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The injection of high-speed cryogenic pellets made of frozen hydrogen-isotopes, represents to date the most effective method to fuel magnetically confined thermonuclear fusion plasmas. Moreover, the injection of very large pellets composed of cryogenic solid of some suitable impurity (typically a noble-gas such as H2, Ne, or H2/Ne, D2/Ne mixtures), shattered in relatively small fragments just before entering the plasma, seems to be the most promising method to reduce the damage risks for the plasma-facing components in case of a plasma disruption. This technology, known as "Shattered Pellet Injection" (SPI), allows to spread out the plasma energy and mitigate possible damage to the in-vessel components, as well as to densify the plasma to suppress the formation of runaway electrons, and/or dissipate their energy. Several techniques to produce and launch cryogenic pellets have been investigated in the past decades. "Pipe gun" injectors are reliable and relatively simple devices are still commonly used today. They make use of single- or two-stage pneumatic light-gas guns to accelerate the pellet at high speeds. In these injectors, the cryogenic pellets are formed “in situ” (i.e., inside the launching barrel), by de-sublimating them directly from the gas phase, i.e., at temperatures and pressures below those of the triple point. The simplest case is pure deuterium pellets (T < 18.7 K, P < 171.3 hPa). The production of good quality solid deuterium, capable of withstanding the mechanical stress during the acceleration of the pellets, is a key issue. To this end the phase transition of deuterium from gas to solid (and vice versa) is modeled with extensive molecular-dynamics (MD) simulations. Moreover, the solid growth from the gas phase is simulated in an ample range of temperatures and pressures, to find the best compromise between growth velocity and mechanical properties of the resulting solid system.

The low-density (LDA) to high-density (HDA) transformation in amorphous Ge at high pressure is studied by first-principles molecular dynamics simulations in the framework of density functional theory. Previous experiments are accurately reproduced, including the presence of a well-defined LDA-HDA transition above 8 GPa. The LDA-HDA density increase is found to be about 14%. Pair and bond-angle distributions are obtained in the 0-16 GPa pressure range and allowed us a detailed analysis of the transition. The local fourfold coordination is transformed in an average HDA sixfold coordination associated with different local geometries as confirmed by coordination number analysis and shape of the bond-angle distributions. © 2016 IOP Publishing Ltd.

The characterization of self-diffusion in MgO grain boundaries is a materials science problem of general interest, being relevant to the stability and reactivity of MgO layers in artificial nanostructures as well as to the understanding of mass transport and morphological evolution in polycrystalline metal oxides which are employed in many technological applications. In addition, atomic transport in MgO is a key factor to describe the rheology of the Earth’s lower mantle. In this work, we tackle the problem using a classical molecular dynamics model and finite-temperature simulations. To this purpose, we first design a stable grain boundary structure, which is meant to be representative of general internal interfaces in nanocrystalline MgO. The Mg and O self-diffusion coefficients along this grain boundary are then determined as a function of temperature by calculating the mean-square ionic displacement in the boundary region. Two different diffusion regimes at low and high temperature are identified, allowing to obtain the relevant activation enthalpies for migration from the temperature dependance of the diffusion coefficients. Our results prove that Mg diffusion along MgO grain boundaries is sufficiently fast to explain the recently reported development of MgO hollow structures during repeated hydrogen sorption cycles in Mg/MgO nanoparticles.

In this work total and partial density of states were calculated for small single wurtzite CdS nanoparticle, quantum dots, with diameter below 2 nm, performing DFT pseudopotentials calculations with generalized gradient approximation. The cluster was previously heated at different temperatures ranging from 100 to 600 K-, by means of Car-Parrinello ab-initio molecular dynamics, finding a stable phase after 340 K. Another stable phase is reached at higher temperature, more than 480 K, where surface migrations play an important role in the determination of final atomic configuration. Density of states was obtained for the structures heated at 100 K, 280 K 330 K, 340 K, 480 K and 570 K and substantial modifications were found after 340 K where the superficial effects cause some alterations of the intermediate electronic states around the band gap. From the partial density of states it has been clearly showed that the atoms on the surface have a main role on the formation of the intermediate states on the band gap. This effect is meaningful for small particles where the fraction of the surface atoms is greater than the fraction of the core atoms.Copyright © 2013 American Scientific Publishers.

