Magnetoelectric (ME) effect is defined as the change in dielectric polarization (P) of a material under an applied magnetic field (H) or an induced magnetization (M) under an external electric field (E). ME materials have attracted number of investigators due to their potential for improving applications such as magnetic field sensors, filters, transformers, memory devices and energy harvesters. It has been shown both experimentally and theoretically that the composite structures consisting of piezoelectric and magnetostrictive phases possess stronger ME coupling in comparison to that of single phase materials. Giant magnetoelectric effect has been reported in variety of composites consisting of bulk-sized ME composites and thin film ME nanostructures. In this dissertation, novel ME composite systems are proposed, synthesized and characterized in both bulk and thin films to address the existing challenges in meeting the needs of practical applications. Two applications were the focused upon in this study, tunable transformer and dual phase energy harvester, where requirements can be summarized as: high ME coefficient under both on-resonance and off-resonance conditions, broad bandwidth, and low applied DC bias.
In the first chapter, three challenges related to the conventional ME behavior in bulk ME composites have been addressed (1) The optimized ME coefficient can be achieved without external DC magnetic field by using a self-biased ME composite with a homogenous magnetostrictive material. The mechanism of such effect and its tunability are studied; (2) A near-flat ME response regardless of external magnetic field is obtained in a self-biased ME composite with geometry gradient structure; (3) By optimizing interfacial coupling with co-firing techniques, the ME coefficient can be dramatically enhanced. Theses co-fired ME laminates not only exhibit high coupling coefficient due to direct bonding, but also illustrate a self-biased effect due to the built-in stress during co-sintering process. These results present significant advancement toward the development of multifunctional ME devices since it eliminates the need for DC bias, expands the working bandwidth and enhances the ME voltage coefficient.
Next, magnetoelectric nanocomposites were developed for understanding the nature of the growth of anisotropic thin film structures. In this chapter following aspects were addressed: (1) Controlled growth of nanostructures with well-defined morphology was obtained. Microstructure and surface morphology evolution of the piezoelectric BaTiO3 films was systematically analyzed. A growth model was proposed by considering the anisotropy of surface energy and the formation of twin lamellae structure within the frame work of Structure Zone Model (SZM) and Dynamic Scaling Theory (DST). In parallel to BaTiO3 films, well-ordered nanocomposite arrays [Pb1.1(Zr0.6Ti0.4)O3/CoFe2O4] with controlled grain orientation were developed and investigated by a novel hybrid deposition method. The influence of the pre-deposited template film orientation on the growth of ME composite array was studied. (2) PZT/CFO/PZT thick composite film and BTO/CFO thin film were synthesized using sol-gel deposition (SGD) and pulsed laser deposition (PLD) techniques, respectively. The HRTEM analysis revealed local microstructure at the interface of consecutive constituents. The interfacial property variation of these films was found to affect the coupling coefficient of corresponding ME nanocomposites. Subsequently, a novel complex three-dimensional ME composite with highly anisotropic structure was developed using a hybrid synthesis method. The influence of growth condition on the microstructure and property of the grown complex composites was studied. The film with highly anisotropic structure was found to possess tailored ferroelectric response indicating the promise of this synthesis method and microstructure.
Based on the laminated ME composites, three types of ME tunable transformer designs were designed and fabricated. The goal was to develop a novel ME transformer with tunable performance (voltage gain and/or working resonance frequency) under applied DC magnetic field. Conventional ME transformers need either winding coil or large external magnetic field to achieve the tunable feature. Considering the high ME coupling of ME laminate, two ME transformers were developed by epoxy bonding Metglas with transversely/longitudinally poled piezoelectric ceramic transformer. The influence of different operation modes toward magnetoelectric tunability was analyzed. In addressing the concern of the epoxy bonding interface, a co-fired ME transformer with unique piezoelectric transformer/magnetostrictive layer/piezoelectric transformer trilayer structure was designed. The design and development strategy of thin film ME transformer was discussed to illustrate the potential for ME transformer miniaturization and on-chip integration.
Lastly, motivated by the increasing demand of energy harvesting (EH) systems to support self-powered sensor nodes in structural health monitoring system, a magnetoelectric composite based energy harvester was developed. The development and design concept of the magnetoelectric energy harvester was systematically discussed. In particular, the first dual-phase self-biased ME energy harvester was designed which can simultaneously harness both vibration and stray magnetic field (Hac) in the absence of DC magnetic field. Strain distribution of the EH was simulated using the finite element model (FEM) at the first three resonance frequencies. Additionally, the potential of transferring this simple EH structure into MEMS scalable components was mentioned. These results provide significant advancement toward high energy density multimode energy harvesting system.
Ph. D.