The aim of this study, which was carried out in two main parts, is to investigate the glass forming ability of Fe-based systems. The first part involves the theoretical modeling to cover the requirement of a predictive model to identify the Fe-based alloy families that have high glass forming ability in the frame of atomistic and thermodynamic approach. The second part involves the experimental investigations to prove the results of the conducted theoretical modeling studies. For this purpose, in the first part, theoretical investigations were performed to identify the third alloying elements that will lead to an increase in the glass forming ability on the base of electronic theory of alloys in pseudopotential approximation for selected Fe- based systems, Fe - (B, Zr, Nb, C, W). In the experimental part, in the frame of the theoretical investigation results, one of the theoretically modeled binary system, and the third alloying elements that were predicted to lead an increase in the glass forming ability of the selected binary system, were determined. As a first step, designated compositions were synthesized by using low grade conventional Fe-B alloy as a raw material by using centrifugal casting technique and copper mold casting method. To compare the results, same compositions were also cast from the high purity elements by using the same technique and method. For the characterization of these cast specimens, DSC, XRD, SEM, EDS and metallographic examination techniques were used. Amorphous structure was successfully obtained in the thin sections of the wedge-cast samples for Fe-B-Nb and Fe-B-W ternary systems.

No description available.

Lack of crystalline order and microstructural features such as grain/grain-boundary in metallic glasses results in a suite of remarkable attributes including very high strength, close to theoretical elasticity, high corrosion and wear resistance, and soft magnetic properties. In particular, low coercivity and high permeability of iron and cobalt based metallic glass compositions could potentially lead to extensive commercial use as magnetic heads, transformer cores, circuits and magnetic shields. In the current study, few metallic glass compositions were synthesized by systematically varying the iron and cobalt content. Thermal analysis was done and included the measurement of glass transition temperature, crystallization temperature, and the enthalpies of relaxation and crystallization. Magnetic properties of the alloys were determined including saturation magnetization, coercivity, and Curie temperature. The coercivity was found to decrease and the saturation magnetization was found to increase with the increase in iron content. The trend in thermal stability, thermodynamic properties, and magnetic properties was explained by atomic interactions between the ferromagnetic metals and the metalloids atoms in the amorphous alloys. Mechanical behavior of iron based metallic glasses was evaluated in bulk form as well as in the form of coatings. Iron based amorphous powder was subjected to high power mechanical milling and the structural changes were evaluated as a function of time. Using iron-based amorphous powder precursor, a uniform composite coating was achieved through microwave processing. The hardness, modulus, and wear behavior of the alloys were evaluated using nano-indentation.

The thesis focus on modelling of float glass making process using viscose film type approximation. Navier-Stokes equations are averaged over one spatial variable. Then the domain with an a priory unknown shape, where the shape is a part of the solution, is transformed to a fixed computational domain. The problem is solved by finite element method using FEniCS software. In the end is discussed an influence of several parameters such as wheels, which regulates thickness of the glass and enforce an inner condition, boundary conditions or spreading coefficient on the numerical result. 1

The glass-forming composition ranges in Al-La-Ni and Mg-TM (Cu, Zn)-Y alloys were predicted using Miedema’s model. Glass-forming abilities of Al-La-Ni alloys and Mg-Cu-RE alloys were studied in terms of reduced glass transition temperature (Trg), supercooled liquid region (∆Tx) and γ parameters. The glass-forming ability parameters of Mg-Cu-RE (RE: rare-earth) alloys were correlated with Mendeleev number. The Miedema model has been used to determine glass-forming composition range in binary Al-La, Al-Ni and La-Ni alloy systems and the ternary Al-La-Ni system by neglecting the ternary interactions. The glass-forming composition range for Al-La, Al-Ni and La-Ni alloy systems extends from 5 to 90 at% La, 30 to 80 at% Ni and 5 to 95 at% Ni, respectively. In these systems the predicted glass-forming composition range is wider than the experimentally observed range. Miedema model, restricting the difference of enthalpy of formation between the amorphous and solid solution phases to within –10000 J/mole to –55000 J/mole gives rise to better