Method of making an insulating member

Abstract: Disclosed herein is an insulating member which comprises a base material having an outer surface and containing a conductor, a chromium oxide containing layer formed on the outer surface of the base material, and a nitride insulating layer provided around the same. The insulating member has high insulability under high-temperature environment, excellent flexibility and no gas adsorption source.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an insulating member which is applicable to an interconnection wire or a wire for winding in a high vacuum apparatus or an apparatus for a high temperature. Description of the Background Art An insulated wire may be applied to equipment such as heating equipment or a fire alarm, which requires safety under a high temperature. An insulated wire is also employed in an automobile under environment which is heated to a high temperature. Such an insulated wire is generally formed by a conductor which is coated with heat resistant organic resin such as polyimide or fluororesin. When an insulated wire is applied to usage requiring high heat resistance or employed under environment requiring a high degree of vacuum, it is impossible to attain sufficient heat resistance or non-outgassing property with only the aforementioned organic coating. In this case, therefore, generally employed is an insulated wire comprising a conductor which passes through an insulator tube of ceramics, an MI cable comprising a conductor which passes through a tube of a heat resistant alloy, such as stainless steel alloy, filled up with fine particles of a metal oxide such as magnesium oxide, or the like. On the other hand, a glass braided tube insulated wire employing an insulating member of glass fiber fabric or the like is known as an insulated wire having high heat resistance and flexibility. However, an insulated wire coated with organic resin can maintain its insulability merely up to a temperature of about 200½C at the most. Therefore, such an insulated wire cannot be used when insulability is required under a high temperature of at least 200½C. Further, the insulated wire which is improved in heat resistance with an insulator tube of ceramics is inferior in flexibility. On the other hand, the MI cable, which is formed by a heat-resistant alloy tube and a conductor, is increased in outer diameter. Thus, the MI cable has a relatively large section with respect to electric energy which is allowed by the conductor passing through the heat-resistant alloy tube. While it is necessary to bend the heat-resistant alloy tube at a prescribed curvature in order to apply the MI cable to a wire for winding which is coiled on a bobbin or the like, such bending operation is difficult. When the MI cable is coiled, further, it is difficult to improve winding density due to the large diameter. When the glass braided tube insulated wire is arranged in a prescribed configuration, the glass fiber generates glass dust, which may serve as a gas adsorption source. When the glass braided tube insulated wire is employed under environment which requires a high degree of vacuum, therefore, it is impossible to maintain the high degree of vacuum due to the gas adsorption source provided by the glass dust. EP-A-0 416 131 discloses an insulated electric wire comprising a base material having an outer surface and containing a conductor, a chromium oxide containing layer formed on the outer surface of the base material, and an oxide insulating layer formed on the chromium oxide layer. The oxide insulating layer is formed by applying a precursor solution of a metal oxide based on the sol-gel method or an organic acid salt pyrolysis method. GB-A-2 182 800 discloses a flat cable comprising a plurality of metallic conductors, a coating of an insulating compound and a continuous polymeric insulating layer. The compound may be nitride. The oxide or nitride coating is provided by sputtering. SUMMARY OF THE INVENTION The present invention has been proposed in order to solve the aforementioned problems of the conventional insulated wires, and an object thereof is to provide an insulated wire, which has advantages of: (a) high insulability under high-temperature environment; (b) excellent flexibility; (c) provision of no gas adsorption source; and (d) selectability for combination of a base material and an inorganic insulating layer which is applicable in various ways. An insulated wire prepared according to the method of the present invention comprises a base material, a chromium oxide containing layer, and a nitride insulating layer. The base material has an outer surface, and contains a conductor. The chromium oxide containing layer is formed on the outer surface of the base material. The nitride insulating layer is formed on the chromium oxide containing layer. This nitride insulating layer is formed by thermal decomposition of an organic metal polymer. According to the present invention, the chromium oxide containing layer is preferably formed by an electrochemical method such as electrolytic plating or