Date of Award
4-1991
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
First Advisor
Susan C. Schneider
Second Advisor
Martin A. Seitz
Third Advisor
Robert F. Brebrick
Fourth Advisor
Robert Blumenthal
Abstract
In solid-state electrochemical cells, the charge transfer processes at the electrode/electrolyte interface play an important role in determining the overall performance of the cell. In this study, the electrochemical behavior of a simple interface between a single crystal of zinc oxide (ZnO) and a single crystal of yttria-stabilized zirconium dioxide (YSZ) has been investigated using small-signal ac immittance spectroscopy (ACIS). From the total cell immittance data, acquired as a function of frequency over a range of temperatures (200°C ≤ T ≤ 900$°C), partial pressures of oxygen (10-4 1 atm.) and dc bias voltage, the physical processes that play a role in the charge transport were identified.
A simple equivalent circuit that best represents the data and is consistent with the physical model has been assigned for the system under investigation. The equivalent circuit parameter values were obtained using an data analysis program, called "CPFMULIT", developed for this work. In all, five semicircular relaxations were observed and each of these relaxations has been associated with a physical process.
At the lowest temperature range the total impedence of the cell was found to be dominated by the bulk ionic resistance of YSZ. The effects of chemisorption of oxygen on ZnO surface were observed at 275°C-450°C. The contribution of hole conduction in YSZ to the total impedance of the system was seen in the immittance data acquired at temperatures above 500°C. The space charge region at the ZnO/YSZ interface was found to be dominated by the space charge on the ZnO side. A simple charge transfer reaction at this interface has been proposed. The mode of charge transfer appears to involve many intermediate steps but the overall electrochemical reaction is possibly of the form
ZnZnOO + VO(YSZ) + e1 = Zni(ZnO) + OO+(YSZ).
The reaction rate was found to be rate limited by the diffusion of zinc interstitials through zinc oxide.