PRECISION SHEET METAL FABRICATION *

 

 

ELECTROMAGNETIC SHIELDING *

 
     
 
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What is Eagle AAA?

 

 

Eagle AAA is a nickel-iron alloy (approximately 80% nickel, 15% iron, and molybdenum) that has very high magnetic permeability. The high permeability makes Eagle AAA very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods. Eagle AAA can have relative permeabilities of 80,000-100,000 compared to several thousand for ordinary steel. In addition it has low coercivity and magnetostriction resulting in low hysteresis loss. Its magnetic properties are similar to other high permeability alloys such as Permalloy, but it is more ductile and workable.


Parts made from Eagle AAA require heat treatment after they are in final form — annealing in a magnetic field in hydrogen atmosphere, which reportedly increases the magnetic permeability about 40 times. The annealing alters the material's crystal structure, aligning the grains and removing some impurities, especially carbon, which obstruct the free motion of the magnetic domain boundaries. Bending or mechanical shock after annealing may disrupt the material's grain alignment, leading to a drop in the permeability of the affected areas, which can be restored by repeating the hydrogen annealing step.

 

What is Magnetic Shielding?


The high permeability of Eagle AAA provides a low reluctance path for magnetic flux, leading to its major use, in magnetic shields against static or slowly varying magnetic fields. Magnetic shielding made with high permeability alloys like Eagle AAA works not by blocking magnetic fields but by shunting them—providing a path for the magnetic field lines around the shielded area. So the best shape for shields is a closed container surrounding the shielded space. The effectiveness of the shielding decreases with the alloy's permeability, which drops off at both low field strengths and, due to saturation, at high field strengths. So Eagle AAA shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. RF magnetic fields above about 100 kHz can be shielded by Faraday shields, ordinary conductive metal sheets or screens which are used to shield against electric fields.

 

History


Magnetic shielding was developed by scientists named Smith and Garnett and patented in 1923 for inductive loading of submarine telegraph cables by The Telegraph Construction and Maintenance Co. Ltd. (now Telcon Metals Ltd.), a British firm which built the Atlantic undersea telegraph cables. The conductive seawater surrounding an undersea cable added a great deal of capacitance to the cable, limiting the bandwidth and slowing signaling speed to 10 - 12 words per minute. The bandwidth could be increased by adding inductance to compensate. This was first done by wrapping the conductors with a helical wrapping of metal tape or wire of high magnetic permeability, which confined the magnetic field. Magnetic shielding was invented by adding copper to the previous high permeability alloy Permalloy to improve ductility. Fifty miles of the fine shielding wire was needed for each mile of cable, creating a great demand for the alloy. The first year of production Telcon was making 30 tons per week. In the 1930s this use declined, but by World War II many other uses were found in the electronics industry (particularly shielding for transformers and cathode ray tubes) as well as the fuzes inside magnetic mines.

 

Uses and properties

 

Eagle AAA is used to shield equipment from magnetic fields. For example:

 

 


 


 

 

 

 
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