Capacitor and method for manufacturing capacitor

Abstract

A capacitor (10) includes a first electrode (111), a dielectric layer (112) formed on a surface of the first electrode, an inorganic conductive layer (120) disposed on the dielectric layer, and a second electrode (131) disposed on the inorganic conductive layer (120). The inorganic conductive layer (120) includes inorganic particles arranged so as to form a layer. The main component of the inorganic conductive layer (120) and the main component of the inorganic particles are zinc oxide. If inorganic particles having a major axis of 6 nm or more among the inorganic particles are defined as inorganic particles P, the mode value in the major axis distribution of the inorganic particles P is 50 nm or less.

Description

Capacitor and method for manufacturing the same

[0001] The present disclosure relates to capacitors and methods for manufacturing capacitors.

[0002] Various capacitors have been proposed in the past. Claim 1 of Patent Document 1 (JP 2017-103412 A) describes "a solid electrolytic capacitor comprising an anode body, a dielectric layer disposed on the surface of the anode body, and a solid electrolyte layer disposed on the surface of the dielectric layer and made of zinc oxide having a conductivity of 1 (S / cm) or more."

[0003] Claim 1 of Patent Document 2 (JP 2020-35890 A) describes "a solid electrolytic capacitor comprising an anode body made of a valve metal, a dielectric layer formed on the surface of the anode body, a semiconductor layer formed on the dielectric layer, and a cathode layer formed on the semiconductor layer, wherein the semiconductor layer is formed using a p-type inorganic semiconductor."

[0004] JP 2017-103412 A JP 2020-35890 A

[0005] By using an inorganic layer instead of a solid electrolyte layer formed using a conductive polymer, it is possible to improve the heat resistance of the capacitor. One of the objects of the present disclosure is to provide a capacitor with high heat resistance.

[0006] According to one aspect of the present disclosure, a capacitor includes a first electrode, a dielectric layer formed on the surface of the first electrode, an inorganic conductive layer disposed on the dielectric layer, and a second electrode disposed on the inorganic conductive layer. The inorganic conductive layer includes inorganic particles arranged to form a layer, and the inorganic conductive layer and the inorganic particles are primarily composed of zinc oxide. When inorganic particles P are defined as inorganic particles having a major axis of 6 nm or more, the mode in the major axis distribution of the inorganic particles P is 50 nm or less.

[0007] According to the present disclosure, a capacitor with high heat resistance can be obtained.

[0008] FIG. 1 is a cross-sectional view schematically showing an example of the structure of a capacitor according to the present embodiment. FIG. 2A is an SEM image of a portion of an example element produced in an example. FIG. 2B is an enlarged view of a portion of FIG. 2A. FIG. 2C is a graph showing the results of evaluating the particles in FIG. 2B. FIG. 3A is an SEM image of a portion of another example element produced in an example. FIG. 3B is an enlarged view of a portion of FIG. 3A. FIG. 3C is a graph showing the results of evaluating the particles in FIG. 3B. FIG. 4A is an SEM image of a portion of an element of a comparative example. FIG. 4B is an enlarged view of a portion of FIG. 4A.

[0009] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and other materials may be applied as long as the invention of the present disclosure can be implemented. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or greater and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0010] (Capacitor) Hereinafter, the capacitor according to this embodiment may be referred to as "capacitor (C)." The capacitor (C) includes a first electrode, a dielectric layer formed on the surface of the first electrode, an inorganic conductive layer disposed on the dielectric layer, and a second electrode disposed on the inorganic conductive layer. The inorganic conductive layer includes inorganic particles arranged to form a layer. The main component of the inorganic conductive layer and the main component of the inorganic particles is zinc oxide. When inorganic particles P are inorganic particles having a major axis of 6 nm or more, the mode in the major axis distribution of the inorganic particles P is 50 nm or less.

[0011] Zinc oxide is sometimes classified as a semiconductor, but can also be used as a conductive material. In this specification, "main component" means that the content is 50% by mass or more. The content of the main component may be in the range of 80 to 100% by mass, 90 to 100% by mass, or 95 to 100% by mass. The inorganic conductive layer and inorganic particles may be made of zinc oxide (including zinc oxide containing a dopant).

[0012] The use of an inorganic conductive layer such as zinc oxide can result in a capacitor with high heat resistance. However, after further investigation, the present inventors discovered that forming an inorganic conductive layer using conventional methods can significantly deteriorate the capacitor's characteristics (e.g., dielectric loss tangent). This is believed to be due to internal stresses generated in the inorganic conductive layer, which in turn damage the dielectric layer. After further investigation, the present inventors discovered that the use of an inorganic conductive layer with a specific structure can suppress the deterioration of the capacitor's characteristics (e.g., dielectric loss tangent). The present disclosure is based on this new finding.

[0013] According to the present disclosure, a capacitor with high heat resistance can be obtained. Furthermore, according to the present disclosure, a capacitor with a low dielectric loss tangent can be obtained. Although the reason for this is not clear at present, it is thought that the internal stress of the inorganic conductive layer is reduced by forming at least a portion of the inorganic conductive layer from minute inorganic particles arranged to form a layer.

