Resin impregnated oxide superconductor and process for producing same

Abstract: There is disclosed an oxide superconductor capable of assuring a high trapped magnetic field and maintaining its performance for a long period of time without being affected by internal or external force such as thermal strain or by corrosive environments. The oxide superconductor comprises a resin impregnated layer incorporated with a filler material having a low value of linear thermal expansion coefficient, or comprises the resin impregnated layer or a resin impregnated layer incorporated with a filler material and covered with a resin layer incorporated with the filler material having a low value of linear thermal expansion coefficient, or comprises an oxide superconductive bulk body having an adhesively covering layer of resin impregnated fabric on the outside surface, or comprises an oxide superconductive bulk body having on the surface thereof a resin or a resin impregnated layer dispersedly incorporated with a filler material. The impregnation of a resin is performed by a method of bringing the resin in liquid form or the resin in liquid form impregnated with a filler material into contact with the bulk body and impregnating the resin into the oxide superconductive bulk body.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconductor which is capable of trapping a high magnetic field and maintaining its performance for a long period of time without being affected by internal or external force such as electromagnetic force or thermal stress or by corrosive environments, and also to a process for producing said oxide superconductor. Regarding a superconducting material, which has a high critical current density as compared with an ordinary conducting material and is capable of passing a large electric current without any loss, research and development have vigorously been conventionally carried out on its application in the field of experimental equipment for nuclear fusion, superconductive MRI for medical diagnosis, magnetic levitation trains, electric generators, energy storage units, magnetometers and the like, and since there have been found metal oxide superconducting materials each having a relatively high critical temperature (T) such as LiTi 2 O 3 , Ba (Bi, Pb)O 3 and (Ba, K)BiO 3 in recent years and there have successively been created copper oxide superconducting materials each having such a relatively high (T) that had never been anticipated before, such as (La, Sr) 2 CuO 4 , REBa 2 Cu 3 O 7 (RE is a rare earth element), Bi 2 Sr 2 Ca 2 Cu 3 O 10 , Ti 2 Ba 2 Ca 2 Cu 3 O 10 and HgBa 2 Ca 2 Cu 3 O 8 , the research has been spurred moreover. A superconducting material has a high critical current density as compared with an ordinary conducting material, and thus is capable of passing a large electric current without any loss. However it is known that in the case of passing such a large electric current, a material is sometimes destroyed depending upon its strength, since a large electromagnetic force acts on a superconductor in question. Accompanying the enhanced characteristics and large scale operation of relatively high temperature bulk superconductor (particularly, copper oxide superconductor), the magnitude of a magnetic field capable of being trapped in a bulk superconductor has recently been drastically enhanced, for instance, to the extent that a magnetic flux density exceeding 5 tesla (T) has come to be trapped (refer to "Superconductor Science and Technology" 11, 1998, pp. 1345 to 1347). Since an electromagnetic force applied to a superconductor increases with an increase in a trapped magnetic field, there has recently been brought about such a situation in that a restriction is imposed on a trapped magnetic field depending upon a material strength. Under such circumstances, importance is attached to the improvement in mechanical properties rather than a further improvement in a superconducting properties (refer to "Physica C" vol. 7, No. 9, 1991, pp. 4989 to 4994). It being so, the following two proposals have been made as a means for reinforcing an oxide bulk superconductor. One proposal includes a method in which Ag is added to a material in question. It is said that by taking such measures, a remarkable improvement is brought about on the mechanical strength of an oxide bulk superconductor (refer to "Japanese Journal of Applied Physics" vol. 70, No. 9, 1991, pp. 4989 to 4994 and "Superconductor Science and Technology" 11, 1998, pp. 1345 to 1347) The other proposal includes a method in which a compression strain is applied in advance to a material in question by fitting a bulk superconducting material with a metallic ring (refer to "Extended Abstract of ISTEC International Workshop" 1998, pp. 115 to 118). It is said that by taking such measures, an improvement is brought about on the trapped magnetic field, since the tensile stress caused at the time of trapping the magnetic field is alleviated by the compression strain which was applied in advance, thereby suppressing the destruction of the material. Nevertheless, the above mentioned methods including the reinforcement with Ag addition and reinforcement with a metallic ring are desired to make further improvement on the aspects of workability and manufacturing cost. Moreover, the problem has been recognized in that the reinforced performance is deteriorated by long-term use under corrosive environment. I.M. Low et al.: "Mechancial and fracture properties of epoxy-modified YBaCuO (123) superconductors". JOURNAL OF MATERIALS SCIENCE LETTERS, Vol. 13 No. 18, pages 1340 - 1342 describe YBaCuO superconductive material the surface of which has been impregnated by immersing in a bath of epoxy resin. M. Kosaki et al.: "Solid insulation and its deterioration", CRYOGENICS, Vol. 38, No. 11, pages 195 - 1104 describe the insulation of super conductive material with various types of coating. SUMMARY OF THE INVENTION To solve the problem described above, the inventors have made researches repeatedly pursuing a method for readily providing at a low cost, an oxide superconductor which is capable of sufficiently withstanding internal or external force such as a large electromagnetic force or a thermal stress accompanying sudden rise or drop in temperature at the time of use, and further capable of exhibiting a high trapped magnetic field for a long period of time without being adversely influenced by a corrosive environment, and the findings as described as a)-f) hereinafter have been obtained in the midway of researches. Other objects of the present indention will be obvious from the text of this specification hereinafter disclosed. In such circumstance, intensive research and investigation were accumulated by the present inventors in order to achieve the above-mentioned objects. As a result, novel information and findings as described hereunder have been obtained. (a) An oxide bulk superconductor is ceramic in the state of pseudo-single crystal. It is difficult in practice to prevent microcracks or micropores from being internally included during the manufacturing step thereof. (b) When such oxide bulk superconductor is subjected to "a strong mechanical impact", "thermal