JP4769301B2 - Pulse magnetization of bulk superconductors - Google Patents

Description

  The present invention relates to a pulse magnetization method of a bulk superconductor for magnetizing a bulk superconductor to make a permanent magnet.

  The discovery of high-temperature superconductors has pioneered application fields in the form of new materials called bulk superconductors. Conventional metal superconductors have been limited to applications at cryogenic temperatures near absolute zero. In other words, conventional metal superconductors have a material with too low specific heat and cannot be applied in bulk. Applications have been realized only in the form of wires or thin films. In contrast, high temperature superconductors have a high operating temperature of about the boiling point of liquid nitrogen and the specific heat is remarkably high, so that the instability of the superconductor does not occur so much and the superconductor can be applied in bulk. It was. What is most expected is an application field in which magnetizing is performed in place of a permanent magnet. A normal permanent magnet can generate a magnetic flux density of several hundreds mT (Tesla) (thousands of G (Gauss)) at most, but a bulk superconductor has succeeded in generating a magnetic flux density as high as 17T.

For example, in the following Non-Patent Document 1, a high-temperature bulk superconductor has been successfully generated with a magnetic flux density of 17 T at 29K. This magnetic flux density is achieved by a technique called cooling in a magnetic field. That is, the bulk superconductor is set to a temperature higher than the critical temperature, and a high magnetic flux density is given from the outside using a superconducting magnet. In this state, the bulk superconductor is cooled (cooled in a magnetic field) to be in a superconducting state so that the magnetic flux density is maintained. However, this method has a drawback that a large superconducting magnet is always required for magnetization. Therefore, a simpler magnetization method is desired.
M.Tomita and M.Murakami, Nature, vol. 421, pp. 517-520 (2003)

As a method for easily generating a magnetic flux density in a bulk superconductor, pulse magnetization has been attracting attention (see, for example, Patent Document 1). This method is a method in which a pulse current is passed through a copper coil near the bulk superconductor to magnetize it. Although this method has the advantage that it can be magnetized by a simple device compared to a superconducting magnet, the magnitude of the magnetic flux density to be magnetized is about several T, which is lower than cooling in a magnetic field. . The cause of the small trapped magnetic flux density is that when the magnetic flux density is magnetized from the outside to the superconductor, the movement of the magnetic flux lines is fast because it is pulsed, and the heat generated by the movement of the magnetic flux lines is large. This is because the temperature increases for a moment, the force for supplementing the magnetic flux is weakened, and the magnetic flux once entering the interior cannot be captured with the decrease of the pulse magnetic field and escapes to the outside. That is, a device for suppressing heat generation is necessary. Therefore, a device for efficiently removing generated heat has been proposed (see Patent Documents 2 and 3). In this method, a thick disk-type bulk superconductor is sandwiched between ring-shaped coppers to release heat, but the achieved magnetic flux density is still insufficient.
Japanese Patent Laid-Open No. 10-154620 JP 2004-319797 A JP 2005-294471 A

  The present invention provides a method for efficiently magnetizing a bulk superconductor by a simple pulse magnetizing method in order to use a bulk superconductor as a permanent magnet having a very large magnetic force, rather than using the bulk superconductor as a conductor. It is intended to provide.

  The invention described in claim 1 (hereinafter referred to as “Claim 1”, etc.) at the time of filing of the present invention is that when the bulk superconductor is magnetized by the pulse magnetization method, the upper part of the bulk superconductor and / or This is a pulse magnetization method of a bulk superconductor characterized in that a copper material is disposed in the lower part. For example, when direct cooling by a refrigerator is used to cool the bulk superconductor, it is often cooled from the top or bottom of the bulk superconductor, so change the shape of the copper material used for the cooling. Can be considered. When cooling from the side, a copper material may be disposed on the upper surface and / or the lower surface of the bulk superconductor.

  Of the present invention, the invention described in claim 2 is the pulse magnetization method of the bulk superconductor according to claim 1, wherein the magnetization is controlled by changing the thickness of the copper material.

  The invention described in claim 3 of the present invention is the pulse magnetization method of a bulk superconductor according to claim 1 or 2, wherein the bulk superconductor is a disk type and the copper material is a disk shape.

  According to a fourth aspect of the present invention, the bulk superconductor includes a powdered rare earth 123 oxide superconductor. The bulk superconductor pulse magnetizing method according to any one of the first to third aspects is provided. is there.

  The invention described in claim 5 of the present invention is the bulk superconductor according to any one of claims 1 to 4 arranged in direct contact with or thermally connected to the bulk superconductor. This is a pulse magnetization method.

  The invention described in claim 6 of the present invention is characterized in that the shape of the copper material is different between the upper and lower surfaces of the bulk superconductor, and the pulse arrival of the bulk superconductor according to any one of claims 1 to 5 Magnetic method.

