KR100392511B1 - Fabrication method of MgB2 superconducting wire - Google Patents

Abstract

Copper and silver, which are often used in the processing of existing superconducting wires, have good ductility and have a tendency to stretch well when drawing. However, when filling with powder, the outer sheathing material preferentially increases, thus effectively transmitting pressure to the inner superconducting core. In the present invention, it is possible to more efficiently transfer pressure to MgB ₂ powder or rod-shaped core filled inside by using a high-strength metal that is a reinforcing material when processing a composite composed of a compound pyroelectric and a metal. And MgB ₂ superconducting wires that can flow a lot of currents because the particles are connected well by fine processing, and the superconducting current flows smoothly, and the reinforcing metal is made of a material with high mechanical strength such as stainless steel. Yield strength of conductors, which is an important parameter in performance It is possible to improve significantly, and to produce a composite superconducting wire having a copper stabilization layer by electroplating a low-cost copper to the wire continuously, resulting in a low-cost production of a superconducting wire having excellent performance.

Description

Manufacturing method of superconducting wire {Fabrication method of MgB2 superconducting wire}

The present invention relates to a method for manufacturing a superconducting wire, and more particularly, a superconducting magnet system of various kinds, such as a magnetic separation device, a single crystal growth device, an MRI, an NMR, attached to a refrigerator and operated by conduction cooling in a temperature range of 20K to 30K. The present invention relates to a method for manufacturing a high-strength superconducting wire which can be applied to a superconducting magnetic levitation train, a superconducting energy storage device, and a high magnetic field superconducting magnet at low cost.

In general, the superconducting wire has a characteristic that the electrical resistance becomes zero below the critical temperature, so that a large current can flow without loss. When used, the superconducting wire is used as a conductor for the superconducting coil so that many superconductors such as transformers, motors, generators, current limiters, etc. It is possible to put the power equipment into practical use, and it will also be used in many energy, transportation, and environmental industries that apply electromagnetic fields such as superconducting power storage devices, superconducting transmission cables, superconducting magnetic levitation trains, and superconducting magnetic separators.

Superconducting wires can be classified according to the critical temperature and the type of material, and are generally classified into low-temperature superconducting wires of metal type and high-temperature superconductors of oxide type. Metal-based low-temperature superconducting wires include alloys and compounds. Nb-Ti superconductors have already been commercialized and used as superconducting coils for MRI and NMR.

On the other hand, Nb ₃ Sn, a typical compound-based superconductor, is used in high magnetic field superconducting magnets or nuclear fusion coils that generate high magnetic fields because the critical magnetic field is higher than that of Nb-Ti. However, all of these superconductors have a critical temperature lower than 20K, so in order to operate a device made of metal-based superconducting wire, most of them are cooled using liquid helium or some cryogenic freezers of 10K or lower are used.

Bismuth system (Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x : Bi-2223, Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x : Bi-2212) and yttrium system (Y1Ba2Cu3Ox) whose critical temperature exceeds liquid nitrogen temperature since 1986 ), Oxide-based superconductors such as thallium (Tl ₁ Sr ₂ Ca ₂ Cu ₃ O _x ) have been discovered.

Among them, Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x is the most pre-wired research among oxide superconductors. It is generally made by Powder-In-Tube method and it is 1km class. Silver sheath wire was developed.

However, bismuth wires have a critical current density of 100,000 A / cm ² or more under liquid nitrogen temperature and magnetic field due to magnetic anisotropy originating from the crystal structure and weak link at the grain boundary. It is difficult to do this, and the higher the operating temperature is, the critical current density decreases significantly with respect to the external magnetic field. Therefore, in order to increase the operating current, a device having a large magnetic field has been proposed to be used at a temperature lower than 25K.

In order for a superconducting device to be practical, performance and economics must be satisfied at the same time. The most important factor in the performance of a superconducting device is the critical current density. This is because the critical temperature and the critical magnetic field are inherent when the superconducting material is found. However, the critical current density varies greatly depending on the manufacturing method.

In general, the value of the critical current density varies greatly depending on which manufacturing method is selected for the superconducting wire. The superconducting wire consisted of superconductor filament and stabilizer. Stabilizers generally have to consider two characteristics. One is that it is easy to process drawing, drawing, etc. when processed in a state of forming a composite with a superconductor due to plastic workability, and it can be said that an alloy or elemental metal is used and excellent mechanical strength is also required. The other is stabilization. Here, stabilization means that the superconducting wire prevents the superconducting state from being destroyed due to internal or external causes of the temperature rise. In general, the superconductor becomes unstable by using metal with low electrical resistance and high thermal conductivity. When the abnormal current cannot flow, it passes the current above the threshold current and transfers the superconductor heat to the surrounding coolant to cool the superconductor's temperature again, restoring the superconducting wire to its original superconducting state, thereby reducing the current. To shed.

