Resin-impregnated superconducting magnet coil and process for its production

Abstract: A superconducting magnet coil contains a coil of superconducting wire (2) and a cured product (3) of a curable resin composition with which the coil has been impregnated, the cured product having a thermal shrinkage factor of 1.5-0.3%, preferably 1.0-0.3%, when cooled from the glass transition temperature to 4.2 K, a bend-breaking strain of 2.9-3.9%, preferably 3.2-3.9%, at 4.2 K and a modulus of 500-1,000 kg/mm² at 4.2 K, or undergoing a thermal stress of 0-10 kg/mm² when cooled from the glass transition temperature to 4.2 K and resisting to quench during superconducting operation. It is produced by winding a superconducting wire (2) to form a coil; impregnating the coil with a curable resin composition of low viscosity which contains for example at least one epoxy resin selected from the group consisting of diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F and diglycidyl ether of bisphenol AF, all having a number-average molecular weight of 350-1,000, a flexibilizer and a curing catalyst, to obtain a curable-resin composition-impregnated coil; and heating the curable-resin-composition-impregnated coil to cure the composition.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a superconducting magnet coil, the insulating layer thereof and a process for making them. (2) Description of the Prior Art In superconducting magnet coils used, by being dipped in liquid helium, in linear motor cars, superconducting electromagnetic propulsion vessels, nuclear fusion reactors, superconducting generators, MRI, pion applicators (for therapy), electron microscopes, energy storage apparatuses, etc., the superconducting wires contained in the coil cause a temperature increase incurred by frictional heat or the like when the superconducting wires are moved by an electromagnetic force or a mechanical force. As a result, the magnet may shift from the superconducting state to the state of normal conduction. This phenomenon is called "quench". Hence, in some cases the gap between the wires of the coil is filled with a resin such as an epoxy resin or the like to fix the wires. The resins such as epoxy resins or the like, used for filling the coil gaps usually have a thermal shrinkage factor of 1.8 to 3.0 % when cooled from the glass transition temperature to liquid helium temperature, i.e. 4.2 K. Meanwhile, the superconducting wires have a thermal shrinkage factor of about 0.3 to 0.4 % under the same condition. When a superconducting magnet coil comprising superconducting wires and a resin used for filling the gaps between the wires is cooled to liquid helium temperature, i.e. 4.2 K, a residual thermal stress appears due to the difference in the thermal shrinkage factors of the superconducting wires and of the resin. As a result, impregnating resins such as epoxy resins or the like get very brittle, and microcracks of several micrometers appear in the resin, due to electromagnetic or mechanical forces a temperature increase of several degrees is induced at the peripheries of the microcracks due to the releasing energy of the residual thermal stress of the resin, and the superconducting wires show a sharp rise in resistance. Finally, the superconducting magnet coil shifts from the superconducting state to the state of normal conduction and causes the undesirable quench phenomenon. In Cryogenics 25 (1985) 307-316 it is disclosed that the premature quench in a composite made of a copper matrix, NbTi filaments and an epoxy matrix can be avoided by minimizing the shear stresses induced by the Lorentz forces during the energization of the magnet. The reduction of the shear stresses is achieved according to this prior art by letting the whole winding body separate from the coil form ("floating coil" concept) and take its natural shape as the magnet is energized. In this document, properties of the epoxy resin used at 4.2 K are indicated, inter alia the Young modulus which is 8000 MPa (815 kg/mm 2 ) . SUMMARY OF THE INVENTION The present invention has been made in view of the above situation. The object of the present invention is to provide a superconducting magnet coil which is resistant to microcrack generation of the impregnant resin and causes substantially no quench during operation, an insulating layer thereof, and a process for making them. The above object is achieved according to the independent claims. The dependent claims relate to prefered embodiments. The superconducting magnet coil of the present invention comprises a coil of a superconducting wire having a thermal shrinkage factor of 0.3 to 0.4 % when cooled to 4.2 K, and a cured product of a curable resin composition with which the coil has been impregnated, having a modulus of 4,900 to 9,810 Mpa (500 to 1000 kg/mm 2 ) at 4.2 K, a thermal shrinkage factor of 1.5 to 0.3 % and particularly of 1.0 to 0.3 %, when cooled from the glass transition temperature to 4.2 K, and a bend-breaking strain of 2.9 to 3.9 %, and preferably 3.2 to 3.9 %, at 4.2 K. The process of the present invention for producing the superconducting magnet coils comprises the following steps: (a) winding a superconducting wire having a thermal shrinkage factor of 0.3 to 0.4 % when cooled to 4.2 K to form a coil, (b) filling the gaps between the superconductors of the coil with a curable resin composition having a viscosity of 10 -3 to 1 Pa·s (0.01 to 10 Poise) at the time of filling to obtain a coil impregnated with the curable resin composition, and (c) heating the impregnated coil to cure the composition and obtain a cured product having a modulus of 4,900 to 9,810 Mpa (500 to 1,000 kg/mm 2 ), a thermal shrinkage factor of 1.5 to 0.3 %, and particularly of 1.0 to 0.3 %, when cooled from the glass transition temperature to 4.2 K, and a bend-breaking strain of 2.9 to 3.9 % at 4.2 K. In accordance with the above, the insulating layer of a superconducting