Capacitor and method for manufacturing capacitor

Abstract

This disclosed capacitor (10) includes a first electrode (111), a dielectric layer (112) formed on the first electrode (111); an electroconductive layer (120) formed on the dielectric layer (112), and a second electrode (131) disposed on the electroconductive layer (120). The electroconductive layer (120) is composed of an inorganic material that contains a metal oxide. The electroconductive layer (120) constitutes a porous part.

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, including capacitors that do not use a solid electrolyte layer made of a conductive polymer in order to improve heat resistance.

[0003] Claim 1 of Patent Document 1 (JP 2017-103412 A) discloses "a solid electrolytic capacitor comprising an anode body, a dielectric layer disposed on a 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."

[0004] Claim 1 of Patent Document 2 (WO 2023 / 171426) discloses "a capacitor including an anode body having a dielectric layer formed on its surface, and a conductive layer made of a metal oxide formed on the dielectric layer, the conductive layer including a first conductive layer formed on the dielectric layer and a second conductive layer formed on the first conductive layer, the second conductive layer having an average thickness greater than the average thickness of the first conductive layer."

[0005] JP 2017-103412 A International Publication No. 2023 / 171426

[0006] When an inorganic conductive layer is formed on a dielectric layer, the dielectric layer may be destroyed by stress from the inorganic conductive layer, and the capacitor characteristics may not be obtained. One of the objects of the present disclosure is to provide a capacitor having a new structure including an inorganic conductive layer.

[0007] One aspect of the present disclosure relates to a capacitor including a first electrode, a dielectric layer formed on the first electrode, a conductive layer formed on the dielectric layer, and a second electrode disposed on the conductive layer, wherein the conductive layer is made of an inorganic material including a metal oxide, and the conductive layer forms a porous portion.

[0008] Another aspect of the present disclosure relates to a method for manufacturing a capacitor including a first electrode, the method including: a first step of forming a dielectric layer having a needle-like structure on the first electrode; a second step of forming a conductive layer along the needle-like structure to form a porous portion; and a third step of forming a second electrode on the conductive layer, wherein the conductive layer is made of an inorganic material including a metal oxide.

[0009] According to the present disclosure, a capacitor having a novel structure including an inorganic conductive layer is provided. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] Fig. 1 is a cross-sectional view schematically showing a capacitor according to embodiment 1. Fig. 2 is a cross-sectional image showing an example of the shape of a conductive layer.

[0011] Below, embodiments according to the present disclosure will be described 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 according to the present disclosure can be implemented. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more 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 equal to or greater than the upper limit.

[0012] (Capacitor) Hereinafter, the capacitor according to this embodiment may be referred to as a "capacitor (C)." The capacitor (C) includes a first electrode, a dielectric layer formed on the first electrode, a conductive layer formed on the dielectric layer, and a second electrode disposed on the conductive layer. The conductive layer is made of an inorganic material containing a metal oxide. The conductive layer forms a porous portion.

[0013] Forming an inorganic conductive layer on a dielectric layer may result in failure to obtain capacitor characteristics. After investigating the reasons for this, the present inventors discovered that if a conductive layer is formed to be a thick, flat layer, stress from the conductive layer may cause cracks in the dielectric layer. Cracks in the dielectric layer may reduce the performance of the capacitor or prevent it from achieving its intended performance. After further investigation, the present inventors discovered that forming a conductive layer to form porous portions can suppress dielectric breakdown. The present disclosure is based on this new finding. While the reason for suppressing dielectric layer breakdown is unclear, it is believed to be due to reduced stress applied to the dielectric layer. Capacitor (C) uses an inorganic conductive layer and is therefore particularly suitable for applications requiring high heat resistance.

[0014] The first electrode of the capacitor (C) may have pores on its surface. This configuration can increase the capacitance. The conductive layer may have a porous portion (porous structure) within the pores.

[0015] The dielectric layer may have a first needle-like structure on its surface. The conductive layer may be formed along the first needle-like structure. In this case, the conductive layer may have a second needle-like structure along the first needle-like structure. The average diameter D of the needle-like portions of the second needle-like structure in the cross-sectional image may be 100 nm or less. The average diameter D is a diameter determined from the cross-sectional image. Specifically, in the cross-sectional image of the capacitor (C), ten needle-like portions of the second needle-like structure are arbitrarily selected, and the diameters of the needle-like portions at those locations are measured. The average diameter D is then determined by arithmetically averaging the ten measured diameters. The cross-sectional image can be obtained by photographing the cross-section of the capacitor (C) using a scanning electron microscope. The average diameter D may be 10 nm or more, or 20 nm or more.