impact due to sudden temperature variation", "a large electromagnetic force (Lorentz force) "or the like, a stress concentration occurs in the aforesaid microcracks or whereby the microcracks or micropores as starting points progress and expand to a relatively large cracks. (c) In the case where the oxide bulk superconductor is exposed for a long time in a corrosive atmosphere containing a large amount of moisture or carbon dioxide gas, the materials for the oxide bulk superconductor are deteriorated, or a reaction phase is formed resulting in the generation of new cracks, which progress and expand to a relatively large cracks. (d) The aforesaid relatively large cracks, when being formed, inhibit the flow of the superconductive current, thus greatly decreasing the trapped magnetic field. (e) However, even if an oxide bulk superconductor is the one which has been apt to be believed that there is by no means any possibility of internal permeation of a coating material or the like because of an extremely high density due to its generally being produced by melting method, contact with a resin in liquid form in an atmosphere of reduced pressure enables said superconductor to maintain a high trapped magnetic field. This is due to the mechanism that the resin permeates not only into the microcracks having openings on the surface, but also into the whole surface layer and further the inside of bulk body through the microcracks having openings, whereby the corrosion resistance of the surface is markedly improved and besides, the mechanical strength of the bulk superconductor itself is drastically enhanced, thereby suppressing, to the utmost, internal force, external stress and the propagation of cracking due to corrosion. (f) In addition, since there is not recognized at all the deterioration due to the resin impregnation, of the superconductivity characteristics of the bulk body matrix, the above-mentioned method is an extremely advantageous means for improving the mechanical properties and corrosion resistance, while maintaining the excellent superconductivity characteristics of the oxide superconductor. Such being the case, on the basis of the above mentioned information and findings, the present inventors previously proposed an oxide superconductor which comprises an oxide superconductive bulk body (for instance, a copper oxide superconductive bulk body containing at least one rare earth element) having a resin impregnated layer, as an oxide superconductor which is minimized in the generation of crack due to external force or internal stress, is hardly adversely influenced by a corrosive environment, and is capable of maintaining a high trapped magnetic field for a long period of time; and further proposed a process for producing the above mentioned oxide superconductor which comprises impregnating a resin into an oxide superconductive bulk body by bringing the resin in liquid form into contact with the bulk body preserved in an atmosphere of reduced pressure {refer to Japanese Patent Application Laid-Open No. 361722/1998 (Heisei 10) and corresponding EP 1011153A}. However, by the research and investigation continued thereafter by the present inventors, it has been clarified that the oxide superconductor comprising an oxide superconductive bulk body having a resin impregnated layer relating to the previous proposal still involves the problem to be further improved as described hereunder. That is to say, even the oxide superconductor which comprises an oxide superconductive bulk body having a resin impregnated layer, and which exhibits markedly high cracking resistance against mechanical shock and thermal stress, sometimes fails as the case may be, to sufficiently exert the expected working effect which is obtained by proving the resin impregnated layer, and which is shown by the prevention of cracking and corrosion of the oxide superconductive bulk body, since microcracks are generated on the resin impregnated layer during the time zone soon after cooling in the case where the superconductor is rapidly cooled to the critical temperature or lower. The object of the present invention taking into consideration the foregoing is to establish a method for readily providing at a low cost, an oxide superconductor which is capable of further sufficiently eliminating the risk of crack generation due to a mechanical strain derived from a large electromagnetic force or due to a thermal strain accompanying sudden rise or drop in temperature at the time of use, and at the same time, which is capable of maintaining a high trapped magnetic field for a long period of time whether under ordinary environment or a corrosive environment. In such circumstance, intensive research and investigation were accumulated by the present inventors in order to achieve the above-mentioned object. As a result, the facts as described in the following items (a) to (f) were elucidated. (a) The precious proposal, that is, the generation of microcracks which is recognized as the case may be, on the resin impregnated layer at the time of rapidly cooling the oxide superconductor having a resin impregnated layer, is a phenomenon originating from the difference in thermal expansion coefficient (thermal contraction coefficient) between the oxide superconductive bulk body and the resin impregnated layer. The resin impregnated layer, which has a linear thermal expansion coefficient higher than that of the oxide superconductive bulk body, contracts to a greater extent at the time of cooling with the more possible result that said layer can no longer withstand the tensile stress arising therefrom, thus leading to final cracking. Therefore, it is indispensable to solve such a problem in order to sufficiently assure the effect on preventing the cracking. (b) However the linear thermal expansion coefficient of said resin impregnated layer can be lowered by dispersedly incorporating therein a filler material which has a low value of linear thermal expansion coefficient and is exemplified by quarts, calcium carbonate, alumina and glass. Thus by properly selecting the combination of the type and blending amount of a filler material, it is made possible to realize a resin material having linear thermal expansion coefficient almost the same as that of the oxide superconductive bulk body. (c) It being so, when a resin layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient is formed on the outside surface of the oxide superconductor having a resin impregnated layer as previously proposed, then said resin impregnated layer is brought to a state wherein it is interposed between the oxide superconductive bulk body having a low value of linear thermal expansion coefficient and the resin layer which contains the filler material and has a low value of linear thermal expansion coefficient. Accordingly, the tensile stress generated at the time of rapid cooling is bound down by the above two, thus suppressing the generation of cracking. (d) In addition, when a resin dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient is directly impregnated into the oxide superconductive bulk body so that said bulk body is equipped with the resin impregnated layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient, then