Date of Award
5-1973
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
First Advisor
Martin A. Seitz
Second Advisor
Robert Blumenthal
Abstract
Metal oxides are proving to be increasingly useful and this has stimulated interest in their properties and characteristics. Applications of these materials, any at elevated temperatures, are found in electrical and electronic components, in fuel cells, in composite materials, and in abrasion and corrosion resistant coatings. Knowledge of characteristics gives guidance as to the suitability of materials for specific applications.
A study of ionic conductivity will provide insight into the aechanislll8 for mass transport and diffusion processes. These processes are very dependent on temperature, and a study of characteristics must include temperature as a parameter. Through addition of an impurity dopant, the number of ionic charge carriers can be controlled, and this in turn controls conductivity.
A concurrent study of permittivity under an applied alternating electric field introduces a factor of time, and provides additional insight regarding the motion of charge carriers. Motion of charge carriers is thus limited to the diffusion length, which is inversely proportional to the frequency of the applied field. The diffusion process is limited by a time factor, and behavior of the material is dependent on frequency.
This study concerned pure and calcium doped cerium dioxide. Cerium dioxide was selected for the study for several reasons. First, it exhibits large departures from stoichiometry, thereby creating point defects and ionic charge carriers. Despite the large changes in stoichiometry, the basic crystal structure remains unchanged, and this avoids added increases to defects. The creation of ionic charge carriers enhances both conductivity and permittivity. Behavior of CeO2 is similar to that of calcium doped zirconium and to yttria doped thorium dioxide, both of which have similar structures and are known to be ionic conductors. Finally, a number of related studies have been conducted in recent years, and these will be useful in interpreting the results of this study.
This study uses an alternating current capacitance bridge for the measurement of capacity and resistance of a disk shaped specimen capacitor as a function of frequency, temperature and calcium concentration. In addition, direct current resistance was measured. From these basic measurements, total conductivity, d-c conductivity, ionic conductivity and permittivity were derived. In turn, a study of these characteristics yielded information regarding mechanism and energy of motion of point defects in cerium dioxide.