[0016] The first needle-like structure is disposed inside the second needle-like structure. The first and second needle-like structures may each be linear or may include a curved portion. The first and second needle-like structures may each include a curved portion. In one aspect, the needle-like structure is a linear structure. The needle-like structure (linear structure) may form a network structure.

[0017] Examples of materials for the first electrode include aluminum, tantalum, niobium, titanium, etc. Examples of metal oxides constituting the conductive layer include zinc oxide, titanium oxide, indium oxide, tin oxide, copper chromium oxide, copper indium oxide, etc. The capacitor (C) may satisfy the following conditions (1) and / or (2): (1) The first electrode is aluminum foil, and the dielectric layer includes an aluminum oxide layer. (2) The metal oxide constituting the conductive layer is zinc oxide.

[0018] When the first electrode is an aluminum foil, the needle-like structure may be formed of an aluminum compound. Examples of the aluminum compound include aluminum oxide and aluminum hydroxide. The aluminum compound can be produced by treating the aluminum foil.

[0019] When the metal oxide constituting the conductive layer is zinc oxide, an element for increasing the conductivity of the conductive layer may be added to the zinc oxide. Examples of the added element include aluminum, gallium, boron, and indium. The content of the added element in the zinc oxide may be 0.1% by mass or more, or 1.0% by mass or more, or 10% by mass or less, or 5% by mass or less.

[0020] The first electrode may be an anode, and the second electrode may be a cathode. The second electrode may be formed of a conductive material. The second electrode may include a metal layer or a carbon layer. The second electrode may include a carbon layer in contact with the conductive layer and a metal layer formed on the carbon layer.

[0021] The capacitor (C) may include a plurality of first electrodes and a plurality of second electrodes. In this case, the first electrodes and the second electrodes may be arranged alternately. A dielectric layer is formed on the surface of each first electrode. A conductive layer is arranged between the dielectric layer and the second electrode.

[0022] (Method for manufacturing electrolytic capacitor) The manufacturing method according to this embodiment may be referred to as "manufacturing method (M)" below. According to the manufacturing method (M), the capacitor (C) can be manufactured. The capacitor (C) may be manufactured by a method other than the manufacturing method (M). The matters described for the capacitor (C) may be applied to the manufacturing method (M), and therefore, redundant explanations may be omitted. The matters described for the manufacturing method (M) may also be applied to the capacitor (C).

[0023] The manufacturing method (M) is a method for manufacturing a capacitor including a first electrode. The manufacturing method (M) includes a first step, a second step, and a third step, in this order. These steps are described below.

[0024] (First Step) The first step is to form a dielectric layer having needle-like structures (first needle-like structures) on the first electrode. The conductive layer is made of an inorganic material containing a metal oxide. The method for forming the dielectric layer is not limited. In one example of a method for forming a dielectric layer when the first electrode is an aluminum foil, the first electrode is first immersed in hot water for several hours to form needle-like structures made of a metal compound of the first electrode. The temperature of the hot water may be in the range of 70 to 100°C. Next, the first electrode is further anodized to form a dielectric layer. In this way, a dielectric layer having needle-like structures on its surface can be formed.

[0025] As described above, the first electrode may have pores on its surface. That is, the surface of the first electrode may be porous. The method for making the surface of the first electrode porous is not limited. For example, the surface of the first electrode may be made porous by etching.

[0026] (Second step) The second step is a step of forming a porous portion by forming a conductive layer along the needle-like structure (first needle-like structure). By forming a thin conductive layer along the first needle-like structure, a second needle-like structure is formed that covers the first needle-like structure. As a result, a porous portion (porous structure) is formed.

[0027] The method for forming the conductive layer may be a gas phase method or a liquid phase method. The conductive layer may be formed using both a gas phase method and a liquid phase method. Examples of the gas phase method include a sputtering method and an atomic layer deposition method (ALD method). Examples of the liquid phase method include a chemical solution deposition method (CBD method) and a sol-gel method. The conductive layer may be formed on the dielectric layer by immersing the first electrode on which the dielectric layer has been formed in a dispersion liquid in which a powder having the same composition as the conductive layer is dispersed.

