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Ethylene-vinyl acetate co-polymers are widely used as cold-flow improving additives in diesel fuels. Although their functionality is undisputed, the mechanisms of action at a molecular level are not fully understood. Theories that EVA acts as both a nucleating agent for small wax crystals and as a crystal growth inhibitor are generally acknowledged, however the understanding of the effect of structure on additive performance is limited. Studies have been carried out using molecular simulation techniques with the aims of improving the understanding of the interactions between EVA and diesel fuels, and assessing the potential of these methods for the analysis of different additive structures and fuel compositions. Firstly, the ability of Monte Carlo simulations to predict the chemical potentials of n-alkanes in organic solvents was studied. This information would be of use in determining the characteristics of different fuel compositions. It was found that the particle insertion method used becomes more efficient as the aspect ratio of the solvent increases relative to that of the solute. Despite the identification of expected linear trends in the chemical potential of n-alkanes, it was not possible to produce high quality quantitative data using this method. The effect of vinyl acetate spacing and solvent environment on the configuration of lone EVA molecules in the liquid phase was studied. Molecular dynamics showed that closely spaced vinyl acetate groups increase the tendency of the molecule to fold at that point, whilst molecules with 2 or 3 ethylene groups in between show resistance to folding. It was shown that n-hexane as a solvent produced more folded configurations than benzene and n-hexadecane respectively. Monte Carlo simulations were also performed but were largely unsuccessful. Finally, molecular dynamics simulations were performed to study the effect of different EVA structures on the spontaneous crystallisation of n-hexadecane, and the interaction of the EVA with the resulting structures at a molecular level. It was shown that different EVAs had varying effects on the crystallisation, with trends attributed to the spacing of the vinyl acetate groups and the length and position of extended aliphatic sections of the EVA molecules. One molecule, with evenly spaced side-groups and no extended aliphatic sections was shown to fully prevent the formation of any crystalline structure within the time of the simulation where all other EVA molecules allowed some degree of wax formation to occur.

Carbon capture and sequestration are considered to be a temporary fix to the climate change global crisis. Following the noble gas tracers injection field experiment at Salt Creek in the state of Wyoming, USA, these tracers may be used to characterise the reservoir as a potential geological sequestration site for carbon dioxide. This study aims to investigate various thermodynamics properties of the five noble gases (Xe, Kr, Ar, Ne, and He) in n-decane at reservoir conditions (340 K – 460 K and 10 MPa – 200 MPa). The study utilises the SKS force field to describe n-decane and both Gibbs Ensemble Monte Carlo and molecular dynamics simulations were used to investigate the solubility, diffusivity, and vapour-liquid equilibrium of the five binary mixtures. The size of the noble gases was found to be important in these nonpolar mixtures where typical interactions are weak and short-ranged. The enthalpies of solvation were calculated and found to be directly correlated to the size of the solute where the energy required for the formation of a cavity to accommodate the solute is compensated by the nature of the intermolecular interaction between solvent and solute. The mixture of Ar + n-decane is of interest particularly because the sigma value for Ar is very similar to that of the CH3 group, resulting in the overall non-mononicity of several thermodynamics properties. Additionally, maxima in enthalpies of solvation were observed in Xe and Kr in n-decane solution at 200 MPa. While these maxima were observed in two different species at similar conditions, they are accommodated by unusually high uncertainties - further investigation is required before definitive conclusions can be drawn. The results from the vapour-liquid equilibrium study of the five noble gas + n-decane binary mixtures were in good agreement with the Peng-Robinson equation of state predictions. What is more, the diffusion coefficient ratios amongst the five noble gases in n-decane were investigated in light of Stoke-Einstein’s relation and Enskog’s hard-sphere relation. Three different radii of solute-solvent interaction were investigated and the best fit was observed when R =radius of solute + radius of gyration of n-decane. Additionally, the diffusion coefficients were utilised in the reservoir simulation to investigate the role of diffusion within the reservoir.