as is the case with the item (c), cracking or corrosion is no longer generated, thereby realizing an oxide superconductor capable of exhibiting a high trapped magnetic field for a long period of time. (e) However because of its increased viscosity, the resin dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient is difficult to form a layer deeply impregnated into the oxide superconductive bulk body. In such a case, sufficient cracking and corrosion resistances can be imparted to an oxide superconductor by further covering the outside surface of the oxide superconductive bulk body having a resin impregnated layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient with a resin dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient. (f) In the case of simultaneously employing the resin impregnation into the bulk body along with the formation of the layer covered with resin impregnated fabric, by dispersedly incorporating a filler material having a low value of linear thermal expansion coefficient such as quartz, calcium carbonate, alumina and glass, in a resin material to be impregnated into the inside of the oxide superconductive bulk body, it is made possible to lower the linear thermal expansion coefficient of said resin material; and further by properly selecting the combination of the type and blending amount of the filler material, there is materialized a resin material having a linear thermal expansion coefficient almost the same as that of the oxide superconductive bulk body. In this case, by constituting the resin impregnated layer to be formed inside the oxide superconductive bulk body, of such impregnated resin layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient, it is made possible to completely clear the concern about the cracking of the resin impregnated layer the generation of which could not have been completely be denied, said cracking being due to the difference in thermal expansion (contraction) coefficient between the resin impregnated layer and the oxide superconductive bulk body. As a result, the above-mentioned advantage leads to further enhancement of stability of properties as well as reliability of the oxide superconductor. The present invention, which has been accomplished on the bases of the aforesaid information and findings, provides the undermentioned oxide superconductor and also a process for producing said oxide superconductor according to claims 1 and 3, and claim 6, respectively. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a graph illustrating the contradistinction of linear thermal expansion coefficients at various temperatures for SmBa 2 CuO y base oxide superconductors in the directions of "ab" and c-axis, respectively, and for epoxy base resin blended with fine powders of quartz at various ratios by weight. Fig. 2 is a graph illustrating the contradistinction of linear thermal expansion coefficient (contraction coefficient at low temperatures) for glass, carbon and polyamide. Fig. 3 is an explanatory illustration showing the configuration of the oxide superconductor obtained in Example 1; Fig. 4 is a schematic view showing the observed state taken on longitudinal cross section of the oxide superconductor obtained in Example 2. Fig. 5 is a schematic view showing the observed state taken on lateral cross section of the oxide superconductor obtained in Example 2. Fig. 6 is a graph showing a result of measurement of the trapped magnetic field of Sm base oxide superconductor as obtained in Example 6. Fig. 7 is a graph showing a result of measurement of the trapped magnetic field of Gd base oxide superconductor as obtained in Example 7. DESCRIPTION OF PREFERRED EMBODIMENT The oxide superconductive bulk body to which the present invention is applied, may be any of hitherto known types thereof. Thus, mention may be made as a preferable oxide superconductive bulk body, of copper oxide superconductive bulk body such as an RE-Ba-Cu-O base superconductor which is known as a relatively high temperature superconductor having a high trapped magnetic field, wherein RE is at least one rare earth element selected from the group consisting of Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb. Of these, mention may be made as a more preferable oxide superconductor, of an oxide superconductive bulk body which is known as a superconductor having a high trapped magnetic field, comprising REBa 2 Cu 3 O y as a parent phase wherein RE is at least one rare earth element selected from the group consisting of Y, Dy, Ho, Er, Tm and Yb and RE 2 BaCuO 5 as a dispersion phase, wherein RE is at least one rare earth element selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb in an amount of at most 50% by volume based on said parent phase; and of an oxide superconductive bulk body comprising RE 1+x Ba 3-x Cu 3 O y as a parent phase, wherein RE is at least one rare earth element selected from the group consisting of La, Nd, Sm, Eu, and Gd, and preferably satisfies the relationships -0.1 < x < 0.2 and 6.5 < y < 7.2, and RE 4-2x Ba 2+2x Cu 2-x O 10-2x as a dispersion phase, wherein RE is at least one rare earth element selected from the group consisting of La and Nd, and preferably satisfies the relationship -0.2 < x < 0.3, or RE 2 BaCuO 5 phase as a dispersion phase, wherein RE is at least one rare earth element selected from the group consisting of Sm, Eu, and Gd in an amount of at most 50% by volume based on said parent phase. The reason for the content of the dispersion phase being defined as at most 50% by volume is that the content of the dispersion phase exceeding 50% by volume reveals a tendency to deteriorate the superconductivity characteristics. In addition, incorporation of Ag when necessary, into the oxide superconductive bulk body is advantageous in further enhancing its strength, and effective in maintaining high trapped magnetic field. However when the amount of Ag exceeds 40% by weight, the superconductivity properties tend to be deteriorated. Accordingly, in the case of incorporating Ag into an oxide superconductive bulk body, the amount thereof is preferably suppressed to at most 40% by weight. Suitable resins for impregnating into the oxide superconductor are exemplified by epoxy base resin, urea resin, phenolic resin, unsaturated polyester resin, polyurethane, alkyd resin, melamine resin and the like. A method for impregnating a resin into the oxide superconductive bulk body, is preferable a method in which the resin in liquid form is brought into contact with the oxide superconductive bulk body maintained at a reduced pressure atmosphere such as under vacuum. However, there is acceptable a method other than the foregoing, such as "a pressurized impregnation method", provided that the method in question enables resin impregnation. At any rate, a method in which the surface of the oxide superconductive bulk body is only coated with the resin fails to impregnate the resin thereinto. This method, even if possible to protect from the corrosive environment, fails to maintain high trapped magnetic field for a long period of time. A filler material to be dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient thereof is preferably a