Reduction of nickel doped fluorite and perovskite structured oxides, The

Abstract

A powerful technique for preparing metal-oxide nanocomposites is internal reduction. The selective reduction of the least redox stable species creates a distribution of fine metallic precipitates in the microstructure, resulting in unique electrochemical and functional properties. Transition metal dopants, when added to fluorite and perovskite structured zirconia-based ceramics, reduce more easily in low oxygen partial pressure than constituents of the parent lattice. Nickel oxide (NiO) dopant is commonly added to ceramics that are used in fuel cells, catalysis, nanoionic devices, and many other electrochemical applications to aid microstructure development. An improved understanding of the mechanism for internal reduction of NiO-doped oxides will enable greater nanostructural control and will lead to smart engineering of nanoionic and catalytic devices. Specifically the studies in this dissertation evaluate the microstructural influences on the overall internal reduction mechanism and reaction kinetics. Microstructural features have unique local chemistry compared to the bulk which can influence the mass transport by creating chemical and electrical gradients. The baseline microstructures are heavily characterized by this work to understand the distribution of NiO dopant prior to internal reduction. The current model describing the mechanism of internal reduction only explains mass transport in single crystals, neglecting space charge effects, and assumes that oxygen species are immobile. To overcome these limitations, internal reduction mechanisms are evaluated in polycrystalline NiO-doped YSZ with some porosity. It is possible to distinguish stages of reduction as they relate to microstructural features through systemic experimentation by varying temperature, oxygen partial pressure, and soak time. Each stage is described by distinct kinetics and magnetic signatures, showing that it is possible to tailor metal-oxide nanocomposites with nanoscale control of features. Redox cycling is performed to describe the reversibility and stability of the reduced microstructure. The solubility of NiO dopant in BZY powders and pellets was examined prior to reduction studies. As a result, this dissertation characterizes the distribution and excess barium nickel oxide phases in NiO-doped BZY. Reduction experiments on NiO-doped BZY powders are used to show that chemical segregation during particle growth leads to selective reduction behavior. By varying the amount of NiO dopant, it is possible to ratchet the amount of excess barium nickel oxide in powders. Although the internal reduction mechanism is not described by this work, a foundation for magnetic characterization of NiO-doped BZY is provided for future studies.