Superconducting ceramic circuits and manufacturing method for the same

Abstract: A superconducting pattern (5) formed from a super­ conducting ceramic film is disclosed. The pattern is made in the form of a coil which is embedded in an insulating ceramic film (6). The insulating film is made of a ceramic material whose thermal expansion coeffi­ cient is approximately equal to that of the coil.

Description: This invention relates to superconducting ceramic circuits and methods of manufacturing the same. It has been known to use metallic materials such as Nb₃Ge to wind a coil to form a superconducting magnet having a high Tc since a metallic material has a high ductility and a high malleability. In recent years, superconducting ceramics which have a high Tc have been attracting the interest of researchers. However, such superconducting ceramics are fragile because of low ductility and malleability so that it is virtually impossible to form a superconducting coil with a wire made of such superconducting ceramics. EP-A-292,125 was cited under Art. 54(3) and (4) EPC by the European Patent Office during examination. The citation describes a multilayer superconductive circuit comprising ceramic insulating layers and patterned ceramic oxide superconductive layers, the insulating layers having through holes for interconnecting the superconductive layers together. SUMMARY OF THE INVENTION Therefore the present invention aims to provide a high Tc superconductor in the form of wires or strips with high reliability and to provide a method of manufacturing high Tc superconductors in the form of wires for strips which method is suitable for mass-production with a high yield. In accordance with a first aspect of the invention, there is provided a multilayered superconductive circuit comprising: a lower superconductive coil made of a superconductive ceramic material; an intervening insulating ceramic layer formed on said lower superconductive coil; and an upper superconductive coil made of a superconductive ceramic material insulated from said lower superconductive coil by said insulating ceramic layer. In accordance with a second aspect of the invention, there is provided a method of manufacturing a multilayered superconductive circuit comprising: forming a first superconductive coil made of a superconductive ceramic material; insulating said first coil by covering the coil with an insulating layer made of a ceramic material; and forming a second superconductive coil made of a superconductive ceramic material. In accordance with an embodiment of the present invention, an oxide mixture for constituting a superconducting ceramic is deposited in the form of a thin film on a substrate. This deposition can be performed by various methods including sputtering such as magnetron sputtering, printing such as screen press printing, spraying such as plasma spraying, or electron beam evaporation. The deposited ceramic film is then fired at 500-1000°C, e.g. 900°C for 15 hours. After gradually cooling at a rate of 200°C/min or slower, e.g. 10°C/min, the ceramic film is further oxidized at 450°C for an hour, if necessary. During the deposition, firing and oxidizing, a magnetic field may be applied to the deposited film in order to expedite the formation of crystalline structure. The superconducting film is irradiated and scanned with a laser beam which is directed along a prescribed pattern in order to partly remove the film. Examples of lasers for this use are YAG lasers (1.06 microns), excimer lasers (KrF or KrCl), Argon lasers, and Nitrogen lasers. YAG lasers can emit infrared light pulses (having a circular cross section) at a frequency of 5-100KHz. The pulse width of YAG lasers is not shorter than 50 nm, and therefore, if fine control of processing depths of laser scribing is desired, excimer lasers are suitable for this application because their pulse width can be as short as 20 microns. By focusing with an optical system, it is possible to reduce the diameter of excimer laser beams to 10-100 microns. A representative pattern that is used is a spiral which forms a superconducting coil. The superconducting pattern on the substrate is covered with an insulating coating. It is important to make the insulating coating from a material whose thermal expansion coefficient is approximately equal to that of the superconducting