Theoretical Studies On Hydrogen Behavior In α-Al₂O₃Tritium Permeation Barrier Material

Tritium permeation barrier (TPB) is one of key scientific and technological issues for preventing the tritium loss from fusion plants. α-Al2O3is of special interest because of low hydrogen permeability. However, from the microcosmic point view, less insight is available regarding interaction of hydrogen with α-Al2O3material and corresponding mechanism of thermodynamics and kinetics, thus the design, processing and optimization of α-Al2O3TPBs rely on trial-and-error, making such TPBs often exhibiting lower efficiency than anticipated based on the bulk properties of this material.Focusing on basic scientific issues of the interaction of hydrogen with α-Al2O3TPB, adsorption, dissociation and diffusion of hydrogen on and into α-Al2O3surfaces, dominant H-related defects and its diffusion in bulk α-Al2O3, and effects of α-Al2O3/FeAl interface on these H-behaviors have been investigated thermodynamically and kinetically in a systematic way by the first-principle calculations and transition state theory. Based on above basic and essential steps of H-permeation in α-Al2O3, mechanisms of α-Al2O3resisting H-permeation under typical TPB working condition, and principles for the optimum performance have proposed from the basic point of view, to guide design, processing and performance optimization of α-Al2O3TPBs for engineering practical applications. The major results are as follows.1. Mechanisms for adsorption, dissociation and diffusion of hydrogen on α-Al2O3surface2. A H2molecule, with parallel configuration, absorbs on α-Al2O3(0001) and α-Al2O3(1-102) surface, and then dissociates heterolytically near room temperature, with one H atom adsorbing on a top Al atom site, and another H atom adsorbing on a top O atom site,which is thermodynamically spontaneous. It is possible that at finite temperatures a number of events involving surface diffusion of such H atoms before their migration into the bulk takes place. Bulk diffusion process of the H atom involves two steps on every O atomic layer of α-Al2O3:(1) the reorientation step in which hydrogen atom remains bonded to the same O atom and (2) the hopping step in which breaking and reforming of O-H bond take place. The rate-limiting barrier of H diffusion into the bulk from the (1-102) and (0001) surface is1.41～1.58eV, while H atoms in bulk α-Al2O3are significantly less stable than on surfaces, thus H diffusion preferentially occurs via surface path rather than bulk one. The rate-limiting barrier of H diffusion into the bulk from the (1-102) surface is0.17eV less than that of the (0001) surface, and the equilibrium constant for adsorbed H on the (1-102) surface in equilibrium with absorbed H in bulk α-Al2O3is larger than that of the (0001) surface, thus significant diffusion of H into bulk can occur more readily from the α-Al2O3(1-102) surface compared to the α-Al2O3(0001) surface, in well agreement with results of infrared spectroscopy, which is related to the different crystal structure of surfaces resulting in the different interference from other hydrogen atoms.3. State, local configuration and diffusion of H-related defects in bulk α-Al2O3We predict that the stable forms of H related defects in α-Al2O3are charged H interstitials (Hiq) and hydrogenation of the bulk VAl3-([VAl3--H+]q) under hydrogen-rich condition, which are less energetically stable than the gas phase H2. As the system of α-Al2O3and H reaches equilibrium, H in α-Al2O3is mainly in present of Hi+state, which is the most stable among H-related defects, and also likely to exist in the form of [VAl/3--H+] and Ho+. Hi+is the predominant diffusion species in α-Al2O3, and [VAl3--H+]2-and Ho+can release trapped hydrogen during high temperature annealing, and then contribute to the H-transport in α-Al2O3. The Hi+diffusion process, involving H-reorientation around oxygen atom from an octahedral interstitial site to an adjacent one followed by H-hopping within the adjacent one, has a barrier of1.26eV, lower than suface-to-subsurface diffusion barriers of H in α-Al2O3, thus it is rather reasonable that the overall diffusivity of H in α-Al2O3is governed by the surface-to-subsurface diffusion. Hi+will be trapped by the VAl3-, Vo0, Oi2-, increasing the activation energy of H migration and decreasing the H mobility, which is favored for low H-transport in α-Al2O3TPB. Local vibration mode and local structures for Hi+,[VAl3--H+]2-and Ho+have been obtained and is very helpful for us to identifying actual local structures responsible for the IR observed peaks.3. The effect of α-Al2O3/FeAl interface on stability and diffusion of hydrogen in α-Al2O3partThe interfacial binding involves cation-anion and metal-metal interactions. Due to the localized distributions of the interface on density of sate of α-Al2O3part of α-Al2O3/FeAl, H-surface interaction on the α-Al2O3/FeAl resembles that on pure α-Al2O3(0001) case, H interstitials in the α-Al2O3part is significantly less stable than on surface, and in interface region consisting of the only Al-oxide (Al/O interface) or the Al, Fe mix-oxide (Al/Fe/O interface), and in FeAl part are significantly more stable. The α-Al2O3component takes a critical role in preventing hydrogen diffusion during the α-Al2O3/FeAl TPB operation. H diffusion into the α-Al2O3part of both slabs must overcome a rate-limiting barrier of about1.66-2.02eV at surface-to-subsurface step, like in α-Al2O3case. For the bulk path, the migration of H atom can occur more readily in the α-Al2O3part of the slab with the Al/O interface compared to that with the Al/Fe/O interface, while H atoms in the former are less stable, thus the α-Al2O3/FeAl barrier with Al/Fe/O interface is predicted to be more effective in resisting H-permeation against the underlying steel. The adhesion work decreasing and hydrogen-related structural damage do not occur in the interface region, suggesting that the appearing or trapped H atoms in such interface region will not be weak linked and accelerate the failure of α-Al2O3/FeAl TBP.4. Mechanisms, in atomic scale, of α-Al2O3resisting hydrogen isotopic permeation under typical TPB working conditionsWe have combined thermodynamic concepts, chemical potentials of atoms in the system of H and α-Al2O3, density functional theory calculations with rate theory of transition state theory to identify the kinetic and thermodynamic mechanism of α-Al2O3resisting H-permeation. The high formation energy of Hi+is the dominate term in the activation energy for H-transport in bulk α-Al2O3, suggesting that the low H concentration (low stability) in α-Al2O3is the thermodynamical bottleneck for H-permeation through α-Al2O3. As for the kinetic mechanism, H-permeation through α-Al2O3is governed by the H reorientation around oxygen in the third atomic layer of α-Al2O3(1-102) surface at the hydrogen pressure of above17kPa, whereas that is suppressed by H2dissociation above the site between B-site Al and D-site Al on the α-Al2O3(1-102) surface at the hydrogen pressure lower than1kPa. Our DFT calculations successfully reproduce hydrogen permeation and tritium imaging plate experiment observations. These results are very important to comprehend the results from the common method based on Fick’s law.5. Principles for optimum suppressing-performance of α-Al2O3TPBThermodynamical principle for optimum suppressing H-permeation for α-Al2O3TPB would be to decrease energetical stability of Hi+, and kinetics principle would be to enhance migration barrier for H,+.