Tape-formed oxide superconductor

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a tape-formed oxide superconductor and, more particularly, it relates to a tape-formed oxide superconductor used for superconducting magnets, superconducting cables and electric power machines and instruments.

[0003] 2. Description of the Related Art

[0004] Since critical temperature of oxide superconductors exceeds the temperature of liquid nitrogen, their applications to superconducting magnets, superconducting cables and electric power machines and instruments are expected and various studies have been carried out vigorously.

[0005] In order to apply superconductors to the above fields, it is necessary to fabricate a wire having a high critical current density (Jc) and a long size. In order to obtain a long tape, it is necessary to form an oxide superconductor on a metal tape from a viewpoint of mechanical strength and flexibility. Since oxide superconductors change their superconducting characteristics by their crystalline alignment, it is necessary to improve an in-plane alignment and it is therefore necessary to form an oxide superconductor on a tape-formed in-plane aligned substrate. In order to improve the critical current density in that case, it is necessary that a c-axis of the oxide superconductor is aligned vertically to the substrate plane and that its a-axis (or b-axis) is subjected to an in-plane aligned almost parallel to the direction of current (hereinafter, referred to as “c-axis alignment” and “a-axis in-plane alignment”, respectively) so as to keep the quantum connectivity of the good superconducting state.

[0006] When an oxide superconducting layer on a tape-formed substrate by means of sputtering, pulse laser vapor deposition (PLD), vapor deposition or chemical vapor deposition using metalorganic salts (MOCVD), the crystal alignment of the substrate is usually in random polycrystals and, therefore, the oxide superconductor formed on this substrate is also affected by the substrate and is unable to have a high alignment.

[0007] Because of such a reason, there has been investigated a method of using biaxially textured Ni (100) substrates as the tape-formed substrates.

[0008] In this method, a nickel substrate subjected to a cold rolling is heated in vacuo to make it highly aligned and that is called RABiTS (trade mark; Rolling-Assisted Biaxially Textured-Substrates). There has been reported a method where cerium is sedimented on the biaxially textured nickel substrate in an atmosphere of inert gas of high temperature by means of an electron beam evaporation, hydrogen is made present during this sedimentation whereby a thin film of an epitaxial layer of CeO2 is formed, then a thick film of YSZ (yttria-stabilized zirconia) is formed thereon at high temperature in vacuo by sputtering and the resulting one is used as a substrate.

[0009] In that method, a layer of YBCO (superconductor of an Y—Ba—Cu—O type; hereinafter, the same abbreviation will be used) is formed by a pulse laser vapor deposition on the above substrate (refer, for example, to John Emathis, et al., Jpn. J. Appl. Phys., Vol. 37 (1988), pages L1379-1382).

[0010] In that case, the CeO2 layer on the biaxially textured Ni substrate is aligned so as to depress the reaction of the Ni substrate with YSZ and to prevent the oxidation of the Ni substrate to form an island-formed NiO on nickel while the YSZ layer is aligned so as to depress the reaction with the superconducting layer due to the function as a buffer layer for preventing the diffusion of Ni whereby the reduction in the superconducting characteristic is prevented and the matching with the superconducting layer is maintained. Thus, when the biaxially textured Ni substrate and the YSZ are directly deposited, Ni in the substrate reacts with Zr in the YSZ at their interface whereby no epitaxial growth takes place while, when YSZ is directly aligned on the Ni substrate, the oxidation of the Ni substrate occurs whereby no epitaxial growth takes place. Accordingly, the YSZ is layered via the CeO2 layer which does not react with the Ni substrate placed on the substrate in the former case while, in the latter case, it is layered via CeO2 which is able to grow under a reductive atmosphere whereby the diffusion of the element constituting the substrate into the superconducting layer is prevented. Since CeO2 is easily broken, a thick film of YSZ is formed on the thin film of CeO2.

[0011] In the above method, YBCO is formed on YSZ having a good matching with YBCO but, since CeO2 has a better crystallographic matching with YBCO than YSZ and further since CeO2 is better in terms of reactivity with an MOD solution too, there has been reported a method where a thin film of CeO2 is further formed on the YSZ and a YBCO layer is formed thereon by an MOD (metal organic deposition) to give a five-layered structure of biaxially textured Ni substrate/CeO2/YSZ/CeO2/YBCO (A. P. Malozemoff, et al., Eucas Conference, Sep. 14-17, 1999).

