WO2006121299A2

WO2006121299A2 - Organometallic precursor and production of superconducting oxide thin films

Info

Publication number: WO2006121299A2
Application number: PCT/KR2006/001771
Authority: WO
Inventors: Gye-Won Hong; Hee-Gyoun Lee; Byeong-Joo Kim
Original assignee: Korea Polytechnic University
Priority date: 2005-05-12
Filing date: 2006-05-12
Publication date: 2006-11-16
Also published as: KR20060117088A; WO2006121299A3; KR100665587B1

Abstract

Disclosed herein are method for preparing an organometallic precursor solution by dissolving a metal salt and a metal oxide in an aqueous solution of an organic acid (S 1 ), evaporating the solvent (S 2) and dissolving the formed viscous jelly in organic solvent (S 3). The organometallic precursor solution is applied to a substrate (S 4) whose surface is textured (a ceramic single crystal substrate or a metal-ceramic composite substrate in which a textured ceramic thin film is formed on a metal substrate ) to produce an oxide superconductor. According to the method for preparing the organometallic precursor solution, an organic acid is used as a raw material other than trifluoroacetic acid, thereby enabling production of an oxide superconducting thin film with excellent superconductivity at reduced costs without being largely affected by the presence of moisture in air even during storage and processing, such as coating, in air.

Description

METHOD FOR PREPARING ORGANOMET ALLIC PRECURSOR SOLUTION, ORGANOMET ALLIC PRECURSOR

SOLUTION PREPARED BY THE METHOD, AND METHOD

FOR PRODUCING OXIDE SUPERCONDUCTING THIN FILM

BY METALORGANIC DEPOSITION USING THE

ORGANOMETALLIC PRECURSOR SOLUTION

Technical Field

[1] The present invention relates to a method for preparing an organometallic precursor solution, an organometallic precursor solution prepared by the method, and a method for producing an oxide superconducting thin film by metalorganic deposition using the organometallic precursor solution. More specifically, the present invention relates to a method for preparing a precursor solution essential for the production of an oxide superconductor by metalorganic deposition, an organometallic precursor solution prepared by the method, and a method for producing an epitaxial oxide superconducting thin film using the organometallic precursor solution on a substrate whose surface is textured (a ceramic single crystal substrate or a metal-ceramic composite substrate in which a textured ceramic thin film is formed on a metal substrate).

Background Art

[2] Since the discovery of oxide superconductors with superconductivity at 77 K or higher temperatures, numerous methods for processing the oxide superconductors in the form of long rod wires, which are convenient to utilize in various industrial fields, have been developed. Particularly, a number of proposals have been made to produce thin or thick oxide superconducting films that possess superior superconductivity in terms of critical current density ( Jc ) , zero resistance ( Tc ) and texture. Production methods of oxide superconductor films are largely divided into physical and chemical methods.

[3] Physical production methods include reactive evaporation, magnetron sputtering, e- beam deposition, and laser ablation. Although such physical methods lead to the production of high-quality oxide superconducting thin films, they require high vacuum environments so that they necessitate the use of expensive equipment.

[4] In contrast, chemical production methods include metalorganic chemical vapor deposition (MOCVD) and metalorganic deposition (MOD). Such chemical production methods are widely employed in a variety of industrial fields for the production of oxide and ceramic thin films. Particularly, metalorganic deposition (MOD) can be employed to produce high-quality oxide superconducting thin films at atmospheric pressure or under low vacuum, thus leading to low production costs.

[5] According to metalorganic deposition (MOD), a diluted solution of an organometallic compound is applied to a ceramic single crystal substrate or a biaxially textured substrate whose surface is coated with an epitaxially grown ceramic (i.e. a ceramic single crystal substrate or a metal substrate on which a ceramic thin film is epitaxially coated) by dip or spin coating. The coating thus formed is converted into a metallic compound through a single annealing step or multiple annealing steps.

