JPH06334384A - Composite magnetic shielding material - Google Patents

Abstract

PURPOSE:To decrease a leakage magnetic field from a gap and to contrive to obtain the attenuation factor of a composite magnetic shielding material in the same extent as the case of the attenuation factor of an integrally constituted material even if split superconductors are used for the magnetic shielding material by a method wherein the superconductors split into a plurality of pieces are combined with each other, parts of the superconductors are overlapped each other and a ferromagnetic material is inserted in the gap in the overlap. CONSTITUTION:A magnetic shielding material consisting of a plurality of pieces of cylindrical or square cylindrical body type superconductors 1, which are opened in both ends thereof or are opened in one end are opened in the other, is constituted into a structure wherein parts of the superconductors are overlapped each other in their axial direction and a ferromagnetic material 2 is inserted in a gap in the overlap. It is desirable that the strength of a leakage magnetic field from the overlap is controlled in such a way that it is smaller than that of a leakage magnetic field from the end part of the openings in the superconductors 1. As the superconductors 1, a sintered body or a thick film, a thin film or the like formed on a substrate can be used. As the ferromagnetic material 2, a Permalloy, an Ni-Fe alloy or the like can be used, but it is desirable that the initial relative magnetic permeability muof the material 1 is 1000 to 10000. As the superconductors 1 can be split, a large space can be magnetically shielded by the comparatively small superconductors 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic shield body utilizing the superconducting phenomenon (eliminating magnetic flux), and particularly, the superconductor comprises a sintered body of a high temperature oxide superconductor, a thick film and a thin film. The present invention relates to a composite magnetic shield body in which the strength of the leakage magnetic field from the overlapping portion is smaller than the strength of the leakage magnetic field from the opening end of the magnetic shield even if the magnetic field is partially overlapped with each other in the axial direction.

[0002]

2. Description of the Related Art Recently, studies have been conducted to apply an oxide superconductor to a magnetic shield container by utilizing the superconducting phenomenon of eliminating its magnetic flux. In this case, the superconducting shield does not take an external magnetic field into the superconductor and eliminates it toward the outside to prevent the magnetic field from penetrating into the internal space, and the magnetic shield effect is orders of magnitude greater than that of a ferromagnetic material. That is, the ferromagnetic shield has a limit due to the presence of remanent magnetization, and the magnetic shield in a region exceeding this limit must wait for the superconducting magnetic shield. However, for example, in Nb-based metal superconductors, liquid helium needs to be used as a coolant, so there is a cost barrier in magnetic shield construction, and superconducting shields have been put to practical use except for a small part. The reality is that there is none.

Recently, studies have been activated to measure the magnetic field emitted from the brain to elucidate the mechanism of the brain, elucidate headache, and examine the brain. Conventionally, MRI and positron C
Although the search for the inside of the brain such as T is practiced clinically,
There are restrictions on the resolution, time resolution, spatial resolution, radiation used, and the like. Especially, in order to clarify the function of the brain, it is necessary to measure the EEG and the magnetic field of the brain. The strength of the brain magnetic field is an extremely weak magnetic field of 10 to the 9th power (1/10 ⁹ ) gauss, and the geomagnetism is 0.3 gauss. To detect this signal, SQUID (superconducting quantum interference device) Called a magnetometer and a superconducting magnetic shield are essential.

The present inventors have previously clarified the effectiveness of a magnetic shield for biomagnetism measurement, which is a combination of an oxide high-temperature superconductor and a ferromagnetic magnetic shield (Japanese Patent Application No. Hei 3).
-242134). When measuring the human brain magnetic field, it should be clinically applicable to the target patient without fear. In particular, it is desirable for the clinician to measure the SQUID magnetometer while looking at the patient's face, which results in the need for a large superconducting magnetic shield. For example, the diameter is 2.5 m and the length is 5 m. Although it is technically possible to manufacture such a superconductor integrally, it is too costly and not practical.

Further, if the divided superconductors are combined and magnetically shielded, the magnetic flux leaks from the divided portions and a sufficient magnetic shield effect cannot be obtained. In addition, when trying to obtain an extremely weak magnetic field space such as in biomagnetism measurement,
For example, the ratio of the diameter D to the depth L preferably requires L / D = 2 or more. Further, the superconductor needs to be put in a cryostat in order to be used after being cooled to the liquid nitrogen temperature. For example, even if a superconducting cylindrical container having a diameter of 2.5 m is manufactured, the effective inner diameter is reduced to about 1.5 m in order to cool the container and insulate the container to maintain an internal space at room temperature. Furthermore, if the L / D of this container is 2 or more, a superconducting container having a length of 5 m or more must be manufactured. When firing such a large superconductor, the desired sintering temperature of the oxide superconductor is extremely limited, and thus a large electric furnace capable of accurately controlling the temperature is required. If it is attempted to manufacture the superconductor integrally as described above, the cost is enormous and practically impossible.

