An ab initio study is performed for the initiation of chemistry in high explosive crystals from a solid-state physics viewpoint. Specifically, we are looking for the relationship between the defect-induced deformation of the electronic structure of solids, electronic excitations, and chemical reactions under shock conditions. Band structure calculations by means of the Hartree- Fock method with correlation corrections were done to model an effect of a strong compression induced by a shock/impact wave on the crystals with and without edge dislocations. Based on the results obtained, an excitonic mechanism of the earliest stages for initiation of high explosive solids is discussed with application to cyclotrimethylene trinitramine (also known as RDX) crystal. Experimental verification of the validity of the proposed model is reported for RDX and heavy metal azides. Thus, the key role of electronic excitations facilitated by edge dislocations in explosive solids is established and analyzed. Practical applications of the suggested mechanisms are discussed.

In a previous paper, we presented a general theoretical treatment of the effect of stress on defect diffusion in Si (M. S. Daw, W. Windl, N. N. Carlson, M. Laudon, and M. P. Masquelier, to be published in Phys. Rev. B). In this paper, we discuss the calculation of the parameters governing the stress dependence of the diffusivity, which are volume quantities, and present the fully anisotropic volume tensor for vacancy formation in Si.

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.

In this work we propose an analytical expression for the complex dielectric function that includes both discrete and continuum exciton effects. The model is based on the work of Elliott and the proposed model has been applied to modeling the experimental data for the hexagonal GaN. We have obtained good agreement with the experimental data. The model assumes Lorentzian broadening in order to obtain dielectric function equations in analytically closed form. We show that Lorentzian broadened dielectric function decays more slowly than the experimental data for hexagonal GaN at the low energy side. This indicates that the broadening of the absorption edge in GaN is not purely Lorentzian. The agreement with the experimental data can be improved using adjustable broadening modification.

Self-interstitial Diffusion in α-Zirconium-Zr is studied using Molecular Dynamic (MD) and molecular static (MS) simulation using Ackland's many-body inter-atomic potential. The basal crowdion configuration is found to be the ground state. The diffusion process in Zr is complex. Four types of diffusion jumps can be identified, two in-plane and two out-of plane. The in-plane migration mechanism is dominated by one-dimensional crowdion motion along the [1120] directions, interrupted by occasional out-of-plane and on-line or off-line jumps. The mean lifetime before rotation of the crowdion is reported as a function of temperature. The activation energies for the diffusion processes are obtained. The diffusional anisotropy factor Dc/Da is also obtained, and compares well with experiment results.

Atomistic simulations realized on an f.c.c. crystal containing atomic size surface defects (step and groove) show that the defects are privileged sites for dislocation nucleation. Before nucleation, an elastic shear, precursor of the dislocation, appears in the plane in zone with the step where the dislocation will be nucleated. In order to explain the strong localization of the localized elastic precursor shear, we have analyzed the stress concentration near the surface defects using the continuum point force approach. For the step case, the origin of the localized shear is related to an increase in the interplanar separation due to the stress concentration.

The current experimental and theoretical knowledge of new polaronic-type excitons in ferroelectric oxides – charge transfer vibronic excitons (CTVE) – is discussed. It is shown that Hartree-Fock-type INDO calculations as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE-concept. Single CTVE as well as a new phase of strongly correlated CTVEs are analysed.

We present the results of phase field simulations of dendritic growth into pure undercooled melts, at growth velocities up to 35 m s−1. We find that, at low growth velocities, dendrite morphologies are broadly self-similar with increasing growth velocity. However, above ≈ 15 m s−1 the initiation of side-branching moves closer to the dendrite tip with increasing growth velocity. This appears to be related to the level of kinetic undercooling at the tip. Once side- branch initiation begins to occur within 1-2 radii of the tip, profound morphological changes occur, leading to severe thinning of the dendrite trunk and ultimately repeated multiple tip- splitting. This process can be invoked to explain many of the observed features of spontaneous grain refinement in deeply undercooled metallic melts.

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.

We examine the relative stability and adhesion of nonstoichiometric (polar) Al/WC interfaces and WC(0001) surfaces using Density Functional Theory as implemented in a planewave, pseudopo- tential formalism. Relaxed atomic geometries and the ideal work of adhesion were calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. Based on the surface and interfacial free energies, we find that both the clean surface and the optimal interface geometry are W-terminated. However, the largest adhesion energies are obtained with the C-termination, consistent with an argument based on surface reactivity.

The kinetics of grain growth in multicrystalline materials is determined by the interplay of curvature driven grain boundary motion and interfacial stress balance at the vertices of the grain boundaries. A comprehensive way to treat both effects in one model is given by the time dependent Ginzburg Landau model or phase field model. The paper presents the application of a multi phase field model, recently developed for solidification processes to grain growth of a multicrystalline structure. The specific feature of this multi phase field model is its ability to treat each grain boundary with its individual characteristics dependent on the type of the grain boundary, its orientation or the local pinning at precipitates. The pinning effect is simulated on the nanometer scale resolving the interaction of an individual precipitate with a curved grain boundary. From these simulations an effective pinning force is deduced and a model of driving force dependent grain boundary mobility is formulated accounting for the pinning effect on the mesoscopic scale of the grain growth simulation. 2-D grain growth simulations are presented.