[6] Cima et al. in U.S Patent No. 5,231 ,074 reported the production of a 0.1 micron- thick Y Ba Cu O thin film having a Jc of 10 A/cm at 77 K and zero applied

1 2 3 7-x ^{& rr} magnetic field on LaAlO and SrTiO single crystals by MOD process. Specifically, the Y Ba Cu O thin film is prepared by dissolving a metal salt (e.g., a metal acetate)

1 2 3 7-x in a fluorine-containing organic acid (e.g., trifluoroacetic acid) to prepare a diluted solution of an organometallic compound, applying the dilution to a ceramic single crystal substrate or a biaxially textured substrate on which a biaxially aligned ceramic is epitaxially grown (i.e. a ceramic single crystal substrate or a metal substrate on which a ceramic thin film is epitaxially formed) , followed by annealing at a controlled temperature in a controlled atmosphere to produce a YBCO superconducting thin film. [7] Shi et al. have succeeded in producing a superconducting thin film having a Jc of at least 1 x 10 A/cm at 77K using a diluted solution of an organometallic compound containing no fluorine. Specifically, the dilution is prepared by dissolving yttrium trimethyl acetate (Y-TMA), barium hydroxide and Cu-TMA (copper trimethyl acetate) in a mixed solution of propionic acid and an amine until the concentration of oxides reaches 0.1-0.5 mol/1, and diluting the resultant solution in an alcohol or xylene up to a final viscosity of 10-100 cp ('Deposition and interface structures of YBCO thin films via a non-fluorine sol-gel route', Physica C 371 (2002) 97-103, 'Fluorine-free sol gel d eposition of epitaxial YBCO thin films for coated conductors', Physica C 392-396

(2003) 941-945)).

[8] Apettrii et al. have succeeded in producing a superconducting thin film having a Jc of at least 1 x 10 A/cm at 77 K using a dilution of a fluorine-free organometallic compound in dimethylformamide, which is prepared by dissolving yttrium (Y)-nitrate, Ba-nitrate and Cu-nitrate in a poly aery lie acid and diluting the solution in dimethylformamide (Preparation of YBCO thin films by fluorine-free polymer-based chemical solution deposition', Applied superconductivity conference, paper number 1MJ06

(2004) Oct. 3-8, Jacksonville, FL, U.S.A.).

[9] For the production of REBa Cu O (where RE is a rare earth element selected from Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or a combination thereof) based oxide superconducting thin films on a template in which a ceramic thin film is epitaxially formed on a (100)<001> -textured nickel, copper or alloy thereof, or a template in which a biaxially textured ceramic thin film is formed on stainless steel or Hastelloy by ion beam assisted deposition (IBAD), by metalorganic deposition, it is required that the stoichiometric ratio of metal ions contained in an organometallic precursor solution be substantially equal to that of the constituent metal ions of the oxide superconducting thin film. This requirement enables effective conversion of an organometallic compound thin film into an epitaxial oxide superconducting thin film during annealing. [10] Further, when processing factors, such as oxygen partial pressure (PO ), water vapor pressure (P ), annealing temperature, annealing time and gas flow rate, are ef-

H2O fectively controlled after application of an organometallic precursor solution to an substrate, an organometallic precursor is converted into a superconductor and a superconducting thin film having excellent superconductivity is epitaxially grown on the substrate.

[11] In conventional metalorganic deposition (MOD) methods, however, trifluo- roacetate (TFA) polymers of rare earth elements, barium and copper, which are prepared by bonding TFA with corresponding metal ions, have been used as precursors to produce oxide superconducting thin films.

[12] According to a conventional method for preparing a precursor solution, yttrium

(Y)-acetate, barium (Ba)-acetate and copper (Cu)-acetate are dissolved in a TFA solution in accordance with a cationic ratio (e.g., Y : Ba : Cu = 1 : 2 : 3) of a final superconducting product, and then the solvent is evaporated by distillation, followed by re-dissolution and polymerization by refluxing to prepare a precursor solution in which Y, Ba and Cu cations are present in a 1 : 2 : 3 ratio. The precursor solution is then applied to a substrate.

[13] In this method, the Y, Ba and Cu acetates as starting materials are dissolved in an aqueous solution of TFA, and then the solution is polymerized to prepare a cationic polymer with a composition of Y, Ba and Cu in a ratio of 1 : 2 : 3, followed by distillation and purification to prepare a TFA polymer of Y, Ba and Cu. The TFA polymer is then diluted with methanol to prepare a dilution, which is applied to a substrate. The overall process is termed a 'TFA-MOD method'.