electroless plating. The chromium oxide containing layer serving as an underlayer for the nitride insulating layer preferably has an outermost layer which serves as an adhesion layer for the nitride insulating layer. To this end, the outermost layer is preferably prepared from CrO 3-x (1.5 ≦ x ≦ 2.5). The outermost layer of the chromium oxide containing layer, which is formed by an electrochemical method, is defined by such a layer of chromium oxide having excellent adhesion. According to the present invention, the nitride insulating layer preferably contains silicon nitride and/or aluminum nitride. According to the present invention, further, the base material is preferably made of copper or copper alloy, in view of high conductivity and a low cost. In consideration of usage under a high temperature, the base material may be formed by a conductor which is coated with nickel, chromium, silver, iron or iron alloy such as stainless steel, or titanium or titanium alloy. In this case, a layer of such a metal or alloy can be formed on a surface of copper or copper alloy by plating or a cladding method. According to the present invention, a metal oxide insulating layer may be formed by a sol-gel method between the chromium oxide containing layer and the nitride insulating layer. The sol-gel method is a method of forming a sol of a precursor for a metal oxide by hydrolyzing and dehydrating/polycondensing a hydrolyzable compound having metal-oxygen-organic group bonding such as metal alkoxide or metal carboxylic acid ester and forming a metal oxide through a gel by appropriate heat treatment. It is known that a chromium-plated layer is formed on a conductor of copper or copper alloy as an excellent adhesion layer. When such a chromium-plated layer is to be coated with an insulating nitride ceramics layer of silicon nitride or the like which is prepared by heat treatment of a precursor solution for a metal oxide, however, such nitride ceramics hardly exhibits adhesion to the chromium-plated layer. The inventors have empirically found this fact. When an insulated wire is prepared by directly forming a thin film of ceramics such as a nitride on the surface of a conductor of copper or the like, the thin film of ceramics, serving as an insulating layer, has insufficient adhesion to the base material. According to the present invention, therefore, a chromium oxide containing layer having an outermost layer of chromium oxide is formed on the outer surface of a base material. A layer of insulating nitride ceramics having excellent adhesion is provided on the outermost layer of the chromium oxide containing layer. According to the present invention, the chromium oxide containing layer is preferably formed by an electrochemical method, as hereinabove described. When the chromium oxide containing layer is formed by electroplating, the electrolytic bath is preferably prepared by adding a small amount of organic acid to an aqueous solution of chromic acid. This electrolytic bath is different from a Sargent bath, mainly containing chromic acid and sulfuric acid, which is known as an electrolytic bath generally employed for chrome plating, as follows: Mineral acid which is mixed into an electrolytic bath is adapted to dissolve chromium oxide formed on a plated surface in electroplating. Therefore, a glossy metal chromium layer is plated through a Sargent bath. In a chromium oxide containing layer formed in the present invention, on the other hand, it is necessary to preferentially deposit and apply chromium oxide. According to the present invention, therefore, organic acid is employe din place of mineral acid. According to the present invention, the as-formed layer, which is mainly composed of chromium oxide, preferably has a rough surface, since the same is further coated with an intermediate layer such as a nitride insulating layer or a metal oxide insulating layer. In a preferred embodiment of the present invention, such preferential formation of chromium oxide and the rough surface can be attained by performing plating at a current density which is different from that for general gloss plating. In general, gloss plating is performed at a current density of 10 to 60 A/dm², depending on the treatment temperature. In the preferred embodiment of the present invention, however, a current density of 100 to 200 A/dm² is employed to form a chromium oxide containing layer having a rough surface. According to the present invention, the nitride insulating layer is formed by thermally decomposing an organic metal polymer. Such an organic metal polymer can be prepared from alkyl aminosilicate such as polysilazane, for example. This heat treatment is preferably performed under an atmosphere of ammonia or nitrogen jet. The organic metal polymer can be substantially completely decomposed into a nitride by such heat treatment at a temperature of about 700°C. In the insulated wire according to the present invention, the chromium oxide containing layer is formed on the outer surface of the base material, and the nitride insulating layer is formed on the chromium