  According to a seventh aspect of the present invention, the copper material is a copper plate. The bulk superconductor pulse magnetizing method according to any one of the first to sixth aspects.

  Of the present invention, the invention described in claim 8 is the pulse superposition method of the bulk superconductor according to any one of claims 1 to 7, wherein an exciting coil is arranged around the bulk superconductor and the copper material. It is.

According to the present invention, the magnitude of the magnetic flux density magnetized in the bulk superconductor by the pulse magnetization method can be increased.
In the pulse magnetization method according to the method of the present invention, the movement of the magnetic flux lines is controlled, the heat generated by the movement of the magnetic flux lines is suppressed, and the magnetization effect is improved. Further, in the present invention, by changing the thickness of the copper material, the movement of the magnetic flux lines can be finely controlled, and the effect of magnetization can be improved.

It is a figure which shows the method of pulse magnetization. It is a figure which shows the model in a finite element method. It is a figure which shows the model and calculation example which perform a numerical simulation with a finite element method. It is a figure which shows the model and calculation example which perform a numerical simulation with a finite element method. It is a figure which shows the result of the simulation of the time dependence of the penetration depth of magnetic flux. In this invention, it is a figure which shows the state which has arrange | positioned the copper plate from which thickness differs on the upper surface and lower surface of a bulk superconductor.

Explanation of symbols

1 Copper plate with different thickness at the periphery and center 2 Bulk superconductor

  A bulk superconductor is one of the utilization forms of an oxide superconductor, and means a bulk (bulk) superconductor. In general, when a bulk superconductor is magnetized (made a magnet), it is cooled and magnetized in a superconducting coil that produces a strong magnetic force, but an expensive superconducting coil is required. This is called cooling in a magnetic field as described above. On the other hand, the pulse magnetization method can be performed using a pulse coil using an inexpensive copper wire. For example, in the pulse magnetization method, a bulk superconductor is cooled to around 230 to 250 degrees below freezing point and a strong pulse magnetic field of about 10 milliseconds is applied to cause the bulk superconductor to capture the magnetic flux density (4 to 5 T). Degree). When a strong pulse magnetic field is applied, a strong magnetic flux density instantaneously invades, but the bulk rapidly generates heat. Since the superconducting characteristics of the bulk superconductor deteriorate due to temperature rise, the magnetic flux density escapes and the finally captured magnetic flux density also decreases.

  As described above, since the cause of the low magnetic flux density by the pulse magnetization method is heat generation, the present invention is devised to suppress the heat generation itself. For this purpose, the present invention is characterized in that when the bulk superconductor is magnetized by the pulse magnetization method, a copper material is disposed on the upper and / or lower portions of the bulk superconductor. For example, if pulsed magnetization is performed by placing a copper material such as a copper plate on the upper or lower part of the bulk superconductor or both the upper and lower parts, the movement of the magnetic flux lines is controlled, and the heat generated by the movement of the magnetic flux lines It is suppressed and the effect of magnetization is improved. In the present invention, by changing the thickness of the copper material, it is possible to finely control the movement of the magnetic flux lines and improve the magnetization effect.

  In pulsed magnetization of a bulk superconductor, heat is generated because the movement of magnetic flux lines is too fast, so it is considered necessary to make the movement of magnetic flux lines slow. In order to slow the change of the magnetic flux lines, a large coil and a large capacitor bank may be prepared. However, the magnetizing device becomes too large, and the purpose of easily magnetizing cannot be achieved. Therefore, in the present invention, a copper material, for example, a copper plate, is disposed on the upper or lower part of the bulk superconductor or on both the upper and lower parts. Due to the presence of this copper plate, a shield current flows through the copper plate to prevent the magnetic flux density from increasing due to a sudden change in the pulse magnetic field, and hence the movement of the magnetic flux lines becomes much slower, and heat is generated in the bulk superconductor. Can be suppressed. Of course, the copper plate can also be expected to release heat generated by the bulk superconductor.

  The moving speed of the magnetic flux lines can be controlled by changing the thickness of a copper material, for example, a copper plate. For example, if it is desired to increase the moving speed of the magnetic flux lines near the surface, the shielding effect is reduced by reducing the thickness of the copper plate in the vicinity, and as a result, the moving speed of the magnetic flux lines is increased. Further, if it is desired to reduce the moving speed of the magnetic flux lines near the center of the bulk, the thickness of the copper plate is increased near the center. As a result, the shielding effect by the shielding current is increased, and the magnetic flux lines move slowly. Therefore, although it is disadvantageous for increasing the magnetic flux density, heat generation can be suppressed. Therefore, in the present invention, a method of controlling the magnetization by changing the thickness of the copper material is also preferable.