[0012] The metal organic deposition is a method where a metalorganic salt is applied and then thermally decomposed; thus, a solution in which an organic compound having metal component is uniformly dissolved is applied on a substrate and heated to conduct a thermal decomposition whereupon a thin film is formed on the substrate. It does not require a vacuum process and is able to achieve a high deposition rate at a low cost whereby it is suitable for the manufacture of long tape-formed oxide superconducting wires.

[0013] Since the MOD uses a metalorganic salt as a starting material, it is also applicable to an RE (123) superconductor, i.e. an RE1+XBa2−XCu3OY superconductor (here, RE means Y, Nd, Sm, Gd, Eu, Yb or Ho; this will be used in the same meaning hereinafter as well) and to an intermediate layer such as CeO2. When organic salts are thermally decomposed, alkaline earth metal (such as Ba) carbonate is usually produced and, since a high-temperature thermal treatment of 800° C. or higher is necessary for the formation of an oxide superconductor by a solid state reaction via the carbonate, there has been intensively carried out a method in recent years where an organic salt containing F (such as a TFA salt [trifluoroacetate]) is used as a starting material for subjecting to a thermal treatment in a steam atmosphere and to a control of steam partial pressure whereupon an RE (123) superconductor is formed.

[0014] In that method where the TFA salt is used as a starting material, no nucleation is resulted in a precursor and the RE (123) superconductor can be epitaxially grown from the substrate by the reaction of the steam with an amorphous precursor containing fluorine.

[0015]
FIG. 8 shows a tape-formed oxide superconductor 10 in a five-layered structure where CeO2, YSZ, CeO2 and RE (123) superconducting layer formed by an MOD method are sequentially formed on the above-mentioned biaxially textured Ni substrate. In the drawing, there is shown a structure where a first intermediate layer 12 comprising CeO2, a second intermediate layer 13 comprising YSZ and a third intermediate layer 14 comprising CeO2 are formed on the biaxially textured Ni substrate 11 and, on the third intermediate layer 14, an RE (123) superconducting layer 15 formed by an MOD method using a TFA salt is deposited.

[0016] In the above-mentioned tape-formed oxide superconductor 10 in a five-layered structure, the CeO2 first intermediate layer, the YSZ second intermediate layer, the CeO2 third intermediate layer and the superconducting layer are epitaxially grown on a biaxially textured Ni substrate and, in addition, the reaction among the elements constituting the biaxially textured Ni substrate and the superconducting layer is depressed whereby it is possible to prevent the lowering of the superconducting characteristic and, in principle, that is an excellent method for the manufacture of a tape-formed superconductor.

[0017] However, with regard to the biaxially textured Ni substrate used in this method, it is necessary that a cold rolled Ni substrate is subjected to a thermal treatment in vacuo for giving a high alignment and there is a disadvantage that, during the recrystallization by such a thermal treatment, grain growth takes place making the size as big as 100 μm or more and an influence thereby affects the superconducting layer whereupon, actually, the expected Jc value is not achieved.

[0018] Thus, in the (surface) grain boundary constituting the biaxially textured Ni substrate, the disorder in the crystal structure occurs and, therefore, the intermediate layers and the superconducting layer formed on it are also affected by that and such an influence becomes bigger especially when the growth directions of the adjacent crystal grains are greatly different.

[0019] Now, a consideration will be made for the effect of the grain size on Jc of a long tape when the variation among the growth directions is constant. Jc of a tape material is determined at the area where Jc is lowest and, even if the variation in the growth directions is same, the result when the grain size is big is that numbers of grains in the vertical direction to the current become small and the influence of the reduction in Jc value of grain boundary having many growth directions on the reduction in the Jc value as a whole becomes big whereby the Jc value is apt to become small. On the other hand, when the grain size is small, numbers of the grains in a vertical direction become big and the influence of grain boundary having low Jc value having many growth directions on the reduction in Jc as a whole is nearly the same as that of the variation and that does not cause a big reduction. In our study, there was also obtained a result where the more the crystal size, the lower the Jc value.