The present invention has been made under such a technical background, in which superconductors divided into a plurality of pieces are combined, a part of them is overlapped, and a leakage magnetic field from a gap which is further overlapped is combined. An object of the present invention is to provide a composite magnetic shield body that can be dramatically reduced.

[0007]

SUMMARY OF THE INVENTION A composite magnetic shield body according to the present invention comprises a magnetic shield body composed of a plurality of both-end opening or one-end closing / one-end opening cylindrical or prismatic superconductors in the axial direction thereof. Are partially overlapped with each other, and a ferromagnetic material is inserted in the gap. In addition, it is preferable that the leakage magnetic field strength from the overlapping portion is smaller than the leakage magnetic field strength from the front end portion.

As the superconductor in the present invention, a sintered body, a thick film or a thin film formed on a substrate can be used, and as a ferromagnetic material, permalloy, Ni--Fe alloy, soft magnetic material ( Fe-Cr-Si-Al alloy) or the like can be used, but the initial relative permeability μ of the ferromagnetic material is 1000 to 1
0000 or more is desirable.

In general, even if both ends of the superconducting magnetic shield of one body are opened, when a magnetic field tries to enter the opening, a shield current flows in the circumferential direction of the cylinder to prevent the magnetic field from entering. In this case, a path for a current through which the superconducting current flows can be easily formed in the circumferential direction, but if the container is divided in the circumferential direction, the flow of the superconducting current is blocked, so that the shielding effect is significantly reduced. From such a viewpoint, in the present invention, a plurality of containers which are not divided in the circumferential direction are partially overlapped in the axial direction, and the ferromagnetic material is arranged in the gap. The present invention as described above has been discovered as a result of various studies on various combinations of the superconductor magnetic shield and the ferromagnetic magnetic shield.

The shielding mechanism by the composite magnetic shield body of the present invention constructed as above will be described for better understanding of the present invention. Therefore, the following description does not limit the scope of the invention. Now, when considering the two components of the external magnetic field strength in the axial direction and the radial direction of the cylinder or the rectangular cylinder, the leakage magnetic field strength from the overlapping part can be considerably attenuated only by the superconductor magnetic shield. As for the radial component, it is presumed that the ferromagnetic material inserted in the gap can significantly reduce the radial component. In this way, the leakage magnetic field strength from the superposed portion can be attenuated to 1,000,000th or less of the external magnetic field strength.

FIG. 1 shows the superposition of superconductors and the relationship between the gaps. FIGS. 2 to 6 show an example of a composite magnetic shield using two or three superconductors. An example of superposition in the case of forming a shield body is shown. Further, FIG. 7 shows a shield effect S of the leakage magnetic field of the superposition part by computer simulation using the finite element method, where h is the superposition length of the superconductor shown in FIG. 1 and x is the gap. It shows the result of calculating the relationship between'and h / x. S'is a shield effect defined by the ratio of the external magnetic field strength to the internal magnetic field strength of the superposed portion, and S '(axial direction) and S'when the external magnetic field is applied from the axial direction and the radial direction of the cylindrical container, respectively. '(Radial direction). The shield effect is the P shown in FIG. 8 and FIG. 9 showing the setting position of the magnetic field strength for the calculation of the magnetic shield effect.
It was determined from the magnetic field strength at each point of (1, 2, 3) and Q (1, 2, 3). For example, S ′ (axial direction) = external magnetic field strength / internal magnetic field strength. x is usually sufficiently smaller than the inner diameter of the superconducting cylinder (square tube) (for example, the diagonal length in the case of a square tube), and is, for example, 1/10 to 1/20 or less.
For example, for a cylindrical container with a diameter of 10 cm, x is 5 mm
It is a degree.

