43 research outputs found
On the Relationship between Water Adsorption and Surface Chemistry in Soda-lime Silicate Glasses
No full textUnderstanding how the surface structure affects the bioactivity
and degradation rate of the glass is one of the primary
challenges in developing new bioactive materials. Here, classical
and reactive molecular dynamics simulations are used to
investigate the relationship between local surface chemistry
and local adsorption energies of water on three soda-lime
silicate glasses. The compositions of the glasses, (SiO2)65 x-
(CaO)35(Na2O)x with x=5, 10, and 15, were chosen for their
bioactive properties. Analysis of the glass surface structure,
compared to the bulk structure, showed that the surface is rich
in modifiers and non-bridging oxygen atoms, which were
correlated with local adsorption energies. The reactivity of the
glasses is found to increase with higher Na2O content,
attributed to elevated Na cations and undercoordinated species
at the glass surfaces. The current work provides insights into
the relationship between the surface structure, chemistry, and
properties in these bioactive glasses and offers a step toward
their rational design
Einfluss von Struktur und Topologie auf das Verformungsverhalten und den Bruch von Oxidgläsern
No full textOxide glasses are ubiquitous in daily life. They have proven useful as bioactive materials, optical fibers, flexible substrates and displays, solar modules, and many other applications. This enormous number of current applications is due to the sheer number of possible compositions made by the possibility of combining different elements from the periodic table. Nevertheless, their usage is limited by their brittleness and low resistance to damage due to the lack of large and clear deformation (shearing) mechanisms to dissipate stress. Computer simulations, including atomistic simulations, serve as an important tool to understand and reveal the deformation mechanisms at atomic and nanometric scales,
thus guiding the development and design of glasses with superior properties.
The objective of this thesis are to understand the role of structure, modifiers, and processing on oxide glasses’ deformation and fracture behavior. Large-scale atomistic simulations were performed to reveal these deformation mechanisms at the atomic scale in binary metaphosphate glasses modified with different modifiers, silica, and sodium silicate glasses with varying sodium content. The effect of modifier type on deformation-induced structural anisotropy and deformation behavior was studied in metaphosphate glasses. On the other hand, the effect of the modifier content and topology on the deformation behavior and fracture was investigated in sodium silicate glasses.
In the metaphosphate glasses, the presence of modifiers with high field strength leads to higher mechanical properties when compared to glass compositions with low field strength modifiers. The origin of the structural transient and persistent anisotropy observed in all metaphosphate glasses was shown to be the same, which was discussed at different structural levels. At the short-range structure, it originates from the alignment of P–O bonds. The next structural level involving neighboring tetrahedra is captured by the orientation of the P–P bonds. These short-range alignments of the P–O and P–P bonds lead to the changes in the medium-range order as captured by the orientation of the chain along the tensile axis and orthogonal to the loading axis. The tensile Young’s moduli of the anisotropic glasses obtained by either pre-tension or pre-compression measured along the pre-deformation axis is lower than the pristine glass due to a stretching of the structure as indicated by the remaining plastic strain. The tensile Young’s moduli of the pre-deformed metaphosphate glasses by pre-tension showed a higher Young’s modulus than the glasses pre-deformed in compression when measured in the same direction as the pre-deformation one.
On the other hand, understanding the mechanical behavior of silicate glasses with different compositions and pre-loading modes is highly relevant to many technological applications. Therefore, silicate glasses with different compositions and subjected to different type of deformations at room temperature were studied. The atomic-scale mechanisms of the deformation of these glasses showed that during compression and shear at higher strains, a significant number of atoms that switched bonds was found, which was not the case in tension due to the fracture of the samples. These atoms that have a change in their bonding topology are localized within shear bands in the shear deformation and are homogeneously distributed in samples deformed in compression. The tensile mechanical behavior of the predeformed glasses showed that the pre-deformation decreased the material strength, and an increasing ductility was observed in the case of pre-compression. This originates from a homogeneous persistent change in the bonding with pre-compression, which was not the case on pre-tension and pre-shear. The tensile deformation mechanism of pre-deformed glasses was found to be due to the appearance and coalescence of cavities during the deformation. The presence of these cavities is affected by both composition and pre-deformation, which was also discussed for the first time at the atomic scale. The results presented in this thesis highlight the importance of the local events in controlling the macroscopic glass properties. Thus, providing insights at the atomic scale needed for further development of oxide glasses.Oxidgläser sind in unserem täglichen Leben allgegenwärtig. Sie haben sich als bioaktive Materialien, optische Fasern, flexible Substrate und Displays, Solarmodule und viele andere Anwendungen bewährt. Diese enorme Zahl an aktuellen Anwendungen ist auf die schiere Anzahl an möglichen Zusammensetzungen zurückzuführen, die sich aus der Möglichkeit ergeben, verschiedene Elemente aus dem Periodensystem zu kombinieren. Ihre Verwendung wird jedoch durch ihre Sprödigkeit und geringe Widerstandsfähigkeit gegen Beschädigungen eingeschränkt, da es keine großen und eindeutigen Verformungsmechanismen (Scherung) zum Abbau von Spannungen gibt.
