Electrolytic capacitors

Abstract

This invention provides an electrolytic capacitor in which the increase in leakage current over time is suppressed, even when the anode is made of powder laminated foil. [Solution] The device comprises an anode, a cathode facing the anode, a separator interposed between the anode and the cathode, and an electrolyte interposed between the anode and the cathode. The anode consists of a foil body of valve metal, a powder layer made of valve metal powder laminated on the foil body of the anode, and a dielectric film. The cathode consists of a foil body of valve metal and a conductive layer laminated on the foil body of the anode. The electrolyte contains water, alcohols, and compounds having carboxyl groups.

Description

Technical Field

[0001] The present invention relates to an electrolytic capacitor that obtains capacitance by the dielectric polarization action of a dielectric film and stores and discharges electric charges.

Background Art

[0002] An electrolytic capacitor includes valve action metals such as tantalum or aluminum as an anode body and a cathode body. The anode body has a dielectric film on its surface. An electrolyte is interposed between the anode body and the cathode body. The electrolyte adheres closely to the uneven surface of the anode body and functions as a true cathode.

[0003] Capacitors are used in various applications. For example, in the field of power electronics, in a power supply circuit that converts the power of an AC power supply into DC power by a converter circuit and then converts this DC power into desired AC power by an inverter circuit, a smoothing capacitor is provided to suppress and smooth the DC pulsation output from the converter circuit before inputting it into the inverter circuit. Also, a decoupling capacitor is provided in the vicinity of a semiconductor switching element such as gallium nitride for the stable operation and noise removal of the semiconductor switching element. With the increase in power in the field of power electronics, the demand for higher capacitance of capacitors has been increasing.

[0004] In this regard, an electrolytic capacitor has the advantage that its specific surface area can be increased by expanding the surface area of the anode body, and it is easier to obtain a larger capacitance compared to other types of capacitors such as film capacitors. An electrolytic capacitor has an electrolyte in the form of an electrolytic solution. By increasing the specific surface area of the anode body, the contact area between the electrolytic solution and the dielectric film of the anode body increases. Therefore, it is easier to further increase the capacitance of the electrolytic capacitor.

[0005] As a method for expanding the surface area, etching is known to increase the surface area beyond the projected area. For example, by passing a direct current through an acidic aqueous solution containing halogen ions such as hydrochloric acid, numerous tunnel-shaped pits are formed in the thickness direction of the anode. These tunnel-shaped etching pits are further expanded by passing a direct current through an acidic aqueous solution such as nitric acid. Alternatively, by passing an alternating current through an acidic aqueous solution containing halogen ions such as hydrochloric acid, numerous sponge-like or spongy etching pits are formed, consisting of a series of fine voids that expand in a spatial manner.

[0006] In recent years, powder laminated foil has been proposed as an anode body with an expanded surface area using a method different from etching (see, for example, Patent Document 1). Powder laminated foil has a foil body made by stretching a valve metal and a powder layer laminated on the surface of the foil body. The powder layer is made by attaching powder of the valve metal to the foil body and heating and sintering it in a vacuum or reducing atmosphere.

[0007] This powder layer has a porous structure consisting of voids between aggregated valve-acting metal particles. This powder layer enlarges the anode surface. The powder layer is easily formed densely, resulting in a large surface area for the anode, further increasing the capacitance of the electrolytic capacitor. Therefore, powder laminated foil has attracted attention in recent years. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2011-204729 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The electrolyte in electrolytic capacitors sometimes contains water, for example, to reduce the equivalent series resistance (ESR) of the electrolytic capacitor. If the anode is damaged and the base metal of the valve metal is exposed, or if water comes into contact with the dielectric film and the dielectric film deteriorates and dissolves due to hydration, the leakage current of the electrolytic capacitor will increase.

[0010] Therefore, the electrolyte of electrolytic capacitors contains a solute with a regenerative chemical action, and the increase in leakage current of the electrolytic capacitor is suppressed by regenerative chemical action that outweighs hydration degradation. However, powder laminated foil anodes are more susceptible to damage than anodes enlarged by etching, and electrolytic capacitors equipped with powder laminated foil anodes experience a more severe increase in leakage current over time than electrolytic capacitors equipped with anodes enlarged by etching.

[0011] The reason for the significant increase in leakage current in electrolytic capacitors equipped with powder bed foil anodes is that the electrostriction that occurs in the dielectric film when voltage is applied causes the sintered layer to expand and contract, stressing the powder bed foil. This breaks the sintered bonds of the valve metal particles, making the valve metal base metal more susceptible to exposure. It is presumed that the reparative chemical reaction cannot keep up with this exposure of the valve metal base metal due to the stress applied to the powder bed foil, leading to an increase in the leakage current of the electrolytic capacitor. In particular, while electrostriction of the dielectric film occurs in conventional tunnel-shaped etching pits or sponge-like or spongy etching pits when voltage is applied, the effect of electrostriction is especially large in powder bed foil due to its structure that aggregates the valve metal particles.