magnet coil of the present invention comprises (a) a coil of a superconducting wire having a thermal shrinkage factor of 0.3 to 0.4 % when cooled to 4.2 K, and comprising a plurality of thin superconducting wires and a stabilizer selected from copper and aluminum which is thermally or electrically in contact with the superconducting wires, and (b) a cured product of a curable resin composition with which the coil has been impregnated, having a modulus of 4,900 to 9,810 Mpa (500 to 1000 kg/mm 2 ) at 4.2 K, a thermal shrinkage factor of 1.5 to 0.3 % when cooled from the glass transition temperature to 4.2 K, and a bend-breaking strain of 2.9 to 3.9 % at 4.2 K. The insulating layer of the present invention is obtained by impregnation of a coil of superconducting wire with the curable resin composition and curing of the resin composition. According to the present invention, the cured product of the curable resin composition with which the coil has been impregnated, preferably undergoes a thermal stress of 0 to 98.1 MPa (0 to 10 kg/mm 2 ) when cooled from the glass transition temperature to 4.2 K and resists to quench during superconducting operation. According to a prefered embodiment of the process for producing the superconducting magnet coil the curable resin composition comprises (i) at least one epoxy resin selected from diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F and diglycidyl ether of bisphenol AF, all having a number-average molecular weight of 350-1,000, (ii) a flexibilizer and (iii) a curing catalyst. In accordance with another prefered embodiment, step (b) includes the steps of covering the outer surface of the coil with a release film or a perforated film, placing the film-covered coil in a mold, and effecting vacuum impregnation, and, if necessary, pressure impregnation, of the coil with the curable resin composition. Preferably, step (c) includes the step of curing the composition under pressure, and, if necessary, further comprises the step of clamping the curable-resin-composition-impregnated coil before the step of curing. According to get another prefered embodiment, the superconducting wire has a composite structure (a) a plurality of thin superconducting wires and (b) a stabilizer selected from copper and aluminum which is in thermal or electrical contact with the wires. Preferably, in step (a), the composite superconductor wire is subjected to surface treatment with a coupling agent before winding. The curable resin composition according to the present invention can also be preferably used in switches for permanent current which are required in superconducting magnet coils for linear motor cars, MRI, energy storage and nuclear fusion devices. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a race track-shaped, round superconducting magnet coil 1. Fig. 2 is a cross-sectional view of the coil of Fig. 1 when cut at the line II-II'. Fig. 3 is a fragmentary enlarged view of Fig. 2 of a conventional race track-shaped superconducting magnet coil. Fig. 4 is a perspective view of a saddle-shaped superconducting magnet coil. Fig. 5 is a cross-sectional view of the coil of Fig. 4 when cut at line V-V'. DETAILED DESCRIPTION OF THE INVENTION The superconducting wire used in the present invention has no particular restriction and can be any wire as long as it may have superconductivity. There can be mentioned, for example, alloy superconductors such as Nb-Ti and the like; intermetallic compound superconductors such as Nb 3 Sn, Nb 3 Al, V 3 Ga and the like; and oxide superconductors such as LaBaCuO, YBaCuO and the like. Ordinarily, the superconducting wire has a composite structure comprising (a) the above superconductor and (b) a metal of normal conduction such as Cu, cupro-nickel (CuNi), CuNi-Cu, Al or the like. That is, the superconducting wire includes an ultrafine multiconductor wire obtained by embedding a large number of thin filament-like superconducting wires into a metal of normal conduction as a matrix, a straight twisted wire obtained by binding a large number of superconducting material wires into a straight bundle and twisting the bundle with the straightness being maintained, a straight wire obtained by embedding a straight superconducting material wire into a straight normal conductor, and an internal cooling type conductor having inside a passage for cooling medium. The resin for impregnation of superconducting magnet coils used in the present invention has no particular restriction and can be any resin as long as it can give a cured product having a thermal shrinkage factor of 1.5-0.3 % when cooled from the glass transition temperature to liquid helium temperatur, i.e. 4.2 K, a bend-breaking strain of 2.9-3.9 % at 4.2 K and a modulus of 4,900 to 9,810 MPa (500-1,000 kg/mm 2 ) at 4.2 K, and particularly a cured product having a thermal shrinkage factor of 1.0-0.3 % when cooled from the glass transition temperature to liquid helium temperature, i.e. 4.2 K, a bend-breaking strain of 2.9-3.9 % at 4.2 K and a modulus of 4,900 to 9,810 MPa (500-1,000 kg/mm 2 ) at 4.2 K. When the cured product of the resin has a thermal shrinkage factor larger than 1.5 % and a modulus larger than 9,810 Mpa (1,000 kg/mm 2 ), the stress applied to the superconducting magnet during the superconducting operation surpasses the strength of the cured product. As a result, in the cured product cracks are generated, and quench occurs due to the releasing energy of the stress. When the cured product has a thermal shrinkage factor smaller than 0.3 %, the stress applied to the superconducting magnet during the superconducting operation surpasses the strength of the cured product due to the difference in thermal shrinkage factor between the cured product and the superconductor of the magnet. As a result, in the cured product cracks are generated, and quench tends to occur due to the releasing energy of the stress. When the modulus is