[14] However, since the conversion of the TFA polymer into oxides and oxyfluorides requires long term-calcination and the TFA polymer contains fluorine, an environmentally hazardous substance, the TFA-MOD method has limited applicability (Cima et al., 'Preparation of highly textured oxide superconducting films from MOD precursor solutions', U.S Patent No. 5,231,074; and Smith et al., 'Controlled conversion of metal oxyfluorides into superconducting oxides', U.S Patent No. 6,610,428). [15] In addition to TFA polymers, fluorine-free raw materials, such as Y-TMA, barium hydroxide and Cu-TMA, for the production of an oxide superconducting thin film by metalorganic deposition are dissolved in propionic acid and an amine to prepare a solution with a composition of Y, Ba and Cu in a ratio of 1 : 2 : 3, which is used to produce an oxide superconducting thin film (Y. Xu et al., IEEE Trans. Appl. Supercond. 11 (1), 2865-2868, (2001), and D. Shi et al., Physica C, 354 (2001) 71-76). However, the disadvantage of this method is that the steps of applying the coating solution to a substrate and annealing the coated substrate at 200-250⁰C must be repeated to produce a 0.3 micron-thick film. Another disadvantage is a long conversion time of the starting materials into an oxide superconductor.

[16] There is, thus, a need for a metalorganic deposition method that uses organometallic raw materials containing fluorine as starting materials, avoids the need to repeat additional coating of a solution and annealing when it is intended to produce a thin film having a thickness not less than 0.3 microns, and requires a short conversion time of the starting materials into an oxide superconductor. Specifically, there exists a need for a metalorganic deposition method that enables the production of a final superconducting thin film having a thickness not less than 0.3 microns and a high Jc not lower than 0.5 x 10 A/cm .

[17] On the other hand, it is preferred that high-temperature superconducting thin films have a high critical temperature and a high critical current density for practical use thereof. In addition, high-temperature superconducting thin films must be able to be produced in an economical manner. Of conventional deposition methods, metalorganic deposition (MOD) has received considerable attention as the most economical method. According to metalorganic deposition (MOD), high-temperature superconducting thin films are produced by applying an organometallic compound precursor solution to a ceramic single crystal substrate or a substrate whose surface is coated with a biaxially aligned ceramic, followed by one or more multiple annealing steps. Various organic solvents have been tested in order to produce superconducting thin films having excellent superconductivity by metalorganic deposition. However, little is known about suitable organic solvents other than trifluoroacetates that can be used to produce superconducting thin films having excellent superconductivity in a reproducible manner. Since the nature of trifluoroacetate solutions may be greatly varied depending on the amount of impurities, for example, moisture content, present in air, there is a disadvantage in that the processing conditions, such as moisture content, for the production of superconducting thin films must be carefully controlled.

Disclosure of Invention

Technical Problem [18] Therefore, the present invention has been made in view of the above problems, and it is one object of the present invention to provide a method for preparing an organometallic precursor solution using an organic solvent as a raw material other than trifluoroacetates, thereby enabling production of an oxide superconducting thin film with excellent superconductivity at reduced costs without being largely affected by the presence of moisture in air even during storage and processing, such as coating, in air.

[19] It is another object of the present invention to provide an organometallic precursor solution prepared by the method.

[20] It is yet another object of the present invention to provide a method for producing an oxide superconducting thin film by metalorganic deposition using the organometallic precursor solution.

Technical Solution

[21] In accordance with one aspect of the present invention for achieving the above objects, there is provided a method for preparing an organometallic precursor solution that is used to produce an oxide superconductor, the method comprising the steps of: mixing a metal salt and a metal oxide as starting materials with an organic acid and water with stirring, and completely dissolving the mixture under heating until the solution becomes transparent (step Sl); evaporating the solvent until the transparent solution becomes a viscous jelly to prepare an organometallic compound (step S2); and dissolving the organometallic compound in an organic solvent to prepare a precursor solution for the production of an oxide superconducting thin film (step S3).

[22] FIG. 1 is a block diagram illustrating a method for preparing an organometallic precursor solution according to a preferred embodiment of the present invention.

[23] The metal salt is selected from the group consisting of metal nitrates, carbonates, hydroxides, chlorides, and acetates. These metal salts may be used alone or in combination thereof. The metal oxide can be selected from the group consisting of REOs (rare earth oxides), BaO, CuO, (RE¹ RE² _χ )Ba_zCu O , Y124, Bi-2212, Bi- 2223, Tl- 1234, Tl-2223, Hg- 1234, and mixtures thereof. In the metal oxide (RE¹ RE² l-x )B ₂ a 2 Cu 3 O y , x and y satisfy the relationships 0 < x < 1 and 6.5 < y < 7, and RE and

RE are each independently selected from the group consisting of Y, La, Pr, Nd, Pm, Sm, Eu. Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof.