[0020] In addition, since the biaxially textured Ni substrate is ferromagnetic and, further, its mechanical strength is small, when superconducting magnet or superconducting cable is constituted using an oxide superconductor made into tape form, there is a problem that the influence of external field becomes big resulting in a reduction in the Jc value.

SUMMARY OF THE INVENTION

[0021] The present invention has been achieved for solving the above-mentioned problem and is to provide a tape-formed oxide superconductor having an excellent superconducting characteristic and being suitable for use as superconducting magnet and superconducting cable by the use of a metal substrate having an improved quantum connectivity of the superconducting layer, i.e. improved Jc value due to high c-axis alignment and a-axis in-plane alignment, being non-magnetic or weakly magnetic and having a big mechanical strength.

[0022] In order to achieve the above object, a tape-formed oxide superconductor which is the first feature of the present invention is characterized in that,

[0023] on a tape-formed metal substrate of non-magnetism or weak magnetism and high strength, there are sequentially deposited

[0024] an intermediate layer having a high alignment, depressing the diffusion of elements constituting the metal substrate into the superconducting layer and depressing the reaction with the elements constituting the superconducting layer formed by deposition of particles generated from the target onto a metal substrate together with irradiation of ion from an inclined direction to the metal substrate and

[0025] an oxide superconducting layer formed by coating of metalorganic salts followed by subjecting to a thermal decomposition.

[0026] An object can also be achieved by the tape-formed oxide superconductor which is the second feature of the present invention where the above intermediate layer is made in a two-layered structure. Thus, it is a tape-formed oxide superconductor where,

[0027] on tape-formed metal substrate of non-magnetism or weak magnetism and high strength, there are sequentially deposited

[0028] a first intermediate layer having a high alignment, depressing the diffusion of elements constituting the metal substrate into the superconducting layer and depressing the reaction with the elements constituting the superconducting layer formed by deposition of particles generated from the target onto a metal substrate together with irradiation of ion from an inclined direction to the metal substrate,

[0029] a second intermediate layer having a good matching to the oxide superconductor and

[0030] an oxide superconducting layer formed by coating of metalorganic salts followed by subjecting to a thermal decomposition.

[0031] According to the present invention, an intermediate layer is formed by deposition of the particles generated form a target onto a substrate together with irradiation of ion to the metal substrate and, therefore, it is possible to directly form a YSZ, etc. having a good alignment on the substrate, the above-mentioned formation of NiO on the substrate is prevented and it is no longer necessary to coat the YSZ via a CeO2 layer which does not react with the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]
FIG. 1 is a cross sectional view of the tape-formed oxide superconductor according to the present invention;

[0033]
FIG. 2 is a graph showing an example and comparative examples of influence of the thermal treatment temperature on critical temperature (Tc) of the YBCO (123) layer of the tape-formed oxide superconductor according to the present invention;

[0034]
FIG. 3 is a graph showing an example of measuring result of the full width of half maximum by X-ray diffraction of YSZ, CeO2 and YBCO of the tape-formed oxide superconductor according to the present invention;

[0035]
FIG. 4 is a graph showing an example and Comparative Examples 1 and 2 of dependency of Jc value of the tape-formed oxide superconductor according to the present invention on external magnetic field;

[0036]
FIG. 5 is a graph showing the changes in the Jc value in the longitudinal direction of the tape-formed oxide superconductor according to the present invention;

[0037]
FIG. 6 is a graph showing the changes in the Jc value in the longitudinal direction of the tape-formed oxide superconductor in Comparative Example 1;

[0038]
FIG. 7 is a graph showing the changes in the Jc value in the longitudinal direction of the tape-formed oxide superconductor in Comparative Example 2; and

[0039]
FIG. 8 is a cross-sectional view of the conventional oxide superconductor using a biaxially textured Ni substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] As to the substrate used in the present invention, a tape-formed metal substrate which is of non-magnetism or weak magnetism and a high strength is used. The substrate is usually non-textured and includes polycrystals having small crystal grains of around 10 nm such as rolled plate, e.g. Hastelloy which is known as a heat- and acid-stable alloy containing Mo, W, Cr, Fe, etc. as alloy elements besides Ni.