[0014] The inorganic conductive layer is a layer having electrical conductivity. The inorganic conductive layer is made of an inorganic material or substantially made of an inorganic material. Here, "substantially made of an inorganic material" means that the content of the inorganic material is 95% by mass or more (e.g., 99% by mass or more). The thickness of the inorganic conductive layer is not particularly limited. Among the components constituting the inorganic conductive layer, components other than zinc oxide are not particularly limited, and may be inorganic materials other than zinc oxide (e.g., zinc compounds other than zinc oxide).

[0015] At least the surface of the inorganic conductive layer (the surface on the second electrode side) is composed of inorganic particles arranged to form a layer. The entire inorganic conductive layer may be composed of inorganic particles arranged to form a layer. The surface of the inorganic conductive layer (the surface on the second electrode side) or the entire inorganic conductive layer may be composed of an aggregate of inorganic particles. The inorganic particles may be conductive. At least some of the inorganic particles may be in contact with each other. At least some of the inorganic particles may be bonded to each other.

[0016] In this specification, inorganic particles constituting the inorganic conductive layer, those having a major axis of 6 nm or more, may be referred to as “inorganic particles P.” More specifically, the inorganic particles P are particles having a major axis of 6 nm or more as measured in a scanning electron microscope image.

[0017] The mode (the longest diameter with the greatest frequency) in the long diameter distribution of the inorganic particles P is 50 nm or less. The mode may be 40 nm or less, or 22 nm or less. The mode may be 6 nm or more, or 13 nm or more.

[0018] The average aspect ratio of the inorganic particles P may be 1.5 or less, or 1.4 or less. By setting the average aspect ratio to 1.5 or less, the internal stress of the inorganic conductive layer can be reduced, and damage to the dielectric layer can be suppressed. The aspect ratio of the inorganic particles can be calculated using the following formula: Aspect ratio = (major axis of inorganic particle) / (minor axis of inorganic particle). The average major axis of the inorganic particles P may be 30 nm or less, or 25 nm or less. By setting the average major axis of the inorganic particles P to 30 nm or less, the internal stress of the inorganic conductive layer can be reduced, and damage to the dielectric layer can be suppressed. The average major axis of the inorganic particles P may be 6 nm or more, or 9 nm or more. The major axis of the largest peak in the major axis distribution of inorganic particles P having a major axis of 6 nm or more among the inorganic particles may be 50 nm or less.

[0019] The inorganic conductive layer satisfies the following condition (C1). In addition to condition (C1), condition (C2) and / or (C3) may also be satisfied. By satisfying condition (C1), a capacitor with a low dielectric tangent can be obtained. In the following conditions (C1) to (C3), the mode in the major axis distribution of the inorganic particles P, the average aspect ratio of the inorganic particles P, and the average major axis of the inorganic particles P may be replaced with the ranges described above. (C1) The mode in the major axis distribution of the inorganic particles P having a major axis of 6 nm or more is 50 nm or less. (C2) The average aspect ratio of the inorganic particles P having a major axis of 6 nm or more is 1.5 or less. (C3) The average major axis of the inorganic particles P having a major axis of 6 nm or more is 30 nm or less.

[0020] It is also believed that a capacitor (C') using an inorganic conductive layer that must satisfy condition (C2) and / or (C3) will have good characteristics. The inorganic conductive layer of capacitor (C') may or may not satisfy condition (C1).

[0021] The mode, average aspect ratio, average major axis, and peak of the major axis distribution of inorganic particles P are determined by analysis using a scanning electron microscope. Specifically, a portion of the cross section of a first electrode on which an inorganic conductive layer is formed, including the inorganic conductive layer, is arbitrarily selected to obtain a scanning electron microscope image. The magnification is set to 100,000 times or more. Next, a 400 nm x 400 nm range is arbitrarily selected from the obtained scanning electron microscope image. If the surface layer of the first electrode is porous, a 400 nm x 400 nm range in the porous portion of the surface layer is selected. That is, if the surface layer of the first electrode is porous, inorganic particles present in the porous portion (pore portion) of the surface layer are analyzed. Next, the major axis (maximum diameter) and minor axis in the image are measured for each inorganic particle present in the selected range. The minor axis is the maximum diameter in the direction perpendicular to the major axis. Next, the aspect ratio of each inorganic particle is calculated using the above formula. However, particles with a major axis of less than 6 nm are not analyzed.

[0022] The major axis distribution, average aspect ratio, and average major axis are determined from the measurement results for all observable inorganic particles P present in an image within a 400 nm x 400 nm range. The major axis distribution is obtained by creating a graph with the major axis on the horizontal axis and the number of inorganic particles P on the vertical axis. The average aspect ratio is determined by arithmetically averaging the measured aspect ratios. The average major axis is determined by arithmetically averaging the measured major axes. Note that if the number of inorganic particles P present in an image within a 400 nm x 400 nm range is less than 50, the range of images used for analysis is expanded until the number of inorganic particles P is 50 or more. In other words, the mode of the major axis distribution, average aspect ratio, average major axis, and peak of the major axis distribution are determined by analyzing 50 or more (preferably 100 or more) arbitrarily selected inorganic particles P. As described above, the mode of the major axis distribution, average aspect ratio, average major axis, and peak of the major axis distribution are each analytical values ​​in a scanning electron microscope image. The peak of the major axis distribution is the maximum value of the curve obtained by fitting the graph of the major axis distribution.

