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Title: Study of magnetoelectric composites for sensor and transducer applications
Authors: Li, Tongle
Degree: M.Phil.
Issue Date: 2005
Abstract: Magnetoelectric materials are potentially useful in magnetic field sensors and magnetoelectric transducers due to their intrinsic effect upon the conversion between magnetic and electric signals without the aid of any external sources of power. Since single-phase materials have very small magnetoelectric effect and their operating temperatures are often below the room temperature, they cannot be used in any application. This makes it necessary to develop two-phase composite materials by using the product property of the magnetostrictive effect in a magnetostrictive material and the piezoelectric effect in a piezoelectric material. That is, a magnetic field applied to such a magnetoelectric composite induces a magnetostrictive strain in the magnetostrictive material phase that subsequently stresses the piezoelectric material phase to generate a piezoelectric voltage or charge proportional to the applied magnetic field. While the existing bulk 0-3 [e.g., cobalt ferrite (CFO)/barium titanium (BaTiO3), nickel ferrite (NFO)/BaTiO3, etc.] and laminated [e.g., ferrite/lead zirconate titanate (PZT), Terfenol-D/PZT, etc.] composite systems possessing an increased magnetoelectric effect of 3-230 times that in single-phase materials, they suffer from high eddy-current losses, limited operational bandwidth and/or high mechanical brittleness. In this study, a novel magnetoelectric laminated composite system consisting of at least one layer of transversely (or thickness-) polarized polyvinylidene fluoride (PVDF) piezoelectric polymer sandwiched between two layers of longitudinally (or length-) magnetized Terfenol-D/epoxy pseudo-1-3 magnetostrictive composite was developed to provide a high magnetoelectric effect over a wide range of frequencies with a reduced mechanical brittleness as compared with the previously reported alloy- and/or ceramic-based magnetoelectric material systems. To facilitate the development of the proposed longitudinally magnetized- transversely polarized (L-T) magnetoelectric composite system, the two constituent material phases were studied. Terfenol-D/epoxy pseudo- 1-3 magnetostrictive composites were fabricated by embedding and aligning Terfenol-D particles with a size distribution of 10-300 um in a Spurr epoxy matrix using six Terfenol-D volume fractions ranging from 0.22 to 0.72. Their quasistatic and dynamic magnetic and magnetomechanical properties were measured as functions of magnetic bias field (HBias), frequency (f) and Terfenol-D volume fraction (vf). The observed HBias-dependent data was explained in terms of domain-wall motions. The f-dependent data indicated an insignificant eddy-current effect for frequencies up to and beyond 100 kHz. The vf -dependent data suggested an optimal device performance and cost by using composites with vf >= 0.5. The quasistatic and dynamic dielectric and electromechanical properties of a 0.1-mm thick, longitudinally drawn, transversely (or thickness-) polarized PVDF piezoelectric polymer were also measured and discussed. Existing physical quasistatic and dynamic models were refined and extended to predict the magnetoelectric voltage sensitivity (MEv=dV/dH) and magnetoelectric charge sensitivity (MEQ=dQ/dH) of the L-T laminated composite system under different combinations of geometric configurations, dimensions and material phases. The models were applied to design and analyze three types of L-T magnetoelectric laminates. These included: 1) Laminates with a transversely polarized PVDF layer sandwiched between two longitudinally magnetized Terfenol-D/epoxy pseudo- 1-3 composite layers in the transverse direction (denoted as L-T laminates); 2) Laminates with four transversely polarized PVDF layers connected electrically in series and mechanically in series and sandwiched between two longitudinally magnetized Terfenol-D/epoxy pseudo- 1-3 layers in the transverse direction (denoted as L-T series laminates); and 3) Laminates with four transversely polarized PVDF layers connected electrically in parallel and mechanically in series and sandwiched between two longitudinally magnetized Terfenol-D/epoxy pseudo- 1-3 layers in the transverse direction (denoted as L-T parallel laminates). The results revealed that L-T series laminates and L-T parallel laminates, respectively, are capable of enhancing MEv and MEQ of the ordinary L-T laminates by four-fold. In particular, the L-T parallel laminates have an additional benefit of higher capacitances and thus lower electrical impedances for interfacing with electronics. Three types of magnetoelectric laminates were fabricated with dimensions of 12 mm (length) * 6 mm (width) * t (thickness), where t=2tMS+ntPE, tMS = 1mm is the thickness of a single magnetostrictive layer, tPE = 0.1 mm is the thickness of a single piezoelectric layer and n is the number of piezoelectric layers. The quasistatic and dynamic magnetoelectric characteristics of these laminates were characterized and compared with those predicted by the physical models with good agreements. For the L-T laminates, the maximum quasistatic MEv and MEQ were found to be 12 mV/Oe and 0.74 nC/Oe, respectively, under a relatively low HBias of 0.6 kOe, while the maximum resonance MEv and MEQ as observed at ~61 kHz were ~20 times larger than the non-resonance values. The L-T series laminates exhibited an improved MEv of 42 mV/Oe, corresponding to 3.5 times that in the L-T laminates. The L-T parallel laminates displayed an enhanced MEQ of 2.54 nC/Oe, reaching ~3.4 times that in the L-T laminates. Therefore, it is practically viable to use the proposed laminates in magnetic field sensors and magnetoelectric transducers.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Pages: xviii, 168 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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