Showing papers in "Journal of Computer-aided Materials Design in 2007"

An overview of spatial microscopic and accelerated kinetic Monte Carlo methods

Abstract: The microscopic spatial kinetic Monte Carlo (KMC) method has been employed extensively in materials modeling. In this review paper, we focus on different traditional and multiscale KMC algorithms, challenges associated with their implementation, and methods developed to overcome these challenges. In the first part of the paper, we compare the implementation and computational cost of the null-event and rejection-free microscopic KMC algorithms. A firmer and more general foundation of the null-event KMC algorithm is presented. Statistical equivalence between the null-event and rejection-free KMC algorithms is also demonstrated. Implementation and efficiency of various search and update algorithms, which are at the heart of all spatial KMC simulations, are outlined and compared via numerical examples. In the second half of the paper, we review various spatial and temporal multiscale KMC methods, namely, the coarse-grained Monte Carlo (CGMC), the stochastic singular perturbation approximation, and the τ-leap methods, introduced recently to overcome the disparity of length and time scales and the one-at-a time execution of events. The concepts of the CGMC and the τ-leap methods, stochastic closures, multigrid methods, error associated with coarse-graining, a posteriori error estimates for generating spatially adaptive coarse-grained lattices, and computational speed-up upon coarse-graining are illustrated through simple examples from crystal growth, defect dynamics, adsorption–desorption, surface diffusion, and phase transitions.

Core structure and Peierls potential of screw dislocations in α -Fe from first principles: cluster versus dipole approaches

Abstract: A comparison between the dipole and cluster approaches for the study of the core structure and Peierls potential of 1/2 \(\langle 111\rangle\) screw dislocations in bcc metals is presented. It is based on first principles electronic structure calculations in alpha iron carried out within the DFT framework using localized basis functions as implemented in the SIESTA code. The effect of the energetic model is first investigated on the {211} and {110} generalized stacking fault energy (γ) surfaces which are known to be closely related to the dislocation core properties. All DFT results yield similar shapes—characteristic of a non-degenerate core structure—and the effect of the exchange-correlation functional is shown to be larger than the discrepancies between SIESTA and planewave-pseudopotential results. The core structure is found to be non-degenerate, with an excellent agreement between the various approaches on the deviation from the linear elasticity theory of the atomic positions. In the dipole approach, the interaction between dislocations is dominated by elastic effects, but significant anisotropic core–core interactions are evidenced, which strongly affect the energetics of the system when a triangular array of dipoles is used. For the calculation of the Peierls potential a very good agreement is obtained between the cluster approach and the dipole approach, provided that a quadrupolar-like arrangement is used. Similar calculations are performed with the EAM potential proposed by Mendelev et al. [Philos. Mag. 83, 3977 (2003)] for iron; the comparison between the two sets of results is briefly discussed.

The Dislocation Bias

Abstract: Bias factors for dislocations quantify the preferential diffusion of self-interstitials relative to the diffusion of vacancies to dislocations. These parameters are essential for rate theory computer codes that model nucleation and growth of voids, helium bubbles, dislocation loops, and the evolution of microstructures in materials subject to high dose irradiations. Compact formulae for these factors are derived and the accuracy of several approximations is explored.

Phase-field model during static recrystallization based on crystal-plasticity theory

Abstract: A numerical model and computational procedure for static recrystallization are developed using a phase-field method coupled with crystal-plasticity theory. In this model, first, the microstructure and dislocation density during the deformation process of a polycrystalline metal are simulated using a finite element method based on strain-gradient crystal-plasticity theory. Second, the calculated data are mapped onto the regular grids used in the phase-field simulation. The stored energy is calculated from the dislocation density and is smoothed to avoid computational difficulty. Furthermore, the misorientation required for nucleation criteria is calculated at all grid points. Finally, phase-field simulation of the nucleation and growth of recrystallization is performed using the mapped data. By performing a series of numerical simulations based on the proposed numerical procedure, it has been confirmed that the recrystallization microstructure can be reproduced from the deformation microstructure.