[0023] The mode in the major axis distribution is determined by the following procedure. The major axis (nm) of the inorganic particles P is determined in nm units by rounding the measured value (nm) to the nearest tenth. The mode in the major axis distribution is the major axis with the highest frequency when comparing the frequencies for each nm. When there are multiple modes, the arithmetic mean of these modes is used as the mode.

[0024] The main component of the inorganic conductive layer is zinc oxide (ZnO). Zinc oxide is sometimes classified as a semiconductor, but can also be used as a conductive material. In this specification, "main component" means that the content is 50% by mass or more. The content of the main component may be in the range of 80 to 100% by mass, 90 to 100% by mass, or 95 to 100% by mass. The inorganic conductive layer may be made of zinc oxide.

[0025] Zinc oxide may contain oxygen vacancies. By containing oxygen vacancies, the electrical conductivity of zinc oxide can be increased. Zinc oxide is ZnO １－ｘ(0≦x≦0.25), where x may be greater than 0. In this specification, the term "ZnO" encompasses zinc oxide containing oxygen vacancies unless otherwise specified. Furthermore, for the sake of simplicity, zinc oxide to which a dopant has been added may be referred to as "zinc oxide" or "ZnO."

[0026] The zinc oxide constituting the inorganic conductive layer may or may not contain a dopant. By adding a dopant, it is possible to improve the conductivity of the zinc oxide.

[0027] The zinc oxide may contain a dopant. The inclusion of a dopant in the zinc oxide can increase the electrical conductivity of the zinc oxide. The dopant is not particularly limited, and known dopants may be used. The dopant may be at least one element selected from the group consisting of Al, Ga, B, and In. The content of the dopant in the zinc oxide may be 0.1 atomic % or more, or 0.5 atomic % or more of Zn, and may be 10 atomic % or less, or 5.0 atomic % or less of Zn.

[0028] The first electrode may be an anode and the second electrode may be a cathode, examples of which are described below.

[0029] The first electrode may be mainly composed of a valve metal. For example, the first electrode may be mainly composed of aluminum or tantalum. An example of the first electrode is aluminum foil. Another example of the first electrode is a tantalum sintered body obtained by sintering tantalum particles.

[0030] The first electrode may have a porous portion in at least a portion of its surface layer, or may have a porous portion in the entire surface. In this case, the dielectric layer may be formed on at least a portion of the surface of the porous portion, or may be formed on the entire surface. Furthermore, at least a portion of the inorganic conductive layer may be disposed in at least a portion of the voids in the porous portion, or may be disposed in all of the voids. By using a first electrode having a porous portion, the electrostatic capacitance can be increased.

[0031] The inorganic conductive layer may be a layer formed by heating a material containing zinc. It is believed that the use of such an inorganic conductive layer can particularly reduce the internal stress of the inorganic conductive layer. The zinc-containing material may be a material containing a zinc compound (e.g., a zinc salt). Zinc oxide can be produced by heating and thermally decomposing the zinc compound. Examples of methods for forming the inorganic conductive layer will be described later.

[0032] (Method for manufacturing a capacitor) The manufacturing method according to this embodiment may be referred to as "manufacturing method (M)" below. Manufacturing method (M) is a method for manufacturing a capacitor. According to manufacturing method (M), capacitor (C) can be manufactured. However, capacitor (C) may also be manufactured by a method other than manufacturing method (M). Since the matters described for capacitor (C) can be applied to manufacturing method (M), duplicated explanations may be omitted. The matters described for manufacturing method (M) may also be applied to capacitor (C).

[0033] The manufacturing method (M) includes a first step of forming an inorganic conductive layer on a dielectric layer formed on the surface of a first electrode, and a second step of forming a second electrode on the inorganic conductive layer. The inorganic conductive layer is mainly composed of zinc oxide. The first step includes a step (a) of disposing a solution containing a zinc compound on the dielectric layer, and a step (b) of heating the solution to form the inorganic conductive layer.

[0034] (Step (a)) In step (a), a solution in which a zinc compound is dissolved is placed on the dielectric layer. The solvent of the solution is selected depending on the type of zinc compound. The solvent may be at least one selected from the group consisting of water, an organic solvent, or a mixture thereof. The solvent may be a liquid in which the zinc compound dissolves and which evaporates during heating in step (b). In other words, the solvent may be a liquid whose boiling point is lower than the heating temperature in step (b).

[0035] The zinc compound used is a compound that forms zinc oxide upon heating in step (b). The zinc compound may be a salt. An additive may be added to the solution as needed. For example, an additive may be added to the solution to increase the solubility of the zinc compound.

[0036] (Step (b)) In step (b), an inorganic conductive layer is formed on the dielectric layer by heating the solution on the dielectric layer. In step (b), zinc oxide can be produced by thermally decomposing a zinc compound. For example, when the zinc compound is zinc acetate, zinc acetate is thermally decomposed by heating to produce zinc oxide. Heating conditions are selected depending on the solution. The heating method is not particularly limited, and known heating methods (e.g., heating with a heater) may be used. The atmosphere in which the heating step of step (b) is performed is not particularly limited, and the heating step may be performed in air or in an atmosphere other than air (e.g., in an inert gas or water vapor). Step (b) may also be performed under reduced pressure.

