Orientador: Alex Antonelli / Dissertação (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

No description available.

Rotational-translational decoupling, a phenomenon commonly observed in supercooled liquids, has been a topic of great interest. Despite its prevalence, the underlying cause of this phenomenon, often attributed to dynamic heterogeneity, has not been conclusively elucidated. This thesis investigates and evaluates how dynamic heterogeneity may lead to this decoupling using simultaneous single-molecule rotational and translational measurements. In the experimental study, single molecule fluorescence imaging experiments are performed on the ideal probe N,N’-dipentyl-3,4,9,10-perylenedicarboximide in high molecular weight polystyrene near its glass transition temperature. A novel trajectory linking method based on hierarchical clustering is developed to facilitate single molecule tracking even in imaging data where specific molecules cannot be observed visually for a substantial number of frames. This linking algorithm then allows molecules to be localized over full movies, such that rotational and translational measurements can be compared over comparable timespans. The investigation of translational dynamics using such long trajectories, which was not previously achieved, reveals that both rotational-translational decoupling and translational enhancement persist on the single molecule level, supporting the hypothesis that temporally heterogeneous dynamics experienced by the probe molecules is a contributing factor in observed rotational-translational breakdown in both ensemble and single molecule studies. A tendency towards dynamical convergence between subgroups with fast and slow dynamics is observed, demonstrating temporal heterogeneity at the single molecule level. In comparison to rotational dynamics, translational dynamics was discovered to have a longer lifetime. Other key observations facilitated by the linked trajectory analysis include that apparent diffusion coefficient of probe molecules decreases with longer observation time, a finding inconsistent with normal diffusive behavior. To investigate the origin of this anomalous slowing in single molecule studies existing alongside the observed overall enhancement in translational motion, temporally heterogeneous models with multiple types of correlation were studied via simulations. The results emphasize the critical role that bias in translational and rotational measurements can play when investigating and observing dynamic heterogeneity, as nearly all models including dynamic heterogeneity show increasing diffusion coefficient with increasing number of dynamic environments explored. Strikingly, translational enhancement is evident in single molecule translational simulations even when slow dynamics are reinforced via positive correlation in the models. A comparison of the diffusion coefficient evolution between simulations and experiments reveals that the sub-diffusive continuous time random walk model is the most plausible candidate to account for the set of observations seen in experiment.

The heterogeneous dynamics exhibited by supercooled liquids near the glass transition temperature (𝑇_𝑔) has been a topic of much research over the past several decades. In particular, the advent of single molecule (𝖲𝖬) methods has permitted great insight into the extent of both spatial and temporal heterogeneities in these systems, information which is either difficult or impossible to access via ensemble approaches. Despite this, the related phenomenon of rotational-translation decoupling, whereby the translational motion observed in supercooled systems is enhanced relative to Debye-Stokes-Einstein predictions, is difficult to study with 𝖲𝖬 approaches. This is due to the very low localization uncertainty required to accurately report the extremely slow translational motion in supercooled systems near 𝑇_𝑔. In this thesis, a new approach for quantifying rotational dynamics in supercooled liquids is introduced which leverages fluorescence intensity fluctuations due to out-of-plane fluorophore rotations. Unlike linear dichroism (LD) measurements, the most common experiment used to access rotational dynamics, this technique does not require a polarizing optical element, thus improving localization precision in the acquired images. This intensity fluctuation-based approach is shown to report comparable rotational correlation timescales (𝝉_𝘤) and information on dynamic heterogeneity to that typically extracted via LD measurements. On a probe-by-probe basis, rotational correlation times obtained from simultaneous measurement of LD (𝝉_𝘤,𝘓𝘋) and intensity fluctuations (𝝉_𝘤,𝘐 ) are found to be only moderately well-correlated. We postulate that this is a consequence of dynamic heterogeneity due to temporal dynamic exchange, the process in which a probe (and its surroundings) undergoes sudden changes in dynamics. This hypothesis is explored through simulations, which reveal that the Pearson R correlation coefficients associated comparing log 𝝉_𝘤,𝘐 and log 𝝉_𝘤,𝘓𝘋 increases as the time between dynamic exchange increases. The information obtained from such simulations is then used to estimate the exchange timescales from experimental data. When examined in concert with experimentally measured degrees of relaxation non-exponentiality - generally considered a metric of heterogeneity in an interrogated supercooled liquid – this permits access to previously inaccessible information regarding the breadth of the distribution of underlying timescales experienced by these supercooled systems. In addition to this work focused on rotational dynamics, we also aim to further clarify information contained in 𝖲𝖬 experiments characterizing translational dynamics, towards the goal of full understanding of rotational-translational decoupling. Here, two widefield fluorescence imaging setups are optimized to minimize localization uncertainty, and differences in how localization uncertainties manifest in perceived translational motion near 𝑇_𝑔 are examined. The setup with greater localization uncertainty reports faster translational dynamics compared to the other optical setup, suggesting significant influence of the localization noise floor on perceived dynamics and highlighting the importance of maximizing the signal to noise ratio of 𝖲𝖬 experiments aiming to study the underlying cause of rotational-translational decoupling.

