Method for the production of an electrical conductor having an inorganic insulation

Abstract: Disclosed herein is an inorganic insulating member which comprises a conductor containing Ni or Ni alloy at least in its outer surface, an oxide layer of Ni or Ni alloy formed through oxidation treatment of the outer surface of the conductor, and an insulating inorganic compound layer which is formed on the oxide layer of Ni or Ni alloy. The insulating inorganic compound is hardly separated, and the inorganic insulating member is improved in heat resistance and insulability.

Description: The present invention relates to a method for producing inorganic insulated electrical conductor according to the preamble of claim 1. An insulated electrical conductor (i. e. insulating member) such as an insulated wire is generally 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 polyamide or fluororesin. Such a resin-coated wire can merely withstand a temperature of about 300°C at the most. However, a wire which is employed in a high vacuum apparatus must have high heat resistance against baking, small emission characteristics as to gas and water which are absorbed for achieving and maintaining a high degree of vacuum, and small gas emission caused by thermal decomposition. It is impossible to satisfy such requirements for heat resistance and non-outgassing property with the conventional wire which is coated with an organic material. 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 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 (Mineral Insulated 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. On the other hand, a glass braided tube insulated wire employing an insulating member of glass fiber fabric is known as an insulated wire having heat resistance and flexibility. Further, wires coated with in organic materials are studied, and there has been proposed an alumite-coated wire prepared by alumite-working the surface of an aluminum conductor for forming an Aℓ 2 O 3 film on its surface, and a wire which is formed by electrolysis. However, the aluminum-coated wire and the wire which is formed by electrolysis are inferior in heat resistance to a wire employing a metal such as Cu, since the material for the conductors thereof is restricted to aluminium. Further, such conventional wires have only low breakdown voltages and high gas emission characteristics due to porous films. In the case of the MI cable, on the other hand, the overall diameter is increased as compared with the conductor diameter leading to an inferior space factor. Thus, it is impossible to feed a high current. In the glass braided tube insultated wire, further, fine glas powder is generated and the conductor is disadvantageously exposed due to mesh displacement. US-A-2 975 078 discloses the features of the preamble of claim 1. JP-A-02 301 909 discloses an inorganic insulating member having an Al alloy layer, an oxide layer of the Al alloy and an inorganic insulator layer. EP-A-0 292 780 discloses an electric wire coated by gel film which is formed by applying a solution obtained by hydrolyzing and dehydrating/condensing alkoxide onto an outer part of a conductor. GB-A-2 220 295 refers to superconducting articles having a generally substoichiometric oxygen insulation between superconducting strands. It is the object of the present invention to provide a method for producing an inorganic insulating member, which is excellent in heat resistance and insulability. The invented method for producing this inorganic insulating member comprises the features of claim 1. The oxide layer of Ni or Ni alloy is formed through oxidation treatment of Ni or Ni alloy forming the outer surface of the conductor. Such oxidation treatment is preferably performed in a vapor phase containing oxygen. According to the present invention, the insulating inorganic compound layer of Al 2 O 3 or SiO 2 can be formed on the oxide layer of Ni or Ni alloy by hydrolyzing and polycondensing metal alkoxide or metal carboxylate, for example. The insulating inorganic compound layer can alternatively be formed by thermally decomposing an organic metal polymer. According to the present invention, the insulating inorganic compound layer may contain fine particles of ceramics. The method according to the present invention is applied to produce a wire for a high temperature or an insulated lead wire, for example. However, the present invention is not restricted to such usage. Fig. 1 is a sectional view showing a first embodiment produced according to the present invention. Referring to Fig. 1, an Ni oxide layer 2 is formed around an Ni conductor 1, and an insulating inorganic compound layer 3 is formed around the Ni oxide layer 2. Fig. 2 is a sectional view showing a second embodiment produced according to the present invention. Referring to Fig. 2, an Ni alloy oxide layer 12 is formed around an Ni alloy conductor 11. An insulating inorganic compound layer 13 is formed around the Ni alloy oxide layer 12. Fig. 3 is a sectional view showing a third embodiment produced according to the present invention. Referring to Fig. 3, a diffusion preventing layer 24 of carbon, for example, is provided around a Cu conductor 20. An Ni layer 21 is formed around the diffusion preventing layer 24. An Ni oxide layer 22 is formed around the Ni layer 21, and an insulating inorganic compound layer 23 is formed around the Ni oxide layer 22. According to the present invention, it is possible to employ a metal having higher heat resistance than Aℓ, which is generally employed for a conductor. At least the outer surface of a conductor employed in the present invention is made of Ni or Ni alloy. Although the overall conductor may be made of Ni or Ni alloy, such a material has low conductivity. While Aℓ has conductivity of 60 % IACS, those of Ni and Ni alloy