pattern. The insulating coating is non-superconducting and preferably made of a ceramic material similar to that of the superconducting pattern. A portion of the insulating coating just above the end of the super-conducting coil is removed to provide an opening. Then, another superconducting ceramic film is superimposed on the insulating film and patterned into an upper spiral form in order to be coupled with the lower superconducting coil through the opening. The windings of the upper and lower spirals have to be in opposite directions so that their inductances are summed rather than cancelled out. Preferred examples of the insulating films used in contact with the superconducting pattern are represented by the stoichiometric formula, referred to "non-superconducting ceramics" hereinafter, [(A′ p A″ 1-p ) 1-x (B′ q B″ 1-q ) x ] y (CU r X 1-r ) z O w , where A′ is one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements or lanthanides, B′ is one or more alkaline earth metals, i.e. Ba, Sr and Ca, A″, B″ and X are selected from a group consisting of Mg, Be, Al, Fe, Co, Ni, Cr, Ti, Mn and Zr, and 0.1≦x≦1; y = 2.0 - 4.0, preferably 2.5 - 3.5; z = 1.0 - 4.0, preferably 1.5 - 3.5; and w = 4.0 - 10.0, preferably 6.0 - 8.0. The numbers p, q and r are chosen to be 0.99 to 0.80 so that the total proportion of A″, B″ and X is 1-20 vol% in the ceramic material, particularly in case of Mg and Al, the proportion may be 1-5%. Preferred examples of the superconducting ceramic materials used in accordance with the present invention for forming coils or other patterns are represented by the stoichiometric formula, (A 1-x B x ) y Cu z O w , where A is one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements or lanthanides, B is one or more alkaline earth metals, i.e. Ba, Sr and Ca, and 0<x<1; y = 2.0 - 4.0, preferably 2.5 - 3.5; z = 1.0 - 4.0, preferably 1.5 - 3.5; and w = 4.0 - 10.0, preferably 6.0 - 8.0. The non-superconducting ceramic film and the superconducting ceramic film can be formed by a spraying method for example. Namely, nitrates, hydrochlorides, and/or oxalates of the elements are dissolved in water and neutralized by ammonia in order to produce extremely fine particles. These fine particles are coated on a surface, dried and fired in an oxidizing atmosphere such as an excited oxygen atmosphere. This firing process can be carried out at relatively low temperatures by making use of an O₃ atmosphere. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1(A) is a perspective view showing the manufacturing process of a superconducting coil embodying the present invention. Fig.1(B) is a cross sectional view through A-A′ showing the superconducting coil in Fig.1(A). Fig.2(A) is a perspective view showing the manufacturing process of another superconducting coil in accordance with another embodiment of the present invention. Fig.2 (B) is a cross sectional view through A-A′ showing the superconducting coil in Fig. 2(A). DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to Figs.1(A) and (B), a superconducting coil in accordance with the present invention is illustrated. A circular substrate 1 is made of a ceramic material such as Alumina, Silicon Oxide, Aluminium Nitride, Zirconia, Yttria or YSZ(Yttria stabilized Zircon), or a metal or a quartz glass. YSZ, Yttria or Zirconia are particularly suitable having regard to the desirability of matching the coefficient of thermal expansion rate of the substrate 1 to that of a superconducting film formed thereon as hereinafter described. The differential coefficient of thermal expansion between the underlying substrate 1 and the overlying ceramic thin film to cover them should be as small as possible preferably within 50% of the coefficient of thermal expansion of the ceramic thin film. If the substrate 1 and the ceramic thin film have substantially different thermal coefficients, the strain caused between them may hinder the formation of a superconducting structure because of the recrystallization of the ceramic films. The surface of the substrate may be coated with a "non-superconducting ceramic" material prior to formation of the superconducting film. Prescribed amounts of oxides, carbonates and/or halides are mixed together in a ball mill, compacted and fired. The fired mixture is deposited on the substrate 1 at a substrate temperature of 650°C