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The nature of the dominant point defect and semiconductor character in paramagnetic Cr2O3 depends on temperature and oxygen partial pressure.

Samples of Ni-30Cr alloy were oxidized at different temperatures from 500 to 900 °C in a nominal oxygen partial pressure of 5 × 10−6 atm. The parabolic rate constants for growth of the oxide scales, which were confirmed to be chromia, were in agreement with the literature following an Arrhenius law. The semiconductor character of the chromia scale was investigated in order to reveal the nature of the dominant point defects responsible for the diffusion process during oxidation. An n-type semiconductor was found at low temperature (500 °C), and p-type semiconductor at high temperature (900 °C). Our results suggest that oxygen vacancies and chromium vacancies are dominant in grown chromia of n- and p-type, respectively.

International audience; Stress corrosion cracking is a well-known phenomenon in oxide glasses. However, how amorphous phase separation (APS) alters stress corrosion cracking, and the overall mechanical response of an oxide glass is less known in literature. APS is a dominant feature concerning many multicomponent systems, particularly the ternary sodium borosilicate (SBN) glass systems. Its three constituent oxides have significant industrial relevance, as they are the principal components of many industrial oxide glasses. Simulations and experimental studies demonstrate the existence of a two-phase metastable miscibility gap. Furthermore, theory suggests the possibility of three-phase APS in these oxide glasses. Literature already details the mechanisms of phase separation and characterizes SBN microstructures. Realizing that glasses are structurally sensitive materials opens a number of other questions concerning how the mesoscopic APS affects the continuum behavior of glasses, including dynamic fracture and stress corrosion cracking. This paper reviews current literature and provides a synthetic viewpoint on how APS structures of oxide glasses alter physical, mechanical, dynamic fracture, and stress corrosion cracking properties

International audience

The iron titanium oxide ilmenite (FeTiO3) is a technologically and economically important mineral in the industrial preparation of titanium-based pigments and spintronic devices. In this study, atomistic simulation techniques based on classical pair potentials are used to examine the energetics of the intrinsic and extrinsic defects and diffusion of Fe2+ ions in FeTiO3. It is calculated that the cation anti-site (Fe‒Ti) cluster is the most dominant defect, suggesting that a small amount of cations exchange their positions, forming a disordered structure. The formation of Fe Frenkel is highly endoergic and calculated to be the second most stable defect process. The Fe2+ ions migrate in the ab plane with the activation energy of 0.52 eV, inferring fast ion diffusion. Mn2+ and Ge4+ ions are found to be the prominent isovalent dopants at the Fe and Ti site, respectively. The formation of additional Fe2+ ions and O vacancies was considered by substituting trivalent dopants (Al3+, Mn3+, Ga3+, Sc3+, In3+, Yb3+, Y3+, Ga3+, and La3+) at the Ti site. Though Ga3+ is found to be the candidate dopant, its solution enthalpy is &gt, 3 eV, suggesting that the formation is not significant at operating temperatures.

Radiation damage lowers the melting point of metal tungsten, an effect that needs to be reckoned with for fusion reactors., Materials exposed to extreme radiation environments such as fusion reactors or deep spaces accumulate substantial defect populations that alter their properties and subsequently the melting behavior. The quantitative characterization requires visualization with femtosecond temporal resolution on the atomic-scale length through measurements of the pair correlation function. Here, we demonstrate experimentally that electron diffraction at relativistic energies opens a new approach for studies of melting kinetics. Our measurements in radiation-damaged tungsten show that the tungsten target subjected to 10 displacements per atom of damage undergoes a melting transition below the melting temperature. Two-temperature molecular dynamics simulations reveal the crucial role of defect clusters, particularly nanovoids, in driving the ultrafast melting process observed on the time scale of less than 10 ps. These results provide new atomic-level insights into the ultrafast melting processes of materials in extreme environments.

International audience; Stoichiometric circular shaped interstitial dislocation loop energies are calculated in stoichiometric UO2 by empirical potential simulation. The Burgers vector directions studied are < 110 > and < 111 >. The main structural properties of each type of interstitial dislocation loop are determined, including stacking fault energy. Defect energies are compared and a maximum size for stable < 111 > dislocation loops before transition to < 110 > dislocation loops is given. A model of dislocation loop energy based on elasticity theory is then fitted on the basis of these simulation results. (C) 2016 Elsevier B. V. All rights reserved.

Recently it has been suggested that dislocations, generated by radiation damage, may increase the rate of fission gas diffusion from the fuel grains, an affect which is at present not incorporated into fuel performance codes. Therefore, we perform molecular dynamics simulations employing empirical potentials to investigate the diffusion of Xe atoms around edge dislocations in UO2 to establish the importance of this pathway for fission gas release. The results suggest that for isolated atoms near the dislocation the activation energy for Xe diffusion is dramatically reduced relative to the bulk. However, Xe atoms diffusing along the dislocation cluster together to form small bubbles, these bubbles incorporate all of the isolated mobile Xe atoms thereby inhibiting fast diffusion of Xe along the dislocation core.