Computer-aided design of transformation toughened blast resistant naval hull steels: Part I

Abstract: A systematic approach to computer-aided materials design has formulated a new class of ultratough, weldable secondary hardened plate steels combining new levels of strength and toughness while meeting processability requirements. A theoretical design concept integrated the mechanism of precipitated nickel-stabilized dispersed austenite for transformation toughening in an alloy strengthened by combined precipitation of M2C carbides and BCC copper both at an optimal ∼3 nm particle size for efficient strengthening. This concept was adapted to plate steel design by employing a mixed bainitic/martensitic matrix microstructure produced by air-cooling after solution-treatment and constraining the composition to low carbon content for weldability. With optimized levels of copper and M2C carbide formers based on a quantitative strength model, a required alloy nickel content of 6.5 wt% was predicted for optimal austenite stability for transformation toughening at the desired strength level of 160 ksi (1,100 MPa) yield strength. A relatively high Cu level of 3.65 wt% was employed to allow a carbon limit of 0.05 wt% for good weldability, without causing excessive solidification microsegregation.

Prototype evaluation of transformation toughened blast resistant naval hull steels: Part II

Abstract: Application of a systems approach to computational materials design led to the theoretical design of a transformation toughened ultratough high-strength plate steel for blast-resistant naval hull applications. A first prototype alloy has achieved property goals motivated by projected naval hull applications requiring extreme fracture toughness (C v > 85 ft-lbs or 115 J corresponding to K Id≥ 200 ksi.in1/2 or 220 MPa.m1/2) at strength levels of 150–180 ksi (1,030–1,240 MPa) yield strength in weldable, formable plate steels. A continuous casting process was simulated by slab casting the prototype alloy as a 1.75′′ (4.45 cm) plate. Consistent with predictions, compositional banding in the plate was limited to an amplitude of 6–7.5 wt% Ni and 3.5–5 wt% Cu. Examination of the oxide scale showed no evidence of hot shortness in the alloy during hot working. Isothermal transformation kinetics measurements demonstrated achievement of 50% bainite in 4 min at 360 °C. Hardness and tensile tests confirmed predicted precipitation strengthening behavior in quench and tempered material. Multi-step tempering conditions were employed to achieve the optimal austenite stability resulting in significant increase of impact toughness to 130 ft-lb (176 J) at a strength level of 160 ksi (1,100 MPa). Comparison with the baseline toughness–strength combination determined by isochronal tempering studies indicates a transformation toughening increment of 65% in Charpy energy. Predicted Cu particle number densities and the heterogeneous nucleation of optimal stability high Ni 5 nm austenite on nanometer-scale copper precipitates in the multi-step tempered samples was confirmed using three-dimensional atom probe microanalysis. Charpy impact tests and fractography demonstrate ductile fracture with C v > 80 ft-lbs (108 J) down to −40 °C, with a substantial toughness peak at 25 °C consistent with designed transformation toughening behavior. The properties demonstrated in this first prototype represent a substantial advance over existing naval hull steels. Achieving these improvements in a single design and prototyping iteration is a significant advance in computational materials design capability.

A multilattice quasicontinuum for phase transforming materials: Cascading Cauchy Born kinematics

Abstract: The quasicontinuum (QC) method is applied to materials possessing a multilattice crystal structure. Cauchy-Born (CB) kinematics, which accounts for the shifts of the crystal basis, is used in continuum regions to relate atomic motions to continuum deformation gradients. To avoid failures of the CB kinematics, QC is augmented with a phonon stability analysis that detects lattice period extensions and identifies the minimum required periodic cell size. This augmented approach is referred to as Cascading Cauchy-Born kinematics. The method is analyzed for both first- and second-order phase transformations, and demonstrated numerically on a one-dimensional test problem.

Differential formation of allophane and imogolite: experimental and molecular orbital study