[0012] The present invention was proposed to solve the above problems, and its objective is to provide an electrolytic capacitor in which the increase in leakage current over time is suppressed, even when the anode body is a powder laminated foil. [Means for solving the problem]

[0013] To solve the above problems, the electrolytic capacitor of this embodiment comprises an anode, a cathode facing the anode, a separator interposed between the anode and the cathode, and an electrolyte interposed between the anode and the cathode. The anode consists of a foil body of valve metal, a powder layer made of valve metal powder and laminated on the foil body of the anode, and a dielectric film. The cathode consists of a foil body of valve metal and a conductive layer laminated on the foil body of the cathode. The electrolyte contains water, alcohols, and compounds having carboxyl groups.

[0014] The amount of water contained in the electrolyte may be 10 wt% or less relative to the electrolyte.

[0015] The conductive layer may contain carbon. [Effects of the Invention]

[0016] According to the present invention, the increase in leakage current over time in an electrolytic capacitor using powder laminated foil as the anode is suppressed. [Modes for carrying out the invention]

[0017] The following describes an electrolytic capacitor according to an embodiment of the present invention. However, the present invention is not limited to the embodiments described below.

[0018] (Overall structure) An electrolytic capacitor is a passive element that stores and discharges electric charge by obtaining capacitance through the dielectric polarization effect of a dielectric film. This electrolytic capacitor has a capacitor element. The capacitor element comprises an anode, a cathode, a separator, and an electrolyte. The anode has a dielectric film on its surface. The separator is interposed between the anode and the cathode, insulating them from each other. The anode and cathode face each other via the separator. The electrolyte is impregnated into the voids within the capacitor element, particularly between the anode and the cathode. The electrolyte is in close contact with the dielectric film and acts as a true cathode.

[0019] Capacitor elements have either a flat plate structure or a wound structure. A flat plate capacitor element is a laminate in which flat plates of anode and cathode are alternately stacked with a separator in between. A wound capacitor element is a laminate in which strip-shaped anode and cathode are alternately stacked with a separator in between, and the strips are wound so that the short sides of the strips are along the winding axis and the long sides of the strips are wrapped around it. Lead terminals for the anode and cathode are drawn out from this capacitor element. The lead terminals are connected to the anode and cathode, respectively. The lead terminals are conductors that electrically connect the electrolytic capacitor to the mounting board. The lead terminals are electrically and mechanically connected to the anode and cathode by stitch connections, cold welding, ultrasonic welding, or laser welding.

[0020] (Anode) In such an electrolytic capacitor, the anode is a powder laminated foil. The powder laminated foil comprises a foil body formed by stretching a valve metal and a powder layer of the valve metal laminated on the surface of the foil body. The valve metals include aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony. The purity of the valve metal is preferably 99.9% or higher for the anode foil, and other elements such as silicon, iron, copper, manganese, magnesium, chromium, zinc, titanium, vanadium, gallium, nickel, boron, and zirconium may also be present.

[0021] The powder layer is formed on one or both sides of the foil. The powder layer consists of layers of valve metal powder. The valve metal powder may be the same type as that of the foil or a different type. The powder layer is either a sintered layer formed by sintering the valve metal powder, or a deposited layer formed by depositing the valve metal powder.

[0022] The sintered layer is produced by attaching a powder of valve-acting metal to a foil body and heating and sintering it in a vacuum or reducing atmosphere or the like. The vapor deposition layer is produced, for example, by a resistance heating type vapor deposition method or an electron beam heating type vapor deposition method. In this vapor deposition layer, the valve-acting metal is heated and evaporated by resistance heat or electron beam energy, and the vapor of the valve-acting metal particles is deposited on the surface of the foil body to form a film. Also, the anode body may be produced by pressure molding and sintering a powder of valve-acting metal into a foil shape. This anode body has no core part and is entirely composed of an extended surface layer.

[0023] The dielectric film is formed on one or both sides of the anode body. The dielectric film is typically an oxide film formed on the surface layer of the anode body, and if the anode body is made of aluminum, it is an aluminum oxide layer. In the chemical conversion treatment for forming the dielectric film, a voltage is applied to the anode body in a chemical conversion solution aiming at a desired withstand voltage. In the chemical conversion treatment, in order to obtain a withstand voltage of 1V, it is preferable to form a dielectric film with a thickness of 0.9 to 1.5 nm. The chemical conversion solution is a solution free of halogen ions, and for example, a phosphoric acid-based chemical conversion solution such as ammonium dihydrogen phosphate, a boric acid-based chemical conversion solution such as ammonium borate, and an adipic acid-based chemical conversion solution such as ammonium adipate.