smaller than 4,900 MPa (500 kg/mm 2 ), the glass transition temperature tends to be lower than room temperature and, when the superconducting magnet has been returned to room temperature, in the cured product cracks are generated due to the low strength; when the magnet is recooled to 4.2 K and reoperated, the cracks become nuclei of further crack generation, and the superconducting magnet causes quench. When the bend-breaking strain is smaller than 2.9%, the cured product has low adhesion to the superconductor and, after the cooling or during the operation of the superconducting magnet, peeling takes place between the superconductor and the cured product. As a result, the thermal conductivity between them is reduced, even slight cracking leads to a temperature increase, and the superconducting magnet tends to incur quench. For increasing the bend-breaking strain of a thermosetting resin, that is, for toughening a thermosetting resin, there are a number of methods available In the case of an epoxy resin, for example, there are (1) a method of subjecting an epoxy resin to preliminary polymerization to obtain an epoxy resin having a higher molecular weight between crosslinked sites, (2) a method of adding a flexibilizer (e.g. polyol, phenoxy resin) to an epoxy resin to increase the specific volume of the latter, (3) a method of introducing a soft molecular skeleton into an epoxy resin by using a curing agent such as elastomer-modified epoxy resins, long-chain epoxy resins, long-chain amines, acid anhydrides, mercaptans or the like, (4) a method of using an internal plasticizer such as branched epoxy resins, polyamide-amines, dodecyl succinic anhydrides or the like, (5) a method of using, in combination with an epoxy resin, a monofunctional epoxy resin to give rise to internal plasticization, (6) a method of using an epoxy resin as a main component and a curing agent in proportions deviating from the stoichiometric amounts to give rise to internal plasticization, (7) a method of adding a plasticizer (e.g. phthalic acid ester) to give rise to external plasticization, (8) a method of dispersing butadiene rubber particles, silicone rubber particles or the like in an epoxy resin to form an islands-in-the-sea structure, (9) a method of introducing, into an epoxy resin, an acrylic resin, an urethane resin, a polycaprolactone, an unsaturated polyester or the like to form an interpenetrating network structure (IPN structure), (10) a method of adding, to an epoxy resin, a polyether having a molecular weight of 1,000-5,000 to form a microvoid structure, and so forth. Of these methods, the methods (1) and (2) are preferable in view of the low thermal shrinkage and high toughness of the improved epoxy resin. Specific examples of the improved epoxy resin obtained according to the above method are epoxy resins obtained by curing an epoxy resin of high molecular weight with an acid anhydride, epoxy resins obtained by curing an epoxy resin of high molecular weight with a catalyst alone, epoxy resins obtained by adding a flexibilizer to an epoxy resin and curing the resin with an acid anhydride, epoxy resins obtained by adding a flexibilizer to an epoxy resin and curing the resin with a catalyst alone, and maleimide resins obtained by adding a flexibilizer. The epoxy resin usable in the present invention can be any epoxy resin as long as it has at least two epoxy groups in the molecule. Such epoxy resins are, for example, bifunctional epoxy resins such as diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol AF, diglycidyl ether of bisphenol AD, diglycidyl ether of hydrogenated bisphenol A, diglycidyl ether of 2,2-(4-hydroxyphenyl)nonadecane, 4,4'-bis(2,3-epoxypropyl)diphenyl ether, 3,4-epoxycyclohexylmethyl-(3,4-epoxy)-cyclohexanecarboxylate, 4-(1,2-epoxypropyl)-1,2-epoxycyclohexane, 2-(3,4-epoxy)-cyclohexyl-5,5-spiro(3,4-epoxy)-cyclohexane-m-dioxane, 3,4-epoxy-6-methylcyclohexylmethyl-4-epoxy-6-methylcyclohexanecarboxylate, butadiene-modified epoxy resins, urethane-modified epoxy resins, thiol-modified epoxy resins, diglycidyl ether of diethylene glycol, diglycidyl ether of triethylene glycol, diglycidyl ether of polyethylene glycol, diglycidyl ether of polypropylene glycol, diglycidyl ether of 1,4-butanediol, diglycidyl ether of neopentyl glycol, diglycidyl ethers of propylene oxide adducts of bisphenol A, diglycidyl ethers of ethylene oxide adducts of bisphenol A, and the like; trifunctional epoxy resins such as tris[p-tris[p-(2,3-epoxypropoxy)-phenyl]methane, 1,1,3-tris[p-(2,3-epoxypopoxy)-phenyl]butane and the like; and polyfunctional epoxy resins such as glycidylamines (e.g. tetraglycidyldiaminodiphenylmethane, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, diglycidylamine, tetraglycidyl-m-xylylenediamine, tetraglycidyl-bis(aminomethylcyclohexane), phenolic novolac type epoxy resins, cresol type epoxy resins, and the like. It is also possible to use a polyfunctional epoxy resin obtained by reacting epichlorohydrin with at least two polyhydric phenols selected from (a) bis(4-hydroxyphenyl)methane, (b) bis(4-hydroxyphenyl)ethane, (c) bis(4-hydroxyphenyl)propane, (d) tris(4-hydroxyphenyl)alkanes and (e) tetrakis(4-hydroxyphenyl)alkanes, because the resin has a low viscosity before curing and gives easy working. Specific examples of tris(4-hydroxyphenyl)alkanes are tris(4-hydroxyphenyl)methane, tris(4-hydroxyphenyl)ethane, tris(4-hydroxyphenyl)propane, tris(4-hydroxyphenyl)butane, tris(4-hydroxyphenyl)hexane, tris(4-hydroxyphenyl)heptane, tris(4-hydroxyphenyl)octane, tris(4-hydroxyphenyl)nonane, etc. There can also be used tris(4-hydroxyphenyl)alkane derivatives such as tris(4-hydroxydimethylphenyl)methane and the like. Specific examples of tetrakis(4-hydroxyphenyl)alkane are tetrakis(4-hydroxyphenyl)methane, tetrakis(4-hydroxyphenyl)ethane, tetrakis(4-hydroxyphenyl)propane, tetrakis(4-hydroxyphenyl)butane, tetrakis(4-hydroxyphenyl)hexane, tetrakis(4-hydroxyphenyl) heptane, tetrakis(4-hydroxyphenyl)octane, tetrakis(4-hydroxyphenyl)nonane