The manufacturing method of the superconducting wire will vary depending on the type and state of the superconductor used. In general, in the case of a manufacturing method using a superconductor powder as a raw material, a powder is filled in a stabilizing metal tube to form a billet, and the billet is baked by swaging, drawing, drawing, rolling, or the like. The finished wire is processed by heat treatment.

More specifically, in the case of Bi-2223, which is a typical representative oxide-based superconducting wire, as shown in FIG. 1, Bi-2212 precursor powder 1a is placed in a stabilization tube 2a made of silver or silver alloy. After filling, it is subjected to singing and drawing (3) to reduce the diameter and gradually increase the length. In the case of multicore wires, a hexagonal cross section is made by passing a hexagon die at an appropriate diameter, and a bundle of hexagonal wires is laminated in a large diameter silver (or silver alloy) tube. Thus, the reduced diameter (3) or the laminated billet is plastically processed and finally rolled to increase the density of the superconductor (4). In general, Bi-2223 wire is subjected to a heat treatment after rolling (5), so that the precursor powder (Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x ) is the final superconducting phase (Bi ₂ Sr ₂ Ca ₂ Cu ₃ ) during the heat treatment. As the phase transformation into O _x ) 1b and the plate-shaped particles are oriented, the critical current characteristic is improved.

However, the existing Bi-2223 wire rod manufactured as described above has a problem that it is difficult for the final superconductor tissue to generate a complete single phase because the starting material is multicomponent and has a complicated phase transformation process, and the impurity phase exists to limit the flow of current. There was this.

In addition, the reproducibility of wire characteristics is problematic because there are too many process factors affecting the critical current density in the manufacturing of existing Bi-2223 wires and it is difficult to control it completely. Due to the use, high mechanical strength required for high magnetic field superconducting magnets cannot be satisfied, and wire rod manufacturing cost due to expensive raw material price is the biggest obstacle to practical use (AP Malozemoff et al, IEEE Trans. on Appl.Supercond., Vol. 9, (1999) p.2469).

That is, a summary of the problems of the existing Bi-2223 silver sheath wire and its manufacturing method, which have been suitably developed as a wire for superconducting magnets in the temperature range of 20 to 30 K, are as follows.

Existing Bi-2223 superconducting wire and its manufacturing method are expensive by using silver as a stabilizing sheath material (or coating material), and since silver itself is weak in strength, there is a limit to improving the yield strength of the wire even when alloying. There was. In addition, wire rod filled with precursor Bi-2212 powder should be heat treated for more than 100 hours, and various kinds of impurity phases are produced together with Bi-2223 phase even if the heat treatment temperature deviates slightly from the production temperature of Bi-2223 phase. Significantly lowers. In addition, in the manufacturing process, when the carbon concentration in the raw material powder is high or carbon dioxide in the air is adsorbed to the powder and released into the gas during heat treatment, there is a problem that the critical current characteristic is significantly reduced due to the bubbling phenomenon that causes the silver sheath to swell. (Refer to Ha Hong-hu et al, Proceedings of the Superconducting Magnetic Materials Research Society of Spring 2001, "Bubbling Controlled Heat Treatment of Bi-2223 / Ag High-Temperature Superconducting Wires").

The present invention was created to solve the above conventional problems, the purpose of which is to manufacture a superconducting wire with excellent stability and mechanical strength at low cost compared to the conventional, in particular in the temperature range of 20 ~ 30K It is an object of the present invention to provide a method for manufacturing a superconducting wire, in which a superconducting wire that can be optimally used as an attached conductive cooling superconducting magnet conductor can be manufactured using MgB ₂ powder and high strength metal.

In January 2001, Akimitsu, Japan, confirmed that an intermetallic compound called MgB ₂ exhibited superconducting properties with zero electrical resistance near 39K temperature (J. Nagamatsu et al, Nature, Vol. 410 (2001). At present, much research has been conducted to understand the synthetic properties and physical properties of this material, and to date, it has been confirmed that the material has no magnetic anisotropy and exhibits superconductivity even with the powder itself. It has been reported that bulk lumps having excellent properties at high temperature and high pressure are produced.