[0041] The intermediate layer on the substrate, i.e. the intermediate layer in the first feature of the present invention or the first intermediate layer in the second feature of the present invention, can be formed by deposition of the particles generated from a target onto the substrate together with irradiation of ion in an inclined direction to the substrate.

[0042] That method has been known as an IBAD method (ion beam assisted deposition).

[0043] When an oxide superconducting layer is formed on a substrate comprising polycrystals, it is usual that a superconducting layer having a good alignment is not obtained and there is also a problem that the substrate reacts with the elements which constitute the superconductor whereupon the crystal structure of the superconductor is disintegrated and the superconducting characteristic is deteriorated. In order to avoid such a problem, it is necessary to form an intermediate layer between the substrate and the oxide superconducting layer.

[0044] As to such an intermediate layer, there has been carried out that YSZ, MgO or SrTiO3 made into film by a common gas state reaction is used but, although the c-axis is aligned in a right angle to the substrate plane in the intermediate layer, an a-axis (or b-axis) does not take place an in-plane alignment in nearly the same direction in the substrate plane and, therefore, an a-axis (or b-axis) of the oxide superconducting layer formed thereupon also does not take place an in-plane alignment in nearly the same direction and there is no improvement in Jc.

[0045] The IBAD method solves such a problem and, when the constituting particles sputtered from the target by means of sputtering are deposited on the substrate, the deposition is carried out by irradiation of both argon ion and oxygen ion generated by an ion gun from an inclined direction (e.g., 45°). According to that method, it is possible to give an intermediate layer having high c-axis alignment and a-axis in-plane alignment to the film surface on the substrate (Japanese Patent Unexamined Publication Nos. 4-329867 and 4-331795) but, since the oxide superconducting layer coated on the intermediate layer is formed by a pulse laser vapor deposition, there is a problem that cost is high whereby it is not suitable for the manufacture of a long wire.

[0046] As to the intermediate layer in the present invention, that which comprises one or more selected from CeO2, Y2O3, YSZ and Zr2Rx2O7 (where Rx is Y, Nd, Sm, Gd, Eu, Yb, Ho, Tm, Dy, Ce, La or Er; this has the same meaning hereinafter as well) is appropriate.

[0047] When the intermediate layer is made into a two-layered structure, it is preferred that YSZ or Zr2Rx2O7 which is efficiently aligned by the IBAD method is used as the first intermediate layer while, as the second intermediate layer, CeO2 or RE2O3 which has a good matching with YBCO is used.

[0048] In order to form the YBCO on the intermediate layer or on the second intermediate layer with an alignment (i.e., having high c-axis alignment and a-axis in-plane alignment), there is adopted an MOD method where a metalorganic salt which is capable of a quick filming is applied followed by subjecting to a thermal decomposition without the necessity of a vacuum process.

[0049] As to the metalorganic salt, it is preferred to use a material comprising trifluoroacetate, octylate, naphthenate or acetate while, as to the YBCO, RE1+XBa2−XCu3OY is appropriate.

[0050] The oxide superconductor of the present invention can be formed by coating a metalorganic salt followed by subjecting to a thermal decomposition and it is also possible to make the oxide superconducting layer in multi-layers. In that case, a step of coating a mixed solution of metalorganic salt containing a predetermined molar ratio of metal elements constituting the oxide superconductor followed by calcining to form an oxide superconductor precursor is repeated so that plural layers of the oxide superconductor precursor are formed and then subjected to a thermal treatment for crystallization.

[0051] In the formation of the above superconductor in a multi-layered structure, it is preferred that the calcination is carried out at the temperature of lower than 400° C. except at least the oxide superconductor precursor of the outermost layer. This means that, in the formation of an oxide superconductor by crystallization of the calcined film in a three-layered structure for example, the calcination of the first and the second layers is carried out at the temperature of lower than 400° C. and there is no problem if that of the third layer is carried out at lower or higher than 400° C.