Molecular dynamics (MD) was used to establish a mechanistic basis for the experimentally observed reduction in liquidus and solidus temperatures below the melting point of the end-members for the mixed oxides ( Th , U ) O 2 , ( Th , Pu ) O 2 and ( Pu , U ) O 2 . This dip is found at ∼ 5 % additions of the oxide with higher melting point to the oxide with the lower melting point. There are many causes suggested for the dip; here the distribution of the cation Frenkel energy for the mixed oxides caused by the local environment is proposed as a contributor. Furthermore, a variant of the moving interface method which yields information on the position of the solidus and liquidus boundaries, is used to predict the phase diagrams of these systems.

Synroc-Z is a new wasteform being developed to be specifically suited to waste from the advanced EURO-GANEX reprocessing route. This type of waste contains nuclides that release significant amounts of heat during their decay. Here, predictions for radiogenic heat generation from EURO-GANEX wasteforms based on Synroc-Z ceramic and R7T7 glass materials are presented. During reprocessing, any actinides present in the waste-stream are removed to leave only fission products. Simulation results are given demonstrating that, although initially classified as high level waste, the effects of decay mean the wasteforms effectively become intermediate level after 140 years of storage. Furthermore, many of the issues arising from alpha decay will be avoided. During the first 600 years, it is shown that the main source of self heating comes from the decay of Cs-137 and Sr-90. Our calculations show how the significant radiogenic heating by these wasteforms must be considered when defining package sizes and waste loadings. These effects are discussed for both ceramic and glass wasteforms and demonstrate that Synroc-Z provides more flexibility allowing larger packages even at higher waste loadings than R7T7.

An unexplained discrepancy exists between the experimentally measured and theoretically calculated magnetic moments of Mn in \ensuremath{\alpha}-Fe. In this study, we use density functional theory to suggest that this discrepancy is likely due to the local strain environment of a Mn atom in the Fe structure. The ferromagnetic coupling, found by experiment, was shown to be metastable and could be stabilized by a 2% hydrostatic compressive strain. The effects of Mn concentration, vacancies, and interstitial defects on the magnetic moment of Mn are also discussed. It was found that the ground-state, antiferromagnetic (AFM) coupling of Mn to Fe requires long-range tensile relaxations of the neighboring atoms along $\ensuremath{\langle}111\ensuremath{\rangle}$ which is hindered in the presence of other Mn atoms. Vacancies and Fe interstitial defects stabilize the AFM coupling but are not expected to have a large effect on the average measured magnetic moment.

In this work interfaces between (Na2O)x(SiO2)1−x glasses (for x=0.0, 0.1 and 0.2) and TiO2 crystals are simulated using molecular dynamics and empirical potentials. Interfaces are presented for the distinct terminat- ing surfaces of TiO2 with Miller indices ≤ 2, the properties of which have been investigated using atomistic models. Simulations showed that par- tially ordered layers had been induced in the glass close to the interfaces, with successive oxygen-rich and cation-rich planes being noted. The first silicate layer in contact with the crystal tended to be highly-structured, with Si ions occupying well-defined positions that depend on the orien- tation of the crystal at the interface, and showing 2-dimensional ordering depending on glass composition. Finally, interface energies were calcu- lated. These indicated that the interface formation may stabilise a crystal surface in comparison to maintaining a free surface. Results are pre- sented suggesting that the structural flexibility of the glass network allows it to conform to the crystal, thereby providing charge compensation and avoiding large relaxation of the crystal structure close to the interfaces. Such interfacial properties could be crucial to improving phenomenologi- cal models of glass-crystal composite properties.

Molecular dynamics simulations and genetic algorithms are used to identify the mechanisms by which oxygen is transported through the Sr1−3x/2LaxTiO3 family of perovskites as a function of x. Across this compositional range the relative stability of ordered structures and random arrangements of cations and vacancies on the A sublattice is established. O and Ti Frenkel pair formation is then predicted for the considered compositions. These results show that Ti defects are more favourable for all La-containing compositions, but have a larger defect volume than O defects for low x. Oxygen diffusion at high temperature is determined using Molecular dynamics simulations. Two types of oxygen transport mechanisms are identified, that each contribute to oxygen diffusion in ordered structures. These mechanism involve transient O and Ti defects respectively, and provide means for O ions to swap positions. Depending on cation ordering, they can be correlated in space, which causes qualitatively different diffusion patterns.

