Thesis (P.hD (Physics)) --University of Limpopo, 2013 / Titanium dioxide (TiO2) nanoparticles, nanowires, nanosheets and nanoporous are of great interest in many applications. This is due to inexpensive, safety and rate capability of the material. It has being considered as a replacement of graphite anode material in rechargeable lithium batteries. Much experimental work on pure and lithiated nanostructures of TiO2 has been reported, mostly with regards to their complex microstructures. In this work we employ molecular dynamics (MD) simulation to generate models of TiO2 nano-architectures including: nanosheet, nanoporous, nanosphere and bulk. We have successfully recrystallised all four nanostructures from amorphous precursors; calculated radial distribution functions (RDFs), were used to confirm crystallinity. Configuration energies, calculated as a function of time, were used to monitor the recrystallisation. Calculated X-Ray Diffraction (XRD) spectra, using the model nanostructures, reveal that the nanostructures are polymorphic with TiO2 domains of both rutile and brookite in accord with experiment. Amorphisation and recrystallisation was successful in generating complex microstructures. In particular, bulk and nanoporous structures show zigzag tunnels (indicative of micro-twinning) while nanosphere and nanosheet shows zigzag and straight tunnels in accord with experiment. All model nanostructures of TiO2 were lithiated with different lithium content. RDFs, microstructures, configuration energies, calculated as a function of time and XRDs of all lithiated structures are presented. / University of Limpopo Research Office,The Royal Institution(Ri),Granfield University,Materials Modelling Centre,UCL,and the CHPC

Nanowires are an exciting class of novel materials that have potential applications in areas including biological sensing, photonics, and electronics. The promise of these future applications relies on the production of nanowires of controlled size, shape, and crystal structure, in reasonable quantities, and further, ultimately requires that the nanowires be mechanically stable in the application environment. This research is aimed at understanding the mechanical behavior of metallic nanowires, through the use of atomistic simulations. At the nanometer scale, where the surface-area-to-volume ratio is substantial, the effects of free surfaces have a primary influence on the physical properties of a material. Surface energy arises from unsatisfied bond coordination at the surface of a solid and results in a surface stress as the surface atoms contract into the bulk of the material to increase their local electron density. The magnitude of surface energy and surface stress is dependent on the orientation of the surface and the compliance of the structure. In bulk materials, the effects of surfaces are negligible; however, at the nanometer scale, surface effects become quite significant. In metallic nanowires, these surface effects strongly influence mechanical properties, and the characteristics of plastic deformation. The mechanical testing of nanowires is precluded by the difficulties of accurately applying and measuring forces on the nanometer scale. For this reason, computational simulations are a primary tool for investigating the mechanical behavior of nanowires. In this work, we have performed atomistic simulations to examine the mechanical response of silver nanowires. We have conducted studies to determine the deformation characteristics of experimentally observed nanowire geometries subjected to tensile and bending loads. We have also developed a technique to probe the energetics of mechanical deformation, in order to elucidate the energetically favored deformation pathways in nanowires. Our results show that nanowires may be tailored for specific mechanical requirements based on geometry and free surface orientation and provide insight to the effect of free surfaces in the mechanical deformation of nanometer scale structures.

The research presented in this dissertation pertains to the evaluation of stiffness of carbon nanotubes (CNTs) in a multiscale framework and modeling of the interfacial mechanical behavior in CNT-polymer nanocomposites. The goal is to study the mechanical behavior of CNTs and CNT-polymer interfaces at the atomic level, and utilize this information to develop predictive capabilities of material behavior at the macroscale. Stiffness of CNTs is analyzed through quantum mechanical (QM) calculations while the CNT-polymer interface is examined using molecular dynamics (MD) simulations. CNT-polymer-matrix composites exhibit promising properties as structural materials and constitutive models are sought to predict their macroscale behavior. The reliability of determining the homogenized response of such materials depends upon the ability to accurately capture the interfacial behavior between the nanotubes and the polymer matrix. In the proposed work, atomistic methods are be used to investigate the behavior of the interface by utilizing appropriately chosen atomistic representative volume elements (RVEs). Atomistic simulations are conducted on the RVEs to study mechanical separation with and without covalent functionalization between the polymeric matrix and two filler materials, namely graphite and a (12,0) Single Wall zig zag CNT. The information obtained from atomistic studies of separation is applicable for higher level length scale models as cohesive zone properties. The results of the present research have been correlated with available experimental data from characterization efforts.