Accordingly, an object of the present invention is to provide a method of manufacturing a new superconducting wire which can solve the above-mentioned problems by using MgB ₂ powder and high strength metal as new superconducting materials recently discovered as described above.

1 is a process diagram illustrating a method of manufacturing Bi-2223, which is a typical representative oxide-based superconducting wire,

2 is a cross-sectional view of an MgB2 superconducting billet (Billet) according to an embodiment of the present invention,

FIG. 3 is a flowchart illustrating a process for manufacturing an MgB2 superconducting wire as the final product of the present invention with the MgB2 superconducting billet, and is a process diagram shown in the longitudinal section of FIG.

4 is a view for explaining the stabilizing material coating method according to the present invention,

5 and 6 are views showing the results of measuring the critical current of the MgB ₂ superconducting wire 50 as a final target manufactured without heat treatment by a four-terminal conduction method.

※ Explanation of code for main part of drawing

20: MgB ₂ Superconducting Billet 21: MgB ₂ Powder

22: stainless steel tube 23: stopper

24: stabilized copper 30: shaft diameter processed wire rod

40: rolled wire 50: final MgB ₂ superconducting wire

61 cold rolling process 62 rolling process

63: electroplating process 71: pay-off spool

72: take-up spool 73: electroplating tank

In order to achieve the above object, a method of manufacturing a superconducting wire according to the present invention includes a first step of filling a superconducting material of MgB ₂ material in a high strength metal tube having a yield strength of 300 MPa or more; A second step of performing cold axis diameter processing to reduce the diameter while maintaining the cross-sectional shape of the resultant of the first step; A third step of rolling the shaft diameter processed wire rod; And a fourth step of plating a metal stabilizer having low electrical resistance and thermal conductivity on the surface of the rolled wire.

The high-strength metal tube is preferably any one of stainless steel, nickel, inconel, and titanium metal tube, the superconducting material of the first step is MgB ₂ powder, magnesium powder and boron powder in an atomic ratio of 1: 2 Consisting of mixed powder, in which the amount of the magnesium powder in the mixed powder is further added to 10% or less from the original amount, or a bar obtained by compressing MgB ₂ powder with hydrostatic pressure, the cross section of the bar After drawing to form a hexagon, it is preferable to selectively use any one of a plurality of rods which have been drawn in the metal tube with a plurality of drawn bars. In addition, the present invention is characterized in that the step of selectively heat-treating the rolled wire rod in the third step in the third step before performing the plating in the fourth step.

As described above, the present invention provides a commercially available superconductor MgB ₂ powder, a powder in which magnesium and boron raw powder are mixed at a ratio of 1: 2, or a rod obtained by processing the powder into rods using hydrostatic pressure technology to a high-strength metal tube. The present invention relates to a method of manufacturing a superconducting wire material of a composite by filling, in which a high strength metal (eg, stainless steel, Inconel high strength alloy steel, Tubes made of titanium or nickel). Since MgB ₂ superconducting bulk is manufactured under high pressure and high pressure, the characteristics are improved. Therefore, MgB ₂ cores are densified through saging or drawing plastic processing using a high-strength metal tube and finally filled by rolling. It further improves the density of MgB ₂ . In addition, the high-strength metal can be used as an external sheath material in the composite processing with the metal powder, but because of its high electrical resistance, it cannot satisfy the characteristics as a stabilizing material. Therefore, in the present invention, after winding the final processed wire to the spool and continuously electroplating copper for stabilization to form a superconducting wire consisting of MgB ₂ superconducting core / reinforcing metal / stabilizer layer MgB excellent mechanical strength and stabilization characteristics _{2 A} superconducting wire was produced.

In general, copper or silver has a good ductility and tends to stretch well when drawing, but the external sheath material is preferentially increased when filling the powder, so that pressure cannot be effectively transmitted to the inner core. On the other hand, according to the present invention when using the high-strength metal that is a reinforcing material for processing as a sheath material for the plastic processing of the MgB ₂ powder or rods filled therein, the pressure can be more efficiently transferred to the superconducting core to make the structure dense. . Since the superconductor particles are well connected by the dense processing, the superconducting current flows smoothly and can flow a lot of currents. The finished wire can be heat treated as needed to make the structure more dense. On the other hand, the reinforcing metal may be said to be unsuitable as a stabilizing material because of its high mechanical strength but high electrical resistance and low thermal conductivity. Therefore, in the present invention, by manufacturing a superconducting wire having a copper stabilization layer by continuously electroplating copper cheaper than silver on the wire, the superconducting magnet which requires high stability in order to lower the specific resistance value of the stabilization characteristics of copper In this case, if necessary, the wire is heat-treated at a temperature lower than the decomposition temperature of MgB ₂ to lower the electrical resistivity value.