  In actual magnetization, a method of gradually injecting magnetic flux lines into the bulk superconductor by applying a pulse magnetic field several times is employed. At that time, if heat generation is small by the method of the present invention, it is considered that the magnetic flux density that can be captured is sufficiently high and magnetization can be performed efficiently.

  The bulk superconductor used in the present invention is as follows. That is, a bulk superconductor is produced using RE-123 (RE = rare earth) represented by Y-123 (yttrium 123) produced by a melting method. Specifically, the raw materials are mixed, usually formed into a tablet shape, a seed crystal is placed in the center, and melted at a temperature exceeding 1000 degrees to cause crystal orientation. At present, almost single crystal bulk superconductors having a diameter of 10 cm and a height of 3 cm have been made. Since the bulk superconductor can have a larger cross-sectional area than a thin film or wire, the critical current can be set to several thousand A.

  The finite element method (FEM) used in the embodiment is a widely known method of numerical analysis, and the analysis target region is a relatively simple shape called “finite element”. Is divided into a large number of sub-regions, and an unknown variable to be obtained is approximated by a relatively simple function such as a linear function on each element. Then, on each element, the potential to be obtained is expanded by an approximate function, and the potential is obtained by using the variational principle so that the energy in the analysis target is minimized. This finite element method is a technique often used in electromagnetic field structural analysis (see, for example, Nakata, Takahashi “Finite Element Method of Electrical Engineering” (Morita Publishing 1982) and “Overview of Finite Element Method” (Science 1980). ).

  Using the powdered Y-123 (yttrium 123) oxide superconductor as a raw material, the method of the present invention is verified for a single crystal bulk superconductor produced by a conventionally known melting method. For the verification, a numerical simulation using the finite element method was performed. A pulse magnetic field is applied to a disc-shaped superconductor having a diameter of 100 mm and a thickness of 10 mm from the outside to increase the magnetic flux density by about 2T in 1 second. This is shown in FIG. As shown in FIG. 1, a pulse magnetic field is applied to the superconductor in the direction of the arrow using a pulse magnet.

  The external and internal magnetic flux densities at this time were calculated using the finite element method. Since the calculation is complicated this time, the effect of heat generated by the movement of magnetic flux is ignored. As the pulse magnetic field, a pulse magnetic field of about 100 milliseconds is applied, and a pulse magnetic field is applied in the same manner a plurality of times with a time interval. Some heat generation has a time interval, so the accumulation of temperature rise is negligible. Magnetization is sequentially accumulated until saturation and further penetrates into the interior.

  FIG. 2 shows a model in the finite element method used in the above calculation example. From the symmetry of the disc-type superconductor, and for the purpose of shortening the numerical calculation time of the simulation, one degree (°) was cut out and used only as a model. Of course, the same calculation result as when all (360 °) are modeled can be obtained. As shown in FIG. 2, there is an air layer around the thin bulk superconductor disk, and the whole is surrounded by the air layer. In the case of such a highly symmetric shape, the amount of calculation can be greatly reduced by creating a model by extracting only one degree angle and performing a simulation by the finite element method.

  3 and 4 show a model and a calculation example for performing a numerical simulation by the finite element method. 3 and 4 use a model cut out only once from FIG. The whole is divided into four layers. The model is divided into an inner layer including the bulk superconductor and an outer layer not including the bulk superconductor with respect to the horizontal axis of the disk-type bulk superconductor. The inner layer is divided into three layers with respect to the vertical axis of the bulk superconductor. FIG. 3 is a side view of a model and calculation example in which the top and bottom are air layers. The sandwiched layer is a bulk superconductor. There is one outer layer, which is an air layer. It can be seen that when the external magnetic flux density is increased to 2T, the magnetic flux penetrates inside. That is, the magnetic flux lines indicated by arrows on the surface of the superconductor are concentrated at the boundary between the inner layer and the outer layer and penetrate into the inside. However, the magnetic flux lines do not penetrate to near the center of the bulk superconductor in the horizontal axis direction. The direction of the arrow is the direction of the magnetic flux lines. Here, the movement of magnetic flux can be made slow by making the upper and lower air layers into disk-shaped copper plates. FIG. 4 shows a side view of a model and calculation example using a disk-shaped copper plate.

  In the model, the disk-shaped copper plate is calculated as being in contact with the disk-type bulk superconductor, but it is not necessarily electrically connected. However, in terms of heat, the direct contact or connection between the two is useful for quickly removing the heat generated in the superconductor from the superconductor to the copper plate and keeping the temperature of the superconductor at a low temperature. Therefore, in practice, the copper plate is preferably in direct contact with or connected to the bulk superconductor.

