WO2008072808A1

WO2008072808A1 - A manufacturing method of precursor solution with improved viscosity

Info

Publication number: WO2008072808A1
Application number: PCT/KR2006/005462
Authority: WO
Inventors: Jai-Moo Yoo; Young-Kuk Kim; Jae-Woong Ko; Kook-Chae Chung
Original assignee: Korea Institute Of Machinery & Materials
Priority date: 2006-12-11
Filing date: 2006-12-14
Publication date: 2008-06-19
Also published as: KR100820747B1

Abstract

The present invention relates to a precursor solution for manufacturing a superconducting wire or tape, and, more particularly, to a method of preparing a precursor solution having improved viscosity characteristics for manufacturing a superconducting wire or tape having improved viscosity characteristics. The method of preparing a precursor solution having improved viscosity characteristics includes the step of dissolving yttrium acetate, copper acetate, barium trifluoroacetate, and an additive in a solvent to form a solution; and adding metal carboxylate to the solution.

Description

A MANUFACTURING METHOD OF PRECURSOR SOLUTION WITH

IMPROVED VISCOSITY

Technical Field

The present invention relates to a precursor solution for manufacturing a superconducting wire or tape, and, more particularly, to a method of preparing a precursor solution having improved viscosity characteristics for manufacturing a superconducting wire or tape having improved viscosity characteristics.

Background Art

Generally, a superconducting oxide wire or tape, for example, YBa₂Cu₃O_7-X, has high current transport capacity and excellent critical current characteristics in high magnetic fields. Therefore, it is expected that such a superconducting oxide wire or tape will enable large-capacity power equipment to have a small size, high efficiency and large capacity when it is applied to power cables, industrial motors, generators, and the like in the future. In order to apply the superconducting oxide wire or tape to these various application fields, a superconducting wire or tape having a high critical current value must be able to be manufactured using an economical method.

Currently, research to form a superconductive oxide layer using various methods, such as pulsed laser deposition (PLD), metalorganic chemical vapor deposition (MOCVD), thermal evaporation, electron beam evaporation, and the like, is being conducted. However, these methods, all of which are high- vacuum deposition methods, are economically disadvantageous in that they need an expensive high-vacuum system and incur high maintenance costs. Meanwhile, a method of manufacturing a superconducting wire or tape having excellent critical current characteristics at a low cost may include a metalorganic deposition (MOD) method.

The metalorganic deposition method is a method of finally forming a superconducting layer using a precursor solution composed of rare earth metal salts, alkaline earth metal salts and transition metal salts through a solution coating process and a heat treatment process. In order to manufacture a superconductive thin film wire or tape using a metalorganic deposition method, a precursor solution, which can increase the thickness of a precursor thin film formed after heat treatment because it has excellent flow characteristics, such as viscosity etc., must first be developed. Various types of precursor solutions have been developed for the manufacture of a superconductive thin film wire or tape using a metalorganic deposition method. However, except for several cases, the developed precursor solutions are limited in the practical use thereof to the improvement of critical current characteristics.

For example, methods of forming a superconductive layer through a sol- gel method using metal alkoxide were disclosed. However, since the methods have problems in that heat treatment temperature is high (900 ^°C or more) and carbonates, which hinder superconducting properties, are produced, the methods have been limitedly used.

Meanwhile, in a MOD-TFA method, which is a metalorganic deposition method developed in the 1990s, a precursor solution, which does not produce carbonates even though it is heat-treated at a temperature of 800 ^°C or less, was used because the precursor solution is synthesized using metal trifluoroacetate.

However, the MOD-TFA method has problems in that a large amount of HF gas is generated in the heat-treatment process because the precursor solution includes a large amount of fluorine, and a precursor thin film obtained through the method has many pores or cracks. For this reason, in the method, the precursor solution must be heat-treated for a long time (20 hours or more), and the thickness of the obtained precursor thin film is also limited to 0.6 μm for one coating.