The upper threshold of hydrogen adsorption in Li-doped and hydrogenated carbon nanotube densely packed arrays is calculated to check the ability of such systems to fulfill the target indicated by the United States Department of Energy (DOE). To this aim, model potential parameters have been obtained by Density Functional Theory and have been used to calculate the adsorption isotherms in honeycomb arrays containing up to seven tubes by means of Grand-Canonical Monte Carlo simulations. A hybrid model has been developed involving both atomistic potentials for short-range interactions and integrated potentials for hydrogen interacting with distant tubes. In the pressure range explored, it is shown that the hydrogen adsorption performances of Li-doped carbon nanotubes arranged in close packed honeycomb arrays, while being enhanced with respect to pristine carbon nanotubes, are still well below the DOE targets. © Springer Science+Business Media 2013.

We analyze from a computational viewpoint the conformational and dynamical properties of the interesting oligopeptide that is reported to bind specifically to titania surfaces. First-principles simulations based on Density Functional Theory (DFT) and classical Molecular Dynamics (MD) are carried out to investigate both the structure and the stability of single amino acids and of the full peptide on the (101) surface of TiO2 in the anatase crystalline structure. Moreover the strength of the adhesion is estimated by pulling the peptide away from the surface. Our results shed light on the role played by some amino acids that are known to be essential in selective adsorption on TiO2, as well as on the peptide structural conformation upon the surface.Copyright © 2013 American Scientific Publishers.

The long-term objective of the European Fusion Development Agreement (EFDA) fusion materials programme is to develop structural and armor materials in combination with the necessary production and fabrication technologies for reactor concepts beyond the International Thermonuclear Experimental Reactor. The programmatic roadmap is structured into four engineering research lines which comprise fabrication process development, structural material development, armor material optimization, and irradiation performance testing, which are complemented by a fundamental research programme on "Materials Science and Modeling." This paper presents the current research status of the EFDA experimental and testing investigations, and gives a detailed overview of the latest results on materials research, fabrication, joining, high heat flux testing, plasticity studies, modeling, and validation experiments. (C) 2013 Elsevier B.V. All rights reserved.

We describe two examples of application focusing on first-principles molecular dynamics as an effective tool to unravel the atomic-scale structure of condensed-matter systems. The first application is on disordered network-forming materials and the second is on silicon-doped fullerenes. We show that an accurate modelling of interatomic forces based on density functional theory, when combined with an account of the temperature evolution, is an unavoidable prerequisite for analyzing and interpreting experimental results on a quantitative basis. In the case of disordered systems, we describe the basic structural features of amorphous GeSe4 and highlight the predominant chemical order in this system. The effect of adding or removing an electron charge on the stability of Si-doped fullerenes is exemplified by considering the finite temperature evolution of heterofullerenes. © (2008) Trans Tech Publications, Switzerland.

Tight binding molecular dynamics is used to predict the structure and the total energy of the most relevant intrinsic point defects in C54 and C49 TiSi2. The comparison between the relative formation energies of point defects of the two phases in contact with a Si substrate suggests that the metastable C49 form has a higher concentration of point defects. In particular, we point out that Si vacancies and (010) stacking faults should be quite common in the C49 structure. This issue could be important in explaining the kinetic advantage of the latter phase in film growth by solid state reaction.

The total energy of equilibrated pyramids layer by layer films and flat prepyramids in the epitaxial growth of Ge on Si (001) was investigated. Molecular dynamics (MD) simulations were applied to equilibrate the samples at atomic scale by the efficient code. It was shown that pyramids are more stable than Frank-van der Merwe (FM) films due to the partial strain relief.

The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technologies is pursued within the EFDA long-term fusion materials programme. This paper reviews the progress of work within the programme in the area of tungsten and tungsten alloys. Results, conclusions, and future projections are summarized for each of the programme's main subtopics, which are: (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. It gives a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments. (C) 2012 Elsevier B.V. All rights reserved.