Υπέρλεπτη υφή της μικροδομής μεταλλικών υάλων Cu-Zr

This work is dedicated in the investigation of the fine details of the microstructure of Metallic Glasses. The study was conducted mainly by means of Molecular Dynamic Simulations on several compositions of Cu-Zr Bulk Metallic Glasses, in comparison with experimental data, revealing their properties dependency with the existence of several Icosahedral-like clusters._______________________________________The study comprises the exploration of the microstructure evolution under three different vitrification techniques; rapid quenching, thin film deposition and mechanical alloying. The Cu-Zr Metallic Glasses were analysed during every vitrification procedure and the formation of the Icosahedral-like clusters was investigated revealing their importance on the Glass Forming Ability of the Cu-Zr systems._____________________Furthermore, the Icosahedral-like clusters were found to be usually distorted and interpenetrated while when the stoichiometry of these nanostructures obeyed the system’s composition their size followed a sequence of magic numbers named therefore SuperClusters. It came out that these structural units embrace all the details of the microstructure of every system reflecting its ultrafine texture and thus, they can be considered as the basic structural units of the Metallic This work is dedicated in the investigation of the fine details of the microstructure of Metallic Glasses. The study was conducted mainly by means of Molecular Dynamic Simulations on several compositions of Cu-Zr Bulk Metallic Glasses, in comparison with experimental data, revealing their properties dependency with the existence of several Icosahedral-like clusters._______________________________________The study comprises the exploration of the microstructure evolution under three different vitrification techniques; rapid quenching, thin film deposition and mechanical alloying. The Cu-Zr Metallic Glasses were analysed during every vitrification procedure and the formation of the Icosahedral-like clusters was investigated revealing their importance on the Glass Forming Ability of the Cu-Zr systems._____________________Furthermore, the Icosahedral-like clusters were found to be usually distorted and interpenetrated while when the stoichiometry of these nanostructures obeyed the system’s composition their size followed a sequence of magic numbers named therefore SuperClusters. It came out that these structural units embrace all the details of the microstructure of every system reflecting its ultrafine texture and thus, they can be considered as the basic structural units of the MetallicΗ εργασία αυτή είναι αφιερωμένη στην υπέρλεπτη υφή της μικροδομής των μεταλλικών υάλων. Η μελέτη έγινε κυρίως με τη βοήθεια προσομοιώσεων Μοριακής Δυναμικής διαφόρων στοιχειομετριών Cu-Zr, σε σύγκριση με πειραματικά δεδομένα, αποκαλύπτοντας την εξάρτηση των ιδιοτήτων τους με την ύπαρξη πληθώρας νανοδομών Εικοσαεδρικού τύπου._______________________Η μελέτη συμπεριλαμβάνει τη διερεύνηση της εξέλιξης της μικροδομής κατά την υαλοποίηση με τρεις διαφορετικές τεχνικές˙ με γρήγορη ψύξη υγρού, με εναπόθεση αερίου και με μηχανική κραματοποίηση. Τα μεταλλικά γυαλιά Cu-Zr αναλύθηκαν κατά τη διάρκεια κάθε τεχνικής υαλοποίησης και διερευνήθηκε ο σχηματισμών των Εικοσαεδρικού τύπου νανοδομών αποκαλύπτοντας τον πολύ σημαντικό ρόλο τους ως προς την δυνατότητα υαλοποίησης των συστημάτων Cu-Zr.___________________________________________Επιπλέον, οι Εικοσαεδρικού τύπου νανοδομές βρέθηκαν να είναι συνήθως παραμορφωμένες και διεισδύοντες ενώ όταν η στοιχειομετρία αυτών των μικροδομών απαντούσε τη στοιχειομετρία του συστήματος το μέγεθος τους ακολουθούσε μία ακολουθία μαγικών αριθμών κι ονομάστηκαν ως Υπερσυσσωματώματα. Αποδείχθηκε ότι αυτές οι δομικές μομάδες εμπεριέχουν όλες τις λεπτομέρειες της μικροδομής του εκάστοτε συστήματος και μπορούν να θεωρούνται ως η βασική δομική μονάδα των Μεταλλικών Γυαλιών, αντανακλώντας τη Τάξη Μεσαίας Εμβέλειας. Τελικά, η δημιουργία και επαναδημιουργία των Εικοσαεδρικού τύπου νανοδομών και των Υπερσυσσωματωμάτων βρέθηκε να είναι ο τρόπος με τον οποίο το σύστημα απορροφάει την εξωτερικά εφαρμοζόμενη παραμόρφωση.__________________________Τα αποτελέσματα της παρούσας Διδακτορικής Διατριβής μπορούν να χρησιμοποιηθούν στο σχεδιασμό Μεταλλικών Γυαλιών κατάλληλων για πληθώρα τεχνολογικών εφαρμογών

