Dissertations / Theses on the topic 'Supercooled liquids'
To see the other types of publications on this topic, follow the link: Supercooled liquids.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Supercooled liquids.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Sibik, Juraj. "Terahertz spectroscopy of glasses and supercooled liquids." Thesis, University of Cambridge, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708270.
Full text
2
Stevenson, Jacob D. "Cooperative motions in supercooled liquids and glasses." Diss., [La Jolla] : University of California, San Diego, 2009. http://wwwlib.umi.com/cr/ucsd/fullcit?p3355232.
Full textAbstract:
Thesis (Ph. D.)--University of California, San Diego, 2009.Title from first page of PDF file (viewed June 2, 2009). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 78-91).
3
Nielsen, Johannes K. "Fluctuations and linear response in supercooled liquids /." Roskilde : IMFUFA, RUC, 1998. http://hdl.handle.net/1800/451.
Full text
4
Manke, Kara Jean. "Toward ultra-broadband photoacoustic spectroscopy of supercooled liquids." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/98789.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 163-176).
Theoretical understanding of the dramatic slow-down of structural relaxation dynamics in supercooled liquids leading to the glass transition is limited in part by the lack of data in these systems. Between the melting point and the glass transition, the viscosity of supercooled liquids can change by 16 orders of magnitude or more and few experimental techniques can access this broad a range of frequencies and time scales. Photoacoustic spectroscopy is an ideal technique for probing the dynamics of supercooled liquids because it provides direct access to mechanical relaxation. Currently, photoacoustic spectroscopy of supercooled liquids has been carried out in the range of 10 MHz to 1 GHz with impulsive stimulated light scattering (ISS) and 8 GHz to 200 GHz with picosecond ultrasonics. In this work, I present advances in photoacoustic techniques with the goal of extending the range of frequencies accessible by these methods. To achieve higher-frequency acoustic generation, experiments on semiconductor superlattices (SLs) composed of alternating layers of gallium arsenide (GaAs) and aluminum arsenide (AlAs) and multiple quantum well structures (MQW) of indium gallium nitride (InGaN) and gallium nitride (GaN) as photoacoustic transducers are presented. The results demonstrate that InGaN/GaN SLs can be employed as broadband photoacoustic transducers, generating acoustic frequencies up to 2.5 THz. A simple transient reflectivity technique for the detection and reconstruction of short (- 2 ps) acoustic strain pulses is also presented. Using a time-domain analogue of Brillouin light scattering, data on the acoustic velocity and attenuation rate of supercooled liquid DC704 at frequencies of - 6 GHz and - 12 GHz, a region which can be hard to access with both ISS and PU approaches, is shown. Finally, the slow rise or "Mountain mode" component of ISS signal from DC704, which arises from slow components of the density response at timescales from 10- 4 s to 10- 7 s is examined. Comparison with the broadband compliance spectrum of DC704 demonstrates that the slow rise signal does not directly reflect the relaxing elastic compliance, but contains contributions from other slow degrees of freedom that couple to the density.
by Kara Jean Manke.
Ph. D.
5
Hedges, Lester Owen. "Aspects of Dynamic Heterogeneity in Models of Supercooled Liquids." Thesis, University of Nottingham, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491128.
Full textAbstract:
In this thesis we study dynamic heterogeneity in glass-forming systems by means of kinetically constrained models (KC1\Is) and atomistic molecular dynamics simulations. KCr-Is are idealised models of supercooled liquids in which glassy dynamics are the result of local dynamical constraints. By studying a range of models with varying complexity we show that KC1\Is readily capture the key dynamical ingredients of supercooled liquids. Using the framework of r-Ionte Carlo with absorbing r-Iarkov Chains (r.ICAr-IC) we develop an advanced algorithm that can improve on traditional numerical methods by many orders of magnitude for the simulation of a particular KC1\1. \Ve use the two-vacancy assisted triangular lattice gas, or (2)-TLG, to investigate the relationship between structure and dynamics in a supercooled liquid. KCr-Is are used to help interpret recent atomistic simulations that consider supercooled dynamics in terms of transitions between low-energy configurations, or 'metabasins', of the potential energy landscape. Our results imply that dynamic facilitation may be a suitable mechanism for such transitions. We analyse distributions of persistence and exchange times in a computationally efficient atomistic model. For sufficient supercooling we observe a striking de-coupling behveen the two distributions, as previously predicted from KCr.Is. Finally, ,ve study the dynamics of KCr-Is under the influence of an external field, both in the form of confinement and a gravitational field. \Ve compare our results to recent experimental and numerical studies.
6
Bhattacharya, Deepanjan. "Fast Scanning Calorimetry Studies of Supercooled Liquids and Glasses." Thesis, The George Washington University, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3646950.
Full textAbstract:
This dissertation is a compilation of research results of extensive Fast Scanning Calorimetry studies of two non-crystalline materials: Toluene and Water.
Motivation for fundamental studies of non-crystalline phases, a brief overview of glassy materials and concepts and definitions related to them is provided in Chapter 1. Chapter 2 provides fundamentals and details of experimental apparata, experimental protocol and calibration procedure.
Chapter 3 & 4 provides extensive studies of stable non-crystalline toluene films of micrometer and nanometer thicknesses grown by vapor deposition at distinct deposition rates and temperatures and probed by Fast Scanning Calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor -deposited samples of toluene during heating with rates in excess 100,000 K/s follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics correlates with the surface roughness scale of the substrate, which is interpreted as evidence for kinetic anisotropy of the samples. We also show that out-of-equilibrium relaxation kinetics and possibly the enthalpy of vapor-deposited (VD) films of toluene are distinct from those of ordinary supercooled (OS) phase even when the deposition takes place at temperatures above the glass softening (Tg). The implications of these findings for the formation mechanism and structure of vapor deposited stable glasses are discussed.
Chapter 5 and 6 provide detailed Fast Scanning Calorimetry studies of amorphous solid water in bulk and confining geometry (ultrathin films and nano-aggregates). Bulk-like water samples were prepared by vapor-deposition on the surface of a tungsten filament near 140 K where vapor-deposition results in low enthalpy glassy water films. The vapor deposition approach was also used to grow nano-aggregates (2- 20 nm thick) and multiple ultrathin (approximately 50 nm thick) water films alternated with benzene and methanoic films of similar dimensions. When heated from cryogenic temperatures, the ultrathin water films underwent a well manifested glass softening transition at temperatures 20 degrees below the onset of crystallization. The thermograms of nano-aggregates of ASW films show two endotherms at 40 and 10 K below the onset temperatures of crystallization. However, no such transition was observed in bulk-like water samples prior to their crystallization. These results indicate that water in confined geometry demonstrates glass softening dynamics which are dramatically distinct from those of the bulk phase. We attribute these differences to water's interfacial glass transition which occurs at temperatures tens of degrees lower than that in the bulk. Implications of these finding for past studies of glass softening dynamics in various glassy water samples are discussed in chapter 5 and 6.
7
Chakrabarti, Dwaipayan. "Slow Dynamics In Soft Condensed Matter : From Supercooled Liquids To Thermotropic Liquid Crystals." Thesis, Indian Institute of Science, 2006. https://etd.iisc.ac.in/handle/2005/292.
Full textAbstract:
This thesis, which contains fourteen chapters in two parts, presents theoretical and computer simulation studies of dynamics in supercooled liquids and thermotropic liquid crystals. These two apparently diverse physical systems are unified by a startling similarity in their complex slow dynamics. Part I consists of six chapters on supercooled liquids while Part II comprises seven chapters on thermotropic liquid crystals. The fourteenth chapter provides a concluding note.
Part I starts with an introduction to supercooled liquids given in chapter 1. This chapter discusses basic features of supercooled liquids and the glass transition and portrays some of the theoretical frameworks and formalisms that are widely recognized to have contributed to our present understanding.
Chapter 2 introduces a new model of binary mixture in order to study dynamics across the supercooled regime. The system consists of an equimolar mixture of the Lennard-Jones spheres and the Gay-Berne ellipsoids of revolution, and thus one of its components has orientational degrees of freedom (ODOF). A decoupling between trans-lational diffusion and rotational diffusion is found to occur below a temperature where the second rank orientational correlation time starts showing a steady deviation from the Arrhenius temperature behavior. At low temperatures, the optical Kerr effect (OKE) signal derived from the system shows a short-to-intermediate time power law decay with a very weak dependence on temperature, if at all, of the power law exponent as has been observed experimentally. At the lowest temperature investigated, jump motion is found to occur in both the translational and orientational degrees of freedom.
Chapter 3 studies how the binary mixture, introduced in the previous chapter, explores its underlying potential energy landscape. The study reveals correlations between the decoupling phenomena, observed almost universally in supercooled molecular liquids, and the manner of exploration of the energy landscape of the system. A significant deviation from the Debye model of rotational diffusion in the dynamics of ODOF is found to begin at a temperature at which the average inherent structure energy of the system starts falling as the temperature decreases. Further, the coupling between rotational diffusion and translational diffusion breaks down at a still lower temperature, where a change occurs in the temperature dependence of the average inherent structure energy.
Chapters 4-6 describe analytical and numerical approaches to solve kinetic models of glassy dynamics for various observables. The β process is modeled as a thermally activated event in a two-level system and the a process is described as a β relaxation mediated cooperative transition in a double-well. The model resembles a landscape picture, conceived by Stillinger [Science 267, 1935 (1995)], where the a process is assumed to involve a concerted series of the β processes, the latter being identified as elementary relaxations involving transitions between contiguous basins. For suitable choice of parameter values, the model could reproduce many of the experimentally observed features of anomalous heat capacity behavior during a temperature cycle through the glass transition as described in chapter 4. The overshoot of the heat capacity during the heating scan that marks the glass transition is found to be caused by a delayed energy relaxation. Chapter 5 shows that the model can also predict a frequency dependent heat capacity that reflects the two-step relaxation behavior. The high-frequency peak in the heat capacity spectra appears with considerably larger amplitude than the low-frequency peak, the latter being due to the a relaxation. The model, when simplified with a modified description of the a process that involves an irreversible escape from a metabasin, can be solved analytically for the relaxation time. This version of the model captures salient features of the structural relaxation in glassy systems as described in chapter 6.
In Part II, thermotropic liquid crystals are studied in molecular dynamics simulations using primarily the family of the Gay-Berne model systems. To start with, chapter 7 provides a brief introduction to thermotropic liquid crystals, especially from the perspective of the issues discussed in the following chapters. This chapter ends up with a detail description of the family of the Gay-Berne models.
Chapter 8 demonstrates that a model system for calamitic liquid crystal (comprising rod-like molecules) could capture the short-to-intermediate time power law decay in the OKE signal near the isotropic-nematic (I-N) phase transition as observed experimentally. The single-particle second rank orientational time correlation function (OTCF) for the model liquid crystalline system is also found to sustain a power law decay regime in the isotropic phase near the I-N transition. On transit across the I-N phase boundary, two power law decay regimes, separated by a plateau, emerge giving rise to a step-like feature in the single-particle second rank OTCF. When the time evolution of the rotational non-Gaussian parameter is monitored as a diagnostic of spatially heterogeneous dynamics, a dominant peak is found to appear following a shoulder at short times, signaling the growth of pseudonematic domains. These observations are compared with those relevant ones obtained for the supercooled binary mixture, as discussed in chapter 2, in the spirit of the analogy suggested recently by Fayer and coworkers [J. Chem. Phys. 118, 9303 (2003)].
In chapter 9, orientational dynamics across the I-N transition are investigated in a variety of model systems of thermotropic liquid crystals. A model discotic system that consists of disc-like molecules as well as a lattice system have been considered in the quest of a universal short-to-intermediate time power law decay in orientational relaxation, if any. A surprisingly general power law decay at short to intermediate times in orientational relaxation is observed in all these systems. While the power law decay of the OKE signal has been recently observed experimentally in calamitic systems near the I-N phase boundary and in the nematic phase by Fayer and coworkers [J. Chem. Phys. 116, 6339 (2002), J. Phys. Chem. B 109, 6514 (2005)], the prediction for the discotic system can be tested in experiments.
Chapter 10 presents the energy landscape view of phase transitions and slow dynamics in thermotropic liquid crystals by determining the inherent structures of a family of one-component Gay-Berne model systems. This study throws light on the interplay between the orientational order and the translational order in the mesophases the systems exhibit. The onset of the growth of the orientational order in the parent phase is found to induce a translational order, resulting in a smectic-like layer in the underlying inherent structures. The inherent structures, surprisingly, never seem to sustain orientational order alone if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. The Arrhenius temperature dependence of the orientational relaxation time breaks down near the I-N transition and this breakdown is found to occur at a temperature below which the system explores increasingly deeper potential energy minima. There exists a remarkable similarity in the manner of exploration of the potential energy landscape between the Gay-Berne systems studied here and the well known Kob-Andersen binary mixture reported previously [Nature, 393, 554 (1998)].
In search of a dynamical signature of the coupling between orientational order and translational order, anisotropic translational diffusion in the nematic phase has been investigated in the Gay-Berne model systems as described in chapter 11. The translational diffusion coefficient parallel to the director D// is found to first increase and then decrease as the temperature drops through the nematic phase. This reversal occurs where the smectic order parameter of the underlying inherent structures becomes significant for the first time. The non-monotonic temperature behavior of D// can thus be viewed from an energy landscape analysis as a dynamical signature of the coupling between orientational and translational order at the microscopic level. Such a view is likely to form the foundation of a theoretical framework to explain the anisotropic translation diffusion.
Chapter 12 investigates the validity of the Debye model of rotational diffusion near the I-N phase boundary with a molecular dynamics simulation study of a Gay-Berne model system for calamitic liquid crystals. The Debye model is found to break down near the I-N phase transition. The breakdown, unlike the one observed in supercooled molecular liquids where a jump diffusion model is often invoked, is attributed to the growth of orientational pair correlation. A mode-coupling theory analysis is provided in support of the explanation.
Chapter 13 presents a molecular dynamics study of a binary mixture of prolate ellipsoids of revolution with different aspect ratios interacting with each other through a generalized Gay-Berne potential. Such a study allows to investigate directly the aspect ratio dependence of the dynamical behavior.
In the concluding note, chapter 14 starts with a brief summary of the outcome of the thesis and ends up with suggestion of a few relevant problems that may prove worthwhile to be addressed in future.
8
Chakrabarti, Dwaipayan. "Slow Dynamics In Soft Condensed Matter : From Supercooled Liquids To Thermotropic Liquid Crystals." Thesis, Indian Institute of Science, 2006. http://hdl.handle.net/2005/292.
Full textAbstract:
This thesis, which contains fourteen chapters in two parts, presents theoretical and computer simulation studies of dynamics in supercooled liquids and thermotropic liquid crystals. These two apparently diverse physical systems are unified by a startling similarity in their complex slow dynamics. Part I consists of six chapters on supercooled liquids while Part II comprises seven chapters on thermotropic liquid crystals. The fourteenth chapter provides a concluding note.
Part I starts with an introduction to supercooled liquids given in chapter 1. This chapter discusses basic features of supercooled liquids and the glass transition and portrays some of the theoretical frameworks and formalisms that are widely recognized to have contributed to our present understanding.