[0024] (Cathode body) The cathode body includes a foil body obtained by stretching a valve-acting metal and a conductive layer laminated on the surface of the foil body. The foil body has a long strip shape in a wound type electrolytic capacitor and a flat plate shape in a multilayer type electrolytic capacitor. The purity of the valve-acting metal is preferably 99% or more with respect to the cathode body.

[0025] The conductive layer is formed on one or both sides of the foil body. As the conductive layer, it is only necessary to mainly contain an inorganic substance or an inorganic compound. Examples of the inorganic substance or inorganic compound include titanium, zirconium, tantalum, niobium, nitrides or carbides thereof, aluminum carbide, carbon materials, and composite materials or mixed materials thereof. Specifically, a carbon layer which is a conductive layer of a carbon material, a conductive layer of titanium nitride, a conductive layer of titanium carbide, a conductive layer obtained by mixing titanium and a carbon material, and a conductive layer obtained by compositing aluminum carbide (Al4C3) and titanium oxide (TiO2) are mentioned.

[0026] The conductive layer containing carbon material, i.e., the carbon layer, contains graphite, carbon black, or a mixture thereof as the carbon material. Examples of graphite include natural graphite, artificial graphite, and graphitized Ketjenblack. Examples of carbon black include Ketjenblack, acetylene black, channel black, and thermal black. In addition, the carbon layer may contain activated carbon, carbon nanohorns, or fibrous carbon as the carbon material. Activated carbon is made from natural plant tissue such as coconut shells, synthetic resins such as phenol, and fossil fuels such as coal, coke, and pitch. Examples of fibrous carbon include carbon nanotubes (hereinafter referred to as CNTs) and carbon nanofibers (hereinafter referred to as CNFs).

[0027] Carbon materials may be subjected to porous treatments such as activation treatment and opening treatment. Conventional known activation treatments such as gas activation methods and chemical activation methods can be used for porous treatment. Examples of gases used in gas activation methods include water vapor, air, carbon monoxide, carbon dioxide, hydrogen chloride, oxygen, or mixtures thereof. Examples of chemicals used in chemical activation methods include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, alkaline earth metal hydroxides such as calcium hydroxide, inorganic acids such as boric acid, phosphoric acid, sulfuric acid, and hydrochloric acid, or inorganic salts such as zinc chloride. Heat treatment may be applied as needed during this activation treatment.

[0028] Methods for forming a conductive layer on a foil include vacuum deposition, sputtering, ion plating, CVD, coating, electroplating, and electroless plating. In the case of coating, an inorganic substance or inorganic compound is dispersed in a dispersion solvent to prepare a slurry, and the slurry is applied to the foil and dried using methods such as slurry casting, doctor blade method, or spray atomization.

[0029] Slurries are prepared by dispersing inorganic substances or inorganic compounds in a solvent and adding a binder. The inorganic substances or inorganic compounds may be pulverized using a grinding method such as a bead mill or ball mill to adjust the particle size. Solvents for the slurry include methanol, alcohols such as ethanol and 2-propanol, hydrocarbon solvents, aromatic solvents, amide solvents such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF), water, and mixtures thereof.

[0030] Dispersion methods include mixers, jet mixing, ultracentrifugation, and other ultrasonic treatments. In the dispersion process, the inorganic substances or inorganic compounds and binders in the mixed solution are subdivided and homogenized, and dispersed in the solution. Examples of binders include styrene-butadiene rubber, polyvinylidene fluoride, or polytetrafluoroethylene.

[0031] Examples of deposition methods include resistance heating deposition and electron beam heating deposition. In the deposition method, inorganic materials or inorganic compounds are evaporated by electric heating in a vacuum, or by irradiating inorganic materials or inorganic compounds with an electron beam in a vacuum to evaporate them, thereby forming a film of inorganic materials or inorganic compounds on a foil. In the sputtering method, a target made of inorganic materials or inorganic compounds and a foil are placed in a vacuum chamber, and an inert gas is introduced into the vacuum chamber and a voltage is applied to cause the plasma-generated inert gas to collide with the target, depositing the inorganic materials or inorganic compound particles knocked out from the target onto the foil.

[0032] Alternatively, the conductive layer may be manufactured by forming it into a sheet using a papermaking process and then placing it on a foil body. The papermaking sheet is prepared by dispersing the inorganic substance or inorganic compound to be contained in the conductive layer in a dispersion solvent, adding a binder as needed, then performing vacuum filtration and drying, and finally peeling the deposited material off the filter paper.

