Solid electrolytic capacitor

Abstract

To improve reliability for a long term, in a solid electrolytic capacitor using a porous anode body containing tantalum.SOLUTION: A solid electrolytic capacitor includes: a porous anode body containing tantalum; a dielectric layer covering at least a portion of the anode body; and a solid electrolyte layer covering at least a portion of the dielectric layer. The dielectric layer contains a tantalum element, an oxygen element, and a first element other than tantalum element and oxygen element. The dielectric layer includes a first layer that covers at least a portion of the anode body. The first layer includes an IA layer that covers at least a portion of the anode body, and an IB layer that covers at least a portion of the IA layer. The content of the first element in the IB layer is higher than the content of the first element in the IA layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to solid electrolytic capacitors. [Background technology]

[0002] A solid electrolytic capacitor includes, for example, a solid electrolytic capacitor element and a resin outer casing or case that seals the solid electrolytic capacitor element. The solid electrolytic capacitor element includes, for example, an anode body, a dielectric layer formed on the surface of the anode body, and a solid electrolyte layer that covers at least a portion of the dielectric layer. The anode body may be made of a metal foil containing a valve metal or a porous sintered body of particles containing a valve metal (e.g., a porous sintered body containing elemental tantalum). When the anode body is a sintered body, the dielectric layer is formed by subjecting the anode body to a chemical conversion treatment using, for example, a chemical conversion solution containing an acid or its salt.

[0003] Patent Document 1 proposes a method for producing an anode foil for a solid electrolytic capacitor having a dielectric film by chemically treating a valve metal having micropores, characterized in that the chemical treatment includes a step of performing primary chemical formation with an electrolyte containing at least one acid selected from the group consisting of oxalic acid, adipic acid, boric acid, phosphoric acid, silicic acid, and salts thereof, and a step of performing secondary chemical formation with an electrolyte having a different composition from the primary chemical formation electrolyte. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-124068 Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional solid electrolytic capacitors using porous anode bodies containing tantalum tend to have larger leakage currents when exposed to high temperatures than those using anode foils, resulting in a decrease in capacity and a loss of long-term reliability. [Means for solving the problem]

[0006] One aspect of the present disclosure provides a battery comprising: a porous anode body containing tantalum; a dielectric layer covering at least a portion of the anode body; and a solid electrolyte layer covering at least a portion of the dielectric layer, the dielectric layer contains tantalum, oxygen, and a first element other than tantalum and oxygen; the dielectric layer includes a first layer covering at least a portion of the anode body, the first layer includes an IA layer covering at least a portion of the anode body and an IB layer covering at least a portion of the IA layer, The present invention relates to a solid electrolytic capacitor, wherein the content of the first element in the IB layer is higher than the content of the one element in the IA layer. [Effects of the Invention]

[0007] In a solid electrolytic capacitor using a porous anode body containing tantalum, the long-term reliability can be improved. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view of a solid electrolytic capacitor according to an embodiment of the present disclosure. [Figure 2] FIG. 10 is a graph showing the relationship between the depth of the dielectric layer and the intensity of the oxygen element and the first element (phosphorus element) in a time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis of the dielectric layer formed on the anode body of Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0009] In Patent Document 1, an aluminum foil is used as an anode body, and the dielectric layer formed on the surface of the aluminum foil by chemical conversion contains an oxide containing aluminum. On the other hand, in the case of a porous anode body containing tantalum (such as a sintered body of tantalum-containing particles), the dielectric layer formed on the surface by chemical conversion contains an oxide containing tantalum. In addition to containing an oxide containing tantalum, such a dielectric layer must be formed deep inside the anode body because the entire anode body is porous. Therefore, in the case of a porous anode body containing tantalum, it is difficult to form a homogeneous dielectric layer over the entire surface, and such a dielectric layer is more susceptible to deterioration due to volume change when exposed to a high-temperature environment than a dielectric layer formed on the surface of aluminum foil. Therefore, when a porous anode body containing tantalum is used, a large leakage current is likely to occur.

[0010] (Technology 1) In view of the above, a solid electrolytic capacitor according to one aspect of the present disclosure includes a porous anode body containing tantalum, a dielectric layer covering at least a portion of the anode body, and a solid electrolyte layer covering at least a portion of the dielectric layer. The dielectric layer includes elemental tantalum, elemental oxygen, and a first element other than tantalum and oxygen. The dielectric layer includes a first layer covering at least a portion of the anode body. The first layer includes an IA layer covering at least a portion of the anode body and an IB layer covering at least a portion of the IA layer. The content of the first element in the IB layer is higher than the content of the one element in the IA layer.

[0011] The first element is, for example, a doping element introduced into a dielectric layer formed by chemical conversion. In the present disclosure, the dielectric layer includes a first layer composed of an IA layer covering at least a portion of the anode body and an IB layer covering at least a portion of the IA layer. The IA layer and the IB layer have different contents of the first element. This layer configuration can reduce leakage current when the solid electrolytic capacitor is exposed to a high-temperature environment. Reducing leakage current allows the solid electrolytic capacitor to maintain a high capacity for a long period of time, thereby improving the long-term reliability of the solid electrolytic capacitor.

[0012] Although the details of why the above-described layer structure reduces leakage current are unclear, it is believed that the presence of the first element and the above-described layer structure of the dielectric layer make the dielectric layer relatively flexible. Forming such a dielectric layer on the surface of a porous anode containing tantalum is thought to alleviate stress applied to the dielectric layer when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby suppressing deterioration of the dielectric layer (such as the occurrence of cracks). As a result, it is thought that leakage current is reduced when the solid electrolytic capacitor is exposed to a high-temperature environment.

