Methods of manufacturing ceramic material superconductors

Abstract: A patterned superconductor is formed by sputtering or otherwise depositing onto a substrate (1) surface a ceramic film (2) which is subsequently melted by firing or by laser irradiation to produce a high Tc superconducting film. By tracing a laser beam (4) of sufficient power or intensity to cause the film to sublimate a pattern (3) can be scribed into the film either before or after firing, a superconducting device such as a superconducting coil (5) can be readily obtained.

Description: BACKGROUND OF THE INVENTION This invention relates to methods of manufacturing ceramic material superconductors by light irradiation. It has been known to use metallic materials such as Nb₃Ge to wind a coil to form a superconducting magnet since such metallic materials have high ductility and malleability. However, such metallic materials do not have a high Tc and to operate as superconductors such metallic materials require the use of liquid helium in order to function as superconductors. In recent years, superconducting ceramics which have a high Tc have attracted the interest of researchers. However, superconducting ceramics which have a high Tc are fragile and have low ductility and malleability and therefore it has been quite impossible to form a superconducting coil from a wire made of such superconducting ceramics. Applied Physics A, vol 40, no 4, 1986, pp235-239 teaches laser-induced chemical etching of non-superconducting ceramic solid solutions of PbTi 1-x Zr x O₃. OBJECTS AND SUMMARY OF THE INVENTION It is therefore the object of the invention to provide a high Tc superconductor in any desired form and to provide a method of manufacturing a high Tc superconductor in any desired form which method is suitable for mass-production. It has been found that laser scribing of a ceramic film can be easily achieved since the thermal conductivity of the film is relatively small and the film will undergo sublimation when irradiated. The present invention resides generally in the concept of providing a layer of a prescribed material on a substrate, treating the said layer by for example thermal oxidation or firing to convert it into a superconducting layer and scribing a pattern into the layer by irradiation in order to obtain a superconducting ceramic with desired characteristics. It is unimportant to the working of the invention whether the pattern is scribed into the layer before or after it is converted into a superconducting layer since the invention takes advantage of the fact that the material forming the layer will undergo sublimation when irradiated by a suitable source and proposes to provide hitherto unobtainable high Tc superconducting structures. In accordance with one embodiment of the present invention which is described hereinafter, an oxide (or carbonate) mixture for constituting a superconducting ceramic is first deposited in the form of a thin film onto a substrate by sputtering, printing such as screen press printing or other methods. When deposited by sputtering, the resultant film adopts an amorphous or other structure which gives rise to abundant lattice defects and/or imperfections so that the resistivity of the ceramic material is usually rather high. The deposited ceramic film can be transformed into a desired superconducting ceramic by thermal oxidation or firing. The ceramic film can be processed by laser irradiation before or after firing and can be arranged such that for example a strip of the ceramic material is removed from or remains on the substrate. Any suitable laser can be used to irradiate the layer in order to scribe the desired pattern into the layer and may for example be a YAG laser emitting a circular beam of light with a wavelength of about 1.06 microns pulsed at between 5 and 100 KHz, an excimer laser (KrF, KrCl and so forth), a C0₂ laser or an N₂ laser. When an excimer laser is used, a shaped beam having a circular cross-section with a diameter from 10 to 100 microns is emitted and a beam with a linear cross section typically from 10 to 40 cm wide and from 5 to 100 microns long can be obtained therefrom by expanding and squeezing the circular beam with an optical system. Further features of the invention are set forth with particularity in the appended claims and will become apparent to those possessed with the relevant skills from consideration of the following description, given with reference to the accompanying drawings, of an exemplary embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a perspective view illustrating a first exemplary manufacturing process in accordance with the present invention; and Fig.2 is a perspective view illustrating a second exemplary embodiment of the manufacturing process in accordance with the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS 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 