to a thickness of 0.1 to 50 microns, e.g. 20 microns by magnetron sputtering using a target composed of the fired mixture and in an oxygen atmosphere containing 20% Argon. The deposited film is annealed at 850°C for 8 hours, gradually cooled at 4°C per minute and annealed again at 400°C for 2 hours. As a result, a superconducting structure of the ceramic material which is sublimable is formed with its (a,b) plane in parallel to the plane of the substrate. Next, the surface of the superconducting film is scanned and irradiated with laser pulses from an excimer laser (254nm) of 50 microns diameter along a spiral path 3 while the circular substrate is being rotated. The laser pulse location 4 is shifted from a peripheral position toward the center of the circular substrate 1 along the spiral path 3. By virtue of the laser pulses, the irradiated portion of the superconducting film is removed by sublimation, and a spiral groove 3 is formed which defines a resultant superconducting spiral pattern 5. The peak output power level of the laser pulses is 10⁶ to 10⁸ W/sec. Alternatively, this laser scribing may be performed before the annealing of the ceramic film. The superconducting spiral 5 on the upper surface 1′ of the substrate 1 is isolated by coating an insulating film 6 made of a "non-superconducting ceramic". It is important that the insulating film 6 is selected to have a thermal expansion coefficient approximately equal to that of the superconducting spiral 5. A portion of the insulating film 6 is removed to form an opening in order to expose the inner end 8 of the superconducting spiral 5. A ceramic oxide film is deposited again on the structure, fired and patterned in the same manner to form an upper superconducting ceramic spiral 7. The upper superconducting ceramic spiral 7 is formed to turn in a reverse direction to the lower superconducting ceramic spiral 5. Since the upper and lower spirals 7 and 5 are coupled through the opening at the inner ends 8 of the spirals, a double-layered coil is constructed with a doubled inductance. Referring now to Figs.2(A) and 2(B), another embodiment of the present invention will be described. In this embodiment, a procedure equivalent to that of the previous embodiment is carried out but on the surface of a cylinder 11. Deposition is performed on the cylindrical surface coated with a "non-superconducting ceramic" film 12 with the cylinder 11 being rotated as shown in Figure.2(A). The laser pulses 4 are directed along a path towards the right end of the rotating cylinder from its left end as indicated by the arrow in Figure.2(A). Accordingly, a helical groove 13 is cut into the superconducting film thereby forming a first superconducting helix 15. A first "non-superconducting ceramic" film 17 is formed with an opening at one end 15-1 of the first helix 15. A second superconducting helix 19 and a second "non-superconducting ceramic" film 21 with an opening at one end 19-1 of the second helix 19 are also formed in the same manner. The other end 19-2 of the second helix 19 is coupled with the first helix 15 through the opening of the first non-superconducting film 17. Another opening is formed penetrating through the first and second non-superconducting films 17 and 21. Furthermore, a third superconducting helix 23 is formed which is coupled at one end 23-1 thereof with the second superconducting helix 19 through the opening at its one end 19-1. Then, a triple-layered coil is formed. One terminal is the other end 23-2 of the third helix 23 and the other terminal is the other end 15-2 of the first helix 15. The other terminal is provided by a contact 25 which connects with the first helix 15 through the opening at the other end 15-2. thereof. While the foregoing description is directed to methods of making superconducting ceramic patterns carried by substrates, light weighted superconducting coils which are not supported by substrates can also be formed. For this purpose, temporary substrates are prepared in the form of the substrates 1 and 11 as illustrated in Figs.1(A) and 1(B) and Figs.2(A) and 2(B). The temporary substrates have to be made of materials which are soluble by a suitable solvent, e.g. organic substances, but compatible with "non-superconducting ceramics". On the temporary substrate, a non-superconducting ceramic film 12 is formed to a thickness