[0013] The IA layer and the IB layer may be, but are not necessarily, distinct from each other. The first layer may be formed so that the content of the first element increases from the IA layer side toward the IB layer side. That is, the first layer may have an IA region close to the anode body and an IB region far from the anode body. The same applies to each of the second to n-th layers described below.

[0014] (Technology 2) In the above (Technology 1), the dielectric layer may further include an IIA layer covering at least a portion of the IB layer. In this case, the content of the first element in the IB layer is higher than the content of the first element in the IIA layer. In the dielectric layer, the IA layer, the IB layer, and the IIA layer overlap with layers with low, high, and low first element contents. This further facilitates stress relief when the solid electrolytic capacitor is exposed to a high-temperature environment, further reducing leakage current.

[0015] (Technology 3) In the above (Technology 2), the dielectric layer may further include an IIB layer covering at least a portion of the IIA layer. The IIA layer and the IIB layer constitute a second layer. The content of the first element in the IIB layer is higher than the content of the first element in the IIA layer. In the dielectric layer, the IA layer, the IB layer, the IIA layer, and the IIB layer, as well as layers with low and high first element contents, are alternately stacked. This further facilitates stress relief when the solid electrolytic capacitor is exposed to a high-temperature environment, further reducing leakage current.

[0016] (Technology 4) In the above (Technology 3), the dielectric layer may further include a IIIA layer covering at least a portion of the IIB layer. In this case, the content of the first element in the IIB layer is higher than the content of the first element in the IIIA layer. In this case, as in the above (Technology 2) or (Technology 3), layers with low and high contents of the first element are alternately stacked in the dielectric layer. This further facilitates stress relief when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby further reducing leakage current.

[0017] (Technology 5) In the above (Technology 4), the dielectric layer may further include a IIIB layer covering at least a portion of the IIIA layer. The IIIA layer and the IIIB layer constitute a third layer. The content of the first element in the IIIB layer is higher than the content of the first element in the IIIA layer. In this case, as in the above (Technology 2) to (Technology 4), layers with low and high contents of the first element are alternately stacked in the dielectric layer. This further facilitates stress relief when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby further reducing leakage current.

[0018] (Technology 6) In the above (Technology 5), the dielectric layer may further include an IVA layer covering at least a portion of the IIIB layer. The content of the first element in the IIIB layer is higher than the content of the first element in the IVA layer. In this case, as in the above (Technology 2) to (Technology 5), layers with low and high contents of the first element are alternately stacked in the dielectric layer. This further facilitates stress relief when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby further reducing leakage current.

[0019] (Technology 7) In the above (Technology 6), the dielectric layer may further include an IVB layer covering at least a portion of the IVA layer. The IVA layer and the IVB layer constitute a fourth layer. The content of the first element in the IVB layer is higher than the content of the first element in the IVA layer. In this case, as in the above (Technology 2) to (Technology 6), layers with low and high contents of the first element are alternately stacked in the dielectric layer. This further facilitates stress relief when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby further reducing leakage current.

[0020] (Technology 8) In any one of the above (Technology 1) to (Technology 7), the content of the first element in the IB layer may be 0.5 atomic % or more. In this case, the difference in the content of the first element between the IA layer and the IB layer is likely to be relatively large. This enhances the effect of alleviating stress when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby further reducing leakage current.

[0021] (Technology 9) In any one of the above (Technology 1) to (Technology 8), the content of the first element in the IA layer may be less than 0.5 atomic %. In this case, the difference in the content of the first element between the IA layer and the IB layer is likely to be relatively large. This enhances the effect of alleviating stress when the solid electrolytic capacitor is exposed to a high-temperature environment, thereby further reducing leakage current.

[0022] (Technology 10) In any one of the above (Technology 1) to (Technology 9), the first element may be an element selected from the group consisting of phosphorus, boron, silicon, and carbon, which has the highest content in the dielectric layer. These elements are easily doped into the dielectric layer by chemical conversion using a chemical conversion solution. In this specification, when the dielectric layer contains multiple first elements, the content in each layer is compared, focusing on one first element (in other words, the element with the highest content).

[0023] (Technology 11) In any one of the above (Technology 1) to (Technology 10), the first element may be phosphorus. In this case, flexibility of the dielectric layer is further increased, and leakage current can be further suppressed when the solid electrolytic capacitor is exposed to a high-temperature environment.

[0024] The content of the first element in the dielectric layer is determined by TOF-SIMS. Either a negative mode or a positive mode may be used for the analysis depending on the chemical species being analyzed. The TOF-SIMS analysis is performed while etching the dielectric layer from the outermost surface in the depth direction (e.g., from the outermost surface of the dielectric layer toward the anode body).

[0025] TOF-SIMS analysis of the dielectric layer is carried out under the following conditions after removing minute deposits on the surface of the dielectric layer with a gas cluster ion beam (GCIB). Equipment: TRIFTV nano TOF (ULVAC-PHI) Primary ion conditions: 30Au3200 bunching mode Use Neutralization Gun Primary ion species: Au3+ (=Au trimer) Accelerating voltage: 30 kV Bunching mode (pulsed primary ion beam irradiation) Primary ion irradiation area (= measurement area): 100 μm x 100 μm (when measuring the outermost surface) 50μm×50μm (depth direction analysis) Sputtering conditions Sputtered ion species: Au3+ continuous beam Raster range: 150μm x 150μm

[0026] The solid electrolytic capacitor of the present disclosure will be described in more detail below, including the above (Technology 1) to (Technology 11), with reference to the drawings as necessary. At least one of the above (Technology 1) to (Technology 11) may be combined with at least one of the elements described below, provided that this is not technically inconsistent.

