A SUPERCONDUCTING COIL AND METHOD FOR PRODUCING THE SAME

Abstract: A superconducting coil comprising a support (1,10) and at least one ring-shaped and/or spiral turn (2) of superconductor which is composed of superconducting compound oxide and is supported on a surface of said support.

Description: Background of the Invention Field of the invention The present invention relates to superconducting coil composed of compound oxide type superconducting material and a method for producing the same. The coil according to the present invention is applicable for manufacturing a small and light-weight actuator, motor or generator, electromagnetic valve, sensor or the like. Description of the related art Superconducting coil or solenoid which is realized by metal type superconductor such as Ti-Nb alloy is already known and has been developed for a superconducting magnet or the like. The critical temperature of the metal type superconductors, however, could not exceed 23.2 K of Nb₃Ge which was the the highest Tc for all studies for the past ten years and hence usage of liquidized helium (boiling point of 4.2 K) as cryogen is indispensable to realize superconductivity. Therefore, their actual application is limited to special industrial or scientific fields and still more it is difficult to reduce their size or diameter to less than several 10 mm at the present condition. Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Müller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]. This new oxide type superconducting material discovered by Bednorz and Müller is [La, Ba]₂CuO₄ which is called as the K₂NiF₄-type oxide having a crystal structure which is similar to a known perovskite type oxide. The K₂NiF₄-type oxides show such a high Tc as 30 K which is extremely higher than the known superconducting materials. It was also reported in the news parer that C. W. Chu et al. discovered in the United States of America another type of superconducting material having the critical temperature of abut 90 K in February 1987. This superconductor called as YBCO is a compound oxide of Ba-Y system represented by YBa₂Cu₃O 7-δ . From the discovery of the abovementioned new type of compound oxide type superconductors, the realization of high-temperature superconductors which can realize the superconductivity in cheap and easily available liquidized nitrogen have arisen suddenly. However, the abovementioned compound oxide is a ceramic material so that it is difficult or impossible to adopt directly the conventional technique which is used in the metal type superconductor for producing a solenoid or coil. Therefore, an object of the present invention is to provide a superconducting coil consisting of new type compound oxide which possesses a high critical temperature (Tc) and a method for producing the same. Summary of the Invention The object of the invention is solved by the subject matter of claims 1 and 5. The abovementioned oxides possess preferably perovskite type or quasi-perovskite type crystal structure. The term of quasi-perovskite type means a structure which can be considered to have such a crystal structure that is similar to Perovskite-type oxides and includes an orthorhombically distorted perovskite or a distorted oxygen-deficient perovskite or the like. It is known that the oxygen deficiency in the crystalline structure of the compound oxide is one of the critical factors for realizing the high Tc (the critical temperature) superconductor of the perovskite type or quasi-perovskite type compound oxide. Therefore, the elements α, β, γ and oxygen (O) must be within the abovementioned proportions in order to realize the proper crystalline structure and oxygen deficiency. The element α of IIa group of the Periodic Table is preferably Ba or Sr. From 10 to 80 % of Ba or Sr can be substituted by Mg and/or Ca. The element β of IIIa group of the Periodic Table is preferably at least one of elements selected from a group comprising Y, La, Gd, Ho, Er, Tm, Yb, Dy, Nd, Sm, Eu and Lu. The element γ is Cu (copper) in general. A portion of Cu may be substituted by another element selected from Ib, IIb, IIIb, IVb and VIII groups of the Periodic Table such as Ti, V or the like. Still more, a portion of the oxygen (O) may be substituted by another element such as fluorine. The superconducting thin films according to the present invention may consists of Ba-Y-Cu system compound oxide, Ba-La-Cu system compound oxide and Sr-La-Cu system compound oxide such as YBa₂Cu₃O 6.7 , Sr 0.15 La 1.85 CuO 3.5 or the like. As preferred compound oxide, it can be mentioned the following compound oxide represented by the general formula: Ln₁Ba₂Cu₃O 7-δ in which, Ln stands for an element selected from a group comprising Y, La, Gd, Ho, Er, Tm, Yb and Lu, and δ is a number which satisfies 0 < δ < 1 and including the following systems: Y₁Ba₂Cu₃O 7-x , Ho₁Ba₂Cu₃O 7-x , Lu₁Ba₂Cu₃O 7-x , Sm₁Ba₂Cu₃O 7-x , Nd₁Ba₂Cu₃O 7-x , Gd₁Ba₂Cu₃O 7-x , Eu₁Ba₂Cu₃O 7-x , Er₁Ba₂Cu₃O 7-x , Dy₁Ba₂Cu₃O 7-x , Tm₁Ba₂Cu₃O 7-x , Yb₁Ba₂Cu₃O 7-x , La₁Ba₂Cu₃O 7-x , (La, Sr)₂CuO 4-x in which x is a number which satisfies a range of 0 < x < 1. Of course, the superconductor of the the present invention is applicable to the other type compound oxides having another system including the following compound oxide represented by the formula: Θ₄(Φ 1-q ,Ca q ) m Cu n O p+r in which Θ stands for Bi or Tl, Φ stands for Sr when Θ is Bi and stands for Ba when Θ is Tl, m and n are numbers each satisfying ranges of 6 ≦ m ≦ 10 and 4 ≦ n ≦ 8 respectively, p = (6+m+n), q is a number which satisfies a range of 0 < q < 1, and r is a number which satisfies a range of -2 ≦ r ≦ +2. This system is considered to be a single phase of the following compound oxide or a mixed phase consisting mainly thereof: Bi₄Sr₄Ca₄Cu₆O 20-r , Bi₂Sr₂Ca₂Cu₃O 10-r , Tl₄Ba₄Ca₆O 20-r , Tl₂Ba₂Ca₂Cu₃O 10-r , in which r is a number which satisfied a range of -2 ≦ r ≦ +2. According to another aspect of the present invention, it is also provided a method for producing a method for producing a superconducting coil, characterized by the steps comprising placing a superconducting