Study On Negative Giant Magnetostrictive SmNdFe Materials

Pseudobinary RR’Fe2 Laves phase compounds (R, R’= rare earths,) have attracted much interest because of their giant magnetostriction and low magnetocrystalline anisotropy at room temperature, thus are regarded as important functional materials used in transducers and actuators. SmNdFe system possesses the largest negative saturation magnetostriction and contains no heavy-rare-earth elements, which make it a good candidate as new type giant magnetostrictive material. In the present work, structural, magnetic and magnetostrictive properties for SmNdFe polycrystals have been studied, as well as the mechanical and piezomagnetic properties under different bias magnetic fields. In addition, to enhance ductility and electrical resistivity, magnetostrictive composites have been prepared by mixing Smo.88Nd0.12Fe1.93 particles with epoxy resin and curing them at external magnetic fields. The effects of the preparation process on microstructure, magnetostriction and electrical resistivity for bonded composites have been systematically investigated. Main results are summarized as follows:Magnetostriction performance for SmNdFe alloys can be improved by directional solidification technique. Matrix of Smo.9Nd0.1Fe1.8 alloy is the cubic Laves phase with the easy magnetization direction along<111> at room temperature. Smo.9Nd0.1Fe1.8 powders exhibit a saturation magnetization Ms of 52.077 Am2/kg and a magnetocrystalline anisotropy K1 of-5.78×105erg/cm3. Magnetostriction (λ∥-λ⊥) at 1.0 T for the directionally solidified Smo.9Nd0.1Fe1.8 polycrystal is as high as-1462×10-6. which is 21% higher than the as-cast ones (λ∥-λ⊥=-1208×10-6). The dependences of elastic modulus and piezomagnetic constant on the bias magnetic field are also clarified. Under the parallel bias field parallel, compressive stress-strain curves of SmNdFe alloys are nearly linear. Young’s modulus decreases with the increase of field, which implies that "softening" phenomenon is induced by bias magnetic field. However, under transverse bias field, compressive stress-strain curves are non-linear and exhibit obvious hysteresis during unloading process. Young’s modulus increases with the increase of field, which means that "hardening" phenomenon is induced.Influences of particle volume fraction and molding magnetic field on the microstructure. magnetostriction and electrical resistivity for SmNdFe composites are revealed. Bonded composites with large magnetostriction are successfully prepared. Maximum magnetostriction(λ∥-λ⊥) at 1.3 T reaches as high as-1088×10-6 for the composites with 30% Sm0.88Nd0.12Fe1.93 particles that are cured at 0.6 T. which is 86.6% of that value for the as-cast alloy. Additionally, electrical resistivity for the composites cured under zero field is larger than the ones cured under 0.6 T. which should be ascribed to the directional alignment of SmNdFe particles. Compared with the alloys prepared by directional solidification, bonded SmNdFe composites possess better plasticity and toughness with higher electrical resistivity, which are therefore more suitable for applications under high frequency electromagnetic fields.