Steven M. Valone

The isotope and temperature dependence of self-diffusion for hydrogen, deuterium, and tritium on Cu(100) in the 100–1000 K range

Abstract The quantum mechanical contributions to the diffusion of H, D, and T on the Cu(100) surface are investigated as a function of temperature. The results proceed from a transition state theory calculation of the barrier crossing rate using an anharmonic, temperature-dependent potential between the adatom and the static Cu surface. The temperature-dependent potential is derived from path integral considerations. At room temperature, the present calculations indicate a 0.6, 0.3, and 0.2 kcal/mol decrease in the effective activation energy for H, D, and T, respectively. However, at 100 K, the analogous decreases become 1.8, 1.0, and 0.7 kcal/mol. These values may be compared to the classical activation energy of 11.6 kcal/mol. Note that these corrections qualitatively conform to a 1/m mass dependence in the effective activation energy which is predicted by a harmonic model also presented below. One of the goals of this work is to provide some preliminary framework by which the results of electronic structure calculations may be adjusted for quantum mechanical contributions so as to facilitate comparison with experiment.0

Revisiting the Al/Al?O? interface: Coherent interfaces and misfit accommodation

We study the coherent and semi-coherent Al/α-Al2O3 interfaces using molecular dynamics simulations with a mixed, metallic-ionic atomistic model. For the coherent interfaces, both Al-terminated and O-terminated nonstoichiometric interfaces have been studied and their relative stability has been established. To understand the misfit accommodation at the semi-coherent interface, a 1-dimensional (1D) misfit dislocation model and a 2-dimensional (2D) dislocation network model have been studied. For the latter case, our analysis reveals an interface dislocation structure with a network of three sets of parallel dislocations, each with pure-edge character, giving rise to a pattern of coherent and stacking-fault-like regions at the interface. Structural relaxation at elevated temperatures leads to a further change of the dislocation pattern, which can be understood in terms of a competition between the stacking fault energy and the dislocation interaction energy at the interface. Our results are expected to serve as an input for the subsequent dislocation dynamics models to understand and predict the macroscopic mechanical behavior of Al/α-Al2O3 composite heterostructures.

The influence of substrate motion on the self‐diffusion of hydrogen and its isotopes on the copper (100) surface

Work is presented that examines the effect of substrate motion on the surface self‐diffusion of hydrogen and its isotopes on the Cu(100) surface. Lattice motion, represented as a sum of Lennard‐Jones interactions, is found to increase the diffusion constant of hydrogen and its isotopes at all temperatures examined. The increase varies from 1.3 to 4.0 over the temperature range from 1000 to 110 K. The results agree with the recent calculations of Lauderdale and Truhlar above 150 K. The quantum contribution to the isotope effect is enhanced relative to the values for the frozen substrate. These conclusions are based on approximate path integral calculations in which quantum‐mechanical effects are treated in a semiclassical manner using temperature‐dependent effective potentials. The differences between the present results and those of Lauderdale and Truhlar are attributed to a breakdown of these semiclassical approximations at low temperatures. In the temperature range considered, commonly accepted harmonic...

Possible behavior of a diamond (111) surface in methane/hydrogen systems

A combined numerical and experimental investigation into the behavior of diamond (111) surfaces in plasma CVD reactors is presented. Numerically, semiempirical molecular orbital methods are used as a model of diamond (111) surfaces represented by a 20-atom carbon cluster plus surface species. The abstraction of hydrogen atoms by gas-phase hydrogen atoms, the coverage dependence of the heat of formation for submonolayers of CH{sub 3} and C{sub 2}H groups coadsorbed with H, and the energy change for abstraction of H atoms from the surface by various radicals in the gas-phase are examined. No barrier to abstraction is found, steric effects in achieving clusters of CH{sub 3} groups are large, and C{sub 2}H and atomic oxygen are found to be the most energetically favored for removal of adsorbed H. Experimentally, relative concentrations of atomic H in the near-surface region as a function of added O{sub 2} mole fraction were measured. A weak dependence on O{sub 2} concentration is observed, but does not appear to be significant enough to account for observed changes in growth rate. This suggests that other radical species be investigated for their contribution to diamond film growth.