Abstract: Allophane and imogolite are naturally occurring aluminum silicate soil constituents with nano-ball and nano-tube morphology. Wall of the both materials is composed of Al(OH)3 sheet with orthosilicic acid attached to it. Synthesis of allophane and imogolite can be controlled by addition of alkali and alkaline-earth metal ions. The main reaction product without or with small amounts addition of the metal ions is imogolite, while allophane forms when the metal ions were much added. The effect of metal ions on facilitating allophane formation and inhibition of imogolite formation were greater in the following order of Na, K < Ca, Mg. These metal ions affect the degree of dissociation of Si–OH group of orthosilicic acid, which may causes differential formation of allophane and imogolite. Structure optimization of the proto-imogolite model, precursor of allophane and imogolite, showed that when the Si–OH was undissociated, the shape of proto-imogolite model was transformed to asymmetrical in molecular configuration. This caused curling of the proto-imogolite model, which lead to formation of imogolite tube. On the other hand, when the Si–OH was dissociated, the shape of the proto-imogolite model was transformed to symmetrical configuration. This model curved to make a hollow sphere with placing the orthosilicic acid inside the sphere (allophane). Both of the experimental and molecular orbital calculation results proved that the dissociation of the Si–OH has an important role during the differential formation of allophane and imogolite.

On the formation of cementitious C–S–H nanoparticles

Abstract: This work explores from a theoretical viewpoint the underlying growth mechanisms which govern the formation of the most important hydration product present in cementitious environments, the so called C–S–H (calcium–silicate–hydrate) gel. Aiming at identifying the basic building blocks which make up the cementitious C–S–H gel, we have performed ab-initio calculations at the Hartree Fock (HF) level. Two different growth mechanisms have been identified depending on the amount of Si and Ca ions, which naturally lead to the appearance of tobermorite-like and jennite-like nano-crystals.

Molecular dynamics modeling of cavity strengthening in irradiated iron

Abstract: One of the most important problems in the field of nuclear industry is the relationship between irradiation-induced damage and the resulting induced mechanical response of the target metal and in particular ferritic base steels. In this work molecular dynamics simulation is used to simulate the nanoscale interaction between a moving dislocation and a defect, such as a cavity, as void or He bubble. The stress–strain curves are obtained under imposed strain rate condition using the atomic potentials based on the Fe potential of Ackland et al. 1997 for a void and He bubble as a function of He content and temperature. It appears that a 2 nm void is a stronger obstacle than a He bubble at low He contents, whereas at high He contents, the He bubble becomes a stronger obstacle. With increasing temperature the escape stress decreases and at the same time there is increasing degeneracy in the type of interaction.

Collision cascades in pure δ -plutonium

Abstract: Molecular dynamics simulations of the formation and annealing of large collision cascades in delta-phase plutonium are presented. The defect evolution is followed with time up to 2 ns. Simulations are performed with the MEAM potential at three different temperatures; at 600 K where the pure delta phase is thermodynamically stable; at 300 K where the delta phase can only be maintained in a metastable state with minor additions of gallium or aluminum; and at 180 K where plutonium should transform to the alpha phase. It is found in all three cases that the atomic structure within the cascade evolves through a glass-like state. At 600 K, this structure recovers very slowly; at 300 K it persist up to 2 ns with no discernable trend to recover eventually; and at 180 K the amorphous structure initiated by the collision cascade spreads through the entire crystal and converts it to a glass-like structure.

Mechanism of heat transport in nanofluids

Abstract: We have calculated thermal conductivity of alumina nanofluids (with water and ethylene glycol as base fluids) using temperature as well as concentration-dependent viscosity, η. The temperature profile of η is obtained using Gaussian fit to the available experimental data. In the model, the interfacial resistance effects are incorporated through a phenomenological parameter α. The micro-convection of the alumina nanoparticle (diameter less than 100 nm) is included through Reynolds and Prandtl numbers. The model is further improved by explicitly incorporating the thermal conductivity of the nanolayer surrounding the nanoparticles. Using this improved model, thermal conductivity of copper nanofluid is calculated. These calculations capture the particle concentration-dependent thermal conductivity and predict the dependence of the thermal conductivity on the size of the nanoparticle. These studies are significant to understand the underlying processes of heat transport in nanofluids and are crucial to design superior coolants of next generation.