Computersimulationen, einschließlich atomistischer Simulationen, sind ein wichtiges Instrument, um die Verformungsmechanismen auf atomarer und nanometrischer Ebene zu verstehen und aufzudecken und so die Entwicklung und den Entwurf von Gläsern mit besseren Eigenschaften zu unterstützen.
Ziel dieser Doktorarbeit ist es, die Rolle der Struktur, der Modifikatoren und der Verarbeitung auf das Verformungs- und Bruchverhalten von Oxidgläsern zu untersuchen. Es wurden großmaßstäbliche atomistische Simulationen durchgeführt, um diese Verformungsmechanismen auf atomarer Ebene in binären Metaphosphatgläsern, die mit verschiedenen Modifikatoren modifiziert wurden, sowie in Siliziumdioxid und Natriumsilikatgläsern mit unterschiedlichem Natriumgehalt aufzudecken. In Metaphosphatgläsern wurde die Auswirkung der Art des Modifikators auf die verformungsinduzierte strukturelle Anisotropie und das Verformungsverhalten untersucht. Bei Natriumsilikatgläsern hingegen wurde der Einfluss des Modifikatorgehalts und der Topologie auf das Verformungsverhalten und den Bruch untersucht.
In den Metaphosphatgläsern führt das Vorhandensein von Modifikatoren mit hoher Feldstärke zu höheren mechanischen Eigenschaften im Vergleich zu Glaszusammensetzungen mit Modifikatoren mit niedriger Feldstärke. Es wurde gezeigt, dass der Ursprung der in allen Metaphosphatgläsern beobachteten strukturellen transienten und anhaltenden Anisotropie derselbe ist, was auf verschiedenen strukturellen Ebenen diskutiert wurde. Auf der Ebene der Kurzstreckenstruktur ist sie auf die Ausrichtung der P--O-Bindungen zurückzuführen. Die nächste Strukturebene, an der benachbarte Tetraeder beteiligt sind, wird durch die Ausrichtung der P-P-Bindungen bestimmt. Diese kurzreichweitigen Ausrichtungen der P--O- und P--P-Bindungen führen zu Veränderungen in der mittelfristigen Ordnung, die durch die Ausrichtung der Kette entlang der Zugachse und orthogonal zur Belastungsachse erfasst wird. Die Zug-E-Moduln der anisotropen Gläser, die durch Vorspannung oder Vorkompression entlang der Vorverformungsachse gemessen wurden, sind aufgrund der Streckung der Struktur, die durch die verbleibende plastische Dehnung angezeigt wird, niedriger als die des ursprünglichen Glases. Die Zug-E-Moduln der durch Vorspannung vorverformten Metaphosphatgläser wiesen einen höheren Elastizitätsmodul auf als die durch Druck vorverformten Gläser, wenn sie in der gleichen Richtung wie die Vorverformung gemessen wurden.
