Method of manufacturing mineral insulated wire

Abstract: A method of manufacturing a mineral insulated wire comprises a step of preparing a gel compound formed by dissolving an metal-organic compound in a solvent and adding at least one thermoplastic polymer or its monomer thereto and a step of extruding the gel compound around the outer periphery of a conductor for coating the conductor and thereafter performing heat treatment for sintering the gel compound.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a mineral insulated wire, which can be applied to heat resistant and fire resistant wires, a radiation resistant nuclear wire, a wire for a vacuum apparatus, and the like. Description of the Background Art An MI cable, a glass braided tube insulated wire, an insulated wire passed through a ceramics tube and the like are known as conventional insulated wires. Such conventional insulated wires are disadvantageous in space and configurations thereof are liable to be restricted to round wires. Also known is an insulated wire manufactured by the so-called wet process, such as Nippon Sheet Glass method (LPD method) or a sol-gel method of applying a ceramic precursor solution which is prepared by hydrolyzing metal alkoxide or the like. However, it is industrially difficult to obtain a thick film by such a wet process. In other words, the thickness of a film formed by a single application/baking operation is extremely small, and the application/baking step must be repeated an extremely large number of times in order to attain a sufficient film thickness. EP-A-0 188 370 discloses an electrical wire comprising a copper conductor and an electrical insulating refractory coating. At least a part of the coating is formed by sol-gel method. EP-A- 0 292 780 doscloses an electric wire which is coated of a gel film formed by apllying a solution obtained by hydrolyzing and dehydrating/condesing alkoxide onto an outer part of a conductor. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of efficiently manufacturing a mineral insulated wire by a wet process, which can be easily industrially applied to a thick film. The object is achieved with a method according to claim 1. The organic compound of a metal employed in the present invention is prepared from metal alkoxide, metal organic acid salt, or the like. The metal alkoxide, which is adapted to form SiO₂, Al₂O₃, ZrO₂, TiO₂, MgO or the like, is composed of ethoxide, propoxide, butoxide or the like. The organic acid salt is preferably prepared from metallic salt such as naphthenic salt, caprylic salt, stearic salt, octylic salt or the like. The term "gel compound" used in this specification indicates a precursor state compound, which is mainly formed by a sol-gel method or an organic acid salt thermal decomposition method and converted to ceramics by heat treatment. A sol solution formed by the sol-gel method, for example, contains a ceramic precursor which is an metal-organic polymer compound polymer of a metal having an alkoxide, a hydroxyl group and metalloxane bonds formed by hydrolytic reaction and dehydration/condensation reaction of a compound having hydrolyzable organic group such as metal alkoxide, an organic solvent such as alcohol for serving as a solvent, metal alkoxide of raw material, and small amounts of water and a catalyst required for hydrolytic reaction. The metalloxane bonds grow with progress of the condensation reaction and volatilization of the solvent etc., whereby the sol solution is converted from a liquid state to an agar-type gel state. In this gel state, organic substances, water and the like are held in a network structure having voids defined by the metalloxane bonds, in such an excellent flexible state that a three-dimensional structure is not completed. Such conversion into the gel state is prompted by heating, although no such heating is required. This gel enters a state hardly containing the organic substances, water and the like in the voids of the network structure upon heating or the like, and is converted to the so-called xerogel. The xerogel is further heated for condensation of the hydroxyl group and growth of the metalloxane bonds, to be finally converted to a metal oxide. The metal organic compound employed in the present invention can be prepared from an organic compound of at least one metal selected from a group of Si, Al, Zr, Ti and Mg. The thermoplastic polymer which is added to the solution of the metal-organic compound is prepared from polyacrylic acid, for example, while the monomer is methacrylic acid or diethylene triamine. According to the present invention, the gel compound can contain ceramics powder, which can be prepared from whiskers, mica or the like, for example. The