KR20020064040A - Cu-Sheathed Bi2Sr2CaCu2Ox High-Tc Superconductor Thick Film and Method for Producing the same - Google Patents

Abstract

PURPOSE: A Bi2Sr2CaCu2Ox high temperature superconductive thick film tape is provided to reduce a manufacturing cost and to improve a through-put by forming Bi2Sr2CaCu2O8 high temperature superconductive thick film on a copper tape. CONSTITUTION: A copper is used as a substrate for coating with Bi2212 high temperature superconductive thick film. A precursor power made of a mixed power with Bi2O3, SrCO3 and CaCO3 except for CuO and a roasted powder is coated on the outer surface of the copper substrate using a screen printing or a dipping. Then, Bi2212 high temperature superconductive thick film is formed on the outer surface of the copper substrate by a thermal treatment at the temperature of 820-870 deg.C for 55 seconds or longer.

Description

Bi-Sheathed Bi2Sr2CaCu2Ox High-Tc Superconductor Thick Film and Method for Producing the same}

The present invention relates to a Bi2Sr2CaCu2Ox high-temperature superconducting thick film tape using copper as a substrate and a method of manufacturing the same. In particular, a Bi-based high-temperature superconducting thick film tape is manufactured, but a low-cost copper is used instead of silver, which is expensive to purchase as a metal coating material (substrate). By making Bi2212 high-temperature superconducting wire in large quantities, it is possible to obtain high productivity and economic efficiency by shortening the process time and mass production by reducing the process time.

High-temperature superconducting wire manufacturing technology is a core technology in the field of superconducting technology, and has emerged since the discovery of high-temperature superconductors.

This technology, due to the unique phenomenon that the electrical resistance of the superconductor becomes zero and there is almost no current loss under the critical temperature, various applications are possible and are currently being actively researched, and the US electric power company plans to install the superconducting cable in 2000. Declared.

This indicates that superconducting wire technology has approached the commercialization level.

Applications that this technology can be applied to are power / demand fields such as cables, generators, current limiters, SMESs, light bulbs, and medical fields such as MRI, and the commercialization of devices in each field will bring a change comparable to the industrial revolution. It is expected.

In order for high temperature superconducting wires to be applied, they must be flexible and have strong mechanical properties and the ability to transport large currents by orienting microstructures.

The Y-based high temperature superconductor is weak and vulnerable to moisture, and because the heat treatment temperature for unidirectional growth is higher than the melting point (960.5 ° C.) of the coating material (Ag), it is difficult to form a wire rod.

Therefore, research on manufacturing superconducting wires has been focused on research on coating materials to improve mechanical properties, especially on Bi-based high-temperature superconductors, and on manufacturing process to increase critical current density.

The cladding uses a ductile metal to improve the workability by supplementing the brittleness of the oxide, to increase the cooling effect due to the good heat transfer, and to allow the current to flow instead of losing superconducting properties. Ag, Cu, Au, Pt and the like. Since Au and Pt react with the superconductor to form a second phase, the application is limited and Cu is not used as a coating material because of the problem of oxidation during heat treatment.

Since Ag does not oxidize and has oxygen solubility, Ag is used as a coating material in the case of superconducting wires because when heating the superconductor, oxygen can be supplied to the oxide through the coating material and does not react with the superconducting material. .

Conventional high-temperature superconducting wire manufacturing technology, which is currently in practical use or under development, includes a powder-in-tube (PIT) method and a partial melt growth method.

Although the PIT method is not commercialized, it is the method most commonly used for Bi2Sr2Ca2Cu3Ox (Bi2223) and the partial melting growth method is widely used for Bi2Sr2Ca1Cu2Ox (Bi2212).

Early in the development of high temperature superconductors, the PIT method was attempted to fabricate conductors and various metals were tested as cladding.

As a result, silver coating which is easy to diffuse oxygen without reacting with oxygen was spotlighted as an optimal coating material and was first used in Y-Ba-Cu-O system.

However, the Y-Ba-Cu-O system had a critical current density of 2000-3000 A / cm2 at 77 K due to weak link due to grain boundary and was not successful as a wire rod.

The weak bonding problem was solved to some extent using the Bi-Sr-Ca-Cu-O system, and the weak bonding problem was significantly improved by doping Pb to Bi2223 having a critical temperature of 110 K.