[0027] [Solid electrolytic capacitor] The solid electrolytic capacitor according to the present disclosure includes an anode body, a dielectric layer covering at least a portion of the anode body, and a cathode portion covering at least a portion of the dielectric layer. The cathode portion includes at least a solid electrolyte layer and may include a cathode extraction layer. The cathode portion typically includes the solid electrolyte layer covering at least a portion of the dielectric layer and the cathode extraction layer covering at least a portion of the solid electrolyte layer.

[0028] (anode body) The anode body contains tantalum. Examples of such anode bodies include a sintered body of particles containing tantalum. This sintered body is entirely porous. A porous anode body has a large surface area, so a high capacity can be obtained. The sintered body can be obtained, for example, by sintering a compact of particles containing tantalum. The particles before sintering contain, for example, at least one tantalum material selected from the group consisting of tantalum, a tantalum alloy, and a tantalum compound.

[0029] The anode body has a cathode forming portion. A cathode portion including a solid electrolyte layer is formed on the surface of the cathode forming portion of the anode body. The anode body has a shape of, for example, a rectangular parallelepiped.

[0030] (anode wire) An anode wire for current collection is embedded in the anode body, such as a sintered body. The anode body and the anode wire together form an anode part.

[0031] The anode wire includes a valve metal, such as aluminum, tantalum, niobium, titanium, etc. The anode wire may include tantalum as the valve metal.

[0032] One end of the anode wire is embedded in the anode body, and the other end protrudes outward from the end face of the anode body. The anode wire is used for electrical connection to an external electrode on the anode side. Specifically, the anode wire is connected to, for example, an anode lead terminal connected to an external terminal.

[0033] (dielectric layer) The dielectric layer is an insulating layer that functions as a dielectric. The dielectric layer is formed by anodizing tantalum on the surface of the anode body, for example, by chemical conversion treatment. In the present disclosure, the dielectric layer contains elemental tantalum, elemental oxygen, and a first element other than elemental tantalum and elemental oxygen. The dielectric layer formed by anodizing a porous anode body containing tantalum contains a tantalum compound (e.g., oxide) such as Ta2O5. In the dielectric layer, the tantalum compound is doped with the first element. When the dielectric layer contains the tantalum compound, high capacitance can be ensured, and the first element can reduce leakage current.

[0034] The first element may be contained in the dielectric layer in a state bonded to oxygen or as a monoatomic ion. When the dielectric layer is analyzed by TOF-SIMS, the first element is often detected in the form of an oxide. The oxidation number of the first element in the oxide is not particularly limited.

[0035] In the present disclosure, the dielectric layer includes a first layer covering at least a portion of the anode body. This first layer includes an IA layer covering at least a portion of the anode body and an IB layer covering at least a portion of the IA layer. The content of the first element in the IB layer is higher than the content of the first element in the IA layer. By configuring the first layer covering at least a portion of the anode body with the IA layer and the IB layer having different contents of the first element, as described above, it is possible to reduce the leakage current of the solid electrolytic capacitor when exposed to a high-temperature environment. Furthermore, since the content of the first element in the outer IB layer is higher than the content of the first element in the IA layer, the oxygen defect level in the first layer tends to be more easily stabilized, which is advantageous for further reducing the leakage current.

[0036] The dielectric layer may further include a layer IIA covering at least a portion of the layer IB. The content of the first element in the layer IB may be higher than the content of the first element in the layer IIA. The dielectric layer may further include a layer IIB covering at least a portion of the layer IIA. In this case, the layers IIA and IIB constitute a second layer. The content of the first element in the layer IIB may be higher than the content of the first element in the layer IIA.

[0037] Furthermore, the dielectric layer may include a IIIA layer covering at least a portion of the IIB layer. The content of the first element in the IIB layer may be higher than the content of the first element in the IIIA layer. Furthermore, the dielectric layer may include a IIIB layer covering at least a portion of the IIIA layer. The IIIA layer and the IIIB layer constitute a third layer. The content of the first element in the IIIB layer may be higher than the content of the first element in the IIIA layer.

[0038] Furthermore, the dielectric layer may include an IVA layer covering at least a portion of the IIIB layer. The content of the first element in the IIIB layer may be higher than the content of the first element in the IVA layer. Furthermore, the dielectric layer may include an IVB layer covering at least a portion of the IVA layer. The IVA layer and the IVB layer constitute a fourth layer. The content of the first element in the IVB layer may be higher than the content of the first element in the IVA layer.

[0039] In these cases, layers with low and high first element contents alternate in the dielectric layer. This further alleviates internal stress when the solid electrolytic capacitor is exposed to a high-temperature environment. As a result, leakage current is further reduced. The present disclosure is not limited to the case where the dielectric layer has the first to fourth layers as described above, but also includes cases where the dielectric layer has an nA layer covering at least a portion of the (n-1)B layer and an nth layer covering at least a portion of the (n-1)B layer. For example, when n = 5 (= V), cases where a VA layer covering at least a portion of the IVB layer and a 5th layer covering at least a portion of the IVB layer are also included. n may be 5 or more and 10 or less. The nth layer may have an nA layer covering at least a portion of the (n-1)B layer and an nB layer covering at least a portion of the nA layer, similar to the first to fourth layers. In this case, the relationship between the contents of the first element in each layer may be the same as the first to fourth layers. From the viewpoint of easily ensuring better long-term reliability, it is preferable that the dielectric layer includes at least the first layer and the second layer, and in addition to these layers, it may further include a IIIA layer, a third layer, a third layer and an IVA layer, or a third layer and a fourth layer, etc.