The formation of nano–sized precipitates is considered to be the origin of hardening and embrittlement of ferritic steel used as structural material for pressure vessels of nuclear reactors, since these nanoclusters hinder the motion of dislocations within the grains of the polycrystalline bcc–Fe matrix. Previous investigations showed that these small precipitates are coherent and may consist of Cu, Ni, other foreign atoms, and vacancies. In this work a combination of on–lattice simulated annealing based on Metropolis Monte Carlo simulations and off–lattice relaxation by Molecular Dynamics is applied in order to determine the structure, energetics and thermodynamics of coherent clusters in bcc–Fe. The most recent interatomic potentials for Fe–Cu–Ni alloys are used. The atomic structure and the formation energy of the most stable configurations as well as their total and monomer binding energy are calculated. Atomistic simulation results show that pure (vacancy and copper) as well as mixed (vacancy-copper, copper-nickel and vacancy-copper-nickel) clusters show facets which correspond to the main crystallographic planes. Besides facets, mixed clusters exhibit a core-shell structure. In the case of v_lCu_m, a core of vacancy cluster coated with copper atoms is found. In binary Cum_Ni_n, Ni atoms cover the outer surface of copper cluster. Ternary v_lCu_mNi_n clusters show a core–shell structure with vacancies in the core coated by a shell of Cu atoms, followed by a shell of Ni atoms. It has been shown qualitatively that these core–shell structures are formed in order to minimize the interface energy between the cluster and the bcc-Fe matrix. Pure nickel consist of an agglomeration of Ni atoms at second nearest neighbor distance, whereas vacancy-nickel are formed by a vacancy cluster surrounded by a nickel agglomeration. Both types of clusters are called quasi-cluster because of their non-compact structure. The atomic configurations of quasiclusters can be understood by the peculiarities of the binding between Ni atoms and vacancies. In all clusters investigated Ni atoms may be nearest neighbors of Cu atoms but never nearest neighbors of vacancies or other Ni atoms. The structure of the clusters found in the present work is consistent with experimental observations and with results of pairwise calculations. In agreement with experimental observations and with recent results of atomic kinetic Monte Carlo simulation it is shown that the presence of Ni atoms promotes the nucleation of clusters containing vacancies and Cu. For pure vacancy and pure copper clusters an atomistic nucleation model is established, and for typical irradiation conditions the nucleation free energy and the critical size for cluster formation have been estimated. For further application in rate theory and object kinetic Monte Carlo simulations compact and physically–based fit formulae are derived from the atomistic data for the total and the monomer binding energy. The fit is based on the structure of the clusters (core-shell and quasi-cluster) and on the classical capillary model.

There were three main driving forces behind this thesis: global concern over climate change mainly due to uncontrolled carbon dioxide (CO2) emissions, the excitement over the discovery of graphene and its versatile potential, and the potential to design three-dimensional (3D) or two-dimensional (2D) structures, in our case using unique triptycene molecule. We examined two polymeric materials for CO2 adsorption and suggested simple design of disordered carbons suitable for gas adsorption studies. The approach in each task was to examine structural and adsorption properties of materials using detailed atomistic modelling employing Monte Carlo and Molecular Dynamics techniques and where possible provide experimental measurements to validate the simulations. The thesis is presented as a collection of papers and the work can be divided into three independent projects. The aim of the first project is to utilize graphene as an additive in polymer composites in order to increase separation between the polymer chains increasing available surface area. The matrix used is a polymer of intrinsic microporosity (PIM-1), which possess large surface area and narrow nano-sized ( > 2nm) pore distribution attractive for gas separation membrane applications. Adding a filler can reduce aging of the polymer, and enhance permeability across the membrane, often to the expense of loosing selectivity. Therefore, we investigated the packing of PIM-1 chains in presence of discrete 2D graphene platelets and 3D graphene-derived structures and its effect on composite structure and adsorption properties. We found that additives do not alter structural polymer properties at the molecular level preserving the same adsorption capacity and affinity. Potential permeability increase would benefit from the retention of selectivity in the material. Building on design philosophy of materials with intrinsic microporosity we continued further investigation of 3D graphene-derived structures. The idea is that highly concave molecules or polymer chains pack inefficiently creating microporous materials with sufficient surface area for gas adsorption. 