Process for manufacturing an electrode for a solid electrolytic capacitor

Abstract: A solid electrolytic capacitor is disclosed wherein use is made of an electrode produced by simultaneously forming an oxide film layer and a conductive polymer layer through electrolytic oxidation or other chemical treatments on a metal subjected to a surface roughening treatment, or forming a conductive polymer layer through plasma polymerization or other chemical treatments on a metal successively subjected to surface roughening and anodic oxidation treatments.

Description: The present invention relates to a process of manufacturing an electrode for a solid electrolytic capacitor. A conventional solid electrolytic capacitor comprises an electrode on the anode side composed of a film forming metal, such as aluminum or tantalum, coated with an oxide film; a solid electrolyte, such as manganese dioxide or TCNQ (7,7,8,8-tetracyanoquinodimethane) salt, deposited on the oxide film; and an electrode on the cathode side composed of a conductive metal layer or the like and provided on the reverse side of the oxide film. However, the use of manganese dioxide as the solid electrolyte has drawbacks such as damage to the oxide film of the electrode on the anode side and poor restorability of the oxide film by manganese dioxide. By contrast, the use of a TCNQ salt as the solid electrolyte has advantages such as high conductivity by virtue of usual powdery crystal form, freedom from the above-described damage to the oxide film and superiority to manganese dioxide in the oxide film restoration. However, since the conductivity of the TCNQ salt is fixed, it is difficult to realize a solid electrolytic capacitor complying with desired electrical characteristics. For this reason, a conductive polymer was used as a solid electrolyte formed on the oxide film to develop a solid electrolytic capacitor free from the damage to the oxide film and capable of providing desired electrical characteristics such as a resistance loss which can be freely set in an appropriate range. Examples of this type of solid electrolytic capacitor known in the art include those described in Japanese Patent Laid-Open Nos. 61-10230/1986, 61-47621/1986, 60-70719/1985, 60-245111/1985, 60-245209/1985, 60-250614/1985, 60-244017/1985, 61-2315/1986, 61-47625/1986, 61-22614/1986, 61-22613/1986, 60-22311/1985, and 60-17909/1985. In these solid electrolytic capacitors, use is made of an electrode comprising an oxide film formed on the surface of film-forming metal and, disposed thereon, a conductive polymer through electrolytic polymerization or chemical polymerization. However, in these conventional electrolytic capacitors, since the conductive polymer layer is formed on the oxide film layer in a laminated form, it is difficult to successively adhere the oxide film layer to the conductive layer, so that a small space is formed between these layers. This unfavorably brought about a problem of a lowering in the capacity. An object of the present invention is to provide an electrode for a solid electrolytic capacitor which is excellent in the electrical properties and in the adhesion between an anode oxide layer and a conductive polymer layer and free from the formation of a small space between these layers. Another object of the present invention is to provide an electrode for a solid electrolytic capacitor which is excellent in the adhesion between an oxide film layer and a conductive polymer layer and free from the formation of a small space between these layers. In a first aspect, the invention provides a process of manufacturing an electrode for a solid electrolytic capacitor, wherein said electrode is formed by conducting electrolytic oxidation in an electrolyte containing a monomer of a conductive polymer dissolved therein through the use of a metal having a roughened surface as an anode to thereby simultaneously form an anodic oxidation layer and a conductive polymer layer on said metal. In a second aspect, the invention provides a process of manufacturing an electrode for a solid electrolytic capacitor, wherein said electrode is formed by simultaneously forming through chemical treatments an oxide film layer and a conductive polymer layer on the surface of a metal subjected to a roughening treatment. In the second aspect described above, since the surface roughening treatment and formation of the anodic oxide film layer and the conductive polymer layer is carried out simultaneously while the oxidizing agent is being carried on the metal surface, generation of noxious oxide film, which is likely to be produced by reaction of the metal surface with oxide and moisture in the atmosphere, solvents in the surface treatment and active gases, can be minimized. Thus, a desired electric conductivity of the capacitor can be obtained. The above-described metal used in the present invention may be in various forms, such as cylinder, sheet, or foil. The material of the metal may be any