prediction of glass-forming composition range compared to the original models. The concept of mixing enthalpy and mismatch entropy has been used in order to quantify Inoue’s criteria of glass formation. The mixing enthalpy and normalised mismatch entropy of the ternary Al-La-Ni alloys, calculated by the extended regular solution model, vary between –12 to –40 kJ/mol and 0.16 to 0.65, respectively. The enthalpy contour plot has been constructed to distinguish the glass-forming compositions on the basis of the increasing negative enthalpy of the composition. Six Al rich Al-La-Ni alloys with nominal compositions Al89La6Ni5, Al85La10Ni5, Al85La5Ni10, Al82La8Ni10, Al80La10Ni10 and Al60La20Ni20 three La rich Al-La-Ni alloys with nominal compositions Al34La33Ni33, Al40La40Ni20 and Al25La50Ni25 have been chosen from the Al-La-Ni ternary phase diagram, to study the glass-forming ability of Al-La-Ni ternary alloy system and the correlation between La-based and Al-based glasses. All the alloys have been prepared using arc melting unit. All the alloy ribbons have been prepared using single-wheel vacuum melt-spinning unit. Two different wheel speeds of 20 m/s and 40 m/s were used for preparing ribbons of all the nine alloys. All the Al-La-Ni compositions, excluding equi-atomic composition (Al34La33Ni33) and Al60La20Ni20, give rise to amorphous phases. The supercooled liquid region and reduced glass transition temperature of this system increases with a decrease in Al content and an increase in La content. The glass-forming ability of the Al rich Al-La-Ni alloys is lower than that of the La-rich Al-La-Ni alloys. The glass-forming ability has been explained by taking into account the binary heat of mixing and the atomic radius mismatch of the constituent elements. Preferential crystallisation takes place during the heat treatment of glassy ribbons. The crystalline products are partially influenced by composition and binary heat of mixing between elements. Mg65Cu25Y10 alloy is a classical glass former of a family of Mg-based alloys. The partial or complete substitution of Y with other rare earth elements has been introduced to correlate the Mendeleev Number with the glass-forming ability parameters: reduced glass transition temperatures (Trg = Tg/Tl), supercooled liquid regions (∆Tx = Tx – Tg) and γ-criterion (TX/(Tg + Tm)). Mg-Cu-RE alloys with nominal compositions Mg65Cu25Y10, Mg65Cu25Y5Gd5, Mg65Cu25Y5Nd5, Mg65Cu25Gd10 and Mg65Cu25Nd10 were chosen for this work. The high reduced glass transition temperature, wider supercooled liquid region and higher γ value of Mg-Cu-Gd-Y amorphous alloy compared to Mg-Cu-Y and Mg-Cu-Nd-Y systems indicates that Mg-Cu-Gd-Y alloys possess higher glass-forming ability. The devitrification of all Mg-Cu-RE glassy alloys used for this work give rise to Mg2Cu (oF48) phase, which is known as anti-Laves phase. The glass-forming composition range for binary and ternary Mg-Cu-Y systems was calculated using Miedema’s model. The development of accurate methods of prediction of glass-forming ability in metallic systems is an important challenge. Pettifor has pioneered the Structure Mapping approach to binary intermetallics. The Pettifor approach can be adapted to the designing of bulk metallic glasses (BMGs). This method has been used to design Al-based and Mg-based BMG’s. Pettifor introduced an integer parameter to characterize the elements, which he called the Mendeleev Number. Essentially, Pettifor’s scheme orders the elements in a sequence of increasing electronegativity. With respect to Mendeleev Number, the Mg-Cu-RE system can be regarded as a binary system, because of the closeness of Mg and Cu (Mg:73, Cu:72, Y:25, Gd:27 and Nd:30). For this system, Mendeleev Number is a more effective parameter than atomic size (Mg: 1.60 Å, Cu: 1.27 Å), as a predictor of glass-forming ability. The effect of Y and rare earth elements on glass forming ability is similar. The atomic number of Y (39) is away from that of the rare earth elements and the Mendeleev Number of Y (25) comes in between those of the rare earth elements. Mg-Zn-Y system is an interesting system for researchers because of higher strength of these alloys. This system draws the crystallographers’ attention due to its composition-dependent structure variations. The Mg-rich RS/PM Mg-Zn-Y alloys yield superior mechanical properties. Therefore, the Mg-rich Mg-Zn-Y system has been chosen to study the microstructural evolution, even though the theoretical calculations for the glass-forming composition range for the Mg-Zn-Y system shows that this system is not a good glass former. Mg-Zn-Y system with nominal compositions Mg97Zn1Y2, Mg97Zn2Y1, Mg97−xZn1Y2Zrx and Mg92Zn6.5Y1.5 were chosen to study the microstructural evolution of these alloys. A small increase in Zn amount (above 2 at.%) in Mg-rich Mg-Y system results in quasicrystalline particles embedded in the matrix, whereas the addition of Zn up to 2 at.% leads to microstructural changes in the α-Mg solid solution.

