Generally, superconductors enable a large amount of current to flow without loss, and can be used to make strong magnets, and thus they are applied in various fields, such as magnetic levitation trains, MRI systems, etc. Moreover, superconductors can be used to develop extremely fast, efficient and sensitive sensors and electronic devices, which cannot be realized using conventional devices.
Among such superconductors, a high-temperature superconductor, made of BSCCO (bismuth strontium calcium copper oxide) or YBCO (yttrium barium copper oxide), exhibits superconductivity even at a temperature lower than a boiling point (77 K) of liquefied nitrogen (LN2). Therefore, a high-temperature superconductor is advantageous in that it can be cooled using liquefied nitrogen as a coolant at low cost, compared to a low-temperature superconductor cooled using liquefied helium (LHe) as a coolant.
However, since a high-temperature superconductor exists as an oxide, it has insufficient ductility and is easily damaged, thus weakening the flow of current. Therefore, methods of using the high-temperature superconductor in the form of a superconducting wire have been researched.
In the superconducting wire, since a superconductor and a stabilizing substrate are bonded to each other, when current equal to or greater than critical current is applied to the superconductor, it passes to the stabilizing substrate, and thus the superconducting wire is protected.
Here, methods of forming a stabilizing layer may include a powder filling method, a vacuum thin film deposition method, and the like.
Korean Patent No. 10-0122105, registered on Sep. 2, 1997, discloses a method of manufacturing a high-temperature superconducting strip using the filling pressure of refractory powder through a hot working process.
FIG. 1 is a process view showing a conventional method of manufacturing a high-temperature superconducting strip using the filling pressure of refractory powder through a hot working process.
First, a superconducting strip is formed by putting bismuth-based powder into a silver tube and then drawing and rolling the bismuth-based powder. As shown in FIG. 1A, the superconducting strip is cut to a desired length, and then the cut pieces of superconducting strip are temporarily layered in a desired number. Then, as shown in FIG. 1B, the temporarily layered superconducting strips are covered with an adhesive strip, which is a very thin organic adhesive. This adhesive strip serves to stably maintain superconducting strips in a desired form while the superconducting strips are temporarily layered in a thickness direction.
Subsequently, as shown in FIG. 1C, the temporarily multi-layered superconducting strips are spirally disposed in a container, and are buried in powder compressed using a press, to conduct hot work. Then, as shown in FIG. 1D, the surface of the powder is pressed, and thus three-dimensional pressure occurs, thereby forming pressure for layering or contacting the superconducting strips.
Finally, as shown in FIG. 1E, the superconducting strips disposed in the container are directly put into a heating furnace where the strips are bonded to each other through a hot working process.
The superconducting strip manufactured using the above method is advantageous in that it has high critical current density, but is problematic in that an expensive silver tube must be used to manufacture the superconducting strip, it takes a lot of time to manufacture the superconducting strip, and the manufacturing cost thereof is high.
In order to overcome the above problem, technologies of manufacturing a superconducting strip by depositing Ag or Au on a superconducting layer and then soldering a stabilizing substrate (Cu) thereto have been proposed.
Korean Patent No. 10-0750664, registered Aug. 13, 2007, discloses an apparatus for continuously laminating superconducting strips by applying compressive stress thereto.
As shown in FIG. 2, a superconducting strip 10 is passed through a solder supply unit 200 which holds melted solder, and thus solder is adhered to the superconducting strip 10. The superconducting strip 10, on which solder is adhered, passes through a preheating unit 300 together with two reinforcement strips 20 to be preheated to a predetermined temperature.
The preheated superconducting strip 10 and reinforcement strips 20 pass through a tension control press 400 where they are pressed and thus bonded with each other by the solder disposed therebetween. The tension control press 400 is configured to control tension such that the tension of the reinforcement strips 20 is greater than that of the superconducting strip 10. Consequently, the superconducting sheet 30, which is manufactured under the condition of causing tension difference, is subjected to compressive stress over the entire region. That is, since the reinforcement strips 20 are bonded with the superconducting strip 10 in a state in which they are stretched, compressive stress is also applied to the superconducting strip in the same amount as tensile stress applied to the reinforcement strips 20.
This method of manufacturing a superconducting sheet by bonding a superconducting strip 10 with reinforcement strips 20 using solder is advantageous in that the manufacturing cost thereof can be decreased, and the damage to a superconducting layer in the superconducting sheet can be minimized because compressive stress is applied to the entire superconducting sheet, but is problematic in that a stabilizing substrate is detached from the superconducting strip at the time of heat-treatment due to the melted solder, and the superconducting strip is heated at the time of soldering.
In addition, a method of manufacturing a superconducting sheet by coating a superconducting thin film with a metal, such as silver (Ag), etc., and then completely covering the superconducting thin film with copper (Cu) through a wet-plating process is proposed. The method is advantageous in that, since the superconducting thin film is completely covered, it is hardly damaged and its thickness can be easily adjusted, but is problematic in that environmental pollution problems occur and process costs increase because a wet-plating process is additionally conducted, it takes a lot of time to deposit a thick superconducting thin film, and the superconducting thin film cannot be wound at various curvatures.