We study the optical properties of the nonpolar (110) surface of cubic Boron Nitride calculated within the first-pinciple DFT-LDA scheme. The reflectance anisotropy (RA) spectrum is analyzed in relation to the better known spectrum of the GaAs(110) surface. The influence of the ionicity on the surface relaxation, the surface states character and the surface optical spectra, is studied. Comparisons with existing data for the surface under study and results to the GaN(110) surface are also given.

Palladium clusters have been chosen to represent a typical supported heterogeneous catalyst and their interaction with hydrocarbons has been investigated theoretically. The calculations were performed through density functional theory and the Becke-Lee-Yang-Parr hybrid (B3LYP) functional was adopted to calculate exchange and correlation energy. An effective core potential basis set (ECP on core electrons and Dunning/Huzinaga on outer electrons) was found sufficiently accurate to reproduce experimental data. Clusters containing up to seven Pd atoms were considered and their interaction with hydrogen, methane and ethane and their fragments was analyzed and a kinetic study of the system was performed. Transition states structures and energies were calculated through quantum mechanics and kinetic constants were derived from a statistic thermodynamic approach. On the basis of such information, a kinetic model that accounts for ethane transformations. Finally the kinetic scheme was embedded in a plug flow reactor model and simulations were performed to test the validity of the developed mechanism. In this way information obtained at the atomic scale were adopted to study phenomena occurring on the much higher reactor scale.

We present a method to calculate impact ionization and Auger recombination rates within density functional theory and a screened-exchange approach and implement it in the all- electron FLAPW method. We investigate the dependence of the overlap matrix elements as a function of the states involved along the main symmetry lines of the Brillouin zone. Our results for the final impact ionization rates along the main symmetry lines of the Brillouin zone. Our results for the final impact ionization rates along τ — X and τ —L directions for GaAs show a strong anisotropy imposed by energy and momentum conservation and related to the use of a realistic and accurate sX-LDA band structure.

We present a theoretical model in order to describe both thermal and electronic in-plane transports in quantum dot superlattice. The model takes into account the modifications of electron and phonon transport due to the space confinement caused by the mismatch in electronic and thermal properties between dot and host materials. The developed model provides the analysis of the in-plane superlattice electronic and thermal properties versus quantum dot size and their arrangement. Numerical calculations were carried out for a structure that consists of multiple layers of Si with regimented germanium quantum dots. The simulation results of the lattice thermal conductivity are in a good agreement with experimental data.

The activation-relaxation technique (ART) is a method for finding saddle points in high-dimen- sional energy landscapes. ART has already been applied to a wide range of materials including amorphous semiconductors, Lennard-Jones glasses, and proteins. In spite of its successes, a number of fundamental questions remain to be answered regarding the biases associated with its sampling of the saddle points. We present here results of a detailed analysis of the biases in the simulation of amorphous silicon. We focus in particular on the biases of the method in sampling saddle points, the completeness of the sampling and the sensitivity of these quantities to variations of the different parameters.

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.

We have carried out long time scale simulations where the “dimer method” [G. Henkelman and H. Jónsson, J. Chem. Phys. 111, 7010 (1999)] is used to find the mechanism and estimate the rate of transitions within harmonic transition state theory and time is evolved by using the kinetic Monte Carlo method. Unlike traditional applications of kinetic Monte Carlo, the atoms are not assigned to lattice sites and a list of all possible transitions does not need to be specified beforehand. Rather, the relevant transitions are found on the y during the simulation. An application to the diffusion and island formation of Al adatoms on an Al(100) surface is presented.

The simulation of defect dynamics (e.g., transient enhanced diffusion of boron in the presence of silicon interstitials) is a technologically relevant challenge for computational materials science. The dynamics of defect structures in bulk unfolds as a sequence of thermally induced structural transitions. Identifying and characterizing reaction paths, as well as extracting dynamical quantities (e.g., diffusion constants) is important for modeling the macroscopic properties of real materials. Applying real-time multiresolution analysis (RTMRA) to various dynamical quantities using simple Haar wavelets, we have developed a computationally cheap data compression scheme to handle the massive data sets generated in molecular dynamics (MD) simulations; data storage has been reduced hundredfold with no loss of relevant information. More importantly, the same RTMRA techniques are developed into a sophisticated event detection scheme capable of solving three major challenges to multiscale MD simulations, specifically, (1) identifying meta-stable structures against the background of thermal vibrations, (2) detecting infrequent events, e.g., structural transitions, in the presence of thermal noise, and (3) accurately identifying transition times to further enhance recently emerging MD acceleration techniques.

Models of semi-solid processing currently utilise constitutive equations to describe slurry rheology. However, using the techniques of molecular dynamics it may be possible to go beyond this approach to obtain direct relationships between applied shear rates, the resultant microstructural changes and rheology.

The results of a simple molecular dynamics simulation of semi-solid flow within a Couette rheometer are presented and compared with experimental data for the system Sn -15% Pb. We find that the model correctly predicts the apparent viscosity under steady state conditions, up to solid fractions of ≈ 40%.