The MuShield Advantage
Selecting Magnetic Shielding Materials
When selecting magnetic shielding materials, there are several parameters which need to be considered. The most critical is determining the strength or flux density of the magnetic field to be shielded. Other factors, such as shield geometry, required attenuation, and mechanical stability are also important. Once the field strength has been determined, either by direct measurement with a gauss meter or by mathematical modeling, the appropriate magnetic shielding alloy can be selected.
The ability to conduct magnetic lines of flux is called permeability, and in a magnetic shield, the degree of permeability is expressed numerically. The standard is free space , which has a permeability value of one. In comparison, MuShield’s magnetic shielding materials range in permeability from 200-350,000. Knowing the permeability value of the magnetic shielding materials you are choosing from is imperative when selecting the proper materials for your magnetic shield.
For most magnetic shielding applications, a high permeability material known throughout the industry as mumetal or HyMu 80 is the magnetic shielding material of choice. These materials meet industry specs ASTM A753 Alloy Type 4, and MIL-N-14411 Composition 1, are the most readily available of all the magnetic shielding alloys and distribute the highest permeability.
For magnetic shielding applications involving strong magnetic fields, (usually over 25 gauss) and requiring a moderate amount of attenuation, a medium permeability alloy known in the magnetic shielding industry as Alloy 49 is used. Alloy 49 conforms to spec ASTM A753 Alloy Type 2, or MIL-N-14411 Composition 3. This material is used for stronger magnetic fields because while its permeability is not as high as mumetal’s, the saturation induction of Alloy 49 is double that of mumetal. When saturation occurs in a magnetic shield, the permeability asymptomatically approaches one, which as mentioned earlier, is the permeability of free space. In other words, the magnetic shielding affect of the material no longer exists.
A common application for the Alloy 49 is in multi stage cylinders. The outside layer is made using Alloy 49 material, while the inside layer is made from mumetal with a minimum of a 1/2″ gap between the two layers. If the mumetal was used on its own, it would simply saturate due to the strong magnetic field. The Alloy 49 is used to dampen the field, allowing the mumetal to absorb the weakened field the magnetic shield was designed to block.
In severe cases, low permeability materials such as low carbon steel or pure ingot iron can be used to prevent saturation. While the materials have low initial permeability, they exhibit a tremendous ability to withstand strong magnetic fields without saturating. Often times, Alloy 49 or Silicon Irno magnetic shielding materials are combined with the mumetal, forming a multistage magnetic shield that can withstand flux densities that exceed 50 gauss.
In some scenarios, there is a need for a magnetic shield that deploys all three types of materials. High permeability mumetal, medium permeability Alloy 49, and low permeability steel are used together to form a magnetic shield that yields high attenuation of high flux magnetic fields. A magnetic shield used in this scenario would have the low permeability material closest to the field, the Alloy 49 material as the center material, and then the mumetal closest to that which is being shielded. Similar to the two-stage design mentioned earlier, the magnetic shields should be insulated from each other with at least a 1/2″ gap.
To wrap this up, the significant parameter to remember is a material’s permeability. Permeability is the materials ability to align magnetically to the applied (ambient) magnetic field. It is expressed a ratio, comparing the materials molecular magnetic alignment caused by the applied magnetic flux field. Some of the materials mentioned earlier can be seen below:
Initial Permeability Ratio
|High Permeability per ASTM A753 Alloy Type 4 (mumetal)||80,000:1|
|Medium Permeability per ASTM A753 Alloy Type 2 (Alloy 49)||20,000:1|
|Low Permeability (Low Carbon Steel, Silicon Iron)||200:1|