Magnetostriction of Field-Structured Composites
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Magnetostriction of Field-Structured Composites James E. Martin, Robert A. Anderson, and Gerald Gulley1 Sandia National Laboratories Albuquerque, New Mexico 87185, U.S.A. 1 Dominican University River Forest, IL 60305, U.S.A. ABSTRACT Field-structured magnetic particle composites are an important new class of materials that have great potential as both sensors and actuators. These materials are synthesized by suspending magnetic particles in a polymeric resin and subjecting these to magnetic fields while the resin polymerizes. If a simple uniaxial magnetic field is used, the particles will form chains, yielding composites whose magnetic susceptibility is enhanced along a single direction. A biaxial magnetic field, comprised of two orthogonal ac fields, forms particle sheets, yielding composites whose magnetic susceptibility is enhanced along two principal directions. A balanced triaxial magnetic field can be used to enhance the susceptibility in all directions, and biased heterodyned triaxial magnetic fields are especially effective for producing composites with a greatly enhanced susceptibility along a single axis. Magnetostriction is quadratic in the susceptibility, so increasing the composite susceptibility is important to developing actuators that function well at modest fields. To investigate magnetostriction in these field-structured composites we have constructed a sensitive, constant-stress apparatus capable of 1 ppm strain resolution. The sample geometry is designed to minimize demagnetizing field effects. We have demonstrated field-structured composites with nearly 10,000 ppm strain, and have shown that at large magnetic fields a structural phase transition occurs within the composite. These experimental results are compared to microscopic, self-consistent field simulations of magnetostriction in these complex, disordered materials. INTRODUCTION There is a need for soft actuators that have a much larger strain response than piezoelectrics, and that can respond in microseconds. To meet these needs we are developing efficient Field-Structured Magnetostrictive Elastomers (FSMEs). [1,2] These are materials whose magnetic permeability is sensitive to tensile or compressive strains, as discussed below. These materials contract in a magnetic field, and have the potential for many actuator applications, for example as artificial muscles for robots. The inverse magnetostriction effect is also useful, enabling stress/strain sensors based on permeability (or permittivity) changes. Related applications include magnetic- or electric-field-tunable capacitors for RF applications. Magnetostriction refers to the tendency of magnetic materials to deform in a uniform magnetic field. In fact, there are two causes of deformation. First, the material will tend to elongate along the field to increase the penetration of the applied field into the body, enhancing the macroscopic internal field (field penetration is opposed by the demagnetizing field of the magnetized solid [3]). This effect is strongly dependent on the
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