The absence of long-range order in glassy chalcogenides provides the convenience of changing the elemental ratios and hence the properties over a wide range. The interesting properties exhibited by chalcogenide glasses make them suitable materials for Phase Change Memories (PCM) and other applications such as infrared optical devices, photo-receptors, sensors, waveguides, etc. One of the most remarkable properties of chalcogenides is their electrical switching behavior. Reversible (threshold type) or irreversible (memory type) switching from a high resistance OFF state to a low resistance ON state in glassy chalcogenides occurs at a critical voltage called the threshold/switching voltage (VT). Investigations on the switching behavior and its composition dependence throw light on the local structural effects of amorphous chalcogenide semiconductors and also help us in identifying suitable samples for PCM applications. Thermal analysis by Differential Scanning Calorimetry (DSC) has been extensively used in glass science, particularly for measurements of thermal parameters such as enthalpy of relaxation, specific heat change, etc., near glass transition. Quite recently, the conventional DSC has been sophisticated by employing a composite temperature profile for heating, resulting in the Temperature Modulated DSC (TMDSC) or Alternating DSC (ADSC). Measurements made using ADSC reveal thermal details with enhanced accuracy and resolution, and this has lead to a better understanding of the nature of glass transition. The thermal parameters obtained using DSC/ADSC are also vital for understanding the electrical switching behavior of glassy chalcogenides. The motivation of this thesis was twofold: The first was to develop a novel, high voltage programmable power supply for electrical switching analysis of samples exhibiting high VT, and second to investigate the thermal and electrical switching behavior of certain Se-Te based glasses with Ge and Sb additives. The thesis contains seven chapters: Chapter 1: This chapter provides an overview of amorphous semiconductors (a-SC) with an emphasis on preparation and properties of glassy chalcogenides. The various structural models and topological thresholds of a-SC are discussed with relations to the glass forming ability of materials. The electronic band models and defect states are also dealt with. The essentials of electrical switching behavior of chalcogenides are discussed suggesting the electronic nature of switching and the role of thermal properties on switching. Chapter 2: The second chapter essentially deals with theory and practice of the experimental techniques adopted in the thesis work. The details of the melt-quenching method of synthesizing glassy samples are provided. Considering the importance, the theory of thermal analysis by DSC & ADSC, are discussed in detail, highlighting the advantages of the latter method adopted in the thesis work. The instrumentation and electronics, developed and used for electrical switching analysis are also introduced at a block diagram level. Finally, the methods used for structural analysis are briefed. Chapter 3: This chapter is dedicated to the design and development details of the programmable High Voltage dc Power Supply (HVPS: 1750 V, 45 mA) undertaken in the thesis work. The guidelines used for power supply topology selection, the specifications and block diagram of the HVPS are provided in that sequence. The operation of the HVPS is discussed using the circuit diagram approach. The details of software control are also given. The performance validations of the HVPS, undertaken through voltage & current regulation tests, step & frequency response tests are discussed. Finally, the sample-test results on the electrical switching behavior of representative Al20As16Te64 and Ge25Te65Se10 samples, obtained using both the current & voltage sweep options of the HVPS developed are illustrated. Chapter 4: Results of the thermally induced transitions governed by structural changes which are driven by network connectivity in the GexSe35-xTe65 (17 ≤ x ≤ 25) glasses, as revealed by ADSC experiments, are discussed in this chapter. It is found that the GexSe35-xTe65 glasses with x ≤ 20 exhibit two crystallization exotherms (Tc1 & Tc2), whereas those with x ≥ 20.5, show a single crystallization reaction upon heating (Tc). The glass transition temperature of GexSe35-xTe65 glasses is found to show a linear, but not-steep increase, indicating a progressive and not an appreciable build-up in network connectivity with Ge addition. The exothermic reaction at Tc1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at Tc2 is associated with the additional crystallization of rhombohedral Ge-Te phase. It is also found that the first crystallization temperature Tc1 of GexSe35-xTe65 glasses of lower Ge concentrations (with x ≤ 20), increases progressively with Ge content and eventually merges with Tc2 at x = 20.5 (<r> = 2.41); this behavior has been understood on the basis of the reduction in Te-Te bonds of lower energy and an increase in Ge-Te bonds of higher energy, with increasing Ge content. Chapter 5: This chapter deals with the electrical switching studies on GexSe35-xTe65 (17 ≤ x ≤ 25) glasses, with an emphasis on the role of network connectivity/rigidity on the switching behavior. It is found that the switching voltage (VT) increases with Ge content, exhibiting a sudden jump at x=20, the Rigidity Percolation Threshold (RPT) of the system. In addition, the switching behavior changes from memory to threshold type at the RPT and the threshold switching is found to be repetitive for more than 1500 cycles. Chapter 6: In this chapter, the results of thermal analysis (by ADSC) and electrical switching investigations on SbxSe55-xTe45 (2 ≤ x ≤ 9) are discussed. It is found that the addition of trivalent Sb contributes very meagerly to network growth but directly affects the structural relaxation effects at Tg. Further, SbxSe55-xTe45 glasses exhibit memory type electrical switching, which is understood on the basis of poor thermal stability of the samples. The metallicity factor is observed to outweigh the network factor in the composition dependence of VT of SbxSe55-xTe45 glasses. Chapter 7: The chapter 7 summarizes the results obtained in the thesis work and provides the scope for future work. The references are cited in the text along with the first author’s name and year of publication, and are listed at the end of each chapter in alphabetical order.

