JP2006108172A - Electrolytic capacitor and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolytic capacitor having a small leakage current, and to provide a manufacturing method therefor. <P>SOLUTION: In the electrolytic capacitor 100, an anode 1 comprises a base 1a, which is made of a porous sintered body of niobium particles, a surface layer 1b which is made of niobium boride and formed on the base 1a, and an anode node 1c which is partly embedded in the base 1a, with a dielectric layer 2 which is made of niobium oxide formed by anodization being formed on the surface layer 1b. An electrolytic layer 3 made of polypyrrole is formed on the dielectric layer 2, and a cathode 4 is formed on the electrolytic layer 3. A conductive adhesive layer 5 and a cathode terminal 6 are formed on the upper surface of the cathode 4. An anode terminal 7 is weld-connected to the top of the anode lead 1c of the anode 1. A mold exterior resin 8 is formed around a second conductive layer 4b, the cathode terminal 6, and the anode terminal 7, such that the ends of the cathode terminal 6 and the anode terminal 7 are drawn out. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solid electrolytic capacitor and a method for manufacturing the same.

  Niobium oxide has high insulation properties, and its dielectric constant is about 1.8 times that of tantalum oxide, which is the material of conventional solid electrolytic capacitors. Therefore, niobium oxide is a dielectric material for next-generation high-capacity solid electrolytic capacitors. Attention has been paid. Here, niobium oxide having a high insulating property can be easily formed on the anode by anodizing with a niobium substrate as the anode. Note that the niobium oxide formed at this time is amorphous.

In conventional solid electrolytic capacitors using niobium oxide, the niobium oxide formed by anodic oxidation is used as a dielectric layer. The dielectric layer made of niobium oxide is not affected by heat treatment such as a reflow process. It was easy to receive, and the stability of the capacitance was inferior to that of solid electrolytic capacitors using other dielectric materials such as tantalum oxide. Therefore, a solid electrolytic capacitor in which a niobium nitride region is formed in niobium oxide constituting the dielectric layer has been proposed in order to suppress a decrease in capacitance (see, for example, Patent Document 1).
JP 11-329902 A

  However, even in a solid electrolytic capacitor using niobium oxide having a niobium nitride region formed as described above, there is a problem that leakage current between the anode and the cathode increases after heat treatment such as a reflow process.

The present invention has been made to solve the above problems,
One object of the present invention is to provide a solid electrolytic capacitor with low leakage current.

  Another object of the present invention is to provide a method of manufacturing a solid electrolytic capacitor with a small leakage current.

  In order to achieve the above object, a solid electrolytic capacitor according to a first aspect of the present invention includes an anode having a surface layer made of niobium boride on a substrate made of niobium, and a dielectric made of niobium oxide formed on the anode. A body layer and a cathode formed on the dielectric layer.

  In the solid electrolytic capacitor according to the first aspect, as described above, the surface layer made of niobium boride is formed between the base and the dielectric layer made of niobium oxide. Since oxygen atoms hardly diffuse in the surface layer made of niobium boride, the surface layer made of niobium boride can prevent the oxygen atoms in the dielectric layer from diffusing toward the substrate. As a result, even when a heat treatment such as a reflow process is performed, oxygen can be prevented from diffusing from the dielectric layer to the anode, so that the thickness of the dielectric layer is difficult to decrease, and the insulating property of the dielectric layer also decreases. Hateful. Therefore, in the invention of the first aspect, a solid electrolytic capacitor with a small leakage current can be obtained.

  Since oxygen atoms are less diffusible in amorphous niobium boride than in crystalline niobium boride, the crystal structure of niobium boride constituting the surface layer is more amorphous. Further, it is possible to suppress the oxygen atoms in the dielectric layer from diffusing toward the substrate.