It was found that pressing increased the critical current density by increasing the orientation of the microstructures, and in line with the efforts for the development of the powder process and the cold working process, the critical current density value increased significantly. 1 is a simplified illustration of a Bi-based high-temperature superconducting wire manufacturing process using silver-coated powder in tube (PIT) method. Currently, 69,000 A / cm 2 (77 K) in a short Bi2223 / Ag specimen using this technique, 50,000 A / cm2 or more (ASC, USA) is manufactured, and EURUS develops, manufactures and sells the technology to produce Bi2223 / Ag tape up to 3 km, which has a critical current of 26 ± 5 A at 77 K. .

Silver coated Bi2Sr2CaCu2Ox high temperature superconductors with high critical current density are made by partial melting method. The principle of this method is to heat the tape coated with Bi2212 powder on silver by immersion or doctor-blade until it is partially melted and then regenerate Bi2212 in the slow cooling copper.

However, the Bi2212 phase has several problems because it separates into a liquid phase and a non-superconducting phase when it melts and a Bi2212 superconducting phase has to be formed from these phases when cooling.

In particular, the non-superconducting phase in the liquid phase is not consumed during cooling and remains, which prevents the flow of current, hinders the orientation of Bi2212, and makes it difficult to switch to the Bi2212 single phase. In the non-superconducting phase, the (Sr, Ca) CuO2 phase (known as 1: 1 phase) grows largely in the liquid phase and reacts slowly with the liquid phase during cooling, always remaining after the end of the process.

Since these 1: 1 phases are particularly problematic when heat treated in air, many researchers are investigating ways to reduce or eliminate 1: 1 phases in partial melting.

Possible methods are to change the composition of the precursor powder or to change the oxygen partial pressure in the atmosphere.

Since changing the composition may lead to the appearance of other unwanted superconducting phases, most studies are directed to changing the partial pressure of oxygen to form a second phase that is highly reactive with the liquid phase rather than a 1: 1 phase. .

In the method of manufacturing high-temperature superconducting wire using silver coating, silver accounts for 75% of the total material cost. In particular, in the case of the PIT method, the manufacturing cost and investment cost are high due to the multi-step manufacturing process.

According to the US DOE, the economical price of wire rod is $ 0.01 / ampere-meter, whereas the production cost of silver-coated wire is currently $ 1 / ampere-meter.

Such high cost has been an obstacle to the commercialization of superconducting devices using high temperature superconducting wires.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and a Bi2Sr2CaCu2O8 (Bi2212) high temperature superconducting thick film is coated on a copper tape by coating and heat-treating a Bi2O3-SrCO3-CaCO3 mixture precursor having no copper component on the copper tape by using net printing or dipping. It is an object of the present invention to provide a manufacturing method for manufacturing a large amount of Bi2212 high temperature superconducting wire in a low cost and fast time.

1 is an exemplary view showing a Bi-based high-temperature superconducting wire manufacturing process using a silver coating powder in tube (PIT) method of the prior art.

Figure 2 is an exemplary view showing a Bi-based high temperature superconducting wire manufacturing method using the copper-clad Cu-free precursor method of the present invention.

FIG. 3 is a Bi2212 thick film prepared by printing a Bi2SrCaOx precursor powder on a copper plate by using a copper-clad Cu-free precursor method of the present invention, followed by heat treatment at 820 ° C. for 1 minute to 870 ° C. for 50 seconds to 830 ° C. for 3 minutes in air. Exemplary diagram showing an X-ray diffraction pattern of.

4 is a Bi2212 thick film prepared by printing a Bi2SrCaOx precursor powder on a copper plate by using a copper-clad Cu-free precursor method of the present invention and then heat-treating at 820 ° C. for 1 minute to 870 ° C. for 50 seconds to 830 ° C. for 3 minutes. An illustration showing the temperature-resistance relationship of

Figure 5 is an exemplary view showing the surface microstructure of the Bi2212 thick film prepared by printing the Bi1.5SrCaOx precursor powder on a copper plate using a copper-clad Cu-free precursor method of the present invention and then heat-treated in 845 ℃, air for 50 seconds.

6 is an exemplary view showing the temperature-resistance relationship of a Bi2212 thick film prepared by printing a Bi1.5SrCaOx precursor powder on a copper plate using a copper-clad Cu-free precursor method of the present invention and then heat-treating the same at 845 ° C. for 50 seconds in air.

