The Application of Ferromagnetic Shape Memory Alloys in Power Generation Devices

By the inverse magnetoplastic (IMP) effect, twin boundaries in magnetic shape memory alloys (MSMA) can be reoriented by applying a mechanical stress. The result of this applied stress is a plastic deformation, as well as a change of magnetization. Used in conjunction with a coil, this changing magnetic field is harnessed to produce an electrical signal. With the application of a static bias field, as well as the removal of the applied stress, the twin boundaries reorient themselves and restore the transducer\u27s initial state. This process was repeated via a reciprocating force applied by a variable speed motor, and was performed at room temperature and with compact permanent magnets. The transducer used was an off-stoichiometric Ni2MnGa MSMA single crystal with approximate dimensions 5.4 x 3.9 x 3.1 mm3. With voltage measured as a function of time, the power can be computed using the known resistance of the coil. It is expected that the power output will exhibit a quadratic increase with the load application frequency. This behavior is demonstrated by the power generation device and is within the associated statistical error of the average power outputs. It is also shown that the twinning microstructure of the transducer plays a large role in the exhibited voltage peaks. Seen in the stress-strain plots for off-stoichiometric Ni2MnGa samples, a threshold stress level must first be reached prior to the rapid motion of twin boundaries, a phenomenon that is in this paper termed twin snapping. The magnitude and duration of these twin snaps are directly proportional to the output voltage, and these are shown to evolve each time the transducer is cycled. It is demonstrated that while much optimization has yet to be performed, MSMAs are a viable material for use in power generation devices. With their ability to be cycled at relatively high frequencies, as well as their large strains and associated changing magnetization, MSMAs provide the required prerequisites for their use in power generation applications