Andererseits ist das Verständnis des mechanischen Verhaltens von Silikatgläsern mit unterschiedlichen Zusammensetzungen und Vorspannungen für viele technische Anwendungen von großer Bedeutung. Daher wurden Silikatgläser mit unterschiedlichen Zusammensetzungen untersucht, die bei Raumtemperatur verschiedenen Arten von Verformungen ausgesetzt waren. Die Mechanismen der Verformung dieser Gläser auf atomarer Ebene zeigten, dass während der Kompression und der Scherung bei höheren Dehnungen eine signifikante Anzahl von Atomen gefunden wurde, die ihre Bindungen wechselten, was bei der Spannung aufgrund des Bruchs der Proben nicht der Fall war. Diese Atome, die eine Änderung ihrer Bindungstopologie aufweisen, sind bei der Scherverformung innerhalb von Scherbändern lokalisiert und in den unter Druck verformten Proben homogen verteilt. Das mechanische Zugverhalten der vorverformten Gläser zeigte, dass die Vorverformung die Festigkeit des Materials verringerte, während bei der Vorkompression eine zunehmende Duktilität beobachtet wurde. Dies ist auf eine gleichmäßige, anhaltende Veränderung der Bindung bei Vorkompression zurückzuführen, was bei Vorspannung und Vorscherung nicht der Fall war. Es wurde festgestellt, dass der Mechanismus der Zugverformung von vorverformten Gläsern auf das Auftreten und Zusammenwachsen von Hohlräumen während der Verformung zurückzuführen ist. Das Vorhandensein dieser Hohlräume wird sowohl von der Zusammensetzung als auch von der Vorverformung beeinflusst, was zum ersten Mal auch auf atomarer Ebene diskutiert wurde.
Die in dieser Arbeit vorgestellten Ergebnisse unterstreichen die Bedeutung lokaler Ereignisse für die Kontrolle der makroskopischen Glaseigenschaften. Sie liefern somit Erkenntnisse auf atomarer Ebene, die für die weitere Entwicklung von Oxidgläsern erforderlich sind
Atomistic origins of deformation-induced structural anisotropy in metaphosphate glasses and its influence on mechanical properties
No full textGlasses are generally isotropic. However, structural anisotropy can be induced through processing. Here,
molecular dynamics simulations were used to study the deformation behavior of metaphosphate glasses
and the atomistic origins of the deformation-induced structural anisotropy. The anisotropy observed in the
metaphosphate glasses originates from a preferred orientation of the tetrahedral units at both short- and
medium-range, depending on the loading mode. The mechanical behavior of the glasses showed that the
Young’s modulus of the anisotropic glasses is lower than that of pristine glasses. Pre-deformed glass shows a
clear directional dependence with respect to the axis of the pre-deformation. In general, the Young’s moduli of
the pre-deformed glasses are lower than those of pristine glasses. These findings provide insights into the origin
of deformation-induced anisotropy in metaphosphate glasses and its influence on their mechanical properties,
thus providing important insight for the rational design of oxide glasses with tailored material properties
Short-term Euro-Dollar exchange rate forecasting using regression models
No full textIn this project, our goal is to investigate mathematical and statistical models to forecast the short-term exchange rate. Notably, we are considering the 15-minutes time frame Euro-Dollar (EUR/USD) currency pair as the object of the project.
There are several different major currency pair such as EUR/USD USD/JPY, GBP/USD, USD/CHF, AUD/USD, and USD/CAD.
We chose EUR/USD since it is the most traded currency pair in the market; however, most of our work can be applied to other currency pair with some modifications.
In addition, we will also focus on polynomial regression models, which we hypothesized to be a better fit given the non-linear nature of the data
The origin of phase separation in binary aluminosilicate glasses
No full textThe quest for hard and tough transparent oxide glasses is at the core of
glass science and technology. Aluminosilicate glasses exhibiting nanoscale
phase separation emerge as promising candidates for such materials.
Nevertheless, proper control of the phase separation represents a daunting
challenge due to its elusive origins. Here we employ large-scale molecular
dynamics simulations and structural analysis to unravel the underlying
mechanisms of the phase separation in aluminosilicate. The observed phase
separation originates from an arrangement of SiO and AlO polyhedra,
which manifests from the second coordination shell and extends to higher
shells. This specific arrangement is driven by repulsion between the polyhedra,
reaching its maximum at around 50 mol% of AlO. This behavior becomes
pronounced around and below the glass transition temperature. This work sheds
light on the origin of phase separation and provides a route for further
exploration across other compositions to develop glasses with adapted
mechanical performance.Comment: Main + Supp (Under review
The origin of phase separation in binary aluminosilicate glasses
No full textThe quest for hard and tough transparent oxide glasses is at the core of glass science and technology.