oxide containing layer. The chromium oxide containing layer is excellent in adhesion to the base material, as well as to a layer such as the nitride insulating layer or a metal oxide insulating layer. Therefore, high adhesion can be attained as compared with a case of directly forming a nitride insulating layer or a metal oxide insulating layer on the outer surface of the conductor. Thus, the insulated wire according to the present invention has heat resistance and insulability, as well as excellent flexibility. The nitride insulating layer formed on the chromium oxide containing layer has a smooth outer surface. Thus, it is possible to obtain a high breakdown voltage which is proportionate to the film thickness and reduce a gas adsorption source, whereby the insulated wire provides a high degree of vacuum in a high vacuum apparatus. In the insulated wire according to the present invention, the nitride insulating layer is formed on the chromium oxide containing layer. Since any type of nitride insulating layer can be formed on the chromium oxide containing layer with excellent adhesion, it is possible to combine a nitride insulating layer which is suitably applied in various ways. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing Example 1 of the present invention; Fig. 2 is a sectional view showing Example 2 of the present invention; and Fig. 3 is a sectional view showing Example 3 of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 (a) Formation of Chromium Oxide Containing Layer Electrolytic plating was performed on the outer surface of a nickel-plated copper wire of 1.8 mm in wire diameter. The electrolyte was prepared to contain 200 g/ℓ of chromic anhydride and 20 g/ℓ of acetic acid. As to plating conditions, the base material was used as a cathode under a bath temperature of 50°C with a current density of 150 A/dm² and a treatment time of 2 minutes. Thus, a chromium oxide containing layer was formed on the outer surface of the nickel-plated copper wire with a thickness of about 1 µm. (b) Preparation of Coating Solution 15 mℓ of dichlorosilane and 40 mℓ of triethylamine were heated in an autoclave for 5 hours, to prepare polysilazane. The as-obtained polysilazane was diluted with 100 mℓ of tetrahydrofuran, to prepare a coating solution. (c) Coating The wire obtained in the above process (a) was dipped in the coating solution obtained in the process (b). The wire, whose outer surface was thus coated with the coating solution, was heated under a nitrogen atmosphere at a temperature of 700°C for 10 minutes. The steps of dipping the wire in the coating solution and heating the same were repeated 10 times. Thus, an organic metal polymer was applied onto a chromium oxide containing layer and heated to prepare a nitride insulating layer. Fig. 1 shows the as-obtained insulated wire. Referring to Fig. 1, a nickel-plated layer 2 is formed on the outer surface of a copper wire 1. A chromium oxide containing layer 3 is formed on the nickel-plated layer 2. A nitride insulating layer 4 prepared by heat treating a precursor for a metal nitride is provided on the chromium oxide containing layer 3. In this Example, the nitride insulating layer 4 was made of silicon nitride. Further, a layer defined by the chromium oxide containing layer 3 and the nitride insulating layer 4 was about 5 µm in thickness. In order to evaluate insulability of the as-obtained insulated wire, the breakdown voltage was measured. The breakdown voltage of this insulated wire was 500 V under the room temperature, and 300 V under a temperature of 800°C. When this insulated wire was wound on the outer peripheral surface of a cylinder of 3 cm in diameter, no crack was caused in the insulating layer. Example 2 (a) Formation of Chromium Oxide Containing Layer A copper wire clad with stainless steel (SUS304) was prepared with a wire diameter of 1.8 mm. The stainless steel layer was 200 µm in thickness. This copper wire, clad with stainless steel, was used as a base material, so that its surface was chrome-plated with an electrolyte containing 200 g/ℓ of chromic anhydride and 20 g/ℓ of acetic acid. As to plating conditions, the base material was used as a cathode under a bath temperature of 50°C, with a current density of 150 A/dm² and a treatment time of 2 minutes. Through such chrome plating, a chromium oxide containing layer was formed on the surface of the copper wire, clad with stainless steel, with a thickness of about 1 µm. (b) Preparation of Coating Solution Tris(N-methylamino)methylsilane was heated in an autoclave at 500°C for 3 hours, to prepare polysilazane. 