Hereinafter, a method of manufacturing a superconducting wire according to a preferred embodiment of the present invention with reference to the accompanying drawings will be described in detail.

2 is a cross-sectional view of an MgB2 superconducting billet (Billet) according to an embodiment of the present invention, Figure 3 is a view illustrating a process for producing a MgB2 superconducting wire as a final product of the present invention having the MgB2 superconducting billet Figure 2 Is a process diagram shown in the longitudinal section.

First, as shown in FIG. 2, MgB ₂ powder 21, which is commercially available as a new superconducting material, is filled into a cylindrical stainless steel tube 22, and the tube is made of a marge 23 made of the same material as the tube 22. Closing both ends of (22) to prepare a MgB ₂ superconducting billet (20). At this time, a rod material obtained by densely compressing the MgB ₂ powder 21 at a hydrostatic pressure may be inserted into the stainless steel tube 22. The tube 22 may be a high-strength metal such as nickel, inconel, or titanium instead of stainless steel. Tube can be used.

Subsequently, the cold shaft diameter processing of the MgB ₂ superconducting billet 20 is performed by a saging and / or drawing process. In the case of such processing, in particular, in the case of a singing process, the powder 21 may be subjected to a vibration impact. Since it can leak out, the MgB ₂ superconducting billet as shown in FIG. 3 is processed by rolling a number of times by a method such as a groove roller and processing both ends of the tube 22 having the march _23. (20) is subjected to cold axis diameter processing 61 to reduce the diameter while maintaining a circular cross section by a method such as saging or drawing to form a wire diameter 30 processed shaft diameter. At this time, since the MgB ₂ (21) can be deteriorated by the heat of processing, it is cooled and processed sufficiently with water every time of processing.

On the other hand, in the case of making a multi-core superconducting wire rod, the MgB ₂ powder 21 is drawn to a hexagonal cross section using a hexagonal die at an arbitrary diameter of a bar material compressed at high density with hydrostatic pressure, and then cut into appropriate lengths. MgB ₂ superconducting billets are made by stacking the rods again in a large diameter metal tube, that is, the stainless steel tube 22, and cold billing diameter 61 is performed in the same manner as above.

Subsequently, as described above, the wire rod 30 having the shaft diameter is rolled (62) to form a wire rod 40 having a tape shape. The tape wire 40 is heat-treated for 1 to 10 hours at 800 ° C. to 900 ° C. in an argon atmosphere, which is an inert gas, to make the MgB ₂ structure more dense, but the heat treatment process produces a superconducting magnet that requires high stability. If it is to be manufactured, it may be selectively performed as necessary. In addition, since the Mg of the MgB ₂ can be decomposed and evaporated during the heat treatment, Mg particles are put together with the argon gas into the heat treatment furnace to maintain the MgB ₂ composition. However, the above heat treatment process may be omitted to simplify the process and reduce cost.

Finally, the tape wire 40 is wound around a pay-off spool 71, as shown in FIG. 4, to coat the stabilizing copper 24 on the surface of the tape wire 40. The copper layer is then passed through the tape wire 40 through an electroplating bath 73 in which copper ions 24 are dissolved while maintaining a constant tension in the take-up spool 72. The electroplating process 63 for coating is completed to complete the MgB ₂ superconducting wire 50 as the final object. In the present invention, the copper 24 is electroplated as a stabilization layer on the surface of the tape wire 40. 63), but it is also possible to coat aluminum with high electrical and thermal conductivity, such as copper 24, by an electroplating process.

5 and 6 show the results of measuring the critical current of the MgB ₂ superconducting wire 50 as a final target manufactured without heat treatment by a four-terminal conduction method, and FIG. 5 shows a critical current under 4.2K temperature and magnetic field. 6 is a graph showing the results of the measurement of the critical current under a temperature of 20 K and a magnetic field, and the criterion of the threshold current Ic is defined as the current value at the point where an electric field of 1 μV / cm appears. As shown in the figure, it can be seen that at a temperature of 4.2K and 20K, respectively, the critical current Ic can carry a relatively large current to 310A and 100A.

Referring to the manufacturing method of the superconducting MgB ₂ superconducting wire according to another embodiment of the present invention.

