JPH0365680B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a magnetic shielding device, and particularly to a magnetic shielding device used at low temperatures below room temperature and at extremely low temperatures. With the rapid development of low temperature and cryogenic engineering in recent years, the need for precise measurements below room temperature has increased, and therefore the removal of environmental magnetic noise has become essential. Here, I will give a few examples that require a high degree of magnetic shielding. Magnetic testing of spacecraft structural materials to observe interstellar space, planets such as Venus and Mars, and even the moon's weak magnetic field distribution, geophysics, especially rock magnetism, and SQUID (superconducting quantum Micromagnetic field space has become indispensable for detecting magnetic fields generated from the human body using interferometers. Moreover, these requirements are not only required at room temperature, which is the living space, but are particularly required at temperatures below room temperature. In order to meet the above-mentioned requirements for highly sophisticated magnetic shielding, it is first important to construct the shielding device using a material that has a high magnetic shielding effect at low temperatures below room temperature and at extremely low temperatures. In addition, this material must have good strength and weldability at low temperatures and cryogenic temperatures, and low thermal shrinkage, as well as good structural stability and thermal conductivity. A soft magnetic material is generally used as a magnetic shield material, but in order to meet the above requirements, it is necessary to have a face-centered cubic lattice structure. Materials with a body-centered cubic lattice structure other than the face-centered cubic lattice structure cannot be used below room temperature because of their low-temperature brittleness. Therefore, permalloy (Ni-Fe alloy) is used as a soft magnetic material with a face-centered cubic lattice structure.
is attracting attention in the low-temperature and cryogenic regions. However, in magnetic shielding devices made of permalloy (JIS-PC material, etc.) that has been conventionally used for magnetic shielding, the shielding effect changes significantly with temperature changes. In other words, even if the shielding effect is high near room temperature, the shielding effect decreases significantly as the temperature decreases, or conversely, even though the shielding effect is high in the extremely low temperature region, the shielding effect decreases significantly as the temperature decreases. The disadvantage is that the shielding effect is extremely reduced as the shielding effect increases. Such a magnetic shielding device cannot be used in a wide temperature range because the usable temperature range is narrow. However, a magnetic shielding device that can be used in the entire temperature range below room temperature is eagerly awaited in the field of low temperature and cryogenic engineering. The present invention has been made in response to the above-mentioned need, and an object of the present invention is to provide a magnetic shielding device that has a high shielding effect over the entire temperature range from low temperatures below room temperature to extremely low temperatures. The magnetic shielding device for low temperatures and cryogenic temperatures of the present invention is a cylinder with both ends open made of a material having high magnetic permeability below room temperature, and the inside of the cylinder is a magnetically shielded space filled with a cooling medium. In the magnetic shielding device made of Ni74 to Ni74 by weight, the material has high magnetic permeability below room temperature.
It is characterized in that the residual magnetic field inside the cylinder is reduced by using an alloy consisting of 80% Mo, 0.5 to 4.5% Cu, 0.5 to 6.5% Cu, and the balance Fe and a small amount of impurities. A shielding device made of an alloy outside this composition range has a high shielding effect near room temperature, but is not preferable because the shielding effect decreases significantly as the temperature decreases below room temperature. Also, the more preferable composition range is 23%2Mo+Fe.
+Cu27%. The residual magnetic field inside the shield cylinder filled with a cooling medium and made of an alloy having the above composition is preferably 1 milliGauss (hereinafter mG) or less, more preferably 100 microgauss (hereinafter μG) or less, and even more preferably It is less than 10μG. This is because in the field of low temperature and cryogenic engineering, a micromagnetic field space of several mG or less is required, more preferably 100μG or less, and even more preferably
This is because 10μG or less is required. The alloy constituting the magnetic shielding device for low temperatures and cryogenic temperatures of the present invention has a face-centered cubic lattice structure, and has good strength and weldability at low temperatures and cryogenic temperatures, as well as good structural stability. Further, the thermal contraction is not large and the thermal conductivity does not pose any problem in use. In order to keep the residual magnetic field inside the shield cylinder below 1 mG, it is sufficient to have a single layer of high magnetic permeability material for the cylinder, and it is sufficient to keep the residual magnetic field below 100 μG, even 1 μG, although it depends on the shape of the shield device. In order to achieve the following, two or more layers are preferred. As is clear from the examples below, in order to reduce the residual magnetic field to 1 μG or less, a two-layer structure is sufficient for a shield device made of an alloy with the most preferable composition. It is also possible to aim for Hereinafter, the present invention will be explained based on examples. Example 1 As shown in Figure 1, the outer diameter is 42 mm and the inner diameter is 40 mm.
A cylinder with a length of 200 mm and both ends open was fabricated from a material having the composition shown in Table 1, subjected to an optimal heat treatment in a hydrogen atmosphere, and used to measure the magnetic shielding effect. The shielding effect at this time was evaluated by measuring the residual magnetic field at the center point of the center of the cylinder, that is, at point P in the figure, when 460 mG of geomagnetism was applied from the outside of the case in the direction of the arrow in Figure 1. evaluated. To measure the residual magnetic field, use a Gaussmeter for 100μG or more, and a SQUID for 100μG or less.

