PhD Proposal by Yulian Yao
THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Friday, June 14, 2019
in Love 210
will be held the
DISSERTATION PROPOSAL DEFENSE
"Electric-Field-Induced Phase Transitions in Antiferroelectric PbZrO3 Thin Films"
Prof. Nazanin Bassiri-Gharb, Advisor, ME/MSE
Prof. Rosario Gerhardt, MSE
Prof. Hamid Garmestani, MSE
Prof. Josh Kacher, MSE
Prof. Eric Vogel, MSE
xPbZrO3-(1-x)PbTiO3 (PZT) thin films are widely used in microelectromechanical systems (MEMS) and memory applications for their high electromechanical response and large reversible spontaneous polarization, respectively. While PbTiO3 rich PZT compositions are extensively studied for these applications, studies near or at the antiferroelectric end member, PbZrO3 (PZO), are much more limited. In PZO, adjacent dipoles are oriented antiparallel in the  orthorhombic direction resulting in a net-zero macroscopic polarization. Applying a high external electric field aligns the dipoles resulting in a structural transition from an antiferroelectric (AFE-orthorhombic) to ferroelectric (FE-rhombohedral) state and in an isotropic volume expansion of ~0.26%. Prior studies on bulk PZO single crystals have shown that with increasing external electric field, PZO undergoes multiple structural changes from AFE-orthorhombic to FE-orthorhombic to FE-rhombohedral I to FE-rhombohedral II. However, the multiple phase transitions have not been previously observed in PZO thin films, hindered by poor dielectric breakdown strength or possible non-observed due to the growth crystallographic orientation of the films.
To improve the breakdown strength of PZO thin films, previous work has concentrated on complex doping approaches with up to four cations for the B-site in the perovskite cell. Despite the enhanced dielectric properties, doping increases the complexity in processing and any deviation from the targeted doping concentration may result in a dramatic decrease in functional response. Here we show that an alternative approach to increase the breakdown strength is by increased A-site cation content in the precursor solution. We demonstrate that with increased dielectric breakdown strength and control of the crystallographic orientation of the films, multiple phase transitions can be observed in PZO thin films. Control of the phase transitions results in changes in the functional response, which can be also further tuned by simple addition of dopants. This proposed work aims to improve the scientific understanding of processing conditions, structural phase transitions and electromechanical response in pure PZO thin films and guide future studies on AFE materials with more complex structures or compositions.
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2. Fesenko, O.E., R.V. Kolesova, and Y.G. Sindeyev, The structural phase transitions in lead zirconate in super-high electric fields. Ferroelectrics, 1978. 20(1): p. 177-178.