  In the solid electrolytic capacitor according to the first aspect, preferably, the boron concentration in the surface layer is in the range of 1 atomic% to 10 atomic%. When the boron content is less than 1 atomic%, the effect of suppressing the diffusion of oxygen atoms is reduced. Further, when the boron content exceeds 10 atomic%, niobium boride constituting the surface layer is easily crystallized. In this case, oxygen is easily diffused, and the leakage current is liable to increase. Therefore, the leakage current can be further reduced by setting the boron concentration in the surface layer in the range of 1 atomic% to 10 atomic%.

  According to a second aspect of the present invention, there is provided a method for producing a solid electrolytic capacitor comprising: an anode having a surface layer made of niobium boride on a substrate by electrolytically treating the substrate made of niobium in a molten salt containing boron. A step of forming a dielectric layer made of niobium oxide on the anode by anodizing the anode in an aqueous solution, and a step of forming a cathode on the dielectric layer.

  In the method for producing a solid electrolytic capacitor according to the second aspect, as described above, by subjecting the substrate made of niobium in the molten salt containing boron to electrolytic treatment, the boron in the molten salt is reduced on the surface of the substrate, Niobium on the substrate surface can be borated. Thereby, a surface layer made of niobium boride can be easily formed on the surface of the substrate. As a result, oxygen atoms in the dielectric layer are less likely to diffuse toward the substrate, so that it is difficult to reduce the thickness of the dielectric layer, and it is possible to suppress a decrease in the insulating properties of the dielectric layer. Therefore, in the invention of the second aspect, a solid electrolytic capacitor with a small leakage current can be easily manufactured.

  Hereinafter, the present invention will be described in more detail based on the embodiments. However, the present invention is not limited to the following embodiments, and can be appropriately modified and implemented without departing from the scope of the present invention. It is a thing.

  FIG. 1 is a cross-sectional structure diagram of a rectangular parallelepiped solid electrolytic capacitor according to an embodiment of the present invention.

  As shown in FIG. 1, in the solid electrolytic capacitor 100, the anode 1 has an outer shape of about 3.3 mm × about 2.7 mm × about 1.7 mm made of a porous sintered body of niobium particles having a particle size of about 2 μm. A rectangular parallelepiped base 1a, a surface layer 1b made of niobium boride formed on the base 1a so as to cover the periphery of the base 1a, and a linear anode lead made of niobium partially embedded in the base 1a 1c.

  A dielectric layer 2 made of niobium oxide is formed on the surface layer 1b so as to cover the periphery of the surface layer 1b. An electrolyte layer 3 made of polypyrrole or the like is formed on the dielectric layer 2 so as to cover the periphery of the dielectric layer 2, and a cathode 4 is formed on the electrolyte layer 3 so as to cover the periphery of the electrolyte layer 3. Is formed. The cathode 4 includes a first conductive layer 4a made of carbon paste formed so as to cover the periphery of the electrolyte layer 3, and a second conductive layer 4b made of silver paste formed so as to cover the periphery of the first conductive layer 4a. It consists of and.

  A conductive adhesive layer 5 is formed on the upper surface of the periphery of the cathode 4, and a cathode made of an iron foil having a thickness of about 0.1 mm whose surface is nickel-plated on the conductive adhesive layer 5. Terminal 6 is formed. On the anode lead 1c exposed from the substrate 1a, an anode terminal 7 made of an iron foil having a thickness of about 0.1 mm whose surface is nickel-plated is connected by welding. In addition, a mold exterior resin 8 is formed around the second conductive layer 4b, the cathode terminal 6, and the anode terminal 7 so that the end portions of the cathode terminal 6 and the anode terminal 7 are drawn to the outside. Thereby, the solid electrolytic capacitor 100 according to the embodiment of the present invention is configured.

  Next, a method for manufacturing a solid electrolytic capacitor according to an embodiment of the present invention shown in FIG. 1 will be described.

  First, a niobium particle powder having a particle size of about 2 μm is sintered to form a base body 1a made of a porous sintered body having an outer shape of about 3.3 mm × about 2.7 mm × about 1.7 mm. At this time, a part of the linear anode lead 1c made of niobium is embedded in the substrate 1a.