[0040] The content of the first element in the IB layer may be 0.5 atomic % or more, 1.0 atomic % or more, or 1.5 atomic % or more. The content of the first element in the IB layer within this range is advantageous in further reducing leakage current. To ensure higher capacity, the content of the first element in the IB layer may be 5.0 atomic % or less. The content of the first element in each of the IB, IIIB, IVB, and nB layers may be selected from the ranges described above for the content of the first element in the IB layer.

[0041] The content of the first element in the IA layer may be less than 0.5 atomic %, 0.4 atomic % or less, or 0.1 atomic % or less. The content of the first element in the IA layer is 0 atomic % or more. The content of the first element within the above range is advantageous in further reducing leakage current. The content of the first element in each of the IIA layer, IIIA layer, IVA layer, and nA layer may be selected from the range described above for the content of the first element in the IA layer. Each of the IA layer, IIA layer, IIIA layer, IVA layer, and nA layer may not contain the first element. This case also includes cases where the content of the first element in each layer is below the detection limit.

[0042] In the first layer, an interface may or may not be formed between the IA layer and the IB layer. On the other hand, an interface is usually observed between the first layer and the second layer (or the IIA layer). When no interface is formed between the IA layer and the IB layer, the average thickness T1 of the first layer may be divided into two equal layers, the IA layer on the anode body side, and the IB layer on the opposite side of the anode body. In the present disclosure, the IA layer and the IB layer thus divided also have the difference in content of the first element as described above. The same can be said for the second to n-th layers as well as the first layer.

[0043] The content of the first element at the center of the thickness of the IA layer is r1, and the content of the second element at the center of the thickness of the IB layer is r2. In this case, the ratio of r2 to r1 (=r2 / r1) may be 10 or more, or may be 15 or more. When the ratio r1 / r2 is in this range, it is possible to further reduce leakage current while ensuring higher capacity.

[0044] The first element may include at least one element selected from the group consisting of elements of Groups 1 to 14 of the periodic table. In particular, the first element may be an element selected from the group consisting of phosphorus, boron, silicon, and carbon, which has the highest content in the dielectric layer. The first element may be phosphorus.

[0045] The average thickness T of the entire dielectric layer may be 15 nm or more and 300 nm or less. When the thickness T is in this range, a higher capacitance can be obtained. Even when the thickness T is relatively small, such as 200 nm or less (e.g., 15 nm or more and 200 nm or less, or 20 nm or more and 150 nm or less), the layer structure of the dielectric layer and the concentration gradient of the first element are advantageous in further reducing leakage current.

[0046] The thickness of the dielectric layer and each layer constituting the dielectric layer is determined from an image taken with a scanning electron microscope (SEM). The average thickness of the dielectric layer is determined by measuring the thickness of the dielectric layer at any number of locations (for example, 10 locations) and averaging the measurements. For example, an anode body on which a dielectric layer is formed is used as the measurement sample. In this case, a sintered body of tantalum particles with a side length of 10 mm or more and 50 mm or less is used as the anode body. Hereinafter, this sample will be referred to as Sample A. An anode body on which a dielectric layer is formed, obtained in the process of manufacturing a solid electrolytic capacitor, may also be used as the sample. Hereinafter, this sample will be referred to as Sample B.

[0047] More specifically, a cross section parallel to the thickness direction of sample A or sample B is first exposed, and an image is taken using an SEM. The thickness of the dielectric is determined at any multiple locations within the field of view of any multiple locations on this image. The sample for taking the above cross-sectional image is obtained by the following procedure: First, sample A or sample B is embedded in a curable resin, and the curable resin is cured. The cured product is wet- or dry-polished to expose a cross section parallel to the thickness direction of the dielectric (a cross section in which the layer structure of the dielectric can be confirmed). The exposed cross section is smoothed by ion milling to obtain the sample for imaging. The thickness (average thickness) of each of the first and second layers is also determined in the same manner as for the thickness of the dielectric.

[0048] The dielectric layer is formed, for example, by anodizing the anode body using a chemical conversion solution. For example, the anode body may be chemically treated using a chemical conversion solution ia to form an IA layer so as to cover at least a portion of the anode body, and the anode body having the IA layer may be chemically treated using a chemical conversion solution ib to form an IB layer. In this case, the chemical conversion solution ib contains a higher concentration of the first element than the chemical conversion solution ia. Alternatively, the anode body may be chemically treated using the first chemical conversion solution, and the first layer having the IA layer and the IB layer may be formed by appropriately diffusing the first element. These chemical conversion solutions may contain the first element in any form. The chemical conversion solution may contain the first element, for example, as a compound or salt containing the first element, as an acid, or as an ion (monoatomic ion, polyatomic ion, etc.).

[0049] For example, when the first element is phosphorus, an aqueous solution containing phosphoric acid (such as orthophosphoric acid) or a phosphate may be used as the chemical conversion solution. When the first element is boron, an aqueous solution containing boric acid or a borate may be used as the chemical conversion solution. When the first element is silicon, an aqueous solution containing silicic acid or a silicate may be used as the chemical conversion solution. When the first element is carbon, an aqueous solution containing a polycarboxylic acid (such as an aliphatic polycarboxylic acid) such as oxalic acid or adipic acid, or a salt thereof may be used as the chemical conversion solution. Examples of salts include, but are not limited to, potassium salts and sodium salts.