material which has a low value of linear thermal expansion coefficient, a high thermal conductivity, a low heat generation in curing a resin and a high mechanical strength. Such a filler material is exemplified by quartz, calcium carbonate, alumina, hydrate alumina, glass, talc and calcined gypsum. Any of the filler materials, which is used by dispersing and incorporating it in the resin, is preferably in the form of fine powders so as to be uniformly dispersed. The content of the filler material to be dispersedly incorporated in the resin is preferably regulated according to the linear thermal expansion coeffcient of the objective oxide superconductive bulk body. Even a slight decrease in the linear thermal expansion coefficient of the resin material brings about appreciable effect, however the filler material is preferably incorporated in such an amount that the linear thermal expansion coefficient of the resin material is almost equalized to that of said bulk body. Fig. 1 is a graph which shows the results of investigation on the linear thermal expansion coefficients of SmBa 2 Cu 3 Oy base oxide superconductive bulk body in the direction of "ab" and c-axis at each of a variety of temperatures, and in comparison therewith, on those of epoxy resin (composed of 100 parts by weight of epoxy resin of bisphenol type A as the principal ingredient and 32 parts by weight of an aromatic polyamine as the curing agent) blended with fine powders of quartz as the filler material at various ratios by weight on the basis of the epoxy resin of bisphenol type A at each of a variety of temperatures. It can be seen from the Fig. 1 that as the blending ratio of the filler material increases, the linear thermal expansion coefficient of the resin material comes close to that of the oxide superconductive bulk body, and also that at a blending ratio of the filler material of approximately 200 parts by weight based thereon, the linear thermal expansion coefficient of the resin material is almost equalized to that of said bulk body. In the case where the resin which is dispersedly incorporated with the filler material is impregnated into the oxide superconductive bulk body, it is possible to adopt a method comprising bringing said resin in liquid form and said bulk body preserved in an atmosphere of reduced pressure such as vacuum into contact with each other, and to adopt pressurized impregnation method. On the one hand, in the case where the layer of the resin dispersedly incorporated with the filler material is installed on the outside surface of the oxide superconductive bulk body having the resin impregnated layer, there may be applicable a method comprising coating said resin dispersedly incorporated with the filler on the outside surface thereof. As the resin material to be applied in this case, there are suitably usable thermosetting resins such as the epoxy base resin same as the resin material for impregnation as mentioned above, urea resin, phenolic resin, unsaturated polyester resin, polyurethane resin, alkyd resin and melamine resin. As the coating method, there is preferably usable a method comprising immersing the oxide superconductive bulk body having the resin impregnated layer into the resin which is dispersedly incorporated with a filler material so that the resin adheres to said bulk body, more preferably a method comprising bringing said resin dispersedly incorporated with a filler material and the oxide superconductive bulk body preserved in an atmosphere of reduced pressure into contact with each other by means of immersion method. By allowing the liquid resin containing the filler material to adhere to said bulk body, bubbles possibly accompanying the filler material can by removed therefrom, thereby enabling the formation of a sound resin-covered layer free from a pinhole or the like. On the one hand, in the case of covering the outside surface of the oxide superconductive bulk body with a resin-impregnated fabric, there is usable as the fabric constituting the covering layer, any of woven fabric and non-woven fabric (including the laminated fabric by uniformizing the direction of fibers) without specific limitation of the material itself (Japanese paper and non-woven fabric fall within the scope of usable materials). Taking into consideration the strength, chemical resistance and the like, however, it can be said that examples of preferable raw material for the fabric include glass fiber, carbon fiber, ceramic fiber (Al 2 O 3 fiber, SiC fiber, Si-C-O fiber, SiO 2 fiber, Si-Ti-C-n-O fiber and the like), metallic fiber (titanium fiber, aluminum fiber and the like) and polyamide base synthetic high-polymeric fiber. In particular, it is believed that the polyamide base synthetic high- polymeric fiber is a particularly preferable material with respect to the fact that since it has a relatively high contraction coefficient when cooled to a low temperature, the resultant contraction force suppresses the electromagnetic expansion force (Lorentz force) caused on the oxide superconductive bulk body, thereby more effectively inhibiting the propagation of microcracks. In this connection, Fig. 2 is a graph illustrating the contradistinction of contraction coefficients at low temperatures for glass, carbon and polyamide. In particular, it has been confirmed that glass fiber has a linear thermal expansion coefficient almost same as that of Y-Ba Cu-O base oxide superconductive bulk body in the direction of c-axis and besides, carbon fiber has a linear thermal expansion coefficient almost same as that of Y-Ba Cu-O base oxide superconductive bulk body in the direction of ab plane. In addition, suitable resins to be impregnated into the covering layer of fabric are thermosetting resins such as epoxy base resin, urea resin, phenolic resin, unsaturated polyester resin, polyurethane resin, alkyd resin and melamine resin. As the method for impregnating the resin into said covering layer of fabric there is preferably usable a method comprising bringing a resin in liquid form and the oxide superconductive bulk body which is wrapped and covered with fabric and which is preserved in an atmosphere of reduced pressure such as vacuum into contact with each other. Another method such as pressurized impregnation method, however, is acceptable, if resin impregnation is possible. In preparing the oxide superconductor according to the present invention comprising the oxide superconductive bulk body which has the resin impregnated covering layer of fabric on the outside surface thereof, and which has the resin impregnated layer on the surface portion of said covering layer, there is adoptable a method wherein in the first place, the oxide superconductive bulk body in an atmosphere of reduced pressure or increased pressure is brought into contact with a resin in liquid form to carry out a treatment of resin impregnation in the surface portion thereof, thereafter the bulk body is tightly wrapped with the fabric, and then the fabric layer of the bulk body thus wrapped is impregnated with the resin. It is more advantageous from the standpoint of production efficiency, however, to adopt a method wherein both the covering layer of fabric and the surface portion