Oxygen Permeability and Its Role in Governing Life of YbDS Environmental Barrier Coatings

The growth kinetics of thermally grown oxide (TGO) silica in Yb-disilicate (YbDS) environmental barrier coatings (EBCs) significantly affects the durability of EBCs. The oxygen permeability can control the TGO growth kinetics and thus could play an essential role in determining EBCs life. Therefore; the oxygen permeability constant of YbDS and TGO is systematically evaluated and quantified in terms of thermodynamics using defect reactions and the parabolic rate constant (kp); respectively. Dry oxygen and wet oxygen conditions as well as different temperatures; partial pressures and top coat modifiers are investigated. The results offer evidence that the oxygen permeability constant for the YbDS top coat is an order of magnitude higher than for the TGO. As such; the TGO hinders the oxidant diffusion stronger; proving to be the diffusion rate controlling layer. Moreover; water vapor strongly increases the oxygen permeability with defect reactions playing a key role. It is suggested that the mass transfer through the top coat is primarily by outward ytterbium ion diffusion and inward oxygen ion movement; with the latter being dominant; particularly in wet environments. The effect of top coat modifiers on oxidant permeation is composition sensitive and seems to be related to their interaction with oxygen ions and their mobility.

The Conundrum of Relaxation Volumes in First-Principles Calculations of Charged Defects in UO2

The defect relaxation volumes obtained from density-functional theory (DFT) calculations of charged vacancies and interstitials are much larger than their neutral counterparts, seemingly unphysically large. We focus on UO2 as our primary material of interest, but also consider Si and GaAs to reveal the generality of our results. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defects in periodic DFT. We probe the dependence of the proposed energy corrections to charged defect formation energies on relaxation volumes and find that corrections such as potential alignment remain ambiguous with regards to its contribution to the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprising individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights the issue that, as is well-known for defect formation energies, the defect formation volumes depend on the choice of reservoir. We show that considering the change in volume of the electron reservoir in the formation reaction of the charged defects, analogous to how volumes of atoms are accounted for in defect formation volumes, can renormalize the formation volumes of charged defects such that they are comparable to neutral defects. This approach enables the description of the elastic properties of isolated charged defects within an overall neutral material.