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A set of oxide and chalcogenide tetrahedral glasses are investigated using molecular dynamics simulations. It is shown that unlike stoichiometric selenides such as GeSe$_2$ and SiSe$_2$, germania and silica display large standard deviations in the associated bond angle distributions. Within bond-bending constraints theory, this pattern can be interpreted as a manifestation of {\it {broken}} (i.e. ineffective) oxygen bond-bending constraints. The same analysis reveals that the changes in the Ge composition affects mostly bending around germanium in binary Ge-Se systems, leaving Se-centred bending almost unchanged. In contrast, the corresponding Se twisting (quantified by the dihedral angle) depends on the Ge composition and is reduced when the system becomes rigid. Our results establishes the atomic-scale foundations of the phenomelogical rigidity theory, thereby profoundly extending its significance and impact on the structural description of network glasses., Comment: 5 pages, 4 figures

We calculate the concentration-concentration partial structure factor S-CC(k) and the charge-charge structure factor S-zz(k) of liquid SiO2, amorphous SiSe2 and liquid GeSe2 using first-principles molecular dynamics. These systems are characterized by the occurrence of intermediate range order, as evidenced by a first sharp diffraction peak (FSDP) at low k values in the total neutron structure factor. We show that a FSDP in the concentration-concentration partial structure factor S-CC(k) is generally associated with a small departure from chemical order. This feature tends to vanish either when sufficiently high levels of structural disorder set in, or, oppositely, when the chemical order is essentially perfect. For none of these networks, a FSDP is observed in the charge-charge structure factor S-zz(k), i.e., fluctuations of charge do not occur over intermediate range distances. The constraint of charge neutrality is at the very origin of the appearance of fluctuations of concentration. These are observed when the atoms occur in configurations with different coordinations.

The microscopic origin of the intermediate phase in two prototypical covalently bonded A(x)B(1-x) network glass forming systems, where A=Ge or Si, B=Se, and 0

In a search for the physical origin of the first sharp diffraction peak (FSDP) in the concentration-concentration partial structure factor S-CC (k) of disordered network-forming materials, we perform first-principles molecular dynamics simulations of liquid GeSe4 (l-GeSe4) and liquid SiSe2 (l-SiSe2). These systems are designed to provide clues on the relationship between the appearance of an FSDP in the S-CC(k) structure factor and the degree of chemical order. Short-range chemical order is more pronounced in l-GeSe4 and in l-SiSe2 than in liquid GeSe2. For the latter system, our level of theory does not reproduce the FSDP in the experimentally observed SCC(k) structure factor. We find that a distinct FSDP shows up in the partial structure factor SCC (k) for l-GeSe4. In SCC (k) for l-SiSe2, we also find a feature at the FSDP location, although it is smaller compared to GeSe4. Given the tight correlation existing between chemical order and ionicity, these results suggest that the ionic character of the bonds plays a crucial role in inducing concentration fluctuations at intermediate-range distances.

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.

Corrosion property of iron in high temperature stagnant liquid lead has been studied using molecular dynamics method. The method was used to predict the limit values of the injected oxygen into the liquid lead for maximum corrosion inhibition of iron. It is from experimental results, in order to inhibit the corrosion at possible lowest rate then a stable self-healing protective iron oxide layer should be developed at the surface of steel continuously. In this research we investigated the iron corrosion and it can be predicted that the protective oxide layer may be formed by injecting oxygen within the range of 5.35 × 10-2 wt% to 8.95 × 10-2 wt% (for observed temperature 750 C). The oxygen 5.35 × 10-2 wt% is the lower limit to prevent high dissolution of iron while the oxygen content of 8.95 × 10-2 wt% is the upper limit to avoid high precipitation of lead oxide. We also guess that effect of oxygen injection into liquid lead creates a thin oxygen barrier that separating the liquid lead and iron oxides from direct interaction. The iron oxides layer and oxygen barrier then may be regarded as double corrosion inhibition. © 2013 Elsevier Ltd. All rights reserved.

The interface of biological molecules with inorganic surfaces has been the subject of several recent studies. Experimentally some amino acids are evidenced to play a critical role in the adhesion and selectivity on oxide surfaces; however, detailed information on how the water molecules on the hydrated surface are able to mediate the adsorption is still missing. Accurate total energy ab initio calculations based on dispersion-corrected density functional theory have been performed to investigate the adsorption of selected amino acids on the hydrated ZnO(101¯0) surface, and the results are presented and discussed in this paper. We have also investigated the role played by water in the determination of the most energetically favorable adsorption configurations of the selected amino acids. We have found that for some amino acids the most energetically favorable configurations involve the deprotonation of the molecule if the water screening is not effective. © 2015 American Chemical Society.

After a brief review of the basic properties of TiSi 2 phases recently obtained by molecular dynamics simulations, we show the self-diffusing species at high temperatures in both the C54 and C49 structures to be silicon, by hopping through a first neighbours network. The kinetic parameters and the diffusion paths for Si diffusion by vacancies and interstitials are reported, pointing out that diffusion in the C49 structure by SiI vacancies is strongly favoured, due to the very low formation energy they display.

In this paper we show that the driving force leading to the metal-semiconductor structural phase transition occurring in epitaxial FeSi2 with film thickness, originates from a local Jahn-Teller distortion. This effect can be straightforwardly seen by the analysis of the site-projected density of states during a variable cell molecular dynamics for the bulk configuration. We point out that the evolution of the local density of states is a reliable and powerful tool solely provided by tight-binding molecular dynamics. [S0163-1829(99)00905-4].

In this paper we report a molecular dynamics simulation at constant pressure and constant temperature of the structural phase transition occurring in epitaxial FeSi2 from the fluorite phase (metallic and pseudomorphic) to orthorhombic one (semiconductor and bulk stable). The evolution of the electronic density of states is carefully monitored during the transformation and we can show that the Jahn-Teller coupling between the density of states at the Fermi level and the lattice deformation drives the metal-semiconductor transition.

We point out that the predictive power of tight binding potentials is not limited to obtaining fairly accurate total energy calculations and very satisfactory structural evolutions by molecular dynamics simulations. They also allow for a nice physical picture of the links between bonding and stability in different structures, which is particularly helpful in the case of binary suicides

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.