With the emergence of nanotechnology, mankind has obtained the capability to manipulate materials at nanoscale and this led to the invention of a new group of novel materials like carbon nanotubes, graphene and quantum nanodots. Silicene nanoribbons (SiNRs) are one of the newest members of this nanomaterial family which has been synthesized very recently by deposition on silver substrates. A SiNR sheet is made up of a layer of two dimensional honeycomb structure solely composed of silicon atoms. In this thesis, structural and mechanical properties of SiNR are being investigated with the help of classical empirical molecular dynamics simulation technique. In the first part of this thesis, structural properties of SiNR sheets have been investigated. This investigation has been carried out by performing classical molecular dynamics simulations using atomistic many-body potential energy functions at many different SiNR sheet lengths and widths, at low and room temperatures with and without periodic boundaries. It has been found that SiNR sheets do not have perfectly flat honeycomb geometry. It has also been found that finite length models are more likely to form tubular structures resembling distorted silicon nanotubes at room temperature. In the second part of this thesis, mechanical properties of SiNRs have been investigated. Mechanical properties of silicene nanoribbons of varying width have been investigated under 5% and 10% uniaxial strain via classical Molecular-Dynamics simulations at 1 K&deg / and 300 K&deg / temperatures by the aid of atomistic many-body potential energy functions. It has been found that under strain, SiNRs show such material properties: they are very ductile, they have considerable toughness and despite their low elasticity, they have a very long plastic range before fragmentation.

In a 2005 article in Science [45], Park et al. measured in vacuum the friction between a coated atomic-force-microscope tip and the clean two-fold surface of an AlNiCo quasicrystal. Because the two-fold surface is periodic in one direction and aperiodic (with a quasiperiodicity related to the Fibonacci sequence) in the perpendicular direction, frictional anisotropy is not unexpected; however, the magnitude of that anisotropy in the Park experiment, a factor of eight, is unprecedented. By eliminating chemistry as a variable, the experiment also demonstrated that the low friction of quasicrystals must be tied in some way to their quasiperiodicity. Through various models, we investigate generic geometric mechanisms that might give rise to this anisotropy.

The metallic binary-alloy fuel Uranium-Zirconium is important for use in the new generation of advanced fast reactors. Uranium-Zirconium goes through a phase transition at higher temperatures to a (gamma) Body Centered Cubic (BCC) phase. The BCC high temperature phase is particularly important since it corresponds to the temperature range in which the fast reactors will operate. A semi-empirical Modified Embedded Atom Method (MEAM) potential is presented for Uranium-Zirconium. This is the first interatomic potential created for the U-Zr system. The bulk physical properties of the Uranium-Zirconium binary alloy were reproduced using Molecular Dynamics (MD) and Monte Carlo (MC) simulations with the MEAM potential. The simulation of bulk metallic alloy separation and ordering phenomena on the atomic scale using iterative MD and MC simulations with interatomic potentials has never been done before. These simulations will help the fundamental understanding of complex phenomena in the metallic fuels. This is a large step in making a computationally acceptable fuel performance code, able to replicate and predict fuel behavior.

Static strain aging is an important concept in metalurgy that refers to the hardening of a material that has undergone plastic deformation and then is aged for a certain period of time. A theory proposed in the late 1940s by Cottrell and Bilby explains this phenomenon as being caused by the pinning of dislocations by impurities (e.g., carbon atoms in solid solution) that migrate to the vicinity of the line defect. In the course of this PhD work, the atomistic mechanism behind the static strain aging phenomenon in bcc iron has been studied by means of computer simulations. Given the fact that diffusion in the solid state proceeds slowly, thus preventing the use of molecular dynamics at low temperatures (when the effect of the dislocation stress field on carbon diffusion is more pronounced), we have preferentially employed a method coupling molecular statics with atomistic kinetic Monte Carlo. Three major points have been addressed by this thesis: (i) the effect of the stress field of an edge or screw dislocation on a carbon atom diffusing nearby; (ii) the diffusion of a carbon atom in the tight channel found in the dislocation core (pipe diffusion); and (iii) the equilibrium carbon distribution in a Cottrell atmosphere. The main effect of the dislocation stress field outside the dislocation core consists of biasing carbon diffusion, such that some transitions become more likely than others. This effect is expected to drive the early stages of Cottrell atmosphere formation, when the mutual interaction between carbon atoms is negligible. Right in the dislocation core, as expected, carbon was seen to diffuse faster than in the bulk. Carbon concentration in the neighborhood of an edge or a screw dislocation was modeled by an approach based in statistical physics using the binding energies calculated by molecular statics, revealing a good agreement with experimental data obtained by atom probe techniques.