In order to overcome the problems with the metal trifluoroacetate-based precursor solution, the International Superconductivity Technology Center (ISTEC) in Japan synthesized a precursor solution, which can obtain a precursor thin film having no pores or cracks even when heat treatment is conducted for several hours, by substituting a precursor for a transition metal component in the precursor solution with naphthenates, ethyl hexanoate, or decanoates. However, in the precursor solution synthesized by substituting a transition metal component in the precursor solution with naphthenates, ethyl hexanoate or decanoates, in order to form a flawless precursor film through heat treatment for several hours, the thickness of the precursor film is limited to 0.4 IM or less for one coating. Therefore, in order to obtain a high critical current value, coating and heat treatment must be conducted 6 ~ 7 times or more. Generally, the thickness of a solution that can be obtained when the solution is applied on a substrate can be calculated using the Landau-Liebig equation below: t = 0.94(ηU / r_Ly )²ⁿ /r^U6Lv (pg)^{U 2} t: thickness, η: viscosity, Y_LV: surface tension, p: density, g: gravitational acceleration

Accordingly, when the thickness of a superconducting film can be increased through the control of the viscosity characteristics of a solution through only one coating and heat-treating process, the critical current value of a superconducting thin film wire or tape can also be increased. For this, conventionally, efforts to increase the concentration of the solution such that the thickness of the precursor thin film is increased and efforts to increase the viscosity of the solution using organic additives having high viscosity, such as ethylene glycol, polypyrrolidone, and the like, thus finally increasing the thickness of the precursor thin film that can be obtained through one solution coating process, have been made.

However, in order to increase the concentration of the precursor solution such that the thickness of the precursor solution is increased, the precursor solution is made in a solid state or a gel state having low flowability by heating the precursor solution under reduced pressure for a predetermined amount of time and thus removing a solvent from the precursor solution, and then the concentration of the precursor solution must be adjusted by introducing a suitable solvent into the precursor solution again. In this case, there are problems in that it is difficult to control the characteristics of the precursor solution because the reaction between the precursors dissolved in the precursor solution is accelerated by being heated under reduced pressure, and cracks or wrinkles occur in the precursor film formed by applying the precursor solution and then heat-treating it. Further, when organic additives having high viscosity are introduced into the precursor solution, there is a problem in that, although the viscosity of the precursor solution can be easily adjusted without performing an additional concentration adjustment process, it is difficult to improve the critical current characteristics thereof because of a problem of residual carbon caused by the introduction of organic materials. That is, if a precursor solution having excellent flow characteristics can be prepared using a method that entails no problem of residual carbon due to the introduction of the organic materials without adjusting the concentration of the precursor solution, the thickness of a superconducting film can be increased through only one coating and heat-treating process, so that the critical current value of a superconducting thin film wire or tape can be increased, thereby economically performing a process of manufacturing a superconducting wire or tape.

Disclosure of the Invention

Technical tasks to be solved by the invention

Accordingly, the present invention has been made to overcome the above problems occurring in the prior art, and an object of the present invention is to provide a method of preparing a precursor solution having improved viscosity characteristics, in which a precursor solution having excellent viscosity is prepared using a method that entails no problem of residual carbon due to the introduction of the organic materials without adjusting the concentration of the precursor solution, so that the thickness of the superconducting film can be increased through only one coating and heat-treating process, thereby obtaining a superconducting film having a high critical current value.

Technical Solution

In order to accomplish the above object, the present invention provides a method of preparing a precursor solution having improved viscosity characteristics, comprising the steps of dissolving yttrium acetate, copper acetate, barium trifluoroacetate, and an additive in a solvent to form a solution; and adding metal carboxylate to the solution.

The additive may include alkaline materials, such as ammonia, pyridine, bipyridine and the like; organic acids, such as acetic acid, propionic acid, butyric acid and the like; alkali metal salts, which are easily dissolved in a solvent, such as sodium hydroxide, sodium acetate, sodium propionate, sodium butyrate and the like; and a combination thereof.

