Textured polymer bonded composites with Ni–Mn–Ga magnetic shape memory particles
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Cited by 119 publications
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Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation
Composites consisting of magnetic shape memory ͑MSM͒ particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni 50.9 Mn 27.1 Ga 22.0 ͑at. %͒ fibers. The crystal structure of the martensite is tetragonal ͑5M͒ with c Ͻ a = b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression ͑height reduction͒, the MSM particles have a ͑004͒-fiber texture in the direction of compression. This is unambiguous evidence for stress induced twin boundary motion within the MSM particles. The term magnetic shape memory ͑MSM͒ effect refers to a magnetic field induced structural phase transformation. At the same time, also a magnetic field induced twin boundary motion is commonly referred to as MSM effect. 1-3 Large magnetic field induced strains can be achieved in MSM alloys, e.g., in the Heusler alloy Ni 2 MnGa up to 10%. 4 Besides employing single and polycrystals to utilize the MSM effect in actuators or dampers, also composites, i.e., MSM particles embedded in a polymer matrix, were proposed. 5,6 It was shown previously that compressed composite samples can be magnetized easier in the compressed direction and it is more difficult to magnetize them in the perpendicular directions. 6,7 This behavior was regarded as an indication for stress induced twin boundary motion within the MSM particles.In order to verify this, the texture of the MSM particles before and after compression is investigated in this work. The texture is measured by using diffraction of high-energy synchrotron radiation. The advantage of this method is that the texture of the whole sample volume is measured.The MSM particles used were prepared by mortar grinding of crucible melt-extracted and subsequently annealed Ni 50.9 Mn 27.1 Ga 22.0 ͑at. %͒ fibers. The preparation of the composite was done by mixing these MSM particles with a two component polyester resin ͑about 20 vol % MSM particles͒ followed by a vacuum treatment to remove air bubbles. Curing of the polyester matrix was done without a magnetic field applied. The composite was disk shaped with a height of 2.79 mm and a diameter of about 9 mm. A cylindrical sample with a diameter of 3.03 mm and a height of 2.79 mm was cut out from the middle of the composite. Detailed information on preparation of MSM particles and composites are given by Scheerbaum et al. 6 For the synchrotron texture measurements the beam line BW5 ͑about 100 keV͒ at DESY-HASYLAB in Hamburg ͑Germany͒ was used. The synchrotron radiation...
Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation
Composites consisting of magnetic shape memory ͑MSM͒ particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni 50.9 Mn 27.1 Ga 22.0 ͑at. %͒ fibers. The crystal structure of the martensite is tetragonal ͑5M͒ with c Ͻ a = b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression ͑height reduction͒, the MSM particles have a ͑004͒-fiber texture in the direction of compression. This is unambiguous evidence for stress induced twin boundary motion within the MSM particles. The term magnetic shape memory ͑MSM͒ effect refers to a magnetic field induced structural phase transformation. At the same time, also a magnetic field induced twin boundary motion is commonly referred to as MSM effect. 1-3 Large magnetic field induced strains can be achieved in MSM alloys, e.g., in the Heusler alloy Ni 2 MnGa up to 10%. 4 Besides employing single and polycrystals to utilize the MSM effect in actuators or dampers, also composites, i.e., MSM particles embedded in a polymer matrix, were proposed. 5,6 It was shown previously that compressed composite samples can be magnetized easier in the compressed direction and it is more difficult to magnetize them in the perpendicular directions. 6,7 This behavior was regarded as an indication for stress induced twin boundary motion within the MSM particles.In order to verify this, the texture of the MSM particles before and after compression is investigated in this work. The texture is measured by using diffraction of high-energy synchrotron radiation. The advantage of this method is that the texture of the whole sample volume is measured.The MSM particles used were prepared by mortar grinding of crucible melt-extracted and subsequently annealed Ni 50.9 Mn 27.1 Ga 22.0 ͑at. %͒ fibers. The preparation of the composite was done by mixing these MSM particles with a two component polyester resin ͑about 20 vol % MSM particles͒ followed by a vacuum treatment to remove air bubbles. Curing of the polyester matrix was done without a magnetic field applied. The composite was disk shaped with a height of 2.79 mm and a diameter of about 9 mm. A cylindrical sample with a diameter of 3.03 mm and a height of 2.79 mm was cut out from the middle of the composite. Detailed information on preparation of MSM particles and composites are given by Scheerbaum et al. 6 For the synchrotron texture measurements the beam line BW5 ͑about 100 keV͒ at DESY-HASYLAB in Hamburg ͑Germany͒ was used. The synchrotron radiation...
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...
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