Structure-property relationships and phase stability in the tin-modified lead zirconate titanate crystalline solution series
Forst, Donald William
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https://hdl.handle.net/2142/19230
Description
Title
Structure-property relationships and phase stability in the tin-modified lead zirconate titanate crystalline solution series
Author(s)
Forst, Donald William
Issue Date
1996
Doctoral Committee Chair(s)
Viehland, Dwight D.
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)
Physics, Condensed Matter
Engineering, Materials Science
Language
eng
Abstract
This thesis will try to explain the influence of Sn on the phase stability and the transformational characteristics of high zirconium content lead zirconate titanate (PZT) ceramics. First, microstructural studies were done as a function of titanium content. An incommensurate phase was observed across much of the compositional range and was defined by the appearance of superlattice reflections along the $$ direction which were not rational multiples of the fundamental periodicity along that direction. The incommensurate modulation wavelength, $\lambda,$ was found to increase as the Ti content and the temperature were increased. Another phase region was found with decreasing temperature, designated as the metastably-locked incommensurate state, where the polarization modulations are believed to be pinned by the quenched Sn impurities. This metastably-locked incommensurate region was previously believed to be tetragonal, however results presented in this thesis will show it actually has orthorhombic symmetry.
The dielectric properties of Sn-doped lead zirconate titanate ceramics have been extensively studied as a function of Sn and Ti contents. No macroscopic symmetry changes were found near the dielectric maximum and strong deviations from Curie-Weiss behavior were found far above the dielectric maximum. In addition, slim-loop antiferroelectric-ferroelectric P-E curves were observed in the incommensurate region, indicating a thermally reversible electrically-induced AFE-FE transformation. Hysteresis was found to develop on cooling into the metastably-locked incommensurate region. Investigations revealed that the electrically-induced strain was decoupled from the polarization due to polarization switching by the modulation of the incommensurate structure.
Thermal-expansion investigations as a function of composition have revealed the existence of negative deviations from linearity at temperatures far above that of the dielectric maximum. Coupled with Curie-Weiss plots and the appearance of F-spots to high temperatures, an oxygen tilt mechanism is proposed to account for the nonlinearity. Evidence of the importance of compositional fluctuations in PZST was found by studying the effect of annealing (grain growth) on the microstructure-property relationships. SnO$\sb2$ inclusions were found to develop in unannealed specimens. The effects on the properties that were observed with lesser grain growth included a reduction in electrically-induced polarization and strain responses, a reduction in dielectric constant, and a decrease in modulation wavelength.
Finally, structure-property relationships were performed as a function of Sn content. It was found that at a critical Sn content, antiferroelectric and ferroelectric phases coexist. The presence of a multi-cell cubic state was not evident until Sn-contents greater than 20 at.%, whereas an incommensurate phase was detected above $\sim$15%. At extremely high Sn contents ($>$50%), large compositional fluctuations were observed, giving rise to a range of incommensurately modulated AFE phases, as well as substantial SnO$\sb2$ inclusions.
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