[0052] In the above case, it is more preferred that the calcination temperature is within a range of 250-350° C. except at least the oxide superconductor precursor of the outermost layer.

[0053] Further, it is preferred that, during the crystallizing thermal treatment, partial pressure of vapor in the introducing gas in the crystallizing thermal treatment atmosphere before arriving the crystallization of the oxide superconductor precursor at least at the outermost layer is made 4.0% by volume or less. It means for example that, when a calcined film in a three-layered structure is crystallized to form an oxide superconductor, the partial pressure of steam in the introducing gas during the crystallizing thermal treatment for the first and the second layers until the crystallization of the third layer is initiated is made 4.0% by volume or less.

[0054] In the above case, it is more preferred that the partial pressure of steam in the introducing gas in the crystallizing thermal treatment atmosphere or the partial pressure of steam in the introducing gas in the crystallizing thermal treatment atmosphere before arriving the crystallization of the oxide superconductor precursor of at least the outermost layer is made within a range of 0.5-3.0% by volume. With regard to the introducing gas in the crystallizing thermal treatment atmosphere, it is preferred to use a mixed gas comprising steam, oxygen and gas which does not react with the oxide superconductor. The partial pressure of steam in the introducing gas in the crystallizing thermal treatment atmosphere may be raised either with a certain gradient or by a step-by-step manner provided that the above-mentioned condition is satisfied.

[0055] After completion of the crystallizing thermal treatment, it is preferred to introduce dry gas for depressing the deterioration of characteristics by the reaction of the steam with the superconductor.

[0056] In the case of formation of a long tape-formed oxide superconductor, there is adopted a method where delivering device and winding device for metal substrate tape of non-magnetism or weak magnetism and high strength are arranged in the inner area of a vacuum container and the substrate tape is continuously supplied from the delivering device to form an intermediate layer on a heating holder by means of the above IBAD method followed by being wound by the winding device.

[0057]
FIG. 1 shows a cross-sectional structure of the tape-formed oxide superconductor 1 of the present invention where the intermediate is in a two-layered structure.

[0058] In the drawing, a Hastelloy substrate 2 comprising polycrystals of non-magnetism or weak magnetism and high strength is used as the metal substrate tape and YSZ particles generated from a target is deposited on the substrate 2 together with irradiation of ion from an inclined direction to the substrate 2 to form the first intermediate layer 3. The first intermediate layer 3 has high c-axis and a-axis in-plane alignments to the substrate 2.

[0059] After that, the second intermediate layer 4 comprising CeO2 having a good matching to RE (123) on the first intermediate layer 3 comprising YSZ and then an metalorganic salt containing F is coated followed by subjecting to a thermal decomposition on the second intermediate layer 4 to form a filmed RE (123) layer 5. Both those second intermediate layer 4 and the RE (123) layer 5 also have high c-axis and a-axis in-plane alignment according to the crystalline alignment of each of the layers beneath them.

EXAMPLES

Example 1

[0060] On a Hastelloy tape having an average crystal particle size of about 10 nm and having 10 m length, 10 mm width and 0.1 mm thickness was coated a first intermediate of YSZ of 1 μmm thickness at room temperature using an IBAD method and then a second intermediate layer of CeO2 was formed thereon in a thickness of 0.5 μm by a sputtering method to prepare a compounded substrate.

[0061] After that, on the second intermediate layer was coated a YBCO layer having a thickness of 0.15 μm using a solution of salt of trifluoroacetic acid (TFA) as a material according to the following procedure.

[0062] First, M-TFA salts (M=Y, Ba, Cu) were mixed with a methanolic solution to adjust the solution concentration to 0.25 mol/liter. The mixed solution was coated on the compounded substrate by a roll coat method. After that, it was heated slowly up to 400° C. in an atmosphere containing steam to give an M—(O,F) precursor.

[0063] Then it was sintered at 725-800° C. for 1 hour in a low-oxygen atmosphere (10−3 atm) containing steam and subjected to an oxygen annealing treatment at 450° C. for 1 hour.

[0064] Tc value, Jc value and in-plane alignment were measured for the tape-formed oxide superconductor prepared as such.

[0065] Result of the measurement is shown in FIGS. 2-7.