Chapter 2 introduces a new model of binary mixture in order to study dynamics across the supercooled regime. The system consists of an equimolar mixture of the Lennard-Jones spheres and the Gay-Berne ellipsoids of revolution, and thus one of its components has orientational degrees of freedom (ODOF). A decoupling between trans-lational diffusion and rotational diffusion is found to occur below a temperature where the second rank orientational correlation time starts showing a steady deviation from the Arrhenius temperature behavior. At low temperatures, the optical Kerr effect (OKE) signal derived from the system shows a short-to-intermediate time power law decay with a very weak dependence on temperature, if at all, of the power law exponent as has been observed experimentally. At the lowest temperature investigated, jump motion is found to occur in both the translational and orientational degrees of freedom.
Chapter 3 studies how the binary mixture, introduced in the previous chapter, explores its underlying potential energy landscape. The study reveals correlations between the decoupling phenomena, observed almost universally in supercooled molecular liquids, and the manner of exploration of the energy landscape of the system. A significant deviation from the Debye model of rotational diffusion in the dynamics of ODOF is found to begin at a temperature at which the average inherent structure energy of the system starts falling as the temperature decreases. Further, the coupling between rotational diffusion and translational diffusion breaks down at a still lower temperature, where a change occurs in the temperature dependence of the average inherent structure energy.
Chapters 4-6 describe analytical and numerical approaches to solve kinetic models of glassy dynamics for various observables. The β process is modeled as a thermally activated event in a two-level system and the a process is described as a β relaxation mediated cooperative transition in a double-well. The model resembles a landscape picture, conceived by Stillinger [Science 267, 1935 (1995)], where the a process is assumed to involve a concerted series of the β processes, the latter being identified as elementary relaxations involving transitions between contiguous basins. For suitable choice of parameter values, the model could reproduce many of the experimentally observed features of anomalous heat capacity behavior during a temperature cycle through the glass transition as described in chapter 4. The overshoot of the heat capacity during the heating scan that marks the glass transition is found to be caused by a delayed energy relaxation. Chapter 5 shows that the model can also predict a frequency dependent heat capacity that reflects the two-step relaxation behavior. The high-frequency peak in the heat capacity spectra appears with considerably larger amplitude than the low-frequency peak, the latter being due to the a relaxation. The model, when simplified with a modified description of the a process that involves an irreversible escape from a metabasin, can be solved analytically for the relaxation time. This version of the model captures salient features of the structural relaxation in glassy systems as described in chapter 6.
In Part II, thermotropic liquid crystals are studied in molecular dynamics simulations using primarily the family of the Gay-Berne model systems. To start with, chapter 7 provides a brief introduction to thermotropic liquid crystals, especially from the perspective of the issues discussed in the following chapters. This chapter ends up with a detail description of the family of the Gay-Berne models.
Chapter 8 demonstrates that a model system for calamitic liquid crystal (comprising rod-like molecules) could capture the short-to-intermediate time power law decay in the OKE signal near the isotropic-nematic (I-N) phase transition as observed experimentally. The single-particle second rank orientational time correlation function (OTCF) for the model liquid crystalline system is also found to sustain a power law decay regime in the isotropic phase near the I-N transition. On transit across the I-N phase boundary, two power law decay regimes, separated by a plateau, emerge giving rise to a step-like feature in the single-particle second rank OTCF. When the time evolution of the rotational non-Gaussian parameter is monitored as a diagnostic of spatially heterogeneous dynamics, a dominant peak is found to appear following a shoulder at short times, signaling the growth of pseudonematic domains. These observations are compared with those relevant ones obtained for the supercooled binary mixture, as discussed in chapter 2, in the spirit of the analogy suggested recently by Fayer and coworkers [J. Chem. Phys. 118, 9303 (2003)].
In chapter 9, orientational dynamics across the I-N transition are investigated in a variety of model systems of thermotropic liquid crystals. A model discotic system that consists of disc-like molecules as well as a lattice system have been considered in the quest of a universal short-to-intermediate time power law decay in orientational relaxation, if any. A surprisingly general power law decay at short to intermediate times in orientational relaxation is observed in all these systems. While the power law decay of the OKE signal has been recently observed experimentally in calamitic systems near the I-N phase boundary and in the nematic phase by Fayer and coworkers [J. Chem. Phys. 116, 6339 (2002), J. Phys. Chem. B 109, 6514 (2005)], the prediction for the discotic system can be tested in experiments.
Chapter 10 presents the energy landscape view of phase transitions and slow dynamics in thermotropic liquid crystals by determining the inherent structures of a family of one-component Gay-Berne model systems. This study throws light on the interplay between the orientational order and the translational order in the mesophases the systems exhibit. The onset of the growth of the orientational order in the parent phase is found to induce a translational order, resulting in a smectic-like layer in the underlying inherent structures. The inherent structures, surprisingly, never seem to sustain orientational order alone if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. The Arrhenius temperature dependence of the orientational relaxation time breaks down near the I-N transition and this breakdown is found to occur at a temperature below which the system explores increasingly deeper potential energy minima. There exists a remarkable similarity in the manner of exploration of the potential energy landscape between the Gay-Berne systems studied here and the well known Kob-Andersen binary mixture reported previously [Nature, 393, 554 (1998)].
In search of a dynamical signature of the coupling between orientational order and translational order, anisotropic translational diffusion in the nematic phase has been investigated in the Gay-Berne model systems as described in chapter 11. The translational diffusion coefficient parallel to the director D// is found to first increase and then decrease as the temperature drops through the nematic phase. This reversal occurs where the smectic order parameter of the underlying inherent structures becomes significant for the first time. The non-monotonic temperature behavior of D// can thus be viewed from an energy landscape analysis as a dynamical signature of the coupling between orientational and translational order at the microscopic level. Such a view is likely to form the foundation of a theoretical framework to explain the anisotropic translation diffusion.
Chapter 12 investigates the validity of the Debye model of rotational diffusion near the I-N phase boundary with a molecular dynamics simulation study of a Gay-Berne model system for calamitic liquid crystals. The Debye model is found to break down near the I-N phase transition. The breakdown, unlike the one observed in supercooled molecular liquids where a jump diffusion model is often invoked, is attributed to the growth of orientational pair correlation. A mode-coupling theory analysis is provided in support of the explanation.
Chapter 13 presents a molecular dynamics study of a binary mixture of prolate ellipsoids of revolution with different aspect ratios interacting with each other through a generalized Gay-Berne potential. Such a study allows to investigate directly the aspect ratio dependence of the dynamical behavior.
In the concluding note, chapter 14 starts with a brief summary of the outcome of the thesis and ends up with suggestion of a few relevant problems that may prove worthwhile to be addressed in future.
9
Mizuno, Hideyuki. "Molecular Dynamics Simulation Studies of Dynamical Properties of Supercooled Liquids." 京都大学 (Kyoto University), 2012. http://hdl.handle.net/2433/157540.
Full text
10
Ninarello, Andrea Saverio. "Computer simulations of supercooled liquids near the experimental glass transition." Thesis, Montpellier, 2017. http://www.theses.fr/2017MONTS071/document.
Full textAbstract:
La compréhension du mécanisme de la formation du verre est l'un des importants problèmes ouverts en recherche sur la matière condensée. De nombreuses questions restent sans réponse, en raison d'une énorme augmentation des temps de relaxation pendant le processus de refroidissement qui ne permet pas l'exploration des propriétés d'équilibre des liquides surfondus à très basses températures. Les simulations numériques des liquides surfondus sont actuellement en mesure d'atteindre l'équilibre à des températures comparables à la température du crossover de la théorie de couplages de modes, qui est bien supérieure à la température de transition vitreuse expérimentale. En conséquence, les simulations plus lentes que les expériences pour équilibrer un liquide surfondu par un facteur d'environ huit ordres de grandeur. Les progrès réalisés pour combler cet écart ont été lents et résultent essentiellement d'améliorations de l'architecture des ordinateurs. Dans cette thèse, nous résolvons en partie le problème de la thermalisation à basse température de liquides surfondus dans des simulations numériques. Nous combinons l'utilisation d'un algorithme Monte Carlo, connu sous le nom d'algorithme de swap, avec la conception de nouveaux modèles de formateurs de verre. Nous examinons systématiquement des nombreux systèmes, à la fois des mélanges discrets de particules, ainsi que des systèmes a polydispersité continue. Nous discutons le rôle que la polydispersité et la forme du potentiel entre particules jouent pour éviter la cristallisation et parvenir efficacement à des régimes de température inexplorés. De plus, nous étudions les processus dynamiques à l’œuvre pendant une simulation de swap Monte Carlo. Nous démontrons que, dans certains cas, notre technique permet de produire des configurations équilibrées à des températures inaccessibles même dans des expériences. Dans ce régime de température complètement nouveau, nous examinons plusieurs questions ouvertes concernant la physique de la transition vitreuse. Nous montrons qu'un fluide de sphères dures peut être équilibré jusqu'à la densité critique du jamming, et même au-delà. Nous mesurons l'entropie configurationelle dans un liquide refroidi à très basse température. Nous mettons en évidence une forte dépendance dimensionnelle, qui suggère l'existence d'une transition vitreuse idéale à une température finie en trois dimensions et à son absence en deux dimensions. Nous détectons l'augmentation de l'ordre amorphe quantifié par une longueur statique point-to-set pendant la formation du verre. Nous mesurons les exposants critiques introduits dans la théorie de champ moyen des verres beaucoup plus proche de la température critique prédite dans la théorie. Enfin, nous révélons l'absence de transition géométrique caractérisant le paysage d’énergie potentiel au travers de la température du crossover de la théorie de couplages de modes.Les modèles et les algorithmes développés dans cette thèse déplacent les études des liquides surfoundus vers un territoire entièrement nouveau, en réduisant l'écart entre la théorie et les expériences, ce qui nous amène plus proche de la solution du problème de la transition vitreuseUnderstanding the mechanisms that lead to glass formation is one of the open problems for the condensed matter research. Numerous questions remain unanswered, because the tremendous increase of relaxation times during the cooling process prevents the exploration of equilibrium properties of supercooled liquids at very low temperature. Computer simulations of glass-forming liquids are nowadays able to reach equilibrium at temperatures comparable to the Mode-Coupling crossover temperature, which is well above the experimental glass transition temperature. As a consequence, simulations lag eight orders of magnitude behind experiments in terms of equilibration times. Progress to close this gap has been slow, and stems mostly from hardware improvements.In this thesis we make an important step to close this gap. We combine the use of a Monte Carlo algorithm, known as the swap algorithm, with the design of novel glass-forming models. We systematically test numerous models using both discrete mixtures and polydisperse systems. We discuss the role that polydispersity and particle softness play in avoiding crystallization and in efficiently reaching previously unexplored regimes. We study the dynamical processes taking place during swap Monte Carlo simulations. We demonstrate that in some cases our technique is able to produce thermalized configurations at temperatures inaccessible even by experiments.In this newly accessible regime, we investigate some open questions concerning the glass transition. We show that a hard sphere fluid can be equilibrated at, and even beyond, the jamming packing fraction. We measure the configurational entropy in extremely supercooled liquid, finding a strong dimensional dependence that supports, on the one hand, the existence of an ideal glass transition at a finite temperature in three dimensions and, on the other hand, its absence in two dimensions. We detect the increase of amorphous order quantified through a static point-to-set length throughout the glass formation. We measure the critical exponents introduced in the mean-field theory of glasses much closer to the supposed ideal glass transition. Finally, we reveal the absence of a sharp geometric transition in the potential energy landscape across the Mode-Coupling crossover.The models and the algorithms developed in this thesis shift the computational studies of glass-forming liquids to an entirely new territory, which should help to close the gap between theory and experiments, and get us closer to solve the long-standing problem of the glass transition
11
Elenius, Måns. "Computer Simulations of Simple Liquids with Tetrahedral Local Order : the Supercooled Liquid, Solids and Phase Transitions." Doctoral thesis, Stockholms universitet, Numerisk analys och datalogi (NADA), (tills m KTH), 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-27876.
Full textAbstract:
The understanding of complex condensed matter systems is an area of intense study. In this thesis, some properties of simple liquids with strong preference for tetrahedral local ordering are explored. These liquids are amenable to supercooling, and give complex crystalline structures on eventual crystallisation. All liquids studied are simple, monatomic and are similar to real metallic liquids. The vibrational density of states of a glass created in simulation is calculated. We show a correspondence between the vibrational properties of the crystal and the glass, indicating that the vibrational spectra of crystals can be used to understand the more complex vibrational spectra of the glass of the same substance. The dynamics of supercooled liquids is investigated using a previously not implemented comprehensive measure of structural relaxation. This new measure decays more slowly in the deeply supercooled domain than the commonly used measure. A new atomic model for octagonal quasicrystals is presented. The model is based on findings from a molecular dynamics simulation that resulted in 45˚ twinned β-Mn. A decoration is derived from the β-Mn unit cell and the unit cell of the intermediate structure found at the twinning interface. Extensive simulations are used to explore the phase diagram of a liquid at low densities. The resulting phase diagram shows a spinodal line and a phase coexistence region between a liquid and a crystalline phase ending in a critical point. This contradicts the old conclusion of the Landau theory -- that continuous transitions between liquids and crystals cannot exist The same liquid is explored at higher densities. Upon cooling the liquid performs a first order liquid-liquid phase transition. The low temperature liquid is shown to be strong and to have very good glass forming abilities. This result offers new insights into fragile to strong transitions and suggests the possibility of a good metallic glass former.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: In progress.
12
Elenius, Måns. "Computer simulations of simple liquids with tetrahedral local order the supercooled liquid, solids and phase transitions /." Stockholm : Numerical Analysis and Computer Science (NADA) (together with KTH), Stockholm University, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-27876.
Full textAbstract:
Diss. (sammanfattning) Stockholm : Stockholms universitet, 2009.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: In progress. Härtill 5 uppsatser.
13
Li, Lilong Wu Yue. "Dynamics and structure in metallic supercooled liquids and glasses studied by NMR." Chapel Hill, N.C. : University of North Carolina at Chapel Hill, 2005. http://dc.lib.unc.edu/u?/etd,335.
Full textAbstract:
Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2006.Title from electronic title page (viewed Oct. 10, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Curriculum in Applied and Materials Sciences." Discipline: Applied and Materials Sciences; Department/School: Applied and Materials Sciences.
14
Wu, Jianlan 1976. "Slow dynamics in supercooled liquids : matrix formalism, mode coupling and glass transition." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/28695.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2004.Vita.
Includes bibliographical references (p. 155-169).