[0033] The foil body may be a plain foil without a spreading layer, or a spreading layer may be formed on the surface of the foil body, and a conductive layer may be laminated so as to be embedded in this spreading layer. As the spreading layer, a sintered layer or a vapor-deposited layer may be selected, as well as an etched layer obtained by etching the cathode foil. That is, the spreading layer has a porous structure and consists of tunnel-shaped pits, sponge-like pits, or densely packed powder or voids between particles.

[0034] Tunnel-shaped etching pits are holes carved in the direction of the foil thickness. These tunnel-shaped etching pits are typically formed by passing a direct current through an acidic aqueous solution containing halogen ions, such as hydrochloric acid. The tunnel-shaped etching pits are further expanded by passing a direct current through an acidic aqueous solution containing nitric acid, for example. Sponge-like etching pits cause the expanded layer to become a sponge-like layer with a series of fine voids. These sponge-like etching pits are formed by passing an alternating current through an acidic aqueous solution containing halogen ions, such as hydrochloric acid.

[0035] To allow the conductive layer to penetrate more deeply into the expanded layer, the conductive layer and the foil body may be pressed together by press working. In press working, for example, the cathode body consisting of the conductive layer and the foil body is sandwiched between press rollers and press pressure is applied. A press pressure of approximately 1 to 20 kN / cm is desirable. Press working can be done by pressing together the conductive layer while laminating it onto the foil body, or by pressing together the carbon layer and the cathode foil after lamination. By allowing the conductive layer to penetrate deeper into the expanded layer, a cathode body can be obtained in which the conductive layer is more closely attached to the cathode foil.

[0036] Furthermore, the foil body may have a thin oxide film of about 1 to 10V formed by a natural oxide film or a chemical conversion treatment. The natural oxide film is formed when the cathode body reacts with oxygen in the air.

[0037] (electrolyte) An electrolyte is a mixture obtained by dissolving a solute in a solvent and adding additives as needed. This electrolyte is composed of a combination of water, a solvent that undergoes esterification, and an anionic component. Specifically, the electrolyte contains water and alcohols as solvents, and a carboxylic acid as the anionic component.

[0038] Alcohols include monohydric alcohols, polyhydric alcohols, and oxyalcohol compounds. Examples of monohydric alcohols include ethanol, propanol, butanol, pentanol, hexanol, cyclobutanol, cyclopentanol, cyclohexanol, and benzyl alcohol. Examples of polyhydric alcohols and oxyalcohol compounds include ethylene glycol, propylene glycol, diethylene glycol, glycerin, methyl cellosolve, ethyl cellosolve, methoxypropylene glycol, dimethoxypropanol, and alkylene oxide adducts of polyhydric alcohols such as polyethylene glycol and polyoxyethylene glycerin.

[0039] Examples of carboxylic acids include oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, 1-methylazelaic acid, undecanediic acid, dodecanediic acid, tridecanediic acid, t-butyladipic acid, 11-vinyl-8-octadecenediic acid, resorcinic acid, phloroglucic acid, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, and pyromellitic acid.

[0040] In addition to alcohols, other types of solvents may be mixed. Examples of other types of solvents include water or aprotic organic polar solvents. Representative examples of aprotic organic polar solvents include sulfones, amides, lactones, cyclic amides, nitriles, and sulfoxides. Examples of sulfones include dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane. Examples of amides include N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, and hexamethylphosphoric amide. Examples of lactones and cyclic amides include γ-butyrolactone, γ-valerolactone, δ-valerolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, and isobutylene carbonate. Examples of nitriles include acetonitrile, 3-methoxypropionitrile, and glutalonitrile. Examples of sulfoxides include dimethyl sulfoxide.

[0041] In addition to carboxylic acids, other types of anionic components may be mixed. Examples of other types of anionic components include organic acids, inorganic acids, and composite compounds of organic and inorganic acids.

[0042] Examples of organic acids include carboxylic acids such as oxalic acid, succinic acid, glutaric acid, pimelic acid, suberic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, maleic acid, adipic acid, benzoic acid, toluic acid, enanthic acid, malonic acid, 1,6-decanedicarboxylic acid, 1,7-octanedicarboxylic acid, azelaic acid, undecanediic acid, dodecanediic acid, tridecanediic acid, t-butyladipic acid, 11-vinyl-8-octadecenediic acid, resorcinic acid, phloroglucic acid, gallic acid, gentisic acid, protocatechuic acid, pyrocatechuic acid, trimellitic acid, and pyromellitic acid, as well as phenols and sulfonic acids.