of 10-5000 microns, e.g. 20 microns for example. Thereafter, the procedures of the foregoing examples are repeated on the "non-superconducting ceramic" film 12 in order to make a multi-layered spiral coil and a multi-layered helical coil. After completing the formation of the coils, the temporary substrates are removed with a solvent. If a further increased large number of turns is desired, a further number of superconducting ceramic layers, e.g. several tens of superconducting ceramic layers, with coils respectively formed thereon may be provided. Although the helices are connected in series in Fig.2(B), the helices can be formed without interconnection therebetween, but the superconducting patterns on the ceramic layer can be designed to produce an individual pair of terminals for each helix so that the connection among the helices can be arbitrarily arranged. Superconducting ceramics for use with the described embodiments may also be prepared in consistence with the stoichiometric formulae (A 1-x B x ) y Cu z O w , where A is one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements, B is one or more elements of Group IIa of the Periodic Table, e.g., the alkaline earth metals including Beryllium and Magnesium, and 0<x<1; y = 2.0 - 4.0, preferably 2.5-3.5; z = 1.0 - 4.0, preferably 1.5 - 3.5; and w = 4.0 - 10.0, preferably 6.0 - 8.0. also, superconducting ceramics for use with the described embodiments may be prepared consistent with the stoichiometric formulae (A 1-x B x ) y Cu z O w , where A is one or more elements of Group Vb of the Periodic Table such as Bi, Sb and As, B is one or more elements of Group IIa of the Periodic Table, e.g., the alkaline earth metals including Beryllium and Magnesium, and x = 0.3 - 1; y = 2.0 - 4.0, preferably 2.5 - 3.5; z = 1.0 - 4.0, preferably 1.5 - 3.5; and w = 4.0 - 10.0, preferably 6.0 - 8.0. Examples of this general formula are BiSrCaCuCu₂O x and Bi₄Sr₃Ca₃Cu₄O x . Tc onset and Tco samples confirmed consistent with the formula Bi₄Sr y Ca₃Cu₄O x (y is around 1.5) were measured to be 40-60°K, which is not so high. Relatively high critical temperatures were obtained with samples conforming to the stoichiometric formulae Bi₄Sr₄Ca₂Cu₄O x and Bi₂Sr₃Ca₂Cu₂O x . The number designating the oxygen proportion is 6-10 e.g. around 8.1. Hereinbefore, the description has been made of the embodiments in which superconducting spirals or helices are formed by eliminating unnecessary intervening portions between adjacent turns of the spirals or helices. However, superconducting patterns can also be produced by introducing one or more elements selected from a group consisting of Mg, Be, Al, Fe, Co, Ni, Cr, Ti, Mn and Zr into unnecessary portions of superconducting thin films by ion implantation in order to modify these portions to be non-superconducting, so that a superconducting pattern composed of a non-implanted portion remain superconductive. By use of such a method, smooth layered structures can be formed. While a description has been made for several embodiments, the present invention should be limited only by the appended claims and should not be limited by the particular examples. For example, each spiral or helix may be formed with a metallic spiral or helix by, before or after forming a superconducting film, forming an underlying or overlying metallic film which is to be processed by laser scribing together with the superconducting film. For the avoidance of doubt it is to be noted that all references herein and in the appended claims to the Periodic Table and to specific Groups of the Periodic Tables are to be understood to refer to the Japanese Periodic Table and to the Groups thereof as described in the Iwanami "Physics and Chemistry Dictionary". As compared with the Group designations of the Periodic Table as described in "The Penguin Dictionary of Science" for example, which are the Group designations commonly accepted throughout Europe, Groups Ia, IIa, VIII, Ib, IIb and O are the same as in the Japanese and European Periodic Tables, Groups IIIa, IVa, Va, VIa and VIIa of the Japanese Periodic Table correspond respectively to Groups IIIB, IVb, Vb, VIb and VIIb of the European Periodic Table, and Groups IIIb, IVb, Vb, VIb and VIIb of the Japanese Periodic Table correspond respectively to Groups IIIa, IVa, Va, VIa and VIIa of the European Periodic Table.