[24] Chloroacetic acid (CA), dichloroacetic acid (DCA) or trichloroacetic acid (TCA) can be used as the organic acid, and methyl alcohol or ethyl alcohol can be used as the organic solvent.

[25] The organic solvent may contain water in an amount of 10-40%. The concentration of the metal ions in the final precursor solution for the production of an oxide superconducting thin film is in the range from 1 M to 5 M.

[26] In accordance with another aspect of the present invention, there is provided a method for producing an oxide superconducting thin film by metalorganic deposition, the method comprising the steps of: mixing a metal salt and a metal oxide as starting materials with an organic acid and water with stirring, the starting materials having the same cationic ratio as that of a rare earth element-barium-copper oxide superconducting thin film, and completely dissolving the mixture under heating until the solution becomes transparent (step Sl); evaporating the solvent until the transparent solution becomes a viscous jelly to prepare an organometallic compound (step S2); dissolving the organometallic compound in an organic solvent to prepare a precursor solution for the production of an oxide superconducting thin film (step S3); applying the organometallic compound precursor solution to a substrate to form a chlorine- containing organometallic compound thin film (step S4); annealing the chlorine- containing organometallic compound thin film while varying the annealing conditions (e.g., heating rate, conversion temperature, annealing time, P , gas flow rate and

HzO oxygen partial pressure) to convert the chlorine-containing organometallic compound thin film into a RE-Ba-Cu oxide (step S5); and oxygen-annealing the RE-Ba-Cu oxide to convert the RE-Ba-Cu oxide into an oxide superconducting thin film having a critical current density not lower than 1 x 10 A/cm at 77 K at zero applied magnetic field (step S6).

[27] FIG. 2 is a block diagram illustrating a method for producing an oxide superconductor according to a preferred embodiment of the present invention.

[28] Step S5 may further include the sub-step of heating the metal chloride thin film at the lowest oxygen partial pressure where the final oxide superconductor can stably exist. The oxygen partial pressure is adjusted to greater than 100 parts per million (ppm) and lower than 1 atm.

[29] The flow rate of oxygen-containing gases is controlled within 50-500 cm /cm -min.

Water at 10-100⁰C can be passed through the oxygen-containing gases so as to allow the gases to contain moisture. Preferably, water at 20-70⁰C is passed through the oxygen-containing gases so that the moisture content of the gases can reach a maximum.

[30] In step S5, heating may be performed at a rate of 2-400°C/hr at the temperature range of 695-735⁰C. Preferably, heating may be performed at a relatively low rate of 5-100°C/hr. In step S5, the conversion of the organometallic compound into the superconducting compound may be performed at 715-755⁰C. Preferably, active conversion of the organometallic compound into the oxide can be achieved at 725-745⁰C.

[31] In step S4, the substrate can be applied by various coating techniques, including dip coating, spin coating, slot-die coating and spray coating. The substrate is applied in such a manner that the surface of the substrate, where the organometallic compound precursor solution is applied, has a biaxially aligned texture. The substrate may be a single crystal ceramic substrate having a (100)<001> orientation or a metal substrate.

[32] The single crystal ceramic substrate having a (100)<001> orientation may be made of a material selected from the group consisting of SrTiO , LaAlO , zirconia, stabilized zirconia (YSZ), MgO, CeO , rare earth element oxides, and mixtures thereof. In addition, the surface of the ceramic substrate, where the organometallic compound precursor solution is applied, can be substantially lattice-matched to the final oxide superconductor.

[33] Further, the rare earth element can be selected from the group consisting of Y, La,

Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and mixtures thereof.

