Reversible Magnetic Field Induced Strain in Ni₂MnGa‐Polymer‐Composites

Abstract

Magnetic shape memory (MSM) alloys are a new class of materials for sensor and actuator applications. Ni-Mn-Ga single crystals show large magnetic-field induced strain (MFIS) in moderate magnetic fields below 1 T caused by reorientation of martensitic variants by twin boundary motion. The maximum possible strain e ¼ 1-(c/a) is given by the lattice parameters a and c of the martensite unit cell resulting in e ¼ 6% for 5M martensite and e ¼ 11% for 7M martensite. [1][2][3] However, the main disadvantage of single crystals is their high brittleness. One possible solution is the synthesis of textured polycrystalline Ni-Mn-Ga for which a MFIS of 1% has been reported. [4] Due to constraints by grain boundaries, the twinning stress s twin for moving twin boundaries is still much higher for polycrystals (s twin > 15 MPa) compared to that in single crystals (s twin < 2 MPa). A strong texture, large grains and a mechanical training can decrease s twin . [4,5] The main disadvantage of single-and polycrystals is their complex preparation.An alternative to single-and polycrystals are Ni-Mn-Ga polymer composites. [6][7][8] Single crystalline particles embedded in a stiffness-matched polymer matrix allow overcoming the disadvantages. A thin polymer film between the single crystalline particles allows the particles to strain und reduces s twin . Another advantage of composites is the decrease of eddy currents due to a non-conducting polymer matrix, which enables high frequency applications, and the simple preparation of textured bulk materials.In previous works, we showed that melt-extracted and subsequently annealed fibres exhibit a bamboo-like grain structure and an MFIS of 1%. [6] However, only small amount of grains in the fibre are active. Different crystallographic orientations of grains as well as still existing grain boundaries hinder activation of the whole fibre. A separation of fibres into nearly single crystalline particles by breaking them alongComposite materials consisting of magnetic shape memory alloy particles and a polymer matrix combine the advantages of both material classes: the high achievable magnetic field induced strain (MFIS) of 6% of Ni-Mn-Ga with a ductile matrix. Engineering the particle-matrix interface as well as matching stiffness of polymer matrix is of importance for achieving high reversible MFIS to use this material as actuator or damper. We investigated those properties for Ni 50.9 Mn 27.1 Ga 22.0 and Ni 50.3 Mn 24.6 Ga 25.1 polymer composites. Particles were produced by gently crushing melt-extracted and subsequently annealed fibres. At room temperature, the Ni 50.9 Mn 27.1 Ga 22.0 particles exhibit a 5M martensitic structure, while the Ni 50.3 Mn 24.6 Ga 25.1 particles are austenitic. These particles were embedded into the polymer, either a stiff epoxy resin or a soft polyurethane. In response to an external applied magnetic field, the particles tend to relocate within the polyurethane due to its very low Young's modulus and magnetostatic interaction between particles. Slightly stiffer pol...

Effect of annealing and silylation on the strength of melt-spun Ni–Mn–Ga fibres and their adhesion to epoxy

“…In contrast, for the annealed fibres it led to an IFSS increase of 67%. This trend confirms observations of KauffmannWeiss et al [21] on untreated and silane treated Ni-Mn-Ga fibres produced by crucible melt-extraction embedded in an epoxy matrix for pull-out tests. For as-spun fibres, in contrary, a possible effect of the silylation is within the experimental scatter.…”

Review and perspectives: shape memory alloy composite systems

“…First, with respect to the magnetic domain reorientation, the response of NiMnGa reinforced polymer systems has been extensively studied. In such composites, utilization of a matrix with stiffness matched to that of the particulates is essential as: (i) twin boundary motion (which has been experimentally observed via neutron-diffraction [137]) is suppressed if the matrix response is too stiff [243,165] and (ii) with soft materials, particulate motion and/or localized yielding arise and should be avoided [243,213]. Similarly, good bonding between matrix and inhomogeneity is also needed and both pull-out [213] and single fiber fragmentation tests [153] have been performed indicating improved performance by using silane treatments [213,153].…”

Comparison of magnetic field controlled damping properties of single crystal Ni-Mn-Ga and Ni-Mn-Ga polymer hybrid composite structures

“…So far, the costs and the difficulties related to manufacturing of Ni-Mn-Ga single crystals and the brittle nature of the material [4] have hindered its widespread use. As a viable alternative to bulk single crystals, magnetic shape memory (MSM)-hybrid composite structures have been proposed [5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Previously, such Ni-Mn-Ga composite structures have been produced from particulates manufactured either by crushing [7][8][9][10][11][12][13][14] or cutting [15] of single crystals or from specialized processes such as spark erosion [9] or melt-spinning [16]. However, these do not solve the complexity related to single crystal manufacturing.…”