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Design, synthesis and characterization of novel, lead-free multiferroic and relaxor materials

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Thesis type
(Thesis) M.Sc.
Date created
Author: Wu, Hsin Yu
New multiferroic solid solutions of (1-x)BiFeO3-xBaHfO3 (BF-BaHf) with x = 0.05 - 0.95 has been prepared by conventional solid-state reaction in the form of ceramics. X-ray powder diffraction revealed that the perovskite structure of BF-BaHf transfers from rhombohedral to cubic symmetry with the increasing concentration of BaHf, resulting in the formation of solid solution. Compared with pure BF, the ferromagnetism in the BF-BaHf solid solution is substantially enhanced by the structural distortion due to A-and B-site co-substitutions, with remanent magnetization Mr = 0.0469 ????" ⁄????. ???? in x = 0.15. Meanwhile, ferroelectric domains were observed by transmission electron microscopy for x = 0.15. This confirms the beneficial role of BaHf substitution in enhancing magnetic properties while maintaining ferroelectric properties of the BF-based multiferroic materials. Another new multiferroic solid solution of pseudo-binary system (1-x)[0.8BiFeO3- 0.2BaHfO3]-xBaTiO3 [(1-x)(0.8BF-0.2BaHf)-xBaT] has been successfully synthesized by solid state reaction with x = 0.15 - 0.95. All the composition studies showed a gradual phase transition from rhombohedral to tetragonal phase as the BaT concentration increases. A morphotropic phase boundary is found in the composition range of 0.70 < x < 0.80. Additionally, weak ferromagnetism at room temperature was observed in the composition range of 0.30 ≤ x ≤ 0.40. The dielectric loss tangent shows a significant decrease as the BaT component increases. Summarily, a phase diagram of the (1- x)[0.8BiFeO3-0.2BaHfO3]-xBaTiO3 solid solution has been established. Single crystals of lead-free BaZr0.275Ti0.725O3 (BZT) have been grown successfully by a high temperature solution growth method after 32 trials. The optimal flux combination to grow the BZT single crystals is a mixture of BaCO3, BaCl2 and B2O3. The size of the as- grown crystals varies from 0.5 to 1 nm. X-ray diffraction indicates a pure perovskite phase. Polarized light microscopy has shown that the crystals are of cubic symmetry at room temperature, suggesting that they are of relaxor behaviour, as initially expected. The BZT single crystal composition is determined by X-ray photoelectron spectroscopy to be consistent with the nominal composition. The atomic concentrations ratio of the elements Ti and Zr are 72.5 % and 27.5%, respectively. This work demonstrates that the BZT single crystal can be grown by high temperature solution growth method and this is the first successful growth.
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Thesis advisor: Ye, Zuo-Guang
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