Description of Drawings

[34] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[35] FIG. 1 is a block diagram illustrating a method for preparing an organometallic precursor solution according to a preferred embodiment of the present invention;

[36] FIG. 2 is a block diagram illustrating a method for producing an oxide superconductor according to a preferred embodiment of the present invention;

[37] FIG. 3 is a schematic temperature-time profile of annealing in a method for producing a superconducting thin film according to the present invention;

[38] FIG. 4 is a graph showing the results of X-ray diffraction analysis for a superconducting thin film produced by a method of the present invention;

[39] FIG. 5 is a (103) pole figure of a superconducting thin film produced by a method of the present invention;

[40] FIG s. 6a to 6d are photographs showing the microtextures of a superconducting thin film produced by a method of the present invention;

[41] FIG. 7 is a graph showing the measurement results of critical transition temperature of a superconducting thin film produced by a method of the present invention; and

[42] FIG. 8 is a graph showing the measurement results of critical current of a superconducting thin film produced by a method of the present invention.

Best Mode

[43] Hereinafter, a method for preparing an organometallic precursor solution, an organometallic precursor solution prepared by the method, and a method for producing an oxide superconducting thin film by metalorganic deposition using the organometallic precursor solution according to the present invention will be explained in detail with reference to the following examples and the accompanying drawings.

[44] FIG. 3 is a schematic temperature-time profile of annealing in a method for producing a superconducting thin film according to the present invention, FIG. 4 is a graph showing the results of X-ray diffraction analysis for a superconducting thin film produced by a method of the present invention, FIG. 5 is a (103) pole figure of a superconducting thin film produced by a method of the present invention, FIG s. 6a to 6d are photographs showing the microtextures of a superconducting thin film produced by a method of the present invention, FIG. 7 is a graph showing the measurement results of critical transition temperature of a superconducting thin film produced by a method of the present invention, and FIG. 8 is a graph showing the measurement results of critical current of a superconducting thin film produced by a method of the present invention.

[45] The present invention provides a method for preparing a solution suitable to produce a highly textured oxide superconductor by metalorganic deposition using a chlorine-containing organic acid. The present invention also provides an oxide superconductor having a Jc not lower than 1 x 10 A/cm using a solution prepared by the method. The present invention also provides a method for producing the oxide superconductor.

[46] The oxide superconducting thin film produced by the method of the present invention exhibits superconductivity, i.e. conducts electricity without any resistance at a temperature not lower than the boiling point (77 K) of liquid nitrogen. In addition, the oxide superconducting thin film produced by the method of the present invention is epitaxially grown on a biaxially aligned substrate (including a ceramic substrate), and as a result, the oxide superconducting thin film possesses a critical current density not lower than 1 x 10⁵ A/cm² (at 77K, self-field).

Mode for Invention

[47] Example 1

[48] 0.04 moles of Y 1 Ba 2 Cu 3 O 7-x as an oxide ^c powder was weig °hed and added to a mixed solution of water (100 cc) and dichloroacetic acid (DCA, 50 cc). The mixture was dissolved under heating to 8O⁰C until the solution became transparent.

[49] After complete dissolution of the powder, the solvent was evaporated at reduced pressure at 5O⁰C to obtain a viscous jelly.

[50] Heating was continued until the flowability of the solution substantially disappeared, followed by cooling.

[51] The jellied compound thus obtained was dissolved in 40 cc of methyl alcohol at room temperature to prepare a precursor solution ('Solution A') for the production of an oxide superconducting thin film by metalorganic deposition. In Solution A, an organometallic complex in which DCA was attached to Y, Ba and Cu atoms was dissolved in methyl alcohol.

[52] Example 2

[53] 15 cc of water was dissolved in Solution A to prepare a precursor solution

('Solution B') for the production of an oxide superconducting thin film by metalorganic deposition.

[54] Example 3

[55] 0.04 moles of Y-acetate, 0.08 moles of Ba-acetate and 0.12 moles of Cu-acetate were weighed so that the ratio Y : Ba : Cu was 1 : 2 : 3, and added to a mixed solution of water (100 cc) and dichloroacetic acid (DCA, 50 cc). The mixture was dissolved under heating to 8O⁰C until the solution became transparent. [56] After complete dissolution of the powder, the solvent was evaporated at reduced pressure at 5O⁰C to obtain a viscous jelly. [57] Heating was continued until the flowability of the solution substantially disappeared, followed by cooling. [58] The jellied compound thus obtained was dissolved in 40 cc of methyl alcohol and

15 cc of water at room temperature to prepare a precursor solution ('Solution C) for the production of an oxide superconducting thin film by metalorganic deposition. [59] Example 4

[60] 0.04 moles of Eu 1 Ba 2 Cu 3 O 7-x as an oxide powder was weighed and added to a mixed solution of water (100 cc) and dichloroacetic acid (DCA, 50 cc). The mixture was dissolved under heating to 8O⁰C until the solution became transparent.