[0037] In the manufacturing method (M), the steps (a) and (b) may be repeated. The number of times a cycle consisting of one step (a) and one step (b) is performed may be 2 or more, or 5 or more, and may be 40 or less, or 15 or less. By repeating the cycle, it is possible to increase the thickness of the inorganic conductive layer.

[0038] Examples of zinc compounds include zinc acetate (e.g., zinc acetate dihydrate) and zinc nitrate (e.g., zinc nitrate hexahydrate). Examples of solvents include alcohol, acetone, acetonitrile, ethyl ether, 2-ethoxyethyl acetate, tetrahydrofuran, and water. Examples of alcohols include methanol, ethanol, isopropyl alcohol, 2-methoxyethanol, 1-hexanol, cyclohexanol, 1,2-propanediol, and 1,3-propanediol. Multiple zinc compounds may be dissolved in the solution. The solvent may be a mixed solvent. A compound containing a dopant may be dissolved in the solution. An example of a compound containing a dopant is aluminum nitrate (e.g., aluminum nitrate nonahydrate). An additive to increase the solubility of the zinc compound may be added to the solution. Examples of such additives include monoethanolamine, diethanolamine, triethanolamine, and mercaptopurine.

[0039] An example of the solution is prepared by dissolving zinc acetate and aluminum nitrate in 2-methoxyethanol. It is preferable that monoethanolamine be added to the 2-methoxyethanol. The concentration of zinc acetate may be in the range of 0.01 to 10 mol / L (e.g., 0.1 to 5 mol / L). Aluminum nitrate may be added so that the amount of dopant falls within the above-mentioned range. The concentration of monoethanolamine may be in the range of 0.01 to 10 mol / L (e.g., 0.1 to 5 mol / L).

[0040] When zinc acetate is used as the zinc compound, the heating temperature in step (b) may be 150° C. or higher, or 200° C. or higher, and 400° C. or lower, or 300° C. or lower. In this case, the heating time may be 1 minute or longer, or 5 minutes or longer, and 30 minutes or shorter, or 15 minutes or shorter.

[0041] The manufacturing method (M) may further include a heat treatment step after steps (a) and (b). The heat treatment step is carried out at a temperature higher than that of the heating step in step (b). Heat treatment at a high temperature can improve the conductivity of the inorganic conductive layer. The heat treatment temperature in the heat treatment step may be 250°C or higher and 500°C or lower. In this case, the heat treatment time in the heat treatment step may be 10 minutes or higher and 5 hours or lower. Conditions other than the temperature and time may be the same as those described for step (b).

[0042] The manufacturing method (M) may include, instead of steps (a) and (b), step (A) of disposing a material containing a zinc compound on a dielectric layer, and step (B) of forming the inorganic conductive layer by heating the material. The heating in step (B) can thermally decompose the zinc compound to produce zinc oxide.

[0043] Step (A) may be performed by placing a solution containing a dissolved zinc compound on the dielectric layer and then drying the solution. After performing step (a), step (A) can be performed by proceeding partway through step (b). The subsequent step (b) corresponds to step (B). As with steps (a) and (b), steps (A) and (B) may be repeated in manufacturing method (M).

[0044] After the first step, a second step is performed in which a second electrode is formed on the inorganic conductive layer. A capacitor (capacitor element) is obtained by the second step. The method for forming the second electrode is not particularly limited. Examples of the method for forming the second electrode will be described later.

[0045] Thereafter, other processes may be performed as necessary. In one example manufacturing method, first, a first lead is connected to the first electrode, and a second lead is connected to the second electrode. Next, the capacitor element, a portion of the first electrode, and a portion of the second electrode are sealed with a sealing resin (exterior body). These processes can be performed by known methods.

[0046] Examples of components of the capacitor (C) are described below. The components of the capacitor (C) are not limited to the following examples. Components used in known capacitors may be applied to components other than those characteristic of the present disclosure.

[0047] (First Electrode) The first electrode includes a valve metal. Examples of valve metals include aluminum (Al), titanium (Ti), tantalum (Ta), niobium (Nb), zirconium (Zr), and hafnium (Hf). The first electrode may be a foil of the valve metal. Alternatively, the first electrode may be a sintered body of particles containing the valve metal. The particles may be particles of the valve metal, particles of an alloy containing the valve metal, or particles of a compound containing the valve metal. The first electrode may be an aluminum foil or a sintered body of tantalum particles.

[0048] The first electrode may have a porous portion on the surface layer. When the first electrode is a foil, the porous portion can be formed on the surface layer of the foil by etching the foil.

[0049] When the first electrode is a sintered body of particles containing a valve metal, a porous portion exists in the surface layer of the sintered body. When the first electrode is a sintered body, the capacitor (C) may include a lead (e.g., a lead wire) partially embedded in the sintered body. The lead may include a valve metal. The valve metal contained in the lead and the valve metal contained in the first electrode may be different or the same. In one example of the capacitor (C), the first electrode is a tantalum sintered body, and the lead is a tantalum lead.

[0050] The method for forming the sintered body (first electrode) is not particularly limited, and known methods may be used. In one example of a method for forming the sintered body, raw material particles are first pressure-molded into a predetermined shape to obtain a molded body. At this time, a portion of the lead wire is embedded in the molded body. The median diameter (D50, volume basis) of the raw material particles may be 0.05 μm or more and 0.5 μm or less. The median diameter of the raw material particles can be determined using a laser diffraction particle size distribution measuring device. Next, the molded body is sintered to obtain a sintered body (first electrode) in which a portion of the anode body is embedded.

