Date of Award
4-1990
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
First Advisor
Robert N. Blumenthal
Second Advisor
Robert F. Brebrick
Abstract
Nonstoichiometric cerium dioxide, CeO2-x, is generally regarded as an n-type defect semiconductor at elevated temperatures and low oxygen partial pressures (PO2). At high PO2's, below about 800°C, p-type behavior has been reported but not extensively studied. Electrical conductivity and thermodynamic measurements have therefore been performed in order to characterize the defect structure and electrical behavior in the vicinity of the n to p transition. The dc electrical conductivity of single and polycrystalline CeO2-x, the latter with two different impurity levels, was measured in the temperature range of 600-1000°C and from 1-10-4 atm of oxygen partial pressure. Partial conductivities and respective activation energies were determined by applying the law of mass action. Using the form $\sigma$ = $\sigma\sb{\rm o}$e$\sp{\rm -(E/kT)}$, activation energies of 2.57 and 1.15 eV were obtained for $\sigma\sb n$ and $\sigma\sb p$ respectively. The activation energy for ionic conductivity in the single crystal was 0.63 eV when fit to the form $\sigma$ = ${\sigma\sb{\rm o}\over\rm T}$e$\sp{\rm -(E/kT)}$. The ionic conductivity in the polycrystalline materials was affected by the grain boundaries. Electrical conductivity measurements were also made between 10$\sp{-2}$ and 10$\sp{-19}$ atm at 800 and 900$\sp\circ$C on one of the polycrystalline samples. Near-continuous data within this P$\sb{\rm O\sb2}$ region revealed one feature at $\approx$10$\sp{-6.5}$ atm not previously reported. Thermodynamic measurements by electrochemical coulometric titration were made at 800$\sp\circ$C between 10$\sp{-4}$ and 10$\sp{-19}$atm. High P$\sb{\rm O\sb2}$ data analyzed according to mass action equations indicated a significant number of holes compared to that of electrons are present in the high oxygen partial pressure region. Since the observed conductivity was only n-type at 800$\sp\circ$C, it was concluded that the drift mobility of the electrons was much greater than that of the holes. Thermodynamic constant composition measurements of CeO$\sb{2-x}$ for $x < 10\sp{-2}$ revealed a decrease in the partial molar enthalpy with decreasing x for $x < 10\sp{-2.8}$. This decrease in the partial molar enthalpy is tentatively attributed to a defect reaction which involves holes.