INSULATED ELECTRIC WIRE

Abstract: The insulated electric wire of the invention is applicable for wirings and windings used in a high-vacuum or high-temperature environment, that is within high-vacuum or high-temperature equipment. This wire comprises a base member (1), an anodically oxidized layer (2), and an oxide insulating layer (3). The base member (1) includes a conductor and has at least on the outer surface thereof an aluminum or aluminum-alloy layer. The anodically oxidized layer (2) is formed on the surface thereof. The oxide insulating layer (3) is formed on the anodically oxidized layer (2) by the sol-gel method or by the organic acid salt thermal decomposition method. The insulated electric wire exhibits excellent heat-resistant insulating property and flexibility, and does not adsorb gases.

Description: SPECIFICATION Field of the Invention The present invention relates to an insulated wire, and more particularly, it relates to an insulated wire such as a distribution wire, a wire for winding or the like which is employed under high-vacuum environment or high-temperature environment such as a high-vacuum apparatus or a high-temperature service apparatus. Background of the Invention An insulated wire may be applied to equipment such as heating equipment or a fire alarm, for which safety under a high temperature is required. Further, the insulated wire is also used under environment in an automobile, which is heated to a high temperature. An insulated wire formed by a conductor which is coated with heat resistant organic resin such as polyimide, fluorocarbon resin or the like has generally been used as such an insulated wire. As to application for which high heat resistance is required, or employment under environment for which a high degree of vacuum is required, mere organic coating is insufficient in view of heat resistance, gas emission property and the like. Thus, an insulated wire of such a form that a conductor is inserted in an insulator tube of ceramics, an MI cable (Mineral Insulated Cable) of such a form that a conductor is inserted in a heat resistant alloy tube of a stainless steel alloy etc. which is filled with metal oxide powder of magnesium oxide etc., or the like has been employed for such application. A fiber-glass braided insulated wire employing textile glass fiber as an insulating member etc. is listed as an insulated wire for which flexibility is required with heat resistance. In the aforementioned insulated wire coated with organic resin having heat resistance, the highest temperature at which insulability can be maintained is about 200°C at the most. Therefore, it has been impossible to employ such an organic insulated coated wire for application for which guarantee for insulability is required under a high temperature of at least 200°C. Further, the insulated wire which is improved in heat resistance through an insulator tube of ceramics has disadvantages such as inferior flexibility. The MI cable is formed by a heat resistant alloy tube and a conductor, and hence the outer diameter of the cable is increased with respect to the conductor radius. Thus, the MI cable has a relatively large section with respect to electric energy allowed by the conductor which is passed through the heat resistant alloy tube. In order to use the MI cable as a wire for winding which is wound on a bobbin etc. in the form of a coil, however, it is necessary to bend the heat resistant alloy tube in prescribed curvature. In this case, bending performed on the heat resistant alloy tube involves difficulty. When the MI cable is wound in the form of a coil, further, it is difficult to improve winding density since the tube of its outer layer is thick as compared with the conductor. EP-A-0 292 780 describes an electric wire which has a coating of a gel film onto an outer part of a conductor. EP-A-188 369 describes an insulation of an oxide keying layer and a further electrically insulating refractory layer. Both can be from Al₂O₃. Further, when the fiber-glass braided insulated wire having heat resistance is employed and worked into a prescribed configuration in response to its application, the network of the braid is disturbed to cause a breakdown. In addition, dust of glass is generated from the glass fiber. This glass dust may serve as a gas adsorption source. Therefore, when the fiber-glass braided insulated wire is used under environment for which a high degree of vacuum is required, it has been impossible to maintain a high degree of vacuum due to the gas adsorption source provided by the glass dust. Disclosure of the Invention Accordingly, the present invention has been proposed in order to solve the aforementioned problems, and its object is to provide an insulated wire comprising the following items: (a) It has high insulability under environment of a high