The work presented in this doctoral thesis employs colloidal suspensions to explore key open problems in condensed matter physics. Colloidal suspensions, along with gels, polymers, emulsions and liquid crystals belong to a family of materials that are collectively labelled as soft matter. Compositionally, colloidal suspensions consist of particles whose size ranges from a few nanometers to a few microns, dispersed in a solvent. A hallmark feature of these systems is that they exhibit Brownian motion, which makes them suitable for investigating statistical mechanical phenomena. Over the last fifteen years or so, colloids have been used extensively as model systems to shed light on a wide array of such phenomena typically observed in atomic systems. The chief reason why colloids are good mimics of atomic systems is their large size and slow dynamics. Unlike atomic systems, the dynamics of colloids can be probed in real time with single-particle resolution, which allows one to establish the link between macroscopic behavior and the microscopic processes that give rise to it. Yet another important feature is that colloidal systems exhibit various phases of matter such as crystals, liquids and glasses, which makes them versatile model systems that can probe a broad class of condensed matter physics problems. The work described in this thesis takes advantage of these lucrative features of colloidal suspensions to gain deeper insights into the physics of glass formation as well as shear-induced anisotropic grain growth in polycrystalline materials. The thesis is organized into two preliminary chapters, four work chapters and a concluding chapter, as follows. Chapter 1 provides an introduction to colloidal suspensions and reviews the chief theo-retical concepts regarding glass formation and grain boundary dynamics that form an integral part of subsequent chapters. Chapter 2 describes the experimental methods used for performing the work presented in the thesis and consists of two parts. The first part describes the protocols followed for synthesizing the size-tunable poly (N-isoprolypacrylamide) (PNIPAm) particles used in our study of shear-induced grain growth. The second part describes the instrumentation and techniques, such as holographic optical tweezers, confocal microscopy, rheology and Bragg diffraction microscopy, used to perform the measurements described in the thesis. Chapter 3 deals with our work on the dynamical facilitation (DF) theory of glass forma-tion. Despite decades of research, it remains to be established whether the transformation of a liquid into a glass is fundamentally thermodynamic or dynamic in origin. While obser-vations of growing length scales are consistent with thermodynamic perspectives, the purely dynamic approach of the DF theory has thus far lacked experimental support. Further, for glass transitions induced by randomly freezing a subset of particles in the liquid phase, theory and simulations support the existence of an underlying thermodynamic phase transi-tion, whereas the DF theory remains unexplored. In Chapter 3, using video microscopy and holographic optical tweezers, we show that dynamical facilitation in a colloidal glass-forming liquid grows with density as well as the fraction of pinned particles. In addition, we observe that heterogeneous dynamics in the form of string-like cooperative motion, which is consid-ered to be consistent with thermodynamic theories, can also emerge naturally within the framework of facilitation. These findings suggest that a deeper understanding of the glass transition necessitates an amalgamation of existing theoretical approaches. In Chapter 4, we further explore the question of whether glass formation is an intrinsi-cally thermodynamic or dynamic phenomenon. A major obstacle in answering this question lies in determining whether relaxation close to the glass transition is dominated by activated hopping, as espoused by various thermodynamic theories, or by the correlated motion of localized excitations, as proposed in the Dynamical Facilitation (DF) approach. In Chapter 4, we surmount this central challenge by developing a scheme based on real space micro-scopic analysis of particle dynamics and applying it to ascertain the relative importance of hopping and facilitation in a colloidal glass-former. By analysing the spatial organization of excitations within cooperatively rearranging regions (CRRs) and examining their parti-tioning into shell-like and core-like regions, we establish the existence of a crossover from a facilitation-dominated regime at low area fractions to a