Magnetoelastic dampers have been proposed for use in suppressing vibrations of large motors and transformers that operate at temperatures below 100 K. [Also see "Progress in Magnetoelastic Vibration Dampers," (NPO-20887) on page 57 of this issue.] These dampers would be made of magnetostrictive materials - specifically, Tb/Dy alloys. It would not be necessary to use the rare and expensive high-purity grades of Tb/ Dy; the commercial grade would suffice and may in fact be preferable.
Conventional dampers for such applications are based on viscoelasticity. Because viscoelastic damping involves movements of atoms and these movements become very small at low temperatures, viscoelastic dampers become ineffective at low temperatures.
In magnetostrictive materials, vibrational energy becomes damped through the movement of magnetic domains and the consequent dissipation of vibrational energy as heat. Because this effect is not diminished at low temperatures, magnetostrictive materials can be expected to be effective in damping vibrations at low temperatures. In a recent quasi-static experiment, a Tb/Dy alloy specimen was found to dissipate 30 percent of vibrational energy per stress-and-strain cycle at a temperature of 77 K. Moreover, it may be possible to increase the dissipation factor through the addition of small amounts of N, Ta, and/or other elements to Tb/ Dy alloys and through changes in the processing conditions.
This work was done by Jennifer Dooley, Brent Fultz, John Voccio, and Robert Chave of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-20988.