temperature. (b) It is excellent in flexibility. (c) It comprises no gas adsorption source. An insulated wire according to one aspect of the present invention comprises a base material, an anodic oxide film, and an oxide insulating layer. The base material includes a conductor, and has a surface layer of either an aluminum layer or an aluminum alloy layer at least on its outer surface. The anodic oxide layer is formed on the surface layer. The oxide insulating layer is formed on the anodic oxide layer by a sol-gel method. When the base material is worked into a composite conductor, a material containing either copper or a copper alloy etc. is illustrated by way of example for the core of the base material. In this case, the base material is preferably prepared by a pipe clading method. The oxide insulating layer preferably contains at least either silicon oxide or aluminum oxide. An insulated wire according to another aspect of the present invention comprises a base material, an anodic oxide layer, and an oxide insulating layer. The base material includes a conductor, and has a surface layer of either an aluminum layer or an aluminum alloy layer at least on its outer surface. The anodic oxide layer is formed on the surface layer. The oxide insulating layer is formed on the anodic oxide layer by an organic acid salt pyrolytic method. The core of the base material may contain either copper or a copper alloy. In this case, the base material is preferably prepared by a pipe clading method. The organic insulating layer preferably contains at least either silicon oxide or aluminum oxide. In a word, the oxide insulating layer of the present invention is a layer which is formed by applying a solution containing a ceramics precursor onto the anodic oxide layer and thereafter completely bringing the ceramics precursor into a ceramics state. The solution containing the ceramics precursor indicates a solution formed of metal organic compound high polymers having an alkoxide group, a hydroxy group and metalloxan bonding, which is generated by hydrolysis and dehydration/condensation reaction of a compound having a hydrolyzable organic group such as metal alkoxide, and contains an organic solvent such as alcohol, which is a solvent, the metal alkoxide of the raw material, and a small amount of water and a catalyst which are required for the hydrolysis. Or, it indicates a solution which is obtained by mixing/dissolving metal organic compounds (Metal-organic Compounds) in a proper organic solvent. Further, the metal organic compounds mentioned herein exclude those in which elements directly bonded with metal atoms are all carbon although the same are understood in various meanings in various countries, while those employed in the present invention are restricted to those in which thermal decomposition temperatures are lower than the boiling points of the metal organic compounds under the atmospheric pressure, since a metal oxide film is obtained by thermally decomposing the metal organic compounds by heating. In one aspect of the present invention, an anodic oxide film is formed on an aluminum layer or an aluminum alloy layer, and an insulating oxide film is formed on the anodic oxide film by the sol-gel method, which is a solution method. The sol-gel method is a method of applying a solution prepared by hydrolyzing and dehydrating/condensing metal alkoxide onto an outer surface to be formed, applying the solution onto a base material and thereafter treating the same under a prescribed temperature, thereby forming an oxide insulating layer. The film formed by the sol-gel method is of an oxide which is brought into a ceramics state. This oxide is preferably formed by heat treatment under an atmosphere in an oxygen gas current in the sol-gel method. The oxide insulating layer thus brought into a ceramics state exhibits excellent heat resistance/insulability under a high temperature of at least 500°C. In another aspect of the present invention, an anodic oxide film is formed on an aluminum layer or an aluminum alloy layer, and an insulating oxide film is formed on the anodic oxide film by an organic acid salt pyrolytic method, which is a solution method. The organic acid salt pyrolytic method is a method of obtaining a metal oxide by pyrolyzing organic acid salt, i.e., metallic salt such as naphthenic acid, capric acid, stearic acid, octylic acid or the like and causing pyrolysis. A film formed by the organic acid salt pyrolytic method is of an oxide which is brought into a ceramics state. This oxide is preferably formed by heat treatment under an atmosphere in an oxygen gas current in the organic acid salt pyrolytic method. The oxide insulating layer thus brought into a ceramics state exhibits excellent heat resistance/insulability under a high temperature of at least 500°C. The anodic