[0066]
FIG. 2 shows an influence of the thermal treatment temperature on the critical temperature (Tc) of the YBCO (123) layer and, at the same time, that is shown as compared with the case where CeO2 layer was coated on a YSZ single crystal or an LaAlO3 single crystal.

[0067] The result was that, when the sintering time was 1 hour, unreacted BaF2 remained at 725° C. while, at 800° C. or higher, formation of BaCeO3 was noted. In the YBCO film formed at 750° C. and at 775° C., no other phase was noted by X-ray diffraction but the membrane was confirmed to have c-axis and a-axis alignment (in-plane alignment).

[0068]
FIG. 3 shows the result of measurement of the full width of half maximum (FWHM) and the X-ray diffraction was carried out in such a manner that X-ray was applied upon the (103) surface of the YBCO (123) layer at an angle of incidence of θ, a counter is placed at the position in an angle of 2θ to this incoming X-ray and intensity of diffraction to a horizontal angle β to a vertical plane including the incoming and diffracted X-ray was measured.

[0069] The result was that the full width of half maximum of YSZ is 8.80 while those of CeO2and YBCO film were 5.6° and 4.7°, respectively showing an improvement in the alignment whereby it is apparent that the above tape-formed oxide superconductor has high c-axis alignment and a-axis in-plane alignment.

[0070]
FIG. 4 shows a dependency of Jc value of the sample cut into the length of 10 mm on the external magnetic field and the Jc value was measured by a DC four-probe method at the voltage standard of 1 μV/cm.

[0071] It is apparent from the result that the above tape-formed oxide superconductor has a good characteristic as compared with the case of Comparative Example 1 which will be mentioned later where the first intermediate layer of YSZ is coated by an IBAD method on the Hastelloy tape and then a YBCO layer is coated by a laser vapor deposition method on the second intermediate layer of Y2O3. This is presumably because of an improvement in the grain alignment since an MOD method is a process similar to thermal equilibrium as compared with a PLD method. It is also apparent that, as compared with the case where a YBCO layer is formed by an MOD method on a compounded substrate prepared by coating CeO2 and YSZ on a biaxially textured Ni substrate which will be mentioned in the Comparative Example 2, the product has such high Jc values to the external magnetic field that similar 77.3 K at 10 mm length, Jc>1 MA/cm2 at 0 T and Jc>105 A/cm2 at 2 T. FIG. 5 shows a Jc distribution of a 10-meter tape in the longitudinal direction. It is apparent from the result that a uniform Jc distribution is achieved at 1 MA/cm2 or more.

Comparative Example 1

[0072] A first intermediate layer of YSZ was formed in a thickness of 1 μm by an IBAD method on a Hastelloy tape having an average crystal particle size of about 10 nm and having 10 m length, 10 mm width and 0.1 mm thickness and then a second intermediate layer of Y2O3 was formed in a thickness of 0.5 μm thereon by a laser vapor deposition method whereupon a compounded substrate was prepared.

[0073] After that, a YBCO layer having a thickness of 0.15 μm was formed on the second intermediate layer by a laser vapor deposition method.

[0074] Dependency of the Jc values on the external magnetic field and the in-plane alignment of the tape-formed oxide superconductor prepared as such were measured.

[0075] Result of the measurement is shown in FIGS. 4 and 6. The Jc values were measured by the same method as in the Example.

[0076] It is apparent from the result that, although the variation in the Jc values is small in the case of 10 mm, the Jc values were better in the Example.

Comparative Example 2

[0077] An epitaxial layer thin film of CeO2 was formed in a thickness of 0.5 μm on a biaxially textured Ni substrate (RABiTS) having an average crystal particle size of about 50 μm or more and having 10 mm length, 10 mm width and 0.1 mm thickness and then a thick film of YSZ in a thickness of 1 μm was formed thereon at high temperature and in vacuo by a sputtering method whereupon a compounded substrate was prepared.

[0078] A YBCO layer having a thickness of 0.15 μm was formed on the compounded substrate by the same manner as in the Example, i.e. a TFA salt solution was used by an MOD method.

[0079] Results of the Jc values of the tape-formed oxide superconductor prepared as such are shown in FIGS. 4 and 7.