The metal ion concentration in the precursor solution may be in the range of 1 to 3M, based on the total ion concentration in the precursor solution. The metal carboxylate, which can increase the viscosity of the precursor solution, includes rare earth acetate, such as samarium acetate, and transition metal acetate, such as zirconium acetate.

The amount of the metal carboxylate may be in the range of 0.1% to 30%, based on the amount of yttrium.

The viscosity of the precursor solution may be 70 cP or more.

The solvent may be selected from among alcohols, such as methanol, ethanol and the like, esters, such as ethyl acetate, ethyl butyrate and the like, ketones, such as acetone, propanone and the like, and mixtures thereof. A precursor film, formed through a single application of the precursor solution and the calcination thereof at a temperature of 450 ~ 650 ^"C, may have a thickness of 1 μm or more.

The calcination of the precursor solution may be performed after the precursor solution is left at a temperature of 150 ~ 220 "C for at least 30 minutes. A superconducting layer formed by finally heat-treating the precursor film in the calcination of the precursor solution may have a critical current of 50 A/cm or more and a critical current density of 1 MA/cm² or more.

Hereinafter, the present invention will be described with reference to the attached drawings. As described above, in order to form an YBa₂Cu₃0_7-x based superconducting film, the method of preparing a precursor solution according to the present invention includes the steps of synthesizing a precursor solution having excellent flowability by dissolving yttrium acetate, copper acetate, barium trifluoroacetate and an additive in a solvent to form a stable solution, and then adding other metal salts to the solution; and calcinating the synthesized precursor solution.

That is, the synthesized precursor solution is applied on a substrate, calcinated, and then heat-treated additionally, thereby forming an YBa₂Cu₃0_7-x based superconducting film having a few defects and an excellent critical current value.

The solvent for synthesizing the precursor solution may be selected from among alcohols, such as methanol, ethanol and the like, esters, such as ethyl acetate, ethyl butyrate and the like, ketones, such as acetone, propanone and the like, and mixtures thereof. If possible, it is advantageous to use a solvent having low moisture content. Yttrium acetate, copper acetate, or barium trifluoroacetate may be used as the precursor for synthesizing the precursor solution. In particular, unlike a conventional MOD-TFA solution composed of only metal trifluoroacetate, in the present invention, the amount of fluorine in the precursor solution can be decreased by about 70% by substituting metal trifluoroacetate with yttrium acetate and copper acetate containing no fluorine. The additive for forming the stable solution may include alkaline materials, such as ammonia, pyridine, bipyridine and the like; organic acids, such as acetic acid, propionic acid, butyric acid and the like; alkali metal salts, which are easily dissolved in a solvent, such as sodium hydroxide, sodium acetate, sodium propionate, sodium butyrate and the like; and a combination thereof.

The prepared precursor solution may be applied using various coating methods, such as dip coating, slot die coating, spin coating, gravure coating, and the like. When the precursor solution is applied on the surface of a metal substrate formed with a single-crystal buffer layer having excellent orientation using the above methods and is then calcinated, the precursor solution is changed into an oxyfluoride film. In this calcination, the precursor solution is left at a temperature of 150 ~ 220 ^°C for 30 minutes ~ 2 hours, is heated to a temperature of 450 ~ 650 ^°C at a heating rate of 4 "C ~ 30^°C/min, and is then cooled in an electric furnace. In this case, if the time for which the precursor solution is left at a temperature of 150 ~ 220 ^°C is excessively decreased, a large amount of carbon remains in the precursor film formed through the calcination, or a large number of cracks or pores occur on the surface of the precursor film in the process of heating the precursor solution to a temperature of 450~650^°C . Therefore, it is required that the precursor solution be left in the calcination process for at least

30 minutes. Further, it is advantageous that the heating rate be high in the process of heating the precursor solution to a temperature of 450-650 ^°C .

FIG. 1 is a graph showing the X-ray diffraction pattern of a precursor film obtained through calcination, and FIG. 2 is a photograph showing the surface and section of a precursor film obtained through a calcination process.