[0050] The IIA to IVA layers and the nA layer can be formed by chemically treating an anode body having the IB to IIIB layers or the (n-1)B layer formed thereon using the respective chemical conversion solutions, similar to the case of the IA layer or the first IA layer. The second to fourth layers or the nth layer can be formed by chemically treating an anode body having the first to fourth layers formed thereon using the respective chemical conversion solutions, similar to the case of the first layer.

[0051] The concentration of the first element in the chemical conversion solution forming each of the IA layer, the IIA layer, the IIIA layer, the IVA layer, and the nA layer may be 0% by mass or more and 0.01% by mass or less. When the concentration of the first element in the chemical conversion solution is in this range, the content of the first element in adjacent layers is likely to differ, which is advantageous in further reducing leakage current.

[0052] The concentration of the first element in the chemical conversion solution forming each of the IB, IIB, IIIB, IVB, and nB layers may be 0.10% by mass or more and 2.00% by mass or less. The concentration of the first element in the chemical conversion solution forming each of the first to nth layers may be selected from this range. When the concentration of the first element in the chemical conversion solution is in this range, each layer is easily doped with the first element, and differences in the content of the first element between adjacent layers are likely to occur. This is therefore advantageous in further reducing leakage current.

[0053] The anode body may be anodized while a voltage is applied to the anode body. The voltage (also referred to as oxidation voltage) applied to the anode body may be 30 V or more and 100 V or less, 50 V or more and 100 V or less, or 70 V or more and 100 V or less. The anode body may be anodized at a constant voltage, or may be anodized while increasing the voltage at a predetermined rate, or a combination of these. For example, oxidation may be performed by increasing the voltage at a predetermined rate and maintaining the increased voltage constant for a predetermined period of time. The oxidation voltage is the voltage of the anode body relative to the counter electrode. The rate of increase in voltage may be, for example, 0.5 V / h or more and 15 V / h or less, or 1 V / h or more and 10 V / h or less. The voltage maintained after the increase in voltage may be within the above oxidation voltage range. The time for which the constant voltage is maintained may be 5 hours or more and 50 hours or less, or 10 hours or more and 30 hours or less.

[0054] The anode body may be formed at room temperature or under heating. The temperature of the forming solution during the formation may be 20°C or higher and 100°C or lower, 35°C or higher and 100°C or lower, or 50°C or higher and 100°C or lower.

[0055] When a concentration gradient of the first element in each layer is generated using an anodizing solution for forming each of the first to nth layers, the degree of the concentration gradient may be adjusted by adjusting the anodizing conditions (voltage, temperature, time, etc.).

[0056] (Solid electrolyte layer) The solid electrolyte layer is formed on the cathode forming portion of the anode body via a dielectric layer, and may be a laminate of two or more layers of different solid electrolytes.

[0057] The solid electrolyte constituting the solid electrolyte layer is not particularly limited, and a solid electrolyte used in a known electrolytic capacitor may be used. The solid electrolyte is disposed so as to cover at least a portion of the dielectric layer. The solid electrolyte may be formed using, for example, at least one of a manganese compound and a conductive polymer. The conductive polymer may include, for example, a conjugated polymer and a dopant. The conductive polymer may include a self-doping conductive polymer.

[0058] Examples of conjugated polymers include polypyrrole, polythiophene, polyaniline, and derivatives thereof. These may be used alone or in combination. The conjugated polymer may also be a copolymer of two or more monomers. Note that the derivative of a conjugated polymer refers to a polymer having a conjugated polymer as its basic skeleton. An example of a polythiophene derivative is poly(3,4-ethylenedioxythiophene).

[0059] The dopant can be selected depending on the conjugated polymer, and known dopants may be used. Examples of dopants include compounds capable of generating anions (e.g., aromatic sulfonic acids (e.g., naphthalenesulfonic acid, p-toluenesulfonic acid, etc.) or their salts), polyanions (e.g., polymer-type polyanions (e.g., polystyrenesulfonic acid)), etc. Examples of solid electrolytes include polypyrrole doped with aromatic sulfonic acids and poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonic acid (PSS).

[0060] The solid electrolyte layer may be formed, for example, by polymerizing a precursor of a conjugated polymer (such as a raw material monomer) on a dielectric layer in the presence of a dopant as needed. The solid electrolyte layer may be formed by applying a liquid composition containing the conjugated polymer (and a dopant as needed) to the dielectric layer and then drying the applied composition.

[0061] (Cathode extraction layer) The cathode extraction layer is a conductive layer. The cathode extraction layer is disposed so as to cover at least a portion of the solid electrolyte layer. The configuration of the cathode extraction layer is not particularly limited, and a known cathode extraction layer may be employed. The cathode extraction layer may include, for example, a carbon layer formed on the solid electrolyte layer and a metal particle-containing layer formed on the carbon layer. The carbon layer may include a conductive carbon material such as graphite and a resin. The metal particle-containing layer may include metal particles (e.g., silver particles) and a resin. The metal particle-containing layer may be a silver particle-containing layer formed from a silver paste containing silver particles or silver alloy particles.

[0062] The cathode extraction layer may include a metal foil. The metal foil may be a valve metal (aluminum, tantalum, niobium, etc.) or an alloy containing a valve metal. If necessary, the surface of the metal foil may be roughened. The surface of the metal foil may be provided with a chemical conversion coating, or may be provided with a coating of a metal (dissimilar metal) different from the metal constituting the metal foil or a non-metal. Examples of dissimilar metals and non-metals include metals such as titanium and non-metals such as carbon (e.g., conductive carbon).

