Abstract

Cerium dioxide, Ce02, one of the more abundant of the rare earth oxides, ls mined In only a few locals. One place ls the Union of South Africa which possesses massive monazite deposits from which Ce02 is refined. Commercially, this oxide finds use as a glass polish and catalyst, while the cerium metal, Ce, ls used as a coloring agent In glass. In solid state research, however, Ce02 has received attention due to its large departures from stoichiometry in the fluorite phase and its high temperature semi-conductive nature which is controlled by this stoichiometry. This wide range of variable composition has permitted the measurement of many very sensitive and less defect sensitive properties of the compound. Correlation of the properties of electrical conductivity, weight change, and oxygen dissociation pressure has been attempted In an effort to elucidate the defect behavior of Ce02. But unanimous agreement concerning the exact nature of the characteristic nonstoichiometric defect in Ce02 has not been achieved. Opinion appears divided equally between the cerium Interstitial and oxygen vacancy as the responsible defect. Earliest studies of Ce02, conducted approximately thirty years ago, can be used to point up one of the basic problems that has accompanied rare earth research. That is, that alI rare earths, since they have similar outer electron configurations, have similar chemical characteristics with each other, and hence are not easily obtained In a state of high purity. Subsequently, with Increased purity In materials, refinements In the observed behavior of the more defect sensitive properties of Ce02 have been noted. Ce02 of 99.999% purity (with - respect to other rare earths and 99.9% overall purity) has been prepared in Iimited quantity for laboratory use. This degree of purity has permitted observation of the electrical behavior of Ce02 above 800°C where impurity defects at this low concentration are of second order as compared to nonstoichiometric defects. The previous problem highIights the possible use of control led impurity doping to alter and regulate the electrical behavior of Ce02. The ensuing pages wiII present a study of electrical conductivity of Ce02 with controlled impurity doping as an experimental parameter. A discussion of the selection of appropriate dopants for this study is included. Doping as a method to test stoichiometric defect models wiII be considered. The technique used for this study was the preparation of sintered bars of high purity Ce02 with controlled additions of several different dopants. The specific property investigated was electrical -conductivity which is very sensitive to both nonstoichiometric and impurity defects. Conductivity measurements employing a four probe D.C. method were ' conducted under equilibrium conditions by varying specimen temperature • in various oxygen partial pressures at one atmosphere total pressure. The results of these experiments have been used to provide insight into the nature of the impurity defect and its interaction with the nonstoichiometric defect. Analysis and modeling of the experimental data yield the fol lowing quantities: I. Activation energy of the nonstoichiometric defect charge carrier as a function of dopant concentration. 2. Activation energy of the Impurity defect charge carrier as a function of dopant concentration. 3. The magnitude and temperature dependence of the impurity defect charge carrier mobility. 4. The magnitude and temperature dependence of the impurity defect diffusion coefficient. 5. Equilibrium relationships between nonstolchiometric defect concentrations and the amount of dopant added. 6. Equilibrium relationships between impurity defect concentration and the amount of dopant added.