JP4061576B2 - Melt electrodeposition synthesis method of superconducting boride MgB2 - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a superconducting boride MgB ₂ (Tc = 39K) superconducting thin film and a method for producing a superconducting wire.
[0002]
[Prior art]
A superconducting thin film is used for a highly sensitive magnetometer as a superconducting quantum interference device (SQUID). The superconducting wire is used as a superconducting electromagnetic magnet to generate a strong magnetic field. Both are widely used in academic research in addition to medical CT scanning devices.
[0003]
[Problems to be solved by the invention]
Recently, a boride MgB ₂ showing the highest Tc = 39K as an intermetallic compound was discovered, and its application is expected as an alternative to the conventional A15 type intermetallic compound (about TC = 15K) (A15 type: Nb). ₃ ) A name for classifying a crystal structure common to A ₃ B compounds such as ₃ Ge).
[0004]
However, MgB ₂ is difficult to synthesize even a single-phase bulk sample due to the extreme vapor pressure difference between the constituent elements Mg and B. A wire material manufacturing technique essential for practical use of a superconductor or a thin film manufacturing technique necessary for device formation has not been obtained so far.
[0005]
[Means for Solving the Problems]
The present invention provides a method for synthesizing MgB ₂ from a melt into a thin film by an electrodeposition method (electrodeposition method) with a simple apparatus configuration. By optimizing this method, it is possible to grow MgB ₂ with a uniform thickness for any shape of substrate metal. Therefore, an MgB ₂ wire can be produced by plating MgB ₂ on a metal wire previously formed into a linear shape. It is also possible to grow on a flat substrate. That is, it is effective for the production of a system that requires a thin film structure, such as a Josephson element using MgB ₂ . The precipitation shape of MgB ₂ is determined by the shape of the substrate material. If it is on a plate substrate, it becomes a thin film, and if it is a linear substrate, it becomes a wire.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
MgB ₂ is a compound of boron having a very high melting point, as opposed to Mg having a very high vapor pressure. In a series of trials for synthesizing a high vapor pressure material conducted there, the present invention is based on an electrodeposition method. A method for synthesizing MgB ₂ was found.
[0007]
That is, in the present invention, unlike the film forming method using an advanced and expensive apparatus such as vacuum deposition which is currently actively used, an MgB ₂ film can be easily produced with only a few apparatuses for plating. enable.
[0008]
In addition, it is possible to form a film on the substrate surface, which is impossible by vacuum deposition. This means that, for example, an MgB ₂ superconductor magnet can be produced by plating a metal wire that has been formed into a coil shape from the beginning.
[0009]
【Example】
Commercially available reagents MgCl ₂ , KCl and MgB ₂ O ₄ are weighed in a total amount of 2 at a molar ratio of 5: 5: 1 (hereinafter referred to as powder sample). A platinum wire having a diameter of 1 mm is fixed to one end of the alumina boat so as to run over the bottom of the boat. Similarly, a graphite rod was secured to the opposite end of the boat. A platinum wire having a diameter of 0.3 mm (hereinafter referred to as a lead wire) is pressure-bonded to each of the platinum wire and the graphite rod.
[0010]
After filling the boat with the powder sample, it is placed in a horizontal electric furnace equipped with a quartz furnace core tube. Attach the tube to a rubber stopper that plugs one end of the core tube and connect it to the argon gas line. In addition to the exhaust tube, a two-hole insulator is inserted into the rubber plug at the other end of the core tube. Lead wires are routed outside the core tube through insulators and connected to a DC power source. Be careful not to touch the leads together. The platinum wire is bonded to the negative electrode, and the graphite rod is bonded to the positive electrode.
[0011]
The furnace temperature is raised to 600 ° C. while flowing argon gas for about 1 liter per minute. The powder sample becomes a melt. When a 5V DC voltage is applied to the lead wire and energization of several tens of milliamps is confirmed, the lead wire is allowed to stand for several hours. After a few hours, return the furnace temperature to room temperature and remove the boat.
[0012]
Soak the boat in distilled water to remove the flux. After removing the flux, a platinum wire coated with a black material (MgB ₂ ) was obtained. A magnetic susceptibility / temperature curve of MgB ₂ grown on a platinum wire is shown in FIG. The measurement magnetic field is 20 gauss, and shows the respective curves of cooling without magnetic field (ZFC) and cooling in magnetic field (FC).
[0013]
The no-magnetic-field cooling curve shows that the temperature is raised to Tc or higher, cooled without applying a magnetic field after clearing the superconducting state, then heated by applying a magnetic field of 0.002 T, and the magnetization profile obtained during that time Is shown. On the other hand, the cooling curve in the magnetic field shows the magnetization profile obtained when cooling from Tc (0.002 T) or higher with the magnetic field applied. The characteristic of a superconductor is that the curve of cooling without magnetic field and the curve of cooling in magnetic field are greatly different as shown in the figure.
[0014]
【The invention's effect】
Compared with a large-scale film forming process using a conventional vacuum vapor deposition apparatus, by using the simple apparatus of the present invention, it is possible to produce a MgB ₂ thin film and a wire which are difficult to synthesize at a very low cost.
[0015]
According to the present invention, when an MgB ₂ wire having a high Tc is applied to a superconducting magnet, a superconducting magnet using a refrigerator that does not use liquid He becomes more widespread. In particular, CT scans used in the medical field can be operated at low cost, and a large economic effect is expected such as widespread use in small hospitals.
[0016]
Further, according to the present invention, it is expected that SQUID elements used for MRI (magnetic resonance imaging) can be mass-produced at low cost.
[Brief description of the drawings]
FIG. 1 is a magnetic susceptibility / temperature curve of MgB ₂ grown on a platinum wire, as shown in FIG.
FIG. 2 is a diagram of an apparatus for producing MgB ₂ by the method of the present invention.

Claims (2)

A powder mixture obtained by adding magnesium borate to a mixture of magnesium halide and potassium chloride is filled in a heat-resistant boat, and a negative electrode and a positive electrode are inserted into the mixture, and these are placed in a heating furnace. A superconducting boride MgB _{2 comprising} heating in an inert gas atmosphere to melt the mixture in the boat to form a melt, and then applying a direct current between both electrodes to deposit MgB ₂ on the negative electrode. The melt electrodeposition synthesis method. As an electrode material, a metal electrode of Fe, Co, Ni, Cu, Pd, Ag, Pt, or Au that is stable to the melt, or graphite, highly doped silicon, or tin that is chemically stable to the melt 2. A process according to claim 1, wherein an oxide non-metal is used.

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