We studied the structure and mechanical properties of DNA i-motif nanowires by means of molecular dynamics computer simulations. We built up to 230 nm-long nanowires, based on a repeated TC5 sequence from NMR crystallographic data, fully relaxed and equilibrated in water. The unusual C●C+ stacked structure, formed by four ssDNA strands arranged in an intercalated tetramer, is here fully characterized both statically and dynamically. By applying stretching, compression and bending deformations with the steered molecular dynamics and umbrella sampling methods, we extract the apparent Young’s and bending moduli of the nanowire, as well as estimates for the tensile strength and persistence length. According to our results, i-motif nanowires share similarities with structural proteins, as far as their tensile stiffness, but are closer to nucleic acids and flexible proteins, as far as their bending rigidity is concerned. Curiously enough, their tensile strength makes such DNA fragments tough as mild steel or a nickel alloy. Besides their yet to be clarified biological significance, i-motif nanowires may qualify as interesting candidates for nanotechnology templates, due to such outstanding mechanical properties.

The remarkable ability of magnesium to store significant quantities of hydrogen has fostered intense research efforts in the last years in view of its future applications where light and safe hydrogen-storage media are needed. Magnesium material, characterized by light weight and low cost of production, can reversibly store about 7.7 wt% hydrogen. However, further research is needed since Mg has a high operation temperature and slow absorption kinetics that prevent the use in practical applications. For these reasons a detailed study of the interface between Mg and MgH2 is needed. Further insights are gained by characterizing the Mg-MgH2 system from both the experimental and the numerical point of view.

The study of the MgH2-Mg phase transformation in powder samples has been performed to gain detailed metallographic information. A method for studying this phase transformation by cross sectional samples scanning electron microscopy observation of partially transformed material has been developed. This method exploits the peculiar features of this system where the MgH2 phase is insulating and the Mg is a metallic conducting phase. This difference can induce a contrast between the two phases owing to the different secondary emission yield. Further insights are gained by characterizing Mg-MgH2 interfaces by means of accurate first-principle molecular dynamics simulations based on the density-functional theory. Extensive electronic structure calculations are used to characterize the equilibrium properties and the behavior of the surfaces in terms of total energy considerations and atomic diffusion.

A tight-binding hamiltonian is used to study the electronic properties of covalently-bonded, crossed (5,5) metallic nanotubes with increasing degree of disorder in the junction region. At one extreme, ideal junctions between coplanar nanotubes with a minimal number of topological defects show a good ohmic behavior. Upon increasing disorder, ohmic conduction is suppressed in favor of hopping conductivity. At the opposite extreme, strongly disordered junctions as could be obtained after electron-beam irradiation of overlayed nanotubes, display weak localization and energy quantization, indicating the formation of a quantum dot contacted to metallic nanowires by tunnel barriers.

We present the recent extensions of our stochastic microstructure evolution model including multiphase domain evolution and dislocation plasticity. The model was implemented by means of numerical simulations based on the velocity Monte Carlo algorithm. It describes the evolution of a two-dimensional microstructure by tracking the motion of triple junctions, i.e., the vertices where three grain boundaries (GBs) meet. GBs can be modeled as straight, curved, or discretized segments; the misorientation dependence of both grain-boundary energies and mobilities can be included to represent different textures.

We discuss a microstructure evolution framework which couples atomic-level information about extended-defect interactions into a mesoscopic model; the latter, in turn, describes the dy-namic evolution of a statistical population of grain boundaries and dislocations. Atomistic simulations are carried out by means of molecular dynamics simulations on both isolated and interacting dislocations, grain boundaries, triple junctions, microcracks; the reference material for such studies is, at present, Silicon with the Stillinger-Weber potential. The mesoscale model describes the motion of discrete triple junctions (and, consequently, of the continuous network of adjoining grain boundaries) embedded in a continuous medium containing a homogenous, evolving distribution of dislocations.

Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (Stone- Wales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen is physisorbed between the graphite planes, but cannot diffuse across a graphitic plane.

Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (Stone- Wales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen can be physisorbed between the graphite planes, but cannot diffuse across a graphitic plane.

