Graduate Thesis Or Dissertation
 

Factors influencing the crystallization, phase and oxygen vacancy concentration in zirconia

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  • In order to achieve a better understanding of the processes that occur during formation and sintering of zirconia, various chemical and physical techniques were used. Along with Perturbation Angular Correlation spectroscopy, that allowed us to investigate microscopic properties inside the nanometer-size zirconia grains, such techniques as Scanning Electron Microscopy and X-ray diffraction were used for determining the size of particles before and after sintering, and Neutron Activation Analysis was employed for measuring the impurity levels in zirconia powders. By controlling the initial conditions and heat treatment of the powders, we investigated the dependence of formation of the charged defects on the existing molecular structure and morphology of zirconia particles. During the study, it was discovered that at low temperature the PAC frequencies of tetragonal zirconia behave very similarly for all materials that were used in this study. If stabilization is achieved by heavy doping, there are shifts and line-broadening due to the presence of dopants but no obvious differences in the essential physics. One material included in this group is Nb-doped zirconia that has no oxygen vacancies. It was concluded that there are no detectable oxygen vacancies in our pure or lightly doped tetragonal zirconia powders before they are heated into the temperature region where sintering occurs. Vacancies are incorporated as the samples are heated above 1050°C, the temperature at which sintering becomes important. The oxygen vacancies in samples that have been heated to 1200°C remain when cooled. We see no vacancy concentration dependence on the atmosphere for samples not doped with +5 valent elements in order to reduce the vacancy density at 1200°C. In several instances, samples that had been heated to a maximum temperature of 1050°C or 1100°C contained a vacancy density that was small (<100 ppm) but measurable. A reduced oxygen pressure increased the oxygen vacancy density by a measurable amount in these samples. Samples that are tetragonal at 800°C are well-sintered after being heated to 1200°C. Samples that are monoclinic below 1170°C are very poorly sintered at 1200°C and contain few vacancies. Flowing Cl in the system as the samples are sintering retards the densification of the grains. These samples had the smallest density of oxygen vacancies.
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