[0063] The cathode extraction layer may be electrically connected to one end of a cathode lead terminal that is continuous with the external terminal. The cathode lead terminal is bonded to the cathode extraction layer via a conductive adhesive applied to the cathode extraction layer. An anode wire protruding from the anode body may be electrically connected to one end of the anode lead terminal.

[0064] The other end of the anode lead terminal and the other end of the cathode lead terminal are each drawn out from the resin exterior body or the case. The other end of each terminal exposed from the resin exterior body or the case is used for soldering to a substrate on which the electrolytic capacitor is to be mounted. In addition to drawing out each lead terminal, at least one end face of the anode portion and the cathode portion may be exposed from the outer surface of the sealing body and electrically connected to an external electrode.

[0065] The capacitor element is sealed in a resin outer casing or case. For example, the capacitor element and the resin material of the outer casing (e.g., uncured thermosetting resin and filler) may be placed in a mold, and the capacitor element may be sealed in the resin outer casing by transfer molding, compression molding, or the like. At this time, the other ends of the anode lead terminal and the cathode lead terminal connected to the anode wire drawn from the capacitor element are exposed from the mold.

[0066] FIG. 1 is a cross-sectional view schematically illustrating an example of a solid electrolytic capacitor according to the present disclosure. The solid electrolytic capacitor 100 shown in FIG. 1 includes a capacitor element 110, an anode lead terminal 120, a cathode lead terminal 130, an outer casing 101, and a conductive layer 141. The capacitor element 110 includes an anode portion 111, a dielectric layer 114, and a cathode portion 115. The anode portion 111 includes an anode body 113 and an anode wire 112. The anode body 113 is a sintered body of valve action metal particles and has a rectangular parallelepiped shape. A portion of the anode wire 112 protrudes from a first surface of the anode body 113 toward a front surface 100f of the solid electrolytic capacitor 100. The other portion of the anode wire 112 is embedded in the anode body 113. A dielectric layer 114 is formed on the surface of the anode body 113 and on the portion of the anode wire 112 facing the anode body 113.

[0067] Cathode section 115 includes solid electrolyte layer 116 disposed so as to cover at least a portion of dielectric layer 114, and cathode extraction layer 117 formed on solid electrolyte layer 116. Cathode extraction layer 117 includes, for example, a carbon layer formed on solid electrolyte layer 116, and a metal particle-containing layer formed on the carbon layer. The metal particle-containing layer is formed using, for example, a metal paste (such as a silver paste).

[0068] The anode lead terminal 120 includes an anode terminal portion 121 and a lead connection portion 122. The anode terminal portion 121 is exposed on the bottom surface 100b of the solid electrolytic capacitor 100. The lead connection portion 122 is connected to the anode wire 112. The cathode lead terminal 130 includes a cathode terminal portion 131 and a connection portion 132. The cathode terminal portion 131 is exposed on the bottom surface 100b of the solid electrolytic capacitor 100. The connection portion 132 is electrically connected to the cathode extraction layer 117 by a conductive layer 141.

[0069] [Example] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0070] Example 1 Samples for evaluation were prepared in the following manner, and their properties were evaluated.

[0071] (i) Preparation of the first electrode Tantalum metal particles were molded into a rectangular parallelepiped so that one end of the anode wire made of tantalum metal was embedded in the tantalum metal particles, and the molded body was then sintered in a vacuum to obtain an anode part including an anode body made of a porous tantalum sintered body and an anode wire with one end portion embedded in the anode body and the remaining portion protruding from one surface of the anode body.

[0072] (ii) Formation of oxide film (dielectric layer) An aqueous solution containing sodium phosphate at a concentration of 0.01% by mass was prepared as the anodizing solution ia. A formation tank was filled with the anodizing solution ia, and the anode body was immersed in it. The anodizing solution had a temperature of 50°C to 100°C. The upper end of the wire not immersed in the solution was connected to the counter electrode, and the anodizing voltage was increased at a rate of 1 V / h to 10 V / h until it reached 50 V. The increased voltage was maintained for 10 hours to 30 hours, thereby performing anodizing. In this way, an oxide film of tantalum oxide (IA layer) was formed on part of the surface of the anode body.

[0073] Next, an aqueous solution containing 2.00% by mass of sodium phosphate was prepared as the chemical conversion solution ib. A chemical conversion tank was filled with the chemical conversion solution ib, and the anode element on which the IA layer had been formed was immersed. A phosphorus-containing tantalum oxide oxide film (IB layer) was formed in the same manner as for the IA layer. In this manner, an anode body provided with an oxide film (dielectric layer) having a first layer (IA layer and IB layer) was produced.

[0074] (iii) Formation of a solid electrolyte layer An aqueous dispersion (first treatment liquid) containing a self-doping polythiophene-based polymer (Mw: approximately 10,000) was prepared. The concentration of the polythiophene-based polymer in the first treatment liquid was 1% by mass or more and 4% by mass or less. The tantalum sintered compact having the oxide film obtained in (ii) above was immersed in the first treatment liquid for approximately 30 seconds to 60 seconds, and then the tantalum sintered compact was removed from the dispersion liquid. Next, the tantalum sintered compact removed from the dispersion liquid was heated at a temperature of 140°C to 180°C for 10 minutes to 20 minutes, thereby forming a first solid electrolyte layer.