and the like. It is also possible to use tetrakis(4-hydroxyphenyl)alkane derivatives such as tetrakis(4-hydroxydimethylphenyl)methane and the like. Of these, useful are diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol AF, diglycidyl ether of bisphenol AD, and diglycidyl ethers of higher-molecular-weight bisphenols A, F, AF and AD, because they have a low thermal shrinkage factor. Particularly preferable are diglycidyl ethers of higher-molecular-weight bisphenols A, F, AF and AD wherein the number n of the repeating units has a value of 2-18. The above polyfunctinal epoxy resins may be used in combination of two or more. If necessary, the polyfunctional epoxy resin may be mixed with a monofunctional epoxy resin such as butyl glycidyl ether, styrene oxide, phenyl glycidyl ether, allyl glycidyl ether or the like in order to obtain a lower viscosity. However, the amount of the monofunctional epoxy resin added should be small because, in general, the monofunctional epoxy resin has an effect for viscosity reduction but brings about an increase in the thermal shrinkage factor. The acid anhydride used in the present invention has no particular restriction and can be any ordinary acid anhydride. Such acid anhydrides are e.g. methylhexahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, tetrahydrophthalic anhydride, nadic anhydride, methylnadic anhydride, dodecylsuccinic anhydride, succinic anhydride, octadecylsuccinic anhydride, maleic anhydride, benzophenonetetracarboxylic anhydride, ethylene glycol bis(anhydrotrimellitate), glycerol tris(anhydrotrimellitate), etc. They can be used alone or in combination of two or more. The maleimide used in the present invention can be any maleimide as long as it is an unsaturated imide containing in the molecule the group having the formula (I), wherein D is a bivalent group containing a carbon-carbon double bond. Such unsaturated imides are, for example, bifunctional maleimides such as N,N'-ethylenebismaleimide, N,N'-hexamethylene-bismaleimide, N,N'-dodecamethylene-bismaleimide, N,N'-m-xylylene-bismaleimide, N,N'-p-xylylene-bismaleimide, N,N'-1,3-bismethylenecyclohexane-bismaleimide. N,N'-1,4-bismethylenecyclohexane-bismaleimide, N,N'-2,4-tolylene-bismaleimide, N,N'-2,6-tolylene-bismaleimide, N,N'-3,3'-diphenylmethane-bismaleimide, N,N'-(3-ethyl)-3,3'-diphenylmethane-bismaleimide, N,N'-(3,3'-dimethyl)-3,3'-diphenylmethane-bismaleimide, N,N'-(3,3'-diethyl)-3,3'-diphenylmethane-bismaleimide, N,N'-(3,3'-dichloro)-3,3'-diphenylmethane-bismaleimide, N,N'-4,4'-diphenylmethane-bismaleimide, N,N'-(3-ethyl)-4,4'-diphenylmethane-bismaleimide, N,N'-(3,3'-dimethyl)-4,4'-diphenylmethane-bismaleimide, N,N'-(3,3'-diethyl)-4,4'-diphenylmethane-bismaleimide, N,N'-(3,3'-dichloro)-4,4'-diphenylmethane-bismaleimide, N,N'-3,3'-diphenylsulfone-bismaleimide, N,N'-4,4'-diphenylsulfone-bismaleimide, N,N'-3,3'-diphenylsulfide-bismaleimide, N,N'-4,4'-diphenylsulfide-bismaleimide, N,N'-p-benzophenone-bismaleimide, N,N'-4,4'-diphenylethane-bismaleimide, N,N'-4,4'-diphenylether-bismaleimide, N,N'-(methyleneditetrahydrophenyl)bismaleimide, N,N'-tolidinebismaleimide, N,N'-isophorone-bismaleimide, N,N'-p-diphenyldimethylsilyl-bismaleimide, N,N'-4,4'-diphenylpropane-bismaleimide, N,N'-naphthalene-bismaleimide, N,N'-p-phenylene-bismaleimide, N,N'-m-phenylene-bismaleimide, N,N'-4,4'-(1,1'-diphenyl-cyclohexane-bismaleimide, N,N'-3,5-(1,2,4-triazole)bismaleimide, N,N'-pyridine-2,6-diyl-bismaleimide, N,N'-5-methoxy-1,3-phenylene-bismaleimide, 1,2-bis(2-maleimidoethoxy)ethane, 1,3-bis(3-maleimidopropoxy)propane, N,N'-4,4'-diphenylmethane-bisdimethylmaleimide, N,N'-hexamethylene-bisdimethylmaleimide, N,N'-4,4'-(diphenylether)-bisdimethylmaleimide, N,N'-4,4'-(diphenylsulfone)-bisdimethylmaleimide, N,N'-bismaleimide of 4,4'-diaminotriphenyl phosphate, N,N'-bismaleimide of 2,2'-bis[4-(4-aminophenoxy)-phenyl]propane, N,N'-bismaleimide of 2,2'-bis[4-(4-aminophenoxy)-phenylmethane, N,N'-bismaleimide of 2,2'-bis[4-(4-amino-phenoxy)-phenylethane and the like; polyfunctional maleimides obtained by reacting maleic anhydride with an aniline-formalin reaction product (a polyamine compound), 3,4,4'-triaminodiphenylmethane, triaminophenol or the like; monomaleimides such as phenylmaleimide, tolylmaleimide, xylylmaleimide and the like; various citraconimides; and various itaconimides. These unsaturated imides can be used by adding to an epoxy resin, or can be cured with a diallylphenol compound, an allylphenol compound or a diamine compound or with a catalyst alone. The flexibilizer used in the present invention can be any flexibility-imparting agent as long as it can impart flexibility, toughness and adhesion. Such flexibilizers are, for example, diglycidyl ether of linoleic acid dimer, diglycidyl ethers of polyethylene glycols, diglycidyl ethers of polypropylene glycols diglycidyl ethers of alkylene oxide adducts of bisphenol A, urethane-modified epoxy resins polybutadiene-modified epoxy resins, polyethylene glycols, polypropylene glycols, polyols (e.g. hydroxyl group-terminated polyesters), polybutadienes, alkylene oxide adducts of bisphenol A, polythiols, urethane prepolymers, polycarboxyl compounds phenoxy resins and polycaprolactones. The flexibilizer may be a low viscosity compound such as caprolactone or the like, which is polymerized at the time of curing of the impregnant resin and thereby exhibits flexibility. Of the above flexibilizers, polyols, phenoxy resins or polycaprolactones are preferable in view of the high toughness and low thermal expansion. The catalyst used in the present invention has no particular restriction and can be any compound as long as it has an action of accelerating the reaction of an epoxy resin or a maleimide. Such compounds are, for example, tertiary amines such as trimethylamine, triethylamine, tetramethylbutanediamine, triethylenediamine