In FIG. 8, the inner diameter of the both-end open container is Di, and the inner diameter of the one-end open / one-end closed container is Do. From Figure 7,
When a ferromagnetic material is inserted in the overlapping portion (Fig. 8), S'when a uniform magnetic field is applied from the radial direction of the cylindrical container.
It can be seen that (radial direction) is remarkably improved as compared with the case where there is no ferromagnetic material (FIG. 9). Moreover, from FIG. 7, h /
Regarding x, for example, consider a case where a one-end open / one-end closed container having L / D of about 1 and a both-end open container are combined to form a composite magnetic shield body of L / D = 2. The leakage magnetic field strength from the opening end at the divided portion (L / D = 1 position) is attenuated by about 1/1000 in the axial direction, and the leakage magnetic field strength from the divided portion may be reduced to less than this. , Fig. 7
Then, h / x ≧ 3 (S ′ ≧ 10 ³ ) is selected. Similarly, when three containers are combined in the same manner, the attenuation of the leakage magnetic field strength at the position L / D = 2 from the opening end is about 1/10 ⁶ in the axial direction, and h / x ≧ 4 is selected. To do. In this way, h / x can be appropriately selected so that the leakage magnetic field strength from the divided portion is smaller than the leakage magnetic field strength from the opening end portion. The thickness t of the ferromagnetic material is t ≦ x, and the smaller x is, the more the leakage magnetic field can be prevented.
mm is about mm, and h is about several tens of mm to 100 mm. A material having a large initial relative permeability μ of the ferromagnetic material is preferable, and for example, permalloy (μ to 80000), Fe-
A Ni alloy (μ to 10,000) or the like can be selected. Since the superconductor is cooled to the liquid nitrogen temperature, the ferromagnetic material is also used at the liquid nitrogen temperature. Therefore, μ is slightly lower than the value at room temperature, but there is no problem at all. The interposition of the ferromagnetic material is preferably most effective when it is placed in the center of the gap between the superconductors.

As shown in FIGS. 2 to 6, there are several methods for superposing superconductors, but the same shielding effect can be obtained in any combination as long as the ratio of h and x is satisfied. However, the present invention is not limited to FIGS.
According to the model calculation, for example, a superconducting magnetic shield container 1 having a diameter of 20 cm and a container 1 having both ends having a diameter of 20 cm are combined as shown in FIG. 2, a permalloy having a thickness of 1 mm is used as the ferromagnetic material 2, and h / x = 10. Then, the leakage magnetic field strength from the divided parts is attenuated to less than one millionth of the external magnetic field strength,
The attenuation rate at the bottom of the container that combines two superconductors is about 100.
It is 1 / 10,000, and it is possible to obtain exactly the same attenuation factor as that of the one-piece superconducting magnetic shield container.

According to the composite magnetic shield of the present invention, a space having a very weak magnetic field, such as a magnetic shield room using a superconducting magnetic shield, can be created. For example, it is possible to fabricate a magnetically shielded chamber by producing three rectangular tube shield bodies each having a side length of 2 m and a height of 2 m and combining them with the present invention. If this is applied to biomagnetism measurement, the patient and the doctor can simultaneously enter the superconducting magnetic shield room and perform measurement in a magnetic field environment with an attenuation rate of 1 / 1,000,000 and extremely low noise.

The present invention is particularly suitable for biomagnetic measurement, but is not limited to this and can be applied to various fields such as physical experiments and device experiments requiring magnetic shielding.

[0016]

According to the present invention as described above, the following effects can be obtained. (1) Even if the divided superconductors are used, it is possible to obtain the same degree of attenuation as in the case of the integrated body. (2) Since the superconductor can be divided, it is possible to magnetically shield a large space with a relatively small superconductor. Especially, when a large space such as a magnetically shielded room is to be secured by using a superconductor, the manufacturing cost can be greatly reduced. (3) The amount of the ferromagnetic substance to be inserted is small, and it is not necessary to use a large amount of expensive ferocious substance and thus the cost is low.

[0017]

Example 1 Bi: Pb: Sr: Ca: Cu = 1.8
Using an oxide superconductor powder having a composition of 4: 0.34: 1.91: 2.06: 3.06, an inner diameter of 10 cm and a length of 25
cm, thickness 5 mm, one-end opening / one-end closing type container, and inner diameter 12 cm, depth 25 cm, thickness 5 mm both-end opening type container and inner diameter 10 cm, depth 40 cm, thickness 8 mm one-side opening / one-end closing type container The container was molded by cold isostatic pressing and fired to produce a superconducting magnetic shield container.
The critical temperature Tc of this superconductor was 105K. As shown in FIG. 2, the two containers were combined, the overlapping portion h was 3 cm and 5 cm, and the shielding effect was measured under each overlapping condition. The gap x between the superconductors is 5 mm and h / x = 6 and 10. As a ferromagnetic material, permalloy having a length h ′ = 5 cm, a thickness t = 1 mm and a thickness of 2 mm (initial relative permeability μ = 5000 at room temperature)
The ring of 0) was used, and it inserted in the center of the gap of a superconductor. The superconducting magnetic shield container of the composite magnetic shield body thus assembled and the latter integrated container are cooled to the liquid nitrogen temperature, and the two channels of the axial direction and the radial direction SQU are used.
The magnetic shield effect was measured using the ID. here,
As the external magnetic field, a Helmholtz coil with a diameter of 1 m was used, and a magnetic field of about 0.3 gauss was applied in the axial direction and the radial direction of the cylindrical container. The magnetic shield effect S is defined by the ratio of external magnetic field strength / internal magnetic field strength. The measurement position is depth L / diameter D from the open end
The ratio was about 2 in the center of the container. In the magnetic shield container of one body, Sa (axial direction) was about 10 ⁶ and Sr (radial direction) was 1.3 × 10 ³ .