[0028] The second step may include forming a first conductive layer by atomic layer deposition and forming a second conductive layer on the first conductive layer by chemical solution deposition, which facilitates uniform formation of the conductive layer, particularly within the pores.

[0029] In one example of forming a zinc oxide layer by chemical solution deposition, an aqueous solution of zinc nitrate and hexamethylenetetramine is used. The zinc oxide layer can be formed by immersing the first electrode, on which the dielectric layer has been formed, in the aqueous solution. The temperature of the aqueous solution is maintained in the range of 55 to 95°C, for example. The immersion time is in the range of 3 to 24 hours, for example.

[0030] (Third Step) The third step is a step of forming a second electrode on the conductive layer. The method for forming the second electrode is not limited, and it may be formed by a known method. The second electrode may be formed by vapor deposition or may be formed using a conductive paste. The second electrode may include a carbon layer and / or a metal layer (e.g., a silver particle layer). The carbon layer may be formed using a carbon paste containing conductive carbon particles. The metal layer may be formed using a metal paste containing metal particles (e.g., silver particles).

[0031] In one example of the third step, a carbon layer is first formed on the conductive layer using a carbon paste containing conductive carbon particles, and then a silver particle layer is formed on the carbon layer using a silver paste containing silver particles, thus forming the second electrode.

[0032] As mentioned above, the metal oxide may be zinc oxide, and the first electrode may be aluminum foil.

[0033] Examples of the configuration and constituent members of the capacitor (C) are described below. Known constituent members may be used for constituent members other than those characteristic of the present disclosure.

[0034] (First Electrode) The first electrode can be formed using a valve metal, an alloy containing a valve metal, a compound containing a valve metal, or the like. These materials may be used alone or in combination of two or more. Examples of valve metals that are preferably used include aluminum, tantalum, niobium, and titanium. The first electrode may be formed using a foil of the above material (e.g., a metal foil such as aluminum foil).

[0035] The first electrode may have a porous portion on its surface. The first electrode having a porous portion on its surface can be obtained, for example, by roughening the surface of a metal foil containing a valve metal. The roughening may be performed by electrolytic etching or the like.

[0036] (Dielectric Layer) The dielectric layer is an insulating layer that functions as a dielectric. The dielectric layer may be formed by anodizing a valve metal on the surface of the first electrode (e.g., a metal foil). The dielectric layer is formed so as to cover at least a portion of the first electrode. The dielectric layer is usually formed on the surface of the first electrode. When a porous portion exists on the surface of the first electrode, the dielectric layer is formed on the surface of the porous portion of the first electrode.

[0037] A typical dielectric layer includes an oxide of a valve metal. The first electrode may be formed of a valve metal, and the dielectric layer may be formed of a compound (e.g., an oxide) of the valve metal. For example, if tantalum is used as the material of the first electrode, the dielectric layer may include a tantalum oxide layer. If aluminum is used as the material of the first electrode, the dielectric layer may include an aluminum oxide layer.

[0038] The dielectric layer may include a dense layer portion without an acicular structure and a coarse portion with an acicular structure, in which case the acicular structure is formed on the dense layer portion.

[0039] (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 of only metal (a vapor deposition layer or a metal foil). Examples of conductive carbon include graphite, carbon black, graphene flakes, and carbon nanotubes. Examples of metal paste include a silver paste containing silver particles.

[0040] (Leads and exterior body) The capacitor (C) may include other components as necessary, for example, leads and exterior body. The leads and exterior body are not particularly limited, and known leads and exterior bodies may be used.

[0041] The capacitor (C) may include only one capacitor element. Alternatively, the capacitor (C) may include multiple capacitor elements. For example, the capacitor element may include multiple capacitor elements connected in parallel. The multiple capacitor elements are typically connected in parallel in a stacked state and covered with an exterior body.

[0042] Examples of embodiments according to the present disclosure will be described in detail below with reference to the drawings. The components described above can be applied to the components of the examples described below. The examples described below can be modified based on the above description. The matters described below may also be applied to the above embodiments. In the embodiments described below, components that are not essential to the capacitor of the present disclosure may be omitted. Note that the following drawings are schematic and may differ from the actual configuration.