of those used in usual electrolytic capacitors, such as aluminum, tantalum, or titanium. A metal sheet per se may be used as the metal. Alternately, a sinter or the like prepared by press-molding metal powder and sintering the resultant molding may also be used. The conductive polymer used in the present invention may be a polymer having conjugated double bonds in its main chain, and examples thereof include polyacetylene, polythiophene, polypyrrole, poly-para-phenylene, polyaniline, polyimidazole, polythiazole, and polyfuran. Among them, preferred are an aniline polymer and polymers having a nitrogen, oxygen, oxygen or sulfur atom as a hetero atom and selected form five- and six-membered heterocyclic ring compounds having a conjugated π-electron system, such as polyaniline polythiophene, and polypyrrole. Preferable examples of the monomer used in the synthesis of the above-described conductive polymers include pyrrole and thiophene compounds having no substituent at the 2- and 5-positions of a ring skeleton structure of pyrrole or thiophene. Among them, specific examples of the pyrrole compound include pyrrole, N-methylpyrrole, N-ethylpyrrole, N-n-propylpyrrole, N-n-butylpyrrole, N-phenylpyrrole, N-toluylpyrrole, N-naphthylpyrrole, 3-methylpyrrole, 3,5-dimethylpyrrole, 3-ethylpyrrole, 3-n-propylpyrrole, 3-n-butylpyrrole, 3-phenylpyrrole, 3-methoxypyrrole, 3-naphthylpyrrole, 3,5-dimethoxypyrrole, 3-ethoxypyrrole, 3-n-propoxypyrrole, 3-phenoxypyrrole, 3-methyl-N-methylpyrrole, 3-methoxy-N-methylpyrrole, 3-chloropyrrole, 3-bromopyrrole, 3-methylthiopyrrole, and 3-methylthio-N-methylpyrrole. Specific examples of the thiophene compound include 2,2'-bithiophene, 3-methyl-2,2'-bithiophene, 3,3'-dimethyl-2,2'-bithiophene, 3,4-dimethyl-2,2'-bithiophene, 3,4-dimethyl-3',4'-dimethyl-2,2'-bithiophene, 3-methoxy-2,2'-bithiophene, 3,3'-dimethoxy-2,2'-bithiphene, 2,2',5',2"-terthiophene, 3-methyl-2,2',5',2"-terthiophene, and 3,3'-dimethyl-2,2',5',2"-terthiophene. The oxidizing agent is used in an amount of usually 0.001 to 10,000 times by mole, preferably 0.005 to 5,000 times by mole based on the above-described compound having conjugated double bonds, depending upon the amount of formation of the polymer. Compounds exhibiting polymerization activity against the above-described compound having conjugated double bonds may be used as the oxidizing agent alone or in a combination of two or more of them. In general, metal salts having a strong acid residue, a halogen, or a cyano, peroxides, etc. are used, and specific examples thereof include Fe(ClO₄)₃, Fe(BF₄)₃, Fe₂(SiF₆)₃, Cu(ClO₄)₂, Cu(BF₄)₂, CuSiF₆, FeCl₃, CuCl₂, K₃[Fe(CN)₆], RuCl₃, MoCl₅, WCl₆, (NH₄)₂S₂O₈, K₂S₂O₈, Na₂S₂O₈, NaBO₃, and H₂O₂. These compounds may have water of crystallization and may be used in the form of an aqueous solution. Examples of the dopant added to the above-described conductive polymers include BF 4 - , ClO 4 - , PF 6 - , AsF 6 - , CF₃SO 3 - , I⁻, Br⁻, Cl⁻, and F⁻. As described above, when an oxide film and a conductive polymer layer are simultaneously formed on a roughened surface of a metal, the surface of contact of the oxide film with the conductive polymer is very uneven. This contributes to not only an increase in the area of contact between the oxide film and the conductive polymer but also a remarkable improvement in the adhesion between the oxide film and the conductive polymer. Further, this increase in the adhesion between the oxide film and the conductive polymer. Further, this increase in the adhesion substantially prevents formation of a small space between the oxide film layer and the conductive polymer layer. Further, since the growth of the oxide film is hindered by formation of the conductive polymer, the thickness of the oxide film is remarkably reduced. The above-described improvement in the contact area and the adhesion and reduction in the thickness of the oxide film contribute to an increase in the capacity per unit volume of a capacitor. In particular, the increase in the adhesion improves the resistance to mechanical stress, which suppresses the peeling of the oxide film layer from the conductive polymer layer, so that a lowering in the capacity is prevented. Specifically, in the electrode of the present invention, as shown in Fig. 1, the boundary between an oxide film layer 1 formed on the surface of a metal 3 and a conductive polymer layer 2 is very uneven, while as shown in Fig. 2 the conventional electrode has no significant unevenness. Further, the thickness of the oxide film per se according to the present invention is much smaller than that of the conventional oxide film. Fig. 1 is a schematic view of a boundary between an oxide film layer and a conductive polymer layer in a solid electrolytic capacitor with an electrode obtained according to the first and second aspects of the present invention; Fig. 2 is a schematic view of a boundary between an oxide film layer and a conductive polymer layer in a conventional