Panel for a magnetic shielding cabin, magnetic shielding cabin and method for the production of a panel and a magnetic shielding cabin

Abstract

A panel for a shielding cabin having a base plate made of a non-magnetic material and at least one sheet layer made of a soft magnetic material is provided. The base plate is stuck to at least one sheet layer by a viscoelastic adhesive. The adhesive has a glass transition temperature of −80° C. to −60° C.

Claims

1. A panel for a shielding cabin comprising: a base plate made of a non-magnetic material and at least one sheet layer made of a soft magnetic material, the base plate being connected to the at least one sheet layer by a viscoelastic adhesive, the adhesive having a glass transition temperature of −80° C. to −60° C.

2. A panel according to claim 1, wherein the adhesive has a Shore A hardness according to DIN 53505 within a range of 30 to 60 and a tensile shear strength according to DIN 53504 of at least 0.3 N/mm.sup.2.

3. A panel according to claim 1, wherein the adhesive has a Shore A hardness according to DIN 53505 within a range of 35 to 45 and a tensile shear strength according to DIN 53504 of at least 0.9 N/mm.sup.2.

4. A panel according to claim 1, wherein the adhesive is a silane-modified polymer (SMP) adhesive.

5. A panel according to claim 1, wherein the adhesive is a polyurethane adhesive.

6. A panel according to claim 1, wherein the base plate is made of an oscillation-damping material.

7. A panel according to claim 6, wherein the base plate is made of medium-density fibreboard (MDF).

8. A panel according to claim 1, wherein the base plate is not made of aluminium.

9. A panel according to claim 1, wherein the soft magnetic material comprises a nickel-iron alloy containing 70 to 82 wt % nickel, 0 to 11 wt % copper, 0 to 6.5 wt % molybdenum, 0 to 6 wt % chromium and the rest iron, the minimum iron content being 9 wt %.

10. A panel according to claim 9, wherein the nickel-iron alloy contains 76 to 78 wt % nickel, 4.0 to 5.0 wt % copper, 2.8 to 3.8 wt % molybdenum and at least 13 wt % iron.

11. A panel according to claim 9, wherein the nickel-iron alloy contains 79 to 81 wt % nickel, 4.6 to 5.6 wt % molybdenum and at least 13 wt % iron.

12. A panel according to claim 9, wherein the nickel-iron alloy contains 80 to 82 wt % nickel, 5.5 to 6.5 wt % molybdenum and at least 10 wt % iron.

13. A panel according to claim 1, wherein the sheet layer comprises a plurality of sheets arranged side by side on the base plate.

14. A panel according to claim 13, wherein the panel comprises at least two sheet layers arranged one on top of the other, wherein the sheet layers each comprise a plurality of sheets arranged side by side and the plurality of sheets in adjacent layers extend crosswise in relation to one another and are attached to one another by means of the adhesive.

15. A panel according to claim 1, wherein the panel has one linear dimension greater than 0.8 m.

16. A magnetic shielding cabin according to claim 15, wherein the magnetic shielding cabin comprises a plurality of shielding shells, at least one of these shells having panels according to claim 1.

17. A magnetic shielding cabin comprising panels according to claim 1.

18. A magnetic shielding cabin according to claim 17, wherein at least one of the panels has an effective permeability of greater than 20,000 after installation in the shielding cabin when shielding a magnetic flow density of 1.4±0.5 μT RMS.

19. A magnetic shielding cabin according to claim 17, wherein the mechanical vibration of at least one of the panels when installed has a maximum acceleration amplitude of 0.3 ms.sup.−2 RMS within a frequency range of 0 to 500 Hz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments are explained in greater detail below with reference to the drawings.

(2) FIG. 1 shows a schematic view of a panel according to the invention.

(3) FIG. 2 shows the set-up for measuring a bending parameter k of a panel.

(4) FIG. 3 shows the relative change in permeability values in two measurement frames made of Mumetall caused by using different adhesives.

(5) FIG. 4 shows the relative change in permeability values of an SMP adhesive as a function of the contact pressure selected.

(6) FIG. 5 shows the oscillation behaviour of the side walls of comparison shells (VS) and test shells according to the invention (TS) according to any embodiment.

(7) FIG. 6 shows results of DSC measurements (10 K/min) of an adhesive according to any embodiment and of a comparison adhesive.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(8) FIG. 1 shows a schematic view of a cross-section of a panel 10 according to any embodiment.