Topological changes in microstructure are strictly related to the microscopic evolution of triple junctions (TJ). The three-sided grain disappearance, usually called T2 process, is here investigated via 3D-atomistic modeling. In particular the stability of a three-sided grain insertion in a triple junction in silicon is studied within the framework of Molecular Dynamics simulations. The Stillinger-Weber interatomic potential is adopted and constant-traction border conditions are considered to ensure a proper embedding of the atomistic region in a virtually infinite bulk continuum. Dealing with the T2-event, the critical radius below which the three- sided inner grain become unstable is evaluated to be three to four times the lattice constant of silicon. Moreover, we show that the instability sets in through the amorphization of the central shrinking grain.

We studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.

Triple junctions are crucial elements in microstructural evolution: for example, their mobility can be rate-limiting if lower than that of grain boundaries. However, very little is known about their atomic-level structure and properties. We studied the atomic structure of multiple-twin triple junctions in silicon, formed by the convergence of two {111} and one {221} symmetric-tilt grain boundaries. Molecular dynamics simulations with the Stillinger-Weber potential and constant-traction border conditions were performed on several triple junction configurations, obtained by different combinations of the three grain boundaries. All the configurations have a positive excess line energy, a measurable volume contraction and display regions of opposite, tensile and compressive, residual stress. Moreover, we tried to elucidate the role of triple junctions as being the seeds of the only microscopic events that can lead to topological changes in the microstructure. Such events, usually dubbed T1 and T2 in mesoscopic models, correspond to grain switching (in the Ashby-Verrall sense) and grain-disappearance events, respectively. We present preliminary results for the atomic-scale modelling of both classes of topological events and discuss the connection between atomistic and mesoscopic modelling of microstructural evolution.

Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (EF(vac)=6.10 eV, EF(SW)= 5.55 eV).

An atomic-scale model of liquid silicon has been cooled from high temperatures down in the temperature range between the amorphous and the crystalline melting temperatures by nanosecond scale molecular dynamics simulations with the Stillinger-Weber potential. Tetrahedrally coordinated sites have been identified, in the supercooled liquids, by using a few structural order parameters. The local structure and the stability of these crystalline-like regions (c-type sites and clusters) have been characterized. These have been regarded as candidates for crystalline embryos.

We present a stochastic microstructure evolution model applicable to grain growth and its recent extensions, in particular relative to dislocation plasticity. The model is implemented by means of numerical simulations based on the velocity Monte Carlo algorithm. It describes the evolution of a two-dimensional microstructure by tracking the motion of triple junctions, i.e. the vertices where three grain boundaries meet. Grain boundaries can be modeled as straight or curved segments; the misorientation dependence of both grain-boundary energies and mobilities can be included, as well as grain rotation. We show simple examples of normal, abnormal and oriented grain growth. The model is already capable of dealing with a two-phase (liquid-solid) system, to simulate both grain growth and grain dissolution in the liquid. Finally, we report preliminary results of a recent extension of the model to include mechanical deformation from dislocation plasticity.

The crystalline disordered phase obtained by mechanical alloying of elemental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering process leading to the L12 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni3Al model, reproduced via molecular dynamics using a many-body potential, have been used to interpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600 °C) and occurs simultaneously with the release of internal strain. Numerical simulations performed under different conditions show that the activation energy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an explanation for the considerable lowering of this energy in samples obtained by ball milling.

The order-disorder transformation in CU3Au has been investigated using a combined Molecular Dynamics and Cluster Variation Method approach. Free energy minimization has been performed using the Natural Iteration technique. The calculated temperature dependence of enthalpy, lattice dynamics, short-range and long-range order parameters have been successfully compared with experimental data.

The behaviour of two different systems, NiZr2 and Cu3Au, submitted to a chemical disordering process under various thermodynamical conditions has been investigated by Molecular Dynamics simulations. The role played by volume expansion and elastic softening in the onset of the transition has been discussed in the framework of the percolation model (C.Massobrio and V.Pontikis, Phys.Rev.B 45 (1992) 2484), correlating the elastic instability with the percolation of the distorted regions which establishes in the lattice as a consequence of the external perturbations (introduction of defects, volume expansion, chemical disorder etc.).

We report a first observation, of the behavior of clusters of deuterium atoms confined in a lattice vacancy in a Pd crystal, either at equilibrium or under the influence of externally applied lattice strains. Molecular dynamics simulations have been performed on both Pd perfect crystals and Pd crystals with a single vacancy, showing the energetic behavior and the dynamical values of the interatomic distance among deuterium atoms belonging to clusters of different sizes confined within the vacancy. At low temperatures, the smallest interatomic D-D distances are of the order of the D2 molecular bond length and are obtained during the transitory regimes induced by hydrostatic strains applied to the lattice.