gel compound is preferably heated when the same is extruded around the outer periphery of a conductor. According to the present invention, further, it is preferable to cause dehydration/condensation reaction and polycondensation reaction for gelation by adding water and an acid catalyst, in order to form the gel compound. The ceramics precursor prepared from metal alkoxide or the like is dehydrated/condensed by heating or the like, to be converted to a gel state. The gel-state ceramics precursor is increased in viscosity and brought into an extrusible jelly state. Such a gel compound is extruded to coat the outer periphery of the conductor. Thereafter the gel compound is heated for facilitating reaction, and further converted to ceramics. Since such a gel compound is employed in the present invention, it is possible to form a thick coating layer around the conductor through a single step. Further, since the gel compound has high viscosity, ceramics particles or the like can be added and homogeneously mixed into the same with no problem of precipitation or the like. Thus, it is possible to reinforce the ceramics film and improve insulability by homogeneously adding the ceramics particles etc. to the gel compound. When the metal organic compound contained in the gel compound employed in the present invention is prepared from an organic compound of a metal such as Si, Al, Zr, Ti or Mg, it is possible to obtain a mineral insulating film having excellent insulability. The thermoplastic polymer which is added to the gel compound can be prepared from silicone resin. When silicone resin is thus employed, it may be possible to improve flexibility of the gel compound, as well as to improve adhesion of the gel compound to the conductor when the same is converted to ceramics. In this case, the content of silicone resin is preferably 15 to 70 parts. An effect attained by addition of silicone resin is reduced if the content thereof is less than 10 parts, while it is difficult to completely convert the gel compound to ceramics if the content exceeds 70 parts. The conductor is preferably formed of Ni, or Cu which is coated with stainless steel, in order to improve oxidation resistance. Alternatively, the conductor is preferably formed of Al having an oxide film of Al, in order to improve adhesion with the film which has been converted to ceramics. According to the inventive method, as hereinabove described, it is possible to easily industrially form a thick mineral insulating layer. In the method according to the present invention, further, the ceramics precursor is converted to ceramics to obtain the mineral insulating layer, whereby heat treatment can be performed at a lower temperature as compared with a melt coating method and the conductor can be prevented from deterioration of characteristics in manufacturing, while heat treatment equipment can be simplified. These 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 DRAWING Fig. 1 is a sectional view showing such a state that the outer periphery of a conductor is coated with a gel compound according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 A solution prepared by diluting 15 mM of tetraethyl orthosilicate with 50 mM of ethanol was added as alkoxide of silicone to a solution prepared by diluting 40 mM of diethylene triamine with 600 mM of water, and mixed at the room temperature. Then the mixture was stirred at the room temperature for several minutes to start whitening, and gelled in about 10 minutes. This gel was aged in a thermostat of 30°C for about eight hours, to obtain a gel compound. Then, a nickel-plated copper wire of 1 mm in wire diameter was vapor-degreased with triperchloroethylene. Thereafter such a gel compound 2 was applied onto the nickel-plated copper wire 1 by extrusion in a thickness of 30 µm, as shown in Fig. 1. As to the extrusion, an outlet temperature (crosshead temperature) of 60°C was employed and heat treatment was continuously performed at 150°C immediately after the application step. A sample of 30 cm in length was obtained from the as-formed insulated wire. Platinum foil members of 0.02 mm in thickness were closely wound on four portions of about 10 mm in length, which were spaced apart at intervals of about 50 mm from each other. An alternating voltage of 60 Hz was applied across the conductor and the metal foil members, whereby a dielectric breakdown was caused at 2.5 kV. The above insulated wire was heated at 500°C for 30 minutes, to obtain a sample of 30 cm in length. Platinum foil members of 0.02 mm in thickness were closely wound on four portions of about 10 mm in length, which were spaced apart at intervals of about 50 mm from each other. An alternating voltage of 60 Hz