Fabrication of reliable, self-biased and nonlinear magnetoelectric composites and their applications
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The magnetoelectric (ME) effect, i.e., the induction of magnetization by an applied electric field (E) or a polarization by an applied magnetic field (H), is of great interest to researchers due to its potential applications in magnetic sensors. Moreover, the ME effect in laminate composites is known to be much higher than in single phase and particulate composites due to combination of the magnetostrictive and piezoelectric effects in the individual layers. Given that the highest ME coefficient have been found in Metglas/piezo-fiber laminate composites, this study was designed to investigate and enhance the magnetoelectric (ME) effect in Metglas/piezo-fiber laminate composites, as well as develop their potential for magnetic sensor applications. To initiate this investigation, a theoretical model was derived to analyze the thickness effect of the magnetostrictive, piezoelectric, epoxy and Kapton layers on the ME coefficient. As a result, the importance of the coupling effect by epoxy layers was revealed. I used spin-coating, vacuum bagging, hot pressing, and screen printing techniques to decrease the thickness of the epoxy layer in order to maintain homogeneity, and to obtain good repeatability of the 16 ME laminates fabricated at one time. This protocol resulted in a more efficient way to induce self-stress to Metglas/PZT laminates, which is essential for increasing the ME coefficient. With an enhanced ME effect in the Metglas/piezo-fiber laminates, magnetic field sensitivity could then be increased. An ME sensor unit, which consisted of a Metglas/PMN-PT laminate and a low noise charge amplifier, had a magnetic field sensitivity of 10 pT/Hz0.5 in a well-shielded environment. Stacking four of these ME laminates could further increase the signal-to-noise (SNR) ratio. I studied the optimized distance between a pair of Metglas/PZT ME laminates. A stack of up to four ME sensors was constructed to decrease the equivalent magnetic noise. The magnetic field sensitivity was effectively enhanced compared to a single laminate. Finally, a number of four Metglas/PZT sensor units array was constructed to further increase the sensitivity. ME laminate composites operated in passive mode have typically required an external magnetic bias field in order to maximize the value of the piezomagnetic coefficient, which has many drawbacks. I studied the ME effect in an Ni/Metglas/PZT laminate at zero bias field by utilizing the remnant magnetization between the Ni and Metglas layers. To further enhance this effect, annealed Metglas was bonded on the Metglas/PZT laminate since it is known that hard-soft ferromagnetic bilayers generate built-in magnetic field in these Metglas layers. As a result, giant αME values could be achieved at a zero bias field at low frequency range or at electromechanical resonance (EMR). The sensor unit consisting of self-biased ME laminate arrays is considerably smaller compared to a unit that uses magnet-biased ME laminates. Introducing the converse ME effect and nonlinear ME effect in Metglas/piezo-fiber laminates affords a variety of potential applications. Therefore, I theoretically and experimentally studied converse ME effects in laminates with longitudinally magnetized and longitudinally poled, or (L-L) mode. The optimum structure for producing the maximum effect was obtained for Metglas/PZT laminates. Additionally, the optimum structure and materials for enhancing the nonlinear ME effect in Metglas/PZT laminates are reviewed herein. In particular, this study revealed that modulating the EMR in laminates with high-Q piezo-fibers could enhance the SNR. The stress effect on nonlinear ME effect is also discussed—namely that magnetic field sensitivities can be enhanced by this modulation-demodulation technique.
- Doctoral Dissertations