In this thesis, slow dynamics of supercooled liquids are investigated in the framework of the mode-coupling theory (MCT). Following the real-time generalized Langevin equation in Newtonian liquids, the dynamic Gaussian factorization scheme leads to mode-coupling (MC) closures. As an alternative approach to the projection operator technique, the matrix formalism based on the complete basis set is developed for studying dynamics in many-particle systems. In a dissipative system, the MC closures have to be obtained from irreducible memory kernels instead of standard memory kernels. The matrix formalism provides a new explanation of this preference by comparing linear and nonlinear relaxation time scales, and generalizes the irreducible memory kernel to higher orders. A simple kinetic spin model, the East model, is used to test the matrix formalism and the mode-coupling closures, where the dynamic Gaussian factorization scheme is replaced by a linear approximation due to the kinetic constraint. Next, slow dynamics in Brownian liquids is studied and generalization of MC closures is derived for both coherent and incoherent intermediate scattering functions. Predictions of nonergodic parameters for these two correlation functions in a hard-sphere colloidal suspension improve as the order of the MC closure increases. New glass-transition phenomena are revealed by applying the standard MC closure to a two-Yukawa colloidal suspension.
by Jianlan Wu.
Ph.D.
15
Gebremichael, Yeshitila. "Spatially heterogeneous dynamics and string-like correlated motion in supercooled liquids and polymers." College Park, Md. : University of Maryland, 2004. http://hdl.handle.net/1903/236.
Full textAbstract:
Thesis (Ph. D.) -- University of Maryland, College Park, 2004.Thesis research directed by: Chemical Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
16
Streu, Kristina. "Structure, Thermodynamics, and Dynamical Properties of Nucleic Acids, Proteins, and Glass-Forming Liquids." Thesis, Boston College, 2016. http://hdl.handle.net/2345/bc-ir:107098.
Full textAbstract:
Thesis advisor: Udayan MohantyThe stabilization of particular conformations of protein and nucleic acid structure is believed to play an important role in many important biological functions. In chapter one, the α -helical conformation and structural stability of single and double stapled all- hydrocarbon cross-linked p53 peptides when bound and unbound to MDM2 are investigated. Our study provides a comprehensive rationalization of the relationship between peptide stapling strategy, the secondary structural stability, and the binding affinity of p53-MDM2 complex. In chapter two, we study counterion-mediated collapse of a strongly charged model polyelectrolyte chain by Group-II divalent metal cations using coarse-grained Brownian dynamics simulations. Polyelectrolyte effects govern the association of counterions with the chain. Large ions are less effective in counterion condensation than small ions. However, upon counterion condensation, the reduction of the backbone charge is independent of size of the metal cations. Above a threshold value of Coulomb strength parameter, counterion release entropy drives the formation of counterion-induced compact states. In chapter three, the nature of surface tension in the random first order theory of supercooled liquid is analyzed within the framework of Landau-Lifshitz fluctuation theory. We show that the surface tension of a droplet satisfies the differential equation 4πr2(dσ)+ 8πrσ(r)− Br1/2 = 0 , where B/ T = 12πkBcv , T is temperature, kB is dr Boltzmann constant, and cv is heat capacity. A consequence is that the slope of the relaxation time at the glass transition temperature, i.e., the fragility index, is expressed as the square of the ratio of heat capacity and configurational entropy of the supercooled liquid. When backbone extended nucleosides are incorporated into a double helix, a unique helical structure is formed. In chapter four, we find that the predicted stability of modified backbone DNA strands in aqueous solution is in good agreement with experimental melting temperature data. The incorporation of extended backbone nucleosides into a duplex results in elongation of the end-to-end chain distance due to the distortion of the B-DNA conformation at the mutated base-pair insertion. We also find that the modified backbone helical twist is approximately 40 degrees, larger than B-DNA helical twist and closer to the twist angle predicted for D-form DNA. The folding of RNA tertiary structure has been described as an equilibrium between partially folded I (intermediate) states, and the fully folded native conformation, or N state. RNA is highly sensitive to the ionic environment due to its negative charge, and tertiary structures tend to be strongly stabilized by Mg2+. There is a need for models capable of describing the ion atmosphere surrounding RNA with quantitative accuracy. In chapter 5, we present a generalized Manning condensation model of RNA electrostatics for studying the Mg2+-induced RNA folding of the 58mer ribosomal fragment
Thesis (PhD) — Boston College, 2016
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Chemistry
17
Gauthier, Laurent. "Exemple d'étude structurale et dynamique en phase liquide et liquide surfondu : quinoléine et lépidine." Université Joseph Fourier (Grenoble), 2000. http://www.theses.fr/2000GRE10151.
Full text
18
Widmer-Cooper, Asaph. "Structure and dynamics in two-dimensional glass-forming alloys." Thesis, The University of Sydney, 2006. http://hdl.handle.net/2123/1320.
Full textAbstract:
The glass-transition traverses continuously from liquid to solid behaviour, yet the role of structure in this large and gradual dynamic transition is poorly understood. This thesis presents a theoretical study of the relationship between structure and dynamics in two-dimensional glass-forming alloys, and provides new tools and real-space insight into the relationship at a microscopic level. The work is divided into two parts. Part I is concerned with the role of structure in the appearance of spatially heterogeneous dynamics in a supercooled glass-forming liquid. The isoconfigurational ensemble method is introduced as a general tool for analysing the effect that a configuration has on the subsequent particle motion, and the dynamic propensity is presented as the aspect of structural relaxation that can be directly related to microscopic variations in the structure. As the temperature is reduced, the spatial distribution of dynamic propensity becomes increasingly heterogeneous. This provides the first direct evidence that the development of spatially heterogeneous dynamics in a fragile glass-former is related to spatial variations in the structure. The individual particle motion also changes from Gaussian to non- Gaussian as the temperature is reduced, i.e. the configuration expresses its character more and more intermittently. The ability of several common measures of structure and a measure of structural ‘looseness’ to predict the spatial distribution of dynamic propensity are then tested. While the local coordination environment, local potential energy, and local free volume show some correlation with propensity, they are unable to predict its spatial variation. Simple coarse-graining does not help either. These results cast doubt on the microscopic basis of theories of the glass transition that are based purely on concepts of free volume or local potential energy. In sharp contrast, a dynamic measure of structural ‘looseness’ - an isoconfigurational single-particle Debye-Waller (DW) factor - is able to predict the spatial distribution of propensity in the supercooled liquid. This provides the first microscopic evidence for previous correlations found between short- and long-time dynamics in supercooled liquids. The spatial distribution of the DW factor changes rapidly in the supercooled liquid and suggests a picture of structural relaxation that is inconsistent with simple defect diffusion. Overall, the work presented in Part I provides a real-space description of the transition from structure-independent to structure-dependent dynamics, that is complementary to the configuration-space description provided by the energy landscape picture of the glass transition. In Part II, an investigation is presented into the effect of varying the interparticle potential on the phase behaviour of the binary soft-disc model. This represents a different approach to studying the role of structure in glass-formation, and suggests many interesting directions for future work. The structural and dynamic properties of six different systems are characterised, and some comparisons are made between them. A wide range of alloy-like structures are formed, including substitutionally ordered crystals, amorphous solids, and multiphase materials. Approximate phase diagrams show that glass-formation generally occurs between competing higher symmetry structures. This work identifies two new glass-forming systems with effective chemical ordering and substantially different short- and medium-range structure compared to the glassformer studied in Part I. These represent ideal candidates for extending the study presented in Part I. There also appears to be a close connection between quasicrystal and glass-formation in 2D via random-tiling like structures. This may help explain the experimental observation that quasicrystals sometimes vitrify on heating. The alignment of asymmetric unit cells is found to be the rate-limiting step in the crystal nucleation and growth of a substitutionally ordered crystal, and another system shows amorphous-crystal coexistence and appears highly stable to complete phase separation. The generality of these results and their implications for theoretical descriptions of the glass transition are also discussed.
19
Widmer-Cooper, Asaph. "Structure and dynamics in two-dimensional glass-forming alloys." Science. School of Chemistry, 2006. http://hdl.handle.net/2123/1320.
Full textAbstract:
Doctor of Philosophy (PhD)The glass-transition traverses continuously from liquid to solid behaviour, yet the role of structure in this large and gradual dynamic transition is poorly understood. This thesis presents a theoretical study of the relationship between structure and dynamics in two-dimensional glass-forming alloys, and provides new tools and real-space insight into the relationship at a microscopic level. The work is divided into two parts. Part I is concerned with the role of structure in the appearance of spatially heterogeneous dynamics in a supercooled glass-forming liquid. The isoconfigurational ensemble method is introduced as a general tool for analysing the effect that a configuration has on the subsequent particle motion, and the dynamic propensity is presented as the aspect of structural relaxation that can be directly related to microscopic variations in the structure. As the temperature is reduced, the spatial distribution of dynamic propensity becomes increasingly heterogeneous. This provides the first direct evidence that the development of spatially heterogeneous dynamics in a fragile glass-former is related to spatial variations in the structure. The individual particle motion also changes from Gaussian to non- Gaussian as the temperature is reduced, i.e. the configuration expresses its character more and more intermittently. The ability of several common measures of structure and a measure of structural ‘looseness’ to predict the spatial distribution of dynamic propensity are then tested. While the local coordination environment, local potential energy, and local free volume show some correlation with propensity, they are unable to predict its spatial variation. Simple coarse-graining does not help either. These results cast doubt on the microscopic basis of theories of the glass transition that are based purely on concepts of free volume or local potential energy. In sharp contrast, a dynamic measure of structural ‘looseness’ - an isoconfigurational single-particle Debye-Waller (DW) factor - is able to predict the spatial distribution of propensity in the supercooled liquid. This provides the first microscopic evidence for previous correlations found between short- and long-time dynamics in supercooled liquids. The spatial distribution of the DW factor changes rapidly in the supercooled liquid and suggests a picture of structural relaxation that is inconsistent with simple defect diffusion. Overall, the work presented in Part I provides a real-space description of the transition from structure-independent to structure-dependent dynamics, that is complementary to the configuration-space description provided by the energy landscape picture of the glass transition. In Part II, an investigation is presented into the effect of varying the interparticle potential on the phase behaviour of the binary soft-disc model. This represents a different approach to studying the role of structure in glass-formation, and suggests many interesting directions for future work. The structural and dynamic properties of six different systems are characterised, and some comparisons are made between them. A wide range of alloy-like structures are formed, including substitutionally ordered crystals, amorphous solids, and multiphase materials. Approximate phase diagrams show that glass-formation generally occurs between competing higher symmetry structures. This work identifies two new glass-forming systems with effective chemical ordering and substantially different short- and medium-range structure compared to the glassformer studied in Part I. These represent ideal candidates for extending the study presented in Part I. There also appears to be a close connection between quasicrystal and glass-formation in 2D via random-tiling like structures. This may help explain the experimental observation that quasicrystals sometimes vitrify on heating. The alignment of asymmetric unit cells is found to be the rate-limiting step in the crystal nucleation and growth of a substitutionally ordered crystal, and another system shows amorphous-crystal coexistence and appears highly stable to complete phase separation. The generality of these results and their implications for theoretical descriptions of the glass transition are also discussed.
20
Coslovich, Daniele. "Connections between structure,dynamics and energy landscape in simple models of glass-forming liquids." Doctoral thesis, Università degli studi di Trieste, 2008. http://hdl.handle.net/10077/2563.
Full textAbstract:
2006/2007The microscopic origin of the glass-transition represents a long-standing open problem in condensed matter physics. Recent theoretical advances and the increasing amount of experimental and simulation data demonstrate the activity of this field of research. In this thesis we address, through molecular dynamics simulations of model glass-forming liquids, a key and yet unsolved issue concerning the description of the glass-transition: the connection between the unusual dynamical properties of glass-formers, their structural properties, and the features of the intermolecular interactions. Toward this end, we consider a broad range of models based on pair interactions. Such models are able to describe both fragile and strong glass-formers and to reproduce different types of local order, including icosahedral and prismatic structures (typical of metallic glasses) as well as tetrahedral ones (typical of network glasses). For these models we provide a systematic characterization of the structure, dynamics, and potential energy surface. The first part of the thesis briefly introduces the theoretical framework concerning the connection between structure and dynamics in fragile and strong glass-formers, as well as the main experimental and simulation results. The state of the art of the description in terms of the potential energy surface is critically reviewed on the basis of recent simulation results. The simulation methods and the optimization algorithms employed in the thesis are then presented, focusing on the stage of object-oriented analysis of the problem of molecular simulations of classical interacting systems. Such analysis constitutes an original aspect of the thesis and provided a unified and effective framework for the development of simulation software. The second part focuses on the main results obtained. The variations of dynamical properties in different systems, with particular reference to the Angell's fragility and to dynamic heterogeneities, are traced back first to the features of the locally preferred structures, then to the properties of the potential energy surface. In particular, we show that the variation of fragility in the models considered can be rationalized in terms of the formation of stable domains formed by locally preferred structures. The analysis of the properties of stationary points (local minima and saddle points) in the potential energy surface allows us to establish a direct connection between fragility, structurally stable domains and energy barriers. On the other hand, the spatial localization features of the unstable modes display qualitative variations in the models considered. The study of the correlation between the spatial localization of the unstable modes and the propensity of motion reveals that the dynamical influence of such modes is typical of the late beta-relaxation - time scale within which the effect of dynamic heterogeneity is maximum. It appears to be easier to identify such connection in fragile, rather than strong, systems. This provides indications on the possible qualitative differences concerning the metabasin structure of the potential energy surface in fragile and strong glass-formers.
XX Ciclo
1980
21
Alvarez, Donado René Alberto 1989. "Estudo de propriedades dinâmicas e termodinâmicas de líquidos formadores de vidros metálicos através de simulações computacionais." [s.n.], 2016. http://repositorio.unicamp.br/jspui/handle/REPOSIP/305733.
Full textAbstract:
Orientador: Alex AntonelliDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
Made available in DSpace on 2018-08-30T22:57:05Z (GMT). No. of bitstreams: 1 AlvarezDonado_ReneAlberto_M.pdf: 4171490 bytes, checksum: e9cef25e59956ed4e6201f408c88c61c (MD5) Previous issue date: 2016
Resumo: Através de simulações de dinâmica molecular (MD) estudou-se o comportamento da viscosidade como função da temperatura para a liga Cu46Zr47Al7 que apresenta uma transição dinâmica frágil - forte. A interação entre as partículas foi modelada pelo potencial Modified Embeddded Atom Method (MEAM). As simulações de dinâmica molecular foram feitas usando as equações de Nosé-Hover e a viscosidade foi calculada pela fórmula de Green-Kubo. Observou-se que para uma temperatura reduzida (Tg/T ) de 0.8, o comportamento da viscosidade muda de frágil para forte. Usando a equação de Vogel-Fulcher-Tammann (VFT) em nossos resultados da simulação, observou-se que os valores da viscosidade calculados acima de 0.8 não são bem descritos por este ajuste, o que pode ser entendido como uma mudança no comportamento da viscosidade depois de atingir essa temperatura. A regressão feita usando a equação do VFT deu um valor limitante inferior para a temperatura de transição vítrea de 650K, o qual é um valor próximo da temperatura de transição vítrea reportada para estas ligas
Abstract: By means of molecular dynamic simulation (MD) we studied the behavior of the viscosity of a Cu46Zr47Al7 alloy, as a function of temperature, which displays a fragile - strong transition. Interactions between particles are modeled using the Modified Embedded Atom Method (MEAM). For the molecular simulations, we used the Nosé-Hoover equations, while the Green-Kubo formula gave us the viscosity. It was observed that, for a reduced temperature (Tg/T ) of 0.8, the behavior of the viscosity changes from fragile to strong. Using the Vogel-Fulcher-Tammann (VFT) equation in our results from the simulation, we noticed that the viscosity values above of 0.8 reaching this temperature. The regression achieved by VFT equation gave us a lowerbound value of 650K for the glass transition, which is very close to the glass transition temperature reported for this kind of alloys
Mestrado
Física
Mestre em Física
1370420/2014
CAPES
22
Cajahuaringa, Macollunco Oscar Samuel 1985. "Estudo das propriedades dinâmicas e estruturais do gálio líquido super-resfriado através de simulações atomísticas." [s.n.], 2011. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277230.
