Graduate Thesis Or Dissertation
 

Dependence of oxygen vacancy concentration in zirconia on processing parameters and reducing conditions

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  • ¹¹¹In/Cd perturbed angular correlation (PAC) gamma spectroscopy was used to study the point defect thermodynamics of tetragonal zirconia (t-zirconia). PAC can be used to measure, among other things, oxygen vacancy defect concentrations in t-zirconia. Higher than expected concentrations of oxygen vacancies in undoped samples were found using PAC. It was hypothesized that negatively charged extended defects grew in during grain growth, and in order to maintain charge-balance extra numbers of positively charged oxygen vacancies were introduced. Further studies using scanning electron microscopy and neutron activation analysis in conjunction with PAC, however, indicated that it was probably impurities rather than extended defects that introduced these extra vacancies. Improved sample-making techniques were also developed, and as a result the expected axial symmetry of the electric field gradient (EFG) produced by a tetragonal lattice was finally observed in t-zirconia by PAC. The axial symmetry, however, was found to be sensitive to impurity and/or dopant content. Models that include this effect are being developed; as soon as this effect can be parameterized, accurate hopping enthalpies for the vacancies can be determined. The EFG of the tetragonal lattice was found to be temperature dependent; this was attributed to formation of vacancy-substitutional cation defect pairs. Binding enthalpies for vacancy-cadmium and vacancy-yttrium pairs were found to be 0.62(3)eV and 0.28(5)eV, respectively. Vacancies were found to bind to cadmium in the nearest-neighbor position, while they were found to bind to yttrium in the next-nearest-neighbor position. A cubic structure was also found to form in undoped t-zirconia under reducing conditions between 600°C and 1500°C. Because PAC yields only information about the very local structure of the Cd PAC probe (a few lattice parameters) and because the Zr-O phase diagram is not well-known in this regime, details of this cubic structure found in reduced t-zirconia remain unclear. Precipitation of a defective zirconium metal phase most likely explains the existence of the observed cubic structure. However formation of cubic defect clusters or stabilization of the cubic phase of zirconia by high oxygen vacancy concentrations cannot yet be ruled out. Oxygen vacancy concentrations of t-zirconia in reducing atmospheres were measured at 1400°C, 1300°C, 1200°C, and 1100°C. At lower oxygen partial pressures (<10⁻¹¹ atm) measurements were consistent with the Brouwer approximation that electrons and oxygen vacancies dominate; vacancy concentrations were proportional to the oxygen partial pressure to the -1/6 power. At higher partial pressures, vacancy concentrations were found to be wholly determined by extrinsic impurity concentrations. Equilibrium constants were found, and a vacancy formation enthalpy determined.
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