EP0398918A1 - Magnetic shield, in particular in devices for biomagnetic examinations, and process for producing it - Google Patents

Abstract

Magnetic screens for biomagnetic analyzes are currently made in the form of compact micrometal and aluminum cells. Helium-cooled superconducting shields, on the other hand, are feasible and cost-effectively exploitable only for cells of relatively small volume. According to the present invention, an enveloping structure (10) of the grid type is formed of a multiplicity of tubes (1) traversed by a coolant and provided with a layer (5) of a superconducting ceramic material at high temperature. In the process for manufacturing a magnetic screen of this type, the complete tubular structure is first produced mechanically, which is then coated, by thermal spraying, with the necessary layers of a high-temperature superconducting material of suitable thickness.

Description

 Magnetic shielding, in particular for devices for bio-magnetic investigations, and methods for their production

The invention relates to a magnetic shield, in particular for devices for biomagnetic examinations. In addition, the invention relates to a method for producing such a magnetic shield.

Magnetic shielding is required for many areas of technology. Particularly in the case of biomagnetic investigations in which the smallest signals are to be measured, there is a requirement to create cells with a relatively large volume shielded. Cabins made of μ-metal and / or aluminum are known for this (see monograph BIOMAGNETISM, Springer Verlag (1981) and 3. Phys. E.: Sei. Instrum. 20 (1987), 151 to 164), the thick-walled and / or are constructed with multiple shells.

Shielding for biomagnetic investigations in particular must ensure interference field shielding, in particular in the frequency range from 1 kHz down to about 0.1 Hz for interference field amplitudes, generally significantly smaller than the earth's field of about 0.5 Oersted. The known ferromagnetic shields, even in combination with eddy current shields, shield with increasing frequency, especially below 100 Hz, worse and worse. A great deal of effort must therefore be expended with such shields in order to be able to separate biomagne signals lying below 10 Hz from interference fields in the environment.

In contrast, superconducting shields are able to shield down to the lowest frequencies. In the form of containers coated with superconducting materials and helium-cooled, they have been known for some time and are also used in sentence. For example, sensitive cryoelectronic circuits are shielded. Such superconducting shields, however, have so far not been economically justifiable for larger areas to be shielded, for example entire rooms of significantly more than 10 m.

The object of the invention is therefore to propose a new magnetic shielding device and a method for its production, with which larger areas as a whole can be shielded with reasonable effort.

The object is achieved in that a grid-like envelope structure is formed from a plurality of tubes through which a coolant flows, and in that the tubes are provided with a layer of a ceramic high-temperature superconductor. Such shielding can be produced comparatively simply with the following process steps:

a) First, the entire pipe construction is created mechanically and

b) subsequently provided with the required layers of a high-temperature superconducting material of suitable thickness by thermal spraying processes.

The invention advantageously uses the new high-temperature superconducting materials (HTSL) with jump temperatures in the area of liquid nitrogen in the field of shielding. Particularly in the case of devices for biomagnetic examinations, in which large examination rooms have to be shielded, considerable savings can be achieved compared to multi-layer μ-metal / aluminum eddy current shieldings. It is advantageous that, when flowing through HTSL-coated pipes, there is a smaller surface area compared to total wall cooling, which for heat radiation or supply. Adequate mechanical stability is nevertheless ensured. Interference fields caused by moving current-carrying or magnetic parts cannot occur with the grid-like envelope structure made of the stable tubes. The HTSL-coated tubes can be used to form superconducting meshes of a predetermined size, which completely shield any change in the magnetic flux caused from outside if the current density falls below the critical one. A largely field-free screen area can thus be achieved in the center of the grid-like envelope structure.

The invention thus provides a technically comparatively simple and material-appropriate solution for a large-volume shielding cabin and the associated manufacturing process. In the latter case, the thermal spraying process can be, for example, a plasma spraying process or a high-temperature flame spraying process, with which closed thick layers 20 .mu.m can be applied in a suitable manner. It has already been proposed to apply HTSL layers to surfaces of almost any design using such spraying methods. However, no casing structure made of continuous tubes is known, in which the high-temperature superconducting layers are both sprayed onto the mechanical construction and also sintered for the formation of the superconducting properties required for the formation, and which can subsequently be cooled with liquid nitrogen in such a way that the high-temperature superconductor does not form any cracks due to the thermal stresses occurring during cooling or heating and is therefore deteriorated or even detached with respect to the current density.