material on a surface of a support, and then heating the superconducting material to a temperature at which said superconducting material is sintered to a superconductor. The term of "placing" means any operation which can form a layer of the superconducting material on a surface of the support and which include coating, depositing, applying, wrapping or the like. The superconductor placed on the support is preferably in a shape of at least one ring and/or of at least one spiral turn of a coil which can be prepared by any one of following processes: (1) At least one ring-shaped or spiral groove is cut previously on an outer surface of the substrate so that the superconducting material is received in the groove. (2) A paste composed of superconducting compound oxide and binder is applied directly on the surface of the substrate. (3) The superconducting material is deposited on said support by physical vapour deposition (PVD) technique or chemical vapour deposition (CVD) technique. In case of (2) and (3), the superconducting material may be applied or deposited on an entire surface of the support as a continuous layer and then a predetermined portion of the continuous layer of superconducting material is removed later to leave a portion of the continuous layer alone in a form of at least one ring-shaped or spiral turn on the support. It is also possible to provide such ring-shaped or spiral turn on the support by using a suitable masking member which is removable after the sintering stage and is arranged between adjacent turns. It is also possible to apply or deposit the superconducting material on a part of the support in a shape of at least one ring or spiral turn of a coil directly. In the first method (1), no binder is required to be added to the superconducting material, resulting in that the material is advantegeously not contaminated, but the groove is indispensable to support the poweder of superconducting compound oxide on a three dimensional support. Still more, it is necessary to leave a room for absorb or compensate shrinkage of the sintered mass which occur during the sintering stage. Such shrinkage can be absorbed by arranging a retractable or in the bottom of the groove. Such retractable or shrinkable spacer which may be made of a low melting point metal foam or an organic compound reduce its volume or is burnt at the sintering stage, so that shrinkage of the sintered mass placed on the spacer in the groove is allowed with no breakage of the sintered mass. The shrinkable spacer must be selected from such materials that do not exert bad influence to the superconducting sintered mass but is preferably selected from materials which is decomposed and produce oxygen at the sintering temperature since the oxygen contents in the superconducting compound oxide is a critical factor for realizing the superconductivity. The superconducting material is preferably a sintered powder which is produced by the steps comprising mixing oxides, carbonates, nitrates or sulfonate of the abovementioned constituent elements α, β and γ such as Ba or Sr, La or Y and Cu, subjecting the resulting mixture to a preliminary heat-treatment so called preliminary sintering operation, and then subjecting the resulting preliminary sintered mass to the final sintering. The sintering temperature is preferably selected in a range between the upper limit which is a melting point of said preliminary sintered mass and a lower limit which is not lower than 100°C from the melting point. Namely, when the sintering temperature is not higher than the the lower limit, sintering reaction can not proceed satisfactorily and the sintered product obtained shows very low mechanical strength. To the contrary, if the sintering temperature exceeds the upper limit, liquid phase is produced during the sintering operation, so that the sintered mass is fused or decomposed resulting in lowering the Tc of the sintered body. The sintering temperature depend on the system of the compound oxide but is effected generally in a range of 600 to 1,200°C. The superconducting material can be a powder which can be produced by pulverizing the abovementioned preliminarily sintered mass or the finally sintered block. The steps of from sintering to pulverization are preferably repeated for several times in order to realize superior superconducting compound oxide having a fine crystalline structure and uniformity which result in reduction of discrepancy ΔT between on set temperature (Tc) where the superconductivity start to be observed and a critical temperature (Tcf) where complete zero resistance is observed. The material powder is not limited to the sintered powder but may be a powder mixture of the abovementioned oxides, carbonates, nitrates or sulfonate of the constituent elements α, β and γ which are not subjected to the sintering operation. The proportion or atomic ratio of constituent elements α, β and γ in the superconducting material is preferably adjusted to be equal to the proportion or atomic ratio of these elements in the finally sintered mass or superconductor which is represented by the abovementioned general formula : (α 1-x β x )γ y O z , Ln₁Ba₂Cu₃O 7-δ , Θ₄(Φ 1-q ,Ca q ) m Cu n O p+r or the like. The second method (2) in which paste of superconducting material mixed with binder is applied on the support and is sintered is basically same as the first method (1) except the binder is admixed with the superconducting material powder. In the second method, the ring shaped or spiral turn or turns can be formed relatively easily than the first method because such turn of coil can be shaped with the paste by the conventional insert-molding technique in which the paste is injected into a molding cavity defined between a pair of mold halves and the support placed in the mold halves. The turns of coil may be made simply by winding a strand or filament of paste around the support directly. In the second method also, it is necessary to absorb variation of dimension occurred by shrinkage of the sintered mass. In practice, in order to absorb such shrinkage, the abovementioned compensation