Graduate Thesis Or Dissertation
 

Perturbed angular correlation investigation of cubic zirconia

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  • Perturbed angular correlation (PAC) spectra of fully yttria-stabilized (cubic) zirconia with Y₂0₃ contents of 35.4 wt. %, 27.8 wt. %, and 16.9 wt. %, respectively, were measured using the isotope ¹⁸¹Ta as probe nucleus. PAC spectra in cubic zirconia are characterized by a wide frequency distribution due to contributions of dopant cations (Y) and oxygen vacancies. The jump rate for oxygen vacancies in the lattice is several orders of magnitude higher than for the cations. In the entire temperature range (24°C to 1300°C) the cations can be considered static, whereas the mobility of the anion vacancies is strongly temperature-dependent. The spectra show that the electric field gradient (EFG) is static at highest (T > 1200°C) and lowest (T < 500°C) temperatures. The contributions to the EFG due to oxygen vacancies average to zero for the highest temperatures because of the fast vacancy jump rate. At the lowest temperatures, the vacancy jump time is greater than the lifetime of the intermediate state (10.8 ns), resulting in a static EFG. A relaxation phenomena arising from diffusion of oxygen vacancies was observed for intermediate temperatures. The relaxation parameter, λ, has a maximum at about 850°C. In the high-temperature region (T > 850°C), λ has an activated form. The activation energy for the oxygen vacancy motion, E[subscript a], is 1.06 eV (± 0.07 eV) and is independent of the Y₂0₃ concentration which contradicts results obtained from ionic conductivity measurements found in the literature. The EFG increases by a factor of three with decreasing temperature from 1200°C to 500°C. It is proposed that this is caused by dynamical screening: at elevated temperatures oxygen vacancies jump into higher-energy positions closer to the PAC probe, reducing the cationic contribution to the EFG. The activation energy for this process is 0.2 eV (± 0.02 eV). PAC measurements on samples heated up to different temperatures show that the material must be annealed for about 24 hours at 1300°C. Even though the Y₂0₃ concentrations were 16.9 wt. % and higher, the samples were inhomogeneous without annealing. A computer simulation of the angular perturbation function, G₂(t), was developed, and the results for a static EFG, the XYZ model, and rapidly jumping vacancies in a simple cubic lattice are presented. The simulation assumes that (i) the atoms of Zr and Y are randomly distributed, (ii) the oxygen vacancies can be described as point ions, (iii) there are no interactions between the oxygen vacancies and no interactions between the vacancies and Zr or Y, (iv) the vacancies can only jump to next-nearest-neighbor sites, and (v) the vacancies never occupy nearest-neighbor sites to the PAC probe. The results from the static EFG and the XYZ model are in agreement with theoretical calculations. The transition probability, W, between two particular EFG states per unit time is proportional to the inverse of the average correlation time, τ[subscript c], and the number of possible EFG states, N.
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