Evolution of structure and electrical properties during crystallization of rapidly solidified bismuth(4)titanium(3)oxygen(12)
Drozdyk, Lorri
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https://hdl.handle.net/2142/23291
Description
Title
Evolution of structure and electrical properties during crystallization of rapidly solidified bismuth(4)titanium(3)oxygen(12)
Author(s)
Drozdyk, Lorri
Issue Date
1989
Doctoral Committee Chair(s)
Payne, David A.
Department of Study
Engineering, Materials Science
Discipline
Engineering, Materials Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Amorphous Bi$\sb4$Ti$\sb3$O$\sb{12}$ was prepared by rapid solidification from the melt. Crystallization and microstructure development during heat treatment were investigated. Dielectric and ferroelectric properties were related to microstructure.
Due to the absence of traditional glass-forming oxides, a twin-roller quencher was used to provide the high quench rate necessary for the retention of the amorphous phase during cooling. Formation of an amorphous phase was confirmed by thermal analysis, x-ray diffraction, electron diffraction, and optical microscopy. The d.c. resistivity was 2 $\times$ 10$\sp{14}$ $\Omega$-cm and the dielectric constant was 84 for amorphous Bi$\sb4$Ti$\sb3$O$\sb{12}$. The relatively high dielectric constant was attributed to the presence of TiO$\sb6$ octahedral units in the amorphous structure. Ferroelectricity was not observed, due to the absence of long range order.
Crystallization behavior during heat treatment was investigated by DSC, XRD, and TEM. Direct transformation to the expected crystalline phase occurred without any major intermediary steps. Transformation kinetics were determined by a variety of techniques. Isothermal heating at low temperatures resulted in crystals typically near 500 A in size, but as large as 1500 A. Fully crystallized material had a dielectric constant which was 4 times that of the amorphous phase. The evolution of dielectric properties during heat treatment and crystallization were consistent with dielectric mixing rules for the observed microstructures.
Fully crystallized material had dense microstructures, with a uniform fine-grain size of $<$1 $\mu$m. For decreasing heat treatment temperature, a decrease in grain size was observed, with an increase in residual strain, and a decrease in occurrence of domains. In addition to the observations for microstrain and domains, features of the dielectric anomaly at the Curie temperature implied increasing internal stress for materials heat treated at lower temperature. Fine-grain materials had dielectric constants which were 2-3 times greater than for single crystal, and d.c. resistivities were 10$\sp{12}$ - 10$\sp{14}$ $\Omega$-cm, depending on the heat treatment conditions. The enhanced dielectric constant which increased with decreasing grain size was attributed to internal stress. Ferroelectric hysteresis was observed for fully crystallized material. Coercive fields for fine-grain material were higher than for single crystal material, and materials with smaller grain sizes had higher coercive fields.
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