[0043] Examples of inorganic acids include boric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, carbonic acid, and silicic acid. Examples of complex compounds of organic and inorganic acids include borogisalicylic acid, borogiolusic acid, borogiglycolic acid, borogimalonic acid, borogisuccinic acid, borogiadipic acid, borogiazelaic acid, borogibenzoic acid, borogimalic acid, borogilacterial acid, borogitartaric acid, borogicitric acid, borogiphthalic acid, borogis(2-hydroxy)isobutyric acid, borogisorcinic acid, borodimethylsalicylic acid, boroginaphthoic acid, borogimandelic acid, and borogis(3-hydroxy)propionic acid.

[0044] In addition to anionic components, cationic components are added to the electrolyte as solutes. Cationic components include ammonium, quaternary ammonium, amidinium quaternary, amines, sodium, potassium, etc. Quaternary ammonium includes tetramethylammonium, triethylmethylammonium, tetraethylammonium, etc. Amidinium quaternary includes ethyldimethylimidazolinium, tetramethylimidazolinium, etc. Amines include primary amines, secondary amines, and tertiary amines. Primary amines include methylamine, ethylamine, propylamine, etc. Secondary amines include dimethylamine, diethylamine, ethylmethylamine, dibutylamine, etc. Tertiary amines include trimethylamine, triethylamine, tributylamine, ethyldimethylamine, ethyldiisopropylamine, etc.

[0045] The anionic and cationic components may be added to the solvent as salts of organic acids, inorganic acids, or complex compounds of organic and inorganic acids, or the anionic acid and cationic base may be added to the solvent separately.

[0046] Furthermore, other additives can be added to the electrolyte. Examples of additives include alkylene oxide adducts of polyhydric alcohols such as polyethylene glycol and polyoxyethylene glycerin, complex compounds of boric acid and polysaccharides (mannitol, sorbitol, etc.), complex compounds of boric acid and polyhydric alcohols, boric acid esters, nitro compounds (o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol, p-nitrobenzyl alcohol, m-nitroacetophenone, etc.), and phosphate esters. These may be used individually or in combination of two or more.

[0047] (Separator) Examples of separators include cellulose and mixed papers such as kraft, Manila hemp, esparto, hemp, cotton, and rayon; polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their derivatives; polyamide resins such as polytetrafluoroethylene resins, polyvinylidene fluoride resins, vinylon resins; polyamide resins such as aliphatic polyamides, semi-aromatic polyamides, and fully aromatic polyamides; polyimide resins; polyethylene resins; polypropylene resins; trimethylpentene resins; polyphenylene sulfide resins; acrylic resins; and polyvinyl alcohol resins. These resins can be used individually or in combination.

[0048] (Capacitor element) Capacitor elements have either a flat plate structure or a wound structure. The foil bodies of the anode and cathode have a long strip shape in wound electrolytic capacitors and a flat plate shape in multilayer electrolytic capacitors. A capacitor element with a flat plate structure is a laminate in which flat plates of anode and cathode are alternately stacked with a separator in between. A capacitor element with a wound structure is a laminate in which strip-shaped anodes and cathodes are alternately stacked with a separator in between, and the strips are wound so that the short sides of the strips are along the winding axis and the long sides of the strips are wrapped around it.

[0049] The electrolyte is impregnated into the voids within the capacitor element by immersing it in the electrolyte. To ensure the electrolyte penetrates even finer voids, vacuum or pressure treatment may be performed as needed. The electrolyte impregnation process may be repeated multiple times. For example, the inside of the capacitor element may be depressurized, and the electrolyte may be injected into the capacitor element while simultaneously pressurizing the electrolyte.

[0050] Lead terminals for the anode and cathode are drawn out from this capacitor element. The lead terminals are connected to the anode and cathode, respectively. The lead terminals are electrical conductors that electrically connect the electrolytic capacitor to the mounting board. The lead terminals are electrically and mechanically connected to the anode and cathode by methods such as stitch connections, cold welding, ultrasonic welding, or laser welding.

[0051] The capacitor element, impregnated with electrolyte and with lead terminals extended, is housed in a case with a closed bottom at one end and an open end at the other, and sealed with a sealing body. The case is made of aluminum, an aluminum alloy containing manganese, or stainless steel, and the sealing body is attached to the opening of the case and crimped from the outside of the case. In addition to being a cylindrical body with a closed bottom at one end and an open end at the other, the case may also be made of, for example, a laminate film. Alternatively, the case may be formed by molding the capacitor element with a resin such as a heat-resistant resin or an insulating resin, or the case may be formed on the capacitor element in the form of a thin film using a method such as dip coating or printing.

[0052] When the electrolytic capacitor is a wound type, after forming the powder layer on the foil body of the anode, a division may be formed before the conversion treatment of the dielectric film. Alternatively, the division may be formed after the conversion treatment. Alternatively, the conversion treatment may be performed before the formation of the division, and then again after the formation of the division. However, since the anode body is wound while opening the division during the element formation process, the conversion treatment is not performed to close the division before the element formation process, nor is repair conversion performed to close the division. Most preferably, a dielectric film is also formed on the inner surface of the division.

