Graduate Thesis Or Dissertation
 

Relaxor characteristic and phase evolution of complex perovskite barium titanate bismuth zinc titanate ceramics

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  • Relaxor characteristic of the solid solution (1-x)BaTiO₃-(x)Bi(Zn₁/₂Ti₁/₂)O₃ (BTBZT) was investigated in this research. X-ray diffraction technique and Raman spectroscopy revealed the crystal structure of BT-BZT at various compositions. For compositions with x ≤ 0.08, the solid solutions exhibited clear tetragonal symmetry and transitioned to pseudo-cubic symmetry as the content of BZT increased. The dielectric permittivity exhibited a transformation from a sharp phase transition to the relaxor-like behavior as the concentration of BZT increased with the onset at composition of 0.92BT-0.08BZT. The polarization as a function of electric field (P-E) of these solid solutions also exhibited the same trend. The BT-rich compositions showed a normal ferroelectric P-E response with a decrease in loop area as the BZT content increased. The composition of x = 0.08 exhibited a pinched hysteresis loop and with a further increase in BZT content, the P-E response was characterized by slim loops. Phase formation of BT-BZT was also studied in order to investigate the optimum processing parameters. The result showed that the formation mechanisms of BT-BZT were different than what was observed in BT. However, addition of BZT accelerated the reaction of the BaCO₃ species and increasing the diffusion kinetics associated with densification and reduced the calcination and sintering temperatures by ~200°C compared to unmodified BaTiO₃. This system provides an example of the important and often overlooked role of additives in the calcination, phase evolution, and densification processes, and provides insight into mechanisms that may be further exploited in this and other important materials systems. Heat treatment, a common tuning properties method, was operated on 0.9BT-0.1BZT. Sintered ceramics were subjected to a series of thermal anneals at temperatures of 900, 1000, 1100 and 1175°C for 50 hrs. A slightly observable change in the crystal structure of as-sintered and annealed samples was observed by using XRD technique. However, SEM images showed a clear evidence of grain growth when the ceramic was annealed up to a temperature of 1175°C. The relaxor-like dielectric behavior exhibits no significant differences between as-sintered and post-annealed ceramics. The samples showed slim loops of polarization as a function of electric field with no dramatic shifts observed at various annealing temperatures. These annealing results can be concluded that the grain size does not significantly affect the dielectric properties, instead, the origin of the dielectric response is linked to the electrical heterogeneous structure of 0.9BT0.1BZT.
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