[0051] (Dielectric Layer) The dielectric layer may be formed so as to cover at least a portion of the first electrode. The dielectric layer may be formed by chemically treating the first electrode. By chemically treating the first electrode, a dielectric layer (a layer of an oxide of a valve metal) is formed on the surface of the first electrode. For example, an aluminum oxide layer is formed by anodizing an aluminum foil. A tantalum oxide layer is formed by anodizing a tantalum sintered body. When a porous portion is present in the surface layer of the first electrode, the dielectric layer is formed on the surface of the porous portion of the first electrode.

[0052] (Inorganic Conductive Layer) The inorganic conductive layer has been described above, so a duplicated description will be omitted.

[0053] (Second Electrode) The second electrode is a conductive layer. The second electrode may be formed using conductive carbon or metal. Specifically, the second electrode may be formed using a carbon paste containing conductive carbon particles or a metal paste containing metal particles. Alternatively, the second electrode may include a layer consisting only of metal (a vapor deposition layer or metal foil). Examples of conductive carbon include graphite, carbon black, graphene flakes, carbon nanotubes, etc. The metal paste is a paste containing metal particles. Examples of metal pastes include silver paste containing silver particles, etc. The carbon paste and metal paste are not particularly limited, and commercially available pastes may be used.

[0054] The second electrode may include a first layer formed on the inorganic conductive layer and a second layer formed on the first layer. In this case, the first layer may be a carbon layer containing conductive carbon, and the second layer may be a metal particle layer (e.g., a silver particle layer) formed from a metal paste. The carbon layer can be formed by applying a carbon paste and then heating. The metal particle layer can be formed by applying a metal paste and then heating.

[0055] (Leads and Exterior Body) The capacitor (C) may include other components as necessary. For example, the capacitor (C) may include leads and an exterior body. The leads and exterior body are not particularly limited, and known leads and exterior bodies may be used.

[0056] (Structure of Capacitor (C)) The capacitor (C) may include only one capacitor element. Alternatively, the capacitor (C) may include multiple capacitor elements. For example, the capacitor (C) may include multiple capacitor elements connected in parallel.

[0057] Examples of embodiments according to the present disclosure will be specifically described below with reference to the drawings. The components described above can be applied to the components of the examples described below. Furthermore, the examples described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiments. Furthermore, in the embodiments described below, components that are not essential for the capacitor according to the present disclosure may be omitted. Note that the following drawings are schematic and may differ from the actual configuration.

[0058] (Embodiment 1) Fig. 1 is a cross-sectional view schematically illustrating a portion of a capacitor according to embodiment 1. The capacitor 10 shown in Fig. 1 includes a first electrode 111, a dielectric layer 112, an inorganic conductive layer 120, and a second electrode 131. While an actual capacitor typically includes leads and an exterior body, Fig. 1 shows only the capacitor element. The inorganic conductive layer 120 includes inorganic particles arranged to form a layer, but the particles are not shown in Fig. 1. Examples of the configuration of the inorganic conductive layer 120 are shown in SEM images in the examples.

[0059] The dielectric layer 112 is formed so as to cover at least a portion of the surface of the first electrode 111. The inorganic conductive layer 120 is formed so as to cover at least a portion of the dielectric layer 112. The second electrode 131 is formed so as to cover at least a portion of the inorganic conductive layer 120. The inorganic conductive layer 120 is the inorganic conductive layer described above. The inorganic conductive layer 120 is in contact with the dielectric layer 112 and the second electrode 131.

[0060] The first electrode 111 shown in FIG. 1 has a porous portion 111a on its surface. While the actual porous portion 111a may have a more complex shape, FIG. 1 shows the porous portion 111a in a simplified form. At least a portion of the inorganic conductive layer 120 is disposed within the voids of the porous portion 111a. The capacitor 10 can be formed without using materials with low heat resistance (such as conductive polymers or electrolytes), and therefore can achieve high heat resistance.

[0061] (Additional Note) The above description discloses the following techniques.

[0062] (Technology 1) A capacitor comprising: a first electrode; a dielectric layer formed on a surface of the first electrode; an inorganic conductive layer disposed on the dielectric layer; and a second electrode disposed on the inorganic conductive layer; the inorganic conductive layer comprises inorganic particles disposed to form a layer; the inorganic conductive layer and the inorganic particles each comprise zinc oxide as a main component; and when inorganic particles P have a major axis of 6 nm or greater, the mode in a major axis distribution of the inorganic particles P is 50 nm or less.

[0063] (Technology 2) The capacitor according to Technology 1, wherein the inorganic particles P have an average aspect ratio of 1.5 or less.

[0064] (Technology 3) The capacitor according to Technology 1 or 2, wherein the inorganic particles P have an average major axis of 30 nm or less.

[0065] (Technology 4) The capacitor according to any one of technologies 1 to 3, wherein the zinc oxide contains a dopant.

[0066] (Technology 5) The capacitor according to any one of Technologies 1 to 4, wherein the first electrode is an anode and the second electrode is a cathode.