Claims: A multilayered superconductive circuit comprising: a lower superconductive coil (5,15) made of a superconductive ceramic material; an intervening insulating ceramic layer (6,17) formed on said lower superconductive coil (5,15); and an upper superconductive coil (7,19) made of a superconductive ceramic material insulated from said lower superconductive coil (5,15) by said insulating ceramic layer (6,17). A circuit according to claim 1, wherein said lower and upper superconductive coils (5,15,7,19) are interconnected through an opening formed in said insulating ceramic layer (6,17). A circuit according to claim 1 or 2, wherein said lower and upper coils (5,15,7,19) are arranged in opposite senses and are coupled in order to form a multilayered coil. A circuit according to any preceding claim, wherein said lower coil (5,15) is formed on an insulating ceramic substrate (1). A circuit according to any preceding claim, wherein said coils (5,15,7,19) are formed on a disk (1). A circuit according to any of claims 1 to 4, wherein said coils (5,15,7,19) are formed on a cylindrical surface (11). A circuit according to any preceding claim, wherein said insulating ceramic layer (6,17) is made of an oxide ceramic material conforming to the stoichiometric formula [(A′ p A″ 1-p ) 1-x (B′ q B″ 1-q ) x ] y (Cu r X 1-r ) z O w , where A′ is one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements or lanthanides, B′ is one or more alkaline earth metals, i.e. Ba, Sr and Ca, A″, B″ and X are selected from a group consisting of Mg, Be, Al, Fe, Co, Ni, Cr, Ti, Mn and Zr, and 0<x<1; y = 2.0 - 4.0; z = 1.0 - 4.0; and w = 4.0 - 10.0, and the numbers p, q and r are chosen so that the total proportion of A″, B″ and X is 1-20 vol%, and wherein said superconductive ceramic coils (5,15, 7,19) is made of materials conforming to the stoichiometric formula, (A 1-x B x ) y Cu z O w , where A is one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements or lanthanides, B is one or more alkaline earth metals, i.e. Ba, Sr and Ca, and 0<x<1; y = 2.0 - 4.0; z = 1.0 - 4.0; and w = 4.0 - 10.0. A method of manufacturing a multilayered superconductive circuit comprising: forming a first superconductive coil (5,15) made of a superconductive ceramic material; insulating said first coil (5,15) by covering the coil (5,15) with an insulating layer (6,17) made of a ceramic material; and forming a second superconductive coil (7,19) made of a superconductive ceramic material. A method according to claim 8, wherein the ceramic material of said insulating layer (6,17) is selected from materials having a thermal expansion coefficient approximately equal to that of said superconductive coils (5,15, 7,19). A method according to claim 8 or 9, wherein said second superconductive coil (7,19) is formed in the opposite sense to that of said first superconductive coil (5,15). A method according to any of claims 8 to 10, wherein said first and second superconductive coils (5,15,7,19) are interconnected to form a double-layered coil. A method according to any of claims 8 to 11, wherein said first superconductive coil (5,15) is formed on a ceramic surface (1). A method according to any of claims 8 to 12, wherein said first superconductive coil is formed on a disk (1). A method according to any of claims 8 to 12, wherein said first superconductive coil (5,15) is formed on a cylinder (11). A method according to any of claims 12 to 14, wherein said ceramic surface, disk (1) or cylinder (11) is comprised of a material having a thermal expansion coefficient within 50% of that of said first superconductive coil (5,15). A method according to any of claims 8 to 15, further comprising: providing a temporary substrate (1,11) of a material which is soluble to a suitable solvent; covering the surface (1′) of said substrate (1,11) with an insulating ceramic layer (12); forming said first superconductive ceramic coil (5,15) on the surface of said insulating ceramic layer (12); and removing said temporary substrate (1,11) with said solvent. A method according to any of claims 8 to 16, further comprising: forming a further insulating layer (21) made of a ceramic material; and forming a further superconductive coil (23) made of a superconductive material on said further insulating layer (21). A method according to claim 17, wherein said steps of forming said further insulating layer (21) and said further superconductive coil (23) are repeated one or more times in order to form a multi-layered coil having four or more layers. A method according to any of claims 8 to 18, wherein at least one of said first and second superconductive coils (5,15,7,19) is formed by: forming a layer comprising said superconductive ceramic material; and removing unnecessary portions (3,13) of said superconductive ceramic material by irradiating said portions (3,13) with a laser. A method according to any of claims 8 to 18, wherein at least one of said first and second superconductive coils (5,15,7,19) is formed by: forming a layer comprising said superconductive ceramic material; and introducing an impurity element into unnecessary portions (3,13) of said superconductive layer in order to hinder superconductivity in said portions (3,13).

Inventors: YAMAZAKI SHUNPEI, YAMAZAKI, SHUNPEI
Applicants: SEMICONDUCTOR ENERGY LAB, SEMICONDUCTOR ENERGY LABORATORY CO., LTD.
Application year: 1988
Main classification: H01L39
IPC codes: H01F41, H01L39, H01F6