  The bulk superconductor basically works to prevent magnetic flux from entering due to the pinning effect, and the magnetic flux density on the surface of the bulk superconductor increases due to the so-called shape effect. On the other hand, if a Lorentz force exceeding the pinning force is applied, the magnetic flux penetrates into the inside, and conversely, the magnetic flux once entered does not go out to the outside due to the pinning force and is captured. In the case of the air layer, it can be seen that the magnetic flux easily enters the inside. In reality, heat is generated by the movement of the magnetic flux, and when the temperature rises, the pinning force decreases, so that the magnetic flux that has entered the corner cannot be captured along with the decrease in the external magnetic field and comes out.

  3 and 4, the part described as “air” is not a part of the model but an air layer, and the part written as “superconductor” is the model. On the simulation screen, this is the display. The upper and lower layers described as “copper” in FIG. 4 mean that the simulation is performed when the copper plate is arranged on the upper part and / or the lower part. It can be considered that an exciting coil is arranged on the right side of the “air” portion.

  FIG. 5 shows the simulation result of the time dependence of the penetration depth from the surface of the bulk superconductor at the front of the magnetic flux that penetrates. As shown in FIG. 5, when the penetration depth of the front of the magnetic flux from the surface of the bulk superconductor is plotted with time, when there is no copper material, the magnetic flux quickly enters the superconductor as time passes. You can see that it is invading. That is, the movement of the magnetic flux is in a fast state, and heat generation due to the movement of the magnetic flux is unavoidable as it is.

Therefore, when the external magnetic field is changed in the same way by attaching disc-shaped copper plates with electrical conductivity of 1 × 10 ⁹ [1 / Ωm] above and below the bulk superconductor, The penetration depth from the superconductor surface was investigated. The results are also shown in FIG. It can be seen that the movement of the magnetic flux lines is suppressed by attaching the disk-shaped copper plate.

As can be seen from FIG. 5, it can be seen that the penetration of the magnetic flux becomes milder when the disc-shaped copper plate is attached, and the velocity of the magnetic flux is reduced compared to when the disc is not attached. The speed of magnetic flux can be obtained from the slope of the plot. Furthermore, FIG. 5 also shows the result of calculating the electric conductivity 10 times to 1 × 10 ¹⁰ [1 / Ωm]. It can be seen that the penetration of the magnetic flux is more gradual. That is, it can be seen that the movement of the magnetic flux can be controlled by attaching a disk-shaped copper plate. Naturally, if the thickness of the disk-shaped copper plate is changed, the movement of the magnetic flux can be finely controlled. That is, if the thickness is reduced, the speed of the magnetic flux can be increased. The reverse is also possible.

  Although the simulation result by the finite element method is shown here, it can be said that the same effect appears in an actual experiment. This is because until now, many of the superconducting phenomena have been elucidated by a technique called the finite element method. Actually, the present invention utilizes a very basic effect of preventing a change in magnetic flux density due to a pulse magnetic field by a shielding current flowing through the copper material when a pulse magnetic field is applied to the copper material. Therefore, its realization is relatively simple. The simulation results show that the effect of suppressing the movement of the magnetic flux lines is high by increasing the conductivity. This is because the shielding current is more likely to flow, and this can be confirmed quantitatively. is made of. Also, control of the movement of magnetic flux lines using a copper material has been attempted in metal superconductors. Therefore, the simulation result of this time is also valid and is considered to accurately reflect the actual experiment.

  FIG. 6 is a diagram showing an example of the arrangement of the bulk superconductor and the copper plate when the pulse magnetization method of the present invention is specifically implemented. The state which attached the copper plate 1 from which thickness differs by the peripheral part and the center part on the upper and lower surfaces of the disk-type bulk superconductor 2 is shown. The arrow indicates the direction of the magnetic field. Since the copper plate is thin in the peripheral portion, the shielding effect against the movement of the magnetic flux lines is small and the speed of the magnetic flux lines is large. On the contrary, since it is thick in the central portion, the speed of the magnetic flux lines is suppressed and the magnetic flux lines are difficult to enter. Therefore, the influence of heat generation is reduced. If the heat generation is reduced, the magnetic flux lines tend to stay in the superconductor, and as a result, a high magnetic flux density can be maintained. In this way, it is possible to control the speed of the magnetic flux lines. It is also possible to put differently shaped copper plates on the top and bottom surfaces.

  The present invention relates to pulse magnetization in which a bulk superconductor is used as a permanent magnet rather than as a conductor, and a permanent magnet having a very large magnetic force is provided. The obtained bulk superconductor can be used as a strong permanent magnet by cooling to a temperature of about liquid hydrogen. As a bulk superconductor pulse magnetized magnet, it can be miniaturized and simplified, so that it is possible to expect alternative demand and new demand in various directions.

Claims (1)

When magnetizing a bulk superconductor by the pulse magnetization method, the central part is thick and the peripheral part is thin on both sides or one side outside the bulk superconductor in a plane substantially perpendicular to the direction of application of the magnetic field. A pulsed magnetization method for a bulk superconductor characterized by arranging a copper material in a shape .

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