[61] After complete dissolution of the powder, the solvent was evaporated at reduced pressure at 5O⁰C to obtain a viscous jelly.

[62] Heating was continued until the flowability of the solution substantially disappeared, followed by cooling.

[63] The jellied compound thus obtained was dissolved in 40 cc of methyl alcohol and

15 cc of water at room temperature to prepare a precursor solution ('Solution D') for the production of an oxide superconducting thin film by metalorganic deposition. In Solution D, an organometallic complex in which DCA was attached to Eu, Ba and Cu atoms was dissolved in methyl alcohol.

[64] Example 5

[65] 0.04 moles of Gd 1 Ba 2 Cu 3 O 7-x as an oxide powder was weighed and added to a mixed solution of water (100 cc) and dichloroacetic acid (DCA, 50 cc). The mixture was dissolved under heating to 8O⁰C until the solution became transparent.

[66] After complete dissolution of the powder, the solvent was evaporated at reduced pressure at 5O⁰C to obtain a viscous jelly.

[67] Heating was continued until the flowability of the solution substantially disappeared, followed by cooling.

[68] The jellied compound thus obtained was dissolved in 40 cc of methyl alcohol and

15 cc of water at room temperature to prepare a precursor solution ('Solution E') for the production of an oxide superconducting thin film by metalorganic deposition. In Solution E, an organometallic complex in which DCA was attached to Gd, Ba and Cu atoms was dissolved in methyl alcohol. [69] Example 6

[70] 0.04 moles of Y 1 Ba 2 Cu 3 O 7-x as an oxide ^ powder was weig °hed and added to a mixed solution of water (100 cc) and trichloroacetic acid (TCA, 0.52 moles ). The mixture was dissolved under heating to 8O⁰C until the solution became transparent. [71] After complete dissolution of the powder, the solvent was evaporated at reduced pressure at 5O⁰C to obtain a viscous jelly. [72] Heating was continued until the flowability of the solution substantially disappeared, followed by cooling. [73] The jellied compound thus obtained was dissolved in 40 cc of methyl alcohol at room temperature to prepare a precursor solution ('Solution F') for the production of an oxide superconducting thin film by metalorganic deposition. In Solution F, an organometallic complex in which TCA was attached to Y, Ba and Cu atoms was dissolved in methyl alcohol. [74] Example 7

[75] 15 cc of water was dissolved in Solution F to prepare a precursor solution

('Solution G') for the production of an oxide superconducting thin film by metalorganic deposition.

[76] Example 8

[77] 0.04 moles of Y Ba Cu O as an oxide powder as an oxide powder was weighed and added to a mixed solution of water (100 cc) and chloroacetic acid (CA, 0.52 moles

). The mixture was dissolved under heating to 8O⁰C until the solution became transparent. [78] After complete dissolution of the powder, the solvent was evaporated at reduced pressure at 5O⁰C to obtain a viscous jelly. [79] Heating was continued until the flowability of the solution substantially disappeared, followed by cooling. [80] The jellied compound thus obtained was dissolved in 40 cc of methyl alcohol at room temperature to prepare a precursor solution ('Solution H') for the production of an oxide superconducting thin film by metalorganic deposition. In Solution H, an organometallic complex in which CA was attached to Y, Ba and Cu atoms was dissolved in methyl alcohol. [81] Example 9

[82] 15 cc of water was dissolved in Solution H to prepare a precursor solution

('Solution I') for the production of an oxide superconducting thin film by metalorganic deposition.

[83] Example 10

[84] Each of Solutions A, B, C, D and E prepared in Examples 1 to 5 was applied to a LaAlO (100) single crystal substrate by spin coating.

[85] The coated substrate was charged into a tube furnace (inner diameter: 5 cm) at

100⁰C, and was than heated from 100⁰C to 500⁰C over 12 hours while passing wet oxygen at a flow rate of 3,000 seem through the tube furnace (see, FIG. 3). The wet oxygen was obtained by passing oxygen through water at 3O⁰C.

[86] The resulting structure was maintained in an air atmosphere having a dew point of

I⁰C for 10 minutes.