A liquid Cooled below its normal freezing temperature is known as a supercooled liquid. On further cooling, supercooled liquids crystallize to thermodynamically stable, ordered structures. Alternatively, if the cooling rate is fast enough, the crystallization may be avoided altogether. Below a particular temperature during rapid cooling the liquid will solidify into a disordered, amorphous phase -also known as the glassy phase of matter. This particular temperature is termed the ”glass transition temperature” (Tg). Unlike a crystalline solid, a glass is neither a thermodynamically stable phase nor does it possess long range molecular ordering. Very slow structural relaxation (in the time scale of ∼ 100 s) is always present in the glassy phase. Thus, this phase is often referred to as a metastable phase of matter. Experimental and theoretical studies related to the behavior of supercooled liquids are the subject matter of many investigations for the last few decades [1]. These studies find their applications in diverse fields such as geology, cryopreservation, glaciology and atmospheric science. However, properties of supercooled liquids and the corresponding amorphous phase are not completely understood at present, particularly for hydrogen bonded (H-bonded) systems. This thesis concerns both the crystallization and the glass formation process of H-bonded systems. The systems of interest are water, the commonly accepted universal solvent, and the aqueous binary mixture of glycerol and water. The technique of molecular probing is often used to study the cooperativety and rotational diffusion of supercooled liquids and for determination of the glass transition temperature. For the present set of work, a molecular probe technique called spin probe ESR is extensively used. Electron paramagnetic resonance or electron spin resonance (EPR/ESR) measures the electronic energy level separation and is well known for the high sensitivity. All of the systems studied in the present set of work are diamagnetic. This issue is circumvented by dissolving paramagnetic spin probe molecules, which are usually organic free radicals with one N-O group, into the systems. Spin probes are added in very low concentrations (~10-3M) to minimize the effect on the host system and also to avoid mutual interactions between them. The unpaired electron delocalized in the direction of the N-O bond serves as the paramagnetic center required for an ESR experiment. The splitting of electron energy level due to the external magnetic field (Zeeman splitting) can give rise to resonance absorption of energy if exposed to a microwave of appropriate frequency. There is also a magnetic coupling (hyperfine) between the spin of the unpaired electron and nuclear spin of the nearby nitrogen atom. The hyperfine coupling splits each electron energy levels, to the first order, symmetrically into three levels. The transitions between these levels -subject to appropriate selection rules -give rise to the ESR spectrum [2]. The spectral shape in a magnetic field sweep ESR experiment appears complex if randomly oriented spin probes are dispersed in an amorphous or polycrystalline solid matrix. The high degree of mobility in probe molecules, present in a liquid solution, can average out the individual anisotropy of magnetic tensors to get a spectrum of three equally spaced liens. Experiments can be performed spanning a spin probe reorientation timescale of 10-7-10-12 s typically in the temperature range of 4.2 -300K. In chapter one we have given a brief overview of the supercooled liquids and the phase transitions related to the present work. Particular emphasis has been given to the dynamical features of the supercooled liquid close to its glass transition temperature and their classification based on the degree of ’fragility’ [3]. Brief general introductions of the systems studied in each of the following chapters are also provided. Then, the details of ESR spectroscopy and a quantum mechanical picture of the method of spin probe ESR have been discussed [4]. A separate section has been devoted to the numerical and analytical methods used to analyze the spectrum to extract information related to the spin probe dynamics [5]. The chapter concludes with a description of the ESR spectrometer. In chapter two we have studied the glass transition and dynamics of the supercooled water by the method of spin probe ESR. The vitrification has been done by direct exposure of the bulk water sample, doped with the spin probe TEMPOL, to the liquid helium flow. The vitrified matrix turns into the ultraviscous liquid above the putative glass transition temperature of ~136 K which further transforms to cubic ice (Ic) above TX ~150 K. The supercooled fraction of water, along with the spin probes which are treated as impurities by the crystallized surroundings, remain trapped inside the veins or triple junctions of the ice grains which serve as the interfacial reservoir of impurities in a polycrystalline ice matrix. The spectra for the entire temperature range have been analyzed with the help of in-depth computation by modelling the reorientation of TEMPOL in terms of the jump angle θs and the rotational correlation time τ [5]. This model, based on a homogeneous mobility scenario of the spin probe, works nicely except in the temperature range of 140-180 K. Dynamical heterogeneity (DH) is apparent