(9) The panel 10 has a base plate 11 made of a non-magnetic material and two sheet layers 12, 13 made of a soft magnetic material. The lower sheet layer 12 is attached to the base plate by a layer 14 of a viscoelastic adhesive, for example a silane-modified polymer adhesive 15. The upper sheet layer is also attached to the lower sheet layer 12 by a layer 16 of silane-modified polymer adhesive 15.

(10) Each of the sheet layers 12, 13 comprises a plurality of sheets or strips 17, 18 arranged side by side, the strips 17, 18 running crosswise in relation to one another. The panel 10 can, however, also comprise one single sheet layer or more than two sheet layers. In panels with three or more sheet layers the strips of adjacent layers run crosswise in relation to one another.

(11) The sheet layers 12, 13 or strips 17, 18 are made of a soft magnetic material such as a nickel-iron alloy like Mumetall, for example. The soft magnetic material has a high permeability that provides magnetic shielding. The base plate 11 is made of medium-density fibreboard (MDF).

(12) According to the invention, a viscoelastic adhesive such as a silane-modified polymer, for example, is used. A silane-modified polymer adhesive 15 is flexible and has a glass transition temperature of between −75° C. and −65° C., a Shore A hardness according to DIN 53505 within a range of 30 to 60 and a tensile shear strength according to DIN 53504 of at least 0.3 N/mm.sup.2. The panel and a shielding cabin constructed of a plurality of such panels have good magnetic shielding and good oscillation damping and are easy to produce.

(13) Silane hybrid adhesives (silane-modified polymers (SMP)) are a class of adhesives with a combination of properties that which are advantageous for sticking shielding wall panels together, for example they adhere to Mumetall surfaces without the use of a primer and are temperature-resistant and UV-stable. In addition, such adhesives are particularly viscoelastic.

(14) These excellent properties distinguish SMP adhesives from PU and silicone adhesives, which also have a high elasticity. SMP adhesives also exhibit a marked viscosity, which has the effect of damping oscillations and shocks. The company Soudal provides SMP adhesive Soudaseal 2K that was found to be particularly suitable for the applications described here. An important parameter for the elasticity of an adhesive is its hardness, i.e. the resistance of the elastomer to the mechanical penetration of another body. As a typical representative of the SMP class, Soudaseal 2K has a Shore A hardness of 40±5 on a scale of 0-100 (DIN 53505) and is therefore relatively soft. At the same time, its adhesion at a specified tensile shear strength of >1.3 MPa (DIN 53504) is exceptionally high and it is therefore suitable for reliably sticking together a plurality of heavy metal sheet layers.

(15) In some applications, a polyurethane adhesive is also suitable for the low-stress adhesion of sheets to a base plate. Due to the portion of amorphous components, however, the softness of this adhesive can be set within a wide range. The higher the amorphous portion, the lower the glass transition temperature. In testing, the PU adhesive Technicoll 8324 1-K-PUR proved both relatively elastic and sufficiently adhesive and its softness was reflected in a glass transition temperature (10 K/min) of approx. −66° C., characteristics similar to those of SMP adhesives. The PU adhesive tested has a Shore A hardness and a tensile shear strength similar to those of the SMP adhesive.

(16) Some polyurethane adhesives require the use of more production steps than SMP adhesives, e.g. the activation of the adhesive and a longer setting time. Adhesive tests showed that the polyurethane adhesive impairs the permeability of the panel more significantly than SMP adhesives at high modulations. Consequently, polyurethane adhesives are less suitable for applications with a requirement for very high magnetic shielding than SMP adhesives.

(17) The base plate can be made of a non-metallic material and also be electrically insulating.

(18) In the test, a base plate made of aluminium proved unsuitable for applications in which it is necessary to avoid resonance frequencies within a range of 0 to 20 Hz as such plates, when they have the dimensions typical of shielding panels (approx. 1.2 m×1.2 m), oscillate resonantly within a range of 0 to 20 Hz. Some magnetic sensors are particularly sensitive in this frequency range. In one embodiment aluminium is therefore excluded as the base plate material.

(19) A panel comprising a base plate made of MDF and sheet layers made of Mumetall sheets was produced using various adhesives and tested. To give the wall panel a sufficient degree of stability and flatness, the bottom sheet layer is stuck to a base plate. The base plate can comprise an oscillation-damping material such as medium-density fibreboard (MDF). A 19 mm thick plate made of medium-density fibreboard (MDF) proved a suitable base plate. In addition to its excellent flatness and rigidity, MDF has strong oscillation-damping properties. The many branched wood fibres in the material have the effect of distributing any energy introduced by mechanical excitation widely throughout the plate and converting it into thermal energy.