[0075] The tantalum sintered compact with the first solid electrolyte layer formed thereon was immersed in an aqueous dispersion containing a conductive polymer (PSS-doped PEDOT) at a concentration of 1% by mass to 4% by mass for approximately 30 to 60 seconds, and then removed from the aqueous dispersion. The tantalum sintered compact removed from the liquid dispersion 2A was then dried by heating at a temperature of 140°C to 180°C for 10 to 20 minutes, thereby forming a solid electrolyte. The second solid electrolyte layer was formed by repeatedly immersing in the aqueous dispersion and drying. In this way, a solid electrolyte layer composed of the first solid electrolyte layer and the second solid electrolyte layer was formed.

[0076] (iv) Formation of cathode extraction layer The tantalum sintered body with the solid electrolyte layer formed thereon obtained in (iii) above was immersed in a dispersion liquid in which graphite particles were dispersed in water, and after removal from the dispersion liquid, it was dried to form a carbon layer on the surface of the solid electrolyte layer. The drying was carried out at a temperature of 180°C for 10 to 30 minutes.

[0077] Next, a silver paste containing silver particles and a binder resin (epoxy resin) was applied to the surface of the carbon layer and dried at a temperature of 60°C to 80°C for 20 to 40 minutes. The binder resin was then cured by heating at 180°C for 30 to 60 minutes, forming a metal particle-containing layer. In this way, a cathode extraction layer composed of the carbon layer and the metal particle-containing layer was formed.

[0078] In this way, a capacitor element including a cathode portion composed of a solid electrolyte layer and a cathode extraction layer was fabricated.

[0079] (v) Assembly of solid electrolytic capacitors The cathode extraction layer of the capacitor element obtained in (iv) above was joined to one end of the cathode lead terminal with a conductive adhesive, and the wire protruding from the capacitor element was joined to one end of the anode lead terminal by laser welding.

[0080] Next, a resin exterior body made of insulating resin was formed around the capacitor element by transfer molding, with the other end of the anode lead terminal and the other end of the cathode lead terminal extending out from the exterior body. In this way, a solid electrolytic capacitor was completed.

[0081] Example 2 An aqueous solution having the same composition as the chemical conversion solution ia was prepared as the chemical conversion solution iia. A chemical conversion tank was filled with the chemical conversion solution iia, and an anode element having a first layer formed thereon obtained in the same manner as in Example 1 was immersed therein. Except for this, an oxide film of tantalum oxide (IIA layer) was formed in the same manner as in the case of the IA layer. In this manner, an anode body provided with a dielectric layer having the first layer and the IIA layer was produced, and a solid electrolytic capacitor was produced using this anode body.

[0082] Example 3 An aqueous solution having the same composition as the chemical conversion solution ib was prepared as the chemical conversion solution iib. A chemical conversion tank was filled with the chemical conversion solution iib, and the anode element on which the first layer and the IIA layer had been formed was immersed in the solution in the same manner as in Example 2. Except for this, an oxide film of tantalum oxide containing phosphorus (the IIB layer) was formed in the same manner as in the IB layer of Example 1. In this manner, an anode body provided with a dielectric layer having a first layer and a second layer (a layer IIA and a layer IIB) was produced, and a solid electrolytic capacitor was produced using this anode body.

[0083] Example 4 An aqueous solution having the same composition as the chemical conversion solution ia was prepared as the chemical conversion solution iiia. A chemical conversion tank was filled with the chemical conversion solution iiia, and the anode element on which the first and second layers had been formed was immersed in the same procedure as in Example 3. A tantalum oxide oxide film (IIIA layer) was formed in the same manner as in the case of the IA layer. In this way, an anode body having a dielectric layer including a first layer (layer IA and layer IB), a second layer (layer IIA and layer IIB), and a IIIA layer was produced, and a solid electrolytic capacitor was produced using this anode body.

[0084] Example 5 An aqueous solution having the same composition as the chemical conversion solution ib was prepared as the chemical conversion solution iiib. A chemical conversion tank was filled with the chemical conversion solution iiib, and the anode element on which the first layer, second layer, and IIIA layer had been formed was immersed in the same procedure as in Example 4. Except for this, an oxide film of tantalum oxide containing phosphorus (IIIB layer) was formed in the same manner as in the case of the IB layer. In this way, an anode body was produced that had a dielectric layer having a first layer (layers IA and IB), a second layer (layers IIA and IIB), and a third layer (layers IIIA and IIIB), and a solid electrolytic capacitor was produced using this anode body.

[0085] Comparative Example 1 An oxide film of tantalum oxide (IA layer) was formed on the surface of the anode body using the same procedure as in Example 1. In this manner, an anode body provided with a dielectric layer having a single IA layer was produced, and a solid electrolytic capacitor was produced using this anode body.

[0086] [evaluation] (1) Analysis of oxide film (dielectric) (1-1)TOF-SIMS analysis The evaluation sample of the anode body obtained in (ii) was subjected to TOF-SIMS analysis using the procedure described above to evaluate the element distribution and measure the content of the first element, thereby analyzing the layer structure of the dielectric layer. In the IA, IIA, and IIIA layers, the first element (phosphorus) was barely detected, or if detected, it was in very small amounts (0.4 atomic % or less or 0.1 atomic % or less). In the IB, IIB, and IIIB layers, the content of the first element (phosphorus) was approximately 0.5 atomic % or more and 2.0 mass % or less. FIG. 2 shows the relationship between the depth of the dielectric layer and the intensity of oxygen atoms (O) and phosphorus elements (P) as the first element in the TOF-SIMS analysis of the dielectric layer of Example 5.

[0087] (1-2)SEM analysis The average thickness T of the oxide film (dielectric layer) was determined using the procedure described above. The average thickness T of the dielectric layer in the example was approximately 20 nm or more and 150 nm or less.

