below the detection limit) at either the surface or interior of the oxide film at electrodes X1 and X2. Phosphate ions were detected at either the surface or interior of the oxide film at electrodes Y1 to Y3.

[0103] (2) Leakage current After the formation of the coating (i-ii), the leakage current values ​​of the first electrodes X1, Y2, and Y3 were measured. The prepared first electrode and counter electrode (SUS316L) were immersed in 0.1 wt% phosphoric acid. A voltage of 70% of the formation voltage was applied between each electrode, and the current flowing through the first electrode was measured and its average value was calculated. The average current value of the first electrode Y1 was set as 100%, and the average current value (leakage current value) of each first electrode was calculated. The results are shown in Table 3. For reference, the average current value of the first electrode X2 is also shown in Table 3.

[0104] [Table 3] [Industrial applicability]

[0105] The electrodes manufactured by the method according to the present invention can be used in electrolytic capacitors for various applications because they suppress leakage current. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of symbols]

[0106] 100: Electrolytic capacitor 10: Capacitor element 11: 1st electrode 111: Metal material (porous sintered body) 112: Electrode wire 113: Oxide film 13:Second electrode 131: Solid electrolyte layer 132: Carbon layer 133: Metal paste layer 20: Exterior 30: First lead terminal 40: Second lead terminal 50: Adhesive layer

Claims

1. The process includes a chemical conversion step in which an electric current is passed 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 solution contains a nitrate compound as the electrolyte, The concentration of the phosphorus compound in the aforementioned chemical solution is less than 0.01% by mass. A method for manufacturing electrodes for electrolytic capacitors, wherein the temperature of the chemical solution in the chemical conversion step is 60°C or higher and 75°C or lower.

2. The method for producing an electrode for an electrolytic capacitor according to claim 1, wherein the chemical conversion solution contains the nitrate compound in a concentration of 0.03% by mass or more and 15% by mass or less.

3. The method for manufacturing an electrode for an electrolytic capacitor according to claim 1 or 2, wherein the metal material is a porous sintered body containing tantalum.

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