[87] After the gas of the tube furnace was changed into argon containing 1,000 ppm oxygen, the temperature of the tube furnace was raised from 500⁰C to 695⁰C over 20 minutes while passing water at 4O⁰C at a flow rate of 3,000 seem through the tube furnace.

[88] Thereafter, the temperature of the tube furnace was elevated from 695⁰C to 715⁰C over 2 hours and maintained at 715⁰C for 12 hours.

[89] After the gas of the tube furnace was changed to dry oxygen, the tube furnace was cooled to 500⁰C over 2 hours. Next, the tube furnace was maintained at 500⁰C for one hour, and allowed to cool to room temperature over 12 hours to produce a superconducting thin film.

[90] FIG. 4 is a graph showing the results of X-ray diffraction analysis for the superconducting thin film. The graph shows that the c-axis of the superconductor crystalline grains was grown in a direction perpendicular to the plane of the substrate.

[91] FIG. 5 is a (103) pole figure of the superconducting thin film. From the figure of

FIG. 5, it could be confirmed that the superconducting thin film had a good (100)<001 > texture.

[92] FIG s. 6a to 6d are surface and cross-sectional scanning electron micrographs

(SEM) of the superconducting thin film. The micrographs show that the superconductor crystalline grains were densely grown.

[93] FIG. 7 is a graph showing changes in the resistance of the Y Ba Cu O oxide su-

1 2 3 7-x perconducting thin film according to the changes in the temperature of the oxide superconducting thin film, indicating that the oxide superconducting thin film exhibited superconductivity at 94 K. The superconducting thin films produced using Solutions C, D and E were measured to have critical currents of 5-11 A at 77K, self- field (FIG. 8). The superconducting thin films had critical current density (Jc) values of 2.5-5.5 x 10 A/cm , as calculated by dividing the critical current values by the cross-sectional areas of the thin films. Results obtained in the superconducting thin films produced using Solutions A and B were similar to those obtained in the superconducting thin film produced using Solution C.

[94] The foregoing embodiments are intended for illustrative purposes only, and partial modifications and variations of the methods described herein will be obvious to those skilled in the art from the foregoing detailed description. Therefore, such modifications and variations are encompassed within the scope and spirit of the present invention.

Industrial Applicability

[95] As apparent from the above description, the present invention provides a method for preparing an organometallic precursor solution using an organic solvent as a raw material other than trifluoroacetates, thereby enabling production of an oxide superconducting thin film with excellent superconductivity at reduced costs without being largely affected by the presence of moisture in air even during storage and processing, such as coating, in air. T he present invention also provides an organometallic precursor solution prepared by the method. T he present invention also provides a method for producing an oxide superconducting thin film by metalorganic deposition using the organometallic precursor solution.

[96] The organometallic precursor solution prepared by the method of the present invention is stable without any change in the characteristics of the precursor solution during storage in air. In addition, only one coating of the organometallic precursor solution enables the production of a final thin film having a thickness not less than 0.3 microns and a critical current density not lower than 1 x 10 A/cm .

Claims

[I] A method for preparing an organometallic precursor solution suitable for the production of an oxide superconductor, the method comprising the steps of: mixing a metal salt and a metal oxide as starting materials with an organic acid and water with stirring, and completely dissolving the mixture under heating until the solution becomes transparent (step Sl); evaporating the solvent until the transparent solution becomes a viscous jelly to prepare an organometallic compound (step S2); and dissolving the organometallic compound in an organic solvent to prepare a precursor solution for the production of an oxide superconducting thin film (step

S3). [2] The method according to claim 1, wherein the metal salt is selected from the group consisting of metal nitrates, metal carbonates, metal hydroxides, metal chlorides, metal acetates, and mixtures thereof. [3] The method according to claim 1, wherein the metal oxide is selected from the group consisting of REOs (rare earth oxides), BaO, CuO, (RE RE )Ba Cu O

, Y124, Bi-2212, Bi-2223, Tl- 1234, Tl-2223, Hg-1234, and mixtures "thereof. y _{j 2}

[4] The method according to claim 3, wherein, in the metal oxide (RE RE )Ba x 1-x 2

Cu O , x and y satisfy the relationships 0 < x < 1 and 6.5 < y < 7, and RE and

RE 2 ar ^Ye each independently selected from the group consisting of Y, La, Pr, Nd,