Table 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 0 are the same 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. Referring to Fig.1, a manufacturing process in accordance with the present invention will be described. In Fig.1(A), a substrate 1 may be made of a ceramic material such as alumina, silicon oxide, silicon nitride, aluminium nitride, strontium titanate, zirconia, yttria or YSZ(yttria stabilized zircon), or a metal such as copper or a copper alloy or glass. YSZ, yttria or zirconia are particularly suitable having regard to the desirability of matching the coefficient of thermal expansion to that of the layer formed thereon as hereinafter described. In any case the material should be selected so that the differential coefficient of thermal expansion between the underlying substrate and the overlying ceramic thin film should be limited preferably within 50% of the thermal expansion coefficient of the ceramic thin film, since if the thermal expansion coefficients of the substrate and the ceramic thin film are substantially different, then the strain developed between them may hinder the recrystallisation of the ceramic film into a superconducting structure during thermal annealing. A ceramic film 2 conforming to YBa₂C₃0₆₋₈ of 0.1 to 50 micron thickness, e.g. 20 microns, is formed on the substrate 1 by sputtering or by a printing method such as screen press printing. Having formed the ceramic film 2 on the substrate 1, the next step is to fire the same at a temperature from 500 to 1000°C e.g. 900°C for 15 hours whereby the ceramic film 2 is transformed into a superconducting structure. Once the structure has thus been prepared a laser beam 1.06 microns in wavelength derived by means of a YAG laser may be used to irradiate parts of the superconducting film 2 in order to produce a pattern thereon. In figure 1, the surface of the film has been scanned from the left edge towards the middle of the structure so as to remove a portion of the film by sublimation and thereby leave a groove 3. To ensure that the portion is removed fully by sublimation the output power of the laser should be from 10⁶ to 10⁷ W/sec; stronger irradiation may damage the underlying substrate 1. Alternatively, the laser patterning can be effected before the structure is fired to transform the ceramic film into a superconducting layer in which oxygen and copper atoms form the structure of the layer. Each circular pulse emitted from the laser is directed towards a predefined irradiation region on the film which region is common to 60 to 80% of the irradiation region of the preceding pulse (and also of the subsequent pulse). The scanning speed is typically 2m/min. The frequency can be from 5 to 30 KHz, e.g. 10 KHz. Instead of a YAG laser, an excimer laser which is capable of emitting a series of intense laser pulses with a rectangular cross section of for example 20 x 30 mm² can be used in which case, the emitted pulse can be shaped into a linear cross-section 10 cm wide and 50 microns thick. The effect of this irradiation is to selectively remove the irradiated portions of the ceramic film by sublimation. The entire structure can be heated during this laser scribing process to a temperature from 300 to 800°C, e.g. 600°C by a halogen lamp in order to enhance sublimation. Fig.2 shows a second embodiment of the invention. In this embodiment, substantially the same procedure is repeated as for the preceding embodiment except for the differences described below. In this embodiment, the substrate 1 is a cylinder and a ceramic film 2 is deposited on the substrate 1 while the substrate 1 is rotating the direction indicated by an arrow 12. After the film has been dried a laser beam, 50 microns in diameter, is radiated continuously or repeatedly towards the substrate. The laser beam is shifted towards the right as seen in the drawing (the axial direction of the cylinder 1) while the substrate 1 is rotated around its axis. By this process, a helical groove 3 is scribed into the layer 2 thereby forming a continuous helical superconductor 5 on the cylinder 1. It will be appreciated that each part of the helix 5-1 is isolated from adjacent parts 5-2 by the helical groove 3 which has been removed by the above described laser sublimation scribing process and thus a superconducting electromagnet in the form of a coil is constructed. When the above described laser scribing process is combined with a laser annealing process a superconductor of superior quality can be obtained. In a laser annealing process, a ceramic film is initially formed on the surface of a substrate by for example sputtering. As mentioned hereinabove the film initially has an amorphous or other non-superconducting structure. By laser annealing, a prescribed portion of the film is melted by irradiation with a laser beam which portion recrystallises into a superconducting atomic structure