First, the magnesium powder and boron powder are ball milled by filling the stainless steel container with ceramic or metal beads in an atomic ratio of 1: 2 with a magnesium content of 0 to 10% more. In the ball milling, the relative amount of the ceramic or metal beads relative to the mixed powder of the magnesium powder and the boron powder is preferably 3 to 4 to 1, and the entire mixture of the mixed powder and the ceramic or metal beads is Preferably from 20% to 40% by volume of the vessel.

The precursor powder obtained as a result of the ball milling of the entire mixture is filled into a stainless steel tube, and subjected to shaft diameter processing and rolling in the same manner as in the above-described embodiment of the present invention to form a tape wire. The tape wire is sintered for 1 to 10 hours at 800 ° C to 900 ° C in an inert atmosphere such as argon gas to produce MgB ₂ compound. The resulting tape wire is electroplated with a copper layer in the same manner as in the above-described embodiment of the present invention to produce a final wire.

As described in detail above, according to the method for manufacturing a superconducting wire according to the present invention, the present invention provides a method for processing a new MgB ₂ superconducting wire, thereby providing a brittle intermetallic compound MgB ₂ having no plastic formability at all. The combination creates a method that can be processed into flexible wire rods. That is, according to the present invention, since a high yield strength can be obtained by using a high strength metal tube such as stainless steel as a reinforcing material, it can be usefully used as a large superconducting magnet conductor that takes a lot of electromagnetic force. Because of this, the bubbling phenomenon appearing in the existing silver sheath Bi-2223 superconducting wire is eliminated at the source. In addition, the existing superconducting wire must be heat treated for a long time in the manufacturing process in order to improve the critical current density characteristics, but the present invention also creates the effect of applying a high critical current even without heat treatment. Since there is a copper or aluminum stabilization layer with high thermal conductivity, when the superconducting device in which the superconducting current flows through the conductor is quenched by external disturbance, the stabilizing layer acts as a path through which a conducting current of more than a threshold current flows. Due to the high thermal conductivity of copper or aluminum, the heat generated by the quench is cooled in the surrounding refrigerant to lower the temperature of the superconductor below the critical temperature. There is an effect that can be prevented in advance.

Claims (12)

A first step of filling a superconducting material of MgB ₂ material in a high strength metal tube having a yield strength of 300 MPa or more; A second step of performing cold axis diameter processing to reduce the diameter while maintaining the cross-sectional shape of the resultant of the first step; A third step of rolling the shaft diameter processed wire rod; And And a fourth step of plating a metal stabilizer having low electrical resistance and high thermal conductivity on the surface of the rolled wire. The method of claim 1, The high-strength metal tube is a method of manufacturing a superconducting wire, characterized in that any one of a metal tube made of stainless steel, nickel, Inconel, and titanium. The method of claim 1, The superconducting material of the first step is a method for producing a superconducting wire, characterized in that the MgB ₂ powder. The method of claim 1, The superconducting material of the first step is a method of producing a superconducting wire, characterized in that the powder consisting of magnesium powder and boron powder in an atomic ratio of 1: 2. The method of claim 4, wherein The method of producing a superconducting wire, characterized in that the addition of the amount of the magnesium powder in the mixed powder to 10% or less from the initial amount. The method of claim 5, The mixed powder is placed in a container with ceramic or metal beads and ball milled, wherein the relative amount of the ceramic or metal beads with respect to the mixed powder during the ball milling is 3 to 4 to 1, and the mixed powder and the beads Method for producing a superconducting wire, characterized in that the mixture is added to 20% to 40% by volume of the container. The method of claim 1, The superconducting material of the first step is a method for producing a superconducting wire, characterized in that the bar material is compressed MgB ₂ powder in hydrostatic pressure. The method of claim 7, wherein After drawing the cross-section of the bar to be hexagonal, the method for producing a superconducting wire, characterized in that the plurality of the drawn bar is filled in the metal tube multi-core. The method of claim 1, In the fourth step, the metal stabilizer is one of copper or aluminum, the plating method of producing a superconducting wire, characterized in that using the electroplating method. The method of claim 1, Method for producing a superconducting wire, characterized in that for performing the plating after the heat treatment of the rolled wire. The method of claim 10, The heat treatment is a method for producing a superconducting wire, characterized in that the sintering treatment for 1 to 10 hours at a temperature of 800 ~ 900 ℃ in an inert gas atmosphere. The method of claim 11, Method for producing a superconducting wire, characterized in that to form an atmosphere containing Mg particles in the inert gas.