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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Perovskite-like Layered Structure A2B2O7 Ferrlelectrics with Super-High Curie Points

Author: NING, Huanpo

Year: 2011

Supervisor(s): Mike Reece

This project focuses on ferroelectric compounds, Sr2Nb2O7, Ca2Nb2O7, La2Ti2O7 and Nd2Ti2O7, in the A2B2O7 family. They have a provskite-like layered structure (PLS), and are known for their super-high Curie points. These materials are very promising candidates for high temperature and lead-free applications. The powders were prepared by mixed oxides route, except for the La2Ti2O7 nanopowder which was prepared by a co-precipitaton method. Due to the high poling fields required to pole the A2B2O7 ceramic samples, grain-oriented (textured) ceramics were fabricated by Spark Plasma Sintering using a two-step method. The degree of texture was evaluated using the Lotgering orientation factor from the X-ray diffraction patterns. Their electrical, dielectric, piezoelectric and ferroelectric properties were characterized. They exhibited high Curie point, high coercive field, and good thermal and electrical stability. Their Curie points Tc were characterized by measuring the temperature dependence of the dielectric constant at different frequencies (100 Hz to 1 MHz). The Tc of NTO, LTO, SNO were 14825 C, 14615 C, and 13275 C, respectively. The Curie point of CNO was above 1550 C. The textured ceramics exhibited improved piezoelectric and ferroelectric response compared with untextured ceramics. The ferroelectric hysteresis loops were measured in silicone oil at 200 °C and 10 Hz. Ferroelectric switching was demonstrated by the peaks in the current-electrical field hysteresis loops. Poling experiments were carried out in silicone oil at 200 °C under DC fields. The piezoelectric constants d33 of the textured LTO, NTO and SNO were 2.60.2 pC/N, 0.60.2 pC/N, 2.80.2 pC/N, respectively. These materials also showed high resistivity (1 M.cm at 600 °C) and the resistivity shows high stability during electrical stability measurement. The effect of cerium, tungsten and vanadium doping on the Curie point and other properties of SNO was investigated. Cerium and tungsten doping on A and B site, respectively, was found to inhibit the grain growth and change the grain morphology. The grains become less anisotropic and less plate-like. The Curie point Tc was reduced after cerium and tungsten doping. Vanadium doping on B site increased the Curie point of SNO, but it has little effect on the grain size and morphology. The factors influencing the Curie point of perovskite-related compounds were explored and discussed. This work successfully prepared some A2B2O7 ceramics and systemically characterised their properties. This can open up the possibility of studying more ferroelectric materials from the PLS family with super-high Curie points.