[0067] (Technology 6) The capacitor according to any one of Technologies 1 to 5, wherein the first electrode is mainly composed of aluminum or tantalum.

[0068] (Technology 7) The capacitor according to any one of Technologies 1 to 6, wherein the first electrode has a porous portion on a surface thereof, the dielectric layer is formed on the surface of the porous portion, and at least a portion of the inorganic conductive layer is disposed in the voids of the porous portion.

[0069] (Technology 8) The capacitor according to any one of Technologies 1 to 7, wherein the inorganic conductive layer is a layer formed by heating a material containing zinc.

[0070] The capacitor (C) according to the present disclosure will be described in more detail by way of examples.

[0071] (Experiment 1) In Experiment 1, a plurality of capacitors (capacitor elements) having different conductive layers were fabricated and evaluated. ZnO layers were used as the conductive layers.

[0072] (Capacitor A1) Capacitor A1 was produced by the following method.

[0073] (1) Formation of tantalum sintered body and dielectric layer Tantalum powder (median diameter: 80 μm) was prepared as the raw material powder for the first electrode (anode). Next, the tantalum powder was filled into a predetermined molding die, and one end of an anode wire was embedded in the tantalum powder. A tantalum wire was used as the anode wire. The tantalum powder was then pressure-molded to obtain a rectangular solid body.

[0074] Next, the compact was sintered under reduced pressure at a temperature in the range of 1300 to 1400° C. In this way, a porous body (tantalum sintered body) in which a part of the anode wire was embedded was obtained.

[0075] Next, the tantalum sintered body was subjected to a chemical conversion treatment (anodization) to form a tantalum oxide layer (dielectric layer) on the surface of the tantalum sintered body.

[0076] (2) Formation of Inorganic Conductive Layer (ZnO Layer) A ZnO layer (inorganic conductive layer) was formed on the dielectric layer on the surface of the tantalum sintered compact by the following procedure. First, a zinc acetate solution was prepared by dissolving zinc acetate dihydrate, aluminum nitrate nonahydrate, and monoethanolamine in 2-methoxyethanol (solvent). The aluminum nitrate nonahydrate is a compound used to dope aluminum into the ZnO layer. The concentration of zinc acetate dihydrate in the zinc acetate solution was 1.0 mol / L. The concentration of monoethanolamine in the zinc acetate solution was 1.0 mol / L. The molar concentration of aluminum nitrate nonahydrate was 0.01 times that of zinc acetate dihydrate.

[0077] Next, the tantalum sintered body on which the dielectric layer was formed was immersed in a zinc acetate solution for 3 minutes and then removed. Thereafter, the tantalum sintered body with the zinc acetate solution attached thereto was heated at 260°C for 10 minutes. This immersion and heating process was repeated 10 times. In this way, an aluminum-doped ZnO layer was formed on the surface of the dielectric layer. Thereafter, the tantalum sintered body on which the ZnO layer was formed was heat-treated at 400°C for 1 hour. In this way, an element a1 on which a ZnO layer was formed was obtained.

[0078] (3) Formation of the Second Electrode A second electrode (cathode) was formed on the ZnO layer of element a1 by the following procedure. First, a carbon paste containing carbon particles and a dispersion medium was prepared. Next, the carbon paste was applied to the ZnO layer and then heated to form a carbon layer on the ZnO layer. Next, a silver paste was applied to the carbon layer and then heated to form a silver particle layer. In this way, a second electrode consisting of a carbon layer and a silver particle layer was formed. Capacitor A1 was fabricated in this manner.

[0079] (Evaluation of Electrical Characteristics) The capacitance X1 and dielectric loss tangent Tanδ of the fabricated capacitor A1 were measured. Furthermore, element a1, before the formation of the second electrode, was immersed in an electrolyte, and the capacitance X0 of element a1 was measured in this state. The capacitance and dielectric loss tangent measurements were performed using the following equipment and conditions: Measurement equipment: LCR meter (Agilent, E4980A) AC conditions: 500 mV, 120 Hz (DC bias: 0). Because the electrolyte reaches deep into the porous portion, the capacitance X0 is considered to be close to the theoretical capacity of element a1. The capacitance acquisition rate was calculated using the following formula: Capacitance acquisition rate (%) = 100 × X1 / X0 (Analysis of the Inorganic Conductive Layer) The cross section of element a1, before the formation of the second electrode, was exposed. The cross section was exposed by breaking element a1, before the formation of the second electrode. Next, the cross section was observed at 100,000 times magnification using a scanning electron microscope (SEM), and an SEM image was obtained. An example of an SEM image is shown in FIG. 2A. FIG. 2A is an SEM image of a portion of an example element produced in the example. As shown in FIG. 2A, the SEM image shows a tantalum sintered body (Ta) and tantalum oxide (Ta) formed on the surface of the tantalum sintered body. ２ O ５ ) and a zinc oxide (ZnO) layer formed on the tantalum oxide were observed. The zinc oxide layer was composed of an aggregate of ZnO particles. The square in Figure 2A represents an arbitrarily selected 400 nm × 400 nm region. An enlarged view of this square is shown in Figure 2B. Figure 2B is an enlarged view of a portion of Figure 2A. Using the image in Figure 2B, the long diameter distribution, average aspect ratio, and average long diameter were determined using the method described above. Figure 2B shows the long diameters of some particles. The ZnO particle analysis was performed on all observable particles in the image in Figure 2B that had a long diameter of 6 nm or more. The number of particles with a long diameter of 6 nm or more was 232. Note that, as shown in Figure 2B, the particles analyzed also included those exposed within the voids in the porous portion. In other words, particles whose cross sections were not visible were also analyzed.