Magnetic origin of nano-clustering and point defect interaction in Fe–Cr alloys: an ab-initio study

Abstract: To understand the behaviour of point defects generated in irradiated FeCr ferritic/martensitic steels and to identify the kinetic pathways of micro-structural evolution of binary model Fe–Cr alloys, we use a combination of density functional theory (DFT) with statistical approaches involving cluster expansion and Monte Carlo simulations. This makes it possible to generate in a systematic way the low-energy configurations required for the subsequent DFT study of intrinsic defects (vacancies, interstitials) and impurity-defect interactions over the entire range of Fe–Cr alloy compositions. In the limit of low Cr concentration, DFT calculations predict that an intermetallic compound Fe15Cr has the lowest negative heat of formation. At higher Cr concentrations, simulations performed using a 4 × 4 × 4 super-cell show that magnetism is responsible for the nano-segregation of the ferromagnetic Fe15Cr and anti-ferromagnetic (α′-Cr) phases giving rise to the formation of clusters characterised by a very low positive heat of formation. We perform a systematic investigation of formation energies of point defects and their energies of interaction with Cr atoms. Further investigation of interaction of interstitial and vacancy defects with impurities (V, Nb, Ta, Mo, W, Al, Si, P, S) also shows a complex picture of interplay between magnetism and short-range ordering that affects the interaction between defects and impurities in the presence of chromium in Fe-rich alloys.

Void coalescence processes quantified through atomistic and multiscale simulation

Abstract: Simulation of ductile fracture at the atomic scale reveals many aspects of the fracture process including specific mechanisms associated with void nucleation and growth as a precursor to fracture and the plastic deformation of the material surrounding the voids and cracks. Recently we have studied void coalescence in ductile metals using large-scale atomistic and continuum simulations. Here we review that work and present some related investigations. The atomistic simulations involve three-dimensional strain-controlled multi-million atom molecular dynamics simulations of copper. The correlated growth of two voids during the coalescence process leading to fracture is investigated, both in terms of its onset and the ensuing dynamical interactions. Void interactions are quantified through the rate of reduction of the distance between the voids, through the correlated directional growth of the voids, and through correlated shape evolution of the voids. The critical inter-void ligament distance marking the onset of coalescence is shown to be approximately one void radius based on the quantification measurements used, independent of the initial separation distance between the voids and the strain-rate of the expansion of the system. No pronounced shear flow is found in the coalescence process. We also discuss a technique for optimizing the calculation of fine-scale information on the fly for use in a coarse-scale simulation, and discuss the specific case of a fine-scale model that calculates void growth explicitly feeding into a coarse-scale mechanics model to study damage localization.

Forbidden pitches in sub-wavelength lithography and their implications on design

Abstract: Moore’s Law has been the most important benchmark for microelectronics development over the past four decades. It has been interpreted to mean that critical dimensions (CD) of a design must shrink geometrically over time. The chip-level integration of devices has been possible through concurrent improvement in lithographic resolution. The lithographic resolution was primarily improved by moving deeper into ultraviolet spectrum of light. However, the wavelength of the optical source used for lithography has not improved for nearly a decade. This has lead to the development of sub-wavelength lithography. The diffraction effects of sub-wavelength lithography were offset by optical proximity correction (OPC), phase shift masking (PSM) and impending move to immersion lithography. Unfortunately, one time benefits from each of these resolution enhancement techniques (RET) have nearly exhausted. In this paper, we explore one important diffraction aspect of sub-wavelength lithography viz. the forbidden pitch phenomenon and its implication on future designs. We studied Forbidden pitches in context of 65 and 45 nm technologies using aerial imaging simulation. Aerial imaging simulation is computationally expensive and is not possible to perform on entire layout structures. Based on results from our simulations on selected patterns, we observe that in absence of any other resolution enhancement technique, many of the current layout patterns will be disallowed in 45 nm technology. Such restrictions significantly mitigate the benefit of migration to 45 nm technology in terms of area, power and performance of a design. We further show that even structured gate array based designs are not immune to this problem.

Static and dynamic properties of the water/amorphous silica interface: a model for the undissociated surface

Abstract: Amorphous silica–water interfaces are found ubiquitously in nanoscale devices, including devices fabricated from silica as well as from silicon that acquire a surface oxide layer. The surface silanol groups serve as hydrogen-bonding sites for a variety of chemical species, and their reactivity enables convenient chemical modification, making silica surfaces strategic in bio-sensing applications. We have extended the popular BKS and SPC/E models for bulk silica and water to describe the hydrated, hydroxylated amorphous silica surface. The parameters of our model were determined using ab initio quantum chemical studies on small fragments. Our model will be useful in empirical potential studies, and as a starting point for ab initio molecular dynamics calculations. At this stage, we present a model for the undissociated surface. Our calculated value for the heat of immersion, 0.6Jm−2, falls within the range of reported experimental values of 0.2–0.8Jm−2. We also study the perturbation of water properties near the silica–water interface. The disordered surface is characterized by regions that are hydrophilic and hydrophobic, depending on the statistical variations in silanol group density. We report non-equilibrium molecular dynamics simulations of Poiseuille flow of water near an amorphous silica surface.