In the following, various embodiments of the present invention for achieving the above described group will be described in more detail in connection with the above description.

As mentioned above, in order to use Cu as a cladding material, the problem of oxidizing Cu during the heat treatment for manufacturing the superconducting wire must be solved.

The solution is to use the oxidized CuO as a source of CuO, which is a component necessary for superconducting synthesis.

Briefly, as illustrated in FIG. 2, Bi2O3, SrCO3, and CaCO3 mixed powder except for CuO is coated on a Cu substrate by heat-treatment by screen-printing or dipping to synthesize a superconducting thick film.

The microstructure is crystallographic, because the exact state-by-temperature condition for the Bi-Sr-Ca-Cu-O system has not been established yet, and the CuO composition of the thick film is changing due to the continuous oxidation of the Cu substrate over time. It is not easy to find the conditions for obtaining superconducting single-phase tissue oriented in the -axis.

Therefore, in the present invention, using a copper (Cu) instead of silver (Ag) as a coating material, the crystallographic c-axis by experimenting at various temperatures while changing the composition change, print thickness change, etc. of Bi2O3, SrCO3, CaCO3 mixed powder coated on Cu It is intended to present a method for forming well-oriented superconducting tissue.

FIG. 3 shows a thick film of Bi: Sr: Ca = 2: 1: 1 mixed powder once printed on a Cu substrate (about 25 μm), heat treated at 820 ° C. for 1 minute, and then heat treated at 870 ° C. for 50 seconds in an air atmosphere. After that, the X-ray diffraction experiment of the specimen heat-treated at 830 ℃ for 3 minutes shows that the Bi2212 high temperature superconducting thick film was successfully prepared on the Cu substrate.

The third heat treatment at 830 ° C. for 3 minutes is to transform the liquid phase remaining after the second heat treatment at 870 ° C. into a superconducting phase.

The CuO or Cu2O peaks were hardly shown in the data, and the main component was Bi2212 phase, which is aligned in the (0 0 l) direction, and a small amount of Bi2SrCuOy (hereinafter, Bi2201) phase exists.

4 is a temperature-resistance curve of the specimen tested in FIG. 3, and it is assumed that the critical temperature is about 58 K and the decrease in resistance below 70 K is a small effect of Bi2201.

Here, the influence of the thickness of the printing layer will be described.

Analysis of XRD patterns of specimens obtained by coating a 1, 2 and 3 mixed powder of Bi: Sr: Ca = 2: 1: 1 on Cu substrates at 870 ° C for 36 sec Samples printed once produced a large amount of Bi2212 phase and a small amount of Bi2201 phase. And it can be seen that these phases are well oriented in the crystallographic c-axis, that is, (00l) direction.

However, in the specimens printed twice and three times, a large amount of Bi2201 phase was formed and most of the remaining Cu was excessively oxidized with CuO.

Two- and three-time screen-printed specimens can be thought of as two types of CuO. First, two- and three-time screen-printed specimens produce more liquid than the one-printed specimen. As a result, the Bi2212 phase is not generated at all by leaving the area where the Bi2212 phase can be formed.

Second is the decomposition of CO3 contained in the starting materials. SrCO3 and CaCO3 contained in the starting material are decomposed at the heat treatment temperature, and the generated CO gas is ejected through the precursor printed on the Cu substrate. However, as CO3 decomposes, the remaining O2 reacts with Cu to oxidize a large amount of Cu. As a result, more O2 is generated and twice as much Cu is oxidized in two times and three times.

The higher the number of printed sheets, the higher the amount of liquid phase and the CuO phase, which means that the heat treatment temperature is high.

In the case of the specimen in which the mixed powder having the composition of the precursor powder was Bi: Sr: Ca = 2: 1: 1 on the copper substrate four times, the temperature at which the Bi2212 superconducting thick film was formed was 820 ° C.

This time, the effects of the composition change of the precursor powder will be examined.

Bi in the precursor powder affects the ease of liquid phase formation.

The higher the heat treatment temperature, the easier it will be to form the liquid phase, but it is more likely that unwanted phases such as Bi-free phases are formed and the Cu substrate tends to be excessively oxidized.