The oxyfluoride film obtained through the calcination process, as shown in the X-ray diffraction results of FIG. 1 and the photograph of the surface of the oxyfluoride film of FIG. 2, is composed of a matrix including yttrium oxyfluoride and barium oxyfluoride and copper oxide (CuO) particles, having a size of several nanometers. In this case, the thickness of the precursor thin film obtained through the calcination process is greatly influenced by the flow characteristics of a precursor solution. In order to control the flow characteristics of the precursor solution, first, the metal ion concentration of the precursor solution must be controlled. The concentration of metal ions included in the precursor solution is in the range of 1 to 3M, and, preferably IM to 1.5M, based on the total ion concentration in the precursor solution.

The viscosity of the prepared precursor solution is in the range of 10 to 150 cP depending on the metal ion concentration thereof. In order to increase the thickness of the precursor thin film obtained through the calcination process, it is effective to increase the viscosity of the precursor solution. However, when the viscosity of the precursor solution is increased to 100 cP or more by adjusting the concentration of the precursor solution, the precursor thin film obtained through the calcination process has many cracks, and thus it cannot be used to manufacture a superconducting wire or tape.

The viscosity of the precursor solution can be improved by adding metal carboxylates, such as rare earth metal carboxylates, transition metal carboxylates, or the like, to the precursor solution. There is a great difference in the viscosity characteristics of the precursor solution depending on the kinds of metal carboxylates that are added. In particular, when samarium acetate and zirconium acetate are added to the precursor solution, the viscosity of the precursor solution is rapidly changed. FIG. 3 is a graph showing the change in the viscosity of the precursor solution depending on the kinds of metal carboxylates that are added, and shows the viscosity characteristics of the precursor solution, as described above.

The reason that the viscosity of the precursor solution is rapidly increased after the addition of the metal carboxylates is determined to be that the added carboxylates are crosslinked with some of the components in the precursor solution, thus increasing the viscosity of the entire precursor solution, considering that carboxyl groups in a molecule serve as a crosslinking agent of a polymer material.

FIG. 4 is a graph showing the change in viscosity of the precursor solution depending on the amount of metal carboxylates that are added.

As shown in FIG. 4, it preferred that the amount of added metal carboxylate be in the range of 0.1 to 30%, particularly 8 to 12%, based on the amount of yttrium. When excessive metal carboxylates are added to the precursor solution, the viscosity of the precursor solution is rapidly increased to 300 cP or more, so that the thickness of the film is excessively increased when the precursor solution is applied on a substrate, with the result that many cracks are formed on the surface of the precursor film after the calcination process.

As such, when the high-viscosity precursor solution, prepared by adding the metal carboxylates thereto, is applied on a substrate and then calcinated, a precursor thin film, having no cracks, having a thickness of maximum 3 μm, can be formed, so that a superconducting wire or tape having excellent critical current characteristics can be manufactured through a final heat treatment process.

Advantageous Effects

The present invention is advantageous in that the viscosity of a precursor solution can be easily improved by adding metal carboxylates to the precursor solution, and a thick precursor film can be obtained.

Further, the present invention is advantageous in that the critical current characteristics of a superconducting wire or tape, manufactured through a metalorganic deposition process, can be greatly improved.

Further, the present invention is advantageous in that, since a superconducting wire or tape having excellent critical current characteristics can be manufactured through only one solution coating and calcination process, the present invention is superior to conventional technologies in economic and industrial aspects, and thus will contribute greatly to the practical application of a superconducting oxide wire or tape in the future.

Brief Description of Drawings

FIG. 1 is a graph showing the X-ray diffraction pattern of a precursor film obtained through calcination; FIG. 2 is a photograph showing the surface and section of a precursor film obtained through calcination;

FIG. 3 is a graph showing the change in the viscosity of a precursor solution depending on the kind of metal carboxylate that is added;

FIG. 4 is a graph showing the change in viscosity of a precursor solution depending on the amount of metal carboxylate that is added; FIG. 5 is a graph showing the results of X-ray diffraction of a superconducting film formed using a samarium-added precursor solution; and

FIG. 6 is a sectional photograph showing a superconducting film formed using a samarium-added precursor solution.