Atomistic modeling of thermodynamic equilibrium and polymorphism of iron

We develop two new modified embedded-atom method (MEAM) potentials for elemental iron, intended to reproduce the experimental phase stability with respect to both temperature and pressure. These simple interatomic potentials are fitted to a wide variety of material properties of bcc iron in close agreement with experiments. Numerous defect properties of bcc iron and bulk properties of the two close-packed structures calculated with these models are in reasonable agreement with the available first-principles calculations and experiments. Performance at finite temperatures of these models has also been examined using Monte Carlo simulations. We attempt to reproduce the experimental iron polymorphism at finite temperature by means of free energy computations, similar to the procedure previously pursued by Müller et al (2007 J. Phys.: Condens. Matter 19 326220), and re-examine the adequacy of the conclusion drawn in the study by addressing two critical aspects missing in their analysis: (i) the stability of the hcp structure relative to the bcc and fcc structures and (ii) the compatibility between the temperature and pressure dependences of the phase stability. Using two MEAM potentials, we are able to represent all of the observed structural phase transitions in iron. We discuss that the correct reproductions of the phase stability among three crystal structures of iron with respect to both temperature and pressure are incompatible with each other due to the lack of magnetic effects in this class of empirical interatomic potential models. The MEAM potentials developed in this study correctly predict, in the bcc structure, the self-interstitial in the (110) orientation to be the most stable configuration, and the screw dislocation to have a non-degenerate core structure, in contrast to many embedded-atom method potentials for bcc iron in the literature.

Energy dependence on fractional charge for strongly interacting subsystems.

The energies of a pair of strongly interacting subsystems with arbitrary noninteger charges are examined from closed- and open-system perspectives. An ensemble representation of the charge dependence is derived, valid at all interaction strengths. Transforming from resonance-state ionicity to ensemble charge dependence imposes physical constraints on the occupation numbers in the strong-interaction limit. For open systems, the chemical potential is evaluated using microscopic and thermodynamic models, leading to a novel correlation between ground-state charge and an electronic temperature.

Use of 2-D-adaptive mesh in simulation of combustion front phenomena

Abstract Solutions to models with different length scales may contain regions such as shocks, steep fronts and other near discontinuities. Adaptive meshing strategies, in which a spatial mesh network is adjusted dynamically so as to capture the local behavior accurately, will be described. The algorithm will be tested on an example of solid-solid combustion.

Theoretical studies of surface diffusion: A strategy for enhanced sampling

Recent studies have demonstrated the feasibility and utility of numerical molecular dynamics simulation methods in the study of surface diffusion. Such methods have yielded surface diffusion constants which are in quantitative agreement with those measured by field ion microscope techniques. This letter describes an attempt to reduce the rather large computational effort involved in these studies by devising improved numerical sampling techniques.

Emission of dislocations from grain boundaries by grain boundary dissociation

In this article, we examine the conditions that favour the emission of Shockley partial dislocations (SPDs) that standoff from a grain boundary (GB) plane by a few lattice parameters as part of the atomic structure of some GBs. To do so, we consider GBs to be formed by the operation of arrays of intrinsic grain boundary dislocations (GBDs) that create the tilt and twist misorientation, and the lattice mismatch between the two crystal grains adjoining the GB. The conditions to be considered that favour SPDs are the following: (1) Frank’s rule, (2) the proper sequential arrangement of partial dislocations to bound an intrinsic stacking fault and (3) the equilibrium stand-off distance (ESD). We apply an isotropic elasticity analysis to compute the ESD, in the absence of an applied stress, for SPDs emerging from asymmetric tilt GBs in two FCC metals, Cu and Al. The ESD is shown to be dependent on the glide plane orientation relative to the GB plane and on the position of the glide planes, relative to the position of the GBDs. An applied stress increases the ESD up to a critical stress that removes the SPDs without limit from the GB. We examine the effect of the stacking fault energy on the ESD and critical stress. The critical stress is effectively linearly dependent on the stacking fault energy. Finally, we present results of atomistic simulations of asymmetric tilt Σ11[1 0 1]{4 1 4}||{2 5 2} GBs in Cu bicrystal models subject to shock loading that behave in a manner similar to the elasticity predictions. The atomistic simulations reveal additional behaviour associated with elastic incompatibility between the two grains in the bicrystal models.

A coverage study of the self-diffusion and chemical diffusion constants for disordered overlayers of Xe on W(110)

Abstract The coverage dependence of the self-diffusion and chemical diffusion of submonolayer, disordered overlayers of Xe on W(110) at 150 K is found to be weak. The activation energies are all approximately 0.7 ± 0.2 kcal/mol for low to moderate ( θ = 0.2 to 0.6) coverages. The pre-exponential factors are on the order of 10 −3 to 10 −4 in the same range of coverages. In all cases, the chemical diffusion constant is found to be slightly larger than the self-diffusion constant. On the average, the velocities of different particles are positively correlated. The equal-time particle fluctuations are on the order of the average number of particles in a fixed probe area within the molecular dynamics cell. Consequently, for these disordered systems, we find the chemical diffusion constant to be comparable in magnitude to the self-diffusion constant.