Next, lithium fluoride was melted substrate 1a - sodium fluoride - performing electrolysis in a molten salt containing boron such as potassium borofluoride (molten _{LiF-NaF-KF-KBF 4} ) - potassium fluoride. Thereby, boron is reduced on the surface of the substrate 1a, and niobium on the surface of the substrate 1a is borated. As a result, a surface layer 1b made of niobium boride is formed on the base 1a so as to cover the periphery of the base 1a. Thereby, the anode 1 composed of the base body 1a, the surface layer 1b, and the anode lead 1c is manufactured.

  Next, the anode 1 is anodized in an aqueous solution such as an about 0.1 wt% phosphoric acid aqueous solution maintained at about 60 ° C. for about 10 hours at a constant voltage of about 8 V so as to cover the periphery of the surface layer 1 b. Then, the dielectric layer 2 made of niobium oxide is formed on the surface layer 1b.

  After the dielectric layer 2 is formed, an electrolyte layer 3 made of polypyrrole or the like is formed on the dielectric layer 2 by polymerization or the like so as to cover the periphery of the dielectric layer 2. Also, a carbon paste is applied on the electrolyte layer 3 so as to cover the periphery of the electrolyte layer 3, and dried at about 80 ° C. for about 30 minutes, thereby forming the first conductive layer 4a made of the carbon paste. Further, a silver paste is applied on the first conductive layer 4a and dried at about 170 ° C. for about 30 minutes to form the second conductive layer 4b made of the silver paste. Thereby, the cathode 4 in which the first conductive layer 4a made of carbon paste and the second conductive layer 4b made of silver paste are laminated is formed.

  Next, about 2 mg of a conductive adhesive is applied on the cathode terminal 6 made of an iron foil having a thickness of about 0.1 mm whose surface is nickel-plated, and then the cathode 4 and the cathode terminal are passed through the conductive adhesive. 6 is brought into contact. Further, the conductive adhesive layer 5 that connects the cathode 4 and the cathode terminal 6 is formed by drying at a temperature of about 60 ° C. for about 30 minutes while pressing the conductive adhesive between the cathode 4 and the cathode terminal 6. Form. Further, an anode terminal 7 made of an iron foil having a thickness of about 0.1 mm whose surface is nickel-plated is welded onto the anode lead 1c. Further, a mold exterior resin 8 is formed around the second conductive layer 4b, the cathode terminal 6 and the anode terminal 7 so that the end portions of the cathode terminal 6 and the anode terminal 7 are drawn to the outside. Thus, the solid electrolytic capacitor 100 according to one embodiment of the present invention is manufactured.

  In the present embodiment, since the surface layer 2 made of niobium boride is formed between the base 1a and the dielectric layer 2 made of niobium oxide, oxygen atoms in the dielectric layer 2 are formed on the base 1a. It can suppress that it spreads toward. Accordingly, even when heat treatment such as a reflow process is performed, oxygen can be prevented from diffusing from the dielectric layer 2 to the anode 1, so that the thickness of the dielectric layer 2 is not easily reduced, and the dielectric layer 2 is insulated. It is difficult to decrease the nature. Therefore, the leakage current between the anode 1 and the cathode 4 of the solid electrolytic capacitor can be reduced.

Further, in the present embodiment, since the substrate 1a of niobium in a molten salt containing boron such as melt LiF-NaF-KF-KBF ₄ are electrolytic treatment, the surface of the substrate 1a boron in the molten salt And niobium on the surface of the substrate 1a can be borated. Thereby, the surface layer 2 made of niobium boride can be easily formed on the surface of the substrate 1a. As a result, oxygen atoms in the dielectric layer 2 are less likely to diffuse toward the substrate 1a, so that the thickness of the dielectric layer 2 is difficult to decrease, and the insulating properties of the dielectric layer 2 are also unlikely to decrease. Therefore, a solid electrolytic capacitor having a small leakage current between the anode 1 and the cathode 4 can be easily manufactured.