of the oxide superconductive bulk body are simultaneously impregnated with the resin in one step by bringing the bulk body the surface of which is wrapped with the fabric is brought into contact with the resin in liquid form, while preserving it in an atmosphere of reduced pressure. In this case, by regulating the period of time of contact with the resin in liquid form, it is made possible to selectively impregnating the covering layer of fabric alone or both the covering layer of fabric and the inside of oxide superconductive bulk body. Needless to say, there is no problem or disadvantage with the performance of the objective oxide superconductor, even when the aforesaid filler material is dispersed in the covering layer of fabric which is impregnated with the resin. As described hereinbefore, the present invention provides at a low cost, an oxide superconductor which is endowed with sufficient corrosion resistance and mechanical characteristics that are capable of sufficiently withstanding internal or external force such as a large electromagnetic force or a thermal stress accompanying sudden rise or drop in temperature at the time of use, and further capable of exhibiting a high trapped magnetic field for a long period of time. Accordingly, the present invention renders itself highly useful technique in the case of, for instance, pitting the oxide superconductive bulk body into practice under high electromagnetic force, or utilizing the oxide superconductive bulk body as a high-temperature superconductive magnet by allowing it to trap a magnetic field. In addition, the incorporation of any of the resin impregnated layer, resin coated layer and covering layer of fabric impregnated with a resin into oxide superconductive bulk body, leads to the working effects not only on suppressing the generation and propagation of cracks originating from micropores or microcracks that are formed at the time of manufacturing the bulk body, and preventing corrosion, but also on alleviating direct mechanical impact applied from outside. In particular according to the present invention, it is made possible to markedly enhance the wording effects on preventing the generation of cracks due to rapid cooling at the time of using the oxide superconductor and further to suppress for a long period of time, the situation of bringing about the deterioration of superconductivity properties. The above-described enhanced working effects are manifested by virtue of the specific countermeasure of covering the outside surface of the oxide superconductive bulk body with the resin layer dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient; incorporating the resin impregnated layer dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient into the surface portion of said bulk body; or tightly adhesively covering the outside surface of said bulk body with the covering layer of resin-impregnated fabric. In summarizing the working effects and advantages of the present invention, the invention readily provides at a low cost, an oxide superconductor which bring about extremely useful working effects from the industrial aspect, including not only the enhancement of superconductive characteristics such assuring high trapped magnetic field, but also the preservation of high trapped magnetic field without being deteriorated even in the case of use under thermal history of repeated cooling and heating, under electromagnetic history of repeated application of electromagnetic force, or in a corrosive environment for a long period of time. In the following, the present invention will be described in further detail with reference to working examples, which however, shall never limit the present invention thereto. [Example 1] There were prepared five kinds of bulk materials comprising YBa 2 Cu 3 O y as a superconductor and Y 2 BaCuO 5 phase dispersed therein in an amount of 0, 10, 20, 30, and 40% by volume, respectively by means of the melt-textured-growth (MTG) process. For the MTG process, according to an ordinary method, each of starting materials of YBa 2 Cu 3 O y was heated to 1100°C for 20 minutes, thereafter was cooled to 1050°C for case back 30 minutes, and after placing SmBa 2 Cu 3 O y crystal therein as a seed, was cooled to 900°C at a cooling rate of 0.5°C /hr. Melt-growth samples were annealed with oxygen at 400°C for 250 hours in a stream of oxygen at atmospheric pressure. Subsequently, each of the bulk superconductors thus obtained was classified into the group without resin impregnation, the group impregnated with a resin in a vacuum vessel under the following condition (1), and the group impregnated with a resin in a vacuum vessel under the following condition (2). Condition (1) ---Bisphenol A type epoxy resin, quartz glass powder (amorphous powder having average size of 15µ) and an aromatic polyamine as the curing agent were mixed at a blending ratio by weight of 100 : 125 : 32, while preheating to 30°C, followed by deaeration under vacuum. Then the bulk superconductor was preheated to 70°C, and was evacuated in a vacuum tank. The mixed resin was poured into the tank so as to cover the bulk body, and was cured by pressurizing over atmospheric pressure, and heating at 80°C for 6 hours and at 120°C for 2 hours. Condition (2) ---Bisphenol A type epoxy, quartz glass powder (amorphous powder having average size of 15µ) and an aromatic polyamine as the curing agent were mixed at a blending ratio by weight of 100 : 125 : 32, while preheating to 30°C. Then the bulk superconductor was preheated to 70°C. The mixed resin was poured into the tank so as to cover the bulk body, followed by deaeration under vacuum in the tank. After sufficient defoaming, the impregnated resin was cured by heating under atmospheric pressure at 80°C for 6 hours and at 120°C for 2 hours. The quartz glass powder which was used in conditions (1) and (2) was pulverized molten quartz glass which was vitrified (made amorphous) by completely melting highly pure quartz at 1900°C. Each of the oxide superconductors in any of the groups thus prepared had the configuration as illustrated in Fig. 3. Subsequently, each of the superconductors as samples was cooled to 100K. A magnetic field with 10T was applied thereto at 100K, then after cooling to 50K, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field distribution on the surface of the sample by the use of a Hall sensor. As a result, it was confirmed that all the samples that were not subjected to resin impregnation under vacuum, had been destroyed. On the other hand, the trapped magnetic field of the samples that were subjected to resin impregnation under vacuum, showed 2.5T, 3T, 4T, 4T, and 3T for the samples having contents of Y211 phase of 0, 10, 20, 30 and 40% by volume, respectively. There was no difference in the values of the trapped magnetic field between the samples that were subjected to resin impregnation under the condition (1) and condition (2). As the result of observation for the structure of the cross section of the samples that were subjected to resin impregnation, it was confirmed that the filler-incorporated resin had been impregnated mainly into microcracks and micropores of the bulk material from the surface