[0053] The division section divides the powder layer. The division section is formed by cracking, splitting, cutting, notching, or carving into the surface of the powder layer. In other words, examples of actual division sections are cracks, fissures, cuts, notches, or carvings. This division section may be deep in the direction from the surface of the powder layer toward the core, reaching the deepest part of the powder layer, not reaching the deepest part of the powder layer, or carving into the foil body. The depth of all division sections does not need to be uniform.

[0054] The divisions generally extend in the width direction of the strip, in other words, along the winding axis of the winding. The divisions extend either completely or partially across the powder layer. That is, some divisions extend from one long side of the powder layer to the other long side. Others extend from one long side of the powder layer to below or beyond the centerline of the anode, but not to the other long side. Others extend from the other long side of the powder layer to below or beyond the centerline of the anode, but not to the other long side. The direction and length of all divisions do not need to be uniform.

[0055] The groove width of the divided portion is preferably 50 μm or less, including 0, when the anode body is flattened without curving. The groove width of the divided portion is the length along the longitudinal direction of the anode body. If the divided portion is formed by cracking, splitting, or cutting, the groove width of the divided portion becomes substantially 0. Substantially 0 means that when the anode body is flattened without curving, the interface of the divided portion is at least partially in contact. If the groove width of the divided portion is 50 μm or less, the decrease in capacitance of the electrolytic capacitor due to the reduction in the surface area of ​​the dielectric film can be suppressed.

[0056] In an anode body with a divided section, the divided section opens first during winding, and multiple divided sections share and bear the bending stress. Therefore, when winding to form the capacitor element, a smooth, well-curved winding of the anode body is possible, and the formation of numerous microcracks in the anode body becomes less likely. Numerous microcracks increase leakage current, reducing the leakage current suppression effect of increasing the moisture content of the capacitor element and using powder laminated foil as the anode body. However, by forming a divided section in the powder layer, the occurrence of microcracks is suppressed, and the leakage current of the electrolytic capacitor is further suppressed.

[0057] (action) Such electrolytic capacitors are equipped with powder laminated foil as the anode. The powder layer of the powder laminated foil has a porous structure consisting of voids between aggregated valve-acting metal particles. This powder layer expands the surface area of ​​the anode, giving it a surface area more than one times its projected area. In other words, this powder layer is the surface expansion layer of the anode. The powder laminated foil anode has a dense and large surface area due to the expansion layer, which increases the capacitance of the electrolytic capacitor.

[0058] However, the powder layer is prone to exposing the valve metal base metal because the bonds between valve metal particles are easily broken by the stress generated by the electrostriction of the dielectric film. While the water contained in the electrolyte reduces the ESR of the electrolytic capacitor, it also causes hydration degradation of the valve metal base metal exposed in the powder layer when it comes into contact with it. Hydration degradation of the valve metal base metal in the powder layer increases the leakage current of the electrolytic capacitor. In particular, although electrostriction of the dielectric film occurs in conventional tunnel-shaped etching pits or sponge-like or spongy etching pits when voltage is applied, the effect of electrostriction is especially large in powder laminated foil due to its structure that aggregates the valve metal particles.

[0059] Therefore, the electrolyte contains a compound having a carboxyl group. The compound having a carboxyl group forms an oxide film on the powder layer through a repair reaction, repairing the exposure of the valve metal base metal. However, this compound having a carboxyl group undergoes an esterification reaction with the alcohols contained in the electrolyte. Due to the esterification reaction, the compound having a carboxyl group disappears. When the concentration of the compound having a carboxyl group in the electrolyte decreases, the repair reaction of the electrolyte is reduced or lost. When the repair reaction of the electrolyte is reduced or lost, hydration degradation in the powder layer due to moisture in the electrolyte becomes dominant, and the increase in leakage current of the electrolytic capacitor cannot be suppressed.

[0060] Furthermore, this electrolytic capacitor contains compounds with carboxyl groups and alcohols in the electrolyte, and the cathode is equipped with a conductive layer. The presence of a conductive layer in the cathode slows down the esterification reaction and suppresses the consumption rate of the compounds with carboxyl groups. As a result, the regenerative chemical reaction by the compounds with carboxyl groups can counteract hydration degradation in the powder layer due to moisture in the electrolyte over a long period of time, and the increase in leakage current of the electrolytic capacitor over time can be suppressed.