Dinitrogen activation plays an important role in the production of many essential nitrogen based compounds needed for all life. Using density functional theory together with dispersion correction (DFT+D), the activation of molecular nitrogen with a gas phase Zr dimer and a Zr dimer loaded C60 is investigated. The present calculations show that the optimised trans-Zr2N2 configuration is planar but this configuration exhibits a butterfly shape when it is supported by C60. Furthermore, it is shown that the activation of dinitrogen is facilitated by the Zr dimer stabilized by C60. Additional calculations are carried out to look at the products of the reactions with H2 and a catalytic cycle for the reduction of N2 to NH3 is constructed. Reaction of two molecules of H2 with Zr2N2 molecule loaded C60 is exothermic while reaction of only one molecule of H2 with free Zr2N2 molecule is exothermic again highlighting the importance of stabilization with C60.Dinitrogen activation plays an important role in the production of many essential nitrogen based compounds needed for all life. Using density functional theory together with dispersion correction (DFT+D), the activation of molecular nitrogen with a gas phase Zr dimer and a Zr dimer loaded C60 is investigated. The present calculations show that the optimised trans-Zr2N2 configuration is planar but this configuration exhibits a butterfly shape when it is supported by C60. Furthermore, it is shown that the activation of dinitrogen is facilitated by the Zr dimer stabilized by C60. Additional calculations are carried out to look at the products of the reactions with H2 and a catalytic cycle for the reduction of N2 to NH3 is constructed. Reaction of two molecules of H2 with Zr2N2 molecule loaded C60 is exothermic while reaction of only one molecule of H2 with free Zr2N2 molecule is exothermic again highlighting the importance of stabilization with C60.

The plasticity of the fluorite structure in UO 2 is investigated with molecular dynamics simulation and empirical potential. The stacking fault energies and the dislocation core structures with Burgers vector a 2 〈 1 1 0 〉 are systematically calculated. All dislocation core structures show a significant increase of the oxygen sub-lattice disorder at temperatures higher than 1500 K. The threshold stress for dislocation glide is found to decrease with increasing temperature but its values is always very high, several GPa at 0 K and several hundred of MPa at 2000 K. A relation between the dislocation mobility dependence with temperature and the increase of the oxygen sub-lattice disorder in the dislocation cores is established.

Atomic scale molecular dynamics simulations of radiation damage have been performed on beryllium. Direct threshold displacement simulations along a geodesic projection of directions were used to investigate the directional dependence with a high spatial resolution. It was found that the directionally averaged probability of displacement increases from 0 at 35 eV, with the energy at which there is a 50% chance of a displacement occurring is 70 eV and asymptotically approaching 1 for higher energies. This is, however, strongly directionally dependent with a 50% probability of displacement varying from 35 to 120 eV, with low energy directions corresponding to the nearest neighbour directions. A new kinetic energy dependent expression for the average maximum displacement of an atom as a function of energy is derived which closely matches the simulated data.

International audience; This article presents a synthesis of the investigations at the atomic scale of the transport properties of defects and fission gases in uranium dioxide, as well as of the transfer of results from the atomic scale to models at the mesoscopic scale, performed during the F-BRIDGE European project (2008-2012). We first present the mesoscale models used to investigate uranium oxide fuel under irradiation, and in particular the cluster dynamics and kinetic Monte Carlo methods employed to model the behaviour of defects and fission gases in UO$_2$ , as well as the parameters of these models. Second, we describe briefly the atomic scale methods employed, i.e. electronic structure calculations and empirical potential methods. Then, we show the results of the calculation of the data necessary for the mesoscale models using these atomic scale methods. Finally, we summarise the links built between the atomic and mesoscopic scale by listing the data calculated at the atomic scale which are to be used as input in mesoscale modelling. Despite specific difficulties in the description of fuel materials, the results obtained in F-BRIDGE show that atomic scale modelling methods are now mature enough to obtain precise data to feed higher scale models and help interpret experiments on nuclear fuels. These methods bring valuable insight, in particular the formation, binding and migration energies of point and extended defects, fission product local-ization, incorporation energies and migration pathways, elementary mechanisms of irradiation induced processes. These studies open the way for the investigation of other significant phenomena involved in fuel behaviour, in particular the thermochemical and thermomechanical properties and their evolution in-pile, complex microstructures, as well as of more complex fuels.

Using molecular dynamics, the thermophysical properties of the (Ux,Th1−x)O2 system have been investigated between 300 and 3600 K. The thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure is explained in terms of defect formation and diffusivity on the oxygen sublattice. Vegard's law is approximately observed for solid solution thermal expansion below 2000 K. Different deviations from Vegard's law above this temperature occur owing to the different temperatures at which the solid solutions undergo the superionic transition (2500–3300 K). Similarly, a spike in the specific heat, associated with the superionic transition, occurs at lower temperatures in solid solutions that have a high U content. Correspondingly, oxygen diffusivity is higher in pure UO2 than in pure ThO2. Furthermore, at temperatures below the superionic transition, oxygen mobility is notably higher in solid solutions than in the end members. Enhanced diffusivity is promoted by lower oxygen-defect enthalpies in (Ux,Th1−x)O2 solid solutions. Unlike in UO2 and ThO2, there is considerable variety of oxygen vacancy and oxygen interstitial sites in solid solutions generating a wide range of property values. Trends in the defect enthalpies are discussed in terms of composition and the lattice parameter of (Ux,Th1−x)O2.