THEORETICAL STUDY OF STRUCTURAL TRANSFORMATIONS AND PROPERTIES OF SELECTED MATERIALS AT EXTREME CONDITIONS

There are several objectives that have been addressed in this thesis. Under a broader heading, the methods that have been explored and applied are density functional theory (DFT), ab initio metadynamics and ab initio molecular dynamics (AIMD). These methods have been employed to analyze structural phase transitions, electronic, vibrational and transport properties of selected materials at high pressure. All the materials that have been considered in this thesis have been studied experimentally by various research groups. Using theoretical methods and the sophisticated computational tools mentioned above, the aim of this thesis is to predict as well explain the experimental observations, thus bridging the gap between experiment and theory. The thesis has been divided as follows. The first project that has been discussed is on the structural phase transition of aluminium triiodide (AlI3). Experimentally, no structural phase transition was reported for crystalline AlI3 at high pressure in spite of getting certain subtle results which hinted at a first order phase transition. Thus, in our study, we employed ab initio metadyanamics to scan the potential energy surface (PES) and find the energetically most stable configuration. Indeed, we found first order structural phase transition at approximately 1.3 GPa which was verified by the Raman spectra as well. The next project was to explore the structure of the superconducting phase of hydrogen sulfide (H2S) which was experimentally observed to have a high superconducting critical temperature of 203 K. However, the crystal structure of the superconducting phase has been ambiguous and has been proposed to be a metastable phase. Therefore, in our study, we performed ab initio metadyanamics to search for metastable phases. At 80 GPa and 80 K, a metastable structure was found. This metastable structure on further ab initio molecular dynamics (AIMD) at 200 GPa and 200 K resulted in a modulated structure whose X-Ray diffraction (XRD) pattern matched excellently with that obtained experimentally. Analysis of the electron-phonon interactions on this modulated structure gave superconducting critical temperatures close to the value obtained experimentally. The third project is based on the electron-phonon interaction and subsequent calculation of superconducting properties of an experimentally synthesized polyhydride of iron, FeH5. The structure was found to have hydrogen in the atomic form, which has long since been proposed to be a criterion for high temperature superconductivity. First principles theoretical calculations revealed FeH5 to be a superconductor at high pressure albeit with a low critical temperature of 51 K at 130 GPa, confirming a hypothesis that the superconductivity of any material is sensitive to several factors that have been discussed in the chapter. The final project deals with the study of structural, electronic and transport properties of glass and molten basalt (igneous rock). This material is amorphous and abundant in the Earth’s mantle. Although several experimental and theoretical studies have been performed on materials that mimic basalt, there is still a lot to be unravelled regarding its structural and transport properties at the mantle conditions. A clear understanding of the structure and transport properties of basalt can explain in depth about the thermochemical evolution of the Earth and origin of life. In the study reported in this thesis, ab initio molecular dynamics simulations were performed on an amorphous model basalt structure (containing the most abundant chemical species, Si, Al, Ca, Mg and O) at the mantle conditions over a range of high pressures. The results that have been reported here are in very good agreement with earlier experimental and theoretical results, confirming that the model basalt considered is indeed a good approximation and can be further improved by considering the minor occurring elements (Na, K, etc.) for future research

An Analysis of the Spatio-Temporal Factors Affecting Aircraft Conflicts Based on Simulation Modelling

The demand for air travel worldwide continues to grow at a rapid rate, especially in Europe and the United States. In Europe, the demand exceeded predictions with a real annual growth of 7.1% in the period 1985-1990, against a prediction of 2.4%. By the year 2010, the demand is expected to double from the 1990 level. Within the UK international scheduled passenger traffic is predicted to increase, on average, by 5.8 per cent per year between 1999 and 2003. The demand has not been matched by availability of capacity. In Western Europe many of the largest airports suffer from runway capacity constraints. Europe also suffers from an en-route airspace capacity constraint, which is determined by the workload of the air traffic controllers, i.e. the physical and mental work that controllers must undertake to safely conduct air traffic under their jurisdiction through en-route airspace. The annual cost to Europe due to air traffic inefficiency and congestion in en-route airspace is estimated to be 5 billion US Dollars, primarily due to delays caused by non-optimal route structures and reduced productivity of controllers due to equipment inefficiencies. Therefore, to in order to decrease the total delay, an increase in en-route capacity is of paramount importance. At a global scale and in the early 1980s, the International Civil Aviation Organisation (ICAO) recognised that the traditional air traffic control (ATC) systems would not cope with the growth in demand for capacity. Consequently new technologies and procedures have been proposed to enable ATC to cope with this demand, e.g. satellite-based system concept to meet the future civil aviation requirements for communication, navigation and surveillance/ air traffic management (CNS/ATM). In Europe, the organisation EUROCONTROL (established in 1960 to co-ordinate European ATM) proposed a variety of measures to increase the capacity of en-route airspace. A key change envisaged is the increasing delegation of responsibilities for control to flight crew, by the use of airborne separation assurance between aircraft, leading eventually to ?free flight? airspace. However, there are major concerns regarding the safety of operations in ?free flight? airspace. The safety of such airspace can be investigated by analysing the factors that affect conflict occurrence, i.e. a loss of the prescribed separation between two aircraft in airspace. This paper analyses the factors affecting conflict occurrence in current airspace and future free flight airspace by using a simulation model of air traffic controller workload, the RAMS model. The paper begins with a literature review of the factors that affect conflict occurrence. This is followed by a description of the RAMS model and of its use in this analysis. The airspace simulated is the Mediterranean Free Flight region, and the major attributes of this region and of the traffic demand patterns are outlined next. In particular a day?s air traffic is simulated in the two airspace scenarios, and rules for conflict detection and resolution are carefully defined. The following section outlines the framework for analysing the output from the simulations, using negative binomial (NB) and generalised negative binomial (GNB) regression, and discusses the estimation methods required. The next section presents the results of the regression analysis, taking into account the spatio-temporal nature of the data. The following section presents an analysis of the spatial and temporal pattern of conflicts in the two airspace scenarios across a day, highlighting possible metrics to indicate this. The paper concludes with future research directions based upon this analysis.