Aluminosilicate glasses exhibiting nanoscale phase separation emerge as promising candidates for such
materials. Nevertheless, proper control of the phase separation represents a daunting challenge due to
its elusive origins. Here we employ large-scale molecular dynamics simulations and structural analysis to
unravel the underlying mechanisms of the phase separation in aluminosilicate. The observed phase separation
originates from an arrangement of SiO4 and AlO polyhedra, which manifests from the second coordination
shell and extends to higher shells. This specific arrangement is driven by repulsion between the polyhedra,
reaching its maximum at around 50 mol% of Al2O3
. This behavior becomes pronounced around and below
the glass transition temperature. This work sheds light on the origin of phase separation and provides a route
for further exploration across other compositions to develop glasses with adapted mechanical performance
The effects of normal mixtures and autocorrelation on the fraction non-conforming
No full textIn this article the effects of mixtures of two normal distributions on the fraction nonconforming are studied in the context of capability analysis. When the output from several processes is mixed, the quality characteristic variables of the resulting mix may result in a normal mixture distribution. This can happen in cases such as monitoring an output from several suppliers, several machines, or several workers. This study considered the independence case and autocorrelated processes for a mixture of two normal distributions, using a autoregressive
model of order one, AR(1). It is shown that the true attained process fraction nonconforming (corresponding to specific values for some capability index) can be very different from what is expected when the data are independent normal random variables.Journal ArticleAuthor's accepted manuscrip
A bibliometrics-enhanced, PAGER-Compliant scoping review of the literature on paralympic powerlifting. Insights for practices and future research
No full textParalympic powerlifting (PP), formerly known as “International Paralympic Committee”
(IPC) powerlifting, is the format of powerlifting adapted for athletes with disabilities, and it differs
from the version for able-bodied athletes in that it consists of bench press only. According to the
mandate of the IPC, PP athletes should be enabled to achieve sporting excellence. As such, rigorous
evidence is needed. However, to the best of our knowledge, there exists no systematic assessment
of the body of scholarly evidence in the field of PP. Therefore, the present study was conducted to
fill in this gap of knowledge, by conducting a scoping review of the literature enhanced by a
bibliometrics analysis and by mining two major scholarly databases (MEDLINE via PubMed and
Scopus). The aim was to provide a review/summary of the findings to date to help practitioners and
athletes. Thirty-seven studies were retained in the present study. These covered the following
thematic areas: (i) warm-up strategies (n = 2); (ii) aspects of training (n = 2); (iii) physiological aspects
and responses (n = 2); (iv) psychological aspects and responses (n = 2); (v) biomechanics of bench
press (n = 8); (vi) recovery strategy (n = 5); (vii) impact of the disability and type of disability (n = 4);
(viii) epidemiology of PP (n = 6); and (ix) new analytical/statistical approaches for kinematics
assessments, internal load monitoring, and predictions of mechanical outputs in strength exercises
and in PP (n = 6). Bibliometrics analysis of the PP-related scientific output revealed that, despite
having already become a paralympic sports discipline in 1984, only in the last few years, PP has
been attracting a lot of interest from the community of researchers, with the first scholarly
contribution dating back to 2012, and with more than one-third of the scientific output being
published this year (2022). As such, this scholarly discipline is quite recent and young. Moreover,
the community dealing with this topic is poorly interconnected, with most authors contributing to
just one article, and with one single author being a hub node of the author network. Distributions
of the number of articles and the authors/co-authors were found to be highly asymmetrical,
indicating that this research is still in its infancy and has great room as well as great potential to
grow. Reflecting this, many research topics are also overlooked and underdeveloped, with the
currently available evidence being based on a few studies
Adaptive multiscale stereo images matching based on wavelet transform modulus maxima
No full textIn this paper we propose a multiscale stereo correspondence matching method based on wavelets transform modulus maxima. Exploitation of maxima modulus chains has given us the opportunity to refine the search for corresponding. Based on the wavelet transform we construct maps of modules and phases for different scales, then extracted the maxima and then we build chains of maxima. Points constituents maxima modulus chains will be considered as points of interest in matching processes. The availability of all its multiscale information, allows searching under geometric constraints, for each point of interest in the left image corresponding one of the best points of constituent chains of the right image. The experiment results demonstrate that the number of corresponding has a very clear decrease when the scale increases. In several tests we obtained the uniqueness of the corresponding by browsing through the fine to coarse scales and calculations remain very reasonable.Journal ArticleFinal article publishe