Further, in the present embodiment, as a molten salt containing boron for use in electrolytic treatment of the substrate 1a, melt LiF-NaF-KF-KBF ₄ lithium fluoride in addition to - potassium fluoride - potassium fluoroborate (LiF-KF Molten salts such as KBF ₄ ) and sodium fluoride-potassium fluoride-potassium borofluoride (NaF-KBF-KBF ₄ ) can also be used.

  In this embodiment, a niobium porous sintered body is used as the substrate 1a. However, the present invention is not limited to this, and for example, a metal foil made of niobium may be used. Further, the substrate 1a may be made of not only niobium alone but also a niobium alloy containing elements such as tungsten, vanadium, zinc, aluminum, molybdenum, hafnium and zirconium.

  In the present embodiment, other conductive polymer such as polythiophene or other conductive material such as manganese dioxide can be used as the electrolyte layer 3 in addition to polypyrrole.

  In the present embodiment, the electrolyte layer 3 is formed between the dielectric layer 2 and the cathode 4. However, the present invention is not limited to this, and the cathode 4 is formed without forming the electrolyte layer 3. It may be formed directly on the dielectric layer 2.

  In the present embodiment, the stacked structure of the first conductive layer 4a and the second conductive layer 4b is used as the cathode 4, but the present invention is not limited to this, and for example, the first conductive layer 4a. Or the single layer structure which consists only of the 2nd conductive layer 4b may be sufficient.

  Next, in order to confirm the effect of the above-described embodiment, the following comparative experiment was performed.

Example 1
First, the base 1a and the carbon electrode are immersed in molten LiF-NaF-KBF-KBF ₄ at about 550 ° C., and the base 1a is used as a cathode, the carbon electrode is used as an anode, and a constant voltage of about 3 V is applied to the base 1a and the carbon. Electrolytic treatment was performed by applying between the electrodes for about 1 hour. Thus, a surface layer 1b made of niobium boride having a thickness of about 0.1 μm was formed on the base 1a so as to cover the periphery of the base 1a.

  In addition, as a result of analyzing the surface layer 1b formed at this time by the X-ray diffraction method, only the peak of metallic niobium was detected, and thus the crystal structure of the surface layer 1b was found to be amorphous.

  Using the anode 1 having the surface layer 1b formed as described above, a solid electrolytic capacitor A1 was produced by the same configuration and method as in the above embodiment.

(Comparative Example 1)
In Comparative Example 1, a solid electrolytic capacitor X1 was produced by the same configuration and method as in Example 1 except that the electrolytic treatment of the substrate in molten LiF-NaF-KF-KBF ₄ was not performed. That is, the surface layer made of niobium boride is not formed on the anode of the solid electrolytic capacitor X1 of Comparative Example 1.

(Comparative Example 2)
In Comparative Example 2, a solid electrolytic capacitor X2 was produced as follows.

First, an anode made of a substrate not subjected to electrolytic treatment in molten LiF-NaF-KF-KBF _{4 is} heat-treated at about 600 ° C. for about 5 minutes in a nitrogen atmosphere of about 300 Torr (about 4 × 10 ⁻⁴ Pa). Thus, a niobium nitride layer was formed on the anode so as to cover the periphery of the anode.

  Next, the anode was anodized at a constant voltage of about 8 V for about 10 hours in an about 0.1 wt% aqueous phosphoric acid solution maintained at about 60 ° C. Thus, a dielectric layer made of niobium oxide having a niobium nitride region was formed on the anode so as to cover the periphery of the anode.

  And the solid electrolytic capacitor X2 similar to Example 1 was produced except using the anode and dielectric layer which were formed in this way. The solid electrolytic capacitor X2 corresponds to a solid electrolytic capacitor in which a niobium nitride region is formed in niobium oxide constituting the dielectric layer described in Patent Document 1.

