|MadSci Network: Physics|
Dear Shawn, We would all love a material with a magnetic permeability > 1,000,000. It sounds as though your task is to magnetically shield an object, so you might want to consider creating a multiple layer shield. (Is this what you meant by layering?) Before delving into the multi-layer shield idea, I'll answer your question about metglas. Metglas is an amorphous material, which means it was formed at a high temperature and then cooled rapidly enough so that long range crystal structures couldn't form. This means that large ferromagnetic domains cannot form, which in turn leads to a high permeability in metglas. Annealing would destroy the amorphous structure and allow it to fall back into a crystalline solid with large ferromagnetic domains. These domains would then resist attempts by an outside magnetic field to reorient themselves, and the permeability would drop. Therefore, you are most interested in maintaining the amorphous structure for a shielding application. In addition, for shielding applications, high permeability is not the whole story. If you have a very strong magnetic field, it can quickly saturate the shield. That is, if you have a high permeability, then the density of the flux lines will more rapidly reach the maximum value sustainable by the shielding material. When this happens, the external flux lines penetrate through the material easily. In this case, increasing the permeability doesn't help you -- your material is already saturated. However, if you want to create an excellent magnetic shield, you should use multiple layers of shield material with an air gap or some other insulator between them. It works like this: Assume you've built a shield to enclose your object, and it reduces the field inside to 1% of the field outside. If you then place another shield inside the first one, it will reduce the field inside both shields to 1% of 1%, or 0.01% of the field outside both shields. You can keep this up for any number of shields, budget and physical space permitting. If you use a high permittivity material like metglas for each layer, well, all the better! You can find more detail on this including equations for creating multilayer shields at: http://www.amuneal.com/pages/magshield-formula.php If you poke around the site, you will find a treasure trove of magnetic shielding knowledge. If you can make it to a library with scientific journals, you can get an excellent and more in depth treatment via the following citation: T.J. Sumner, J. M. Pendlebury, and K. F. Smith, Conventional Magnetic Shielding, J. Physics D. Applied Physics 20 (1987), Pg. 1095-1101. The latter is what I have used in the past, and it is very informative. I hope this helps you! Zack Gainsforth
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