3D propeller-like structures were derived from graphene arms connected through the rigid triptycene and other types of cores. The resulting structures created a large amount of micropores and showed similar CO2/CH4 selectivity to activated carbons reported in the literature. It was shown that rigid triptycene core leads to more open structures. The model was also applied to model commercially available activated carbon to predict n- perfluorohexane adsorption. The fitting to experimental structural information proved to be challenging due to trial and error nature of the approach. Nevertheless, the simple packing procedure and diverse structure design have a great potential to serve as a virtual model for porous carbons that possess pore complexity and does not require any previous experimental data to be build on. The last project concerns CO2 adsorption and selectivity over CH4 and N2 in recently reported triptycene-based polymer. The triptycene shape polymer can form a porous 2D network that can be exfoliated into free-standing sheets and potentially used as a membrane. Sheets stack in the bulk material forming anisotropic channel pores. Additionally it contains fluoro- functional groups, which are known to have a high CO2 affinity. We explored pore structure and chemistry of stacked material for gas adsorption and predicted comparable capacity and CO2 selectivity to other microporous covalent materials such as activated carbons and PIMs. The CH4/N2 selectivity was similar to currently most selective material belonging to MOF family. We showed that fluoro-group have a positive effect on CO2 affinity, however predictions are sensitive to the charges of fluorine atoms assigned by different methods.

Fundamental understanding of the behaviour of nuclear fuel has been of great importance. Enhancing this knowledge not only by means of experimental observations, but also via multi-scale modelling is of current interest. The overall goal of this thesis is to understand the impact of atomic interactions on the nuclear fuel material properties. Two major topics are tackled in this thesis. The first topic deals with non-stoichiometry in uranium dioxide (UO2) to be addressed by empirical potential (EP) studies. The second fundamental question to be answered is the effect of the atomic fraction of americium (Am), neptunium (Np) containing uranium (U) and plutonium (Pu) mixed oxide (MOX) on the material properties.UO2 has been the reference fuel for the current fleet of nuclear reactors (Gen-II and Gen-III); it is also considered today by the Gen-IV International Forum for the first cores of the future generation of nuclear reactors on the roadmap towards minor actinide (MA) based fuel technology. The physical properties of UO2 highly depend on material stoichiometry. In particular, oxidation towards hyper stoichiometric UO2 – UO2+x – might be encountered at various stages of the nuclear fuel cycle if oxidative conditions are met; the impact of physical property changes upon stoichiometry should therefore be properly assessed to ensure safe and reliable operations. These physical properties are intimately linked to the arrangement of atomic defects in the crystalline structure. The first paper evaluates the evolution of defect concentration with environment parameters – oxygen partial pressure and temperature by means of a point defect model, with reaction energies being derived from EP based atomic scale simulations. Ultimately, results from the point defect model are discussed, and compared to experimental measurements of stoichiometry dependence on oxygen partial pressure and temperature. Such investigations will allow for future discussions about the solubility of different fission products and dopants in the UO2 matrix at EP level.While the first paper answers the central question regarding the dominating defects in non-stoichiometry in UO2, the focus of the second paper was on the EP prediction of the material properties, notably the lattice parameter of Am, Np containing U and Pu MOX as a function of atomic fractions.The configurational space of a complex U1-y-y’-y’’PuyAmy’Npy’’O2 system, was assessed via Metropolis-Monte Carlo techniques. From the predicted configuration, the relaxed lattice parameter of Am, Np bearing MOX fuel was investigated and compared with available literature data. As a result, a linear behaviour of the lattice parameter as a function of Am, Np content was observed, as expected for an ideal solid solution. These results will allow to support and increase current knowledge on Gen-IV fuel properties, such as melting temperature, for which preliminary results are presented in this thesis, and possibly thermal conductivity in the future. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