in this temperature range and a more mobile (fast) component, as compared to the one corresponding to the very slow dynamics of TEMPOL at lower temperatures (slow), is observed. The relative weight of the fast and the slow component changes with temperature and above ~180 K the entire spectrum changes into the motionally narrowed triplet. The temperature dependence of the slow component of τ shows a change in slope at a temperature close to the putative glass transition temperature of water. The fast component of τ exhibits a fragile, i.e. non-Arrhenius character at high temperature with a crossover to a strong, i.e. Arrhenius behavior below ~225 K, close to the hypothesized fragile-to-strong crossover (FSC) for water at TFSC ~228 K. The breakdown of the Debye-Stokes-Einstein (DSE) law is observed when the τ values are combined with the available viscosity data of water to evaluate the DSE ratio, paralleling the SE breakdown which has recently been observed in nanoconfined water [6]. The dynamical heterogeneity is thought to be closely associated with the static structural heterogeneities of supercooled water. The existence of large scale structural fluctuations spanning a range of low-and high-density phases of liquid water have been associated with the heterogeneous dynamics sensed by TEMPOL. Motivated by the Arrhenius like behavior of the slow component, it has been identified with the low density liquid (LDL). The fragile nature of the fast component at high temperature may be identified with that of the high density liquid (HDL) which is the predominant fraction in liquid or weakly supercooled water [6]. Chapter three reports the studies on freezing and dynamics of the supercooled water trapped inside the veins of a polycrystalline ice matrix by dissolving spin probes TEMPO and TEMPOL into it. When a millimolar spin probe aqueous solution is cooled below the freezing point of water, the spin probes -driven by the mechanism described above migrate to the liquid environment inside the ice veins. Local concentration of the probe molecules inside the veins can go up to 1-10 M [7]. Bulk crystallization is evident in differential scanning calorimetry (DSC) studies whereas the liquid environment of the spin probe below the bulk freezing is confirmed by its narrow triplet ESR spectrum. A sudden collapse of this narrow triplet into a single broad line indicates the freezing of the trapped water fraction which usually happens well below the DSC freezing point for both the probes. The spin probe detected freezing point of this interstitial water is found to be largely dependent on the properties and the amount of the dissolved probe molecules. An explanation is sought in terms of the ’destructuring effect’ on the tetrahedral ordering of the water H-bond network by both the high local concentration of the spin probes and the hydrogen bond strength, formed between the water and the spin probe molecules through the polar groups of the latter [8, 9]. These two factors are thought to play important roles in determining the reorientational dynamics of the spin probe molecules, as well. The rotational correlation times of the two probes exhibit a crossover owing to the different mobility of their salvation shells in the more ordered supercooled water. The observed relaxation behavior of this confined water using the probe TEMPO, which has little effect on water H-bond network, is found in agreement with the previous experimental investigations on water confined in a nanochannel [10]. In chapter four, the glass transition, relaxation and the free volume of the glycerol-water (G-W) system are studied over the glycerol concentration range of 5 -85 mol% with TEMPO as the spin probe. G-W mixture is intrinsically inhomogeneous due to the well established phase segregation below a critical glycerol concentration of 40 mol%. In the inhomogeneous regime the water molecules tend to form cooperative domains besides the mesoscopic G-W mixture [11]. Samples are quenched by rapid cooling down to 4.2 K inside the spectrometer cryostat. Spectra were recorded on slow heating of the sample in the temperature range of 130 -305 K. The glass transition temperature is correlated to the sharp transition of the extrema separation of the ESR spectrum. The glass transition temperatures are found to follow a concentration dependence which is closely associated to the mesoscopic inhomogeneities of the G-W system. The steady enhancement in fragility of the G-W system with the addition of water is evident from the temperature dependence of the spin probe correlation time τ for the entire concentration range. In the temperature range of 283 -303 K, the DSE law is followed i.e. the spin probe reorientation process is found to be strongly coupled to the system viscosity. In this regime, the τ values have been used along with the available viscosity data to calculate the effective volume V of the spin probe for the entire concentration range. The spin probe effective volume is a measure of the available free volume of the host matrix. A drastic change in the quantity is seen in the vicinity of the 40 mol% glycerol concentration owing to a similar structural change of the matrix due to the formation of mesoscopic scale inhomogeneities below the critical concentration [12]. The thesis concludes with a discussion about the possible future directions of research.