In addition, it is easy to obtain Bi2212 superconducting phase in which the microstructure is well oriented in the crystallographic c-axis if the Bi content is high in the precursor powder. As shown in Fig. 3, the residual liquid phase remains after the second heat treatment, which makes it difficult to obtain Bi2212 single phase.

As shown in the result that the critical temperature decreases slowly in FIG. 4, the specimen heat-treated by printing the precursor powder having Bi: Sr: Ca = 2: 1: 1 has a large amount of Bi that facilitates liquid phase formation at 830 ° C. The liquid phase and the second phase remain even after the post-heat treatment at.

In order to reduce the residual liquid phase and the second phase which adversely affect the superconducting properties, the experiment was performed by reducing the amount of Bi in the precursor powder.

FIG. 5 is a surface microstructure of a specimen obtained by printing a precursor powder having Bi: Sr: Ca = 1.5: 1: 1 on a Cu substrate and heat-processing at 845 ° C. for 50 seconds in air.

And FIG. 6 is a temperature-resistance curve of a specimen having the microstructure of FIG. 5. 845 ℃ is the best temperature and microstructure of specimens heat-treated between 830-870 ℃. The reason why the heat treatment was not performed at 820 ° C. was because surface pores due to gas generation were hardly observed unlike in the case of precursor powders of Bi: Sr: Ca = 2: 1: 1.

The microstructure phase feature is that the Bi2212 superconducting phase grows rapidly parallel to the surface. Unlike the case where the number of Bi moles in the precursor powder was 2, the critical temperature was 76 K even though the post-heat treatment was not performed at 830 ° C., and the resistance decreased rapidly near the critical temperature, resulting in a large reduction in the residual liquid phase and the second phase. It can be seen that the particle size of the superconducting phase is increased.

The specimens that were once heated with Cu precursor powder of Bi: Sr: Ca = 1.3: 1: 1 were heat-treated at 855 ° C for 50 seconds in air, and the amount of Bi was reduced compared to the case where the Bi molar ratio in the precursor powder was 1.5. Because the amount of liquid phase in the state was relatively small, the critical temperature was high as 79 K. When the composition of the precursor powder was Bi: Sr: Ca = 1: 1: 1, the heat treatment temperature increased to 870 ° C or higher, and Bi2212 was not formed. However, the amount of liquid phase partially melted was so small that almost no orientation of the microstructure occurred.

In the precursor powders of Bi: Sr: Ca = x: y: z, the composition regions in which the Bi2212 high temperature superconducting phase could be synthesized on the Cu substrate were x = 1-3, y = 1-2, and z = 0.5-1.5. Increasing the amount of Bi lowers the annealing temperature at which Bi2212 can be synthesized, an increase in the amount of Sr increases the annealing temperature, and a change in the amount of Ca does not affect the annealing temperature but affects the remaining second phase. .

This time we look at the Bi2212 superconducting thick film forming mechanism.

The requirement for the microstructure of the membrane surface to be oriented well in the crystallographic c-axis and to obtain the superconducting phase of the single phase is that the superconducting reaction must proceed in the presence of the liquid phase and that an appropriate amount of liquid phase is present.

In this respect, Bi in the precursor powder affects the ease of liquid phase formation.

If the amount of Bi in the precursor powder is high, it is easy to obtain a Bi2212 superconducting phase in which the microstructure is well-oriented to the crystallographic c-axis, but there is a problem that a residual liquid phase remains, making it difficult to obtain a Bi2212 single phase.

When the Bi content is small, the amount of liquid phase is small during the intermediate reaction, so that it is difficult to obtain the oriented superconducting thick film, and the superconducting reaction is also very slow, causing excessive oxidation of the copper substrate.

The higher the heat treatment temperature, the easier it will be to form the liquid phase, but it is more likely that unwanted phases such as Bi-free phases are formed and the Cu substrate tends to be excessively oxidized.

The process of forming the Bi2212 superconducting phase can be described as follows. In order for the Cu-free precursor powder printed on the copper substrate to melt, it must be supplied with a CuO component.

The CuO component is formed by the copper substrate being oxidized by the oxygen supply through the porous printing layer at the beginning of the heat treatment, and the precursor powder, which is in contact with the CuO layer at the time of copper, is supplied with CuO to start partial melting.