[0061] Furthermore, while the moisture contained in electrolytic capacitors is mainly from the electrolyte, moisture from the air also gets mixed into the capacitor element during the manufacturing process. Therefore, when the capacitor element is removed from the case and the liquid component is extracted from the capacitor element by centrifugal force, the total amount of liquid component extracted from the capacitor element is considered to be the electrolyte, and the amount of moisture contained in this liquid component is considered to be the moisture content of the electrolyte. In order to maintain the low ESR of the electrolytic capacitor while suppressing the increase in leakage current over time, it is preferable that the moisture content of the electrolyte be adjusted to 10 wt% or less based on the electrolyte. [Examples]

[0062] The electrolytic capacitor of the present invention will be described in more detail below based on the following examples. However, the present invention is not limited to the following examples.

[0063] (Example 1) An electrolytic capacitor for high-voltage applications, Example 1, was fabricated, with a cylindrical shape having a diameter of 12 mm and a height of 20 mm, and a rated voltage of 450 V. This electrolytic capacitor of Example 1 comprises an anode body made of powder laminated foil, a cathode body having a carbon layer as a conductive layer, and an electrolyte composed of water, alcohols, and a compound having a carboxyl group.

[0064] A strip of aluminum foil was used as the substrate for the anode. Aluminum powder was laminated onto the aluminum substrate and sintered to form a powder layer. A chemical conversion treatment was then performed on this powder-laminate foil to form a dielectric film. Disruptions were created in the powder layer.

[0065] A strip of aluminum foil was used as the cathode. The aluminum foil was subjected to AC etching to form a surface-expanding layer consisting of sponge-like etching pits on both sides of the foil, thereby obtaining the cathode foil. In the AC etching process, the cathode foil was immersed in an acidic aqueous solution, and an electric current was applied to the substrate to expand the surface on both sides of the aluminum foil. Next, the aluminum foil was subjected to a chemical conversion treatment to form an oxide film on the surface of the surface-expanding layer. In the chemical conversion treatment, after removing the chlorine adhering during the AC etching process, a voltage was applied in an aqueous solution of ammonium dihydrogen phosphate.

[0066] The carbon layer of the cathode body contained carbon black as the carbon material. Specifically, a slurry was prepared by mixing and kneading carbon black powder, styrene-butadiene rubber (SBR) as a binder, and an aqueous solution of carboxymethylcellulose ammonium (CMC-NH3) as a dispersant.

[0067] This slurry was uniformly applied to the cathode foil. The slurry was then heated and dried to evaporate the solvent, and the cathode body was then pressed. In the pressing process, the cathode body was sandwiched between press rollers, and a press line pressure of 5.38 kN / cm was applied to fix the carbon layer onto the cathode foil.

[0068] Lead terminals consisting of aluminum wire and metal wire were used for the anode and cathode. Specifically, the aluminum wire consisted of a flat portion formed by pressing one end of a round bar and an unpressed round bar portion at the other end, with the tip of the round bar portion and the metal wire connected by arc welding or the like. The flat portion was stitched to the cathode and anode.

[0069] A separator was placed between the anode and cathode during winding. A double-layered paper was used as the separator, with kraft paper on the high-density side and a cotton linter and hemp blend on the low-density side.

[0070] The anode, cathode, and separator windings were impregnated with an electrolyte. The electrolyte consisted of water and ethylene glycol (an alcohol) as solvents, with azelaic acid (a compound containing a carboxyl group) added. The water content of the electrolyte was 4.7 wt%. The water content of the electrolyte was measured after the electrolytic capacitor was fabricated, the electrolytic capacitor was disassembled, and the amount of water contained in the liquid component extracted from the capacitor element was measured, with the extracted liquid component considered as the electrolyte.

[0071] After impregnating the winding with electrolyte, the lead terminals leading out from the winding were inserted through through holes formed in a butyl rubber sealing body, and metal wires were led out. The winding and sealing body were then inserted into an aluminum case and sealed with the sealing body. After sealing with the sealing body, the electrolytic capacitor was subjected to an aging treatment.

[0072] (Comparative Example 1) An electrolytic capacitor of Comparative Example 1 was fabricated. The electrolytic capacitor of Comparative Example 1 differs from that of Example 1 in that the cathode body does not have a laminated carbon layer. The other components of Comparative Example 1 are the same as those of Example 1, and it was manufactured using the same manufacturing method and under the same manufacturing conditions. Specifically, the electrolytic capacitor of Comparative Example 1 comprises an anode body made of powder laminated foil, a cathode body without a laminated carbon layer, and an electrolyte composed of water, alcohols, and a compound having a carboxyl group. The cathode body is made of strip-shaped aluminum foil, and by AC etching treatment using the same method and conditions as in Example 1, an expanded surface layer consisting of sponge-like etching pits is formed on both sides of the foil.