and the like; amines such as dimethylaminoethanol, dimethylaminopentanol, tris(dimethylaminomethyl)phenol, N-methylmorpholine and the like; quaternary ammonium salts such as cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, cetyltrimethylammonium iodide, dodecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium iodide, benzyldimethyltetradecylammonium chloride, benzyldimethyltetradecylammonium bromide, allyldodecyltrimethylammonium bromide, benzyldimethylstearylammonium bromide, stearyltrimethylammonium chloride, benzyldimethyltetradecylammonium acetylate and the like; imidazoles such as 2-methylimidazole, 2-ethylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-methyl-4-ethylimidazole, 1-butylimidazole, l-propyl-2-methylimidazole, 1-benzyl-2-methylimidazole, l-cycanoethyl-2-phenylimidazole, 1-cyanoethyl-2-methylimidazole, l-cyanoethyl-2-undecylimidazole, l-azine-2-methylimidazole, 1-azine-2-undecylimidazole and the like; microcapsules of amines or imidazoles; metal salts between (a) an amine or imidazole and (b) zinc octanoate, a cobalt salt or the like; 1,8-diaza-bicyclo[5.4.0]-undecene-7; N-methylpiperazine; tetramethylbutylguanidine; amine tetraphenylborates such as triethylammoniumtetraphenylborate, 2-ethyl-4-methyltetraphenylborate, 1,8-diazabicyclo-[5.4.0]-undecene-7-tetraphenylborate and the like; triphenylphosphine; triphenylphosphoniumtetraphenylborate; aluminumtrialkylacetoacetates; aluminum trisacetylacetoacetate; aluminum alcoholates; aluminum acylates; sodium alcoholates; boron trifluoride; complexes between boron trifluoride and an amine or imidazole; diphenyliodonium salt of HAsF 6 ; aliphatic sulfonium salts; amineimides obtained by reacting an alkyl monocarboxylate with a hydrazine and a monoepoxy compound; and metal (e.g. cobalt, manganese, iron) salts of octanoic acid or naphthenic acid. Of these, particularly useful are quaternary ammonium salts, metal salts between (a) an amine or imidazole and (b) zinc octanoate, a cobalt salt or the like, amine tetraphenyl borates, complexes between boron trifluoride and an amine or imidazole, diphenyliodonium salt of HAsF 6 , aliphatic sulfonium salts, amineimides, microcapsules of amines or imidazoles, etc., because they are relatively stable at room temperature but can cause a reaction easily at elevated temperatures, that is, they are latent curing catalysts. These curing agents are added ordinarily in an amount of 0.1-10% by weight based on the polyfunctional epoxy resin. The stress which a superconducting magnet coil undergoes during operation of the superconducting magnet, includes the residual stress generated at the time of production, the thermal stress applied during cooling and the electromagnetic force applied during operation. First, description is made of the thermal stress applied to the cured resin of a superconducting magnet coil when the coil after production is cooled to liquid helium temperature, i.e. 4.2 K. The thermal stress σ applied to the cured resin of a superconducting magnet coil when the coil after production is cooled to liquid helium temperature, i.e 4.2 K, can be represented by the following formula: wherein α R is the thermal expansion coefficient of the cured resin; α S is the thermal expansion coefficient of the superconducting wire of the coil; E is the modulus of the cured resin; and T is the curing temperature of the resin used for obtaining the cured resin. Since the modulus at temperatures above the glass transition temperature Tg of the cured resin is smaller by about two figures than the modulus at the glass transition temperature Tg or below, the thermal stress applied to the cured resin of superconducting magnet coil when the coil after production is cooled to 4.2 K, can be substantially represented by the following formula (1) holding for when the coil after production is cooled from the glass transition temperature of the cured resin to 4.2 K: Now, the thermal stress σ applied to the cured resin of superconducting magnet coil when the coil after production is cured to 4.2 K, is roughly calculated from the above formula (1), using assumptions that the thermal shrinkage factor of the cured resin when cooled from the glass transition temperature Tg to 4.2 K be 2.0%, the thermal shrinkage factor of the superconducting wire of the coil when cooled under the same condition be 0.3% and the modulus of the cured resin be 9,810 MPa (1,000 kg/mm 2 ) at 4.2 K; the rough calculation gives a thermal stress σ of about 167 MPa (17 kg/mm 2 ). Meanwhile, cured epoxy resins ordinarily have a strengh of 167-196 MPa (17-20 kg/mm 2 ) at 4.2 K. Accordingly, when the superconducting magnet coil after production is cooled to liquid helium temperature, i.e. 4.2 K, the thermal stress σ plus the residual stress generated at the time of coil production allow the cured resin to form microcracks of several micrometers ; the releasing energy of the stress of the cured resin gives rise to a temperature increase of several degress at the peripheries of the microcracks; as a result, the resistance of the superconducting wire is increased rapidly, and there occurs a transition from the superconducting state to a state of normal conduction, I,e the so-called quench phenomenon. In superconducting magnet coils used in linear motor cars, MRI, etc., further an electromagnetic force of at least about 39 MPa (4 kg/mm 2 ) is repeatedly applied during operation at 4.2 K. This force plus the above-mentioned thermal stress and residual stress allow the cured resin to form cracks, and the releasing energy of the stress gives rise to the quench phenomenon. The thermal stress σ applied to the cured resin of superconducting magnet coil when the coil after production is cooled to 4.2 K, is roughly calculated from the formula (1), using a thermal shrinkage factor of the cured resin of 1.5% when cooled to 4.2 K and a modulus of the cured resin of 9,810 MPa (1,000 kg/mm 2 ) at 4.2 K; the rough calculation gives a thermal stress σ of about 118 MPa (12 kg/mm 2 ). When an electromagnetic force of about 39 MPa (4 kg/mm 2 ) is repeatedly applied to the above thermal stress during operation at 4.2 K, the total stress becomes about 157 MPa (16 kg/mm 2 ). Meanwhile, cured epoxy resins ordinarily have a strength of 167-196 MPa (17-20 kg/mm 2 ) at 4.2 K. Therefore, on calculation, this strength can withstand the thermal stress applied to the cured resin of superconducting magnet coil when cooled to 4.2 K and the electromagnetic force repeatedly applied to the cured resin during operation. Various impregnant resins of different thermal shrinkage factors for superconducting magnet coils were actually tested. The tests indicated that when there is used, as an impregnant resin for superconducting magnet coil, a curable resin composition giving a cured product having a thermal shrinkage factor of 1.5-0.3% when cooled from the glass transition temperature to liquid helium temperature, i.e. 4.2 K, a bend-breaking strain of 2.9-3.9% at 4.2 K and a modulus of 4,900-9,810 MPa (500-1,000 kg/mm 2 ) at 4.2 K, the cured resin composition of the superconducting magnet coil generates no cracks when cooled to liquid helium temperature, i.e. 4.2 K. The tests also indicated that no quench appears even in a superconducting operation at 4.2 K wherein an electromagnetic force is further applied. When there is used, in particular, a thermosetting resin composition giving a cured product having a thermal shrinkage factor of 1.0-0.3% when cooled from the glass transition temperature to a liquid helium temperature, i.e. 4.2 K, a bend-breaking strain of 2.9-3.9% and a modulus of 4,900-9,810 MPA (500-1,000 kg/mm 2 ), quench can be prevented with a large allowance even in superconducting operation at 4.2 K in which an electromagnetic force is applied. The present invention is hereinafter described more specifically by way of Examples. However, the present invention is by no means restricted to these Examples. The determination of thermal shrinkage was carried out with a thermomechanical analyzer (TMA) having a sample system provided in a cryostat which can cool a sample to a very low temperature and a measurement system containing a differential transformer with which the change of the dimensions of the sample detected by a detecting rod can be measured. The determination of the bending properties was carried out by immersing the sample in liquid helium using a conventional bend test apparatus equipped with a cryostat which can cool the sample to a very low temperature. The size of the sample was 80 mm x 9 mm x 5 mm. The conditions of the determination were: length between supports: 60 mm head speed: 2 mm/min three-point bending. In the Examples, the abbreviations used for polyfunctional epoxy resins, flexibilizers, curing catalysts and bismaleimides are as follows. DER-332: diglycidyl ether of bisphenol A (epoxy equivalent: 175) EP-825: diglycidyl ether of bisphenol A (epoxy equivalent: 178) EP-827: diglycidyl ether of bisphenol A (epoxy equlvalent: 185) EP-828: diglycidyl ether of bisphenol A (epoxy equivalent: 189) EP-1001: diglycidyl ether of bisphenol A (epoxy equivalent: 472) EP-1002: diglycidyl ether of bisphenol A (epoxy equivalent: 636) EP-1003: diglycidyl ether of bisphenol A (epoxy equivalent: 745) EP-1055: diglycidyl ether of bisphenol A (epoxy equivalent: 865) EP-1004AF: diglycidyl ether of bisphenol A (epoxy equivalent: 975) EP-1007: diglycidyl ether of bisphenol A (epoxy equivalent: 2006) EP-1009: diglycidyl ether of bisphenol A (epoxy equivalent: 2473) EP-1010: diglycidyl ether of bisphenol A (epoxy equivalent: 2785) EP-807: diglycidyl ether of bisphenol F (epoxy equivalent: 170) PY-302-2: diglycidyl ether of bisphenol AF (epoxy equivalent: 175) DGEBAD: diglycidyl ether of bisphenol AD (epoxy equivalent: 173) HP-4032: 2,7-diglycidyl ether naphthalene (epoxy equivalent: 150) TGADPM: tetraglycidylaminodiphenylmethane TTGmAP: tetraglycidyl-m-xylylenediamine TGpAP: triglycidyl-p-aminophenol TGmAP: triglycidyl-m-aminophenol CEL-2021: 3,4-epoxycyclohexylmethyl-(3,4-epoxy)-cyclohexane carboxylate (epoxy equivalent: 138) LS-108: bis-2,2'-(4,4'-[2-(2,3-epoxy)-propoxy-3-butoxypropoxy]-phenyl}propane (epoxy equivalent: 2100) LS-402: bis-2,2'-{4,4'-[2-(2,3-epoxy)propoxy-3-butoxypropoxy]-phenyl}propane (epoxy equivalent: 4600) HN-5500: methylhexahydrophthalic anhydride (acid anhydride equivalent: 168) HN-2200: methyltetrahydrophthalic anhydride (acid anhydride equivalent: 166) iPA-Na: sodium isopropylate BTPP-K : tetraphenylborate of triphenylbutylphosphine 2E4MZ-K: tetraphenylborate of 2-ethyl-4-methylimidazole 2E4MZ-CN-K: tetraphenylborate of l-cyanoethyl-2-ethyl-4-methylimidazole TEA-K: tetraphenylborate of triethylamine TPP-K: tetraphenylborate of triphenylphosphine TPP: triphenylphosphine IOZ: salt between 2-ethyl-4-methylimidazole and zinc octanoate DY063: alkyl alkoholate YPH-201: an amineimide obtained by reacting an alkyl monocarboxylate with a hydrazine and a monoepoxy compound (YPH-201 manufactured by Yuka Shell Epoxy K.K.) CP-66: an aliphatic sulfonium salt of a protonic acid (ADEKA OPTON CP-66 manufactured by ASAHI DENKA KOGYO K.K.) PX-4BT: tetrabutylphosphonium benzotriazolate BF 3 -400: boron trifluoride salt of piperazine BF 3 -100: boron trifluoride salt of triethylamine 2E4MZ-CNS: trimellitic acid salt of 2-ethyl-4-methylimidazole 2E4MZ-OK: isocyanuric acid salt of 2-ethyl-4-methylimidazole MC-CllZ-AZINE: microcapsule of 1-azine-2-undecylimidazole 2E4MZ-CN: 1-cyanoethyl-2-ethyl-4-methylimidazole BDMTDAC: benzyldimethyltetradecylammonium chloride BDMTDAI: benzyldimethyltetradecylammonium iodide HMBMI: N,N'-hexamethylene-bismaleimide BMI: N,N'-4,4'-diphenylmethane-bismaleimide DMBMI: N,N'-(3,3'-dimethyl)-4,4'-diphenylmethane-bismaleimide DAPPBMI: N,N'-bismaleimide of 2,2'-bis[4-(4-aminophenoxy)-phenyl]propane PMI: N,N'-polymaleimide