Product Code Database
Mu-metal or μ-metal is a nickel–iron soft ferromagnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency . The name came from the Greek letter mu (μ), which represents permeability in physics and engineering formulas.
Mu-metal typically has relative permeability values of 80,000–100,000 compared to several thousand for ordinary steel. It is a "soft" ferromagnetic material; it has low magnetic anisotropy and magnetostriction, giving it a low coercivity so that it saturates at low magnetic fields. This gives it low when used in alternating current (AC) magnetic circuits. Other high-permeability nickel–iron alloys such as permalloy have similar magnetic properties; mu-metal's advantage is that it is more ductile, malleable and workable, allowing it to be easily formed into the thin sheets needed for magnetic shields.
Mu-metal objects require heat treatment after they are in final form—annealing in a magnetic field in hydrogen atmosphere, which increases the magnetic permeability about 40 times. The annealing alters the material's crystal structure, aligning the grain boundary 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.
The effectiveness of mu-metal shielding decreases with the alloy's permeability, which drops off at both low field strengths and, due to saturation, at high field strengths. Thus, mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. Because mu-metal saturates at relatively low fields, sometimes the outer layer in such multilayer shields is made of ordinary steel. Its higher saturation value allows it to handle stronger magnetic fields, reducing them to a lower level that can be shielded effectively by the inner mu-metal layers.
Radio frequency (RF) magnetic fields above about 100 Kilohertz can be shielded by Faraday cage: ordinary conductive metal sheets or screens which are used to shield against . Superconducting materials can also expel magnetic fields by the Meissner effect, but require cryogenic temperatures.
The alloy has a low coercivity, near zero magnetostriction, and significant Anisotropy magnetoresistance. The low magnetostriction is critical for industrial applications, where variable stresses in thin films would otherwise cause a ruinously large variation in magnetic properties.
Ceramic ferrites are used for similar purposes, and have even higher permeability at high frequencies, but are brittle and nearly non-conductive, so can only replace mu-metals where conductivity and pliability aren't required.
Telcon invented mu-metal to compete with permalloy, the first high-permeability alloy used for cable compensation, whose patent rights were held by competitor Western Electric. Mu-metal was developed by adding copper to permalloy to improve ductility. of fine mu-metal wire were needed for each 1.6 km 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 for mu-metal declined, but by World War II many other uses were found in the electronics industry (particularly shielding for and ), as well as the inside naval mine. Telcon Metals Ltd. abandoned the trademark "MUMETAL" in 1985. The last listed owner of the mark "MUMETAL" is Magnetic Shield Corporation, Illinois.
|
|