(20) Wall panels for a shielding cabin are produced in this manner and measure up to approx. 1.50 m×1.50 m in size. The panel can consist one or more sheet layers of Mumetall, e.g. four to six sheet layers, each measuring 0.5 mm to 0.75 mm thick, for example. Each sheet layer comprises sheet strips measuring approx. 30 cm×150 cm, for example. The sheets in two adjacent layers are rotated by 90° in relation to one another (“attached cross-wise”).

(21) FIG. 2 shows the test set-up. Two measurement frames (external dimension 25 cm) were cut from annealed Mumetall sheets, which were then stuck together in two layers (2×0.75 mm). The permeability of these measurement frames was then examined under varying degrees of bending. The bending put the Mumetall under strain. An important parameter for the quality of shielding plates is permeability at a modulation of H=1 mA/cm, often designated as μ1. Accordingly, the permeability at a modulation of H=4 mA/cm is often designated as μ4.

(22) Tests were carried out to ascertain the loss of permeability in double-layered measurement frames as a result of adhesion alone, i.e. without any macroscopic mechanical bending. The results are shown in FIG. 3. It shows the relative changes in permeability levels μ1 and μ4, resulting from the comparison of values before and after bonding. It plots six different classes of adhesive including a TESA adhesive foil, a flexible silicone adhesive, a flexible polyurethane (PU) adhesive and an SMP adhesive in the form of PATTEX One for All. The use of the flexible PU adhesive also tested and the SMP adhesive, PATTEX One for All, resulted in a lesser worsening in permeability than the silicone adhesive. The SMP adhesive stood out because both μ1 and μ4 remained almost equally high after adhesion. The PU adhesive tested had only a slight adverse effect on μ1 at least.

(23) Various different SMP adhesives were tested. Adhesion was determined by measuring tensile shear strength on test strips. The results are summarised in Table 1. In the test, two sheet strips were stuck together and then pulled apart. The adhesive Soudaseal 2K exhibited the greatest tensile shear strength of all the adhesives tested.

(24) TABLE-US-00001 TABLE 1 Tensile shear strength Adhesive [N/mm.sup.2] Soudal Fix All Flexi 1.07 Soudal Fix All Crystal 1.40 Heller 2H 1.19 PATTEX PL 300 0.30 Soudaseal High Tack 0.70 Soudaseal 2K 1.83

(25) The influence of contact pressure on permeability when sticking together the Mumetall sheets was examined.

(26) FIG. 4 shows the influence of contact pressure on the relative change in permeability after sticking together two-layered measuring frame. It shows permeability levels μ (H=0.5 mA/cm), μ1 and μ4. At a relatively low contact pressure of 0.44 t/m.sup.2 the relative change in permeability is very low at −1% to +2% irrespective of the modulation examined. At an increased contact pressure of 1 t/m.sup.2, approx. 0.1 bar, the changes become more clearly evident. μ4 decreases by 5%, while μ1 improves slightly by 5%.

(27) An even higher contact pressure of 2 t/m.sup.2 leads to a clear reduction in permeability, above all at higher modulations and μ4, even though initial permeability μ1 does not yet fall excessively at this contact pressure. A value of 2 t/m.sup.2 or 0.2 bar is accepted as an upper limit for a reasonable contact pressure.

(28) A small series of 24 panels were produced and made into a closed test shell for the SMP adhesive Soudaseal 2K. The panels comprised four layers of Mumetall sheets with a thickness of 0.75 mm stuck together at low stress and the oscillation-damping MDF base plate with a thickness of 19 mm. The prior art consists of sticking sheets together without an additional base plate. The test shell was set up inside an existing standard double-shelled cabin.

(29) The first series of tests comprised measurements of the shielding factor of the triple-shelled shielding cabin. The shielding factors and permeability of the standard cabin were already known. Using analytical calculations and after comparison with data from FEM simulation it was possible to determine the wall permeability of the test shell to μ≈21,000±1000.

(30) In addition, oscillation measurements were carried out at the walls of both the test shell and the standard cabin. The method used consisted of mounting an acceleration sensor directly on the wall in question in a relatively central position. Low-frequency excitation was effected manually using a rubber hammer. The results of the comparative oscillation measurements are shown in FIG. 5.