are 25 % IACS and not more than 25 % IACS. In order to improve conductivity, therefore, the outer surface of a Cu conductor may be plated or clad with Ni. When such an Ni-plated or Ni-clad Cu conductor is used under a high temperature for a long time, however, mutual diffusion takes place between Ni and Cu, to form an alloy layer and reduce the conductivity. In order to cope with this, a diffusion preventing layer of e.g. BN may be provided in the interface between Ni and Cu, as shown in Fig. 3. According to the present invention, as hereinabove described, the insulating inorganic compound layer is prepared from SiO 2 or Aℓ 2 O 3 which is obtainable by hydrolyzing and polycondensing metal alkoxide or metal carboxylate. Such metal oxides are extremely dense and have smooth surfaces, whereby the same have high insulability and small gas emission. Further, SiO 2 which is obtained by thermally decomposing organic metal polymers also has high insulability and small gas emission. An insulating inorganic compound layer of such a material has small affinity with Ni or Ni alloy forming the outer surface of the conductor. When this layer is directly applied, therefore, it is impossible to attain high adhesion and the layer is easily separated. Thus, the member cannot be bent. According to the present invention, Ni or Ni alloy forming the outer surface of the conductor is subjected to oxidation treatment for forming an oxide layer of Ni or Ni alloy, so that the insulating inorganic compound layer is formed on this oxide layer. The oxide layer is in extremely close contact with the conductor surface, and has excellent adhesion to the insulating inorganic compound layer. According to the present invention, therefore, the insulating inorganic compound layer is hardly separated, and excellent flexibility is attained when the inventive insulating member is applied to a wire, for example. 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 a first embodiment of the present invention; Fig. 2 is a sectional view showing a second embodiment of the present invention; and Fig. 3 is a sectional view showing a third embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Conductors of (1) an Ni wire of 0.5 mm in wire diameter, (2) Ni - 15 wt.% Cr alloy wire of 0.32 mm in wire diameter, and (3)Ni/BN/Cu clad wire, comprising a Cu wire of 0.38 mm in diameter being clad with an Ni layer of 50 µm in thickness through a carbon layer of 10 µm in thickness, serving as a diffusion preventing layer, were employed to prepare inorganic insulating members according to the present invention. The conductors (1) and (2) were heat treated in the atmosphere at 800°C for 30 minutes for oxidation of the surfaces, thereby forming oxide layers. The conductor (3) was subjected to plasma oxidation treatment in Ar - 10 % O 2 of 10 mTorr (1,33 Pa) for 30 minutes, for forming an oxide layer. The oxidation-treated conductors (1) to (3) were used to prepare wires of Examples 1 to 5. Example 1 Tetrabutyl orthosilicate was hydrolyzed and polycondensed in a solvent of isopropyl alcohol, to prepare a coating solution A. The solution A was applied to the oxidation-treated conductor (3) and heated in the atmosphere at 500°C, to form an insulating inorganic compound layer of SiO 2 . This SiO 2 insulating layer was about 5 µm in thickness. Example 2 Aℓ(NO 3 ) 3 of 8 % was added to the coating solution A, which in turn was applied onto the conductor (1) and heated at 500°C, to form an SiO 2 ·Aℓ 2 O 3 composite layer of 6 µm in thickness. Table 1 shows breakdown voltages and flexibility values of the as-formed wires of Examples 1 and 2. The flexibility values were evaluated in terms of diameter ratios, by winding the wires on circular cylinders of a prescribed diameter and measuring the minimum diameters causing no separation of the insulating inorganic compound layers. Comparative example was prepared from an alumite wire, which was obtained by forming an Aℓ 2 O 3 layer of 10 µm in thickness around a conventional aluminum wire. Breakdown Voltage Flexibility Example 1 600 V 5 D Example 2 700 V 5 D Comparative Example 300 V 50 D As clearly understood from Table 1, the wires of Examples 1 and 2 produced according to the present invention are higher in breakdown voltage and superior in flexibility than the alumite wire of the comparative example. As hereinabove described, the inorganic insulating member produced according to the present invention has an insulating inorganic compound layer which is hardly separated, and is excellent in heat resistance and insulability.

Claims: A method for producing an inorganic insulated electrical conductor comprising: providing a conductor containing Ni or Ni alloy at least in its outer surface; forming an oxide layer of Ni or Ni alloy through oxidation treatment of said outer surface of said conductor; and forming an insulating inorganic compound layer on said oxide layer of Ni or Ni alloy; characterized in that said insulating inorganic compound layer is made of Al 2 O 3 or SiO 2 which is obtainable by hydrolyzing and polycondensing metal alkoxide or metal carboxylate or is made of SiO 2 which is obtained by thermally decomposing an organic metal polymer. A method for producing an inorganic insulated electrical conductor in accordance with claim 1, wherein said oxide layer of Ni or Ni alloy is formed by oxidizing said outer surface of said conductor in a vapor phase containing oxygen. A method for producing an inorganic insulated electrical conductor in accordance with claim 1, wherein said insulating inorganic compound layer contains fine particles of ceramics. A method for producing an inorganic insulated electrical conductor in accordance with claim 1, being applied to produce a heat resistant wire or an insulated lead wire.

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