Full textAbstract:
Orientadores: Alex Antonelli, Maurice de KoningDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
Made available in DSpace on 2018-08-18T18:03:40Z (GMT). No. of bitstreams: 1 CajahuaringaMacollunco_OscarSamuel_M.pdf: 6673580 bytes, checksum: a344f89389b318a79fb73ef903615a6b (MD5) Previous issue date: 2011
Resumo: A física dos líquidos super-resfriados é um dos problemas para o qual ainda não existe uma única teoria que tenha capturado com sucesso todas as características destes materiais, principalmente a origem da dinâmica complexa, e também a possível ocorrência de transições líquido-líquido nesse regime. Nosso trabalho está focado nas propriedades dinâmicas do gálio, que é evidenciada pelas funções de correlação temporais antes e depois da transição de fase líquido-líquido. Primeiro, foram feitas simulações atomísticas no gálio através de dinâmica molecular usando o modied embedded atom model (MEAM) e condições periódicas de contorno em uma super-célula contendo 1152 átomos, com o propósito de mostrar a transição de fase líquido-líquido obtida em recentes pesquisas teóricas. Para estudar a dinâmica do sistema como um todo, calculamos o deslocamento quadrático médio, que revela o platô em tempos intermediários, o qual se torna mais notório após a transição líquido-líquido. Esse comportamento pode ser originado por uma dinâmica espacialmente homogênea ou uma dinâmica espacialmente heterogênea. Para saber qual das duas hipóteses é mais relevante foi medido o parâmetro não-gaussiano de ordem 2, porque este nos fornece informação do grau de heterogeneidade dinâmica do sistema, e que mostrou que nosso sistema possui uma dinâmica heterogênea. Posteriormente, foi calculada a auto-função intermediária de espalhamento usando o método da transformada rápida de Fourier que é mas eciente para tempos de correlação longos. Esta função fornece a correlação na densidade de partículas no espaço recíproco, que mostra também um platô em tempos intermediários. Com o fim de analisar como relaxa cada partícula, baseados na ideia da dinâmica espacialmente heterogênea, foi possível separar as partículas pela sua dinâmica em dois grupos, um grupo que possui a dinâmica mais difusiva e outro que possui a dinâmica menos difusiva. Finalmente, foram caracterizados estruturalmente esses dois grupos e, comparando-os com as funções estruturais do sistema todo, concluímos que as duas fases presentes no líquido depois da transição, o líquido de alta densidade possui uma dinâmica mais difusiva e o líquido de baixa densidade possui uma dinâmica menos difusiva. Foi possível com estes resultados identicar espacialmente as duas fases líquidas e observar claramente os seus próprios domínios e que estes não estão misturados
Abstract: The physics of supercooled liquids still lacks a single theory which can successfully capture all features of these materials, mainly the cause for their complex dynamics and the possibility of liquid-liquid phase transitions in this regime. Our work is focused on the dynamics of liquid gallium, which was investigated through the correlation functions before and after the liquid-liquid phase transition. First, atomistic simulations were performed using the modied embedded atom model (MEAM) and periodical boundary conditions in a 1152-atom supercell, in order to obtain the liquid-liquid phase transition observed in recent previous simulations. To study the dynamics of the whole system, we calculate the mean square displacement, which shows the plateau for intermediate times that becomes much more noticeable after the liquidliquid transition. This behavior can be caused by either a spatially homogeneous dynamics or a spatially heterogeneous dynamics. In order to find out which hypothesis is more relevant for our case, the second order non-Gaussian parameter was determined, since it provides the degree of heterogeneity of the dynamics of the system, showing that system exhibits a heterogeneous dynamics. Later, the self-intermediate scattering function was calculated using the fast Fourier transform method, which is more ecient for long correlation times. This function gives the density particle correlation in reciprocal space, showing a plateau in intermediate times as well. In order to analyze how each particle relaxes, based on the idea of a spatially heterogeneous dynamics, it was possible to gather the particles according to their mobility in two groups, one which has a more diffuse dynamics and another which has a less diffuse dynamics. Finally, these two groups were structurally characterized and by comparing them with the structural functions of the whole system, it was possible to determine that the liquid of higher density has a more diffusive dynamics, whereas the lower density liquid has less diffusive dynamics. From these results we were able to spatially identify the two liquid phases, which clearly display their own domains that do not mix with each other
Mestrado
Física da Matéria Condensada
Mestre em Física
23
SOSSO, GABRIELE CESARE. "A neural network potential for the phase change material gete: large scale molecular dynamics simulations with close to ab initio accuracy." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2013. http://hdl.handle.net/10281/40098.
Full textAbstract:
Phase change materials based on chalcogenide alloys are attracting an increasing interest
worldwide due to their ability to undergo reversible and fast transitions between the amorphous
and crystalline phases upon heating. This property is exploited in rewritable
optical media (CD, DVD, Blu-Ray Discs) and electronic nonvolatile memories of new
concept, the Phase Change Memories (PCM). The strong optical and electronic
contrast between the crystal and the amorphous allows discriminating between the two
phases that correspond to the two bits of binary information zero and one. The material
of choice for applications is the ternary compound Ge2Sb2Te5 (GST). However, the
related binary alloy GeTe has also been thoroughly investigated because of its higher
crystallization temperature and better data retention at high temperature with respect
to GST.
PCM devices, born thanks to the work of Ovshinsky in the late 1960s, offer extremely
fast programming, extended cycling endurance, good reliability and inexpensive,
easy integration. A PCM is essentially a resistor of a thin film of the chalcogenide alloy
with a low field resistance that changes by several orders of magnitude across the phase
change. In memory operations, cell read out is performed at low bias. Programming the
memory requires instead a relatively large current to heat up the chalcogenide and induce
the phase change, either the melting of the crystal and subsequent amorphization
(RESET) or the recrystallization of the amorphous (SET).
In the last few years, atomistic simulations based on density functional theory (DFT)
have provided useful insights into the properties of phase change materials. However,
several key issues such as the thermal conductivity at the nanoscale or the origin of
the fast crystallization, just to name a few, are presently beyond the reach of ab initio
simulations due to the high computational cost. In fact, first principles simulations can
deal at most with 10^2 atoms on the timescale of 10^2 ps, while many properties like thermal
conductivity or direct simulations of the crystallization process require at least 10^3 atoms
on the timescale of 10^3 ps.
The development of reliable classical interatomic potentials is a possible route to
overcome the limitations in system size and time scale of ab initio molecular dynamics.
However, traditional approaches based on the fitting of simple functional forms for the
interatomic potentials are very challenging due to the complexity of the chemical bonding in the crystalline and amorphous phases revealed by the ab initio simulations. A possible
solution has been proposed recently by Behler and Parrinello, who developed highdimensional
interatomic potentials with close to ab initio accuracy employing artificial
neural networks (NN).
In this thesis work, we developed a classical interatomic potential for the bulk phases
of GeTe employing this NN technique. The potential was validated by comparing results
on the structural and dynamical properties of liquid, amorphous, and crystalline GeTe
derived from NN-based simulations with the ab initio data obtained here and in previous
works [10]. The NN potential displays an accuracy close to that of the underlying DFT
framework at a much reduced computational load that scales linearly with the size of the
system. It allows us to simulate several thousands of atoms for tens of ns, which is well
beyond present-day capabilities of DFT MD. The development of a reliable interatomic
potential with close to DFT accuracy is thus a breakthrough in the modeling of phase
change materials.
To date, we employed our NNP in order to investigate three issues:
1. the thermal conductivity of the amorphous phase
2. the viscosity and atomic mobility in the supercooled liquid and overheated amorphous
phases
3. the dynamics of homogeneous crystallization of the liquid and amorphous phases
Thermal conductivity is a key property for the PCM operation, as the phase changes
corresponding to the memory writing/erasing processes strongly depend upon heat dissipation
and transport. Moreover, thermal cross-talks among the different bits is a crucial
reliability issue in PCM. Although experimental data on thermal conductivity are available
for few materials in this class it is unclear whether the value measured in the
bulk phase could also describe the behavior of the material in nanoscaled devices (10-20
nm) which might be smaller than the phonon mean free path. This is particularly relevant
for PCM architectures based on nanostructures employing nanowires, colloidal nanoparticles,
thin bridges and nanotubes. In fact, amorphous materials can also display
propagating phonons with mean free path as long as 0.5 μm. This has been demonstrated
for amorphous Si where propagating modes with long mean free path contribute to
half of the total thermal conductivity. It is therefore of great technological relevance to
assess whether similar propagating modes with long mean free path might be present
in amorphous phase change materials as well. Atomistic simulations can provide crucial
insights into the thermal transport properties of phase change materials suitable to aid
a reliable modelling of the device operation. However, the calculation of the thermal conductivity
in an amorphous system requires very long simulations (on the ns scale) of large
models (thousands of atoms) that are presently beyond the reach of fully DFT simulations.
The use of NN potentials allowed us to compute the mean free path of phonons in
a-GeTe and to assess that ballistic transport is inhibited by disorder even at the nanoscale. The key property that makes some chalcogenide alloys suitable for applications in
PCM is the high speed of the transformation which leads to full crystallization on the
time scale of 10-100 ns upon Joule heating. What makes some compounds alloys so special
in this respect and so different from most amorphous semiconductors is, however, still a
matter of debate. The driving force for crystallization of the supercooled liquid is actually
the free energy difference between the crystal and the supercooled liquid. However, the
crystallization is controlled both by the driving force and by the atomic mobility. The
driving force vanishes at melting and increases upon cooling. A large atomic mobility at
high supercooling can thus boost the crystallization speed. These conditions can actually
be met by fragile liquids. In fact, supercooled liquids are classified as fragile or strong
on the basis of the temperature dependence of their viscosity [16]. An ideal strong liquid
shows an Arrhenius behavior of the viscosity η from the melting temperature Tm down
to the glass transition temperature Tg. On the contrary, in a fragile liquid η is very low
down to a crossover temperature T∗, below which a steep rise of the viscosity (and thus
a steep decrease in the mobility) is observed. If T∗ is sufficiently far from the melting
temperature (Tm), high supercooling and large atomic mobility can be met at the same
time. The question is thus whether phase change materials are fragile liquids or not. Due
to the high crystallization speed it is unfortunately not possible to measure η below Tm
experimentally. We have thus addressed this problem by MD simulations and we have
demonstrated that indeed GeTe is a very fragile liquid (fragility index ∼ 100). Moreover
a breakdown of the Stokes-Einstein relation between the viscosity and the diffusion coefficient
is found, which further increase the atomic mobility down to temperatures very
close to Tg. Hysteretic effects in the heating of the amorphous phase above Tg have been
addressed as well.
Finally, we have performed direct simulations of the homogeneous crystallization of
the supercooled liquid and amorphous phases with 4000-atom models and simulation
times of several ns. Although similar simulations have been previously performed by fully
DFT simulations, the limitations in size (about 200 atoms) and time scale (500 ps)
prevented a reliable estimate of the size of the critical nucleus and of the crystallization
speed, which are instead accessible by our large scale simulations.
This thesis is organised as follows: in the introductory chapter 1 I provide essential
information about phase change materials and phase change memories. Chapter 2 is
dedicated to theory and methods, while chapters 3,4,5 and 6 are devoted to the results
on the thermal conductivity of the amorphous phase of GeTe, on the properties of its
supercooled liquid phase and on the homogeneous crystallization from the melt and the
overheated amorphous phase.
24
TRAMONTO, FILIPPO. "MICROSCOPIC STUDIES OF STATIC AND DYNAMIC PROPERTIES IN QUANTUM LIQUIDS AND GASES." Doctoral thesis, Università degli Studi di Milano, 2015. http://hdl.handle.net/2434/260418.
Full textAbstract:
In this thesis I present studies of a number of quantum many-body Bose systems via Quantum Monte Carlo methods.
We investigated the dynamic structure factor of a hard-sphere Bose system simulated at T=0 at different densities, from the dilute to the strongly interacting regimes. By increasing the density we observed the spectrum evolves from the weakly interacting Bogoliubov to a phonon-maxon-roton dispersion, but also the emergence of a broad multi-quasiparticle component. For a system with sphere radius and density corresponding to superfluid 4He at equilibrium, we found good agreement with the spectrum in the roton region.
In another work, a liquid of distinguishable 4He atoms near freezing at T=1 K was studied to compute the equation of state and static density response function. The results of this study have been used to improve the description of the superfluid-to-solid transition within the Density Functional Theory.
Measurements of crystallization kinetics in supercooled liquid p-H2--o-D2 mixtures showed a slowdown with respect to the pure counterparts. In order to contribute to the interpretation of these results we simulated these metastable mixtures. We found differences in the quantum delocalization of the two isotopic molecules, which result in different effective sizes. We characterized also the differences in the local order around the molecules of each species. These results revealed that the observed slowdown is due to purely quantum effects.
Finally, in a QMC study of ion Ar+ doped 4He nanodroplets at T=1 K, we computed density profiles, energies, and investigated local order around the Ar+ ion. We found stable solid structures around the ion composed of three solvation shells having the shape of platonic solids: an icosahedron, a dodecahedron, and, again, an icosahedron, with 12, 20, and 12 4He atoms, going from the inner to the outer shell respectively. These results confirmed the interpretation of experimental measurements of the abundances of Ar+@4He nanodroplets.
25
Włodarczyk, Patryk. "Experimental and theoretical studies on mutarotation in supercooled liquid state." Doctoral thesis, Katowice : Uniwersytet Śląski, 2012. http://hdl.handle.net/20.500.12128/5307.
Full textAbstract:
Carbohydrates are a vast group of biomolecules, which are crucial
for biochemical, life processes. As their chemistry and physics have
been subject of extensive research, understanding their molecular dynamics
in supercooled and glassy region is far from perfect. In a liquid
state, many carbohydrates undergo chemical reactions classified
as tautomerizations, which are the source of their structural diversity.
In the present dissertation mechanism of mutarotation in few
monosaccharides, i.e. D-fructose, D-ribose and L-sorbose was investigated.
In order to study the mechanism and pathways of mutarotation
in supercooled liquid state, the results obtained from dielectric spectroscopy
and results obtained from calculations (density functional
theory) were compared. The dipole moment analysis performed for
D-fructose and D-ribose was used to determine direction of transformations
observed by means of dielectric spectroscopy. It was concluded
that the last stage of consecutive reactions, i.e. formation of
the most stable tautomer (pyranose) from the chain, after quenching
of a melt, is monitored. For the D-fructose and D-ribose, the most
stable is /^-pyranose form, while for L-sorbose the most stable is apyranose.