oxide film strongly adheres onto the aluminum layer or the aluminum alloy layer. Further, this anodic oxide film exhibits insulability to some extent as an insulator. However, the anodic oxide film has a surface having roughness. Therefore, the outer surface of the anodic oxide film has a large surface area, and provides a gas adsorption source. Therefore, a conductor which is formed with only an anodic oxide film on its outer surface cannot be used under environment for which a high degree of vacuum is required. Further, the anodic oxide film is porous and provided with a large amount of holes passing from its surface toward the base material. Thus, it is generally impossible to obtain insulability which is proportionate to the film thickness by the anodic oxide film. To this end, the inventors have found that it is possible to form a film layer filling up the holes of the anodic oxide film and further covering the irregular surface thereby smoothing the surface, by forming an oxide film on the outer surface of the anodic oxide film through the sol-gel method or the organic acid salt pyrolytic method. Thus, it is possible to obtain a high breakdown voltage which is proportionate to the film thickness, as well as to reduce the gas adsorption source by decreasing the outer surface area. Further, the anodic oxide film is excellent in adhesion with the aluminum layer or the aluminum alloy layer at least forming the outer surface of the base material. Thus, adhesion between the oxide film and the outer surface of the base material is improved as compared with the case of directly forming an oxide film on the outer surface of a conductor by the sol-gel method or the organic acid salt pyrolytic method. Therefore, the insulated wire according to the present invention is provided with heat resistance/insulability, and has good flexibility. Brief Description of the Drawings Figs. 1 and 2 are sectional views showing cross sections of insulated wires according to the present invention in correspondence to respective ones of Examples 1 and 3 as well as 2 and 4. Best Modes of Carrying Out the Invention Example 1 (a) Formation of Anodic Oxide Film A pure aluminum wire of 2 mm in wire diameter was dipped in dilute sulfuric acid of 23 percent by weight, which was maintained at a temperature of 38°C. Thereafter a positive voltage was applied to the aluminum wire, and the outer surface of the pure aluminum wire was anodized under a condition of a bath current of 2.5 A/dm² for 20 minutes. Thus, an anodic oxide film was formed on the outer surface of the pure aluminum wire with a film thickness of about 20 µm. The obtained wire material was dried in an oxygen gas current of 500°C in temperature. (b) Preparation of Coating Solution Used for Sol-Gel Method 1.2 N of concentrated nitric acid was added to a solution, which was prepared by mixing tetrabutylorthosilicate, water and ethanol in mole ratios 8:32:60, in the ratio of 1/100 mole with respect to tetrabutylorthosilicate. Thereafter this solution was heated/stirred at a temperature of 70°C for two hours. Thus, a coating solution used for the sol-gel method was composed. (c) Coating The wire obtained by (a) was dipped in the coating solution of (b). A step of heating at a temperature of 400°C for 10 minutes was performed five times on the wire whose outer surface was thus coated with the coating solution. In an initial stage of this step, a characteristic rough surface, which was formed by anodic oxidation treatment, disappeared from the heat treated surface observed with an electron microscope etc., and such a structure was attained that the rough portions were impregnated with oxides. It has been confirmed that a film was formed in the exterior of the impregnated layer by repeating the step. Finally, this wire was heated in an oxygen gas current of 500°C in temperature for 10 minutes. An insulated coated wire obtained in the aforementioned manner is shown in Fig. 1. Fig. 1 is a sectional view showing the cross section of the insulated wire according to the present invention. Referring to Fig. 1, an anodic oxide film 2 is formed on the outer surface of an aluminum wire 1. An oxide insulating layer 3 is formed on this anodic oxide film 2 by the sol-gel method. In the aforementioned Example 1, this oxide insulating layer 3 is made of silicon oxide. According to the aforementioned Example 1, further, the film thickness of an insulating layer formed by the anodic oxide film 2 and the oxide insulating layer 3 was about 40 µm. The breakdown voltage was measured in order to evaluate insulability of the as-formed insulated wire. Its breakdown voltage was 1.6 kV under the room temperature, and was 1.2 kV under a temperature of 600°C. Also when this insulated wire was wound on the outer peripheral surface of a cylinder having a diameter of 5 cm, no