Polymer Assessment for Magnetic Shape Memory Alloy Composites
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Polymer Assessment for Magnetic Shape Memory Alloy Composites Royale S Underhill, Gregory A Keddy, and Shannon P Farrell Defence R&D Canada - Atlantic, PO Box 1012, Dartmouth, Nova Scotia, B2Y 3Z7, Canada
ABSTRACT Our surrounding environment is teeming with useful energy, waiting to be harnessed (i.e., solar, wind, tidal, etc.). If this energy can be exploited at the point where it is required, then the need to carry additional power sources can be reduced. In recent years, magnetic shape memory alloys (MSMA) and their composites have demonstrated an ability to convert mechanical energy to magnetic energy. Such conversions have lead to the investigation of these alloys for energy harvesting applications. There are a number of issues to address when forming a MSMA/polymer composite. The polymer must be stiff enough to transmit the induced strain through the entire matrix, yet soft enough not to exceed the MSMA blocking stress. Also, the polymer must not dampen any force applied before it can be transmitted to the MSMA particles. Ten polymers have been investigated for MSMA/polymer composites. They include: crosslinked silicones, thermoplastic polyurethanes and triblock copolymers, and crosslinked polyurethanes. The work presented here describes progress in nickel-manganese-gallium (NiMn-Ga)/polymer composite fabrication and characterization. It was found that the viscosity of the polymer played an important role; too viscous and air was trapped, causing bubbles; not viscous enough and the MSMA settled. The polymers that met the criteria for MSMA/polymer composites were Sylgard 186 and an in situ polymerized polyurethane. INTRODUCTION As electronic devices become increasingly complex, power availability has become a major concern in almost all civilian and military designs. Batteries have come to represent a major portion of product weight, while offering only a limited operational lifetime. In situ energy harvesting could help alleviate the strain put on current power storage media. It has been shown that Ni-Mn-Ga magnetic shape memory alloys have the ability to convert mechanical energy into magnetic energy and subsequently, magnetic energy can be converted to electrical energy [1-3]. Significant dimensional changes have been seen in off-stoichiometric Ni2MnGa single crystal alloys when exposed to a magnetic field, stress or changes in temperature [4]. The structural and magnetic transformations depend on composition, thermal, magnetic and stress history, and crystal orientation [4, 5]. For energy harvesting applications, the main driving force is stress anisotropy. When the stress anisotropy is large enough, the application of a sufficient external stress relative to the crystallographic axes of the MSMA will cause rotation and realignment of the crystallographic structure (twin boundary motion). It is thermodynamically economical for the twins to activate and reorient the structure rather than rotate the magnetic domains independently. Structure rotation is translated into strain within the grain, a
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