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 mu metal 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.

This is also known as the shielding factor (S) and is a ratio of the magnetic field strength outside of the magnetic shield (Ha) and the resultant field on the inside of the shield ie Ha/Hi (no units) or S = 20 x log(Ha/Hi) (Db). There are various formula based on the permeability of the material, the shape and size of the shield and the material thickness. In most cases these formulae are only approximate.

For a closed shielding can :

S = 4/3 X (Mu x d/D) where Mu : The permeability(relative)

d : material thickness

D : Shielding Diameter

For a long hollow cylinder in a magnetic transverse field :

S = Mu x d/D

For a cubic shielding box :

S = 4/5 X (Mu x d/a)

a : box side length.

In the case of multiple layer shields with air gaps provided by insulating spacers the shielding factors of the individual shields are multiplied together resulting in excellent shielding factors.

For a double layer shield :

S= S1 x ((S2 x (2 x change in diameter /diameter) )

Mumetal

The three alloys concerned have the following approximate compositions and well known trade names:-

36%NiFe widely known as Supra 36, Nilomag36, Radiometal 36, Magnifer36
50%NiFe widely known as Supra 50, Nilomag 50, Radiometal 50, Magnifer 50
80%NiFe 5%Mo widely known as Mumetal, Magnifer

For shielding applications it is possible to use a combination of 2 or more layers of these alloys to arrive at the correct level of shielding without the shield becoming saturated.

For example; A combination of mu metal and a 50%NiFe is frequently used.

The 50% material has a much higher saturation level and can reduce the effect of any magnetic field substantially, leaving a second layer of mu-metal to further reduce the effect on the shielded item because of its much higher permeability without becoming saturated. This is due to the fact that the 50% material has already reduced the magnetic field considerably before it reaches the mumetal layer.

Relative properties of these three materials are shown below:-

                            Max Permeability                Saturation Flux Density(Tesla)

Mumetal                   400000                                                0.76

50% NIFe                  100000                                                 1.60

36%NiFe*                   25000                                                  1.30

*36% material is used mainly for low cost shieldings

Density: 8.7 [g/cm^3]
Youngs Modulus: 225 [GPa]
Poissons Ratio: 0.29
Yield Strength: 280 [MPa]
Ultimate Tensile Strength: 700 [MPa]
Thermal Conductivity: 19 [W/(m*K)]
Linear Expansion: 1.2 [10^-5m/m/C]
Specific Heat: 460 [J/(kg*K)]
Melting Point: 1440 [?C]
Resistivity: 55 [µ? cm]
Formability: Good
Weldability: Good

Material Characteristics: High initial permeability and maximum permeability with minimum hysteresis losses

Available forms: Sheet (mu metal strip and mu metal foil) bar, tube, wire

This heat treatment increases the magnetic permeability by about 40 times. The annealing process increases the mumetal grain size and hence the magnetic domains. However these enlarged grains are susceptible to severe shocks or further mechanical working and the permeability will decrease in these areas. This can be rectified by a re-anneal
The high permeability of mumetal provides a low reluctance path for magnetic flux leading to its use in magnetic shields against DC and AC fields.

Typical Final Annealed Properties
Hardness : 120 Hv
Tensile Strength : 530 MPa
0.2 % Yield Strength : 160 MPa
Elongation : 45 %
Typical Magnetic Properties(DC) after final heat treatment
Permeability Mumax : 350000-500000
Saturation Induction : 0.76 T
Coercive Force : 0.6 A/m

These high levels of mumetal permeability compare to several thousand for ordinary steel .

If space is at a premium, as well as being more practical for mechanical reasons then a flat sided box fabricated from sheet metal will be the next best option. The corners need to have a large bend radius to minimise flux leakage.

The least favourable shield shape is a flat mu-metal sheet because a flat sheet only covers a portion of the flux path.
In terms of shielding size, the smaller the shield radius the better it will be as a magnetic shield.

It is necessary to ensure magnetic continuity whenever a shield is produced from a number of mumetal pieces, i.e. with lids, overlapping seams. Continuity can be achieved mechanically using friction or via welding.

If the magnetic shield needs holes then the size of the holes should be chosen with care. As a rule magnetic fields can travel into any opening a distance of two times the hole diameter. Shielding tubes can be used to protect holes with large diameters.

There are various types of ways and materials used in Magnetic Shielding depending on the frequency and strengths of the magnetic field.

In order to explain for this instance we will use passive low frequency Magnetic Shielding as an example. Shielding of this nature is commonly made from Mu Metal (a specialty, high nickel content alloy) and is fashioned into a sheet metal component or assembly before applying a heat treatment process to maximise the shielding factor of the material.

It is natural to assume that a Magnetic Shield does exactly what it say’s and acts as a shield deterring the magnetic field away from the protected area. In essence a Magnetic Shield does exactly the opposite, acting as a “sponge” drawing the magnetic field in. It is here that the clever bit happens – the magnetic field is now directed along the path of the shield, bypassing the protected area inside. This allows the field to circle around our protected area before continuing its journey once it leaves the opposing side of the shield.

Providing the magnetic field is lower than the saturation of mu-metal (0.76 Tesla) the protected area remains free of spurious magnetic field. Once this level of saturation is reached and exceeded, leakage of the field into the protected area will once again occur.