Sep
21
Multiple Layer Shields
Whilst mu metal ® is available in many different gauges and indeed a single layer is suitable for most commercial applications, optimal shielding results are obtained when a shield is constructed of multiple layers of mu metal ®.
In theory each layer should be of a light gauge and the shield constructed of many layers (T. Rikitake – Magnetic and Electromagnetic Shielding 1987) in practice this not always possible from a mechanical point of view and it will be necessary to reach a compromise between number of layers for best shielding factor and structural requirements.
Evidence has shown that a multi layer shield of 4 layers has greatly improved shielding characteristics than one single layer of the same internal and external radii (example: 1 x layer of 10mm as opposed to 4 x layers of 1mm with 3 x 2mm air-gap) Adopting this method not only increases the shielding factor, it also decreases the mass and improves economy of the overall shield.
In common practice a multi layered shield will vary from 2 to 5 layers and can go up to 5mm material thickness. It is also possible to have different permutations of material types depending on the level and type of shielding required;
For instance;
a. Mu Metal ® and 50% or 36% Nickel Iron Alloy – The layer of 50% or 36% alloy will always be used next to the highest source of interference as it has a higher level of saturation (1.3 – 1.6 Tesla as opposed to 0.76 Tesla in mu metal ®) and whilst offering this higher level of saturation the material does still offer a permeability value to enhance shielding characteristics of the overall product.
b. Mu Metal ® and Pure Iron – Again the pure iron will always be used next to the highest source of interference. Pure iron offers improved saturation characteristics against 36% or 50% nickel iron alloys (typically 2.2 Tesla) but only offers a very low permeability value.
c. Mu Metal ® and Aluminium/Copper – In addition to negating a magnetic field it may also be required to stop an electrical field. Therefore the final layer of the shield should made from aluminium or copper depending on the frequency involved.
d. Mu Metal ® and Cryogenic Mu Metal ® - In order to achieve shielding at cryogenic temperatures it would be necessary to have a special layer made of cryogenic mu metal ®.
Cryogenic material achieves approximately half the permeability of mu metal® operating at ambient room temperature; however mu metal ® would only operate at approximately 10% of its normal permeability at cryogenic temperature levels.
It is of course possible to design a multi layer shield with any number of permutations of the afore mentioned materials depending upon applications and requirements.
Spacing between the shield layers is obviously constrained to many different parameters; available space, economics, mass restrictions and in very large shields even transport limitations. However if possible then it is recommended to follow the “Dubbers Theory” (D.Dubbers – Nuclear instruments and Methods in Physics Research A243 1986, 511-517) which presents a formula suitable for calculating inter-layer spacing to achieve the best theoretical results.
Each layer of the shield ideally should be closed* (such as a cylinder with a close fitting lid) and these parts should be manufactured in such a way as to allow optimal contact. All of the cylinders in turn should be isolated from each other using a spacer material suitable for your application.
For instance;
In high temperature applications - high temperature silicon sponge
In low temperature and vacuum applications – BAKELITE™ or TUFNOL ™ (thermosetting phenol formaldehyde resin)
In ambient temperature applications – any carbon free, non-conductive, non degradable material can be used (such as Neoprene, Polyimide, Polyurethane, Mylar or any of the above mentioned materials)
Great care should be taken if an inter-layer support frame is to be used between the shield layers. Any fixing anchored to the support frame to be used as means of securing the mu metal layer should be isolated from the mu metal, this could be achieved by the use of, for example – TUFNOL™ sleeves and washers.
Consideration should also be taken when specifying any inter-layer framework to ensure that the materials used lie within the acceptable parameters of materials known not to have an adverse effect on the shielding efficiency due to iron content.
* (Based on a single cylinder) For a closed lid to improve the shielding performance it is necessary for the magnetic field to be running perpendicular to the shield. If the magnetic field is running parallel to the shield it can have the opposite effect and reduce the shield efficiency.