Further details and advantages of the invention result from the following description of the figures of exemplary embodiments with reference to the drawing in conjunction with the patent claims. Show it 1 shows a schematic representation of the principle of a mesh-like envelope structure which essentially forms a spherical envelope,

2 triangular mesh constructed from individual tubes,

3 shows the connection point of cells according to FIG. 2 with pipe fitting pieces and

4 shows the section through a tube coated with a high-temperature superconducting material.

In FIG. 1, a lattice-like envelope structure is denoted by 10, which consists of a large number of individual tubes 1 connected to one another at connecting points 20. A three-dimensional network with polygonal meshes is formed from the tubes 1, for example from quadrilaterals or triangles, which are designated by 25 in FIG. 1. The envelope structure 10 can thus be implemented, for example, as a spherical envelope which surrounds a measuring space.

The grid-like envelope structure 10 is largely closed off. At the bottom there is an enlarged inlet 11 with a distributor ring 12 for liquid nitrogen and at the top an expanded outlet 14 with a gas collecting ring 13. It is essential that no enlarged meshes are allowed to arise through which the magnetic fields to be shielded could penetrate from outside. If necessary, tubular protuberances are made in the almost spherical shielding.

To use such envelope structures as magnetic shielding of a measuring cabin, especially for biomagnetic examinations on the human body, a sufficiently large access must be ensured. Such an access is denoted by 15 in FIG. 1. The same applies here as for the inlet 12 or the outlet Run 14, that no increased magnetic field penetration may be possible by expanding meshes.

In Fig. 2 it can be seen that the tubes 1 can each form specially triangular elementary meshes which complement each other to form a hexagonal network. With such a network, in principle any geometries for a spherical envelope structure, in particular the spherical envelope according to FIG. 1, can be realized. The size of the triangular mesh can be calculated according to the required shielding effect.

The grid-like envelope structure 10 is first constructed mechanically from the tubes 1, which advantageously consist of non-magnetic steel, and welded together. It is important that the pipe sections 1 are connected at the connection points 20 by means of continuous pipe fittings, in such a way that nitrogen can flow through the entire pipe structure in the liquid and possibly also in the gaseous state as a coolant uniformly without bumping, in order to create vibration-related interference fields avoid.

3 shows a pipe fitting piece 30 of this type, which in principle consists of a round distributor with corresponding approaches for accommodating six pipes 1, which ensure that the transition between a pipe 1 and a fitting projection 31 is smooth. After the casing structure has been assembled, the welded transitions on the fittings 30 are smoothed, at least on the outside thereof.

Also with the inlet / distributor ring 11, 12 as well as with the gas collecting ring 13 and outlet 14, smooth and shock-free transitions to the pipes 1 must be ensured.

The entire mechanical structure is with a layer of one Provide high temperature superconductors. Layers based on the four-substance system yttrium-barium-copper oxide (Y-Ba-Cu-0) were tested. This ceramic superconductor has the stoichiometric composition - YBa _o 2Cu3.0-7, -x._ and one

Jump temperature T at approx. 90 K, so that it can be cooled with liquid nitrogen at working temperature T V77 K.

As an alternative to this, the layers of the high-temperature superconducting material (HTSL) can also be based on the currently known five-substance systems such as bismuth strontium calcium copper oxygen (Bi-Sr-Ca-Cu-O) or thallium - Barium-calcium-copper-oxygen (Tl-Ba-Ca-Cu-O) should be built up. These HTSL materials also have step temperatures above the temperature of liquid nitrogen, with step temperatures of 80 K and 110 K in certain Bi-Sr-Ca-Cu-O phases and in certain Tl-Ba-Ca-Cu O phases

Jump temperatures of 105 K and 125 K were found. For better handling in the production of these layers, the bismuth (Bi) or thallium (Tl) can be partially replaced by lead (Pb).

