|Title:||Magnetoelectric effect as a function of lattice coupling and microstructure in ferroelectric/magnetostrictive composites|
|Subject:||Composite materials -- Magnetic properties.|
Composite materials -- Electric properties.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||xxv, 174 p. : ill. ; 30 cm.|
|Abstract:||Magnetoelectric (ME) effect is defined as the induction of dielectric polarization P by an applied magnetic field H (i.e. direct ME effect, or MEH effect: P = αH, where α is called coupling coefficient), and/or the induction of magnetization M by an external electric field E (i.e. converse ME effect, or MEE effect: M = α'E). Among all the composite materials reported in the literature, systems that consist of a piezo-electric phase (represented by BaTiO₃, PZT, PVDF and etc.) and a magnetostrictive phase (represented by Terfenol-D, CoFe₂O₄ and etc.) are most widely investigated. ME effect in composites, normally measured by the ME coupling efficient, α, is influenced by a number of factors including composition and structure of each individual phase, the way that all phases are connected, external electrical/magnetic conditions, and so on. While significant progress has been made on the research and development of ME composites over the last decade, there are still a number of questions remaining to be answered. The research work for this thesis has been focusing on the following three issues, which, as we believe, are critical for better understanding of the structure - property relationship in ferroelectric/magnetostrictive composites: direct observation of lattice coupling, analysis of nonlinear piezoelectric response of the ferroelectric phase and analysis of the percolation effect due to the low-resistivity magnetostrictive phase. More details are given below. First, an experiment was conducted for direct observation of coupled lattice distortion of the ferroelectric and magnetostrictive components in a 0 - 3 type com-posite consisting of PZT and CFO. In the experiment, the composite sample was prepared via conventional ceramic processing. With a static electric field applied on the composite, changes in lattice parameter of both PZT and CFO were observed by means of X-ray diffraction running in an ultra-slow mode. The experimental results have verified a coupled lattice distortion in the composite, providing an experimental evidence for the long-existing assumption that the ME effect is based on mechanical coupling. Based on the lattice distortion under different electrical fields, the ME cou-pling coefficient was estimated and found to match well with experimental data.|
Secondly, the influence of microstructural inhomogeneity on the magnetoelectric coupling effect was studied. It is important to note that the piezoelectric coefficient of PZT is sensitive to electrical field strength. In a 0 - 3 composite, the electrical field around CFO particles could be distorted due to the relatively low-resistivity of CFO. Analysis was made to investigate how such electrical field inhomogeneity would affect the performance of the composite via finite element analysis method. An averaged method was developed to evaluate the overall piezoelectric property. The computation results show that the difference in electrical property between PZT and CFO causes a large weakening in PZT's piezoelectric property. Such weakening effect concentrates in the interaction regions between the two phases, which will finally cause an enlarged weakening in the converse magnetoelectric effect. Thirdly, the influence of percolation effect on the magnetoelectric performance of the composite has been studied by numerical approach. The percolation effect in the composite is also due to the relatively low resistivity of CFO. As a result, there exists an upper limit to the volume fraction of the magnetostrictive phase and consequently a limit to the coupling coefficient as well. Such percolation problem could be solved through modification of the microstructure - in particular the grain boundaries in the composite. Numerical approaches have been made to reveal how effective it could be to improve the coupling effect through modification of the grain boundaries. Experiments were also conducted to verify the effect. In the experiment, composites with a core-shell type of microstructure were prepared. Wrapped with a thin layer of zirconia, CFO particles became electrically resistive and they would not form electrical conduction paths in the composite. With the shell to act as a barrier to electron and vacancy transportation, the leakage current of the composite material was significantly reduced, giving rise to enhanced coupling effect. In addition to the above work, effort has also been made to develop prototype devices using PZT - CFO composites as key materials. One of the important devices that have been developed was a magnetically tunable piezoelectric-transformer. The device are designed with a step-down transformer structure and its response to both AC electric field (with/without DC bias) and an AC magnetic field were investigated respectively, which showed good sensitivity to both DC and AC magnetic field and are potentially useful for magnetic field detection.
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