Effect of minor phase additions on the microstructure and dielectric properties of barium titanate ceramics
Armstrong, Timothy Robert
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https://hdl.handle.net/2142/19069
Description
Title
Effect of minor phase additions on the microstructure and dielectric properties of barium titanate ceramics
Author(s)
Armstrong, Timothy Robert
Issue Date
1989
Doctoral Committee Chair(s)
Buchanan, Relva C.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Materials Science and Engineering
Language
eng
Abstract
Grain growth in BaTiO$\sb3$ can be significantly inhibited by insoluble or slightly soluble second phase additions such as ZrO$\sb2$, or by fluxes in the CaO$\cdot$Al$\sb2$O$\sb3\cdot$B$\sb2$O$\sb3$ system. SEM and TEM observations revealed enhanced microstructural uniformity in these specimens. For the ZrO$\sb2$ modified BaTiO$\sb3$ system, Zr diffused into the BaTiO$\sb3$ lattice when sintered above 1320$\sp\circ$C, resulting in a chemical modification of the tetragonal structure and the development of core-shell grains. TEM analysis of ZrO$\sb2$ modified BaTiO$\sb3$ sintered at temperatures less than 1320$\sp\circ$C, revealed the presence of ZrO$\sb2$ at the grain boundaries as discrete particles ($\approx$0.03 $\mu$m). Modification of BaTiO$\sb3$ with organically derived ZrO$\sb2$ resulted in the formation of substantial chemical inhomogeneities in the microstructure. These inhomogeneities were observed by TEM as large ZrO$\sb2$ particles ($>$75nm) and as a Ti and Zr rich phase, Ba$\sb2$(ZrTi)$\sb5$O$\sb{12}$. In the CaO$\cdot$Al$\sb2$O$\sb3\cdot$B$\sb2$O$\sb3$ modified BaTiO$\sb3$, fluxed samples had an amorphous grain boundary phase, which became microcrystalline upon addition of ZrO$\sb2$. For these samples core-shell grains were observed in specimens sintered at 1175$\sp\circ$C for 2h. More noticeable in the samples were regions near the grain boundaries where significant diffusion of the flux constituents into the BaTiO$\sb3$ occurred, resulting in the formation of a fine domain structure. These fine-grained samples were accompanied by an increase in the internal stress level with decreasing grain size and increasing concentration of core-shell grains. The core-shell grains are comprised of a core of unmodified BaTiO$\sb3$ and a shell of Zr modified BaTiO$\sb3$, Ba(Zr$\sb{\rm x}$Ti$\sb{\rm 1-x}$)O$\sb3$. This condition leads to high internal stress levels, arising from the thermal expansion mismatch between the core and the shell. The stress was found to be compressive in nature and dependent on temperature, time and grain size. The combination of internal stress and core-shell grains resulted in a decrease in the axial ratio $(c/a)$ with decreasing grain size and an essentially flat permittivity response with temperature, a condition consistent with a distribution of Curie points within the ceramic. These modified dielectrics also show a decrease in the aging rate with increasing temperature, resulting from the internal stress condition.
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