Two times (about 50 μm) of precursor powder of Bi: Sr: Ca = 1.5: 1: 1 was printed on Cu substrate, and the surface microstructure of the specimen heat-treated at 850 ° C. for 40, 45, 50, and 55 seconds was analyzed. saw. The reason for the heat treatment at 850 ° C. is to compare with the known state at that temperature, and the reason for printing twice is that the phase change can be easily observed due to the increase in the amount of liquid.

XRD analysis of the specimen heat-treated for 40 seconds consisted of the liquid phase and the second phase (Cu-free phase, (Ca, Sr) O phase), and Bi2212 phase began to nucleate and grow at 45 seconds. After 55 seconds of heat treatment, the entire thick film surface is covered with Bi2212.

In this study, where CuO is supplied by oxidation of a copper substrate and Bi2O3, CaCO3, and SrCO3 are used as precursor powders, this study describes the formation of a Bi2212 superconducting phase oriented well along the c-axis by a certain time copper heating at a given temperature. Two variables must be considered.

The first is that CuO is continuously supplied after partial melting, so the total composition of the partial molten film on the copper substrate changes in the direction of increasing CuO. The second is that the carbonate in the precursor powder rapidly increases when partial melting occurs at the beginning of the heat treatment. It is able to dissolve into a liquid phase and enable a large amount of liquid formation in a short time.

It is unknown whether these two variables act on copper and cause the interaction, but the initial composition of thick film and the expected composition after heat treatment can be inferred from the state diagram, and the partial melting reaction can be accelerated by carbonate decomposition. You can expect a group of prizes.

Inferring from FIG. 3, when the amount of Bi in the mixed precursor powder (BixSr1Ca1Ox) is 2 to 1.3, when the amount of CuO is small, it is a Cu-free phase + (Sr, Ca) O + liquid, but when the amount of CuO is increased, Bi is increased. When the -free phase and Bi2212 phase appear and the CuO content increases further, the CuO + Bi2223 + liquid phase coexists.

However, Bi2223 phase is very slow to form, so in case of short heat treatment time like this experiment, CuO + Bi2212 + liquid phase will maintain quasi-stable state.

As described above, the present invention has a simple process for preparing a raw material powder, a heat treatment time, and a manufacturing process, compared to the Bi-based high temperature superconducting tape manufacturing method using the PIT method. Since it does not require lamps, investment costs are low. In particular, copper is used instead of silver as a coating material, so production costs can be increased.

In view of these advantages, Bi-based high-temperature superconducting wire manufacturing method using a copper tape as presented in the present invention is a very useful technology that can bring a great improvement in economics to the superconducting device using the high-temperature superconducting wire.

Claims (4)

In the method of manufacturing Bi2Sr2CaCu2Ox (Bi2212) high temperature superconducting wire, Screen printing or dipping of Bi2212 high temperature superconducting thick film using copper (Cu) and precursor powder composed of mixed powder or calcined powder except CuO on the outer surface of copper substrate ) Bi2Sr2CaCu2Ox high temperature superconducting thick film tape using copper as a substrate, characterized in that it is made by synthesizing a Bi2212 high temperature superconducting thick film on the surface by heat-treating to a predetermined temperature. The method of claim 1, The composition of the precursor powder capable of forming the Bi2212 high temperature superconducting thick film on the outer surface of the copper substrate is a mol (mol) ratio of Bi 1-3, Sr 1-2, and Ca is 0.5-1.5 Method for producing Bi2Sr2CaCu2Ox high temperature superconducting thick film tape. The method of claim 1, Bi2Sr2CaCu2Ox high temperature superconducting thick film tape using copper as a substrate, characterized in that the heat treatment temperature range in which the Bi2212 superconducting thick film can be formed in the composition region of the copper substrate and the precursor powder is 820-870 ° C. In constructing Bi2Sr2CaCu2Ox (Bi2212) high temperature superconducting wire, Bi2O3, SrCO3, CaCO3 powder, except for CuO, was used for the Bi2212 high temperature superconducting thick film, and Cu was 1-3 mol, Sr 1-2, and Ca 0.5-1.5 mol. A precursor powder composed of powder mixed or calcined at (mol) ratio is coated by screen-printing or dipping, followed by heat treatment at a temperature range of about 820-870 ° C. to synthesize Bi2212 high temperature superconducting thick film on the surface. Bi2Sr2CaCu2Ox high temperature superconducting thick film tape using copper as a substrate.