Showing papers by "Adrian P. Sutton published in 2000"

The bcc-to-9R martensitic transformation of Cu precipitates and the relaxation process of elastic strains in an Fe-Cu alloy

Abstract: High-resolution electron microscopy experiments have been performed to explore the bcc—9R transformation and the subsequent elastic relaxation of Cu precipitates in an Fe—Cu alloy aged at 550°C. It was found that both electron irradiation (at an electron energy of 400 kV) and thermal annealing caused rotation of the close-packed (009)9R planes in twinned 9R Cu precipitates. For 400 kV electron irradiation, such rotations were observed in precipitates smaller than about 12nm in diameter. For specimens cooled from the ageing temperature of 550°C to a given temperature up to −60°C, and then annealed at 400°C, the rotation of (009)9R planes was found to occur only in precipitates above a size which depended on the temperature to which the specimen had been cooled. This critical size ranged from about 9 nm for specimens cooled to 400°C, to 4 nm for specimens cooled to −60°C. It is argued that these critical sizes are indicative of the sizes at which coherent bcc precipitates transform martensitically ...

Hydrolysis of the amorphous silica surface. II. Calculation of activation barriers and mechanisms

Abstract: Using a previously derived model of the dry, amorphous, hydrophilic SiO2 surface, the reactivity of generic defect sites on the surface with respect to water, and the local network rearrangement that accompanies hydrolysis at these sites, is investigated using cluster models. Ab initio methods are used to calculate reaction barriers and reaction pathways. Consequences of the various types of hydrolysis product found are discussed with reference to potential sites for polymer chemisorption on the hydrolyzed, amorphous SiO2 surface.

Current-induced forces in atomic-scale conductors

Abstract: We present a self-consistent tight-binding formalism to calculate the forces on individual atoms due to the flow of electrical current in atomic-scale conductors. Simultaneously with the forces, the method yields the local current density and the local potential in the presence of current flow, allowing a direct comparison between these auantities. The method is applicable to structures of arbitrary atomie geometry and can be used to model current-induced mechanical effects in realistic nanoscale junctions and wires. The formalism is implemented within a simple ls tight-binding model and is applied to two model structurest atomie chains and a nanoscale wire containing a vacancy.

Electromigration of vacancies in copper

Abstract: The total current-induced force on atoms in a Cu wire containing a vacancy are calculated using the self-consistent one-electron density matrix in the presence of an electric current, without separation into electron-wind and direct forces. By integrating the total current-induced force, the change in vacancy migration energy due to the current is calculated. We use the change in migration energy with current to infer an effective electromigration driving force ${\mathbf{F}}_{e}.$ Finally, we calculate the proportionality constant ${\ensuremath{\rho}}^{*}$ between ${\mathbf{F}}_{e}$ and the current density in the wire.

A theoretical study of polyimide flexibility

Abstract: The flexibility of a model polyimide, pyromellitic-dianhydride 1,4-oxydianiline (PMDA-ODA) is investigated using fully ab initio methods. Hartree–Fock, second-order Moller–Plesset theory (MP2) and density-functional theory (DFT) methods are employed along with both numerical and Gaussian basis sets. A hybrid scheme which combines energies and first derivatives is also used and appraised for this system. Energies of monomer fragments of the polyimide as a function of torsional angle are calculated using geometry optimizations. Extensive comparisons are made with maleanil, a smaller fragment of PMDA-ODA.

Microscopy of Metal Oxide Surfaces

Abstract: Widespread application of metal oxides in catalysis, gas sensing, and as substrates for thin film growth has stimulated a strong interest in the atomic and electronic surface structure of these materials. The electronic structure of many metal oxide surfaces is characterised, and complicated from the theoretical modelling point of view, by the presence of strong on-site Coulomb repulsion (strong correlations) between valence electrons localised on the metal ions. Additionally, experimental studies have to deal with the difficulties associated with the electrically insulating nature of many of these oxides. We have overcome these problems through the development of novel experimental and theoretical techniques capable of providing structural and electronic information about strongly correlated insulating metal oxide surfaces.The surfaces of NiO and CoO were investigated through elevated temperature scanning tunnelling microscopy (STM) thereby overcoming problems of low electrical conductivity of the samples.We show atomically resolved elevated temperature STM images of (001) cobalt and nickel monoxide surfaces obtained under similar conditions which show an order of magnitude difference in the atomic corrugation heights.

Diamond CVD Growth Mechanisms and Reaction Rates From First-Principles

Abstract: CVD diamond is an enabling material for diverse applications. In recent years, multiscale modelling of CVD growth in conjunction with experimental studies of the deposition processes has made a substantial progress towards our understanding of the fundamental growth chemistry and material quality. Macroscopic gas phase simulations of the CVD reactor, the mesoscale kinetic Monte-Carlo (KMC) modelling of the crystal growth and nanoscale modelling of the surface chemistry are three main legs in the multiscale hierarchy. In the framework of this methodology we have performed first-principles quantum mechanical calculations of bonding and reaction kinetics of the elementary growth processes and provided critical input in the form of atomistic growth mechanisms and reaction rates for the mesoscale KMC modelling of CVD diamond growth. A key success was achieved by combining first-principles and Monte Carlo studies to elucidate (100) growth mechanisms that have perplexed the diamond growth community for many years.

Monte Carlo Simulation of Vapour Deposition of Nonstoichiometric Amorphous Silica

Abstract: Monte Carlo simulation of deposition of nonstoichiometric amorphous SiOx nanolayers from vapor phase onto a polymer surface is reported. The model developed is based on the network properties of silica and takes into account dangling bonds arising during the real process of deposition. The model is validated via comparison of the radial- and bond angle distribution functions for the simulated Si and SiO2 structures with those obtained from experiment for bulk materials. Porosity of the simulated amorphous layer is characterized by the relative volume of pores and the ratio of the pore surfaces to the pore volume. We found that porosity strongly depends on nucleation sites density (NSD) on the polymer substrate. At NSD lower than 1 nm−2 the porosity may reach as much as 30% of the layer volume, while at NSD higher than 4 nm−2 it decreases down to 3-7%. Possible implications of the obtained results are discussed.