Microstructure and Dielectric Behavior for Pure and Bismuth Doped Strontium-Titanate Polycrystalline Ceramics
Mehrotra, Arun Kumar
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https://hdl.handle.net/2142/71702
Description
Title
Microstructure and Dielectric Behavior for Pure and Bismuth Doped Strontium-Titanate Polycrystalline Ceramics
Author(s)
Mehrotra, Arun Kumar
Issue Date
1982
Department of Study
Ceramics Engineering
Discipline
Ceramics Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
Compositions in the system strontium-bismuth titanate are used in the dielectric ceramics industry where variability in properties has been attributed to the high vapor pressure of bismuth. The purpose of this investigation was to study the development of microstructure and properties obtained for ceramics prepared under controlled but varying processing conditions.
Compositions in the solid solution series were synthesized according to the formula Sr(,1-1.5x)Bi(,x)(SQUARE)(,0.5x)TiO(,3), and sintered for differing conditions of temperature, time, oxygen partial pressure and bismuth or non-bismuth containing atmospheres. The resulting microstructures were evaluated by optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. Parameters analyzed were grain size, pore size and respective distributions, and spatial distributions of the elements within the microstructure. The presence of internal boundary layers was identified in polycrystalline ceramics sintered under controlled atmosphere conditions.
Grain growth in strontium titanate was attributed to oxygen vacancy migration, possibly singly ionized, and was affected by relative surface and bulk diffusion processes. Discontinuous grain growth started when bulk processes predominated over surface processes. Additions of bismuth exerted sufficient solute drag to retard grain growth, and for certain conditions prevented discontinuous grain growth. Solute drag occurred when bismuth was stabilized in the system. If bismuth evaporation was allowed to occur, the tendency for grain growth increased due to insufficient solute drag. Stabilization of bismuth resulted in bismuth rich grain boundaries, otherwise, bismuth deficient boundaries were obtained.
Ceramics with bismuth rich boundaries had anomalous dielectric properties, with an apparent "Curie-point" which was frequency dependent, and dielectric dispersion at low temperatures. This behavior could be explained in terms of a series mixing model based upon resistivity control and microstructural characteristics, which were in accordance with experimental observations.
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