Unraveling different influences of the fraction of the tetragonal phase in oxide films on the corrosion resistance of Zr alloys from the phase transition mechanism.

The mechanism of Sn and Nb influence on the fraction of tetragonal ZrO2 in oxide films on Zr alloys and their influence mechanism on corrosion resistance of Zr alloys, despite decades of research, are ambiguous due to the lack of kinetic knowledge of phase evolution of ZrO2 with doping. Using stochastic surface walking and density functional theory calculations, we investigate the influence of Nb and Sn on the stability of tetragonal (t) and monoclinic (m) ZrO2, and t-m phase transition in oxide films. We found that though Nb and Sn result in similar apparent variation trends in the t-phase fraction in oxide films, their influences on t-m phase transition differ significantly, which is the underlying origin of different influences of the t-phase fraction in oxide films on the corrosion resistance of Zr alloys with Sn and Nb alloying. These results clarify an important aspect of the relationship between the microstructure and corrosion resistance of Zr alloys.

First-Principles Calculations to Investigate the Influence of Irradiation Defects on the Swelling Behavior of Fe-13Cr Alloys.

Ferritic/martensitic (F/M) steels whose matrix is Fe-Cr are important candidate materials for fuel cladding of fast reactors, and they have excellent irradiation-swelling resistance. However, the mechanism of irradiation-swelling of F/M steels is still unclear. We use a first-principles method to reveal the influence of irradiation defects, i.e., Frenkel pair including atomic vacancy and self-interstitial atom, on the change of lattice volume of Fe-13Cr lattice. It is found that vacancy causes lattice contraction, while a self-interstitial atom causes lattice expansion. The overall effect of a Frenkel pair on the change of lattice volume is lattice expansion, leading to swelling of the alloy. Furthermore, the diffusion properties of point defects in Fe-13Cr are investigated. Based on the diffusion barriers of the vacancies and interstitial atoms, we find that the defects in Fe-13Cr drain out to surfaces/grain boundaries more efficiently than those in pure α-Fe do. Therefore, the faster diffusion of defects in Fe-13Cr is one of important factors for good swelling resistance of Fe-13Cr compared to pure α-Fe.

Effects of oxygen chemical potential on the anisotropy of the adsorption properties of Zr surfaces.

The anisotropy of metal oxidation is a fundamental issue, and the oxidation of Zr surfaces also attracts much attention due to the application of Zr alloys as cladding materials for nuclear fuels in nuclear power plants. In this study, we systematically investigate the diagram of O adsorption on low Miller index Zr surfaces by using first-principles calculations based on density functional theory calculations. We find that O adsorption on the basal surface, Zr(0001), is more favourable than that on the prism surfaces, Zr(112[combining macron]0) and Zr(101[combining macron]0), under strong O-reducing conditions, while O adsorption on the prism surface is more favourable than that of the basal surface under weak O-reducing conditions and the O-rich conditions. Our findings reveal that the anisotropy of adsorption properties of O on the Zr surfaces is dependent on the O chemical potential in the environment. Furthermore, the ability of the prism for O adsorption is stronger than that of the basal surface under the O-rich condition, which is consistent with the experimental observation that the oxidation of the prism Zr surface is easier than that of the basal surface. Systematic surveys show the adsorption ability of the surface under strong O-reducing conditions is determined by the low coordination numbers of surface atoms and surface geometrical structures, while the adsorption ability of the surface under weak O-reducing conditions and O-rich conditions is only determined by the low coordination number of surface atoms. These results can provide an atomic scale understanding of the initial oxidation of Zr surfaces, which inevitably affects the growth of protective passivation layers that play critical roles in the corrosion resistance of Zr cladding materials.

Graphite to Diamond: Origin for Kinetics Selectivity.

Under mild static compression (15 GPa), graphite preferentially turns into hexagonal diamond, not cubic diamond, the selectivity of which is against thermodynamics. Here we, via novel potential energy surface global exploration, report seven types low energy intermediate structures at the atomic level that are key to the kinetics of graphite to diamond solid phase transition. On the basis of quantitative kinetics data, we show that hexagonal diamond has a facile initial nucleation mechanism inside graphite matrix and faster propagation kinetics owing to the presence of three coherent graphite/hexagonal diamond interfaces, forming coherent nuclei in graphite matrix. By contrast, for the lack of coherent nucleus core, the growth of cubic diamond is at least 40 times slower and its growth is inevitably mixing with that of hexagonal diamond.

Looking deeper into the structure of mixed electric double layers near the point of zero charge.

Molecular simulations have been carried out using the Metropolis Monte Carlo approach to investigate the structure of planar electric double layers containing counterion mixture within the framework of the unrestricted primitive model. The results reveal that near the point of zero charge, the rise of monovalent salt drastically elevates the collapse of ions regardless of their polarity. In particular, we fail to observe the formation of a strongly correlated liquid in the first counterion layer due to favorable entropic effects, in contrast to the early data from molecular dynamics simulations [corrected] for a spherical electric double layer [R. Messina, E. González-Tovar, M. Lozada-Cassou, and C. Holm, Europhys. Lett. 60, 383 (2002)]. Moreover, the large size of coions is found to be a pivotal factor in determining the reversal of electrophoretic mobility. On the other hand, the repulsive image charge forces thoroughly annihilate this peculiar reversal of mobility within the investigated scope of concentrations, but exert no effect on the emergence of charge reversal. These findings highlight potential applications of coion's characteristics to control gene delivery and colloidal stability as well as to design viral packing and polyelectrolyte self-assembly.

First-principles studies for CO and O(2) on gold nanocluster.

First-principles calculations are performed to study the interaction of gold nanocluster Au(55) with small molecules CO and O(2). We find that the adsorption energy of CO on Au(55) is among 0.5-0.7 eV at different sites and [CO+O(2)] can be coadsorbed on Au(55). Comparisons between Au(55) and Au(32) show that the adsorption energy not only depends on the size of the cluster but also on the geometry of the cluster. Similar with smaller cluster (Au(8) and Au(32)), the energy difference between [CO+O(2)] and [CO(2)+O] on Au(55) is much larger than that in the free gas. Our calculations indicate that the nanocluster Au(55) can enhance the reaction process, CO+O(2)-->CO(2)+O, in which the reaction barrier is only about half electron volts.

Quantum-well states in a double-well system: an example of Cu/Co(Ni)/Cu.

The quantum-well (QW) states in the Cu/Co double-well system are studied by first-principles calculations. We have shown that the monolayer Ni or Co as a heterogeneous spacer in Cu QW can not only disturb the QW states extending into the whole structure, but also create new QW states because of the interfaces introduced, resulting in sub-well-confining electrons. If the QW state energy in two sub-wells is close to each other, these two sub-well QW states can couple together. We have also demonstrated that monolayer Co and Ni spacers play different roles for modulating QW states at different energy levels, which also result in a complicated distribution of QW states. The obtained results are in good agreement with experiment data.