Studies on Ni-Mn-Ga ferromagnetic shape memory alloys and its composites

Abstract

Ferromagnetic shape memory alloys are a new class of active materials which combine the properties of ferromagnetism with those of a diffusionless, reversible martensitic transformation. These materials have been the subject of recent study due to the giant magnetic field-induced strain (MFIS) effect. To fulfill the applications for both sensors and actuators, in this thesis, an extensive study was conducted on Ni-Mn-Ga single crystal and its composites. The main findings are as follows: 1) Magnetomechanical properties of Ni-Mn-Ga single crystal were characterized by measuring the MFIS effect in a wide variety of mechanical and magnetic loading conditions. The crystal presented the maximal dc-MFIS of 5.6% and the minimal ac-MFIS of 0.3% under load-free condition, while the maximal ac-MFIS of 3% was observed under the optimized mechanical load of 1.6 MPa. The MFIS could be hampered completely by the block force of 3.5 MPa. The mechanical properties were also measured in various magnetic bias field conditions. The crystal exhibited super-plastic and pseudo-elastic behaviours under relatively low and high magnetic bias field conditions, respectively. 2) The damping behaviours in Ni-Mn-Ga single crystal were measured in three different crystallographic planes (a-b, a-c, and c-a planes). The anisotropic damping behavior was found in the martensitic phase where the loss factors were 0.03, 0.15 and 0.26 for the a-b, a-c, and c-a crystallographic planes, respectively. 3) Electrical transport properties in Ni-Mn-Ga single crystal were developed experimentally by measuring the electrical resisitivey in different crystallographic directions. The anisotropic resistivity was observed in the martensitic phase and the magnetoresistance (MR) effect of about 25.2%, either positive or negative, was also observed. The mechanism of anisotropic resistivity was explored theoretically by the combination of first-principles calculation and Boltzmann transport theory. 4) Ni-Mn-Ga composites of multilayered and sandwiched types were fabricated by laminating Ni-Mn-Ga single crystal plates with polyurethane (PU) polymer plates. The dc- and ac-MFISs in the composites were measured as functions of both magnetic field and mechanical load, and the results were compared with those of the single crystal. It was found that the load-free dc-MFISs were 5.6, 1.5, and 0.8 %, while the load-free ac-MFISs were 0.3, 0.8, and 0.5 %, in the single crystal, multilayered and sandwiched composites, respectively. The dc-MFISs of all samples and the ac-MFISs of the composites decreased with the increase in mechanical load amplitude, while the ac-MFIS of the single crystal peaked at 1.6 MPa load. 5) The MFIS and magnetoelectric (ME) effects in bilayered and sandwiched composites fabricated by laminating Ni-Mn-Ga single crystal plate(s) and piezoelectric PVDF polymer plate(s) were studied. The MFIS decreased in the composites compared to the single crystal. The largest ME coefficient (αE) of 1.24 and 0.58 V/cm·Oe were observed at an optimal HBias of about 400 and 670 kA/m for the bilayered and sandwiched composites, respectively. The observed ME effect was attributed to a reversible reorientation of martensitic twin variants. Based on the combination of pseudo-piezomagnetism in Ni-Mn-Ga single crystal and piezoelectricity in PVDF polymer, the ME effect was reasonably predicted. 6) The effect of phase transformation on the converse magnetoelectric (CME) properties was studied in a heterostructure of Ni-Mn-Ga and PMN-PT single crystals. The CME effect was minimized and independent of temperature in the martensitic phase, was maximized in the martensitic transformation, and decreased gradually with increasing temperature in the austenitic phase. A giant resonance CME coefficient (αB) of 18.6 G/V was observed at a frequency of 47 kHz upon martensitic transformation.