Shielding Science
|
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
 |
|
SCANNING ELECTRON microscopes such as this are fitted with a second skin of high-permeability nickel alloy
to absorb magnetic fields. |
|
日本語 |
Nickel alloy shields sensitive electron microscopes from magnetic interference By Carroll McCormick
Nickel Magazine, November 2005 -- Electron microscopes are widely used in science labs but also in plants where semiconductors are manufactured (specifically to inspect crystalline structures), and they are highly sensitive. All it takes is a nearby transformer or subway car – some magnetic source – to interfere with that sensitivity. The solution is to build nickel-containing shields that absorb the magnetic fields which surround the microscopes.
New Hampshire, U.S.A.-based The MuShield Company designs and builds magnetic shields from the high-permeability alloy MuMetal, containing 77% nickel, which protects microscopes from magnetic interference. MuMetal forms a "path of least resistance" for the magnetic field and absorbs it.
MuShield’s customers include Japan Electro Optics Ltd., which manufactures electron microscopes. The amount of shielding required depends on the location of the microscope and the amount of interference to which it is exposed. "Electron microscopes already have shields built into them, but sometimes they need additional ones," says David Grilli, senior engineer with MuShield.
A metal’s ability to absorb magnetic interference is known as "permeability," and this is determined by the way the metal is processed. MuMetal is processed in such as way that it has a net magnetic field.
"A material has permeability based on its net electron spin," explains Grilli. "With most materials, the net spins cancel out, whereas with [MuMetal] the net electron spin is more one way than another. Nickel is one of the metals that is magnetic. Some metals, by design, are meant to have a higher degree of permeability in a certain direction. MuMetal is processed so that it doesn’t matter which way the magnetic field goes through."
If the interference is too strong, a MuMetal shield can become saturated and stop working, but for most applications it is the optimum material for redirecting and absorbing magnetic flux. The alloy is available in sheets, foils, round bar stock, plates, forgings and tubing in a variety of thicknesses, diameters, lengths and widths -- all the better for MuShield, which must design and fabricate shields with complex shapes to suit the needs of its various customers. In each case, the shield fits over the microscope like a second housing.
Spherical shapes are ideal but rarely practical, according to MuShield. The next most effective shape is cylindrical with a length-to-diameter ratio of 4 to 1. Rounded surfaces, including high-radius corners, are the most conducive to magnetic absorption. The least-effective shape is a large flat surface, especially as it becomes more and more at a right angle to the magnetic field vector.
The international market in magnetic shielding is valued at US$50-100 million a year, says Grilli, and also includes applications such as fibre optic gyro housings and other medical devices, television camera multiplier tubes, and some computer products. The shields are also used to keep radiation of fields from electronic devices.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: JEOL U.S.A.
The MuShield Company |