[0080] It is apparent from the result that, in Comparative Example 2, although the similar Jc values as in the Example are obtained in the case of a short sample, the Jc values in the case of 10 m length show low Jc value sites in several places and, since the total Jc value is decided by them, the Jc value is small as compared with the Example.

Example 2

[0081] An IBAD compounded substrate comprising Hastelloy/YSZ/CeO2 was used as a substrate. This compounded substrate was prepared by such a manner that a first intermediate layer of YSZ was coated in a thickness of 1 μm at room temperature by an IBAD method on a Hastelloy tape having an average crystal size of about 10 nm and having 10 mm length, 10 mm width and 0.1 mm thickness and then a second intermediate layer of CeO2 in a thickness of 0.5 was formed by a sputtering method thereon.

[0082] On the substrate, TFA metal salts were dissolved in methanol where the molar number ratio of Y:Ba:Cu was made 1:2:3 and the solution concentration was adjusted to 0.25 mol/liter on the basis of Y to prepare a coating solution.

[0083] The coating solution was coated on each substrate by a spin coating method to form a coated film. The coated film on the substrate prepared as such was heated at a slow rate up to 300° C. in an oxygen atmosphere containing steam and cooled down in the furnace and the coating solution was coated again on the substrate by the same method as above to form a coated film, heated at a slow rate up to 400° C. in an oxygen atmosphere containing steam and cooled down in the furnace to prepare a Y—Ba—Cu precursor in a two-layered structure.

[0084] In a crystallizing thermal treatment after the calcination treatment, the above precursor film was heated at a rate of 25° C./minute in a low-oxygen atmosphere of 10−3 atm containing steam, subjected to a crystallizing thermal treatment where the substrate temperature was kept at 750° C., kept for 10 minutes where the atmosphere in the furnace was switched to dry gas and cooled down in the furnace.

[0085] Silver was vapor-deposited on the film prepared as above to form an electrode followed by subjecting to a thermal treatment in an oxygen atmosphere at 450° C. for 1 hour to form a superconducting film.

[0086] As a result of a X-ray diffraction, the main ingredient of the superconducting film was confirmed to be YBCO.

[0087] With regard to the above superconducting film, its Tc value was measured by a DC four-probe method. Further, c-axis and a-axis alignments were measured by means of a YBCO (102) pole figure. Result of measurement of Tc values and c-axis and a-axis alignments is shown in Table 1.

1TABLE 1Example 2Comp. Ex. 3SubstrateIBAD Compounded SubstrateCoating Numbers (times)22Film Thickness (μm)0.40.4Calcination Temp. (first run; ° C.)300400Calcination Temp. (second run; ° C.)400400c-Axis Alignment (%)7440a-Axis Alignment (%)2660Tc (K)9284

Comparative Example 3

[0088] An IBAD compounded substrate was used and a material solution was coated on the substrate by the same method as in Example 2 to form a coated film. The coated film on the substrate prepared as such was heated at a slow rate up to 400° C. in an oxygen atmosphere containing steam and then cooled down in the furnace to give a Y—Ba—Cu precursor. Then a material solution was coated on this calcined film by the same method as above to form a coated film, heated at a slow rate up to 400° C. in an oxygen atmosphere containing steam and cooled down in the furnace to give a Y—Ba—Cu precursor. The Y—Ba—Cu precursor in a two-layered structure prepared as such was then made into a superconductor by the same method as in Example 2.

[0089] As a result of X-ray diffraction, YBCO was confirmed to be a main ingredient in all of the superconducting films.

[0090] Tc values and c-axis and a-axis alignments were measured for the above superconducting films by the same manner as in Example 2. The result is shown in Table 1.

[0091] As will be apparent from the above results for Example 2 and Comparative Example 3, it was noted that, when a comparison was made between the cases where the calcination treatment was carried out at 300° C. and at 400° C. in which the calcined films were formed in two layers, Tc value increased and c-axis alignment was significantly improved when the calcination treatment for the first layer was carried out at 300° C.

