Event Scheduled for Aug 2, 2018
Event: MSE PhD Dissertation Defense - Yomery Espinal
Time: 10:00 am
Details of Event:
PhD Dissertation Defense
Presenter: Yomery Espinal
Major Advisor: Dr. Pamir Alpay
Associate Advisors: Dr. Serge Nakhmanson, Dr. Mark Aindow, Dr. Steven Suib, Dr. Brendan Hanrahan
Date: Thursday, August 2, 2018
Title: Synthesis and Modeling of Ferroelectric Nanocomposites
Nanocomposites made up of a ferroelectric oxide embedded within a non-ferroelectric matrix are a unique class of functional materials that show potential to improve performance in next-generation electronics, energy storage, and non-volatile memory. Dielectric losses and breakdown due to processing and intrinsic properties of a monolithic ferroelectric point to low figures of merit, regardless of the application. Therefore, it is necessary to develop composite materials with an active ferroelectric that is physically confined within a low-loss dielectric matrix while maintaining significant dielectric and pyroelectric properties inherent to ferroelectrics. The purpose of this work is to explore the behavior of ferroelectric nanocomposites as a function of dimensionality, and composition of both components.
To understand the physics that governs the behavior of such ferroelectric nanocomposites experimental and theoretical work were performed. The theoretical studies concentrate on a thermodynamic model that is further supported by mesoscale/finite element analysis while experimental work considers two different nanocomposite configurations. Thermodynamic calculations based on a Landau-Devonshire theory of phase transformation for PbZr0.2Ti0.8O3 films with various interposed dielectrics reveal a dielectric constant-dependent critical thickness where subcritical thicknesses show enhanced dielectric and pyroelectric coefficients due to Curie temperature suppression.
These calculations guide experiments where PbZr0.4Ti 0.6O3 films with a thin HfO2 dielectric layer were grown using chemical solution deposition and atomic layer deposition, respectively. Experiments reveal that the polarization, coercive field and dielectric constant vary within the sample set following a model that describes capacitors-in-series, in contradiction to the theoretical work which assumes large electrostatic interactions at the interface. This is explained by electrostatic decoupling of the layers possibly due to trapped charges at the interface. Despite the limited enhancement in the dielectric response, the bilayer ferroelectric films have impressive loss/leakage properties. Furthermore, these films are shown to have over an order of magnitude difference in resistivity, with retention time greater than 1000 seconds making them attractive for memristor applications. Finally, thin films are fabricated by the electrospray evaporation of BaTiO3 particles followed by the atomic layer deposition of HfO2 which permeated and adhered to the porous particle film. The dielectric properties of these films were found to be tunable between pure BaTiO3 and the composite capacitance. This work spans from theory to fabrication to understand how ferroelectric and dielectric materials can synergistically be combined to create a tunable, functional composite material.
Target Audience: Not Available
Sponsored By: Materials Science and Engineering Program
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