Acoustic assisted actuation of Ni-Mn-Ga ferromagnetic shape memory alloys

Abstract

Ferromagnetic shape memory alloys (FSMA) have been shown in recent work to exhibit large magnetic field induced strains. The material generally requires a large threshold field (of order 3-4 kOe) to initiate the strain. Thus, the power requirements are large and actuators based on these materials could tend to be large. This thesis reports on the effect on the actuation properties of Ni-Mn-Ga single crystals of the use of a sinusoidal stress wave generated by a piezoelectric stack actuator. The piezoelectric drive causes a time varying stress wave in the FSMA that resolves as a shear across the twin-boundaries and aids the twin boundary in overcoming defect-related obstacles. The FSMA shows increased strain and a reduction in threshold field. The effect is most pronounced for crystals showing large initial threshold fields which are associated with high defect strengths or concentrations. For crystals with a large threshold, 4.7 kOe, actuated at 1 Hz, the piezoelectric drive reduces the threshold field by as much as 21% for a piezoelectric driven at 5 kHz and 20 Vrms. As a result of this large threshold reduction, strain output can be more than doubled for magnetic drive amplitudes near the threshold field.

(cont.) Strain improvement is modest, 0.001, for magnetic field drive amplitudes that are near saturation magnetization. The effect of piezo-assisted actuation as a function of the magnetic drive frequency was also investigated. The apparatus was first modeled using finite element simulations to determine its expected resonant behavior. The actuation behavior of the high-frequency apparatus was then tested at Ni-Mn-Ga actuation frequencies up to 500 Hz. It was found that the strain amplification at constant piezoelectric drive tends to decrease with increasing actuation frequency. Maximum benefit is seen for actuation frequencies under 20 Hz and is believed to be the result of both a reduction in piezoelectric energy input per FSMA actuation cycle at elevated actuation frequencies, and the system dynamics. The change in piezoelectric drive performance as a function of temperature was investigated for temperatures below the martensitic transformation, 0-30 °C. Strain output decreases with temperature as a result of lowered twin-boundary mobility. The piezoelectric drive does yield an improvement in strain response for all temperatures investigated, however the effect is maximized at 12.5 °C which is near the inflection point of increasing field-induced strain with temperature.