solid electrolytic capacitor shown for comparison therewith; The present invention will now be described in more detail with reference to the following Examples. Examples of the first aspect of the invention: Example 1 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Constant-current electrolysis was conducted by using the aluminum foil as an anode, platinum as a cathode, and an aqueous hydrofluoroboric acid solution containing dissolved aniline as an electrolyte. The constant-current electrolysis caused an aluminum oxide layer and a polyaniline layer to be simultaneously formed on the surface of the aluminum foil. The product was used as an electrode, and solid electrolytic capacitor A-1 with an electrode obtained according to the invention was prepared by, e.g., spirally enfolding the electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to a similar roughening treatment. Thereafter, electrolytic polymerization was conducted in an aqueous hydrofluoroboric acid solution containing dissolved aniline to form polyaniline on the oxide film. The product was used as an electrode, and comparative solid electrolytic capacitor B-1 was prepared by e.g., spirally enfolding the electrode. The capacities of capacitor A-1 and comparative capacitor B-1 were measured to find out that the capacity of capacitor A-1 was twice that of capacitor B-1. These capacitors A-1 and B-1 were each applied to an ultrasonic vibration test. As a result, it was found that the percentage retention of the capacity after the test to that before the test was 97 % for capacitor A-1 and 80 % for capacitor B-1 . Example 2 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Constant-current electrolysis was conducted by using the aluminum foil as an anode, platinum as a cathode, and an aqueous hydrofluoroboric acid solution containing dissolved pyrrole as an electrolyte. The constant-current electrolysis caused an aluminum oxide layer and a polypyrrole layer to be simultaneously formed on the surface of the aluminum foil. The product was used as an electrode, and solid electrolytic capacitor C-1 with an electrode obtained according to the invention was prepared by e.g., spirally enfolding the electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to a similar roughening treatment. Thereafter, electrolytic polymerization was conducted in an aqueous hydrofluoroboric acid solution containing dissolved pyrrole to form polyaniline on the oxide film. The product was used as an electrode, and comparative solid electrolytic capacitor D-1 was prepared by, e.g., spirally enfolding the electrode. The capacities of the capacitors C-1 and D-1 were measured to find out that the capacity of capacitor C-1 was 2.1 times that of comparative capacitor D-1. These capacitors C-1 and D-1 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 96 % for capacitor C-1 and 81 % for capacitor D-1. Example 3 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Constant-current electrolysis was conducted by using the aluminum foil as an anode, platinum as a cathode, and an aqueous hydrofluoroboric acid solution containing dissolved thiophene as an electrolyte. The constant-current electrolysis caused an aluminum oxide layer and a polythiophene layer to be simultaneously formed on the surface of the aluminum foil. The product was used as an electrode, and solid electrolytic capacitor E-1 with an electrode obtained according to the invention was prepared by, e.g., spirally enfolding the electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to a similar roughening treatment. Thereafter, electrolytic polymerization was conducted in an aqueous hydrofluoroboric acid solution containing dissolved thiophene to form polythiophene on the oxide film. The product was used as an electrode, and comparative solid electrolytic capacitor F-1 was prepared by, e.g., spirally enfolding the electrode. The capacities of the capacitors E-1 and F-1 were measured to find out that the capacity of capacitor E-1 was 1.7 times that of capacitor F-1. These capacitors E-1 and F-1 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 95 % for capacitor E-1 and 82 % for comparative capacitor F-1. Example 4 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Constant-current electrolysis was conducted by using the aluminum foil as an anode, platinum as a cathode, and an aqueous hydroflouroboric acid solution contaning dissolved aniline as an electrolyte. The constant-current electrolysis caused an aluminum oxide layer and a polyaniline layer to be simultaneously formed on the surface of the aluminum foil. The product was used as an electrode, and solid electrolytic capacitor G-1 with an electrode obtained according to the invention was prepared by, e.g., spirally enfolding the electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to a similar roughening treatment. Thereafter, electrolytic polymerization was conducted in an aqueous hydrofluoroboric acid solution containing dissolved aniline to