[0043] 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, a conductive layer 120, and a second electrode 131.

[0044] The dielectric layer 112 is formed so as to cover at least a portion of the surface of the first electrode 111. The 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 conductive layer 120. The first electrode 111 of the example shown in Figure 1 has a porous portion 111a on its surface. The actual porous portion 111a may have a more complex shape, but Figure 1 shows a simplified representation of the porous portion 111a.

[0045] Although minute needle-like structures are formed on the surface of the dielectric layer 112 and the conductive layer 120, the needle-like structures are not shown in FIG. 1 . A cross-sectional image of an example of a conductive layer having a second needle-like structure is shown in FIG. 2 . The cross-sectional image in FIG. 2 was captured using a scanning electron microscope. The needle-like structure seen in FIG. 2 is formed by a first needle-like structure on the surface of the dielectric layer and a second needle-like structure (conductive layer) covering the first needle-like structure. The needle-like structure (second needle-like structure) seen in the image is made of zinc oxide. Inside the second needle-like structure, a first needle-like structure made of an aluminum compound is present. The formation of the second needle-like structure covering the first needle-like structure was confirmed by elemental mapping using an energy dispersive X-ray analyzer attached to the scanning electron microscope. The needle-like structures (linear structures) in FIG. 2 are connected to each other to form a mesh-like structure.

[0046] (Additional Notes) The above description discloses the following technologies. (Technology 1) A capacitor including: a first electrode; a dielectric layer formed on the first electrode; a conductive layer formed on the dielectric layer; and a second electrode disposed on the conductive layer, wherein the conductive layer is made of an inorganic material containing a metal oxide, and the conductive layer forms a porous portion. (Technology 2) The capacitor according to Technology 1, wherein the first electrode has pores on its surface, and the conductive layer forms the porous portion within the pores. (Technology 3) The capacitor according to Technology 1 or 2, wherein the dielectric layer has first needle-like structures on its surface, and the conductive layer is formed along the first needle-like structures. (Technology 4) The capacitor according to Technology 3, wherein the conductive layer has second needle-like structures along the first needle-like structures, and the average diameter of the needle-like portions of the second needle-like structures in a cross-sectional image is 100 nm or less. (Technology 5) The capacitor according to any one of Technologies 1 to 4, wherein the first electrode is aluminum foil, and the dielectric layer includes an aluminum oxide layer. (Technology 6) The capacitor according to any one of Technologies 1 to 5, wherein the metal oxide is zinc oxide. (Technology 7) The capacitor according to Technology 6, wherein an element for increasing the conductivity of the conductive layer is added to the zinc oxide. (Technology 8) A method for manufacturing a capacitor including a first electrode, comprising: a first step of forming a dielectric layer having needle-like structures on a surface of the first electrode; a second step of forming a conductive layer along the needle-like structures to form a porous portion; and a third step of forming a second electrode on the conductive layer, wherein the conductive layer is made of an inorganic material including a metal oxide. (Technology 9) The manufacturing method according to Technology 8, wherein the conductive layer includes a first conductive layer and a second conductive layer formed on the first conductive layer, and the second step includes forming the first conductive layer by atomic layer deposition and forming the second conductive layer on the first conductive layer by chemical solution deposition. (Technology 10) The manufacturing method according to Technology 8 or 9, wherein the metal oxide is zinc oxide.

[0047] The capacitor (C) according to the present disclosure will be described in more detail by way of examples.

[0048] (Experiment 1) In Experiment 1, a plurality of capacitors (capacitor elements) having different dielectric layers and second electrodes were fabricated and evaluated.

[0049] (Capacitor A1) Capacitor A1 was fabricated by the following method: First, an aluminum foil having a porous surface was prepared as a first electrode. The surface of the aluminum foil was made porous by etching.

[0050] Next, the surface of the aluminum foil was treated to form a dielectric layer having an acicular structure on the surface. Specifically, the aluminum foil was subjected to a hot water treatment and an anodizing treatment to form a dielectric layer having an acicular structure on the surface.

[0051] Next, a zinc oxide layer (conductive layer) was formed on the dielectric layer. Specifically, a first ZnO layer (thickness: 10 nm or less) was first formed using atomic layer deposition. Next, a second ZnO layer was grown on the first ZnO layer using chemical solution deposition. Specifically, the aluminum foil on which the first ZnO layer was formed was immersed in an aqueous solution (temperature: 85°C) containing zinc nitrate and hexamethylenetetramine for 22 hours to grow the second ZnO layer. In this manner, the zinc oxide layer was formed. This resulted in the formation of the promotion structure shown in Figure 2.