10 g of the as-obtained polysilazane was diluted with 100 mℓ of tetrahydrofuran, naturally cooled at the room temperature, and thereafter mixed with 3 g of aluminum nitride particles of 1.5 µm in nominal particle diameter, to prepare a coating solution. (c) Coating The wire obtained in the above process (a) was dipped in the coating solution prepared in the process (b). The wire, whose outer surface was thus coated with the coating solution, was heated at 500°C for 10 minutes. The steps of dipping the wire in the coating solution and heating the same were repeated 10 times. Thus, a chromium oxide containing layer was coated with an organic metal polymer, and thermally decomposed to form a nitride insulating layer. Fig. 2 shows this insulated wire. Referring to Fig. 2, a stainless steel layer 12 is formed on the outer surface of a copper wire 11 as a clad layer. A chromium oxide containing layer 13 is formed on the stainless steel layer 12. A nitride insulating layer 14 is formed on the chromium oxide containing layer 13. Nitride particles 15, which are aluminum nitride particles, are dispersed in the nitride insulating layer 14. In this Example, a layer defined by the chromium oxide containing layer 13 and the nitride insulating layer 14 was 12 µm in thickness. In order to evaluate insulability of the as-obtained insulated wire, the breakdown voltage was measured. The breakdown voltage of this wire was 900 V under the room temperature, and 700 V under a temperature of 800°C. When this insulated wire was wound on the outer peripheral surface of a cylinder of 15 cm in diameter, no crack was caused in the insulating layer. Example 3 Electrolytic plating was performed on the surface of a nickel-plated copper wire in a similar manner to Example 1, to form a wire of 0.5 mm in wire diameter having a chromium oxide containing layer on its surface. In this wire, the chromium oxide containing layer was 1.0 µm in thickness. Then, a solution for forming a metal oxide insulating layer was prepared by a sol-gel method. Nitric acid was added to a solution, containing tetrabutyl orthosilicate, water and isobutyl alcohol in mol ratios of 8:32:60, at a rate of 3/100 mol. This mixture was heated at a temperature of 80°C for 2 hours, to prepare a coating solution. This solution was applied onto the aforementioned wire having a chromium oxide containing layer on its surface and heated in the atmosphere at 600°C for 15 minutes, to form a metal oxide insulating layer of 4 µm in thickness. The breakdown voltage of this wire having a metal oxide insulating layer on its surface was 400 V, and it was impossible to wind this wire on a cylinder whose diameter was less than 40 mm. Polysilazane was prepared in a similar manner to Example 1, to form a nitride insulating layer of 7 µm in thickness on the surface of the wire having a metal oxide insulating layer. In this case, the wire exhibited a breakdown voltage of 1400 V, and it was possible to bend the same to a diameter of 20 mm. Another wire was prepared to have a nitride insulating layer of 2 µm in thickness. This wire exhibited a breakdown voltage of 600 V, and it was possible to bend the same to a diameter of 5 mm. Fig. 3 is a sectional view showing a wire of this Example having a chromium oxide containing layer, a metal oxide insulating layer provided thereon and a nitride insulating layer formed thereon. Referring to Fig. 3, a nickel-plated layer 22 is provided around a copper wire 21, and a chromium oxide containing layer 23 is provided around the nickel-plated layer 22. A metal oxide insulating layer 24 is provided around the chromium oxide containing layer 23, and a nitride insulating layer 25 is provided around the metal oxide insulating layer 24.

Claims: Method for preparing an insulating member comprising: a base material having an outer surface and containing a conductor (1); a chromium oxide containing layer (3) formed on said outer surface of said base material; and a nitride insulating layer (4), wherein said nitride insulating layer (4) is obtained by thermal decomposition of an organic metal polymer and is formed on said chromium oxide containing layer. The method in accordance with claim 1, characterized in that said insulating member further comprises a metal oxide insulating layer (24), being formed by a sol-gel method, between said chromium oxide containing layer and said nitride insulating layer. The method in accordance with claim 1, characterized in that said insulating member is an insulated wire. The method in accordance with claim 1, characterized in that said organic metal polymer is alkyl aminosilicate. The method in accordance with claim 1, characterized in that said chromium oxide containing layer (3,23) is formed by electrolytic plating. The method in accordance with claim 1 characterized in that said nitride insulating layer (4,14) contains silicon nitride and/or aluminum nitride. The method in accordance with claim 1, characterized in that said base material (1) is made of copper or copper alloy, or prepared by forming a layer of nickel, chromium or stainless steel on a surface of copper or copper alloy by plating or a cladding method. The method in accordance with claim 1, characterized in that fine particles of ceramics (15) are dispersed in said nitride insulating layer.

Inventors: INAZAWA SHINJI C O OSAKA WORKS, SAWADA KAZUO C O OSAKA WORKS, YAMADA KOUICHI C O OSAKA WORKS, INAZAWA, SHINJI, C/O OSAKA WORKS, SAWADA, KAZUO, C/O OSAKA WORKS, YAMADA, KOUICHI, C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1991
Main classification: H01B7
IPC codes: H01B7, H01B3