The mechanism of mutarotation in supercooled liquid state
was studied by comparing activation energies obtained from dielectric
spectroscopy and calculations. The calculations were made for
internal and external proton transfer scenarios in the L-sorbose and
D-fructose. It was found, that experimentally determined activation
energy is higher than that calculated for external proton transfer,
but much lower than the energy calculated for internal proton transfer.
The unimolecular internal proton transfer as well as bimolecular
external proton transfer may occur simultaneously in a supercooled
liquid sample. Moreover, analysis of structural relaxation times and
rate of mutarotation in the D-fructose leads to the conclusion external
proton transfer in the glassy state should be suppressed. In the present
thesis experimental methods other than dielectric spectroscopy proved
to be useful in the kinetics studies. The rate constants derived from
refractive index measurements differ slightly from those obtained by
means of dielectric measurements. An impact of mutarotation on the
hydrogen bonds structure in monosaccharides has been demonstrated
by monitoring changes in secondary mode dynamics in dielectric spectrum.
The change of relaxation time or dielectric strength during mutarotation
has been shown for all monosaccharides under investigation.
It has been concluded that the change of dielectric strength and
relaxation time of the secondary mode may vary depending on the
type of saccharide.
26
Fuentevilla, Daphne. "A scaled parametric equation of state for the liquid-liquid critical point in supercooled water." College Park, Md.: University of Maryland, 2007. http://hdl.handle.net/1903/7398.
Full textAbstract:
Thesis (M.S.) -- University of Maryland, College Park, 2007.Thesis research directed by: Dept. of Chemical and Biomolecular Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
27
Chowdhury, MD Sadrul. "Computational studies of structure and dynamics in amorphous materials." Thesis, The University of Sydney, 2015. http://hdl.handle.net/2123/13328.
Full textAbstract:
This thesis presents a theoretical study of the shear stress relaxation in amorphous silicon network in 3D system and the microscopic origin of shear stress in inherent structure supercooled liquid mixtures in 2D and 3D glass forming alloys. We present the shear stress relaxation in an amorphous silicon network via single bond rotation. To understand the shear stress relaxation we have generated the distribution of shear stress and stress change in inherent structure in both Cartesian and rotated frames. We see the distributions are Gaussian with zero mean and the variance of the stress is weakly dependent on temperature. We have also generated stress autocorrelation function for a range of temperatures and we see shear stress relaxed through the transitions in the inherent structures. We calculate the participation ratio of stress change in the rotated frame. We see almost 20% particles contribute strongly in the small system for stress change. The participation ratio decreases with increasing system sizes. In molecular dynamic simulation of 2D and 3D glass forming mixtures, we generate the plots of distributions of individual particle shear stress in inherent structures to understand the origin of total shear stress. Both distributions appear to have Gaussian with zero mean and demonstrate that the variance of atomic shear stress is very weak dependence on the parent liquid temperatures. To understand how the local shear stress arises in the inherent structures we have analysed local packing of different local compositions in 2D and 3D liquids but we don’t see any significant correlation with shear stress on them. We then analysed shear stress with force network. We have found a strong correlation with maximum shear stress and force fluctuations in the force network. We also found that the shear stress exhibits a long range anisotropic correlation in the inherent structures of both liquids.
28
Liu, Dazhi Ph D. Massachusetts Institute of Technology. "Studies of liquid-liquid phase transition and critical phenomena in supercooled confined water by neutron scattering." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/53260.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2008.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 69-72).
Small angle neutron scattering (SANS) is used to measure the density of water contained in 1-D cylindrical pores of a mesoporous silica material MCM-41-S. By being able to suppress the homogenous nucleation process inside the narrow pore, one can keep water in the liquid state down to at least 160 K. We observe a density minimum at 210±5 K. This is the first experimental evidence of the existence of the density minimum in supercooled water. We show that the results are consistent with the predictions of molecular dynamics simulations of supercooled bulk water. From a combined analysis of SANS data from both H20 and D20 hydrated samples, we determined the absolute value of the density of water in the 1-D confined geometry. We found that the average density of water inside the fully hydrated MCM-41-S is higher than that of the bulk water. Pore size and hydration level dependences of the density are also studied. The temperature derivative of the density shows a pronounced peak signaling the crossing of the Widom line and confirming the existence of a liquid-liquid critical point at an elevated pressure.
by Dazhi Liu.
Ph.D.
29
Stein, Richard S. L. "A simulation study of dynamic heterogeneity in a supercooled lennard-jones liquid /." May be available electronically:, 2007. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
Full text
30
Schulte, Kathrin [Verfasser]. "Modelling of the Initial Ice Growth in a Supercooled Liquid Droplet / Kathrin Schulte." München : Verlag Dr. Hut, 2017. http://d-nb.info/1135596891/34.
Full text
31
Schlesinger, Daniel. "Molecular structure and dynamics of liquid water : Simulations complementing experiments." Doctoral thesis, Stockholms universitet, Fysikum, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-120808.
Full textAbstract:
Water is abundant on earth and in the atmosphere and the most crucial liquid for life as we know it. It has been subject to rather intense research since more than a century and still holds secrets about its molecular structure and dynamics, particularly in the supercooled state, i. e. the metastable liquid below its melting point. This thesis is concerned with different aspects of water and is written from a theoretical perspective. Simulation techniques are used to study structures and processes on the molecular level and to interpret experimental results. The evaporation kinetics of tiny water droplets is investigated in simulations with focus on the cooling process associated with evaporation. The temperature evolution of nanometer-sized droplets evaporating in vacuum is well described by the Knudsen theory of evaporation. The principle of evaporative cooling is used in experiments to rapidly cool water droplets to extremely low temperatures where water transforms into a highly structured low-density liquid in a continuous and accelerated fashion. For water at ambient conditions, a structural standard is established in form of a high precision radial distribution function as a result of x-ray diffraction experiments and simulations. Recent data even reveal intermediate range molecular correlations to distances of up to 17 Å in the bulk liquid. The barium fluoride (111) crystal surface has been suggested to be a template for ice formation because its surface lattice parameter almost coincides with that of the basal plane of hexagonal ice. Instead, water at the interface shows structural signatures of a high-density liquid at ambient and even at supercooled conditions. Inelastic neutron scattering experiments have shown a feature in the vibrational spectra of supercooled confined and protein hydration water which is connected to the so-called Boson peak of amorphous materials. We find a similar feature in simulations of bulk supercooled water and its emergence is associated with the transformation into a low-density liquid upon cooling.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.
32
Lind, Mary Laura Johnson W. L. "Ultrasonic investigation of the elastic properties and liquid fragility of bulk metallic glasses in the supercooled liquid region /." Diss., Pasadena, Calif. : California Institute of Technology, 2008. http://resolver.caltech.edu/CaltechETD:etd-10262007-104725.
Full text
33
Lerbinger, Matthias. "Réarrangements locaux dans les verres modèles : de la micromécanique aux processus de relaxation dans un liquide surfondu." Thesis, Université Paris sciences et lettres, 2020. http://www.theses.fr/2020UPSLS006.
Full textAbstract:
Dans cette thèse, la méthode de la limite d’élasticité locale est appliquée et approfondie pour étudier d’une part les réarrangements atomiques isolés et irréversibles induits par cisaillement et d’autre part la relaxation structurelle dans un liquide formateur de verre, un mélange binaire Lennard-Jones 2D. La méthode permet d’obtenir la réponse mécanique locale d’un état inhérent de manière directe et non-perturbatrive tout en contrôlant les échelles de longueur et les directions de chargement. Dans la première partie, l’accent est mis sur une petite inclusion du verre. Sa réponse micromécanique est sondée dans la limite athermique quasi-statique. L’influence de l’échelle de longueur, sur laquelle la réponse mécanique est sondée, est discutée. La variation des statistiques de seuil en fonction de la taille de la zone de sondage peut être comprise sur la base d’un argument géométrique simple et d’une hypothèse de maillon faible. Ensuite, en déterminant la limite d’élasticité critique locale avec une résolution angulaire élevée sur la direction de chargement, on observe que seul un nombre fini et discret de réarrangements de cisaillement est accessible, chacun d’eux se caractérisant par un plan faible distinct. De plus, la limite d’élasticité critique montre une grande sensibilité à la pression. On constate que pour l’échelle de longueur étudiée, le critère d’élasticité de Mohr-Coulomb décrit par morceaux la contrainte de cisaillement critique avec précision. Dans la deuxième partie, un lien fort entre la structure et la dynamique des liquides surfondus est établi. La nouveauté du présent travail est la caractérisation de la structure par des seuils de glissement locaux. Une forte corrélation est trouvée entre d’une part les barrières de contrainte dans la direction la plus faible calculée dans l’état inhérent et d’autre part les observables associées à la relaxation de la structure à l’état liquide. Comme attendu, un coefficient de corrélation plus élevé est constaté pour les liquides équilibrés à des températures plus basses, signe que le paysage d’énergie potentielle influence davantage la dynamiqueIn this thesis, the local yield stress method is applied and extended to study single, irreversible atomistic rearrangementsas well as structural relaxation in a model glass-forming liquid, a two dimensional binary Lennard-Jones mixture. Themethod gives access to the local mechanical response of an inherent configuration in a direct and non-perturbativemanner while controlling the length scales and loading directions. In the first part, the focus is on a small inclusion ofthe glass. Its micromechanical response is probed in the athermal quasi-static limit. The influence of the length scale,at which the mechanical response is probed, is discussed. The variation of the threshold statistics with the size of theprobing zone can be understood on the basis of a simple geometric argument and a weakest link assumption. Then, upondetermining the dependence of the local critical yield stress on the shear loading direction with a high angular resolution,it is observed that only a finite and discrete number of shear rearrangements is accessible, each of them having a distinctweak plane. Furthermore, the critical yield stress shows a high sensitivity towards the pressure in the simulation box. It isfound that for the length scale studied, a Mohr-Coulomb yield criterion describes piecewise accurately the critical shearstress. In the second part, a connection between structure and dynamics of model supercooled liquids is established. Thenovelty in the present work is the characterization of the structure through local slip thresholds. A strong correlation isfound between the stress barriers in the softest direction calculated in the as-quenched state and observables associatedto the relaxation of the liquid structure at parent temperature. As expected, a higher correlation coefficient is detected forliquids equilibrated at lower temperatures, as the potential energy landscape increasingly influences the dynamics
34
Rajonson, Gabriel. "Etude et comparaison de moteurs moléculaires artificiels par simulation numérique." Thesis, Angers, 2019. http://www.theses.fr/2019ANGE0029.
Full textAbstract:
Le domaine nanométrique présente et soulève beaucoup de questions. Une d’entre elles concerne les moteurs moléculaires. Est-il possible de concevoir un système pouvant se déplacer au sein d’un milieu ? Si oui comment caractériser son déplacement ? Le déplacement a-t-il une influence sur le milieu ? Le milieu influence-t-il le déplacement du moteur ? Nous nous sommes intéressés ici à l’étude de plusieurs paramètres tels que la taille relative du moteur par rapport à la taille des molécules constituant l’environnement, la masse relative du moteur par rapport à la masse des molécules constituant l’environnement, la vitesse des battements du moteur, l’influence de la température,l’orientation du moteur, les configurations temporelles possibles.Nous avons montré que le ratio de masse relative au moteur et des molécules du milieu, tout comme le ratio des tailles, influencent le déplacement du moteur. Tout moteur avec une seule charnière est soumis au scallop theorem énoncé par E.M. Purcell.Nous avons trouvé un moyen de briser ce théorème en jouant sur la vitesse des battements du moteur.Nous avons ensuite modifié notre moteur pour qu’il possède deux charnières. Nous avons de nouveau étudié l’effet des différentes configurations temporelles possible sur le déplacement du moteur.Pour finir nous avons voulu donner une direction privilégié à notre moteur en le soumettant à un champ électriqueThe nanometric field raises a lot of questions. One of them concerns molecular motors. Is it possible to design a device capable of moving itself inside an environment ? If so how can we characterise its motion ? Does it influence the environment ? Does the environment influence the motor’s motion ? We took an interest here to the study of several parameters such as the ratio of the motor’s relative length to the environment’s molecule length, the ratio of the motor’s relative mass to the environment’s molecule mass, the motor’s folding frequency, thetemperature’s influence, the motor’s orientation, the different time configurations. We showed that the motor’s relative length ratio to the environment’s molecule length, as well as the motor’s relative mass ratio to the environment’s molecule mass, have an influence on the motor’s displacement. Every motor presenting one hinge, is subject to the scallop theorem introduced by E.M. Purcell. We found a way to break this theorem by playing on the motor’s folding frequency. Then we altered our motor in order for it to possess to hinges. We studied the effect of different time configuration on the motor’s displacement. Finally we wanted to give a privileged direction to our motor by submitting it to an electrical field
35
Vest, Julien-Piera. "Dynamique vitreuse sur la sphère S2." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066670/document.
Full textAbstract:
Nous nous sommes intéressés à la description de la dynamique d'un liquide surfondu en étudiant un modèle qui repose sur un ingrédient simple. En partant d'un système de Lennard-Jones monodisperse dans le plan euclidien, nous avons ajouté de la frustration en courbant le plan de sorte à former une sphère de rayon arbitraire. A l'aide d'un algorithme de dynamique moléculaire sphérique, nous avons montré que ce système présentait bien une dynamique vitreuse d'équilibre, caractérisée par la fonction de diffusion intermédiaire incohérente $F_s(k,t)$, qui ralentit fortement et change de comportement à basse température, pour une faible variation de la statique. Le système se comporte comme un verre fort pour les courbures les plus grandes, mais sa fragilité augmente lorsque la courbure diminue. L'allure de $F_s(k,t)$ est également modifiée quand la courbure diminue, ce que nous avons essayé d'expliquer par l'étude de la théorie de couplages de modes (MCT) sur la sphère. Nous avons dérivé l'équation dynamique de MCT sphérique puis étudié la limite aux temps longs de sa solution. On obtient une transition dynamique qui est similaire à celle de la MCT euclidienne, ce qui ne permet pas d'expliquer l'effet de courbure sur $F_s(k,t)$, bien que celle-ci ait une influence sur la valeur de la température de transition. Enfin, nous nous sommes intéressés au rôle des "défauts", dont un nombre minimal de $12$ est imposé par la topologie. A basse température, les défauts tendent à se réunir en structures linéaires, ce qui est prévu théoriquement et observé dans certaines expériences. Les défauts ont une contribution importante à la relaxation, sans pour autant que l'influence des autres particules ne soit négligeableWe are interested in the description of the dynamics of a supercooled liquid through the study of a model which relies on a simple geometrical ingredient. Starting from a monodisperse Lennard-Jones system on the euclidean plane, we add frustration by curving the space to form a sphere of arbitrary radius. Using a molecular dynamics algorithm, we showed that this system indeed behaves like a glassy liquid at equilibrium. The dynamics, caracterized by the self-intermediate scattering function $F_s(k,t)$, slows down strongly and changes shape at low temperature, for a small variation of the statics. The system behaves like a strong glass for high curvatures, but its fragility increases when the curvature decreases. The shape of $F_s(k,t)$ is also modified when the curvature decreases, which we tried to explain theoretically through the study of the mode coupling theory (MCT) on the sphere. We derived the dynamical equation of spherical MCT and studied the long time limit of its solution. We predict a dynamic transition similar to the one predicted by euclidean MCT, which does not allow us to explain the effect of curvature on $F_s(k,t)$, though the curvature has an influence on the value of the transition temperature. Finally, we studied the role of "defects", among which a minimal number of $12$ is imposed by topology. At low temperature, the defects tend to form linear structures, as predicted theoretically and observed in some experiments. The defects have a strong contribution in the relaxation; however, the role of other particles is not negligible
36
Albert, Samuel. "Un point critique thermodynamique dans les verres dévoilé par les réponses d'ordre élevé." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS534/document.