was applied across the conductor and the metal foil members, whereby a breakdown was caused at 1.2 kV. A heating cycle of holding the insulated wire in an atmosphere with a degree of vacuum of 1 x 10⁻⁴ Torr at a temperature of 700°C for 10 minutes and then cooling the same to the room temperature was repeated ten times, to make a breakdown test. A breakdown voltage of 1.2 kV was maintained. Then, a coil was prepared by winding the insulated wire on a cylinder of 100 mm in diameter and then extracting the cylinder. As the result of a breakdown test, the coiled insulated wire maintained a breakdown voltage of 1.2 kV. Example 2 100 mM of methacrylic acid, 5 mM of magnesium isopropoxide and 25 mM of aluminum isobutoxide were dissolved in a mixed solvent of 50 mM of MEK, 10 mM of acetone and 100 mM of p-xylene and mixed at the room temperature, and then 5 g of silicone resin was further mixed into the mixture. This mixture was concentrated at 110°C, to obtain a gel compound. Then, a nickel-plated copper wire of 1 mm in wire diameter was vapor-degreased with triperchloroethylene. Thereafter such a gel compound 2 was applied onto the nickel-plated copper wire 1 by extrusion in a thickness of 30 µm, as shown in Fig. 1. As to the extrusion, an outlet temperature (crosshead temperature) of 120°C was employed and heat treatment was continuously performed at 150°C immediately after the application step. A sample of 30 cm in length was obtained from the as-formed insulated wire. Platinum foil members of 0.02 mm in thickness were closely wound on four portions of about 10 mm in length, which were spaced apart at intervals of about 50 mm from each other. An alternating voltage of 60 Hz was applied across the conductor and the metal foil members, whereby a breakdown was caused at 2.6 kV. The above insulated wire was heated at 500°C for 30 minutes, to obtain a sample of 30 cm in length. Platinum foil members of 0.02 mm in thickness were closely wound on four portions of about 10 mm in length, which were spaced apart at intervals of about 50 mm from each other. An alternating voltage of 60 Hz was applied across the conductor and the metal foil members, whereby a breakdown was caused at 1.8 kV. A heating cycle of holding the insulated wire in an atmosphere with a degree of vacuum of 1 x 10⁻⁴ Torr at a temperature of 700°C for 10 minutes and then cooling the same to the room temperature was repeated ten times, to make a breakdown test. A breakdown voltage of 1.8 kV was maintained. Then, a coil was prepared by winding the insulated wire on a cylinder of 100 mm in diameter and then extracting the cylinder. The coiled insulated wire maintained a breakdown voltage of 1.8 kV, as the result of a breakdown test. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A method of manufacturing a mineral insulated wire, comprising the steps of: preparing a gel compound (1) formed by dissolving a metal-organic compound in a solvent, and extruding said gel compound (1) around the outer periphery of a conductor (2) for coating said conductor and thereafter performing heat treatment for sintering said gel compound, characterized in that either the thermoplastic polymer polyacrylic acid or one of the monomers methacrylic acid or diethylene triamine is added to said solvent. A method of manufacturing a mineral insulated wire in accordance with claim 1, wherein said metal-organic compound is an organic compound of at least one metal selected from a group of Si, Al, Zr, Ti and Mg. A method of manufacturing a mineral insulated wire in accordance with claim 1 or 2, wherein said gel compound contains ceramics powder. A method of manufacturing a mineral insulated wire in accordance with claim 3, wherein said ceramics powder is prepared from whiskers or mica. A method of manufacturing a mineral insulated wire in accordance with claim 1, wherein said gel compound is heated when the same is extruded around the outer periphery of said conductor. A method of manufacturing a mineral insulated wire in accordance with claim 1, wherein said thermoplastic polymer contains silicone resin. A method of manufacturing a mineral insulated wire in accordance with claim 1, wherein dehydration/condensation reaction and polycondensation reaction are caused by adding water and an acid catalyst for gelation in order to form said gel compound.

Inventors: INAZAWA SHINJI, SAWADA KAZUO, YAMADA KOUICHI, INAZAWA, SHINJI, SAWADA, KAZUO, YAMADA, KOUICHI, INAZAWA, SHINJI, C/O OSAKA WORKS OF SUMITOMO ELEC., SAWADA, KAZUO, C/O OSAKA WORKS OF SUMITOMO ELEC., YAMADA, KOUICHI, C/O OSAKA WORKS OF SUMITOMO ELEC.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1990
Main classification: H01B13
IPC codes: H01B7, H01B13, B29C47, B29L31, H01B3