GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Thursday, December 19, 2019

2:00 PM
in MoSE 3201A

will be held the

DISSERTATION DEFENSE

for

Lei Zhang

"Atomic Level Computational Probes on Ionic Defect and Transport Properties in Solid State Ionics"

Committee Members:

Prof. Meilin Liu, Advisor, MSE

Prof. Rampi Ramprasad, MSE

Prof. Ting Zhu, ME/MSE

Prof. Angus Wilkinson, CHEM/MSE

Prof. Jean Luc Bredas, CHEM

Abstract:

Solid state ionics plays a key role in energy conversion purpose. Ionic defect and transport properties at atomic level is precisely probed by a combination of computational techniques, including density functional theory calculations, phonon calculations, molecular dynamics simulations, cluster-expansion and Monte-Carlo modeling. Within this type of material family, key physics including elastic and electrostatic interactions, configurational entropy-enthalpy competition, chemo-mechanical coupling, collective behavior of ion transport are unveiled by advanced computation techniques.

Barium hafnate, i.e. BaHfO₃ is one of the promising proton conducting, as similar as BaZrO₃. The proton conductivity at relatively high temperature range, i.e. 400~700 °C, can be utilized as the electrolyte layer in solid oxide fuel cells (SOFCs). A wide spectrum of relevant properties, including hydration percentage, proton diffusion, stability against CO₂, are sensitive to chemical dopants in the lattice, i.e. Li, Na, K, Rb, Cs on A-site, and Sc, Y, La, Gd, Lu, Al, Ga, In on B-site. Fundamental mechanism behind those properties upon various chemical dopants are investigated to achieve a rational design of doped-BaHfO₃.

Dislocation in Y:BaZrO₃ affects oxygen ion transport, due to the distinct strain and local environment w.r.t bulk lattice. To probe the ion mobility, a reactive molecular dynamics simulation based on ReaxFF are utilized to simulate the large supercell with such microstructural feature. Radial distribution function is used to analyze the local structure feature, mean-square displacement is used to calculate diffusivity. Dislocation is found to have an impact on ionic transport, however, in a different way for oxygen ions and protons. This could be due to the distinct ionic size, as well as electrostatic charge of oxygen ions and protons.

Strain also affects another type of ionic materials, i.e. fluorite-structured CeO₂, which is one of the most common oxygen ion conducting electrolyte in SOFCs, as well as oxygen buffering/catalyzing material. Unlike BaZrO₃ perovskite, which has merely no electronic conductivity, ceria can create intrinsic oxygen vacancies and electron-polarons easily under reducing conditions. We propose a unique way of tuning vacancy-polaron configurational relationship, by applying tensile and compressive epitaxial strain on (111) slab. Interesting stability cross-over on surface/sub-surface vacancy, 1NN/2NN polaron, isolated/dimer vacancies through strain are discovered. This might give a hint of various observed surface vacancy patterns in prepared ceria samples.