Full textAbstract:
L'énigme de la transition vitreuse réside en grande partie dans le fait que lorsqu'un matériau entre dans l'état dit surfondu, sa dynamique ralentit de façon spectaculaire, donnant l'impression d'une transition vers un état solide, sans que pour autant on ne parvienne à constater de transition thermodynamique par les expériences usuelles.Autrement dit, on ne parvient pas à expliquer le ralentissement spectaculaire de la dynamique par la croissance d'une longueur mesurable expérimentalement.Ceci conduit à la prolifération de théories souvent contradictoires quant à l'origine de la dynamique vitreuse.Durant la dernière décennie une piste prometteuse de mise en évidence d'une telle longueur a été proposée : il s'agit de la mesure des réponses diélectriques non linéaires d'ordre 3 du matériau ainsi que de leur évolution en température. En effet, les réponses non linéaires reflètent les effets collectifs caractéristiques de l'ordre amorphe, qui ne se traduisent que dans les fonctions de corrélations d'ordre élevé.Durant cette thèse nous avons construit sur ces bases expérimentales et théoriques, une expérience de mesure des réponses non linéaires d'ordre 5. En exploitant ces résultats sur la réponse diélectrique d'ordre 5 et des résultats précédents sur la réponse d'ordre 3, nous sommes parvenus, en collaboration avec une équipe d'expérimentateurs et de théoriciens, à fournir des indices forts de l'existence d'un point critique thermodynamique dans le Glycérol et le Propylène Carbonate. Ceci constitue une avancée significative dans la compréhension des matériaux vitreux.En particulier, cette découverte permet de poser des contraintes fortes sur les théories existantes et contribue à clore certains débats théoriques ayant eu cours sur plusieurs décenniesThe puzzle of the glass transition mainly resides in the fact that a supercooled liquid undergoes when cooled down, a spectacular dynamics slow down, while no evidence of any kind of thermodynamic transition has been measured through usual means.The absence of any known growth of a length scale that could explain the glassy dynamics leads to a wide range of competing models and theories trying to explain the origins of this dynamics.In the last decade, a promising lead has been put forward, that could allow the community to experimentally access such a growing length scale, through third order non-linear dielectric response measurements, and more in particular this response's temperature dependence. Indeed, non-linear response measurements reflect the collective effects that characterize the amorphous order and translate into high order correlation functions.During this PhD, we have built upon this experimental and theoretical background to design a fifth order non-linear dielectric response measurement experiment. In collaboration with a team of experimentalists and theoreticians, we have used these results in conjunction with third order response measurement results to make a very strong case advocating the existence of a thermodynamic critical point in Glycerol and Propylene Carbonate. This is a very significant advance in the understanding of the behaviour of glassy materials.This ground breaking discovery puts very strong constraints on existing theories and will contribute to end some decades-long theoretical debates within the glassy community
37
Wikfeldt, Kjartan Thor. "Structure, Dynamics and Thermodynamics of Liquid Water : Insights from Molecular Simulations." Doctoral thesis, Stockholms universitet, Fysikum, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-56711.
Full textAbstract:
Water is a complex liquid with many unusual properties. Our understanding of its physical, chemical and biological properties is greatly advanced after a century of dedicated research but there are still many unresolved questions. If answered, they could have important long-term consequences for practical applications ranging from drug design to water purification. This thesis presents results on the structure, dynamics and thermodynamics of liquid water. The focus is on theoretical simulations applied to interpret experimental data from mainly x-ray and neutron scattering and spectroscopy techniques. The structural sensitivity of x-ray and neutron diffraction is investigated using reverse Monte Carlo simulations and information on the pair-correlation functions of water is derived. A new method for structure modeling of computationally demanding data sets is presented and used to resolve an inconsistency between experimental extended x-ray absorption fine-structure and diffraction data regarding oxygen-oxygen pair-correlations. Small-angle x-ray scattering data are modeled using large-scale classical molecular dynamics simulations, and the observed enhanced scattering at supercooled temperatures is connected to the presence of a Widom line emanating from a liquid-liquid critical point in the deeply supercooled high pressure regime. An investigation of inherent structures reveals an underlying structural bimodality in the simulations connected to disordered high-density and ordered low-density molecules, providing a clearer interpretation of experimental small-angle scattering data. Dynamical anomalies in supercooled water observed in inelastic neutron scattering experiments, manifested by low-frequency collective excitations resembling a boson peak, are investigated and found to be connected to the thermodynamically defined Widom line. Finally, x-ray absorption spectra are calculated for simulated water structures using density functional theory. An approximation of intra-molecular zero-point vibrational effects is found to significantly improve the relative spectral intensities but a structural investigation indicates that the classical simulations underestimate the amount of broken hydrogen bonds.Vatten är en komplex vätska med flera ovanliga egenskaper. Vår förståelse av dess fysiska, kemiska och biologiska egenskaper har utvecklats mycket sedan systematiska vetenskapliga studier började genomföras för mer än ett sekel sedan, men många viktiga frågor är fortfarande obesvarade. En ökad förståelse skulle på sikt kunna leda till framsteg inom viktiga områden så som medicinutveckling och vattenrening. Denna avhandling presenterar resultat kring vattnets struktur, dynamik och termodynamik. Fokusen ligger på teoretiska simuleringar som använts för att tolka experimentella data från huvudsakligen röntgen- och neutronspridning samt spektroskopier. Den strukturella känsligheten i röntgen- och neutrondiffraktionsdata undersöks via reverse Monte Carlo metoden och information om de partiella parkorrelationsfunktionerna erhålls. En ny metod för strukturmodellering av beräkningsintensiva data presenteras och används för att lösa en motsägelse mellan experimentell diffraktion och EXAFS angående syre- syre parkorrelationsfunktionen. Data från röntgensmåvinkelspridning modelleras med storskaliga klassiska molekyldynamiksimuleringar, och den observerade förhöjda småvinkelspridningen vid underkylda temperaturer kopplas till existensen av en Widomlinje härrörande från en vätske- vätske kritisk punkt i det djupt underkylda området vid höga tryck. En undersökning av inherenta strukturer i simuleringarna påvisar en underliggande strukturell bimodalitet mellan molekyler i oordnade högdensitetsregioner respektive ordnade lågdensitetsregioner, vilket ger en tydligare tolkning av den experimentella småvinkelspridningen. Dynamiska anomalier i underkylt vatten som har observerats i inelastisk neutronspridning, speciellt förekomsten av lågfrekventa excitationer som liknar en bosontopp, undersöks och kopplas till den termodynamiskt definierade Widomlinjen. Slutligen presenteras densitetsfunktionalberäkningar av röntgenabsorptionsspektra för simulerade vattenstrukturer. En approximation av intramolekylära nollpunktsvibrationseffekter förbättrar relativa intensiteteri spektrumen avsevärt, men en strukturanalys visar att klassiska simuleringar av vatten underskattar andelen brutna vätebindningar.
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 6: Submitted. Paper 7: Submitted. Paper 8: Manuscript. Paper 9: Submitted.
38
Hechler, Simon [Verfasser], and Ralf [Akademischer Betreuer] Busch. "On the thermophysical and structural properties of the Au49Cu26.9Si16.3Ag5.5Pd2.3 bulk metallic glass-forming alloy and their connection to the liquid-liquid transition in the deeply supercooled liquid / Simon Hechler ; Betreuer: Ralf Busch." Saarbrücken : Saarländische Universitäts- und Landesbibliothek, 2018. http://d-nb.info/1173703152/34.
Full text
39
Kempa, Thomas. "Towards Implementation of Metal Nanoclusters as Luminescent Probes for Detection of Single-Particle Dynamics: "Watching Nanoscale Dynamics Unfold"." Thesis, Boston College, 2004. http://hdl.handle.net/2345/442.
Full textAbstract:
Thesis advisor: John T. FourkasOne can extract a tremendous amount of information about the organizational and dynamic states of molecules, in situ and in real-time, through highly sensitive and noninvasive single particle optical probing. The highly efficient, multi-photon excited luminescence from stabilized metal nanoclusters renders these species useful as optical probes that can be used in detecting single particle and molecular dynamics. We generate stable, and monodisperse samples of Ag nanoclusters as small as 1 nm in diameter, and find that through substitution of various stabilizer molecules we can precisely tune the size of the clusters over a 1-6 nm range of diameters, ensuring monodispersity and stability at every stage. These clusters also exhibit highly efficient, polarized luminescence upon two photon excitation at 800 nm and remain highly photostable, not exhibiting the deleterious blinking that occurs with many single-molecule fluorophores. In order to demonstrate the utility of these clusters as single-molecule probes, we track their emission polarization over long periods in deeply supercooled liquids such as 4'(octahydro-4,7-methano-5H-inden-5-yliden) bisphenol dimethyl ether (ODE). Our results suggest that these clusters can detect nanoscale dynamics with high sensitivity
Thesis (BS) — Boston College, 2004
Submitted to: Boston College. College of Arts and Sciences
Discipline: Chemistry
Discipline: College Honors Program
40
Niblett, Samuel Peter. "Higher order structure in the energy landscapes of model glass formers." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/277582.
Full textAbstract:
The study of supercooled liquids and glasses remains one of the most divisive and divided fields in modern physics. Despite a vast amount of effort and research time invested in this topic, the answers to many central questions remain disputed and incomplete. However, the link between the behaviour of supercooled liquids and their energy landscapes is well established and widely accepted. Understanding this link would be a key step towards resolving many of the mysteries and controversies surrounding the glass transition. Therefore the study of glassy energy landscapes is an important area of research. In this thesis, I report some of the most detailed computational studies of glassy potential energy landscapes ever performed. Using geometry optimisation techniques, I have sampled the local minima and saddle points of the landscapes for several supercooled liquids to analyse their dynamics and thermodynamics. Some of my analysis follows previous work on the binary Lennard-Jones fluid (BLJ), a model atomic liquid. BLJ is a fragile glass former, meaning that its transport coefficients have super-Arrhenius temperature dependence, rather than the more usual Arrhenius behaviour exhibited by strong liquids. The difference in behaviour between these two classes of liquid has previously been attributed to differing degrees of structure in the relevant energy landscapes. I have studied models for both fragile and strong glass formers: the molecular liquid ortho-terphenyl (OTP) and viscous silica (SiO$_{2}$) respectively. My results for OTP agree closely with trends observed for BLJ, suggesting that the same diffusion mechanism is applicable to fragile molecular liquids as well as to atomic. However, the dynamics and energy landscape of OTP are made complicated by the molecular orientational degrees of freedom, making the analysis more challenging for this system. Dynamics of BLJ, OTP and silica are all dominated by cage-breaking events: structural rearrangements in which atoms change their nearest neighbours. I propose a robust and general method to identify cage breaks for small rigid molecules, and compare some properties of cage breaks between strong and fragile systems. The energy landscapes of BLJ and OTP both display hierarchical ordering of potential energy minima into metabasins. These metabasins can be detected by the cage-breaking method. It has previously been suggested that metabasins are responsible for super-Arrhenius behaviour, and are absent from the landscapes of strong liquids such as SiO2. My results indicate that metabasins are present on the silica landscape, but that they each contain fewer minima than metabasins in BLJ or OTP. Metabasins are associated with anticorrelated particle motion, mediated by reversed transitions between minima of the potential energy landscape. I show that accounting for time-correlation of particle displacement vectors is essential to describe super-Arrhenius behaviour in BLJ and OTP, but also required to reproduce strong behaviour in silica. I hypothesise that the difference between strong and fragile liquids arises from a longer correlation timescale in the latter case, and I suggest a number of ways in which this proposition could be tested. I have investigated the effect on the landscape of freezing the positions of some particles in a BLJ fluid. This “pinning” procedure induces a dynamical crossover that has been described as an equilibrium “pinning transition”, related to the hypothetical ideal glass transition. I show that the pinning transition is related to (and probably caused by) a dramatic change in the potential energy landscape. Pinning a large fraction of the particles in a supercooled liquid causes its energy landscape to acquire global structure and hence structure-seeking behaviour, very different from the landscape of a typical supercooled liquid. I provide a detailed description of this change in structure, and investigate the mechanism underlying it. I introduce a new algorithm for identifying hierarchical organisation of a landsape, which uses concepts related to the pinning transition but is applicable to unpinned liquids as well. This definition is complementary to metabasins, but the two methods often identify the same higher-order structures. The new “packings” algorithm offers a route to test thermodynamic theories of the glass transition in the context of the potential energy landscape. Over the course of this thesis, I discuss several different terms and methods to identify higher-order structures in the landscapes of model glass formers, and investigate how this organisation varies between different systems. Although little variation is immediately apparent between most glassy landscapes, deeper analysis reveals a surprising diversity, which has important implications for dynamical behaviour in the vicinity of the glass transition.
41
Aubry, Clémantyne. "Multiplatform radar-lidar synergistic retrieval for liquid and mixed-phase clouds." Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPASJ008.