(31) The frequency spectrum of the vibrations of the individual walls is plotted from 0 to 500 Hz, the strength of the vibration being expressed using acceleration a. The drawing at the top right of FIG. 5 illustrates the designation of the four side walls of the outer standard cabin VS and the test shell TS respectively. Side wall 2 is the door side of the cabin. The door side 2 has been omitted from the drawing due to the particular construction around the door and the panel dimensions, which are adjusted accordingly. The dimensions of the individual walls and the distances between the wall mountings vary both between VS and TS and within the individual shells. As a result the resonance frequencies of the individual walls differ significantly. However, a comparison of all the side walls of TS (continuous line) to those of VS (dotted line) reveals that the new wall panel exhibited resonances of lesser amplitude due to the damping MDF components. For example, the highest resonance of all the VS walls was a=0.6 m/s.sup.2 RMS, while the maximum resonance of the TS walls was below 0.2 m/s.sup.2 RMS.

(32) An SMP adhesive has a characteristic glass transition that can be determined by dynamic differential calorimetry (DSC) measurement. The temperature of a small adhesive sample is changed continually and the amount of heat absorbed or given off is determined. Attention should be paid to the details of the rate at which the temperature in the DSC measurement changes since the measured transition temperatures shift depending on the rate. The curves shown and the glass transition temperatures relate to a rate of 10 K/min.

(33) It is possible by means of DSC measurements to identify crystallisation processes and glass transition temperatures, for example. These are characteristic of amorphous and crystalline materials. FIG. 6 shows DSC measurements on Soudaseal 2K, an SMP adhesive, on the left and on Elastosil, a silicon-rubber-based comparison adhesive, on the right.

(34) The glass transition of Soudaseal 2K is clearly visible at approx. −71° C. It manifests itself as a local change in the increase in a limited temperature range. In the same temperature range, by contrast, the silicon-based comparison sample records a strong local increase with a peak at approx. −75° C. This increase is explained by a crystallisation process.

(35) Results for different SMP adhesives and other classes of adhesive are summarised in Table 3. All SMP adhesives tested exhibited a glass transition at similar temperatures, on average at −72.61° C. Amongst the comparison adhesives Technicoll 1-K-PUR, the polyurethane adhesive tested, proved the most similar to the SMP adhesives. It, too, exhibited a glass transition, though at a somewhat higher temperature of approx. −66° C. The 3M 583 adhesive foil had a clearly lower glass transition temperature of −119.38° C. The two silicon-rubber-based adhesives tested (including Elastosil, FIG. 6) differed from the other adhesives tested in the nature of the transition. They exhibited a partially crystalline behaviour in the form of a crystallisation peak.

(36) TABLE-US-00002 TABLE 2 Class of Product Type of Temperature adhesive tested transition [° C.] Silyl modified Soudaseal 2K Glass transition −70.66 polymer Silyl modified Soudaseal High Glass transition −74.40 polymer Tack Silyl modified Soudal Fix All Glass transition −71.74 polymer Crystal Silyl modified Soudal Fix All Glass transition −70.98 polymer Flexi Silyl modified Pattex PL300 Glass transition −74.06 polymer Silyl modified Pattex One For Glass transition −73.80 polymer All Polyurethane Technicoll Glass transition −66.01 1-K-PUR Adhesive foil 3M foil 583 Glass transition −119.38 (synth. elastomer with thermoplastic, resin) 1K silicone rubber, Elastosil Crystallisation −74.99 cures at room temperature 1K silicone rubber, DELO-GUM Crystallisation −73.80 cures at room 3597 temperature

(37) To summarise, a panel is provided for a magnetic shielding cabin in which the individual sheets can be stuck together in a stable but flexible and oscillation-damping manner. This prevents the build-up of local stresses and makes the magnetic conductivity less susceptible to sheet undulations. Moreover, the sheet panels are stuck flexibly to a non-metallic, oscillation-damping base plate, thereby increasing their stability and at the same time reducing their susceptibility to vibrations.

KEY TO DRAWINGS

(38) FIG. 3

(39) μ—stuck/μ—before sticking

(40) Capillary adhesive

(41) TESA foil

(42) Epoxide

(43) Silicone

(44) Polyurethane

(45) PATTEX One for All

(46) FIG. 4

(47) μ—Change after sticking (%)

(48) FIG. 6

(49) Soudaseal 2K—silane hybrid polymer

(50) Glass transition

(51) Onset

(52) Midpoint

(53) Elastosil—silicone

(54) Integral Normalised

(55) Onset

(56) Peak height

(57) Extrapolated peak

(58) Peak width