of a reaction product (a polyamine compound) between aniline and formalin DABPA: diallylbisphenol A PPG: polypropylene glycol KR: ε-caprolactone DGEAOBA: diglycidyl ether of an alkylene oxide adduct of bisphenol A PPO: phenoxy resin CTBN: acrylonitrile-modified carboxyl group-terminated polybutadiene rubber 2PZCN: 1-cyanoethyl-2-phenylimidazole LBO: lithium butoxide PZ: pyridine TEA: triethylamine M2-100: benzalkyonium chloride N-MM: N-methylmorpholine MDI: 4,4'-diphenylmethane diisocyanate, equivalent: 125 LMDI: a mixture of MDI, an MDI derivative whose isocyanate group has been converted to carbodiimide and an MDI derivative whose isocyanate groups have been converted to carbodiimide, which mixture is liquid at room temperature, equivalent: about 140 TDI: a mixture of 80% of 2,4-tolylene diisocyanate and 20% of 2,6-tolylene diisocyanate, equivalent: 87 KR2019: a resin obtained by condensation polymerization of methylphenylsilicone, Examples 1-65 and Comparative Examples 1-6 Each of the resin compositions shown in Tables 1-1 to 1-13 was thoroughly stirred, placed in a mold, and heat-cured under the curing conditions shown in Tables 1-1 to 1-13. Each of the resulting cured products was measured for thermal shrinkage factor when cooled from the glass transition temperature to 4.2 K, and the results are shown in Tables 1-1 to 1-13. Each cured product was also measured for bending properties at 4.2 K, and the bending strain and bending modulus are shown in Tables 1-1 to 1-13. All of the curable resin compositions of Examples 1-65 according to the present invention, when cured, had a thermal shrinkage factor of 1.5-0.3% when cooled from the glass transition temperature to 4.2 K, a bend-breaking strain of 2.9-3.9% at 4.2 K and a modulus of 4,900-9,810 MPa (500-1,000kg/mm 2 ) at 4.2 K. Example 66 and Comparative Example 7 Superconducting wires were wound to form coils of the same material and the same shape. The coils were impregnated with the curable resin compositions of Examples 1-65 and Comparative Examples 1-6, and the impregnated coils were heat-cured under given curing conditions to prepare small race track-shaped superconducting magnet coils. Switches for permanent current were also prepared by impregnation with each of the curable resin compositions of Examples 1-65 and Comparative Examples 1-6 and subsequent heat-curing under given curing conditions. Fig. 1 is a perspective view showing the superconducting magnet coils thus prepared. Fig. 2 is a cross-sectional view of the coil of Fig. 1 when cut at the line II-II'. In any or the coils, the cured product 3 of the curable resin composition filled the space between the conductors 2, and any unfilled portion (e.g. void) was not observed. These coils were cooled to 4.2 K. As shown in Fig. 3, in each of the coils impregnated with each of the curable resin compositions of Comparative Examples 1-6, cracks were generated in the cured resin composition 3; the cracks reached even the enamel insulating layer 5 of each conductor 2, which caused even the peeling 6 of the enamel insulating layer 5. Meanwhile, in the coils impregnated with each of the curable resin compositions of Examples 1-65, neither cracking of the cured resin composition nor peeling of the enamel insulating layer was observed. Example 67 and Comparative Example 8 Superconducting wires were wound to form coils of the same material and the same shape. The coils were impregnated with each of the curable resin compositions of examples 1-65 and Comparative Examples 1-6, and the impregnated coils were heat-cured under given curing conditions to prepare saddle-shaped superconducting magnet coils. Fig. 4 is a perspective view showing the superconducting magnet coils thus prepared. Fig. 5 is a cross-sectional view of the coil of Fig. 4 when cut at line V-V' saddle-shaped superconducting magnet coils 15 were cooled to 4.2 K. In the coils impregnated with each of the curable resin compositions of Comparative Examples 1-6, cracks were generated in the cured resin composition. Meanwhile, in the coils impregnated with each of the curable resin compositions of Examples 1-65, no crack was observed. Examples 68-115 Each of the resin compositions shown in Tables 2-1 to 2-11 was thoroughly stirred, placed in a mold, and heat-cured under the curing conditions shown in Tables 2-1 to 2-11. Each of the resulting cured products was measured for the thermal shrinkage factor when cooled from the glass transition temperature to 4.2 K, and the results are shown in Tables 2-1 to 2-11. Each cured product was also measured for the bending properties at 4.2 K, and the bending strain and bending modulus are shown in Tables 2-1 to 2-11. All of the curable resin compositions of Examples 68-115 according to the present invention, when cured, had a thermal shrinkage factor of 1.8-0.3% when cooled from the glass transition temperature to 4.2 K, a bend-breaking strain of 3.5-4.5% at 4.2 K and a modulus of 4,900-9,810 MPa (500-1,000 kg/mm 2 ) at 4.2 K. As described above, in a superconducting magnet coil impregnated with a curable resin composition giving a cured product having a thermal shrinkage factor of 1.5-0.3% when cooled from the glass transition temperature to liquid helium temperature, i.e. 4.2 K, a bend-breaking strain of 2.9-3.9% at 4.2 K and a modulus of 4,900 to 9,810 MPa (500-1,000 kg/mm 2 ) at 4.2 K, particularly a cured product having a thermal shrinkage factor of 1.0-0.3% when cooled from the glass transition temperature to liquid helium temperature, i.e. 4.2 K, a bend-breaking strain of 2.9-3.9% at 4.2 K and a modulus of 4900-9,810 MPa (500-1,000 kg/mm 2 ) at 4.2 K, no microcracks are generated in the cured product when the superconducting magnet coil after production is cooled to liquid helium temperature, i.e. 4.2 K. Such a superconducting magnet coil causes substantially no quench even during an operation in which an electromagnetic force is applied.