when cooled. If necessary the whole structure, i.e. the substrate and the ceramic film, may be cooled at the same time. After the prescribed portion has been converted to a superconducting portion, another portion of the ceramic film can be removed by the above described laser sublimation method. In this way, a ceramic pattern composed of superconducting regions and non-superconducting (insulating) regions can be easily obtained since the effect of laser irradiation on the ceramic film, i.e. whether the film is annealed or sublimated, is dependent only on the power or intensity of the laser output. Superconducting ceramics in accordance with the present invention also may be prepared in accordance with the stoichiometric formulae (A 1-x B x ) y Cu z 0 w , where A is one or more elements of Group IIIa of the Japanese Periodic Table, e.g., the rare earth elements, B is one or more elements of Group IIa of the Japanese Periodic Table, e.g. the alkaline earth metals including beryllium and magnesium, and where x = 0 to 1; y = 2.0 to 4.0, with y preferably having a value from 2.5 to 3.5; z = 1.0 to 4.0, with y preferably having a value from 1.5 to 3.5; and w= 4.0 to 10.0, with w preferably having a value from 6.0 to 8.0. One example is YBa₂Cu₃0₆₋₈. In addition, superconducting ceramics in accordance with the present invention may be prepared in accordance with the stoichiometric formulae (A₁₋ x B x ) y Cu z 0 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 where x = 0.3 to 1; y =2.0 to 4.0, with y preferably having a value from 2.5 to 3.5; 1.0<z<4.0, with z preferably having a value from 1.5 to 3.5; and w=4.0 to 10.0, with w preferably having a value from 6.0 to 8.0. Examples of this general formula are BiSrCaCuCu₂0 x and Bi₄Sr₃Ca₃Cu₄0 x . Tc onset and Tco samples confirmed consistent with the formula Bi₄Sr y Ca₃Cu₄0 x (y is around 1.5) were measured to be 40-60°K, which is not terribly high, but relatively high critical temperatures were obtained with samples conforming to the stoichiometric formulae Bi₄Sr₄Ca₂Cu₄0 x and Bi₂Sr₃Ca₂Cu₂0 x . The number designating the oxygen proportion is in the range 6 to 10, e.g. around 8.1. While the above description has been made with reference to several specific embodiments, it is to be understood that the present invention is limited only by the appended claims and is not limited to the particular examples described.

Claims: A method of manufacturing a ceramic material superconductor comprising: forming a superconducting material ceramic layer on a surface made of a material which has a thermal expansion coefficient of 50% to 150% of that of said ceramic layer, and irradiating a portion of the ceramic layer with a laser beam so as to cause the irradiated portion of the ceramic layer to be removed from the surface. The method of claim 1 wherein said superconducting ceramic layer forming step comprises depositing said surface by sputtering. The method of any preceding claim wherein said substrate is made of strontium titanate, zirconia, yttria or YSZ (yttria stabilized zircon). The method of any preceding claim wherein said laser beam is applied to the said layer in a sequence of pulses. The method of claim 4 wherein said pulses are radiated on to a portion of said ceramic layer so as to trace a prescribed line thereon. The method of any preceding claim wherein said laser beam is radiated onto said layer so as to form a superconducting coil. The method of claim 6 wherein said surface comprises the periphery of a cylinder which is rotated about its axis during said irradiating step and the location of the said laser beam on the said layer is shifted in the axial direction with respect to said cylinder whereby the laser beam traces a helical path on said layer. The method of any preceding claim, further comprising the step of annealing the layer. The method of any preceding claim in which the ceramic layer is initially treated to convert it into a superconducting ceramic and then patterned by said step of irradiating to remove some of said layer. The method of any of claims 1 to 8 in which the ceramic layer is initially patterned by said step of irradiating and then treated to convert it into a superconducting ceramic. A superconducting component comprising a coil of superconducting ceramic material provided as a deposition layer upon a cylindrical substrate of a material having a thermal coefficient of expansion between 50-150% that of the ceramic material, the coil forming a helical superconducting coil on the lateral area of said cylindrical substrate, each turn of the coil being isolated from adjacent turns of the coil by a helical groove.

Inventors: YAMAZAKI SHUNPEI, YAMAZAKI, SHUNPEI
Applicants: SEMICONDUCTOR ENERGY LAB, SEMICONDUCTOR ENERGY LABORATORY CO., LTD.
Application year: 1988
Main classification: H01L39
IPC codes: C04B41, H01L39, H01F41, H01F6, C04B35, C01G1, H01B13, H01B12