(Evaluation)
Next, the leakage current after heat treatment of the solid electrolytic capacitors A1, X1, and X2 was measured.

  First, each solid electrolytic capacitor was heat-treated in air in a drying furnace set at about 240 ° C. for about 5 minutes, and then a constant voltage of about 5 V was applied between the cathode terminal and the anode terminal for about 20 seconds. Later leakage current was measured.

  The results are shown in Table 1. In addition, the measured value of the leakage current was displayed as an index with the measured value of the solid electrolytic capacitor X1 as 100.

  From the results of Table 1, it was found that in the solid electrolytic capacitor A1 of Example 1, the leakage current was greatly reduced as compared with the solid electrolytic capacitors X1 and X2.

(Example 2)
In Example 2, the solid electrolytic capacitors B1 to B1 were subjected to the same configuration and method as in Example 1 except that the electrolytic treatment time in molten LiF-NaF-KBF-KBF ₄ was set to the respective times shown in Table 2 below. B7 was produced.

  In addition, about the surface layer 1b used for each solid electrolytic capacitor B1-B7, the sample after an electrolysis process was analyzed by the X ray diffraction method, respectively. As a result, only the peak of niobium metal was detected from the surface layer 1b used for the solid electrolytic capacitors B1 to B5, but the peak of niobium boride was detected from the solid electrolytic capacitors B6 and B7. Thus, the crystal structure of the surface layer 1b used for the solid electrolytic capacitors B1 to B5 is amorphous, but the surface layer 1b used for the solid electrolytic capacitors B6 and B7 contains crystalline niobium boride. I found out.

  Further, the solid electrolytic capacitors A1 and B1 to B7 are disassembled, the cross section of the anode 1 is exposed, and the surface layer 1b is analyzed by an X-ray photoelectron spectroscopy (XPS) method to thereby determine the boron concentration in the surface layer 1b. It was measured.

  Each solid electrolytic capacitor was heat-treated in air in a drying furnace set at about 240 ° C. for about 5 minutes, and then a constant voltage of about 5 V was applied between the cathode terminal and the anode terminal for about 20 seconds. Later leakage current was measured.

  The results are shown in Table 2. In addition, the measured value of the leakage current was displayed as an index with the measured value of the solid electrolytic capacitor X1 as 100.

  From Table 2, it was found that in any solid electrolytic capacitor, the leakage current was smaller than that of the solid electrolytic capacitor X1. Further, when the boron concentration in the surface layer 2 is in the range of about 1 atomic% to about 10 atomic%, the leakage current is reduced to about 30% or less of the solid electrolytic capacitor X1. Thereby, it was found that the preferable range of the boron concentration in the dielectric layer 2 is about 1 atomic% to about 10 atomic%.

1 is a cross-sectional structure diagram of a cuboid solid electrolytic capacitor according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Anode 1a Base | substrate 1b Surface layer 1c Anode lead 2 Dielectric layer 3 Electrolyte layer 4 Cathode 4a 1st conductive layer 4b 2nd conductive layer 5 Conductive adhesive layer 6 Cathode terminal 7 Anode terminal 8 Mold exterior resin 100 Solid electrolytic capacitor

Claims (3)

An anode having a surface layer of niobium boride on a substrate of niobium;
A dielectric layer made of niobium oxide formed on the anode;
A solid electrolytic capacitor comprising a cathode formed on the dielectric layer.   The solid electrolytic capacitor according to claim 1, wherein a boron concentration in the surface layer is in a range of 1 atomic% to 10 atomic%. Forming an anode having a surface layer made of niobium boride on the substrate by electrolytically treating the substrate made of niobium in a molten salt containing boron; and
Forming a dielectric layer made of niobium oxide on the anode by anodizing the anode in an aqueous solution;
And a step of forming a cathode on the dielectric layer.

Images