[0088] (2) Evaluation of solid electrolytic capacitors The solid electrolytic capacitors were evaluated as follows. (2-1) Long-term reliability A 1 kΩ resistor was connected in series to the solid electrolytic capacitor, and a voltage of 30 V to 60 V was applied using a DC power supply. The leakage current value (LC value, unit: μA) was measured 40 seconds after the start of voltage application, and the average value (initial LC value) of 20 solid electrolytic capacitors was calculated. Next, the solid electrolytic capacitors were subjected to a high-temperature load test. More specifically, solid electrolytic capacitors charged to a rated voltage of 20 V were placed at a rated temperature of 125°C, and the change in the LC value (average value) over time was measured. The LC value (average value) was calculated as the average value of 20 solid electrolytic capacitors using the same procedure as for the initial LC value. The time (h) until the LC value (average value) became three times the initial LC value (average value) was then calculated. The long-term reliability of the solid electrolytic capacitors was evaluated based on this time. The long-term reliability of Comparative Example 1 and Examples 2 to 5 is expressed as a relative value, with the time in Example 1 set as 100. A solid electrolytic capacitor with a high long-term reliability value can be said to have better long-term reliability than one with a low long-term reliability value.

[0089] The results of the Examples and Comparative Examples are shown in Table 1. In Table 1, C1 is Comparative Example 1, and E1 to E5 are Examples 1 to 5, respectively.

[0090] [Table 1]

[0091] As shown in Table 1, the solid electrolytic capacitors of the examples were superior in long-term reliability to the comparative examples (comparison of C1 with E1 to E5). In particular, when the dielectric layer included at least the first layer and the second layer, the long-term reliability was significantly improved (comparison of E1 and E2 with E3 to E5).

[0092] As shown in Figure 2, in the dielectric layer of Example 5 (E5), as the depth increases from the surface (depth = 0), areas with a high phosphorus content (high intensity) and areas with a low phosphorus content (intensity nearly at baseline) alternate. This distribution of phosphorus corresponds to layers IIIB, IIIA, IIB, IIA, IB, and IA from the surface side of the dielectric layer toward the anode body. It is believed that the alternating layers with different contents of the first element (phosphorus) in this way result in excellent long-term reliability, as shown in Table 1. [Industrial Applicability]

[0093] The solid electrolytic capacitor of the present disclosure has low leakage current when exposed to high-temperature environments, can maintain high capacity, and has excellent long-term reliability. Therefore, the solid electrolytic capacitor is suitable for various applications requiring reliability. However, the applications of the solid electrolytic capacitor are not limited to these. [Explanation of symbols]

[0094] 100: Solid electrolytic capacitor 100b: Bottom of electrolytic capacitor 101: Exterior body 110: Capacitor element 111: Anode part 112: Anode wire 113: Anode body 114: Dielectric layer 115: Cathode 116: Solid electrolyte layer 117: Cathode extraction layer 120: Anode lead terminal 121:Anode terminal part 122: Lead connection part 130: Cathode lead terminal 131: Cathode terminal 132: Connection part 141: Conductive layer

Claims

1. a porous anode body containing tantalum, a dielectric layer covering at least a portion of the anode body, and a solid electrolyte layer covering at least a portion of the dielectric layer, the dielectric layer contains tantalum, oxygen, and a first element other than tantalum and oxygen; the dielectric layer includes a first layer covering at least a portion of the anode body, the first layer includes an IA layer covering at least a portion of the anode body and an IB layer covering at least a portion of the IA layer, a content of the first element in the IB layer being higher than a content of the one element in the IA layer;

2. the dielectric layer further includes a layer IIA covering at least a portion of the layer IB; 2. The solid electrolytic capacitor according to claim 1, wherein the content of the first element in the IB layer is higher than the content of the first element in the IIA layer.

3. the dielectric layer further includes an IIB layer covering at least a portion of the IIA layer; the layer IIA and the layer IIB constitute a second layer; The solid electrolytic capacitor according to claim 2 , wherein the content of the first element in the IIB layer is higher than the content of the first element in the IIA layer.

4. the dielectric layer further includes a layer IIIA covering at least a portion of the layer IIB; The solid electrolytic capacitor according to claim 3 , wherein the content of the first element in the IIB layer is higher than the content of the first element in the IIIA layer.

5. the dielectric layer further includes a IIIB layer covering at least a portion of the IIIA layer; the layer IIIA and the layer IIIB constitute a third layer; The solid electrolytic capacitor according to claim 4 , wherein the content of the first element in the IIIB layer is higher than the content of the first element in the IIIA layer.

6. the dielectric layer further includes an IVA layer covering at least a portion of the IIIB layer; The solid electrolytic capacitor according to claim 5 , wherein the content of the first element in the IIIB layer is higher than the content of the first element in the IVA layer.

7. the dielectric layer further includes an IVB layer covering at least a portion of the IVA layer; the IVA layer and the IVB layer constitute a fourth layer; The solid electrolytic capacitor according to claim 6 , wherein the content of the first element in the IVB layer is higher than the content of the first element in the IVA layer.

8. 8. The solid electrolytic capacitor according to claim 1, wherein the content of the first element in the IB layer is 0.5 atomic % or more.

9. 9. The solid electrolytic capacitor according to claim 8, wherein the content of the first element in the IA layer is less than 0.5 atomic percent.

10. The solid electrolytic capacitor according to any one of claims 1 to 7, wherein the first element is an element selected from the group consisting of phosphorus, boron, silicon, and carbon, the element having the highest content in the dielectric layer.

11. 8. The solid electrolytic capacitor according to claim 1, wherein the first element is phosphorus.

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