Dislocation pair correlations from dislocation dynamics simulations

Abstract: The concept of stochastic fiber process is used in conjunction with the method of dislocation dynamics simulation to compute the pair correlations in three-dimensional dislocation systems. The results show that the pair correlations exhibit oscillatory behavior as a function of the radial distance and that the correlations are highly anisotropic in the crystal and dislocation line orientation spaces. The correlation oscillations are found to have smaller magnitude at higher strain levels and to decay as a function of distance to unity or to asymptotic values slightly below unity in some cases, which indicates that dislocations can be uncorrelated or anti-correlated at long range. The results suggest that incorporating the dislocation pair correlations in density-based kinetic dislocation models is not an easy task because of the need to represent the correlations as functions of three spatial coordinates and two line-orientation coordinates.

Engineering design of a cardiac myocyte

Abstract: We describe a design algorithm to build a cardiac myocyte with specific spatial dimensions and physiological function. Using a computational model of a cardiac muscle cell, we modeled calcium (Ca 2+ ) wave dynamics in a cardiac myocyte withcontrolledspatialdimensions.Themodeledmyocytewasreplicatedinvitrowhen primaryneonateratventricularmyocyteswereculturedonmicropatternedsubstrates. The myocytes remodel to conform to the two dimensional boundary conditions and assume the shape of the printed extracellular matrix island. Mechanical perturbation of the myocyte with an atomic force microscope results in calcium-induced calcium release from intracellular stores and the propagation of a Ca 2+ wave, as indicated by high speed video microscopy using fluorescent indicators of intracellular Ca 2+ .A nal- ysis and comparison of the measured wavefront dynamics with those simulated in the computer model reveal that the engineered myocyte behaves as predicted by the model. These results are important because they represent the use of computer mod- eling, computer-aided design, and physiological experiments to design and validate the performance of engineered cells. The ability to successfully engineer biological cells and tissues for assays or therapeutic implants will require design algorithms and tools for quality and regulatory assurance.

Interatomic potentials for materials with interacting electrons

Abstract: Evidence for the significant part played by magnetism in the picture of interatomic interactions in iron and iron-based alloys has recently emerged from density functional studies of the structure of radiation induced defects. In this paper we examine the range of validity of the currently available model interatomic potentials for magnetic iron, investigate the effect of electron–electron interaction on the strength of chemical bonding between atoms, follow the link between the multi-band Hubbard and the Stoner models, and review the concepts underlying the recent development of a semi-empirical magnetic interatomic potential.

A systems-based approach for integrated design of materials, products and design process chains

Abstract: The concurrent design of materials and products provides designers with flexibility to achieve design objectives that were not previously accessible. However, the improved flexibility comes at a cost of increased complexity of the design process chains and the materials simulation models used for executing the design chains. Efforts to reduce the complexity generally result in increased uncertainty. We contend that a systems based approach is essential for managing both the complexity and the uncertainty in design process chains and simulation models in concurrent material and product design. Our approach is based on simplifying the design process chains systematically such that the resulting uncertainty does not significantly affect the overall system performance. Similarly, instead of striving for accurate models for multiscale systems (that are inherently complex), we rely on making design decisions that are robust to uncertainties in the models. Accordingly, we pursue hierarchical modeling in the context of design of multiscale systems. In this paper our focus is on design process chains. We present a systems based approach, premised on the assumption that complex systems can be designed efficiently by managing the complexity of design process chains. The approach relies on (a) the use of reusable interaction patterns to model design process chains, and (b) consideration of design process decisions using value-of-information based metrics. The approach is illustrated using a Multifunctional Energetic Structural Material (MESM) design example. Energetic materials store considerable energy which can be released through shock-induced detonation; conventionally, they are not engineered for strength properties. The design objectives for the MESM in this paper include both sufficient strength and energy release characteristics. The design is carried out by using models at different length and time scales that simulate different aspects of the system. Finally, by applying the method to the MESM design problem, we show that the integrated design of materials and products can be carried out more efficiently by explicitly accounting for design process decisions with the hierarchy of models.

