The effect of magnetic field orientation on the open-circuit voltage of Ni–Mn–Ga based power harvesters

Abstract

Ni–Mn–Ga is a ferromagnetic alloy that can exhibit the shape memory effect or superelasticity in the presence of a magnetic field. The behavior of the material is largely due to its microstructure, which is thought to be made of tetragonal martensite variants, each exhibiting an innate magnetization aligned approximately with the short side of the unit cell. Because the reorientation strain can be induced and recovered by either magnetic field or mechanical stress, it can be induced at frequencies larger than 1 kHz, which makes the material suitable for high-frequency actuation, sensing, or power harvesting applications. This paper investigates the power harvesting capability of Ni–Mn–Ga wrapped with a pick-up coil under a bi-axial magnetic field. In this work, both experimental tests and numerical simulations are used to identify the optimal direction of the externally applied magnetic field in order to achieve maximum open-circuit voltage output from a particular Ni–Mn–Ga based power harvester. Results suggest that significantly more power can be achieved with the bias field applied at an angle of 10°–20° off the perpendicular to the coil axis and the compressive stress. We believe that this increased power output is due to the saturation of domain walls due to a small component of the magnetic field along the direction of the coil.

On the power and efficiency of Ni2MnGa magnetic shape memory alloy power harvesters

“…The part of the sample with the magnetic easy axis aligned with the external magnetic field (variant 2 or V2)exhibits higher bulk magnetization, while the part of the sample with magnetic easy axes aligned with the compressive stress direction (variant 1 or V1) has magnetization vectors rotated away from the easy axis towards the direction of the applied magnetic field and exhibits lower bulk magnetization [5] as shown in figure 1. When cyclically compressing the Ni 2 MnGa element, the volume fraction of variants V1 and V2 changes [4,6], which alters the MSMA's bulk magnetization and correspondingly yields a change in the MSMA's surrounding magnetic flux density [5,7] that can be harvested, according to Faraday's law of electromagnetic induction.…”

“…The side coil setup [1,8,9] and the surrounding coil setup [7,10,11], as shown in figures 2(a) and (b), respectively, are commonly used for developing MSMA-based power harvesters. In the former configuration the pick-up coil is wrapped around a magnetic core that is part of the magnetic circuit which generates the bias field, while the latter configuration involves placing a pick-up coil directly around the MSMA element.…”

“…Using the surrounding coil setup, Karaman et al [10] reported a maximum peak voltage of 280 mV at an operating frequency of 10 Hz. Nelson et al [11] and Guiel et al [7] also used the surrounding coil setup and reported an increase in the open circuit voltage by approximately 15% and 900%, respectively, when exposing the MSMA to both an axial and a transverse bias field (see figure 2(c)). The axial component of the bias magnetic field causes a larger change in magnetization along the axis of the pickup coil because the magnetization vectors in V1 are aligned with the axial field.…”

“…These differences make it difficult to evaluate which power harvester design is most effective and efficient. The studies that use the side coil setup [1,5,8], report the use of higher operating frequencies (195-1000 Hz) as compared to the studies that use the surrounding coil setup that used significantly lower frequencies (0.5-15 Hz) [7,10,11]. Also, the studies that used the side coil setup [1,5,8], used thinner samples (0.3-1 mm) while the studies that used the surrounding coil setup used thicker samples (3-4 mm) [7,10,11].…”

Analysis and modification of a common energy harvesting system using magnetic shape memory alloys

“…Even in the first type of energy harvesters, there are few works in which the magneto-mechanical loading is not the same as above. For example, in Guiel et al (2018) the effect of the orientation of magnetic field on the energy harvester output is investigated, in fact, magnetic field is 2D. However, in all works, mechanical loading is always unidirectional.…”