Claims: Superconducting magnet coil (1) comprising a coil of a superconducting wire (2) having a thermal shrinkage factor of 0.3 to 0.4 % when cooled to 4.2 K, and a cured product (3) of a curable resin composition with which the coil has been impregnated, having a modulus of 4,900 to 9,810 MPa (500 to 1000 kg/mm 2 ) at 4.2 K, a thermal shrinkage factor of 1.5 to 0.3 % when cooled from the glass transition temperature to 4.2 K, and a bend-breaking strain of 2.9 to 3.9 % at 4.2 K. Superconducting magnet coil (1) according to claim 1, characterized in that the cured resin composition has a bend-breaking strain of 3.2-3.9 % at 4.2 K. Superconducting magnet coil (1) according to claim 1 and/or 2, characterized in that the superconducting wire (2) is covered with one or more polymers selected from polyvinyl formal, polyvinyl butyral, polyesters, polyurethanes, polyamides, polyamide-imides and polyimides. Superconducting magnet coil (1) according to one or several of claims 1 to 3, characterized in that the superconducting wire (2) is covered with at least one film selected from polyester films, polyurethane films, polyamide films, polyamide-imide films and polyimide films. Superconducting magnet coil (1) according to one or several of claims 1 to 4, characterized in that the superconducting wire (2) is made of a Nb-Ti type alloy. Superconducting magnet coil (1) according to one or several of claims 1 to 5, characterized in that the curable resin composition comprises at least one epoxy resin selected from diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, diglycidyl ethers of bisphenol AF and diglycidyl ethers of bisphenol AD, all having a number-average molecular weight of 1,000 to 50,000. Superconducting magnet coil (1) according to one or several of claims 1 to 6, characterized in that the curable resin composition comprises (a) at least one epoxy resin selected from diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, diglycidyl ethers of bisphenol AF and diglycidyl ethers of bisphenol AD, all having a number-average molecular weight of 1,000 to 50,000, (b) a flexibilizer, and (c) a curing catalyst. Superconducting magnet coil (1) according to one or several of claims 1 to 7, characterized in that the curable resin composition comprises an unsaturated imide compound. Superconducting magnet coil (1) according to one or several of claims 1 to 8, characterized in that the cured product (3) undergoes a thermal stress of 0 to 98.1 MPa (0 to 10 kg/mm 2 ) when cooled from the glass transition temperature to 4.2 K and resists to quench during superconducting operation. Superconducting magnet coil (1) according to one or several of claims 1 to 9, characterized in that the superconducting wire (2) has a composite structure comprising (a) the superconductor and (b) a stabilizer selected from copper and aluminum which is thermally or electrically in contact with the superconducting wire (2). Superconducting magnet coil according to one or several of claims 1 to 10, characterized in that the superconducting wire (2) comprises a plurality of thin superconducting wires. Superconducting magnet coil (1) according to claim 10 and/or 11, characterized in that the wires each are made of a Nb-Ti type alloy and are covered with at least one film selected from films of polyesters, polyurethanes, polyamides, polyamide-imides and polyimides. Superconducting magnet coil (1) according to one or several of claims 10 to 12, characterized in that the resin composition comprises at least one epoxy resin selected from diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, diglycidyl ethers of bisphenol AF and diglycidyl ethers of bisphenol AD, all having a number-average molecular weight of 1,000 to 50,000. Superconducting magnet coil (1) according to one or several of claims 10 to 13, characterized in that the cured product (3) undergoes a thermal stress of 0 to 98.1 MPa (0 to 10 kg/mm 2 ) when cooled from the glass transition temperature to 4.2 K and resists to quench during the superconducting operation. Process for producing the superconducting magnet coils (1) according to claims 1 to 14, comprising the following steps: (a) winding a superconducting wire (2) having a thermal shrinkage factor of 0.3 to 0.4 % when cooled to 4.2 K to form a coil, (b) filling the gaps between the superconductors of the coil with a curable resin composition having a viscosity of 10 -3 to 1 Pa·s (0.01 to 10 Poise) at the time of filling to obtain a coil impregnated with the curable resin composition, and (c) heating the impregnated coil to cure the composition and obtain a cured product having a modulus of 4,900 to 9,810 MPa (500 to 1,000 kg/mm 2 ), a thermal shrinkage factor of 1.5 to 0.3 % when cooled from the glass transition temperature to 4.2 K, and a bend-breaking strain of 2.9 to 3.9 % at 4.2 K. Process according to claim 15, characterized in that the cured product (3) of the composition has a thermal shrinkage factor of 1.0 to 0.3 % when cooled from the glass transition temperature to 4.2 K. Process according to claim 15 and/or 16, characterized in that in step (b) the coil is impregnated with a curable resin composition comprising at least one epoxy resin selected from diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F and diglycidyl ethers of bisphenol AF, all having a number-average molecular weight of 350 to 1,000, and a curing catalyst. Process according to one or several of claims 15 to 17, characterized in that step (b) comprises the following steps: covering the outer surface of the coil with a release film, placing the film-covered coil in a mold, and effecting the impregnation of the coil with the curable resin composition under vacuum, and in step (c) the composition is cured under pressure. Process according to one or several of claims 15 to 18, characterized in that in step (b) the outer surface of the coil is covered with a perforated film. Process according to one or several of claims 15 to 19, characterized in that step (c) includes the steps of clamping the impregnated coil and curing the composition under pressure. Process according to one or several of claims 15 to 20, characterized in that in step (a) a composite superconductor wire (2) comprising a plurality of thin superconducting wires and a stabilizer selected from copper and aluminum which is thermally or electrically in contact with the superconducting wires is used to form the coil, and the composite superconductor wire (2) is subjected to a surface treatment with a coupling agent before winding. An insulating layer (5) of a superconducting magnet coil (1), comprising (a) a coil of a superconducting wire (2) having a thermal shrinkage factor of 0.3 to 0.4 % when cooled to 4.2 K, and comprising a plurality of thin superconducting wires and a stabilizer selected from copper and aluminum which is thermally or electrically in contact with the superconducting wires, and (b) a cured product (3) of a curable resin composition with which the coil has been impregnated, having a modulus of 4,900 to 9,810 MPa (500 to 1,000 kg/mm 2 ) at 4.2 K, a thermal shrinkage factor of 1.5 to 0.3 % when cooled from the glass transition temperature to 4.2 K, and a bend-breaking strain of 2.9 to 3.9 % at 4.2 K. The insulating layer (5) according to claim 22, characterized in that the superconducting wires are made of a Nb-Ti type alloy. The insulating layer (5) according to claim 22 and/or 23, characterized in that the cured product (3) is made of a resin composition comprising at least one epoxy resin selected from diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, diglycidyl ethers of bisphenol AF and diglycidyl ethers of bisphenol AD, all having a number-averge molecular weight of 1,000 to 50,000.

Inventors: FUKUSHI KEIJI, HONJO KOO, KOYAMA TORU, MUKOH AKIO, NUMATA SEIJI, SUZUKI MASAO, TAKAHASHI AKIO, FUKUSHI, KEIJI, HONJO, KOO, KOYAMA, TORU, MUKOH, AKIO, NUMATA, SEIJI, SUZUKI, MASAO, TAKAHASHI, AKIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01F6
IPC codes: H01B3, C08G59, H01F6