[0080] The graph of the major diameter distribution obtained by the analysis is shown in Figure 2C. In the graph of Figure 2C, the horizontal axis represents the major diameter of the particles, and the vertical axis represents the number of particles (frequency).

[0081] (Capacitor A2) Element a2 was fabricated in the same manner and under the same conditions as element a1, except that the zinc acetate solution used to form the ZnO layer was changed. Then, capacitor A2 was fabricated in the same manner and under the same conditions as element A1, except that element a2 was used instead of element a1.

[0082] The zinc acetate solution used to prepare element a2 was identical to that used to prepare element a1, except that the concentrations of zinc acetate dihydrate and monoethanolamine were each 1.3 mol / L. The molar concentration of aluminum nitrate nonahydrate was 0.01 times that of zinc acetate dihydrate.

[0083] (Capacitor A3) Element a3 was fabricated in the same manner and under the same conditions as element a1, except that the zinc acetate solution used to form the ZnO layer was changed. Then, capacitor A3 was fabricated in the same manner and under the same conditions as element A1, except that element a3 was used instead of element a1.

[0084] The zinc acetate solution used to prepare element a3 was the same as that used to prepare element a1, except that the concentrations of zinc acetate dihydrate and monoethanolamine were each 1.6 mol / L. The molar concentration of aluminum nitrate nonahydrate was 0.01 times that of zinc acetate dihydrate.

[0085] (Capacitor C1) Element c1 was fabricated using the same method and conditions as element a1, except for the method of forming the ZnO layer. Then, capacitor C1 was fabricated using the same method and conditions as capacitor A1, except for using element c1 instead of element a1. The zinc oxide layer (ZnO layer) of element c1 was formed using the following method.

[0086] First, a seed crystal layer made of ZnO was formed on a tantalum oxide layer (dielectric layer) by atomic layer deposition (ALD). Next, an aluminum-doped ZnO layer was grown on the seed crystal layer by liquid phase epitaxy. Chemical bath deposition (CBD) was used as the liquid phase epitaxy. An attempt was made to form the seed crystal layer by sputtering, but the sputtering method failed to form a seed crystal layer inside the porous material.

[0087] Liquid phase growth was performed according to the following procedure. First, an aqueous solution containing zinc nitrate hexahydrate and hexamethylenetetramine dissolved in a molar ratio of 1:1 was prepared. The concentration of zinc nitrate hexahydrate in the aqueous solution was set to 0.05 mol / L. Furthermore, aluminum nitrate nonahydrate was dissolved in this aqueous solution. The molar concentration of aluminum nitrate nonahydrate was set to 0.01 times that of zinc nitrate hexahydrate.

[0088] Next, 2 L of the aqueous solution was placed in a tank and heated to 85°C. The aqueous solution was circulated at a flow rate of 1 L / min, and the tantalum sintered compact on which the seed crystal layer had been formed was immersed in the aqueous solution to cause liquid phase growth of a ZnO layer. In this way, an aluminum-doped ZnO layer was formed on the dielectric layer, and element c1 was obtained.

[0089] The fabricated elements a2, a3, and c1 were evaluated in the same manner as element a1. Furthermore, the fabricated capacitors A2, A3, and C1 were evaluated in the same manner as capacitor A1.

[0090] An example of an SEM image (magnification: 100,000 times) of element a3 is shown in Figure 3A. Figure 3A is an SEM image of a portion of another example element fabricated in the example. Figure 3B shows an enlarged view of the squared area in Figure 3A (an arbitrarily selected area of ​​400 nm x 400 nm). Figure 3B is an enlarged view of a portion of Figure 3A. Using the image in Figure 3B, the major axis distribution, average aspect ratio, and average major axis were determined by the method described above. Figure 3B shows the major axes of some particles. Analysis of ZnO particles was performed on all observable particles in the image in Figure 3B that had a major axis of 6 nm or more.

[0091] The graph of the major diameter distribution obtained by the analysis is shown in Figure 3C. In the graph of Figure 3C, the horizontal axis represents the major diameter of the particles, and the vertical axis represents the number of particles (frequency).

[0092] An example of an SEM image (magnification: 100,000 times) of element c1 is shown in FIG. 4A . FIG. 4A is an SEM image of a portion of a comparative element. FIG. 4B shows an enlarged view of the squared area in FIG. 4A (an arbitrarily selected 400 nm × 400 nm area). FIG. 4B is an enlarged view of a portion of FIG. 4A . Using the image in FIG. 4B , the major axis distribution, average aspect ratio, and average major axis were determined by the method described above. FIG. 4B shows the major axes of some particles. Analysis of ZnO particles was performed on all observable particles in the image in FIG. 4B that had a major axis of 6 nm or greater. Note that because the number of particles with a major axis of 6 nm or greater present in FIG. 4B was less than 50, another arbitrarily selected area (a 400 nm × 400 nm area) was also analyzed, and 50 particles with a major axis of 6 nm or greater were analyzed.

