Equation of state for He bubbles in W and model of He bubble growth and bursting near W{100} surfaces derived from molecular dynamics simulations.

Molecular dynamics (MD) simulations are performed to derive an equation of state (EOS) for helium (He) bubbles in tungsten (W) and to study the growth of He bubbles under a W(100) surface until they burst. We study the growth as a function of the initial nucleation depth of the bubbles. During growth, successive loop-punching events are observed, accompanied by shifts in the depth of the bubble towards the surface. Subsequently, the MD data are used to derive models that describe the conditions that cause the loop punching and bursting events. Simulations have been performed at 500, 933, 1500, 2000, and 2500 K to fit the parameters in the models. To compute the pressure in the bubble at the loop punching and bursting events from the models, we derive an EOS for He bubbles in tungsten with an accompanying volume model to compute the bubble volume for a given number of vacancies ([Formula: see text]), He atoms ([Formula: see text]), and temperature (T). To derive the bubble EOS, we firstly derive the EOS for a free He gas. The derived free-gas EOS can accurately predict all MD data included in the analysis (which span up to 54 GPa at 2500 K). Subsequently, the bubble EOS is derived based on the free-gas EOS by correcting the gas density to account for the interaction between He and W atoms. The EOS for the bubbles is fitted to data from MD simulations of He bubbles in bulk W that span a wide range of gas density and sizes up to about 3 nm in diameter. The pressure of subsurface bubbles at the loop punching events as calculated using the bubble-EOS and the volume model agrees well with the pressure obtained directly from the MD simulations. In the loop punching model, for bubbles consisting of [Formula: see text] vacancies and [Formula: see text] helium atoms, the [Formula: see text] ratio that causes the event, the resulting increase in [Formula: see text], and the associated shift of the bubble depth are formulated as a function of [Formula: see text] and T. In the bursting model, a bubble must simultaneously reach a certain depth and [Formula: see text] ratio in order to burst. The burst depth and [Formula: see text] are also modeled as a function of [Formula: see text] and T. The majority of the loop punching events occur at bubble pressures between 20 and 60 GPa, depending on the bubble size and temperature. The larger the bubble and the higher the temperature, the lower the bubble pressure. Furthermore, our results indicate that at a higher temperature, a bubble can burst from a deeper region.

Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten.

We report results of object kinetic Monte Carlo (OKMC) simulations to understand the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially pristine, defect-free, and has a (100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 1022to 4 × 1025He/m2 s with 100 eV helium atoms impinging on a (100) surface up to a maximum fluence of 4 × 1019He/m2. In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that in pristine tungsten, as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for the flux of 1022He/m2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. We present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

Multifaceted Modularity: A Key for Stepwise Building of Hierarchical Complexity in Actinide Metal-Organic Frameworks.

Growing necessity for efficient nuclear waste management is a driving force for development of alternative architectures toward fundamental understanding of mechanisms involved in actinide (An) integration inside extended structures. In this manuscript, metal-organic frameworks (MOFs) were investigated as a model system for engineering radionuclide containing materials through utilization of unprecedented MOF modularity, which cannot be replicated in any other type of materials. Through the implementation of recent synthetic advances in the MOF field, hierarchical complexity of An-materials was built stepwise, which was only feasible due to preparation of the first examples of actinide-based frameworks with "unsaturated" metal nodes. The first successful attempts of solid-state metathesis and metal node extension in An-MOFs are reported, and the results of the former approach revealed drastic differences in chemical behavior of extended structures versus molecular species. Successful utilization of MOF modularity also allowed us to structurally characterize the first example of bimetallic An-An nodes. To the best of our knowledge, through combination of solid-state metathesis, guest incorporation, and capping linker installation, we were able to achieve the highest Th wt % in mono- and biactinide frameworks with minimal structural density. Overall, the combination of a multistep synthetic approach with homogeneous actinide distribution and moderate solvothermal conditions could make MOFs an exceptionally powerful tool to address fundamental questions responsible for chemical behavior of An-based extended structures and, therefore, shed light on possible optimization of nuclear waste administration.

Cascade morphology transition in bcc metals.

Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, b, in the defect production curve as a function of cascade energy (N(F) ~ E(MD)(b)). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, µ, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of µ as a function of displacement threshold energy, E(d), is presented for bcc metals.

Ab initio study of H, He, Li and Be impurity effect in tungsten Σ3{1 1 2} and Σ27{5 5 2} grain boundaries.