cracking was caused in the insulating layer. Example 2 (a) Formation of Anodic Oxide Film An aluminum/copper clad wire (its conductivity was 84 % IACS on the assumption that conductivity of pure copper was 100) of 1 mm in wire diameter, having an outer layer of an aluminum (material: JIS nominal 1050) layer of 100 µm in thickness and a core of oxygen free copper (OFC), was dipped in dilute sulfuric acid of 23 percent by weight which was maintained at a temperature of 30°C. Thereafter a positive voltage was applied to the aluminum/copper clad wire, to anodize the outer surface of the aluminum layer under a condition of a bath current of 15 A/dm² for two minutes. Thus, an anodic oxide film was formed on the surface of the aluminum/copper clad wire with a film thickness of about 10 µm. The as-formed wire was dried in an oxygen gas current of 500°C in temperature. (b) Preparation of Coating Solution Used for Sol-Gel Method Tributoxyaluminum, triethanolamine, water and ethanol were mixed in mole ratios 3:7:9:81 under a temperature of about 5°C. Thereafter this solution was heated/stirred at a temperature of 30°C for one hour. Thus, a coating solution used for the sol-gel method was composed. (c) Coating Coating treatment was performed through a method similar to Example 1. An insulated coated wire obtained in the aforementioned manner is shown in Fig. 2. Fig. 2 is a sectional view showing the cross section of the insulated wire according to the present invention. Referring to Fig. 2, an aluminum/copper clad wire having an aluminum layer 11 on the outer surface of a copper core 10 was employed as a base material. An anodic oxide film 2 is formed on the outer surface of this aluminum layer 11. An oxide insulating layer 3 is formed on this anodic oxide film 2 by the sol-gel method. In the aforementioned Example 2, this oxide insulating layer 3 is of aluminum oxide. According to the aforementioned Example 2, further, the film thickness of an insulating layer formed by the anodic oxide film 2 and the oxide insulating layer 3 was about 20 µm. The breakdown voltage was measured in order to evaluate insulability of the as-formed insulated wire. Its breakdown voltage was 1.5 kV under the room temperature, and was 1.0 kV under a temperature of 500°C. Also when this insulated wire was wound on the outer peripheral surface of a cylinder having a diameter of 3 cm, no cracking was caused in the insulating layer. Example 3 (a) Formation of Anodic Oxide Film A pure aluminum wire of 1 mm in wire diameter was dipped in dilute sulfuric acid of 23 percent by weight, which was maintained at a temperature of 35°C. Thereafter a positive voltage was applied to the aluminum wire, to anodize the outer surface of the pure aluminum wire under a condition of a bath current of 5 A/dm² for three minutes. Thus, an anodic oxide film was formed on the outer surface of the pure aluminum wire with a film thickness of about 17 µm. The as-formed wire was dried in an oxygen gas current of 400°C in temperature. (b) Preparation of Coating Solution Used for Organic acid salt pyrolytic method Silicate stearate was dissolved in a mixed solution of 90 ml of toluene, 10 ml of pyridine and 6 ml of propionic acid. Concentration of this solution was so adjusted that metal concentration of silicon was 5 percent by weight. (c) Coating The wire obtained through (a) was dipped in the coating solution of (b). A step of heating at a temperature of 400°C for 10 minutes was performed ten times on the wire whose outer surface was thus coated with the coating solution. Finally this wire was heated in an oxygen gas current of 450°C in temperature for 10 minutes. An insulated coated wire obtained in the aforementioned manner is shown in Fig. 1. Fig. 1 is a sectional view showing the cross section of the insulated wire according to the present invention. Referring to Fig. 1, an anodic oxide film 2 is formed on the outer surface of an aluminum wire 1. An oxide insulating layer 3 is formed on this anodic oxide film 2 by an organic acid salt pyrolytic method. In the aforementioned Example 1, this oxide insulating layer 3 is of silicon oxide. According to the aforementioned Example 1, further, the film thickness of an insulating layer formed by the anodic oxide film 2 and the oxide insulating layer 3 was about 25 µm The breakdown voltage was measured in order to evaluate insulability of the obtained insulated wire. Its breakdown voltage was 1.2 kV under the room temperature, and was 0.8 kV under a temperature of 600°C. Also when this insulated wire was wound on the outer peripheral surface of a cylinder having a diameter of 3 cm, no cracking was caused in the insulating layer. Example 4 (a) Formation of Anodic Oxide Film An aluminum/copper clad wire (its conductivity was 89 % IACS on the assumption that conductivity of pure copper was 100) of 