Magnetoelectric Laminates with Novel Properties for Sensor, Transmitter, and Gyrator Applications

The magnetoelectric (ME) effect is a property that results in power/energy conversion between magnetic and electric forms. Two-phase composites consisting of magnetostrictive and piezoelectric materials have been developed that show remarkable ME voltage/charge coefficients. This extrinsic ME effect is achieved by using mechanical coupling as a medium between the magnetostrictive and piezoelectric phases. As described in this thesis, I investigated the optimization of the material properties of sensors/gradiometers, transmitters, and gyrator applications using ME heterostructures with a multi-push-pull structure.

In applications, ME sensors will need to work in an open environment where there will be a mix of magnetic signals and microphonic noises. Prior research has determined that both passive and active mode ME sensors are affected by vibrational noise in the open environment. Therefore, as described herein, an ME gradiometer consisting of a pair of ME sensors working under H-field modulation (active mode) was developed to address the issue of microphonic noise. The common mode rejection ratio of my ME gradiometer was determined to be 74. Gradiometer curves were also measured, which presented the gradiometer outputs as a function of the normalized distance between the magnetic source and the ME gradiometer. Based on resulting data, the proposed ME gradiometer was confirmed to be capable of significant vibration noise rejection. However, this method is not appropriate for rejecting longitudinal vibrations due to the propagation direction being the same as the magnetic field. To resolve this dilemma, a new ME laminate structure was designed that could better reject vibrational noise. Additionally, two different configurations were developed to measure the gradiometer curve.

Second, in order to understand how much energy can be wirelessly transmitted by ME laminates within a local area, a portable (area ~ 16 cm^2), a very low-frequency transmitter was developed using ME laminate with Metglas/PZT structure. The proposed strain-driven ME laminate transmitter functions as follows: (a) a piezoelectric layer is first driven by alternating current electric voltage at its electromechanical resonance (EMR) frequency; (b) subsequently, this EMR excites the magnetostrictive layers, giving rise to a magnetization change; (c) in turn, the magnetization oscillations result in oscillating magnetic fluxes, which can be detected through the use of a search coil as a receiver. The prototype measurements revealed an induction transmission capabilities in the near field. Furthermore, the developed prototype evidenced a 10^4 times higher efficiency in the near field over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas.

Next, recent efforts in our group resulted in the development of an ME gyrator based on ME heterostructures. Such gyrators facilitate current-to-voltage conversion with high power efficiency. ME gyrators working at their resonance frequency are capable of converting power with an efficiency of > 90 %, which show potential for use in power convertors. Here, we found that the resonance frequency could be tuned through the use of a frequency-modulation technique. Accordingly, this method can be utilized to match the frequency difference between the power supply and the piezoelectric transducer in actual applications, which will increase the power efficiency. Another problematic issue is that the electromechanical coupling factor of piezoelectric transducers is limited by bandwidth. Typically, transducers cannot be impedance matched to a power supply, which significantly reduces power efficiency. Our initial studies have shown that an improved impedance match can be realized by using an ME gyrator to geometrically tune a transducer, which will substantially enhance power efficiency.

The last chapter will mainly focus on ME gyrator applications. Designing linear power amplifiers that operate reliably at high frequency is quite challenging, which is mainly due to the fact that the parasitic impedances of their electronic components tend to dominate at higher frequencies, thereby leading to significant power-efficiency loss. Therefore, ME gyrator may play an important role between the power amplifier and the acoustic transducer to reduce the power loss. In this chapter, we achieved the impedance matching between a piezoelectric transducer and a power supply by implementing geometric changes to the gyrator. Both the power efficiency of an individual ME gyrator and a piezoelectric transducer are > 90%. Therefore, the total power efficiency of the ME gyrator and the piezoelectric transducer also approach > 80% when they got connected together. The second aspect of this chapter pertains to resonance-frequency tuning using three method. Since an ME gyrator will be used to achieve impedance matching, the resonance frequency of the ME gyrator and a piezoelectric transducer may not exactly match. This limitation will be overcome through capacitance tuning of the piezoelectric transducer in order to achieve frequency matching. Finally, an equivalent circuit will be developed that connects a piezoelectric transducer with a gyrator, thereby enabling the impedance of the output port of the transducer and the shifted EMR frequency of the transducer to be modified.