Most engineers think amorphous iron is an exotic material that is hard to come by. But the interest level in this metal is rising because its magnetic properties can promote energy efficiency in electrical machinery and power distribution equipment. So it is timely to review the properties of amorphous iron that make it useful as a component in electrical equipment.
Despite what many engineers think, amorphous iron is actually made in large quantities. Two firms now supply the entire world’s amorphous iron: Metglas in Conway, S.C., and Tokyo (a division of Hitachi Metals), and Advanced Technology & Materials Co. Ltd. (AT&M) in China. About 100,000 tons of amorphous iron is produced annually, with Hitachi Metals supplying the vast majority.
Typical amorphous iron is an alloy of iron with boron and silicon. Amorphous iron comes from these suppliers in the form of a thin (25-microns thick) ribbon or foil. This form factor arises directly from the process used to manufacture the iron: Molten iron drips onto a wheel comprised of pure molybdenum. The molybdenum wheel is kept at a controlled temperature so iron hitting the wheel quenches quickly. The molten iron temperature drops at a rate of about 1 million°C/sec. This extra-fast quench freezes the iron molecules before they have a chance to form crystals, resulting in an amorphous structure that is much less orderly than that of crystalline iron.
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The amorphous iron harvested from the molybdenum wheel is necessarily thin. At thicknesses exceeding about 25 microns, the temperature doesn’t drop as quickly for the internal iron molecules. These internal molecules would have time to form crystals so the resulting metal would lose its uniform amorphous quality.
The disorderly structure of the amorphous iron lets it respond to changes in magnetic fields more readily than is the case for ordinary crystalline iron. The magnetic field change also causes eddy currents in the iron that are an additional source of loss, and the superthin nature of the amorphous iron limits these as well. Thus amorphous iron exhibits much less power loss, typically measured in units of Watts-per-pound or Watts-per-kilogram, for a given magnetic field strength than does crystalline iron.