The HTSL coating is applied by means of plasma spraying or high-speed flame spraying. For this purpose, an intermediate layer is first advantageously applied as an adhesive base, for example made of zirconium oxide, at least on the outside of the tubular grid casing, and then the powdery high-temperature superconducting material is sprayed on under suitable boundary conditions. Suitable implementable holders for guiding the coating device can be hung in various positions in the stable tubular grille casing, so that the HTSL coating can be carried out continuously in successive individual steps.

For the intended use of the magnetic shielding, it would only be necessary that the HTSL coating was only on the outside of the pipe grid shell is applied. However, to ensure the stability of the layers, it has proven to be expedient to provide the individual pipes 1 with the HTSL coating all around.

4 shows an HTSL-coated pipe section 1 in section, which - starting from the inside - consists of a steel wall 2 on which a zirconium oxide layer 3 is applied, which carries the HTSL layer 5. There is a further protective layer 6, for example also made of zirconium oxide, on the HTSL layer 5. Instead of zirconium oxide as a primer and / or protective layer, magnesium oxide, yttrium oxide or silver can optionally also be used.

To produce the magnetic shielding, the HTSL layer is applied according to the mechanical structure in the manner described above and, if necessary, coated with zirconium oxide. The desired superconducting properties are then adjusted in a known manner by sintering the HTSL material in an oxygen atmosphere, in particular the critical current density sufficient above 77 K (boiling point of the liquid nitrogen). The latter can, for example, be carried out continuously, piece by piece, by means of high-frequency heating by means of flat coils guided on corresponding devices, or also using a welding gas / oxygen flame from welding torches. The required sintering conditions can be maintained by non-contact temperature measurement, for example using a bolometer, and by an appropriate control.

It is advantageous to use the whole for thermal insulation

Foam construction from the tubes 1 including the connecting piece with polyurethane. Such a foam layer is designated by 8 in FIG. Due to the structure of the continuous from above

A coherent network of superconducting meshes is created for the tubular structure of the tube, which shields field changes from the outside in that each mesh receives the extensive magnetic flux for itself.

If not only field changes, but also the static field existing at the start of superconductivity, e.g. the earth field, are to be shielded, the static field must be temporarily compensated for by auxiliary coils outside the tubular grille cover 10 during the cooling process to 77 K, but only for the time of the cooling process from T> Tt_ _* to the working temperature of about 77 K must exist.

When the static field components are also shielded, continuous currents arise in the grid-like envelope structure, as a result of which the shielding capacity is limited by the critical currents of the mesh. If the critical currents I of the HTSL material are inadequate, an additional auxiliary outer shell made of μ-metal can also be used as an alternative, which is optionally connected to the high-frequency shielding made of aluminum which is generally required. Both envelopes can be attached to the walls of the room, in the interior of which the grid-like envelope structure 10 is constructed.

In further exemplary embodiments, the mesh size of the tubular grid network 10 can be matched to the current density available for the high-temperature superconductor and the HTSL layer thickness, the overall dimensions of the construction and the required shielding conditions within a central shielding region, ie an inner partial volume of the envelope structure. The shielding structure can be accessed via the open mesh. The input mesh can be enlarged so that easy access is made possible, the network in Surrounding this opening - as indicated with reference to FIG. 1 - is preferred tubular.

A magnetic shield is thus created, which is characterized by the combination of the greatest possible mechanical stability with the lowest temperature gradient in the high-temperature superconductor. As a result, the formation of cracks in the HTSL layer 5 during cooling or heating is avoided, which would lead to a degradation in particular of the current density.

Due to the low thermal mass of the entire structure, the cooling times can be kept short, which ensures rapid readiness for use. Overall, this results in a considerable improvement and cost savings compared to the magnetic shields known hitherto.

