The effect of Niobium on the defect chemistry and corrosion kinetics of tetragonal ZrO₂ : a density functional theory study
Massachusetts Institute of Technology. Department of Nuclear Science and Engineering.
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Abstract Advanced Zirconium based alloys used in the nuclear industry today, such as ZIRLOTM , M5 contain up to wt 1.2% Niobium . Experimental effort to determine the effect of Nb on corrosion behaviour of these alloys has no clear answer to whether Nb improves or degrades the corrosion resistance[8, 48, 201. Even the charge state of Nb as a defect in zirconia is debated. Experimental findings of Froideval et al  indicate charge state between +2 and +4 whereas other authors assume it to be +5 [21, 31, 34]. In order to uncover the role of Nb on the local oxide protectiveness we employed ab initio Density Functional Theory (DFT) calculations, and assessed the effect of Niobium on the point defect equilibria in tetragonal zirconia which is critical in the oxide protectiveness [81 among other phases of zirconia. DFT calculated defect formation energies are adjusted for finite temperature effects by accounting for thermal vibrations. Adjusted defect formation energies are then used to construct Kroger-Vink diagram for defect equilibrium concentrations at applicable p02 levels. The Kr6ger-Vink diagrams for Nb containing zirconia was compared to that of pure t-Zirconia in order to isolate the changes due to Nb. Nb is treated as point defect in the oxide. Among the considered point defects and defect complexes of Nbzr ,Nbi, Nbzr - Vo and Nbzr - Oi, the substitutional point defect Nbzr was found to have the lowest free energy of formation, and the highest equilibrium concentration. Nb substitutional point defect, Nbzr, is found to be stable for Nb+3, Nb+4, Nb+5 charge states while Nb+5 has the highest concentration. The effect of applied external compressive strain on the energetics and stress of different types of defects, and formation energy is quantified as a function of strain. It is observed that the more positively charged the defect, the formation energy increases less as compressive strain is applied. Compared to pure T-ZrO2, ~100 times increase in Zirconium vacancy concentration accompanied by a ~5 times decrease in the doubly charged oxygen vacancy concentration was found due to the presence of Niobium in the high oxygen partial pressure (p02) regime corresponding to oxide/water interface. This change implies slowing down of oxygen diffusion from surface to bulk, while accelerating oxygen exchange on the surface. Diffusion of zirconium ion to the surface will also accelerate as available point defect concentration increases due to Nb. The increased concentration of Nbzr defect with increasing oxygen partial pressure is consistent with our experimental findings in a parallel work in our group.
Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 57-60).
DepartmentMassachusetts Institute of Technology. Department of Nuclear Science and Engineering.; Massachusetts Institute of Technology. Department of Nuclear Science and Engineering
Massachusetts Institute of Technology
Nuclear Science and Engineering.