Semi-Empirical Force-Field Model For The Ti1-XAlXN (0 ≤ x ≤ 1) System

We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti1-xAlxN (0 x 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the models predictions with respect to 0 K equilibrium volumes, elastic constants, cohesive energies, enthalpies of mixing, and point-defect formation energies, for a set of approximate to 40 elemental, binary, and ternary Ti-Al-N structures and configurations. Subsequently, the reliability of the model is thoroughly verified against known finite-temperature thermodynamic and kinetic properties of key binary Ti-N and Al-N phases, as well as properties of Ti1-xAlxN (0 amp;lt; x amp;lt; 1) alloys. The successful outcome of the validation underscores the transferability of our model, opening the way for large-scale molecular dynamics simulations of, e.g., phase evolution, interfacial processes, and mechanical response in Ti-Al-N-based alloys, superlattices, and nanostructures.Funding Agencies|Olle Engkvist foundation; competence center FunMat-II - Vinnova [2016-05156]; Linkoping University ("LiU Career Contract") [LiU-2015-01510]; Swedish research council [VR-2015-04630]; Olle Engkvist foundation [SOEB 190-312]</p

Coalescence dynamics of 3D islands on weakly-interacting substrates

We use kinetic Monte Carlo simulations and analytical modelling to study coalescence of three-dimensional (3D) nanoscale faceted silver island pairs on weakly-interacting fcc(111) substrates, with and without concurrent supply of mobile adatoms from the vapor phase. Our simulations show that for vapor flux arrival rates F &lt; 1 monolayer/second (ML/s) coalescence manifests itself by one of the islands absorbing the other via sidewall facet migration. This process is mediated by nucleation and growth of two-dimensional (2D) layers on the island facets, while the supply of mobile atoms increases the nucleation probability and shortens the time required for coalescence completion. When F is increased above 1 ML/s, coalescence is predominantly governed by deposition from the vapor phase and the island pair reaches a compact shape via agglomeration. The crucial role of facets for the coalescence dynamics is further supported by a mean-field thermodynamic description of the nucleation energetics and kinetics. Our findings explain experimental results which show that two-dimensional film growth morphology on weakly-interacting substrates is promoted when the rate of island coalescence is suppressed. The present study also highlights that deviations of experimentally reported film morphological evolutions in weakly-interacting film/substrate systems from predictions based on the sintering and particle growth theories may be understood in light of the effect of deposition flux atoms on the energetics and kinetics of facet-layer nucleation during coalescence.Funding agencies: Linkoping University ("LiU Career Contract") [Dnr-LiU-2015-01510]; Swedish research councilSwedish Research Council [VR-2015-04630]; Olle Engkvist foundation [SOEB 190-312]; Aforsk foundation [AF 19-137]; Linkoping University</p

Energetic bombardment and defect generation during magnetron-sputter-deposition of metal layers on graphene

In the present work, we elucidate the interplay among energetic bombardment effects in magnetron sputtering and defect generation in two-dimensional (2D) materials. Using deposition of gold (Au) layers on single-layer graphene (SLG) as a model system, we study the effect of pressure-distance (pd) product during magnetron sputtering on the pristine SLG properties. Raman spectroscopy, complemented by X-ray photoelectron spectroscopy, shows that for pd = 8.2 Pa center dot cm, Au layer deposition causes defects in the SLG layer, which gradually diminish and eventually disappear with increasing pd to 82.5 Pa center dot cm. Stochastic and deterministic simulations of the sputtering process, the gas-phase transport, and the interaction of sputtered and plasma species with the substrate surface suggest that defects in SLG primarily emanate from ballistic damage caused by backscattered Ar atoms with energies above 100 eV. With increasing pd, and thereby gas-phase scattering, such high energy Ar species become thermalized and hence incapable of causing atomic displacements in the SLG layer. The overall results of our study suggest that control of backscattered Ar energy is a potential path toward enabling magnetron sputtering for fabrication of multifunctional metal contacts in devices founded upon 2D materials.Peer reviewe

