Pulse-Field Activation of Collinear Magnetic Single Crystals
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Pulse-Field Activation of Collinear Magnetic Single Crystals C.A. Jenkins, R.C. O'Handley, S.M. Allen Massachusetts Institute of Technology, Cambridge, MA.
Abstract Single crystals of nickel-manganese-gallium, a ferromagnetic shape memory alloy (FSMA), show up to 10% elongation in response to an applied magnetic field or mechanical stress. The change in dimension is not a result of magnetostriction but of twin boundary motion through the material. The field-induced extension has previously been reversed only by a compressive mechanical stress from a spring or orthogonal field. The use of a second crystal of Ni-Mn-Ga to provide the reset force is unreported. Data are shown here of the stress and strain behavior of two FSMA crystals connected in series and subjected alternately to a magnetic field. Two samples, one initially in full extension and the other in compression, are arranged in series in a rigid frame such that their total length is held constant. A pulsed magnetic field is alternately applied to the samples, with the output energy of the extending crystal tending to apply a resetting force to the other. This setup shows up to 1.5% resetting strain in single crystal laminates. Introduction Ferromagnetic shape memory alloys (FSMAs) are a class of alloys that change shape in response to an applied mechanical or magnetic stress, with tetragonal lattices showing 6% and orthorhombic lattices exhibiting up to 10% strain under optimum conditions [1]. The strain in an FSMA is due to a change in the orientation of c-axis of the crystal lattice by the motion of what are known as twin boundaries, or planes along which the distorted tetragonal lattice is mirrored. Moving these twin boundaries dissipates energy, which makes FSMAs potentially useful in vibration damping applications [2]. If the elongation is carefully controlled, however, there is excess field energy from the applioed pulse that can be converted to mechanical work. Based on this assumption, an apparatus was designed with two collinear single crystals of the FSMA nickel-manganese-gallium. Each sample was centered in its own individual Helmholtz coil with an acrylic spacer to minimize field interference, and actuated alternately to prove the feasibility of such a two-crystal setup. Possible industrial applications include contactless valves or bistable switches. Unlike in piezoelectric materials, where electrical contact must be maintained in order for a voltage to be applied directly to the active material, isolated FSMAs can respond to a magnetic field originating from a separate coil.
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Procedure Laminations measuring approximately 0.1cm x 2cm x 1cm were cut by spark erosion from single crystals grown at Ames Laboratory by Tom Lograsso, and then mechanically polished down to 0.3µm alumina. The crystals were then wrapped in tungsten wire with a titanium getter in a quartz tube backfilled with argon. They were heat treated at 750°C for 8 hours, then held for two additional hours at 500° and subjected to 13.5 kg compressive force during
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