Density functional theory calculations were performed to investigate the effect of H, He, Li and Be impurities on the intergranular cohesion of W Σ3〈1 1 0〉{1 1 2} and Σ27〈1 1 0〉{5 5 2} grain boundaries (GBs). A rigorous search of unique interstices was performed to reveal a range of impurity behaviors. It was found that He exhibits the largest defect formation energy, however it is Li that shows the strongest tendency to segregate towards the GBs. The embrittling potency was studied using the ideal GB fracture energy (Rice-Wang) criterion (Rice and Wang 1989 Mater. Sci. Eng. A 107 23). The potency was found to depend significantly on the local atomic environment. A site-dependent concentration was used as the weighing factor to obtain an average potency as a function of temperature. It was found that the potency depends only weakly on temperature. In Σ3 H, He, Li and Be decrease fracture energy by 4%, 27%, 30% and 14%, respectively. In Σ27 H, He and Li also decrease cohesion, albeit less severely: 3%, 14% and 7%. On the other hand, Be increases cohesion in Σ27 by 3%.

A search model for topological insulators with high-throughput robustness descriptors.

Topological insulators (TI) are becoming one of the most studied classes of novel materials because of their great potential for applications ranging from spintronics to quantum computers. To fully integrate TI materials in electronic devices, high-quality epitaxial single-crystalline phases with sufficiently large bulk bandgaps are necessary. Current efforts have relied mostly on costly and time-consuming trial-and-error procedures. Here we show that by defining a reliable and accessible descriptor , which represents the topological robustness or feasibility of the candidate, and by searching the quantum materials repository aflowlib.org, we have automatically discovered 28 TIs (some of them already known) in five different symmetry families. These include peculiar ternary halides, Cs{Sn,Pb,Ge}{Cl,Br,I}(3), which could have been hardly anticipated without high-throughput means. Our search model, by relying on the significance of repositories in materials development, opens new avenues for the discovery of more TIs in different and unexplored classes of systems.

High-throughput combinatorial database of electronic band structures for inorganic scintillator materials.

For the purpose of creating a database of electronic structures of all the known inorganic compounds, we have developed a computational framework based on high-throughput ab initio calculations (AFLOW) and an online repository (www.aflowlib.org). In this article, we report the first step of this task: the calculation of band structures for 7439 compounds intended for the research of scintillator materials for γ-ray radiation detection. Data-mining is performed to select the candidates from 193,456 compounds compiled in the Inorganic Crystal Structure Database. Light yield and scintillation nonproportionality are predicted based on semiempirical band gaps and effective masses. We present a list of materials, potentially bright and proportional, and focus on those exhibiting small effective masses and effective mass ratios.

Structures and topological transitions of hydrocarbon films on quasicrystalline surfaces.

Lubricants can affect quasicrystalline coating surfaces by modifying the commensurability of the interfaces. We report results of the first computer simulation studies of physically adsorbed hydrocarbons on a quasicrystalline surface: methane, propane, and benzene on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using novel embedded-atom method potentials generated from ab initio calculations, and standard hydrocarbon interactions. The resulting adsorption isotherms and calculated structures show the films' evolution from submonolayer to condensation. We discover the presence and absence of the fivefold to sixfold topological transition, for benzene and methane, respectively, in agreement with a previously formulated phenomenological rule based on adsorbate-substrate size mismatch.

Evolution of topological order in Xe films on a quasicrystal surface.

We report results of the first computer simulation studies of a physically adsorbed gas on a quasicrystalline surface Xe on decagonal Al-Ni-Co. The grand canonical Monte Carlo method is employed, using a semiempirical gas-surface interaction, based on conventional combining rules, and the usual Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and calculated structures are consistent with the results of LEED experimental data. The evolution of the bulk film begins in the second layer, while the low coverage behavior is epitaxial. This transition from epitaxial fivefold to bulklike sixfold ordering is temperature dependent, occurring earlier (at lower coverage) for the higher temperatures.

Nanotube electronics: large-scale assembly of carbon nanotubes.

Nanoscale electronic devices made from carbon nanotubes, such as transistors and sensors, are much smaller and more versatile than those that rely on conventional microelectronic chips, but their development for mass production has been thwarted by difficulties in aligning and integrating the millions of nanotubes required. Inspired by biomolecular self-assembly processes, we have created chemically functionalized patterns on a surface, to which pre-grown nanotubes in solution can align themselves in huge numbers. This method allows wafer-scale fabrication of millions of carbon-nanotube circuits with single-nanotube precision, and may enable nanotube-based devices, such as computer chips and high-density sensor arrays, to be produced industrially.