towards the inside, whereby the surface layer formed a resin impregnated layer. As mentioned above, cracks were observed in all the samples that were not subjected to resin impregnation under vacuum, but were not observed in all the samples that were subjected to a resin impregnation, the resin thereof is dispersedly incorporated with quartz glass powders as a filler, all of which had relatively high trapped magnetic field. Needless to say, the difference in trapped magnetic field due to the difference in the content of Y211 phase, reflects the difference in the critical current density. [Example 2] There were prepared five kinds of bulk materials comprising Sm 0.9 Ba 2.1 Cu 3 O y as a superconductor and Sm 2 BaCuO 5 phase (Sm211 phase) dispersed therein in an amount of 0, 10, 20, 30, and 40% by volume, respectively by means of the oxygen-controller-melt-growth (OCMG) process. For the OCMG process, each of starting materials of Sm 0.9 Ba 2.1 Cu 3 O y was heated to 1200°C for 20 minutes in flowing mixed gas of argon and oxygen having an oxygen partial pressure of 1%, thereafter was cooled to 1050°C for 20 minutes, and after placing NdBa 2 Cu 3 O y crystal therein as a seed, was cooled to 900°C at a cooling rate of 0.5°C /hr. During the course of the growth of crystal, large cracks were found on the samples having Sm211 phase contents of 0 and 10% by volume, and cracks, although not found visually, were observed on the sample having Sm211 phase content of 20% by volume. The samples in the form of bulk superconductor having Sm211 phase contents of 30 and 40% by volume, wherein no cracks were recognized, were annealed with oxygen at 350°C for 200 hours in flowing of oxygen at atmospheric pressure. Subsequently, each of the bulk superconductors thus obtained was classified into the group without resin impregnation, the group impregnated with a resin in a vacuum vessel under the following condition (3), and the group impregnated with a resin in a vacuum vessel under the following condition (4). Condition (3) ---Bisphenol A type epoxy resin and an aromatic polyamine as the curing agent were mixed at a blending ratio by weight of 100 : 32, while preheating to 30°C, followed by deaeration under vacuum. Then the bulk superconductor was preheated to 70°C, and was evacuated in a vacuum tank. The mixed resin was poured into the tank so as to cover the bulk body, and was cured by pressurizing over atmospheric pressure, and heating at 80°C for 6 hours and at 120°C for 2 hours. Condition (4) ---Bisphenol A type epoxy and an aromatic polyamine as the curing agent were mixed at a blending ratio by weight of 100 : 32, while preheating to 30°C. Then the bulk superconductor was preheated to 70°C. The mixed resin was poured into the tank so as to cover the bulk body, followed by deaeration under vacuum in the tank. After sufficient defoaming, the impregnated resin was cured by heating under atmospheric pressure at 80°C for 6 hours and at 120°C for 2 hours. Each oxide superconductive bulk body that had been subjected to the resin impregnation treatment was smoothened on the surface thereof by slightly grinding the outside surface. The oxide superconductive bulk body having the resin impregnated layer was again placed in a vacuum vessel, and was covered on the outside surface with a filler-incorporated resin layer with a thickness of 1 mm under the condition same as the condition (2) in Example 1. Subsequently, each of the superconductors as samples that were subjected to resin impregnation and covering treatment and the samples that were not subjected to resin impregnation and covering treatment was rapidly cooled to 100K. A magnetic field of 10T was applied thereto at 100K, then after cooling to 50K, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field distribution on the surface of the sample by the use of a Hall sensor. As a result, it was confirmed that all the samples that were not subjected to resin impregnation under vacuum and covering treatment, were destroyed. On the other hand, the trapped magnetic field of the samples that were subjected to resin impregnation under vacuum and covering treatment, showed high values of 8T and 6.5T, for the samples having contents of Sm211 phase of 30 and 40% by volume, respectively. There was no difference in the values of the trapped magnetic field between the samples that were subjected to resin impregnation under the condition (3) and condition (4). As the result of observation for the structure of the cross section of the samples that were subjected to resin impregnation and covering treatment, it was confirmed that the resin had been impregnated mainly into microcracks and micropores of the bulk material from the surface towards the inside, whereby a resin impregnated surface layer was formed, and further the outside surface thereof was covered with a resin layer dispersedly incorporated with quartz glass powders (filler). Fig. 4 is a schematic view for the observed state of the longitudinal sectional view (equivalent to sectional view taken on line A-A in the foregoing Fig. 3) for the sample of the oxide superconductor as obtained in this example, and Fig. 5 is a schematic view for the observed state of the transverse sectional view (equivalent to sectional view taken on line B-B in the foregoing Fig. 3) for the sample of the oxide superconductor as obtained in this example. As mentioned above, cracks were observed in all the samples that were not subjected to resin impregnation under vacuum, but were not observed in all the samples that were subjected to resin impregnation under vacuum and covering treatment, all of which had relatively high trapped magnetic field. The difference in trapped magnetic field due to the difference in the content of Sm211 phase, reflects the difference in the critical current density. In addition, the excellent critical current characteristics of Sm based superconductor is reflected by its markedly improved trapped magnetic field as compared with that of the samples in Example 1. [Example 3] There were prepared five kinds of bulk materials comprising Nd 0.9 Ba 2.1 Cu 3 O y as a superconductor and Nd 3.6 Ba 2.4 Cu 1.8 O x phase (Nd422 phase) dispersed therein in an amount of 0, 10, 20, 30 and 40% by volume, respectively by means of the OCMG process. As melting conditions, each of starting materials of Nd 0.9 Ba 2.1 Cu 3 O y was heated to 1040°C for 20 minutes in flowing mixed gas of argon and oxygen having an oxygen partial pressure of 0.1%, thereafter was cooled to 1010°C for 20 minutes, and after placing MgO single crystal therein as a seed crystal, was cooled to 900°C at a cooling rate of 0.5°C /hr. During the course of the growth of crystal, large cracks were found on the samples having Nd422 phase contents of 0 and 10% by volume, respectively. The samples in the form of bulk superconductor having Nd422 phase contents of 20, 30 and 40% by volume, respectively, wherein no cracks were recognized, were annealed with oxygen at 300°C for 300 hours in flowing oxygen at atmospheric pressure. Subsequently, each of the bulk superconductors thus obtained was classified into the group without resin impregnation, the group impregnated with a filler-incorporated resin