Lead - Free Piezoelectric Based Magnetoelectric Composites

• Main Author: Yang, Su Chul
• Other Authors: Materials Science and Engineering
• Format: Others
• Published: Virginia Tech 2014
• Subjects: Lead-free; Piezoelectric; Magnetoelectric and self-bias.
• Online Access: http://hdl.handle.net/10919/49560

The prime objective of this dissertation is to design, synthesize and characterize lead-free piezoelectric based magnetostrictive components based magnetoelectric (ME) composites that exhibit self-bias characteristics and high amplitude of ME coupling. The secondary goal of this thesis was to lay down the foundation for nanoscale and flexible magnetoelectric devices. Self-biased ME effect was investigated in lead-free three-phase laminate composites. This effect is characterized by non-zero remanent ME responses at zero magnetic bias field (Hbias). It was revealed that the self-biased ME effects can be observed in three-phase laminate composites consisting of piezoelectric material and two dissimilar magnetostrictive materials. On applying Hbias to the laminates in bending mode configuration, the ME responses were found to exhibit hysteretic behaviors with remanent ME responses. The shape of hysteretic ME response could be controlled by adjusting the magnetic interactions and piezoelectric properties. Further, converse magnetoelectric (CME) responses in bending-mode three-phase laminates exhibited hysteretic behaviors with similar magnitudes during Hbias sweep as it was generated directly by applying ac voltage (Vac) without any external Hbias. Lead-free (1 - x) [0.948 K0.5Na0.5NbO3 - 0.052 LiSbO3] - x Ni0.8Zn0.2Fe2O4 (KNNLS-NZF) compositions were synthesized for optimizing ME properties of particulate composites. Island-matrix microstructure was developed to improve the magnitude of ME coupling effect by overcoming the problems found in conventional particulate composites. The structure lead to improvement of ME coefficient with maximum magnitude of 20.14 mV/cm ae as well as decrease of optimum Hbias of < 500 Oe in the composition of 0.7 KNNLS - 0.3 NZF particulate composites. Room-temperature ME phase diagram of (1 - x) BaTiO3 - x BiFeO3 materials (BT - x BFO, x = 0.025 - 1.0) was investigated for designing compositions suitable for thin film devices. The BT - x BFO compositions in narrow range of x = 0.71 - 0.8 were found to exhibit good piezoelectric, dielectric and magnetic properties simultaneously. The room temperature ME coefficient was found to be maximum with high magnitude of 0.87 mV/cmOe in the optimized composition of x = 0.725.This composition was found to consist of local monoclinic distortions with average rhombohedral symmetry as confirmed by detailed structural analysis through Raman spectroscopy and atomic pair distribution functions (PDFs). MnFe2O4 (MFO)-Ni core-shell nanoparticles were synthesized and characterized for developing tunable devices such as memristor. The MFO nanoparticles synthesized by solvothermal method exhibited diameter of 200 nm, mean primary particle size of 15 nm, high saturation magnetization of 74 emu/g and coercivity of 89 Oe. Ni encapsulation on MFO nanoparticles was performed by aqueous ionic coating method. Ni shells with uniform thickness of 1 nm were coated on MFO nanoparticles by this method. In order to develop future nanoscale dual phase energy harvesters and magnetic field sensors, vertically-aligned piezoelectric nanorods were synthesized. In the initial attempt, Pb(Zr0.52Ti0.48)O3 (PZT) was used to verify the feasibility of developing one dimensional (1D) piezoelectric nanostructures with controlled diameter and height. For the 1D nanostructure, well-ordered anodic aluminum oxide (AAO) templates were prepared by two step aluminum anodizing. The PZT nanorods were synthesized by vacuum infiltration of PZT precursor solutions and exhibited uniform diameter of 90 nm and aspect ratio of 10 with vertical in respect to the Pt-Si substrate. The piezo-response of PZT nanorods showed good magnitude owing to the reduced clamping effect from the substrate. Attempt towards the development of flexible tunable devices that possess magnetic field sensing and actuation ability was made in the later part of the thesis. The electroactive polymeric actuators in the form of Polypyrrole (PPy) / Au / Polyvinylidene fluoride (PVDF) / Au / Polypyrrole (PPy) were synthesized and the process flow was optimized. Pore size and thickness of PVDF layer was adjusted by changing the solvent, viscosity and drying temperature. Different types of electrolyte solutions were investigated to improve the strain and response time. The actuators exhibited high deflection of 90 % with fast response of 50% deflection per second. Dual-functional structure in the form of  PPy-MFO / Au / PVDF / Au / PPy-MFO was developed by PPy polymerization including MFO nanoparticles via cyclovoltammetric method. === Ph. D.