Dynamics of 3D-island growth on weakly-interacting substrates

The growth dynamics of faceted three-dimensional (3D) Ag islands on weakly-interacting substrates are investigated-using kinetic Monte Carlo (kMC) simulations and analytical modelling-with the objective of determining the critical top-layer radius R-c required to nucleate a new island layer as a function of temperature T, at a constant deposition rate. kMC shows that R-c decreases from 17.3 to 6.0 angstrom as T is increased at 25 K intervals, from 300 to 500 K. That is, a higher T promotes top-layer nucleation resulting in an increase in island height-to-radius aspect ratios. This explains experimental observations for film growth on weakly-interacting substrates, which are not consistent with classical homoepitaxial growth theory. In the latter case, higher temperatures yield lower top-layer nucleation rates and lead to a decrease in island aspect ratios. The kMC simulation results are corroborated by an analytical mean field model, in which R-c is estimated by calculating the steady-state adatom density on the island side facets and top layer as a function of T. The overall findings of this study constitute a first step toward developing rigorous theoretical models, which can be used to guide synthesis of metal nanostructures, and layers with controlled shape and morphology, on technologically important substrates, including two-dimensional crystals, for nanoelectronic and catalytic applications.Funding Agencies|Linkoping University ("LiU Career Contract") [Dnr-LiU-2015-01510]; Swedish Research Council [VR-2015-04630, VR2014-5790]; Knut and Alice Wallenberg Foundation [KAW 2011-0094]</p

Structure formation in Ag-X (X = Au, Cu) alloys synthesized far-from-equilibrium

We employ sub-monolayer, pulsed Ag and Au vapor fluxes, along with deterministic growth simulations, and nanoscale probes to study structure formation in miscible Ag-Au films synthesized under far-from-equilibrium conditions. Our results show that nanoscale atomic arrangement is primarily determined by roughness build up at the film growth front, whereby larger roughness leads to increased intermixing between Ag and Au. These findings suggest a different structure formation pathway as compared to the immiscible Ag-Cu system for which the present study, in combination with previously published data, reveals that no significant roughness is developed, and the local atomic structure is predominantly determined by the tendency of Ag and Cu to phase-separate.Funding agencies: Linkoping University via the "LiU Research Fellows Program"; Linkoping University via the "LiU Career Contract" [Dnr-LiU-2015-01510]; Swedish Research Council [VR-2011-5312, VR-2015-04630]</p

Formation and morphological evolution of self-similar 3D nanostructures on weakly interacting substrates

Vapor condensation on weakly interacting substrates leads to the formation of three-dimensional (3D) nanoscale islands (i.e., nanostructures). While it is widely accepted that this process is driven by minimization of the total film/substrate surface and interface energy, current film-growth theory cannot fully explain the atomic-scale mechanisms and pathways by which 3D island formation and morphological evolution occurs. Here, we use kinetic Monte Carlo simulations to describe the dynamic evolution of single-island shapes during deposition of Ag on weakly interacting substrates. The results show that 3D island shapes evolve in a self-similar manner, exhibiting a constant height-to-radius aspect ratio, which is a function of the growth temperature. Furthermore, our results reveal the following chain of atomic-scale events that lead to compact 3D island shapes: 3D nuclei are first formed due to facile adatom ascent at single-layer island steps, followed by the development of sidewall facets bounding the islands, which in turn facilitates upward diffusion from the base to the top of the islands. The limiting atomic process which determines the island height, for a given number of deposited atoms, is the temperature-dependent rate at which adatoms cross from sidewall facets to the island top. The overall findings of this study provide insights into the directed growth of metal nanostructures with controlled shapes on weakly interacting substrates, including two-dimensional crystals, for use in catalytic and nanoelectronic applications.Funding Agencies|Linkoping University [Dnr-LiU-2015-01510]; Swedish Research Council [VR-2011-5312, VR-2015-04630, VR2014-5790]; Knut and AliceWallenberg Foundation [KAW2011-0094]</p