in a vacuum vessel under the condition (1) in Example 1, and the group impregnated with a filler-incorporated resin in a vacuum vessel under the condition (2) therein. Subsequently, each of the superconductors as samples was rapidly cooled to 100K. A magnetic field with 10T was applied thereto at 100K, then after cooling to 50K, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field removed on the surface of the samples by the use of a Hall sensor. As a result, it was confirmed that all the samples that were not subjected to resin impregnation under vacuum had been destroyed. On the other hand, the trapped magnetic field of the samples that were subjected to resin impregnation under vacuum, showed 3T, 6.5T and 5T for the samples having contents of Nd422 phase of 20, 30 and 40% by volume, respectively. There was no difference in the values of the trapped magnetic field between the samples that were subjected to resin impregnation under the condition (1) and condition (2). As mentioned above, cracks were observed in all the samples that were not subjected to resin impregnation under vacuum, but were not observed in all the samples that were subjected to resin impregnation under vacuum, all of which had relatively high trapped magnetic field. [Example 4] There were prepared five kinds of bulk materials comprising YBa 2 Cu 3 O y as a superconductor, Y 2 BaCuO 5 phase dispersed therein in an amount of 0, 10, 20, 30, and 40% by volume, respectively and further 10% by weight of Ag by means of the MTG process. As melting conditions, each of starting materials of YBa 2 Cu 3 O y was heated to 1050°C for 20 minutes, thereafter was cooled to 1000°C for 30 minutes, and after placing YBa 2 Cu 3 O y phase therein as a seed crystal, was cooled to 900°C at a cooling rate of 0.5°C /hr. Melt grown samples were annealed with oxygen at 400°C for 250 hours in a flowing of oxygen at atmospheric pressure. Subsequently, each of the bulk superconductors thus obtained was classified into the group without resin impregnation, the group impregnated with a filler-incorporated resin in a vacuum vessel under the following condition (1) in Example 1, and the group impregnated with a resin in a vacuum vessel under the following condition (2) therein. Each oxide superconductive bulk body that had been subjected to the resin impregnation treatment was smoothened on the surface thereof by slightly grinding the outside surface. The oxide superconductive bulk body having the resin impregnated layer was again placed in a vacuum vessel, and was covered on the outside surface with a filler-incorporated resin layer with a thickness of 0.5 mm under the condition same as the condition (2) in Example 1. Subsequently, each of the superconductors as samples was cooled to 100K. A magnetic field of 10T was applied thereto at 100K, then after cooling to 50K, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field distribution on the surface of the sample by the use of a Hall sensor. As a result, it was confirmed that all the samples which were not subjected to resin impregnation under vacuum and which had Y211 contents of 0 and 10% by volume, were destroyed. The trapped magnetic field of the samples which had not been destroyed and which had Y211 contents of 20, 30 and 40% by volume, showed 1.5T, 3T and 2T, respectively. On the other hand, all the samples which were subjected to resin impregnation under vacuum and covering treatment were not destroyed. The trapped magnetic field of the samples subjected to resin impregnation under vacuum, showed 2.5T, 3T, 4.5T, 5T, and 3T for the samples having contents of Y211 phase of 0, 10, 20, 30 and 40% by volume, respectively. From the comparison of the trapped magnetic fields among the samples which were not destroyed, it was seen that the samples which were subjected to resin impregnation under vacuum had higher trapped magnetic fields in all cases. There was no difference in the values of the trapped magnetic field between the samples which were subjected to resin impregnation under the condition (3) and condition (4). It can be seen from the foregoing that Y base samples incorporated with Ag are also improved in trapped magnetic field by the resin impregnation under vacuum. [Example 5] There were prepared five kinds of bulk materials comprising Sm 0.9 Ba 2.1 Cu 3 O y as a superconductor, Sm 2 BaCuO 6 phase (Sm211 phase) dispersed therein in an amount of 0, 10, 20, 30 and 40% by volume, respectively and further 10% by weight of Ag by means of the OCMG process. As melting conditions, each of starting materials of Sm 0.9 Ba 2.1 Cu 3 O y was heated to 1010°C for 20 minutes in a stream of mixed gas of argon and oxygen having an oxygen partial pressure of 1%, thereafter was cooled to 990°C for 20 minutes, and after placing SmBa 2 Cu 3 O y crystal therein as a seed, was cooled to 850°C at a cooling rate of 0.5°C /hr. During the course of the growth of crystal, cracks, although not found visually, were observed on the sample not containing Sm211 phase. The samples in the form of bulk superconductor having Sm211 phase contents of 10, 20, 30 and 40% by volume, respectively, wherein no cracks were recognized, were annealed with oxygen at 350°C for 200 hours in flowing of oxygen at atmospheric pressure. Subsequently, each of the bulk superconductors thus obtained was classified into the group without resin impregnation, the group impregnated with a filler-incorporated resin in a vacuum vessel under the condition (1) in Example 1, and the group impregnated with a resin in a vacuum vessel under the condition (2) therein. Subsequently, each of the superconductors as samples was rapidly cooled to 100K. A magnetic field of 10T was applied thereto at 100K, then after cooling to 50K, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field distribution on the surface of the sample by the use of a Hall sensor. The trapped magnetic field of the samples which were not subjected to resin impregnation under vacuum and which had Sm211 contents of 10, 20, 30 and 40% by volume, showed 5T, 6T, 6T and 4T, respectively. On the other hand, the trapped magnetic field of the samples which were subjected to resin impregnation under vacuum and which had Sm211 contents of 10, 20, 30 and 40% by volume, showed 6T, 9T, 9T and 7T, respectively. There was no difference in the values of the trapped magnetic field between the samples subjected to filler-incorporated resin impregnation under the condition (1) and condition (2). It has been confirmed from the foregoing that Sm base samples incorporated with Ag are also improved in trapped magnetic field by the resin impregnation under vacuum. [Example 6] There was prepared a bulk material comprising Sm 0.9 Ba 2.1 Cu 3 O y as a superconductor, Sm 2 BaCuO 6 phase (Sm211 phase) dispersed therein in an amount of 30% by volume, and further 10% by weight of silver oxide by means of the OCMG process. As melting conditions, the starting material of Sm 0.9 Ba 2.1 Cu 3 O y was heated to 1010°C for 20 minutes in flowing of mixed gas of argon and oxygen having an oxygen partial pressure of 1%, thereafter was cooled to 990°C for 20 minutes, and after placing SmBa 2 Cu 3 O y therein as a seed crystal, was cooled to 850°C at