Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Interest in metal oxides has increased in recent years. This ls partly due to their potential uses in composite materials, as electronic components, and as high temperature abrasion and corrosion resistant coatings. Oxides are also of interest because of their relation to extractive metallurgy and oxidation of refractory metals and super alloys.
If one wishes to make use of a metal oxide or even to prevent its formation, It is necessary to know that materiaI's characteristics. An important characteristic is that of bulk diffusion or mass transport which occurs during many physical processes; e.g. sintering, precipitation, oxidation, creep and annealing. Mass transport is al so an important factor in the operation of fuel celIs, in controlling the properties of ceramics which are used at high temperatures and in controlling the processing of all ceramics. Because of this there Is a need for reliable diffusion coefficients and their controlling factors.
Diffusion coefficients and mechanisms of mass transport can be obtained from the ionic conductivity which is closely related to mass transport. The Ionic conduction mechanism In metal oxides is characterized when the charge transporting species, their mobilities and mechanisms of migration are known along with how these quantities vary with environment and various defect types and concentrations. Chemical additives or dopants are used extensively to vary and control the Ionic conductivity and hence the diffusion rates In ceramics.
The need for a better understanding of iconic conduction processes In pure and doped oxides prompted this investigation in which the ionic fraction of the electrical conductivity In pure and calcia doped cerium dioxide was measured. Cerium dioxide exhibits large departures from stoichiometry while maintaining the cubic fluorite structure. Because of its variable composition In a single phase field cerium dioxide easily lends itself to studies of its electrical conductivity over large ranges of oxygen pressure and temperature. Additions of calcium oxide have been found to increase the electrical conductivity of cerium dioxide such that its conductivity is greater than that of either yttrja doped thoria or calcia stabilized zirconia both of which possess fluorite structures similar to that of ceria. Because of this and also because both yttria doped thoria and calcia stabilized zirconia have long been known to be ionic conductors, Interest in calcia doped ceria has been expressed for Its potential use as an electrolyte in high temperature fuel cells.
The present investigation employs an electrochemical eel I of the type
to measure the ionic transference fractions of pure and calcia doped cerium dioxide as a function of temperature, oxygen pressure and calcia concentration.
These measurements were used in combination with measurements of the total electrical conductivity to obtain the ionic and electronic conductivities from which were obtained Information concerning the defect structure and mechanisms of conduction in cerium dioxide.
Oxygen vacancy diffusion coefficients and motion energies were also obtained from the ionic conductivity.