Electron microscopy studies in electrical ceramics: Dielectric composites and high critical temperature superconductors
Xu, Zhengkui
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Permalink
https://hdl.handle.net/2142/22411
Description
Title
Electron microscopy studies in electrical ceramics: Dielectric composites and high critical temperature superconductors
Author(s)
Xu, Zhengkui
Issue Date
1991
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
This thesis is principally concerned with the microstructural characterization of dielectric composites in the system BaTiO$\sb3$-NaNbO$\sb3$ by transmission electron microscopy (TEM), with the expressed purpose of determining the interrelationships that exist between chemical composition and thermal processing conditions on microstructure development and dielectric properties. The investigation was also extended to recent developments in high T$\sb{\rm c}$ ceramic superconductors in the Bi-Ca-Sr-Cu oxide system.
Fast-fired (t$\sb{\rm s}$ = 1 hr) BaTiO$\sb3$ was of a coarse grain size ($>$10 $\mu$m), containing complicated ferroelectric domain structures which extended to the grain boundaries. Fast-fired composites containing less than 3 mol% NaNbO$\sb3$ were of a fine grain structure ($\sim$0.5 $\mu$m). The domains extended to the grain boundaries, and were of a more simple structure. A core-shell type microstructure was identified, which evolved on further additions of NaNbO$\sb3$ (3-15 mol%) in fast-fired composites. Domains were only observed in the grain core, which consisted of essentially unmodified BaTiO$\sb3$. The shell was comprised of a (Ba,Na)(Ti,Nb)O$\sb3$ crystalline solution. In addition, a continuously connected NaNbO$\sb3$-rich intergranular phase was identified.
The core-shell structure was not observed in water-quenched specimens (t$\sb{\rm s}$ = 10 min), nor in specimens which had been extensively fired (t$\sb{\rm s}$ = 19 hrs). The former consisted of unmodified BaTiO$\sb3$ grains, with domains extending to boundaries, and the grains were encapsulated by a NaNbO$\sb3$-rich intergranular phase; while the latter did not contain any domains, and the grains were cubic at room temperature. Na and Nb were distributed quite uniformly throughout the grain and boundary regions. A model for reactive liquid phase sintering was proposed and substantiated by observed microstructural and microchemical characteristics. Features of the ceramic microstructure were related to improvements in the temperature, voltage and time stability of dielectric constant for the composites.
The evolution of microstructure from melt-quenched Bi-Ca-Sr-Cu oxides was also studied by TEM. Prolonged heat-treatments favored the formation of the high T$\sb{\rm c}$ 2223 phase, which developed from the lower T$\sb{\rm c}$ 2122 phase. Additions of PbO and Sb$\sb2$O$\sb3$ reduced the intergrowth content and improved the superconducting properties. A resolidified liquid phase was detected in close contact with the superconducting grains.
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