Table 1 shows measured values of the shield effect of the composite magnetic shield body in which the divided bodies are assembled. As can be seen from this, when the composite magnetic shield body of the present invention is used, a very good magnetic shield effect can be obtained.

[0019]

Example 2 The composition ratio of superconductivity is Bi: Pb: Sr: C.
a: Cu = 1.5: 0.5: 1.9: 1.9: 2.8 using powder, inner diameter 10 cm, length 40 cm, thickness 1.
One 5 mm one-sided opening / one-sided closed Ni cylindrical pipe, two 10 mm inner diameter, 13 cm long, 1.5 mm thick Ni cylindrical pipe (one is a bottomed one-sided opening / one-sided closed container)
And an inner diameter of 9 cm, a length of 16 cm, and a thickness of 1.5 mm.
g was 50 μm, and a superconducting layer was plasma-sprayed thereon with a thickness of 500 μm to prepare a thick film superconducting magnetic shield container. The Tc of the thick film was 106K. These latter three containers were combined as shown in FIG. 5, and the length h of each overlap was 3 cm. The gap x between the superconducting containers is Ni
Excluding the thickness of the substrate and thick film, it is about 3 mm. As a result. Tested at h / x = 10. As a ferromagnetic material, in the same manner as in Example 1, thickness t = 1 mm and 2 mm, length 3c
A permalloy ring of m was used to insert in the middle of the gap. The magnetic shield effect was measured in the same manner as in Example 1, and the measurement position was at the center of the container with L / D = 3 from the opening end. The one open end / end closing type integral of the container, S (axial direction) at this position is about 8 × 10 ^9, S (radial direction) is 5 × 10 ^4. Table 2 shows the measured values of the shield effect. It can be seen from Table 1 that the magnetic shield effect is about the same as that of the integrated body as in the first embodiment.

[0020]

[Table 2]

[0021]

COMPARATIVE EXAMPLE 1 Two superconducting magnetic shield containers identical to those of Example 1 were used, and when superconductors were not superposed on each other as shown in FIG. 10, the same procedure as in Example 1 was performed as shown in FIG. The same experiment as in Example 1 was carried out when the superconductors were superposed on each other by 5 cm and no ferromagnetic material was inserted in the gap. The results are shown in Table 3. From Table 3, it can be seen that in any case, the magnetic shield effect is remarkably reduced as compared with Example 1.

[0022]

[Table 3]

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing a relationship between an overlapping portion and a gap in a composite magnetic shield body of the present invention.

FIG. 2 is a schematic explanatory view showing an example of a cylindrical or square tube composite magnetic shield body according to the present invention.

FIG. 3 is a schematic explanatory view showing another example of a cylindrical or square tube composite magnetic shield body according to the present invention.

FIG. 4 is a schematic explanatory view showing still another example of the cylindrical or rectangular tube composite magnetic shield body according to the present invention.

FIG. 5 is a schematic explanatory view showing still another example of a cylindrical or square tube composite magnetic shield body according to the present invention.

FIG. 6 is a schematic explanatory view showing still another example of the cylindrical or rectangular tube composite magnetic shield body according to the present invention.

FIG. 7 is a relationship diagram between a shield effect S ′ of a leakage magnetic field and h / x.

FIG. 8 is an explanatory diagram showing setting positions of magnetic field strength for calculation of a magnetic shield effect.

FIG. 9 is an explanatory diagram showing setting positions of magnetic field strength for calculation of a magnetic shield effect.

10 is an explanatory diagram of a superposed portion of the magnetic shield body of Comparative Example 1. FIG.

11 is an explanatory diagram of a superposed portion of a magnetic shield body of Comparative Example 2. FIG.

[Explanation of symbols]

 1 Superconductor 2 Ferromagnet

[Table 1]

Claims (2)

[Claims] 1. A ferromagnetic material is placed in a gap formed by superposing a plurality of magnetic shields made of a superconductor of a cylindrical type or a rectangular tubular type having both ends open or one end closed / one end open in the axial direction. A composite magnetic shield body that is inserted. 2. The leakage magnetic field strength from the overlapping portion is
The composite magnetic shield body according to claim 1, wherein the magnetic field strength is smaller than the leakage magnetic field strength from the front end portion.