[0073] (Reference example 1) An electrolytic capacitor of Reference Example 1 was fabricated. The electrolytic capacitor of Reference Example 1 differs from that of Example 1 in that the anode is not made of powder laminated foil, but rather the anode is enlarged by etching. The other components of Reference Example 1 are the same as those of Example 1, and it was manufactured using the same manufacturing method and under the same manufacturing conditions. A strip of aluminum foil was used as the anode of Reference Example 1. The aluminum foil was subjected to DC etching to form an enlarged layer consisting of tunnel-shaped etching pits. That is, the electrolytic capacitor of Reference Example 1 comprises an anode enlarged by etching, a cathode with a carbon layer laminated, and an electrolyte composed of water, alcohols, and a compound having a carboxyl group.

[0074] (Reference example 2) An electrolytic capacitor of Reference Example 2 was fabricated. The electrolytic capacitor of Reference Example 2 differs from that of Example 1 in that the anode is not made of powder laminated foil, but rather the anode is enlarged by etching. Also, the electrolytic capacitor of Reference Example 2 differs from that of Example 1 in that it has a cathode without a laminated carbon layer. The other components of Reference Example 2 are the same as those of Example 1, and were manufactured using the same manufacturing method and conditions. The anode of Reference Example 2 has the same configuration as the anode of Reference Example 1, and was manufactured using the same manufacturing method and conditions. Also, the cathode of Reference Example 2 has the same configuration as that of Comparative Example 1, and was manufactured using the same manufacturing method and conditions. In other words, the electrolytic capacitor of Reference Example 2 comprises an anode enlarged by etching, a cathode without a laminated carbon layer, and an electrolyte composed of water, alcohols, and a compound having a carboxyl group.

[0075] (Characteristic testing) The capacitance [μF] and leakage current [mA] were measured for the electrolytic capacitors of Example 1, Comparative Example 1, Reference Example 1, and Reference Example 2. For each measurement, a DC voltage of 475V was continuously applied to each electrolytic capacitor at a temperature of 105°C for 3000 hours. The capacitance [μF] and leakage current [mA] were measured both immediately before this high-temperature voltage load test and after 3000 hours had elapsed.

[0076] Capacitance was measured using an LCR meter (Agilent ZM2376, manufactured by NF Circuit Design Block Co., Ltd.). The ambient temperature during measurement was 25°C, with a DC bias of 1.5V, an AC current level of 1.0Vrms sine wave, and a measurement frequency of 120Hz. Leakage current was measured using an oscilloscope after 20 minutes of applying 475V in a 105°C environment.

[0077] The results of this characteristic test are shown in Table 1 below. In the table, the initial values ​​show the measurement results immediately before the high-temperature voltage load test. (Table 1) TIFF2026054908000001.tif146164

[0078] As shown in Table 1 above, the leakage current of the electrolytic capacitor in Comparative Example 1 increased to 180 mA after 3000 hours, indicating that the electrolytic capacitor in Comparative Example 1 is defective. On the other hand, as shown in Table 1 above, the leakage current of the electrolytic capacitor in Example 1 only increased to 10.5 mA after 3000 hours, indicating that the electrolytic capacitor in Example 1 remains in good condition.

[0079] The electrolytic capacitor of Example 1 has a carbon layer as the cathode, a powder laminated foil as the anode, and the electrolyte is composed of water, ethylene glycol (an alcohol), and azelaic acid (a compound having a carboxyl group). As a result, even though the anode is a powder laminated foil, the increase in leakage current seen in Comparative Example 1 is prevented.

[0080] Furthermore, as shown in Reference Examples 1 and 2, in the case of electrolytic capacitors equipped with an anode body enlarged by etching, even if the electrolyte is composed of water, alcohols, and compounds having carboxyl groups, no increase in leakage current over time occurs, regardless of whether or not the cathode body has a carbon layer. In other words, it can be confirmed that the increase in leakage current over time is a problem specific to electrolytic capacitors equipped with anode bodies made of powder laminated foil.

Claims

1. Anode and, A cathode body facing the anode body, A separator interposed between the anode and the cathode, An electrolyte interposed between the anode and the cathode, Equipped with, The anode body is A foil body of valve-acting metal, A powder layer consisting of valve-acting metal powder, which is laminated on the foil body of the anode, Dielectric film and, It has, The cathode body is A foil body of valve-acting metal, A conductive layer laminated on the foil body of the cathode body, It has, The electrolyte contains water, alcohols, and compounds having carboxyl groups. An electrolytic capacitor characterized by the following features.

2. The amount of water contained in the electrolyte is 10 wt% or less relative to the electrolyte. The electrolytic capacitor according to claim 1, characterized by the above.

3. The conductive layer contains carbon. The electrolytic capacitor according to claim 1 or 2, characterized by the above.

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