Degree Type

Dissertation

Date of Award

2008

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

Xiaoli Tan

Abstract

Ferroelectrics are important materials displaying outstanding dielectric, piezoelectric, and ferroelectric properties and are widely used in capacitors, transducers, actuators, filters, sensors, and random access memories. Direct observation of the dynamic process of thermal and/or electric field-induced phase transitions in ferroelectrics is of great importance in understanding the underlying mechanisms of their peculiar properties.

In this work, thermal and electric field in-situ TEM techniques were used to study the dynamic process of antiferroelectric to normal ferroelectric, normal ferroelectric to normal ferroelectric as well as relaxor ferroelectric to normal ferroelectric phase transitions.

A thermal in-situ TEM technique was used to study the antiferroelectric to normal ferroelectric phase transition in 0.98PbZrO3-0.02Pb(Ni1/3Nb2/3)O3 ceramics during heating. The low temperature antiferroelectric phase and intermediate ferroelectric phase are characterized by the 1/4{110}c -type and the 1/2{110}sub>c-type superlattice diffractions, respectively. An incommensurate phase with 1/6.48{110}c satellite diffractions was revealed to exist within a narrow temperature range of 3oC between the antiferroelectric and ferroelectric phases presumably as a result of the competition between two these two types of ordering. The ferroelectric phase is special not only because of its frequency dependence, but also because of its checkerboard type domain morphology.

Electric field in-situ TEM and thermal in-situ TEM techniques are used separately to study the FERL to FERH phase transition in Pb0.99[(Zr0.95Ti0.05)0.982Nb0.018]O3 ceramics. There appear to be different ferroelectric domains associated with the 1/2{111}cc-type superlattice diffraction spots. These two types of domains respond to temperature increase independently. Under the conditions used in this study (thin TEM specimen and field close to ), external electric fields favor the FERH over the FERL phase at room temperature.

A cryo electric field in-situ TEM technique was used to study the relaxor ferroelectric to normal ferroelectric phase transition in 0.92Pb(Mg1/3Nb2/3)O3-0.08Pb(Sc1/2Nb1/2)O3 ceramics. The dynamic phase transition process was observed under TEM for the first time. It is revealed that the electric field-induced relaxor to normal ferroelectric phase transition initiates at grain boundaries. The transition involves the gradual coalescence of polar nanoregions and the successive abrupt formation of large ferroelectric domains. The formed domain/phase walls are roughly along {110} planes. The morphology of the cation ordered domains does not change under applied electric fields. Furthermore, no evidence for strong interactions of these chemical domains with the induced large ferroelectric domains is found in the Sc-doped PMN ceramic.

Copyright Owner

Weiguo Qu

Language

en

Date Available

2012-04-30

File Format

application/pdf

File Size

152 pages

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