A theoretical and experimental investigation of power harvesting using the NiMnGa martensite reorientation mechanism

Magnetic shape memory alloys (MSMAs) can exhibit the shape memory effect when there is a magnetic field in the vicinity of a material point. The microstructure of the MSMAs is comprised of tetragonal martensite variants, each with their preferred internal magnetization orientation. Starting from a random variant orientation, the application of a large enough magnetic field will cause the variants to reorient so that the internal magnetization vectors align with the external field. Then, keeping the magnetic field constant and adding a variable compressive stress in a direction normal to that of the magnetic field, some or all of the martensitic variants may rotate into a stress preferred state. As the variants reorient, the internal magnetization vectors rotate, and the material's magnetization changes. For power harvesting and sensing applications, the change in magnetization induces a current in a pickup coil placed around the MSMA specimen, resulting in an output voltage at its terminals according to Faraday's law of inductance. This paper focuses on the evaluation of the voltage output, both experimentally and numerically, in an attempt to assess the ability of a MSMA thermodynamic based constitutive model, used in conjunction with Faraday's law of induction, to predict the variant reorientation induced voltage output. Assessing the accuracy of the predicted voltage is beneficial for the design of both MSMA based power harvesting devices and MSMA based displacement sensors.