[0052] Next, a silver particle layer (second electrode) was formed on the zinc oxide layer using silver paste, thereby producing a capacitor A1.

[0053] (Capacitor A2) Capacitor A2 was fabricated using the same method and conditions as capacitor A1, except that the second electrode was changed. To form the second electrode of capacitor A2, a carbon layer was first formed on a zinc oxide layer. The carbon layer was formed using carbon paste. Next, a silver particle layer was formed on the carbon layer using the same method as the silver particle layer of capacitor A1. In this way, a second electrode consisting of a carbon layer and a silver particle layer was formed.

[0054] (Capacitor C1) Capacitor C1 was fabricated in the same manner and under the same conditions as capacitor A1, except that the needle-like structures were not formed on the surface of the dielectric layer. The dielectric layer of capacitor C1 was formed by anodizing an aluminum foil.

[0055] (Capacitor C2) Capacitor C2 was fabricated in the same manner and under the same conditions as Capacitor A2, except that the needle-like structures were not formed on the surface of the dielectric layer. The dielectric layer of Capacitor C2 was formed in the same manner and under the same conditions as the dielectric layer of Capacitor C1.

[0056] The capacitance per unit area of ​​the fabricated capacitors was measured. Furthermore, the insulation resistance (resistance between the first and second electrodes) of the fabricated capacitors was measured. Some of the manufacturing conditions and the evaluation results are shown in Table 1. The "needle structure" in Table 1 means that the dielectric layer has the first needle structure and the conductive layer has the second needle structure.

[0057]

[0058] Capacitors A1 and A2 are capacitors (C) according to the present disclosure. Capacitors C1 and C2 are comparative examples. The capacitance of capacitor C1 could not be measured, and capacitor C1 did not exhibit performance as a capacitor. As shown in Table 1, capacitors A1 and A2 had higher capacitance and insulation resistance than capacitors C1 and C2. This result is thought to suggest that the destruction of the dielectric layer was suppressed by the conductive layer forming a porous portion.

[0059] The present disclosure can be used for capacitors. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be interpreted as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to cover all modifications and alterations without departing from the true spirit and scope of the present invention.

[0060] 10: Capacitor 111: First electrode 112: Dielectric layer 120: Conductive layer 131: Second electrode

Claims

1. A capacitor comprising: a first electrode; a dielectric layer formed on the first electrode; a conductive layer formed on the dielectric layer; and a second electrode disposed on the conductive layer, wherein the conductive layer is made of an inorganic material containing a metal oxide, and the conductive layer forms a porous portion.

2. The capacitor according to claim 1, wherein the first electrode has pores on the surface, and the conductive layer forms the porous portion within the pores.

3. The capacitor according to claim 1, wherein the dielectric layer has a first needle-like structure on a surface thereof, and the conductive layer is formed along the first needle-like structure.

4. The capacitor according to claim 3, wherein the conductive layer has second needle-like structures along the first needle-like structures, and the average diameter of the needle-like portions of the second needle-like structures in a cross-sectional image is 100 nm or less.

5. The capacitor of claim 1, wherein the first electrode is aluminum foil and the dielectric layer comprises an aluminum oxide layer.

6. The capacitor according to any one of claims 1 to 5, wherein the metal oxide is zinc oxide.

7. The capacitor of claim 6, wherein an element is added to the zinc oxide to increase the conductivity of the conductive layer.

8. A method for manufacturing a capacitor including a first electrode, comprising: a first step of forming a dielectric layer having a needle-like structure on the surface of the first electrode; a second step of forming a conductive layer along the needle-like structure to form a porous portion; and a third step of forming a second electrode on the conductive layer, wherein the conductive layer is made of an inorganic material including a metal oxide.

9. The manufacturing method according to claim 8, wherein the conductive layer includes a first conductive layer and a second conductive layer formed on the first conductive layer, and the second step includes forming the first conductive layer by atomic layer deposition and forming the second conductive layer on the first conductive layer by chemical solution deposition.

10. The manufacturing method according to claim 8 or 9, wherein the metal oxide is zinc oxide.

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