Claims

Claims 1. Magnetic shielding, in particular for devices for biomagnetic investigations, characterized in that a grid-like envelope structure (10) is formed from a plurality of tubes (1) through which a coolant flows and that the tubes (1) are coated with a layer ( 5) a ceramic high-temperature superconducting material (HTSL) are provided. 2. Magnetic shielding according to claim 1, that the cover structure (10) forms a spherical cover with polygonal meshes (25). 3. Magnetic shielding according to claim 2, d a d u r c h g e k e n n z e i c h n e t that the polygonal meshes are triangles. 4. Magnetic shielding according to claim 2, so that the polygonal meshes are quadrilaterals. 5. Magnetic shielding according to claim 2, so that the polygonal meshes (25) are formed from the tubes (1) with continuous tube fitting intermediate pieces (30) at the connection points (20). 6. Magnetic shielding according to claim 2, characterized in that an expanded inlet or distributor ring (11, 12) and an expanded gas collecting ring (13) and an outlet (14) are present on the ball sleeve (10) below , in which the transitions from the pipe sections (1) to the distributor or collecting ring (12, 13) are bumpless. 7. Magnetic shielding according to claim 1, that the coolant is nitrogen, which is present at the working temperature in the liquid or gaseous state. 8. Magnetic shielding according to claim 5, so that the tubes (1) and the intermediate pieces (30) consist of non-magnetic material, preferably non-magnetizable steel, which is coated with the high-temperature superconducting material (HTSL). 9. Magnetic shielding according to claim 8, so that the pipes (1) and the intermediate pieces (30) made of non-magnetic material, preferably non-magnetizable steel, are provided with an intermediate layer (3) as a primer for the high-temperature superconducting material (HTSL). 10. Magnetic shielding according to claim 8 or 9, d a d u r c h g e k e n n z e i c h n e t that the tubes (1) with the layer of high-temperature superconducting material (HTSL) are provided with a protective layer (6). 11. Magnetic shielding according to claims 9 and 10, that the intermediate layer (3) and / or the protective layer (6) consist of zirconium oxide, magnesium oxide, yttrium oxide or silver. 12. Magnetic shielding according to claim 1, d a d u r c h g e k e n n e e c h n e t that the high-temperature superconducting material (HTSL) on the basis of the four-substance system Yttrium-barium-copper-oxygen (Y-Ba-Cu-0) is constructed and preferably has the stoichiometric composition YBa _j Cu ^ O-. 13. Magnetic shielding according to claim 1, characterized in that the high temperature supra¬ conductive material (HTSL) on the basis of five-substance systems, in particular bismuth-strontium-calcium-copper-oxygen f (Bi-Sr-Ca-Cu -0) or thallium-barium-calcium-copper-oxygen (Tl-Ba-Ca-Cu-0) is built up. 14. Magnetic shielding according to claim 13, that the bismuth (Bi) or the thallium (Tl) is partially replaced by lead (Pb). 15. Magnetic shielding according to one of the preceding claims, so that the entire envelope structure (10) made of tubular construction (1) and connecting piece (20, 30) is foamed with polyurethane for thermal insulation. 16. A method for producing a magnetic shielding according to claim 1 or one of claims 2 to 15, the following process steps a) first, the entire tube construction is mechanically created and b) subsequently provided with the required layers of a high-temperature superconducting material (HTSL) of suitable thickness by thermal spraying processes. 17. The method according to claim 16, d a d u r c h g e¬ k e n n z e i c h n e t that a plasma spraying method is used in process step b). 18. The method according to claim 16, dadurchge¬ indicates that in step b) High speed fla spraying process (so-called hypersonic process) is used. 19. The method according to claim 16, so that the HTSL layers applied to the pipe structure are sintered to adjust the superconducting properties following the coating. 20. The method according to claim 19, so that the sintering of the pipe construction is carried out successively piece by piece. 21. The method according to claim 20, so that the sintering is carried out by high-frequency heating, for example by means of flat coils guided on corresponding devices. 22. The method according to claim 19, d a d u r c h g e¬ k e n n z e i c h n e t that the sintering is carried out with a fuel gas / oxygen flame. 23. The method according to claim 19, so that contactless measurement methods, for example a bolometric temperature determination, are used for the measurement and / or control in order to ensure the required sintering conditions, during the sintering.