[0093] As shown in Figures 2B and 3B, the ZnO layers of elements a1 and a3 were composed of an aggregate of small particles. Furthermore, the small particles had a small aspect ratio. That is, the small particles were not columnar, but were relatively spherical in shape. These characteristics were also observed in the ZnO layer of element a2. On the other hand, the ZnO layer of element c1 was formed of an aggregate of columnar crystals with a large aspect ratio. Furthermore, the average long diameter of the particles constituting the ZnO layers of elements a1 to a3 was significantly smaller than the average long diameter of the particles constituting the ZnO layer of element c1.

[0094] Some of the manufacturing conditions and the evaluation results are shown in Table 1. Tan δ in Table 1 indicates the measured dielectric tangent. The smaller the value of Tan δ, the smaller the loss. Therefore, a small Tan δ is preferable. A high capacitance acquisition rate indicates that the electrostatic capacitance is close to the theoretical capacity. Therefore, a high capacitance acquisition rate is preferable.

[0095]

[0096] Capacitors A1 to A3 are examples of capacitors (C) according to the present disclosure. Capacitor C1 is a comparative example. As shown in Table 1, the capacitance acquisition rates of capacitors A1 to A3 were higher than that of capacitor C1. Furthermore, the dielectric loss tangents Tanδ of capacitors A1 to A3 were significantly smaller than that of capacitor C1. These results are thought to be due to the different morphologies of the particles constituting the ZnO layer.

[0097] The ZnO layer of capacitor C1 is formed by the crystal growth of ZnO in a predetermined direction. During this process, adjacent crystal grains push against each other while growing into a columnar shape. As a result, stress is applied to the dielectric layer, which is thought to have caused significant damage to the dielectric layer. The poor evaluation result of capacitor C1 is thought to be due to the significant damage to the dielectric layer. On the other hand, the ZnO layers of capacitors A1 to A3 are composed of relatively small particles. Therefore, the stress applied to the dielectric layer is thought to be small, which is thought to have caused minimal damage to the dielectric layer.

[0098] Comparing capacitors A1 to A3, it was found that by lowering the concentration of the zinc acetate solution, the average major axis of the particles and the peak position of the major axis distribution tended to become smaller.

[0099] (Experiment 2) In Experiment 2, a zinc oxide layer (ZnO layer) and a manganese dioxide layer (MnO ２ A layer) was formed and evaluated.

[0100] The ZnO layer was formed using the following procedure. First, the zinc acetate solution used in the fabrication of element a1 was prepared. Next, the zinc acetate solution was applied to a silicon substrate by spin coating. Next, the applied zinc acetate solution was heated at 260°C for 10 minutes. This application and heating process was repeated three times to form a ZnO layer. Then, the silicon substrate with the ZnO layer formed thereon was heat-treated at 400°C for 1 hour. In this way, a ZnO layer was formed on the silicon substrate. Note that a similar process is sometimes described as a sol-gel method in some literature.

[0101] MnO ２The layer was formed by the following procedure. First, an aqueous solution of manganese nitrate hexahydrate was prepared. Next, the manganese nitrate aqueous solution was applied to a silicon substrate by spin coating. Next, the applied zinc acetate solution was heated at 260°C for 10 minutes. This application and heating process was repeated three times. In this way, a MnO layer was formed on the silicon substrate. ２ A layer was formed.

[0102] The formed ZnO layer and MnO ２ The conductivity of each of the layers was measured: the ZnO layer had a conductivity of 185 S / cm, the MnO ２ The conductivity of the ZnO layer was 0.5 S / cm. ２ Therefore, it is preferable to use a ZnO layer as the inorganic conductive layer.

[0103] The present disclosure can be used for capacitors.

[0104] 10: Capacitor 111: First electrode 111a: Porous portion 112: Dielectric layer 120: Inorganic conductive layer 131: Second electrode

Claims

1. A capacitor comprising: a first electrode; a dielectric layer formed on a surface of the first electrode; an inorganic conductive layer disposed on the dielectric layer; and a second electrode disposed on the inorganic conductive layer; the inorganic conductive layer comprises inorganic particles arranged to form a layer; the inorganic conductive layer and the inorganic particles each comprise zinc oxide as a main component; and when inorganic particles P are inorganic particles having a major axis of 6 nm or greater, the mode in the major axis distribution of the inorganic particles P is 50 nm or less.

2. The capacitor according to claim 1, wherein the inorganic particles P have an average aspect ratio of 1.5 or less.

3. The capacitor according to claim 1 or 2, wherein the inorganic particles P have an average major axis of 30 nm or less.

4. The capacitor of claim 1 or 2, wherein the zinc oxide contains a dopant.

5. The capacitor of claim 1 or 2, wherein the first electrode is an anode and the second electrode is a cathode.

6. The capacitor according to claim 1 or 2, wherein the first electrode is mainly composed of aluminum or tantalum.

7. A capacitor according to claim 1 or 2, wherein the first electrode has a porous portion on its surface, the dielectric layer is formed on the surface of the porous portion, and at least a portion of the inorganic conductive layer is disposed within the voids of the porous portion.

8. The capacitor according to claim 1 or 2, wherein the inorganic conductive layer is a layer formed by heating a material containing zinc.

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