1 mm in wire diameter having an outer layer formed of an aluminum (material: JIS nominal 1050) layer of 83 µm in thickness and a core of oxygen free copper (OFC) was dipped in dilute sulfuric acid of 23 percent by weight, which was maintained at a temperature of 35°C. Thereafter a positive voltage was applied to the aluminum/copper clad wire, to anodize the outer surface of the aluminum layer under a condition of a bath current of 3.5 A/dm² for two minutes. Thus, an anodic oxide film was formed on the surface of the aluminum/copper clad wire with a film thickness of about 15 µm. The as-formed wire was dried in an oxygen gas current of 300°C in temperature. (b) Preparation of Coating Solution Used for Organic acid salt pyrolytic method An O-cresol solution of aluminum octanate was prepared. Concentration of this solution was so adjusted that metal concentration of aluminum was 4 percent by weight. (c) Coating Coating treatment was performed through a method similar to Example 3. An insulated coated wire obtained in the aforementioned manner is shown in Fig. 2. Fig. 2 is a sectional view showing the cross section of the insulated wire according to the present invention. Referring to Fig. 2, an aluminum/copper clad wire having an aluminum layer 11 on the outer surface of a copper core 10 was employed as a base material. An anodic oxide film 2 is formed on the outer surface of this aluminum layer 11. An oxide insulating layer 3 is formed on this anodic oxide film 2 by the organic acid salt pyrolytic method. In the aforementioned Example 2, this oxide insulating layer 3 is of aluminum oxide. According to the aforementioned Example 2, further, the film thickness of an insulating layer formed by the anodic oxide film 2 and the oxide insulating layer 3 was about 30 µm. The breakdown voltage was measured in order to evaluate insulability of the as-formed insulated wire. Its breakdown voltage was 1.6 kV under the room temperature, and was 1.2 kV under a temperature of 400°C. Also when this insulated wire was wound on the outer peripheral surface of a cylinder having a diameter of 3 cm, no cracking was caused in the insulating layer. Industrial Availability As hereinabove described, the insulated wire according to the present invention is suitable for a distribution wire, a wire for winding etc. which is employed under high-vacuum environment, or high-temperature environment such as a high-vacuum apparatus or a high-temperature service apparatus.

Claims: An insulated wire comprising: a base material (1) including an electrical conductor and having a surface layer of either an aluminum layer or an aluminum alloy layer at least on its outer surface, an anodic oxide layer (2) formed on said surface layer, and an oxide insulating layer (3) formed on said anodic oxide layer by a sol-gel method. An insulated wire in accordance with claim 1, wherein the core of said base material (1) contains either copper or a copper alloy. An insulated wire in accordance with claim 2, wherein said base material (1) includes a base material which is prepared by a pipe clading method. An insulated wire in accordance with claim 1, wherein said oxide insulating layer (3) contains at least either silicon oxide or aluminum oxide. An insulated wire comprising: a base material (1) including an electrical conductor and having a surface layer of at least either an aluminum layer or an aluminum alloy layer at least on its outer surface, an anodic oxide layer (2) formed on said surface layer, and an oxide insulating layer (3) formed on said anodic oxide layer by an organic acid salt pyrolytic method. An insulated wire in accordance with claim 5, wherein the core of said base material (1) contains either copper or a copper alloy. An insulated wire in accordance with claim 6, wherein said base material (1) includes a base material which is prepared by a pipe clading method. An insulated wire in accordance with claim 5, wherein said oxide insulating layer (3) contains at least either silicon oxide or aluminum oxide. An insulated wire comprising: a base material (1) including an electrical conductor and having a surface layer of either an aluminum layer or an aluminum alloy layer at least on its outer surface, an anodic oxide layer (2) formed on said surface layer, and an oxide insulating layer (3) formed by applying a solution containing a ceramics precursor onto said anodic oxide layer and thereafter completely bringing said ceramics precursor into a ceramics state.

Inventors: INAZAWA SHINJI OSAKA WORKS OF, SAWADA KAZUO OSAKA WORKS OF SU, YAMADA KOUICHI OSAKA WORKS OF, INAZAWA, SHINJI OSAKA WORKS OF SUMITOMO ELECTRIC, SAWADA, KAZUO OSAKA WORKS OF SUMITOMO ELECTRIC, YAMADA, KOUICHI OSAKA WORKS OF SUMITOMO ELECTRIC
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: H01B7
IPC codes: C25D11, H01B3, H01B7, C23C18