Full textAbstract:
Les nuages jouent un rôle important dans le cycle de l'eau et le bilan radiatif de la Terre, et tendent à légèrement refroidir le climat. Cependant, de nombreuses incertitudes demeurent concernant leurs rétroactions et leur évolution dans le contexte du réchauffement climatique. Les nuages de phase mixte représentent notamment une part significative de l'effet radiatif des nuages. Ils sont constitués d'un mélange de cristaux de glace, de gouttelettes d'eau surfondues et de vapeur d'eau. Cette coexistence implique des processus complexes et la fraction de liquide et de glace affecte de manière significative leurs propriétés radiatives. Cette complexité les rend difficiles à représenter dans les modèles numériques, introduisant des biais significatifs. Il est donc crucial de mieux comprendre les processus microphysiques de ces nuages pour réduire les incertitudes des prévisions climatiques et météorologiques.Pour observer les nuages, il existe plusieurs types d'instruments, tels que les sondes in situ (directement au contact des hydrométéores) et les instruments de télédétection (observations distantes). Les radars et les lidars nous permettent d'obtenir des informations résolues en distance et peuvent être embarqués à bord d'avions ou de satellites, offrant ainsi couvertures régionale et globale. Les radars nuages travaillent à des fréquences (35 et 95GHz) auxquelles la réflectivité est sensible à la taille des particules, impliquant une réflectivité plus élevée pour les grosses particules nuageuses (les cristaux de glace) que pour les petites particules (les gouttelettes d'eau). Les lidars, quant à eux, fonctionnent habituellement entre 355 et 1064 nm et sont globalement plus sensibles à la concentration des particules. Ainsi, la rétrodiffusion lidar est plus élevée pour les particules très concentrées, telles que les gouttelettes d'eau. Leur synergie permet de tirer avantage des forces et des faiblesses de chacun pour restituer les propriétés des nuages. Cependant, ces propriétés ne sont pas directement accessibles à partir des mesures et des algorithmes de restitution sont donc utilisés pour relier les mesures aux propriétés microphysiques.Cette thèse propose une nouvelle méthode synergique radar-lidar dédiée à la restitution des propriétés des nuages d'eau surfondus, de glace et de phase mixte. Sur la base d'une méthode existante mais dédiée uniquement aux nuages de glace, une nouvelle approche permettant d'inclure à la fois l'eau surfondue et les situations de phase mixte a été développée. La première étape a été d'adapter et d'améliorer la classification servant à identifier la nature des particules observées. Ensuite, de nombreuses adaptations ont été apportées à l'algorithme afin de restituer séparément les propriétés des cristaux de glace et de l'eau surfondue. Cette approche est basée sur les sensibilités différentes du radar et du lidar vis-à-vis des deux types d'hydrométéores : les cristaux de glace dominent le signal radar tandis que l'eau surfondue domine le signal lidar.Afin d'évaluer cette nouvelle méthode, les restitutions sont comparées à des mesures in situ, provenant d'observations colocalisées et de la littérature. La première étude compare les restitutions obtenues à partir des données satellites CloudSat-CALIPSO avec des mesures in situ aéroportées colocalisées. Cette étude montre que les restitutions radar-lidar suivent les mêmes tendances que les mesures in situ et fournissent des résultats prometteurs avec un pourcentage d'erreur moyen de 49 % pour le contenu en eau liquide et 75 % pour le contenu en glace et ce malgré des échelles de mesures différentes et une colocalisation imparfaite. La méthode développée est également appliquée aux plateformes aéroportées Française et Allemande RALI et HALO. Les premiers résultats sont prometteurs et les données in situ colocalisées obtenues lors de campagnes récentes pourront être utilisées pour évaluer davantage l'algorithme et améliorer son paramétrageClouds play an important role in the Earth's water cycle and radiation balance, and tend to cool the climate slightly. However, there are still many uncertainties about their feedbacks and their evolution in the context of global warming. In particular, mixed-phase clouds account for a significant proportion of the cloud radiative effect. They are composed of a mixture of ice crystals, supercooled water droplets and water vapor. This coexistence involves complex processes and the fraction of liquid and ice significantly affects their radiative properties. This complexity makes them difficult to represent in numerical models, which introduces significant biases. For this reason, it is crucial to better understand the microphysical processes of these clouds to reduce the uncertainties in climate and weather forecasts.To observe clouds, several instrument types exist, such as in situ probes (in direct contact with the hydrometeors) and remote sensing instruments (remote observations). Radar and lidar allow us to obtain distance-resolved information. They can be deployed onboard aircraft or satellites, providing regional and global coverage. Cloud radars work at frequencies (35 and 95 GHz) at which the reflectivity is sensitive to particle size, implying higher reflectivity for large cloud particles (ice crystals) than for small particles (water droplets). Lidars, on the other hand, usually operate between 355 and 1064 nm and are generally more sensitive to particle concentration. As a result, lidar backscatter is higher for highly concentrated particles, such as water droplets. Their synergy allows us to take advantage of the strengths and weaknesses of each instrument to retrieve cloud properties. However, these properties are not directly accessible from measurements and retrieval algorithms are therefore used to relate measurements to microphysical properties.This thesis proposes a new radar-lidar synergistic method dedicated to retrieve supercooled water, ice and mixed-phase cloud properties. Based on an existing method dedicated solely to ice clouds, a new approach has been developed to include both supercooled water and mixed-phase situations. The first step was to adapt and improve the classification used to identify the nature of the observed particles. Next, numerous adaptations have been applied to the algorithm to retrieve separately ice crystals and supercooled water properties. This approach is based on the different sensitivities of radar and lidar to the two types of hydrometeors: ice crystals dominate the radar signal while supercooled water dominates the lidar signal.To assess this new method, the retrievals are compared to in situ measurements from co-located observations and the literature. The first study compares retrievals from CloudSat-CALIPSO satellite data with collocated in situ airborne measurements. This comparison shows that the radar-lidar retrievals follow the same trend as the in situ measurements and provide promising results with mean percent error of 49 % for liquid water content and 75 % for ice water content, despite the quite different measurement scales and imperfect collocation. Additionally, this has been applied to the French and German airborne platforms RALI and HALO. These first results are promising and the collocated in situ data collected during recent campaigns can be used to further assess the algorithm and improve its parameterization
42
Aybar, Sultan. "Solidification And Crystallization Behaviour Of Bulk Glass Forming Alloys." Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/12608922/index.pdf.
Full textAbstract:
The aim of the study was to investigate the crystallization kinetics and solidification behaviour of Fe60Co8Mo5Zr10W2B15 bulk glass forming alloy. The solidification behaviour in near-equilibrium and non-equilibrium cooling conditions was studied.
The eutectic and peritectic reactions were found to exist in the solidification sequence of the alloy. The bulk metallic glass formation was achieved by using two
methods: quenching from the liquid state and quenching from the semi-state.
Scanning electron microscopy, x-ray diffraction and thermal analysis techniques
were utilized in the characterization of the samples produced throughout the study.
The choice of the starting material and the alloy preparation method was found to be
effective in the amorphous phase formation.
The critical cooling rate was calculated as 5.35 K/s by using the so-called Barandiaran and Colmenero method which was found to be comparable to the best glass former known to date.
The isothermal crystallization kinetics of the alloy was studied at temperatures
chosen in the supercooled liquid region and above the first crystallization
temperature. The activation energies for glass transition and crystallization events
were determined by using different analytical methods such as Kissinger and Ozawa
methods.
The magnetic properties of the alloy in the annealed, amorphous and as-cast states
were characterized by using a vibrating sample magnetometer. The alloy was found
to have soft magnetic properties in all states, however the annealed specimen was
found to have less magnetic energy loss as compared to the others.
43
Carvajal, Jara Diego Alejandro. "Simulações atomísticas do gálio super-resfriado." [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278433.
Full textAbstract:
Orientador: Maurice de KoningDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-13T12:16:33Z (GMT). No. of bitstreams: 1 CarvajalJara_DiegoAlejandro_M.pdf: 4649880 bytes, checksum: d014cd5c835938e4643e478fa70353be (MD5) Previous issue date: 2009
Resumo: Recentemente tem sido proposta a existência de uma transição líquido-líquido em substâncias puras com o propósito de explicar alguns comportamentos anômalos como os incrementos de funções resposta (compressibilidade isotérmica, coeficiente de expansão térmica, calor específico) ao diminuir a temperatura de um líquido. A existência deste tipo de transição foi demostrada experimentalmente para o fósforo por dispersão de raios X, e através de simulações atomísticas ou de primeiros princípios para a água, o silício, o carbono, etc. A compreensão detalhada deste tipo de transição está intimamente relacionada à questão fundamental de quais fatores físicos controlam as propriedades de um líquido, e portanto o estudo desta é de grande importância para o desenvolvimento de novas tecnologias, a síntese de novos materiais e o controle de suas propriedades. Neste trabalho realizamos simulações atomísticas sobre um sistema de 1152 partículas de Gálio submetidas a um potencial semi-empírico MEAM, com condições periódicas de contorno. Com estas simulações procuramos uma transição líquido-líquido no Gálio através de um processo de eliminação de três teorias. Inicialmente mostramos que o limite de metaestabilidade do líquido super-aquecido tem um comportamento monótono decrescente no plano de fase P-T. Posteriormente nosso sistema apresenta histerese, uma descontinuidade no volume, estruturas locais diferentes, duas fases que fluem, e um calor latente característico de uma transição de fase de primeira ordem. Por todas estas razões concluímos que o sistema simulado apresenta um transição líquido-líquido de primeira ordem. Adicionalmente foram realizadas compressões e expansões isotérmicas para temperaturas diferentes, observando que estes processos também apresentam histerese e que ela diminui com o aumento da temperatura, indicando assim a possível existência de um segundo ponto crítico e a finalização da transição líquido-líquido. Finalizamos o trabalho com a obtenção e a caracterização de uma possível nova fase cristalina do Gálio cuja estrutura ainda não tem sido obtida experimentalmente. Esta fase foi obtida por casualidade durante o estudo da existência de uma transição líquido-líquido no Gálio ao tencionar o Gálio a -1.6GPa. Sua estrutura é do tipo ortorrômbica com uma simetria Cmcm (grupo espacial 63) e sua principal diferença do Gálio-I é a orientação dos dímeros de Gálio, que nesta nova fase estão dispostos paralelamente. Simulações por DFT mostraram que esta nova fase é metaestável a pressão nula e chega a ser estável a pressões negativas abaixo de ~ 1.5GPa.
Abstract: Recently, the existence of a liquid-liquid transition in pure substances has been proposed as an explanation of anomalous behaviors such as the increase of response functions (isothermal compressibility, coefficient of thermal expansion, specific heat) with decreasing temperature displayed by some liquids. The existence of this type of transition has been demonstrated experimentally for phosphorous by X-ray diÿraction, and through atomistic simulations for water, silicon and carbon. The detailed understanding of this type of transition is closely related to the fundamental question of which physical factors control the properties of a liquid. Therefore, the study of this phenomenon is of great importance for the development of new technologies, the synthesis of new materials and the control of their properties. In this work, we carry out a series of atomistic simulations of a system containing 1152 Gallium atoms described by a semi-empirical Modified Embedded-Atom Model subject to periodic boundary conditions. By means of these simulations we search for a liquid-liquid transition in Gallium by means of a process of elimination of three theories. Initially we show that the limit of metastability of the superheated liquid has a decreasing monotonous behavior in the pressure-temperature phase diagram. Subsequently, our system presents hysteresis, a discontinuity in volume, two phases that have different local atomic structures and display diffusion, and a latent heat, all characteristic of a first-order phase transition. For all these reasons we conclude that the simulated system presents a liquid-liquid phase transition of first-order in the supercooled regime. In addition, we carried out several simulations of isothermal compressions and expansions for different temperatures. These results also show hysteresis although it is found to decrease with increasing temperature, thus indicating the possible existence of a second critical point at which the liquid-liquid transition ends. We finish our studies with the discovery and characterization of a crystalline phase of Gallium whose structure has not been observed experimentally. This phase was obtained by chance during the study of the existence of a liquid-liquid transition in Gallium under tension of -1.6GPa. Its structure is of the orthorhombic type with Cmcm symmetry (space group 63). Its main difference with respect to the Gallium-I phase is that in the new phase the Gallium dimers are disposed in a parallel fashion. Subsequent DFT simulations show that this new phase is metastable at zero pressure and predict it to become stable with respect to Gallium-I arrives at negative pressures below ~1.5GPa.
Mestrado
Física da Matéria Condensada
Mestre em Física
44
Guisoni, Nara Cristina. "Polimorfismo líquido e efeito hidrofóbico através de modelos simplificados." Universidade de São Paulo, 2002. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-18022014-163743/.
Full textAbstract:
Desenvolvemos dois modelos estatísticos para água, nos quais diferentes aspectos de sua estrutura são considerados. O modelo geométrico permite diferentes números de coordenação. Em uma aproximação de campo médio mostramos que sob pressão o modelo apresenta linha de coexistência entre fases de baixa e alta densidade, e ponto crítico. A entropia das ligações de hidrogênio tem papel fundamental na definição da inclinação da linha de coexistência. O comportamento do modelo pode estar realcionado como segundo ponto crítico da água super resfriada e com transições líquido-líquido em geral.O modelo da água quadrada é uma versão térmica do modelo do gelo, no qual considera-se a direcionalidade das ligações de hidrogênio. O modelo foi estudado na rede de Bethe e através de simulações de Monte Carlo em três situações diferentes: para a água pura e na presença de solutos polares e apolares. A água quadrada pura não apresenta transição de fase. No modelo para solvente com solutos apolares, medidas de frequência relativa de ligações e do tempo de correlação mostram que o modelo apresenta estruturação da camada de hidratação. Medidas de correlação temporal no modelo de Ising mostram comportamento oposto. Em um estudo preliminar para uma solução com solutos que realizam ligações de hidrogênio não conseguimos encontrar diagramas de coexistência com círculo fechado, para o conjunto de parâmetros utilizados, possivelmente devido à ausência de buracos.We have developed two statistical models for water in which different features of water structure are considered. In the geometrical model different coordination numbers are present and the model allows for translational disorder. A mean-field treatment shows that under pressure the model exhibits phases of different densities and a coexistence line ending in a critical point. Entropy of the hydrogen network plays an essential role in defining the slope of the coexistence line. The model behavior might be related with the second critical point in supercooled water and to liquid-liquid transitions in general. The square water model is a thermal version of the ice model, and takes into account the directionality of the hydrogen bonds. The model was studied on a Bethe lattice and through Monte Carlo simulations, for three different situations: as pure water and in the presence of polar and apolar solutes. Pure square water does not present a phase transition. In the presence of apolar solutes, first shell square water presents ordering, as shown from comparison of relative frequency of bonds, as well as from study of time correlations. The latter was shown to present opposite behavior in case of an Ising system. In a preliminary study for a solution of hydrogen bonding solutes we were unable to find a closed loop for the sets of parameters chosen. Vacancies might need to be included.
45
Brun, Coralie. "Des liquides surfondus aux verres : étude des corrélations à et hors équilibre." Phd thesis, Université Paris Sud - Paris XI, 2011. http://tel.archives-ouvertes.fr/tel-00664998.
Full textAbstract:
Lorsqu'un liquide est refroidit suffisamment vite, la cristallisation peut être évitée. On a alors un liquide surfondu dont le temps de relaxation augmente fortement quand la température diminue vers la température de transition vitreuse Tg. En-dessous de Tg, le systèmeest dans l'état vitreux. Il vieillit : son temps de relaxation augmente au cours du temps. L'existence d'une longueur de corrélation croissante associée au ralentissement de la dynamique des liquides surfondus (ou des verres) est une des grandes questions toujours ouvertes dans la physique de la transition vitreuse. Des arguments théoriques très généraux ont montré que la mesure de la susceptibilité alternative non linéaire d'ordre trois des liquides surfondus (ou des verres) donnait directement accès à la longueur de corrélation dynamique. Nous avons mis au point une expérience à haute sensibilité permettant d'accéder à deux susceptibilités diélectriques non linéaires d'ordre trois près de Tg. Nos résultats obtenus sur du glycrol surfondu sont quantitativement en très bon accord avec les prédictions théoriques. Ils montrent que la longueur de corrélation dynamique augmente lorsque T diminue vers Tg. En dessous de Tg, l'étude du vieillissement d'une des susceptibilités non linéaires nous a permis de mettre en évidence que la longueur de corrélation dynamique augmente au cours du temps. Ces résultats renforcent le scénario selon lequel la transition vitreuse serait liée à un point critique sous-jacent, ce qui expliquerait l'ubiquité du comportement vitreux dans la nature.
