Date of Completion

1-13-2014

Embargo Period

1-14-2015

Keywords

PZT, Piezoelectric, Tricritical Point, Process-Structure-Property Relationship

Major Advisor

George Rossetti, Jr.

Associate Advisor

S. Pamir Alpay

Associate Advisor

Pu-Xian Gao

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Solid solutions of lead zirconate titanate [PbZr1-xTixO3 (PZT)] are extensively used in electromechanical transducers. A maximum in dielectric and piezoelectric response is observed near the morphotropic phase boundary (MPB) separating rhombohedral and tetragonal ferroelectric phases. The origin of the enhanced properties near the MPB remains controversial and has been variously attributed to coexisting rhombohedral and tetragonal ferroelectric phases, to the formation of nanodomains, and/or to lower symmetry monoclinic phases. Hence, the phase diagram of PZT in the region of the MPB remains open to debate.

In this work, dense polycrystalline PZT ceramics prepared by chemical methods were subjected to different time-temperature histories to investigate the origins of the two-phase coexistence and to determine the influence of thermal history on structure, microstructure and dielectric properties. Long annealing (240 hours) above the Curie temperature (Tc) revealed a slow relaxation process that was manifested in changes of structural properties. The changes in structural properties were accompanied by changes in the behavior of the paraelectric to ferroelectric phase transition, the domain structure, and the extrinsic contributions to dielectric permittivity. The changes in all these properties were found to show maxima near the MPB. The combined results showed that PZT ceramics made by a normal ceramic processing were not in their equilibrium state near the MPB. However, no clear evidence of phase decomposition into an equilibrium mixture of tetragonal and rhombohedral phases was found. Instead, the changes in structural and dielectric properties observed on annealing were most consistent with a stress relief mechanism that provided for the coarsening of the domain structure and increased domain wall contributions to dielectric properties.

The results provide evidence that the domain structures and electromechanical properties of PZT compositions near the MPB can be controlled without dopants by processing using differing thermal histories. In addition, results obtained for un-annealed PZT ceramics were used to resolve the long controversial issue of tricritical behavior at the paraelectric to ferroelectric phase transition, to separate intrinsic and extrinsic contributions to the dielectric response, and to quantify reversible and irreversible domain wall motion.

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