Year

2022

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The ultrahigh properties of relaxor‒lead-titanate (relaxor-PbTiO3) crystals have been proved to offer dramatic enhancements to electromechanical devices. The developmental stage of relaxor- PbTiO3 (PT) ferroelectrical crystals comprises three generations. The class of binary Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals represents the first generation, which exhibits giant electromechanical properties and piezoelectric coefficients. The ternary system Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) represents the second generation, which demonstrates a greater coercive field (EC), a higher rhombohedral-to-tetragonal phase transition temperature (Tr-t) and a higher Curie temperature (TC) than those of the first generation. The third generation is Mn-modified Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (Mn:PIN-PMN-PT), which possesses a higher mechanical quality factor (Qm) than those of the first and second generations, while it maintains comparable piezoelectric responses to those of generations I and II.

There are several techniques for the growth of relaxor-PT single crystals. Flux growth is a simple method, but its small output is inconvenient for mass production. The solid-state conversion growth (SSCG) method offers large quantities of single crystals. Furthermore, its operation is simple and costeffective. The quality of the single crystal is low, however, due to porosity and defects. The modified Bridgman method is a straightforward way of synthesizing large quantities of relaxor-PT crystals. Single crystals are directly grown from a molten ingot passing through a temperature gradient and the solidus line of the solid solution phase diagram. Compositional segregation is an unavoidable disadvantage of this method, although several modifications have introduced to overcome this issue. Rare-earth doping and a continuous feeding approach have been confirmed to produce single crystals with only low segregation.

In this work, PMN-PT, Sm-modified PMN-PT, and Mn-modified PIN-PMN-PT single crystals are grown using the modified Bridgman method. A new vertical Bridgman furnace was assembled in the Institute for Superconducting and Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong. Ceramic materials were prepared using the two-step precursor method. Optimal conditions for ceramic synthesis have been studied. The dielectric and piezoelectric properties of ceramic materials were investigated and confirmed to be good when compared them to reference values. PMN-PT single crystal was grown first. The physical appearance of the as-grown crystal was cloudy, and several grains were developed, as seen from cross-sectional view. The bottom part of the as-grown single crystal was also un-melted. The reason for these issues was that the charge was lifted to a position where the raw ceramic was not able to melt completely. Therefore, only the part in tapered section of the Pt crucible was melted, from which the crystallization was started with different grains. Learning from the first growth experiment, the next crystal growths were carefully carried out with operational condition’s changes. The growth procedure was carried out with a higher charge position and higher temperature in the upper zone to generate a greater temperature gradient. Consequently, pure and high quality Sm-modified PMN-PT and Mn-modified PIN-PMN-PT single crystals were grown.

The recent development of relaxor-PT ferroelectric single crystals is also reviewed in this work. This review includes the growth methods, property improvement strategies, and application prospects based on the recent progress.

FoR codes (2008)

091201 Ceramics, 091205 Functional Materials

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.