hopping-dominated one close to the glass transition. Remarkably, this crossover coincides with the change in morphology of CRRs predicted by the Random First-Order Transition theory (RFOT), a prominent ther-modynamic framework. Further, we analyse the variation of the concentration of excitations with distance from an amorphous wall and find that the evolution of these concentration profiles with area fraction is consistent with the presence of a crossover in the relaxation mechanism. By identifying regimes dominated by distinct dynamical processes, our study offers microscopic insights into the nature of structural relaxation close to the glass transi-tion. In Chapter 5, we extend our investigation of the glass transition to systems composed of anisotropic particles. The primary motivation for this is to bridge a long-standing di-vide between theories and simulations on one hand, and experiments on molecular liquids on the other. In particular, theories and simulations predominantly focus on simple glass-formers composed of spherical particles interacting via isotropic interactions. Indeed, even the prominent theory of Dynamical Facilitation has not even been formulated to account for anisotropic shapes or interactions. On the other hand, an overwhelming majority of liquids possess considerable anisotropy, both in particle shape as well as interactions. In Chapter 5, we mitigate this situation by developing the DF theory further and applying it to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Further, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical het-erogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass-formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids. In Chapter 6, we turn our attention away from the glass transition and address the problem of grain growth in sheared polycrystalline materials. The fabrication of functional materials via grain growth engineering implicitly relies on altering the mobilities of grain boundaries (GBs) by applying external fields. While computer simulations have alluded to kinetic roughening as a potential mechanism for modifying GB mobilities, its implications for grain growth have remained largely unexplored owing to difficulties in bridging the disparate length and time scales involved. In Chapter 6, by imaging GB particle dynamics as well as grain network evolution under shear, we present direct evidence for kinetic roughening of GBs and unravel its connection to grain growth in driven colloidal polycrystals. The capillary fluctuation method allows us to quantitatively extract shear-dependent effective mobilities. Remarkably, our experiments reveal that for sufficiently large strains, GBs with normals parallel to shear undergo preferential kinetic roughening resulting in anisotropic enhancement of effective mobilities and hence directional grain growth. Single-particle level analysis shows that the anisotropy in mobility emerges from strain-induced directional enhancement of activated particle hops normal to the GB plane. Finally, in Chapter 7, we present our conclusions and discuss possible future directions.

À l'approche de la transition vitreuse les fondus de polymères montrent une augmentation importante de la viscosité de plusieurs ordres de grandeur. Le but de cette étude est d'acquérir une compréhension plus profonde des propriétés viscoélastiques des fondus de polymères vitrifiables. Les polymères sont modélisés comme des chaînes flexibles en utilisant un modèle de bille-ressort. Les propriétés dynamiques sont analysées dans le cadre de la théorie de couplage de mode idéale. Nous constatons que la température critique de couplage de mode varie avec l'inverse de la longueur de chaîne. En étudiant la fonction de relaxation de cisaillement, nous constatons que les processus de relaxation polymériques, ne sont pas modifiés, mais décalés vers des temps plus importants en approchant la transition vitreuse. / Polymer melts show a remarkable increase of their viscosity by many orders of magnitude on approaching the glass transition. The aim of this study is to gain a deeper insight into the viscoelastic properties of glass forming polymer melts. The polymers are modeled as flexible chains using a bead-spring model. The dynamic properties are analyzed in the framework of the ideal mode-coupling theory. We find that the critical temperature of the ideal mode-coupling theory scales with the reciprocal chain length. By studying the shear relaxation function we find that the polymer relaxation processes are not altered but shifted to later times in the approach of the glass transition.