Multiscale bifurcation and stability of multilattices

Abstract: A lattice-level model is developed for active materials, such as shape memory alloys, that undergo martensitic phase transformations. The model is investigated using equilibrium path following and bifurcation techniques. It is shown that a multiscale stability criterion is essential for correctly interpreting the stability of crystal equilibrium configurations under both thermal- and stress-loading conditions. A two-stage temperature-induced phase transformation is predicted from a cubic B2 phase to an orthorhombic Cmmm phase to a final orthorhombic B19 phase. Under stress-loading conditions, martensitic transformations from the B2 austenite phase to a number of possible martensite phases are identified. These include reconstructive transformations to B11, B33, and C2/m structures and proper transformation to a C2/m monoclinic phase which displays characteristic tension-compression asymmetry. The prediction of both temperature-induced and stress-induced proper martensitic transformations indicates the likelihood that the current model will exhibit shape memory behavior.

Atomistic simulations of Ga atom ordering in Pu 5 at. % Ga alloys

Abstract: We employ molecular dynamics and Monte Carlo (MC) methods to simulate the rearrangement of Ga atoms from a randomly distributed Pu-Ga alloy and study the resultant effect on thermodynamic properties. The results show that all of the first neighbor Ga-Ga bonds are removed at all temperatures considered (200, 400 and 600 K) while the number of 2NN and 3NN bonds increase, and the number of 4NN bonds decreases. These results imply that Ga atoms develop strong short range ordering in the solid solution. The ordering causes an enthalpy decrease about ~3–4 meV/atom for different temperatures in the 5 at. % Ga alloy. This energy change is clearly important in the calculation of the Pu-Ga phase diagram. In addition, MC calculations at 200 K show pronounced Ga segregation.

A framework for automated 3D microstructure analysis & representation

Abstract: Over the past 5 years there have been significant advances in developing serial-sectioningmethods that provide quantitative data describing the structure and crystallography of grain-level microstructures in three dimensions (3D). The subsequent analysis and representation of this information can provide modeling and simulation efforts with a highly-refined and unbiased characterization of specific microstructural features. For example, the grain structure and crystallography of an engineering alloy could be characterized and then translated directly into a 3D volume mesh for subsequent Finite Element Analysis. However, this approach requires a multitude of data sets in order to appropriately sample the intrinsic heterogeneity observed in typical microstructures. One way to circumvent this issue is to develop computation tools that create synthetic microstructures that are statistically-equivalent to the measured structure. This study will discuss the development of software programs that take as input a series of Electron Backscatter Diffraction Patterns from a serial sectioning experiment, and output a robust statistical analysis of the 3D data, as well as generate a host of synthetic structures which are analogous to the real microstructure. Importantly, the objective of this study is to provide a framework towards complete microstructure representation that is consistent with quantifiable experimental data.

Cascade damage evolution: rate theory versus kinetic Monte Carlo simulations

Abstract: The thermal evolution of defects produced by ion irradiation is studied by kinetic Monte Carlo and Rate Theory approaches. An isochronal annealing is simulated to evidence the different thermally activated mechanisms that govern defect evolution. KMC simulations show that in the case of ion irradiation, additional recovery peaks should be expected, in comparison to electron irradiation conditions. A comparison between kMC and RT results indicates that some of these peaks are due to spatially – correlated recombinations that occur at low temperature. Therefore kMC and RT approaches differ at low temperature. However, at higher temperature results obtained by both models are in near perfect agreement. In addition, we studied the influence of vacancy cluster mobility on the evolution of damage. KMC simulations show that the mobility of V2, V3 and V4 clusters does not significantly affect the evolution of defects and can be neglected in these conditions.