This work presents theoretical background for the investigation of nanostructure formation in alkali-metal doped fullerene layers. A number of computational methods are used to describe structural transformation in the fullerene layer. They include tight-binding molecular dynamics, empirical molecular dynamics, Monte-Carlo calculations as well as other methods. The doped fullerene layers show the highest superconducting critical temperature among organic superconductors. A new electrochemical method of synthesis of potassium and rubidium fullerides has been recently developed by Professor Dunsch and coworkers in the department of electrochemistry and conductive polymers at IFW Dresden. The process of electrochemical doping is accompanied by several side effects, and one of them is nanostructure formation at the surface of the fullerene layer. In the present work an explanation is given for the nanostructure formation observed recently by scanning tunnel microscopy. The corresponding model is based on the concept of spontaneous phase separation that has been realized by kinetic Monte Carlo calculations. These calculations predict instability of initially homogeneous alkali-doped fullerene layers. Due to the significant gap in the Madelung energy formation of an alkali-poor and an alkali-reach phase is expected. The results of the Monte Carlo simulations point out that the particle size of the corresponding phases remains in the nanometer range. Interpretation of experimental data for metal deposition on fullerene substrates can be easily given in the framework of the phase separation concept as well. Metal clusters of the size order 50 to 100 nm emerge in course of electrochemical copper deposition on alkali-doped fullerene layers. The electrically conductive paths through the insulating fullerene layer are probably responsible for the inhomogeneous copper deposition under electrochemical conditions. A novel computer program has been developed in course of this work, which is designed as a distributed application. It can be used for diverse conventional and kinetic Monte Carlo calculations. / Die vorliegende Arbeit präsentiert theoretische Arbeiten, die das Ziel haben, die Nanostrukturbildung in dotierten Fullerenschichten zu verstehen. Diverse Rechenmethoden wurden verwendet, um die strukturellen Umwandlungen in der Fullerenschicht zu beschreiben. Die Tight-Binding-Molekulardynamik, die empirische Molekulardynamik und Monte-Carlo-Berechnungen sowie andere Methoden sind eingeschlossen. Die dotierten Fullerenschichten zeigen die höchste supraleitende kritische Temperatur unter den organischen Supraleitern. Eine neue elektrochemische Methode der Synthese von Kalium- und Rubidium-Fulleriden wurde vor kurzem von Professor Dunsch und Mitarbeitern in der Abteilung Elektrochemie und leitfähigen Polymere am IFW Dresden entwickelt. Der Prozess der elektrochemischen Dotierung wird von mehreren Nebenprozessen begleitet, und einer davon ist die Nanostrukturbildung an der Oberfäche der Fullerenschicht. In der vorliegenden Arbeit wird eine Erklärung für die Herausbildung der Nanostrukturen, die mit Hilfe von Rastertunnelmikroskopie beobachtet wurden, gegeben. Das entsprechende Modell basiert auf dem Konzept der spontanen Phasenentmischung und wird durch kinetische Monte-Carlo-Simulationen realisiert. Diese Simulationen sagen Instabilität der zunächst homogenen Alkali-dotierten Fullerenschichten voraus. Wegen des wesentlichen Unterschieds in der Madelungenergie ist die Herausbildung einer Alkalimetall-armen und einer Alkalimetall-reichen Phase zu erwarten. Die Ergebnisse der Monte-Carlo-Simulationen weisen darauf hin, dass die Teilchengröße der entsprechenden Phasen im Nanometer-Bereich bleibt. Im Rahmen des Phasenentmischungskonzepts können auch experimentelle Daten zur Metallabscheidung auf Fullerensubstraten problemlos interpretiert werden. Bei elektrochemischer Kupferabscheidung auf Alkali-dotierten Fullerenschichten entstehen Metallcluster der Größenordnung von 50 bis 100 nm. Die elektrisch leitfähige Pfade, die in einer isolierenden Matrix auftreten, sind wahrscheinlich für die ungleichmäßige Kupferabscheidung unter elektrochemischen Bedingungen verantwortlich. Ein neuartiges Computerprogramm wurde im Rahmen dieser Arbeit entwickelt, das als eine verteilte Anwendung entworfen ist. Damit können diverse konventionelle und kinetische Monte-Carlo-Simulationen durchgeführt werden.