No description available.

Les alginates forment des gels transparents en solution aqueuse en présence de certains ions divalents. Cette propriété est principalement attribuée à la formation de zones de jonction impliquant les séquences (1->4)-a-L-guluronane de chaînes adjacentes. Des oligomères d'alginates ont été utilisés comme briques élémentaires pour la synthèse de polymères biohybrides contenant des chaines pendantes oligo-(1->4)-a-L-guluronane. La rhéologie et la diffusion de la lumière ont permis d'étudier leur gélification ionotropique. De plus, une image atomistique des associations entre chaines latérales a été donnée grâce à la modélisation moléculaire et la microscopie de force atomique. Les polymères biohybrides portant des résidus pendant (1->4)-a-L-guluronane forment des gels en présence de Ca2+. L'addition de blocs guluronane ou mannuronane au gel préformé fait diminuer sa force avec quasiment la même efficacité. L'étude par dynamique moléculaire de séquences (1->4)-a-L-guluronan totalement chargées en présence d'ions Ca2+ suggère qu'environ 8 unités de répétition sont suffisantes pour former spontanément des zones de jonction. De plus, l'analyse conformationnelle de duplexes de chaines (1->4)-a-L-guluronane ayant 12 unités de répétition révèle une grande variété de conformations accessibles, ce qui est consistant avec la difficulté d'obtention de cristaux de Ca2+-guluronate de dimension suffisante pour les études cristallographiques. Les forces d'adhésion entre des homo-oligomères d'alginates en présence de Ca2+ mesuré par spectroscopie de force atomique montrent que la force d'interaction croit selon l'ordre suivant: M-M < M-G or G-M < G-G. Un résultat important est que les blocs mannuronanes, en complexe avec le calcium, peuvent être impliqués dans des associations homotypiques et hétérotypiques. Ce résultat est consistant avec la détection d'agrégats d'oligomères de mannuronanes observés en diffusion de la lumière pendant l'addition de CaCl2. Les blocs M contribuent donc également à la formation du gel mais la force associée est plus faible que celle des blocs G.

Irradiation induces a high concentration of defects in the structural materials of nuclear reactors, which are typically of body-centered cubic Iron (BCC Fe) and its alloys. The primary effect of irradiation is hardening which is caused by the blocking of dislocations with defects and defect clusters like point defects, self-interstitial loops, and voids. The dislocation-defect interactions are atomistic in nature due to the very small length and time scales involved, i.e., of the order of nanometers and picoseconds. To predict the effect of dislocation-defect interactions on the macroscopic mechanical and plastic behavior of the material, it is critically important to develop robust coupling schemes by which accurate atomic level physics of the rate-limiting kinetic processes can be informed into a coarse-grained model such as crystal plasticity. In this thesis we will develop an atomistically informed constitutive model. Relevant atomistic processes are identified from molecular dynamics simulations. The respective unit process studies are conducted using atomistic reaction pathway sampling methods like Nudged Elastic Band method. Stress-dependent activation energies and activation volumes are computed for various rate-liming unit processes like thermally activated dislocation motion via kinkpair nucleation, dislocation pinning due to self interstitial atom, etc. Constitutive laws are developed based on transition state theory, that informs the atomistically determined activation parameters into a coarse-grained crystal plasticity model. The macroscopic deformation behavior predicted by the crystal plasticity model is validated with experimental results and the characteristic features explained in the light of atomistic knowledge of the constituting kinetics. We also investigate on unique irradiation induced defects such as stacking fault tetrahedra, that are formed under non-irradiated condition. This thesis also includes our work on materials with internal interfaces that can resist irradiation induced damage. Overall, the research presented in this thesis involves the implementation and development of novel computational paradigm that encompasses computational approaches of various length and time scales towards robust predictions of the mechanical behavior of irradiated materials.