An experimentally justified confining potential for electrons in two-dimensional semiconductor quantum dots

Abstract: We propose a confinement potential for electrons in a two-dimensional (2D) quantum dot that is more physically motivated and better experimentally justified than the commonly used infinite range parabolic potential or few other choices. Because of the specific experimental setup in a 2D quantum dot involving application of gate potentials, an area of electron depletion is created near the gate. The resulting positively charged region can be most simply modeled as a uniformly charged 2D disk of positive background charge. Within this experimental setup, the individual electrons in the dot feel a confinement potential originating from the uniformly positively charged 2D background disk. Differently from the infinitely high parabolic confinement potential, the resulting 2D charged disk potential has a finite depth. The resulting 2D charged disk potential has a form that can be reasonably approximated as a parabolic potential in the central region of the dot (for low energy states of the electrons) and as a Coulomb potential (that becomes zero at large distances). We study the electronic properties of the 2D charged disk confinement potential by means of the numerical diagonalization method and compare the results to the case of 2D quantum dots with a pure parabolic confinement potential.

Detailed parametric study of Casimir forces in the Casimir Polder approximation for nontrivial 3D geometries

Abstract: We present a parametric numerical study conducted with the finite element code CasimirSim developed by ARC Seibersdorf research. This simulation has already been applied to two dimensional geometries in the past and showed agreement with exact theoretical predictions between 100 nm and 10 μm. In the current investigation the code has been enhanced to compute arbitrary, nontrivial, fully three dimensional geometries for any material given by its density and dielectric polarizability. For calculation of the Casimir energy the simple Casimir Polder r −7 model is used. This approach is known to be of limited accuracy due to the assumption of perfect additivity of dipole interactions. Nonetheless, it can be used to give approximate predictions for sharply curved geometries inaccessible to other approximative schemes such as for example the established Proximity Force Approximation. In the current study, we show in detail the dependence of errors upon physical and numerical parameters. After verification with the plate–plate geometry experimentally relevant geometries such as sphere over plate or crossed cylinders are assessed. Finally, the simulation is applied to the more sophisticated geometries of stacked spherical shells, a gear wheel, and a cantilever, showing up some interesting properties.

Evaluation of interactions between functionalised multi-walled carbon nanotubes and ligand-stabilised gold nanoparticles using surface element integration

Abstract: We report here on the application of Surface Element Integration (SEI) to evaluate the potential energy of interactions between alkane-modified multiwalled carbon nanotubes (MWCNTs) and tetraoctylammonium bromide (TOAB) stabilised gold nanoparticles. The interacting objects are treated as cylinders and spheres, respectively, with corresponding alkyl chains extending perpendicularly from their surfaces. In such case the widely used Derjaguin approximation is invalid. Thus SEI was used to calculate the van der Waals, osmotic and elastic interactions. The results show that it is possible to control the self-assembly process of the gold nanoparticles at the surface of modified MWCNT in terms of size- and type-selectivity.

Effect of simulation conditions on friction in polytetrafluoroethylene (PTFE)

Abstract: We report the results of molecular-dynamics simulations of friction at polytetrafluoroethylene (PTFE) interfaces and show that the calculated tribological properties are robust against significant changes in the sliding speed and the morphology of the polymer.

Self-Irradiation Cascade Simulations in Plutonium Metal: Model Behavior at High Energy

Abstract: The Modified Embedded Atom Method model for Pu metal is revised so that it more accurately captures the behavior of the Ziegler-Biersack-Littmark model of ion-ion interactions. Two revision are tested with somewhat different stiffnesses in the 2-1000 eV range. The revised models show higher damage levels at 20 KeV than an earlier model, suggesting that the behavior of the models above 100 eV is dominating damage production, at least in the earlier stages of the cascade.

Discrete size series of CdSe quantum dots: A combined computational and experimental investigation

Abstract: Ab initio computational studies were performed for CdSe nanocrystals (NCs) over a wide variety of sizes ranging from 8 to 150 atoms in conjunction with recent experimental work. The density functional based calculations indicate substantial relaxations. Changes in coordination of surface atoms were found to play a crucial role in determining the NC stability and optical properties. While optimally (threefold) coordinated surface atoms resulted in stable closed-shell structures with large optical gaps, sub-optimal coordination gave rise to lower stability and negligible optical gaps. These computations are in qualitative agreement with recent chemical etching experiments suggesting that closed shell NCs contribute strongly to photoluminescence quantum yield while clusters with nonoptimal surface coordination do not.