High Power Magnetostrictive Materials from Cryogenic Temperatures to 250 C
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ABSTRACT The rare earths, both in elemental form and in compounds, are widely known as possessing many extraordinary magnetic properties. In this paper, we focus on the huge magnetically induced displacements (magnetostrictions) based upon the element terbium. A proper balance of magnetic anisotropy and magnetostriction, plus a proper choice of crystal axes lead to materials which can switch large quantities of energy between the internal (magnetic) and external (mechanical) states with the application of small triggering magnetic fields. Power densities 2000 times those of conventional magnetostrictive materials and 10-20 times those of typical piezoceramics are available. These materials are particularly valuable for smart systems where large energy transduction is needed such as active structure stiffening and active vibration control.
INTRODUCTION One of the most important ways electrical energy can be converted into mechanical energy is via magnetostrictive materials. These materials do not need high electrical fields and are, in general, rugged, impervious to adverse environmental conditions, and have a record of high reliability. Failure mechanisms are few. Although efficiencies are somewhat lower in magnetostrictive actuators than in electromechanical actuators, magnetostrictive actuators do not suffer from electrical breakdown and malfunctions due to high voltage arcing. One of the longstanding shortcomings of magnetostrictive materials has been their characteristically low saturation strains (-5 x 10-5). Clearly very large strains are needed to deliver the large displacements often required for smart system components. In this paper, we review the magnetostrictive properties of three classes of high power magnetostrictive materials: (1) the hexagonal TbxDyl-x alloy system (0 < x < 1), (2) the body centered cubic (bcc) TbZn compound, and (3) the face centered (Laves Phase) cubic RFe 2 compounds (R = Tb, Dy, and Sm). The first two classes possess high magnetostrictions at cryogenic temperatures and at temperatures up to ~150 K. For the TbxDyl-x system and the TbZn compound, ordering temperatures are -200 K. Saturation magnetostrictions (Xs's) exceed 0.5% at 77 K. 1 ,2 The TbxDyl-x system is ductile and is very useful for compressive loads up to -25 MPa. The magnetization process is rotational and the hysteresis is small. On the other hand, the intermetallic TbZn compound is machinable, does not flow under stress, and is useful up to much higher compressive loads ( > 60 MPa). Because of a higher magnetic anisotropy, however, the hysteresis is larger. The cubic Laves phase RFe 2 compounds are unique. The combination of the highly magnetostrictive rare earths with strongly magnetic iron yields highly magnetostrictive materials with extraordinarily high ordering temperatures (-700 K). These compounds are not ductile and are particularly important at room temperature and above. At 171
Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
these temperatures, the magnetostrictions are approximat
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