a cooling rate of 0.5°C /hr. OCMG processed bulk superconductors were annealed with oxygen at 350°C for 200 hours in flowing of oxygen at atmospheric pressure. Subsequently, the bulk superconductors thus obtained were classified into the group without resin impregnation, the group impregnated with a filler-incorporated resin in a vacuum vessel under the condition (1) in Example 1, and the group impregnated with a resin in a vacuum vessel under the condition (2) therein. Subsequently, each of the superconductors as samples was rapidly cooled to 100K. A magnetic field of 10T was applied thereto at 100K, then after cooling to 77K by immersing in liquefied nitrogen, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field distribution on the surface of the sample by the use of a Hall sensor. Thereafter, the above mentioned step was repeated, and the resultant trapped magnetic field was measured every time of repetition. Fig. 6 illustrates the change in the trapped magnetic field with the number of repetitions on the basis of the trapped magnetic field measured at the first time defined ┌1┘ as the index. It is clearly understood from Fig. 6, that the trapped magnetic field decreases with an increase in the number of repetitions in the case of the samples which were not subjected to resin impregnation under vacuum, whereas the trapped magnetic field does not change at all in the case of the samples which were subjected to filler-incorporated resin impregnation under vacuum. There was no difference in the values of the trapped magnetic field between the samples which were subjected to filler-incorporated resin impregnation under the condition (1) and condition (2). As indicated above, it has been confirmed that the samples which are subjected to resin impregnation under vacuum are, effective in preventing the deterioration of trapped magnetic field due to repeated cooling or repeated application of electromagnetic force. [Example 7] There were prepared bulk materials comprising Gd 0.9 Ba 2.1 Cu 3 O y as a superconductor, Gd 2 BaCuO 5 phase (Gd211 phase) dispersed therein in an amount of 30% by volume, and further 10% by weight of a silver oxide by means of the OCMG method. As melting conditions, the starting material of Gd 0.9 Ba 2.1 Cu 3 O y were heated to 1000°C for 20 minutes in flowing of mixed gas of argon and oxygen having an oxygen partial pressure of 1%, thereafter were cooled to 980°C for 20 minutes, and after placing GdBa 2 Cu 3 O y crystal therein as a seed, were cooled to 850°C at a cooling rate of 0.5°C /hr. After the growth of crystal, the samples in the form of bulk superconductor were annealed with oxygen at 350°C for 200 hours in flowing oxygen at atmospheric pressure. Subsequently, the bulk superconductors thus obtained were classified into the group without resin impregnation, the group impregnated with a filler-incorporated resin in a vacuum vessel under the condition (1) in Example 1, and the group impregnated with a resin in a vacuum vessel under the condition (2) therein. Subsequently, each of the superconductors as samples was cooled to 100K. A magnetic field of 10T was applied thereto at 100K, then after cooling to 77K by immersing in liquefied nitrogen, was gradually lowered, and finally removed. Thereafter, measurements were made of the magnetic field distribution on the surface of the sample by the use of a Hall sensor. Thereafter, the above mentioned step was repeated, and the resultant trapped magnetic field was measured every time of repetition. Fig. 7 illustrates the change in the trapped magnetic field with the number of repetitions on the basis of the trapped magnetic field measured at the first time defined ┌1┘ as the index. It is clearly understood from Fig. 7, that the trapped magnetic field decreases with an increase in the number of repetitions in the case of the samples which were not subjected to resin impregnation, whereas the trapped magnetic field does not change at all in the case of the samples which were subjected to resin impregnation under vacuum. There was no difference in the values of the trapped magnetic field between the samples which were subjected to resin impregnation under the condition (1) and condition (2). As indicated above, it has been confirmed that the samples which are subjected to resin impregnation under vacuum are, effective in preventing the deterioration of trapped magnetic field due to repeated cooling or repeated application of electromagnetic force.

Claims: An oxide superconductor comprising an oxide superconductive bulk body in the state of pseudo-single crystal produced by a melting method, characterized in that said oxide superconductive bulk body has, within the outer surface zone thereof, a resin impregnated layer consisting of the oxide superconductive bulk material and the resin penetrated along microcracks and voids of the bulk body, the resin impregnated layer being covered with a resin layer dispersedly incorporated with a filler material having a linear thermal expansion coefficient smaller than that of the resin to be used. The oxide superconductor as set forth in claim 1, wherein the resin impregnated layer is dispersedly incorporated with a filler material having a linear thermal expansion coefficient smaller than that of the resin to be used. The oxide superconductor comprising an oxide superconductive bulk body in the state of pseudo-single crystal produced by a melting method, characterized in that said oxide superconductive bulk body has, within the outer surface zone thereof, a resin impregnated layer consisting of the oxide superconductive bulk material and the resin penetrated along microcracks and voids of the bulk body, the resin dispersedly incorporated with a filler material having a linear thermal expansion coefficient smaller than that of the resin to be used. The oxide superconductor as set forth in any of the preceding claims, wherein the resin in the resin impregnated layer comprises an epoxy base resin. The oxide superconductor as set forth in any of the preceding claims, wherein the filler material is at least one member selected from the group consisting of quartz, calcium carbonate, alumina, hydrate alumina, glass, talc and calcined gypsum. Process for producing the oxide superconductor as set forth in any of the preceding claims which comprises impregnating the resin, optionally with the filler material dispersedly incorporated therein, into the oxide superconductive bulk body by bringing the resin in liquid form into contact with said bulk body preserved in an atmosphere of reduced pressure and thereafter coating the resin impregnated bulk body with a resin in liquid form which is dispersedly incorporated with the filler material having a linear thermal expansion coefficient smaller than that of the resin to be used.

Inventors: MURAKAMI MASATO, TOMITA MASARU, MURAKAMI, MASATO, TOMITA, MASARU, Murakami, Masato Superconductivity Research Labo., Tomita, Masaru Superconductivity Research Labor.
Applicants: INT SUPERCONDUCTIVITY TECH, RAILWAY TECHNICAL RES INST, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, RAILWAY TECHNICAL RESEARCH INSTITUTE
Application year: 2000
Main classification: H01L39
IPC codes: H01L39