Best Mode for Carrying Out the Invention

[Example]

A precursor solution was prepared based on the following composition. precursor: yttrium acetate, copper acetate, barium trifluoroacetate; additive for forming stable solution: acetic acid, propionic acid; solvent: methanol; and additive for improving viscosity characteristics: samarium acetate.

The prepared precursor solution exhibited a viscosity of 270 cP, and was applied on a nickel-stainless steel alloy substrate formed with a buffer layer having a structure of CeO₂/YSZ/Y₂O₃ and then calcinated. The precursor film formed through the calcination had a uniform surface and had no cracks on the surface thereof. Subsequently, the precursor film formed through the calcination was finally heat-treated under heat treatment conditions (temperature: 780 ^°C, atmosphere: humid Ar/O₂ (300ppm) mixed gas), thereby obtaining a YBa₂Cu₃O_7-X superconducting layer.

FIG. 5 is a graph showing the results of X-ray diffraction of a superconducting film formed using a samarium-added precursor solution, and FIG. 6 is a sectional photograph showing a superconducting film formed using a samarium-added precursor solution.

As the results of measuring the critical current of the superconducting layer obtained in Example, the superconducting layer was found to have a critical current of 273 A/cm and a critical current density of 3.8 MA/cm².

Industrial Applicability

The superconducting wire or tape manufactured using a precursor solution prepared using the preparation method thereof according to the present invention has high current transport capacity and excellent critical current characteristics in high magnetic fields. Therefore, it is expected that the superconducting oxide wire or tape will enable large-capacity power equipment to have small size, high efficiency and large capacity when it is applied to power cables, industrial motors, generators, and the like in the future.

Claims

Claims:

1. A method of preparing a precursor solution having improved viscosity characteristics, comprising: dissolving yttrium acetate, copper acetate, barium trifluoroacetate, and an additive in a solvent to form a solution; and adding metal carboxylate to the solution.

2. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1, wherein the additive includes alkaline materials, such as ammonia, pyridine, bipyridine and the like; organic acids, such as acetic acid, propionic acid, butyric acid and the like; alkali metal salts, which are easily dissolved in a solvent, such as sodium hydroxide, sodium acetate, sodium propionate, sodium butyrate and the like; and a combination thereof.

3. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1, wherein metal ion concentration in the precursor solution is in a range of 1 to 3M, based on total ion concentration in the precursor solution.

4. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1, wherein the metal carboxylate includes rare earth acetate, such as samarium acetate, and transition metal acetate, such as zirconium acetate.

5. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1 , wherein an amount of the metal carboxylate is in a range of 0.1% to 30%, based on an amount of yttrium.

6. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1 , wherein viscosity of the precursor solution is 70 cP or more.

7. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1 , wherein the solvent is selected from among alcohols, such as methanol, ethanol and the like, esters, such as ethyl acetate, ethyl butyrate and the like, ketones, such as acetone, propanone and the like, and mixtures thereof.

8. The method of preparing a precursor solution having improved viscosity characteristics according to claim 1, wherein a precursor film formed through application of the precursor solution one time and calcination thereof at a temperature of 450 ~ 650^°C has a thickness of 1 μm or more.

9. The method of preparing a precursor solution having improved viscosity characteristics according to claim 8, wherein the calcination of the precursor solution is performed after the precursor solution is left at a temperature of 150 - 220 ^°C for at least 30 minutes.

10. The method of preparing a precursor solution having improved viscosity characteristics according to claim 8, wherein a superconducting layer formed by finally heat-treating the precursor film in the calcination of the precursor solution has a critical current of 50 A/cm or more and a critical current density of 1 M A/cm² or more.