Pm, Sm, Eu. Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. [5] The method according to claim 1, wherein the organic acid is chloroacetic acid

(CA). [6] The method according to claim 1, wherein the organic acid is dichloroacetic acid

(DCA). [7] The method according to claim 1, wherein the organic acid is trichloroacetic acid

(TCA). [8] The method according to claim 1, wherein the organic solvent is methyl alcohol or ethyl alcohol. [9] The method according to claim 8, wherein the organic solvent contains water in an amount of 10-40%. [10] The method according to claim 1, wherein the precursor solution for the production of an oxide superconducting thin film has a metal ion concentration of 1 M to 5 M.

[I I] A method for producing an oxide superconducting thin film by metalorganic deposition, the method comprising the steps of: mixing a metal salt and a metal oxide as starting materials with an organic acid and water with stirring, the starting materials having the same cationic ratio as that of a rare earth element-barium-copper oxide superconducting thin film, and completely dissolving the mixture under heating until the solution becomes transparent (step Sl); evaporating the solvent until the transparent solution becomes a viscous jelly to prepare an organometallic compound (step S2); dissolving the organometallic compound in an organic solvent to prepare a precursor solution for the production of an oxide superconducting thin film (step

S3); applying the organometallic compound precursor solution to a substrate to form a chlorine-containing organometallic compound thin film (step S4); annealing the chlorine-containing organometallic compound thin film while varying the annealing conditions (including heating rate, conversion temperature, annealing time, P , gas flow rate and oxygen partial pressure) to convert the

H2O chlorine-containing organometallic compound thin film into a RE-Ba-Cu oxide (step S5); and oxygen-annealing the RE-Ba-Cu oxide to convert the RE-Ba-Cu oxide into an oxide superconducting thin film having a critical current density not lower than 1 x 10 A/cm at 77 K at zero applied magnetic field (step S6).

[12] The method according to claim 11, wherein step S5 further includes the sub-step of heating the metal chloride thin film at the lowest oxygen partial pressure where the oxide superconductor stably exists.

[13] The method according to claim 12, wherein the oxygen partial pressure is greater than 100 parts per million (ppm) and lower than 1 atm.

[14] The method according to claim 11 or 12, wherein, in step S5, the flow rate of oxygen-containing gases is within 50-500 cm /cm -min.

[15] The method according to claim 11 or 12, wherein, in step S5, the oxygen- containing gases contain moisture by passing water at 10-100⁰C through the oxygen-containing gases.

[16] The method according to claim 11 or 12, wherein, in step S5, the heating is performed at a rate of 2-400°C/hr at the temperature range of 695-735⁰C.

[17] The method according to claim 11 or 12, wherein, in step S5, the conversion of the organometallic compound into the superconducting oxide is performed at 715-755⁰C.

[18] The method according to claim 15, wherein, in step S5, the oxygen-containing gases contain moisture by passing water at 20-70⁰C through the oxygen- containing gases.

[19] The method according to claim 16, wherein, in step S5, the heating is performed at a rate of 5-100°C/hr at the temperature range of 695-735⁰C. [20] The method according to claim 17, wherein, in step S5, the conversion of the organometallic compound into the superconducting oxide is performed at

725-745⁰C. [21] The method according to claim 11, wherein, in step S4, the substrate is applied by dip coating, spin coating, slot-die coating or spray coating. [22] The method according to claim 11, wherein, in step S4, the substrate is applied in such a manner that the surface of the substrate, where the organometallic compound precursor solution is applied, has a biaxially aligned texture. [23] The method according to claim 22, wherein the substrate is a single crystal ceramic having a (100)<001> orientation.

[24] The method according to claim 22, wherein the substrate is a metal substrate.

[25] The method according to claim 23, wherein the single crystal ceramic substrate having a (100)<001> orientation is made of a material selected from the group consisting of SrTiO , LaAlO , zirconia, stabilized zirconia (YSZ), MgO, CeO , rare earth element oxides, and mixtures thereof. [26] The method according to claim 22 or 23, wherein the surface of the ceramic substrate, where the organometallic compound precursor solution is applied, is substantially lattice-matched to the oxide superconductor. [27] The method according to claim 11, wherein the rare earth element is selected from the group consisting of Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,

Tm, Yb, Lu, and mixtures thereof.