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.

No description available.

Les argiles sont des minéraux complexes présentant une porosité multi-échelles et une aptitude à gonfler sous atmosphère humide. Ces matériaux ont diverses applications en catalyse, dans le stockage des déchets, dans le bâtiment… Pourtant, les propriétés de l'eau confinées sont encore mal comprises, notamment en raison de la complexité de l'eau elle-même. Le but de ce travail est, en utilisant principalement les simulations moléculaires et les spectrométries vibrationnelles, de comprendre la structure et la dynamique de l'eau confinée dans les argiles.Afin d'évaluer la précision des modèles numériques pour décrire l'eau confinée dans les argiles, et pour comprendre l'origine de ses propriétés structurales et dynamiques, un large part de ce travail est consacrée aux briques constitutives de l'argile : l'eau pure, l'eau interfaciale et l'eau salée. A ce titre, on étudie les propriétés viscoélastiques de l'eau du domaine surfondu jusqu'à la température d'ébullition par dynamique moléculaire classique. On analyse aussi les propriétés de frottement près d'une surface type, et la précision des approches ab initio et des modèles de sels.Dans une seconde partie, on confronte ces résultats aux propriétés de l'eau confinée dans les argiles à basse température et à température ambiante, expérimentalement et numériquement. Les expériences consistent en des mesures exhaustives par spectrométrie d'absorption dans l'infrarouge moyen et lointain, tandis que les calculs sont des simulations de dynamique moléculaire classique. En particulier, on s'intéresse à l'existence de transitions de phases induites par le confinement ou les variations de température / Clay are complex minerals with a multiscale porosity and a remarkable ability to swell under humid atmosphere. These materials have many applications in catalysis, waste management, construction industry... However, the properties of confined water are still not fully understood, due in particular to the complexity of water itself. The aim of this work is, using mainly molecular simulations and vibrational spectroscopy, to understand the structure and the dynamics of water confined in clay minerals. To evaluate the accuracy of numerical models to describe water confined in clay minerals, and to understand the origin of its structural and dynamical properties, a large part of the work was devoted to the building blocks of clays: pure bulk water, water at the surface of a solid, and salt water. To this extent, the viscoelastic properties of water from the deeply supercooled regime to the boiling temperature were investigated using classical molecular dynamics. The evolution of the friction properties of water on a prototypical solid surface was also analyzed, and the accuracy of ab initio approaches and empirical salt models was studied.In a second part, those results were confronted to the properties of water confined in clay minerals at low and room temperature, studied both experimentally and numerically. Experimental work consisted mostly in extensive far- and -mid infrared absorption spectrometry measurements, whereas numerical work mainly consisted in empirical molecular dynamics simulations. Especially, the existence of confinement- or temperature-induced phase transitions of confined water was investigated

No description available.

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 décennies. / The 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.