technique can be adopted. Namely, a shrinkable spacer is placed in a groove of the support and the paste or a strand of paste is applied on the shrinkable spacer, so that the shrinkage at the sintering stage is absorbed by the spacer. In order to minimize such shrinkage, the increment of sintering temperature should be controlled precisely. The binder used in the paste can be selected from known binders which are used in the field of ceramic industry and can contain plasticizer, solvent or the like. In case of the first and second method, an assembly of the support and the turn(s) supported on the support can be covered or wrapped by a case or a bladder so that the sintering is carried out in a HIP. In the third method (3) in which a thin film of superconducting compound oxide is prepared by physical vapour deposition (PVD) or chemical vapour deposition (CVD) technique, the turn or turns of coil is shaped by the conventional lithography technique. Namely, a portion on the support where the thin film of compound oxide is not required to be deposited may be covered by a mask such as photoresist, pellicle or the like. The masking of the substrate can be omitted. In this case, a thin film of compound oxide is deposited on the whole surface of the support and then a part of the deposited thin film layer of compound oxide is removed in such manner that a ring-shaped or spiral continuous superconducting thin film of compound oxide is left about the support. In case of PVD such as sputtering, vacuum deposition, ion plating or the like, a vapour source or target may be composed of a sintered mass of a compound oxide to be deposited on the support. Since a composition of vapour source, substrate temperature, sputtering pressure or the like are critical factors for realizing the superconducting thin film of compound oxide, they should be selected so that the superconductivity is realized in the deposited thin film. The vapour source used in the physical vapour deposition is selected from any material that can produce the thin film of compound oxide on the support. For example, the vapour source or target may be prepared by a sintered product which is produced by the steps comprising mixing oxides, carbonates, nitrates or sulfonate of constituent elements such as Ba or Sr, La or Y and Cu, subjecting the resulting mixture to a preliminary heat-treatment so called preliminary sintering operation, and then subjecting the resulting preliminarily sintered mass to the final sintering at a temperature of 700 to 1,500°C. The physical vapour deposition is preferably carried out in the presence of oxygen gas or reactive gas such as fluorine gas. In case of CVD process, a suitable carrier gas is used for carrying the constituent elements of the compound oxide or their compounds onto a substrate. It is apparent from the description abovementioned that the method according to the present invention is advantageously applicable to mass production of small superconducting coils. For example, when a heat-resisting partition ring such as a metal washer is arranged between adjacent ring-shaped turns on a common support, a large number of coils are produced on the common support simultaneously. The small coils produced according to the present invention can maintain a permanent current thereon, once a predetermined electric current is supplied to the ring-shaped superconducting thin film. Such permanent current can be supplied to the ring-shaped superconducting thin film by any one of known methods such as thermally controlled On-Off switching for permanent current, mechanical On-Off switching for permanent current or magnetically controlled switching. The small superconducting solenoid produced according to the present invention is used in the field of small actuator, motor, electromagnetic valve, sensor, coil or similar devices which functions in relatively cheap cryogen such as liquidized nitrogen, hydrogen or neon. Now, the method for producing a small superconducting coil according to the present invention will be described with reference to attached illustrative drawings. Brief description of the drawings Figure 1 (a) to (d) are perspective views illustrating several embodiments of small superconducting coils supported on a solid core or support according to the present invention, Figure 2 (a) to (d) are similar to Figure 1 but illustrate another embodiment of small superconducting coils supported on a hollow core or support according to the present invention. Figure 1 (a) illustrates a small superconducting coil according to the present invention. This coil comprises a solid cylindrical support 1 and a plurality of ring-shaped turns 2. Figure 1 (b) illustrates another small superconducting coil according to the present invention. This coil comprises a solid cylindrical support 1 and a spiral turn 2′. Figure 1 (c) illustrates a variation of the small superconducting coil of Figure 1 (a) and comprises a solid cylindrical support 1 having a parallel grooves 3 and a plurality of ring-shaped turns 2″ supported in the grooves 3. In Figure 1 (d) which illustrates a still another variation of Figure 1 (a), the solid support 1″ has a rectangular cross section and the hollow turns have also rectangular cross section. Figure 2 (a) to (d) which correspond to Figure 1 (a) to (d) have the same configuration as those of Figure 1 (a) to (d) except that each of the supports 1 in Figure 1 is replaced by a hollow support 10, 10′ and 10″ In a center hole or passage of the hollow support 10, 10′ and 10″, a core or armature (not shown) can be inserted or suitable cryogen for cooling the superconducting coil can be circulated. The small coil according to the present invention can maintain a permanent current thereon, once a predetermined electric current is supplied to the ring-shaped superconducting turn(s). Such permanent current can be supplied to the ring-shaped superconducting thin film by any one of known methods such as thermally controlled On-Off switching for permanent current, mechanical On-Off switching for permanent current or magnetically controlled switching.