46
Cipcigan, Flaviu Serban. "Electronically coarse grained molecular model of water." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28814.
Full textAbstract:
Electronic coarse graining is a technique improving the predictive power of molecular dynamics simulations by representing electrons via a quantum harmonic oscillator. This construction, known as a Quantum Drude Oscillator, provides all molecular long-range responses by uniting many-body dispersion, polarisation and cross interactions to all orders. To demonstrate the predictive power of electronic coarse graining and provide insights into the physics of water, a molecular model of water based on Quantum Drude Oscillators is developed. The model is parametrised to the properties of an isolated molecule and a single cut through the dimer energy surface. Such a parametrisation makes the condensed phase properties of the model a prediction rather than a fitting target. These properties are studied in four environments via two-temperature adiabatic path integral molecular dynamics: a proton ordered ice, the liquid{vapour interface, supercritical and supercooled water. In all these environments, the model predicts a condensed phase in excellent agreement with experiment, showing impressive transferability. It predicts correct densities and pressures in liquid water from 220 K to 647 K, and a correct temperature of maximum density. Furthermore, it predicts the surface tension, the liquid-vapour critical point, density of ice II, and radial distribution functions across all conditions studied. The model also provides insight into the relationship between the molecular structure of water and its condensed phase properties. An asymmetry between donor and acceptor hydrogen bonds is identified as the molecular scale mechanism responsible for the surface orientation of water molecules. The dipole moment is identified as a molecular scale signature of liquid-like and gas-like regions in supercritical water. Finally, a link between the coordination number and the anomalous thermal expansion of the second coordination shell is also presented.
47
Vaněček, Lukáš. "Tvorba laboratorních úloh pro předmět Vybrané partie z obnovitelných zdrojů a ukládání energie." Master's thesis, Vysoké učení technické v Brně. Fakulta elektrotechniky a komunikačních technologií, 2017. http://www.nusl.cz/ntk/nusl-316881.
Full textAbstract:
This master‘s thesis contains a comprehensive laboratory task about saving a heat energy with a use of changing a state of matter. The thesis is written for the object Chosen passages of renewable resources and saving a heat energy. Part one presents a theoretical part of the topic, terminology and relations needed to make this laboratory work. It also contains a created procedure of partial tasks necessary to take laboratory measurements. The second part of the thesis is about measuring and evaluation of acquired values according to procedure described in the theoretical part of the thesis. Different variations considered during the creation of measuring procedure are also named here. The final part of the thesis describes a laboratory equipment necessary to complete the task and an example of filled protocol is attached.
48
Heydari, Golrokh. "Toward Anti-icing and De-icing Surfaces : Effects of Surface Topography and Temperature." Doctoral thesis, KTH, Yt- och korrosionsvetenskap, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-186187.
Full textAbstract:
Icing severely affects society, especially in the Nordic countries. Iceaccumulation can result in critical performance problems and safetyconcerns for instance in road, air and sea transportation, transmissionlines, marine and offshore structures, wind turbines and heat exchangers.Present active ice-combating approaches possess environmental,efficiency and cost drawbacks. Thus, fabricating icephobic surfaces orcoatings impeding ice formation (anti-icing), but facilitating ice removal(de-icing) is desired. However, different conditions in the environmentduring ice formation and growth add to the complexity of the problem.An icephobic surface that works for a certain application might not be agood candidate for another. These surfaces and the challenges are infocus in this thesis.Wetting properties are important for ice formation on surfaces fromthe liquid phase (often supercooled water), where the water repellency ofthe surfaces could enhance their anti-icing effect. Considering this,different hydrophobic and superhydrophobic surfaces with differentchemistry, morphology and roughness scale were prepared. Since anyinduced wetting state hysteresis on hydrophobic surfaces could influencetheir performance, the wetting stability was investigated. In particulardynamic wetting studies of the hydrophobic surfaces revealed whatsurface characteristics benefit a stable wetting performance. Further, theeffect of temperature, particularly sub-zero temperatures, on the wettingstate of flat and nanostructured hydrophobic surfaces was investigated.This was complemented with studies of the wetting stability of sessilewater droplets on flat to micro- and multi-scale (micro-nano) roughhydrophobic samples in a freeze-thaw cycle. To be consistent with mostapplications, all temperature-controlled experiments were performed inan environmental condition facilitating frost formation. Further, antiicingproperties of hydrophobic surfaces with different topography butsimilar chemistry were studied by freezing delay measurements.A dynamic wetting study using hydrophobic samples with similarchemistry but different topography revealed that multi-scale roughnesscould benefit the wetting stability. However, when these surfaces areutilized at low temperatures the wetting hysteresis observed during acooling/heating cycle is significant. Such a temperature-inducedhysteresis is also significant on superhydrophobic surfaces. I attributethis to condensation followed by frost formation facilitating spreading of the supercooled water droplet. The freezing delay measurementsdemonstrate no significant effect of surface topography on anti-icingproperties of hydrophobic surfaces, however the flat surfaces showed thelongest delay. These findings are in agreement with heterogeneous icenucleation theory, suggesting preferential ice nucleation in concave sites,provided they are wetted.In the second part of this thesis, I consider the findings from theprevious part illustrating the limitations of (super)hydrophobic surfaces.The de-icing properties of hydrophilic surfaces with a hydration waterlayer, hypothesized to lubricate the interface with ice, were studied. Heretemperature-controlled shear ice adhesion measurements, down to -25oC, were performed on an adsorbed layer of a polymer, either bottle-brushstructured poly(ethylene oxide) or linear poly(ethylene oxide). The iceadhesion strength was reduced significantly on the bottle-brushstructured polymer layer, specifically at temperatures above -15 oC,whereas less adhesion reduction was observed on the layer formed by thelinear polymer. These findings are consistent with differential scanningcalorimetry (DSC) data, demonstrating that the hydration water, boundto the bottle-brush structured polymer, is in the liquid state at thetemperatures where de-icing benefit is observed. Further, continuingwith the hypothesis of the advantage of surfaces with a natural lubricantlayer for de-icing targets, I studied shear ice adhesion on the molecularlyflat basal plane of hydrophilic mica down to -35 oC. Interestingly, ultralowice adhesion strength was measured on this surface. I relate this to theproposed distinct structure of the first ice-like but fluid water layer onmica, with no free OH groups, followed by more bulk liquid-like layers.This combined with the molecularly smooth nature of mica results in aperfect plane for ice sliding.Isbildning har en stark inverkan på samhället, speciellt i de nordiskaländerna. Isuppbyggnad kan resultera i kritiska prestandaproblem ochsäkerhetsrisker inom t.ex. väg-, luft-, och sjötransport, kraftledningar,marina- och offshorestrukturer, vindkraftverk och värmeväxlare.Nuvarande aktiva isbekämpningsmetoder uppvisar brister i avseende påmiljö, effektivitet och kostnad. Det finns därmed ett behov av attframställa ytor eller ytbeläggningar som förhindrar isbildning (antiisning)eller underlättar borttagandet av redan bildad is (avisning). Dockkompliceras problemet av de många olika förhållanden under vilka is kanbildas. En beläggning som fungerar för en viss tillämpning behöver intenödvändigtvis vara en bra kandidat för en annan. Dessa ytor ochutmaningar relaterade till dem är i fokus i denna avhandling.Vätningsegenskaper är viktiga för isbildning på ytor från vätskefas(ofta underkylt vatten), och det har visats att vattenavstötande ytor i vissasammanhang kan motverka isbildning. Med detta i åtanke framställdesolika hydrofoba och superhydrofoba ytor, med varierande kemi,morfologi och ytråhet. Eftersom en förändring i de hydrofoba ytornasvätningsegenskaper kan påverka deras funktion studerades vätningsstabilitetenför dessa ytor. I synnerhet dynamiska vätningsstudier av dehydrofoba ytorna avslöjade vilka ytegenskaper som är fördelaktiga förvätningsstabiliteten. Vidare studerades hur temperaturen, särskilt undernoll grader, påverkar vätningstillståndet på släta och nanostruktureradehydrofoba ytor. Arbetet kompletterades med studier av vätningsstabilitetenför vattendroppar på släta samt mikro- och multistrukturerade(mikro-nano) hydrofoba ytor under flera frysningsupptiningscykler.För att vara i linje med de flesta tillämpningar, utfördesalla temperaturkontrollerade mätningar i en miljö där frost kunde bildaspå ytorna. Anti-isegenskaperna hos de hydrofoba ytorna med varierandetopografi men samma kemi studerades vidare genom att studera hur långtid det dröjde innan en vattendroppe på ytan fryste vid en visstemperatur.De dynamiska vätningsstudierna på hydrofoba ytor med samma kemimen olika topografi avslöjade att en ytråhet på flera längdskalor kan haen positiv inverkan på vätningsstabiliteten. När dessa ytor är exponeradeför låga temperaturer är dock vätningshysteresen under en nedkylnings-/uppvärmnings-cykel significant. Den temperatur-inducerade hysteresenär också betydande för superhydrofoba ytor. Detta tillskriver jag kondensation på ytan som följs av frostbildning, vilket i sin tur möjliggörspridning av den underkylda vattendroppen på ytan. Mätning avfördröjningen i frysningsförloppet påvisade ingen betydande effekt avyttopografin för hydrofoba ytor, men släta hydrofoba ytor uppvisade denlängsta fördröjningen. Dessa resultat är i överensstämmelse med rådandeheterogen iskärnbildningsteori, som visar på fördelaktig iskärnbildningpå konkava delar av ytan, förutsatt att dessa väts.I den andra delen av avhandlingen utnyttjar jag observationerna frånden första delen vilka illustrerade begränsningarna för superhydrofobaytor, och söker en annan lösning. Avisningsegenskaper för hydrofilastarkt hydratiserade ytor studerades, med hypotesen att hydratiseringkan smörja gränsskiktet med is. Temperatur-kontrolleradeisadhesionsmätningar ned till -25 °C utfördes på adsorberade skikt av enpolymer med många sidokedjor av polyetylenoxid (”bottle-brush”), såvälsom på ett skikt av linjär polyetylenoxid. Isadhesionen blev kraftigtreducerad på ”bottle-brush”-polymeren, speciellt vid temperaturer högreän -15°C. Däremot kunde knappast ingen minskad isadhesion observerasför den linjära polymeren. Dessa observationer överensstämmer meddifferentialskanningskalorimetri (DSC) data, som visar att dethydratiserade vattenskiktet, vilket är bundet till ”bottle-brush”-polymeren, är i vätskeform vid de temperaturer där avisningsfördelar ärobserverade. För att vidare undersöka hypotesen att det vore fördelaktigtmed ett naturligt smörjande skikt på ytan för att uppnå godaavisningsegenskaper, utförde jag isadhesionsmätningar på molekylärtsläta glimmerytor ner till -35 °C. Intressant nog uppmättes extremt lågisadhesion på denna yta. Detta relaterar jag till den föreslagna utprägladehydratiseringsstrukturen, bestående av ett första is-liknande vattenskiktutan fria OH-grupper, följt av ett mer bulkliknande skikt. Detta ikombination med den molekylärt släta naturen hos glimmer resulterar iett perfekt plan för isen att glida på.
QC 20160504
TopNano
49
Mittal, Jeetain. "Structure, thermodynamics and dynamics of confined and supercooled liquids." Thesis, 2007. http://hdl.handle.net/2152/3160.
Full textAbstract:
Static measures such as density and entropy, which are intimately connected to structure, have featured prominently in modern thinking about the dynamics of the liquid state. In this dissertation, we explore the connections between self-diffusivity, density, available space, and excess entropy in two non-trivial problems in liquid state theory, confined and supercooled liquids. We present exact simulation data for the relationship between self-diffusivity and excess entropy for a wide range of simple of simple fluids (i.e. hard-sphere, Lennard-Jones and square-well) confined to pores with a variety of different sizes and fluid-wall interations. Our main finding is that, at a given temperature, self-diffusivity of the confined fluids collapses onto the bulk behavior when plotted versus excess entropy. In other words, the only information required to "predict" the implications of confinement for the single-particle dynamics is the bulk fluid behavior at a given temperature and the excess entropy of the confined fluid. This should prove practically useful given that the bulk behavior is well known for these fluid systems, and the excess entropy of the confined fluids can be readily estimated from classical density functional theory. We also show that the self-diffusivity of the confined fluids approximately collapses onto the data for the corresponding bulk fluid when plotted versus the average packing fraction (which is based on total, rather than center accessible volume). For continuous interaction potentials such as Lennard-Jones, calculation of effective packing fraction requires knowledge of both the number density of the fluid and a temperature-dependent Boltzmann diameter associated with the repulsive part of the interparticle interactions. We suggest a way to calculate this effective diameter, which to a very good approximation, collapse the temperature- and density-dependent data for the self-diffusivity of the bulk Lennard-Jones fluid onto hard-sphere fluid data plotted versus the fluid's effective packing fraction. Finally, we found that the self-diffusivities of several model systems in their supercooled state also scale exponentially not only with the excess entropy, but also with the two-body contribution to the excess entropy obtained from the pair correlation function of the fluid. The latter observation is particularly interesting because it provides direct evidence of a quantitative link between the dynamics and the average structural order of supercooled liquids. Whether such a connection could indeed be discovered is part of a long-standing question in the study of liquids.text
50
Hocky, Glen Max. "Connections between structure and dynamics in model supercooled liquids." Thesis, 2014. https://doi.org/10.7916/D80C4SZD.
Full textAbstract:
In this thesis, we examine the relationship between structure and dynamics in supercooled liquids from five unique perspectives. We first study a static length scale in the liquid and compare its growth on decreasing temperature with the growth of the logarithm of relaxation times, and find them to be almost strongly correlated. We find that this length scale can distinguish between several specially chosen model liquids whose structure at the level of two-body correlations are identical but whose dynamics at a given temperature are quite different. We then study the number of normal modes necessary to capture the rearrangements in a two-dimensional supercooled liquid as it moves between inherent structures. We find that the number of modes is quite small and decreases as the system is further cooled. After that, we study the effect of a frozen amorphous boundary on the dynamics of supercooled liquids and find that the range of the effect plateaus near that system's mode coupling temperature. We also identify a dynamical crossover at a higher temperature from these data by contrasting the relaxation behavior in the directions perpendicular and parallel to the boundary. After this work, we compare particle mobility with the position of particles deemed to be in preferred local packing arrangements. We find that the correlation between slow dynamics and the location of these locally preferred structures is highly dependent on the model investigated. Finally, we study supercooled liquids that have a fraction of particles randomly fixed in equilibrium positions. We find from annealing and rapid heating experiments on these samples behavior reminiscent of experimentally produced ultrastable glasses.