Crises and EU’s global economic power: Τhe trade and investments dimensions

The last decade has been one of the most challenging periods for European integration. The decade started with a sovereign debt crisis that hit hard the Eurozone’s peripheral member states and ended with the economic wreckage caused by the COVID-19 pandemic in 2020. As expected, the global financial crisis and the Euro-crisis have had an impact on the orientation of the EU as a global economic power. Two of the main aspects of the EU’s economic power are its trade and investment power. The aim of this paper is to provide an evaluation of the impact of the global financial and Euro-crisis in the European Union’s performance on these two dimensions of the EU’s power

On the Surface Restructuring of Highly Dilute Alloys and its Effects on Catalytic Performance

Recent studies have shown that highly dilute alloys of platinum group metals (PGMs: Pt, Rh, Ir and Pd) with coinage metals (Cu, Au, and Ag) serve as highly selective and coke–resistant catalysts in a number of important chemical reactions. These materials are composed of trace amounts of a PGM or Ni, whose atoms are embedded in a coinage metal surface, and their catalytic behaviour is governed by the size and shape of the surface clusters of PGM atoms. Therefore, establishing a means of control over the topological architecture of highly dilute alloy surfaces is crucial to achieving catalytic performance tailored to a specific application. This Thesis employs density functional theory, kinetic Monte Carlo and microkinetic modelling in order to investigate ways of manipulating the surface architecture of a number of dilute alloy surfaces towards optimal performance for key catalytic reactions. The latter include the direct dissociations of NO, CO2 and N2, and the reverse events, which are important in, among others, emission control technologies. Also examined is the potential of a Ni/Cu dilute alloy for the NO + CO chemistry, and it is demonstrated that the selectivity toward the desired products can be manipulated by tuning the size of the Ni clusters in the ensemble. The results can guide future theoretical, surface science and catalysis studies on highly dilute alloys, towards the development of superior catalysts that can efficiently accelerate chemistries of industrial significance

Fractal roughness effects on nanoscale grinding

Three-dimensional Molecular Dynamics simulations have been performed to investigate the effects of fractal roughness on nanoscale grinding. The first part of this investigation focuses on the effects of the workpiece rough top surface on the grinding process characteristics, with special emphasis placed on the friction coefficient, the grinding forces and the subsurface temperature. The second part focuses on the alternation of the aforementioned parameters due to the rough abrasive contact surface. Rough surface profiles have been generated using the multivariate Weierstrass-Mandelbrot function while the abrasive has been modelled as a trapezoid. The irregular surface topography has been controlled by tuning the value of the fractal dimension Ds . The aforementioned experiments have been repeated for various values of the cutting depth. Our results indicate that the grinding process parameters are mainly dependent on the cutting depth as well as the abrasive lower surface profile, which also defines the interface contact area between the workpiece and the abrasive, rather than the topography of the workpiece top surface

Constraining supermassive primordial black holes with magnetically induced gravitational waves

Primordial black holes (PBHs) can answer a plethora of cosmic conundra, among which the origin of the cosmic magnetic fields. In particular, supermassive PBHs with masses $M_\mathrm{PBH}>10^{10} M_\odot$ and furnished with a plasma-disk moving around them can generate through the Biermann battery mechanism a seed primordial magnetic field which can later be amplified so as to provide the magnetic field threading the intergalactic medium. In this Letter, we derive the gravitational wave (GW) signal induced by the magnetic anisotropic stress of such a population of magnetised PBHs. Interestingly enough, by using GW constraints from Big Bang Nucleosynthesis (BBN) and an effective model for the galactic/turbulent dynamo amplification of the magnetic field, we set a conservative upper bound constraint on the abundances of supermassive PBHs at formation time, $\Omega_\mathrm{PBH,f}$ as a function of the their masses, namely that $\Omega_\mathrm{PBH,f}\leq 2.5\times 10^{-10}\left(\frac{M}{10^{10}M_\odot}\right)^{45/22}$. Remarkably, these constraints are comparable, and, in some mass ranges, even tighter compared to the constraints on $\Omega_\mathrm{PBH,f}$ from large-scale structure (LSS) probes; hence promoting the portal of magnetically induced GWs as a new probe to explore the enigmatic nature of supermassive PBHs.Comment: 5 pages, 2 figures (comments are welcome

Design optimisation of the feeding system of a novel counter-gravity casting process

The appropriate design of feeders in a rigging system is critical for ensuring efficient compensation for solidification shrinkage, thus eliminating (shrinkage-related) porosity and contributing to the production of superior quality castings. In this study, a multi-objective optimisation framework combined with Computational Fluid Dynamics (CFD) simulations has been introduced to investigate the effect of the feeders’ geometry on shrinkage porosity aiming to optimise casting quality and yield for a novel counter-gravity casting process (CRIMSON). The weighted sum technique was employed to convert this multi-objective optimisation problem to a single objective one. Moreover, an evolutionary multi-objective optimisation algorithm (NSGA-II) has been applied to estimate the trade-off between the objective functions and support decision makers on selecting the optimum solution based on the desired properties of the final casting product and the process characteristics. This study is one of the first attempts to combine CFD simulations with multi-objective optimisation techniques in counter-gravity casting. The obtained results indicate the benefits of applying multi-objective optimisation techniques to casting processe

Grain size effects on nanocutting behaviour modelling based on molecular dynamics simulations

Grain size is one of the most critical factors affecting the mechanical and thermal properties of metallic materials. In this study the effect of the grain size of a workpiece made of pure Aluminium on the nanocutting process has been investigated via means of Molecular Dynamics simulations. The polycrystalline workpiece has been generated starting from a Face-Centred Cubic block of Aluminium atoms which was melted and subsequently quenched under various cooling rates in order to control the average grain size. The case of a monocrystalline workpiece has been considered as well. The generated workpieces were ground by a diamond abrasive. Simulations have been repeated in order to eliminate any statistical errors. The obtained results suggest that the average grain size of the workpiece significantly influences almost every aspect related to the nanocutting process. More specifically, it has been found that the cutting forces increase and the friction coefficient decreases with the grain size. Very small grain sizes lead to lower thermal conductivity and consequently high temperature at the cutting region. Finally, it has been shown that the high residual stresses at the grain boundaries can be relieved as the tool passes on top of the workpiece; this phenomenon resembles heat treatment. In summary, the nanocutting behaviour of polycrystalline materials depends on the average grain size and significantly differs from the case of monocrystalline materials. This should be taken into account in future numerical models of nanocutting processe

Large-scale molecular dynamics simulations of homogeneous nucleation of pure aluminium

Despite the continuous and remarkable development of experimental techniques for the investigation of microstructures and the growth of nuclei during the solidification of metals, there are still unknown territories around this topic. The solidification in nanoscale can be effectively investigated by means of molecular dynamics (MD) simulations which can provide a deep insight into the mechanisms of the formation of nuclei and the induced crystal structures. In this study, MD simulations were performed to investigate the solidification of pure Aluminium and the effects of the cooling rate on the final properties of the solidified material. A large number of Aluminium atoms were used in order to investigate the grain growth over time and the formation of stacking faults during solidification. The number of face-centred cubic (FCC), hexagonal close-packed (HCP) and body-centred cubic (BCC) was recorded during the evolution of the process to illustrate the nanoscale mechanisms initiating solidification. The current investigation also focuses on the exothermic nature of the solidification process which has been effectively captured by means of MD simulations using 3 dimensional representations of the kinetic energy across the simulation domain