Claims: A superconducting coil comprising an insulating substrate (1) and at least one ring-shaped and/or spiral turn (2) of superconductor which is composed of superconducting compound and is supported on a surface of said substrate (1), characterized in that said substrate (1) is made of a material selected from a group comprising silicon, MgO, SrTiO₃, Al₂O₃, sapphire, SiO₂, quartz yttrium stabilized zirkonia and ZnO, and in that said superconducting compound is an oxide represented by the formula: (α₁₋ x β x )γ y O z in which α stands for an element selected from IIa group of the Periodic Table, β stands for an element selected from IIIa group of the Periodic Table and γ stands for an element selected from a group comprising Ib, IIb, IIIb, IVb and VIII group of the Periodic Table, and x, y and z are numbers each having a range of 0.1 ≦ x ≦ 0.9, 1.0 ≦ y ≦ 4.0 and 1 ≦ z ≦ 5 respectively and in that the superconducting compound oxide is in direct contact with the substrate. A superconducting coil as set forth in Claim 1, characterized in that said superconducting material is a compound oxide represented by the general formula: Ln₁Ba₂Cu₃O 7-δ in which, Ln stands for an element selected from a group comprising Y, La, Gd, Ho, Er, Tm, Yb, Nd, Sm, Eu and Lu, δ is a number which satisfies a range of 0 < δ < 1. A superconducting coil as set forth in Claim 1 or 2, characterized in that said superconducting material is a compound oxide represented by the general formula: Θ₄(Φ 1-q ,Ca q ) m Cu n O p+r in which Θ stands for Bi or Tl, Φ stands for Sr when Θ is Bi and stands for Ba when Θ is Tl, m and n are numbers each satisfying ranges of 6 ≦ m ≦ 10 and 4 ≦ n ≦ 8, respectively, p = (6+m+n), q is a number which satisfies a range of 0 < q < 1, and r is a number which satisfies a range of -2 ≦ r ≦ +2. A superconducting coil as set forth in one of Claims 1 to 3, characterized in that said substrate is a hollow cylindrical body (10). A method for producing a superconducting coil according to Claim 1 or 2, characterized by placing said superconducting compound oxide in powder form in a shape of at least one ring-shaped or spiral turn (2) of a coil on the substrate (1) and then heating said superconducting material compound oxide to a temperature at which said material is sintered. A method as set forth in Claim 5, characterized in that a whole surface of the support is covered firstly with said superconducting compound oxide to form a continuous layer of said superconducting material on the substrate (1), and then a predetermined portion of the continuous layer of said superconducting material is removed to leave at least one ring-shaped or spiral turn (2) on the substrate (1). A method as set forth in Claim 5 or 6, characterized in that said substrate (1,10) has a ring-shaped or spiral groove (3) on its outer surface so that the superconducting compound oxide is received in said groove. A method as set forth in one of Claims 5, 6 and 7, characterized in that said superconducting compound oxide is a paste composed of superconducting compound oxide and a binder. A method as set forth in any of Claims 5 to 7, characterized in that said superconducting compound oxide is deposited on said substrate (1,10) by physical vapour deposition technique or chemical vapour deposition technique.

Inventors: NISHIGUICHI, MASANORI C/O YOKOHAMA WORKS OF SUMITO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: H01F7
IPC codes: H01F6, H01B13, C23C14, H01F41, C04B35, C23C16, C01G1, H01L39, H01B12