[Table] Measured. The results at this time are shown in Table 1. From Table 1, a residual magnetic field of 300μG or less is always stably obtained in the temperature range below room temperature (293〓).
A good shielding effect has been obtained. On the other hand, the comparative material has a good shielding effect at room temperature, but the residual magnetic field becomes extremely large as the temperature decreases, and the temperature range in which it can be used as a shielding device is limited to around room temperature. is inappropriate. Example 2 As shown in Figure 2, the outer diameter is 34 mm and the inner diameter is 32 mm.
A cylinder with a length of 200 mm and both ends open was made from a material with the composition shown in Table 1, and a spacer (phosphor bronze) was placed inside the cylinder of Example 1 made from the same material.
A concentric double-shield cylinder was created by installing the cylinder through the cylinder. This was produced in the same manner as in Example 1. The results are shown in Table 2. Numbers in Table-2 are in Table-1
This is the same as the material number shown in .

[Table] From Table 2, the residual magnetic field is 1 μG or less in all cases, and a micromagnetic field space is easily obtained, making it a space suitable for precision measurements. This space is also suitable for the operation of Josephson devices. Example 3 The cylinders with open ends shown in Examples 1 and 2, respectively, and the cylinders with open ends with an outer diameter of 50 mm, an inner diameter of 48 mm, and a length of 200 mm were prepared using alloy material No. 5 shown in Example 1 and Example 2. These were arranged concentrically with spacers (phosphor bronze) interposed between the layers of each cylinder to create a triple shield cylinder. This was evaluated in the same manner as in Example 1. As a result, it was 1 nG or less in all cases of 4.2K, 77K, 196K, and 293K. Although structures with three or fewer layers are widely used,
If there is an allowable space, the shielding effect will be further improved by using four or more layers. Moreover, as is clear from Examples 1, 2, and 3, each time the residual magnetic field increases, the residual magnetic field decreases by 1/1000. Although the specific dimensions of the cylinder are shown in the above embodiments, the same effect of reducing the residual magnetic field can be obtained even if the dimensions are different. As described above, the present invention comprises a multilayer shield cylinder having one or more layers made of a material that has high magnetic permeability below room temperature, so that it is possible to reduce the residual magnetic field inside the shield cylinder filled with a cooling medium. It is possible.

[Brief explanation of drawings]

Figures 1 and 2 show a cylinder with both ends open, which was used to measure the magnetic shielding effect.

Claims (1)

[Claims] 1. A magnetic shielding device comprising a cylinder made of a material with high magnetic permeability below room temperature and open at both ends, with the inside of the cylinder being a magnetically shielded space filled with a cooling medium. , Ni74~80 by weight is a material with high magnetic permeability below room temperature.
%, Mo 0.5 to 4.5%, Cu 0.5 to 6.5%, the balance being Fe and a small amount of impurities to reduce the residual magnetic field inside the cylinder. shield device. 2 In the above alloy, 23%≦2Mo＋Fe＋Cu≦27
%. The magnetic shielding device for low temperatures and cryogenic temperatures according to claim 1. 3. The magnetic shielding device for low and extremely low temperatures according to claim 1 or 2, wherein the residual magnetic field inside the shield tube filled with the cooling medium is 1 milliGauss or less. 4. The magnetic shielding device for low and extremely low temperatures according to claim 1 or 2, wherein the residual magnetic field inside the shield cylinder filled with the cooling medium is 100 microgauss or less. 5. The magnetic shielding device for low and extremely low temperatures according to claim 4, wherein the residual magnetic field inside the shield cylinder filled with the cooling medium is 10 microgauss or less.