[0092] The reason therefor is believed to be the fact that an epitaxial growth from the substrate was inhibited by the production of a impurity layer at the interface between the first and the second layers during the crystallizing thermal treatment. The main cause for the production of the impurity layer is believed to be due to the fact that, when the calcination treatment temperature was high, crystallization in the amorphous precursor film proceeded and, during the crystallizing thermal treatment, the calcined film and crystal grains thereof in the first and the second layers reacted predominantly whereupon the impurity was apt to be produced.

Example 3

[0093] Calcination was carried out at 300° C. by the same manner as in Example 2 except that partial pressure of steam (1.05% by volume) in the introducing gas in the crystallizing thermal treatment atmosphere and the time for the crystallizing thermal treatment were changed, then further calcination was carried out at 400° C. to form calcined films in two layers on the substrate and then a crystallizing thermal treatment was applied to form a superconducting film. As a result of X-ray analysis, YBCO was confirmed to be a main ingredient in the superconducting films.

[0094] Jc value and Ic value (voltage standard: 1 μV/cm) for the above superconducting film were measured by a DC four-probe method. Further, the peak integral intensity and the full width of half maximum corresponding to YBCO (005) plane were determined. The result is shown in Table 2.

2TABLE 2Example 3Comp. Ex. 4SubstrateIBAD Compounded SubstratePartial Pressure of Steam (vol %)1.054.2Time for Thermal Treatment (min.)18060Coating Numbers (times)22Film Thickness (μm)0.40.4Jc (MA/cm2)2.30.7Ic (A)9028Peak Integral Intensity136937507Full Width of Half Maximum (deg.)0.601.10

Comparative Example 4

[0095] Calcination was carried out at 300° C. by the same manner as in Example 3 except that the partial pressure of steam (4.2% by volume) in the introducing gas in the crystallizing thermal treatment atmosphere and the time for the crystallizing thermal treatment were changed, then further calcination was carried out at 400° C. to form calcined films in two layers on the substrate and then a crystallizing thermal treatment was applied to form a superconducting film. As a result of X-ray analysis, YBCO was confirmed to be the main ingredient in the superconducting films.

[0096] Jc value and Ic value, integral intensity and the full width of half maximum of the above superconducting films were measured by the same manner as in Example 3. The result is shown in Table 2.

[0097] It will be apparent from the above results for Example 3 and Comparative Example 4 that there were significant improvements in Jc and Ic values in the case where the films were formed in two layers and crystallizing thermal treatment was carried out by lowering the partial pressure of steam to 1.05% by volume as compared with the case where crystallizing thermal treatment was carried out in which the partial pressure of steam was 4.2% by volume. It was confirmed from the locking curve at that time that, in case a crystallizing thermal treatment is carried out when the partial pressure of steam was 4.2% by volume, the volume of textures c-axis crystals of YBCO lowered.

[0098] The reason therefor is believed to be due to the fact that, when the partial pressure of steam during the crystallizing thermal treatment increases, growing speed of crystals of the YBCO film becomes high whereby crystallinity and the volume of textured c-axis crystals lower.

[0099] As will be apparent from the result of measurement for the above Examples and Comparative Examples, the tape-formed oxide superconductor of the present invention is able to form an intermediate layer and an oxide superconducting layer on the intermediate layer whereby a high Jc value to the external magnetic field is available.

[0100] As mentioned hereinabove, the tape-formed oxide superconductor according to the present invention has high c-axis and a-axis alignments (in-plane alignment) and is also able to prevent the deterioration of superconducting characteristic in the crystal boundary whereby, due to an excellent quantum connectivity in crystal grain boundary, the superconducting characteristic can be improved.

[0101] Further, since a metal substrate of non-magnetism or weak magnetism and high mechanical strength is used, it is possible to provide a tape-formed oxide superconductor suitable for use as superconducting magnet and superconducting cable. Furthermore, since a superconducting layer is formed by an MOD method which is a non-vacuum process, it is possible to give a thick film by crystallization of plural layers of calcined film. That is suitable for the manufacture of long wires and the manufacturing cost therefor can be significantly reduced.

Number	Date	Country	Kind
2000-333843	Oct 2000	JP
2001-145599	May 2001	JP

Tape-formed oxide superconductor

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