| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 87 === In this paper, calcium-modified lead titanate (Pb1-xCaxTiO3, PCT) ceramics, which have different calcium contents, were prepared using oxide-mixing method. We focused on understanding the reasons of the ferroelectrical fatigue by means of X-ray diffractometer, modified Sawyer-Tower circuit, and transmission electron microscopy. The c/a ratio approached to 1 with increasing calcium content. In the polarization-electric curve, we found coercive field increased when the ordering degree was higher for specimens sintered at 1200℃ for 2, 4, and 20 hours.
In the electric fatigue properties, it is shown that the remanent polarization (Pr) increased and coercive field (Ec) decreased after switching up to 2x106 cycles for x=0.5. When x=0.4, the P-E curves show an increasing trend in Pr and a first-decrease-then-increase tendency in Ec after switching up to 2x106 cycles. When x=0.3, the P-E curves reveal an increasing trend in Ec and a first-increase-then-decrease inclination of the Pr after switching up to 3x106 cycles.
There is an ordering structure when x=0.4 and 0.5. The ferroelectric domain boundaries were constrained by the ordering domain that reduced the switchability of ferroelectric domain boundaries at the beginning. After polarization switching, the ferroelectric domain boundaries were releases from the constraining of the ordering domains, and the coercive field decreased.
In the microstructure evolution, most of the grains have few domains and simple domain arrangements for as-sintered specimens of x=0.5. We also observed more complicated domain structure in a few grains after repeated electric field application, comparing with that of as-sintered specimens. When x=0.3, most grains show slightly complicated domain structures inside as-sintered specimens. After switching under electric field, the microstructures became more and more complicated due to domain intersections. The intersections of ferroelectric domains induce large strain fields, which distorted the crystal lattice, and then we observed many microcracks in the materials for 3x106 polarization switching.
In summary, the ordering domain affects the growth and switchability of ferroelectric domain. After repeated electrical switching, the influence of ordering domain decreases. In addition, microstructural investigations indicate that domain boundaries are induced under repeated electric field application and the domain structures become more and more complicated due to domain boundary pinning and nucleation. We observed large internal stress distorting the crystal lattice and many microcracks were found in the materials.
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