form polyaniline on the oxide film. The product was used as an electrode, and comparative solid electrolytic capacitor H-1 was prepared by, e.g., spirally enfoldng the electrode. The capacities of the capacitors G-1 and H-1 were measured to find out that the capacity of capacitor G-1 was 1.5 times that of comparative capacitor H-1. These capacitors G-1 and H-1 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 94 % for capacitor G-1 and 77 % for capacitor H-1 . Example 5 The surface of an aluminum sinter was subjected to a roughening treatment through etching with hydrochloric acid. Constant-current electrolysis was conducted by using the aluminum sinter as an anode, platinum as a cathode, and an aqueous hydrofluoroboric acid solution containing dissolved aniline as an electrolyte. The constant-current electrolysis caused an aluminum oxide layer and a polyaniline layer to be simultaneously formed on the furface of the aluminum sinter. The product was used as an electrode, and solid electrolytic capacitor I-1 with an electrode obtained according to the invention was prepared by, e.g., spirally enfolding the electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum sinter subjected to a similar roughening treatment. Electrolytic polymerization was then conducted in an aqueous hydrofluoroboric acid solution containing dissolved aniline to form polyaniline on the oxide film. The product was used as an electrode and comparative solid electrolytic capacitor J-1 was prepared by, e.g., spirally enfolding the electrode. The capacities of the capacitors I-1 and J-1 were measured to find out that the capacity of capacitor I-1 was 2.3 times that of comparative capacitor J-1. These capacitors I-1 and J-1 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 98 % for capacitor I-1 and 83 % for capacitor J-1. Examples of the second aspect of the invention: Example 6 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Thereafter, aniline was carried on the surface of the aluminum foil subjected to the roughening treatment by coating with aniline or immersion in aniline. The aluminum foil was immersed in an aqueous hydrofluoroboric acid solution containing dissolved cupric fluoroborate, thereby oxidizing the roughened surface of the aluminum foil and, at the same time, chemically polymerizing aniline carried on the roughened surface to form a polyaniline layer. Thus, an electrode was prepared according to the second invention. Solid electrolytic capacitor A-2 was prepared by, e.g., spirally enfolding this electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to a similar roughening treatment. Thereafter, aniline was carried on the surface of the aluminum foil subjected to the roughening treatment by coating with aniline or immersion in aniline. The aluminum foil was then immersed in an aqueous hydrofluoroboric acid solution containing dissolved cupric fluoroborate, thereby chemically polymerizing aniline carried on the roughened surface to form a polyaniline layer. Thus, a comparative electrode was prepared. Comparative solid electrolytic capacitor B-2 was prepared by, e.g., spirally enfolding this electrode. The capacities of capacitors A-2 and B-2 were measured to find out that the capacity of capacitor A-2 was 2.5 times that of comparative capacitor B-2. These capacitors A-2 and B-2 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 97 % for capacitor A-2 and 81 % for capacitor B-2. Example 7 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Thereafter, pyrrole was carried on the surface of the aluminum foil subjected to the roughening treatment by coating with pyrrole or immersion in pyrrole. The aluminum foil was immersed in an aqueous hydrofluoroboric acid solution containing dissolved cupric fluoroborate, thereby oxidizing the roughened surface of the aluminum foil and, at the same time, chemically ploymerizing pyrrole carried on the roughened surface to form a polypyrrole layer. Thus, an electrode was prepared according to the present invention. Solid electrolytic capacitor C-2 was prepared by, e.g., spirally enfolding this electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to similar roughening treatment. Thereafter, pyrrole was carried on the surface of the aluminum foil subjected to the roughening treatment by coating with pyrrole or immersion in pyrrole. The aluminum foil was then immersed in an aqueous hydrofluoroboric acid soultion containing dissolved cupric fluroroboride, thereby chemically polymerizing pyrrole carried on the roughened surface to form a polypyrrole layer. Thus, a comparative electrode was prepared. Comparative solid electrolytic capacitor D-2 was prepared by, e.g., spirally enfolding this electrode. The capacities of capacitors C-2 and D-2 were measured to find out that the capacity of capacitor C-2 was 2.1 times that of comparative capacitor D-2. These capacitors C-2 and D-2 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that befor the test was 95 % for capacitor C-2 and 79 % for capacitor D-2. Example 8 The surface of an aluminum foil having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Thereafter, thiophene was carried on the surface of the aluminum foil subjected to the roughening treatment by coating with thiophene or immersion in thiophene. The aluminum foil was immersed in an aqueous hydrofluoroboric acid solution containing dissolved cupric fluoroboride, thereby oxidizing the roughened surface of the aluminum foil and, at the same time, chemically polymerizing thiophene carried on the roughened surface to form a polythiophene layer. Thus, an electrode was prepared according to the present invention. Solid electrolytic capacitor E-2 was prepared by, e.g., spirally enfolding this electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum foil subjected to similar roughening treatment. Thereafter, thiophene was carried on the surface of the aluminum foil subjected to the roughening treatment by coating with thiophene or immersion in thiophene. The aluminum foil was then immersed in an aqueous hydrofluoroboric acid solution containing dissolved cupric fluoroborate, thereby chemically polymerizing thiophene carried on the roughened surface to form a polythiophene layer. Thus, a comparative electrode was prepared. Comparative solid electrolytic capacitor F-2 was prepared by, e.g., spirally enfolding this electrode. The capacities of capacitors E-2 and F-2 were measured to find out that the capacity of capacitor D-2 was twice that of comparative capacitor F-2. These capacitors E-2 and F-2 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 96 % for capacitor E-2 and 83 % for capacitor F-2. Example 9 The surface of an aluminum sinter having a thickness of 20 µm was subjected to a roughening treatment through etching with hydrochloric acid. Thereafter, aniline was carried on the surface of the aluminum sinter subjected to the roughening treatment by coating with aniline or immersion in aniline. The aluminum sinter was then immersed in an aqueous hydrofluoroboric acid, thereby oxidizing the roughened surface of the aluminum sinter and, at the same time, chemically polymerizing aniline carried on the roughened surface to form a polyaniline layer. Thus, the electrode was prepared according to the present invention. Solid electrolytic capacitor G-2 was prepared by, e.g., spirally enfolding this electrode. An oxide film was formed through electrolytic oxidation on the surface of an aluminum sinter subjected to similar roughening treatment. Thereafter, aniline was carried on the surface of the aluminum sinter subjected to the roughening treatment by coating with aniline or immersion in aniline. The aluminum sinter was then immersed in an aqueous hydrofluoroboric acid solution containing dissolved cupric fluoroborate, thereby chemically polymerizing aniline carried on the roughened surface to form a polyaniline layer. Thus, a comparative electrode was prepared. Comparative solid electrolytic capacitor H-2 was prepared by, e.g. spirally enfolding this electrode. The capacities of capacitors G-2 and H-2 were measured to find out that the capacity of capacitor G-2 was 2.6 times that of capacitor H-2. These capacitors G-2 and H-2 were each applied to an ultrasonic vibration test. The test showed that the percentage retention of the capacity after the test to that before the test was 99 % for capacitor G-2 and 85 % for capacitor H-2. As described above, the present invention can provide an electrode for a solid electrolytic capacitor having excellent characteristics such as remarkably increased capacity and reduced lowering in the capacity caused by mechanical stress.

Claims: A process of manufacturing an electrode for a solid electrolytic capacitor, wherein said electrode is formed by conducting electrolytic oxidation in an electrolyte containing a monomer of a conductive polymer dissolved therein through the use of a metal (3) having a roughened surface as an anode to thereby simultaneously form an anodic oxidation layer (1) and a conductive polymer layer (2) on said metal (3). A process of manufacturing an electrode for a solid electrolytic capacitor, wherein said electrode is formed by simultaneously forming through chemical treatments an oxide film layer (1) and a conductive polymer layer (2) on the surface of a metal (3) subjected to a roughening treatment. A process of manufacturing an electrode for solid electrolytic capacitor according to claim 2, wherein prior to simultaneously forming, an oxidizing agent is carried on the surface of said metal (3) subjected to a roughening treatment.

Inventors: ANDO OSAMU, FUJIMOTO MASAHISA, FURUKAWA NOBUHIRO, IKEDA HIRONOSUKE, NISHIO KOJI, ONO HITOSHI, ANDO, OSAMU, FUJIMOTO, MASAHISA, FURUKAWA, NOBUHIRO, IKEDA, HIRONOSUKE, NISHIO, KOJI, ONO, HITOSHI
Applicants: MITSUBISHI CHEM CORP, SANYO ELECTRIC CO, MITSUBISHI CHEMICAL CORPORATION, SANYO ELECTRIC CO., LTD.
Application year: 1989
Main classification: H01G9
IPC codes: H01G9