An increase in the co-operativity in the motion of particles and a growth of a suitably defined dynamical correlation length seem to be generic features exhibited by all liquids upon supercooling. These features have been observed both in experiments and in numerical simulations of glass-forming liquids. Specially designed NMR experiments have estimated that the rough magnitude of this correlation length is of the order of a few nanometers near the glass transition. Simulations also predict that there are regions in the system which are more liquid-like than other regions. A complete theoretical understanding of this behaviour is not available at present. In recent calculations, Berthier, Biroli and coworkers [1, 2] extended the simple mode coupling theory (MCT) to incorporate the effects of dynamic heterogeneity and predicted the existence of a growing dynamical correlation length associated with the cooperativity of the dynamics. MCT also predicts a power law divergence of different dynamical quantities at the mode coupling temperature and at temperatures somewhat higher than the mode coupling temperature, these predictions are found to be consistent with experimental and simulation results. The system size dependence of these quantities should exhibit finite size scaling (FSS) similar to that observed near a continuous phase transition in the temperature range where they show power law growth. Hence we have used the method of finite size scaling in the context of the dynamics of supercooled liquids. In chapter 2, we present the results of extensive molecular dynamics simulations of a model glass forming liquid and extract a dynamical correlation length ξ associated with dynamic heterogeneity by performing a detailed finite size scaling analysis of a four-point dynamic susceptibility χ4(t) [3] and the associated Binder cumulant. We find that although these quantities show the “normal” finite size scaling behaviour expected for a system with a growing correlation length, the relaxation time τ does not. Thus glassy dynamics can not be fully understood in terms of “standard” critical phenomena. Inspired by the success of the empirical Adam-Gibbs relation [4] which relates dynamics with the configurational entropy, we have calculated the configurational entropy for different system sizes and temperatures to explain the nontrivial scaling behaviour of the relaxation time. We find that the behaviour of the relaxation time τ can be explained in terms of the Adam-Gibbs relation [4] for all temperatures and system sizes. This observation raises serious questions about the validity of the mode coupling theory which does not include the effects of the potential energy (or free energy) landscape on the dynamics. On the other hand, in the “random first order transition” theory (RFOT), introduced by Wolynes and coworkers [5], the configurational entropy plays a central role in determining the dynamics. So we also tried to explain our simulation results in terms of RFOT. However, this interpretation has the drawback that the value of one of the exponents of this theory extracted from our numerical results does not satisfy an expected physical bound, and there is no clear explanation for the obtained values of other exponents. Thus we find puzzling values for the exponents relevant to the applicability of RFOT, which are in need of explanation. This can be due to the fact that RFOT focuses only near the glass transition, while all our simulation results are for temperatures far above the glass transition temperature (actually, above the mode coupling temperature). Interestingly, results similar to ours were obtained in a recent analysis [6] of experimental data near the laboratory glass transition, on a large class of glass-forming materials. Thus right now we do not have any theory which can explain our simulation data consistently from all perspectives. There have been some attempts to extend the RFOT analysis to temperatures above the mode coupling temperature [7, 8] and to estimate a length scale associated with the configurational entropy at such temperatures. We compare our results with the predictions arising from these analyses. In chapter 3, we present simulation results that suggest that finite size scaling analysis is probably the only feasible method for obtaining reliable estimates of the dynamical correlation length for supercooled liquids. As mentioned before, although there exists a growing correlation length, the behaviour of all measured quantities (specifically, the relaxation time) is not in accordance with the behaviour expected in “standard” critical phenomena. So one might suspect the results for the correlation length extracted from the scaling analysis. To find out whether the results obtained by doing finite size scaling are correct, we have done simulations of very large system sizes for the same model glass forming liquid. In earlier studies, the correlation length has been extracted from the wave vector dependence of the dynamic susceptibility in the limit of zero wave vector, but to estimate the correlation length with reasonable accuracy one needs data in the small wave vector range. This implies that one needs to simulate very large systems. But as far as we know, in all previous studies typical system sizes of the order of 10, 000 particles have been used to do this analysis. In this chapter we show by comparing results for systems of 28, 000 and 350, 000 particles that these previous estimates are not reliable. We also show that one needs to simulate systems with at least a million particles to estimate the correlation length correctly near the mode coupling temperature and this size increases with decreasing temperature. We compare the correlation length obtained by analyzing the wave vector dependence of the dynamic susceptibility for a 350, 000particle system with the results obtained from the finite size scaling analysis. We were only able to compare the results in the high temperature range due to obvious reasons. However the agreement in the high temperature range shows that the finite size scaling analysis is robust and also establishes the fact that finite size scaling is the only practical method to extract reliable correlation lengths in supercooled liquids. In chapter 4, we present a free energy landscape analysis of dynamic heterogeneity for a monodisperse hard sphere system. The importance of the potential energy landscape for particles interacting with soft potentials is well known in the glass community from the work of Sastry et al. [9] and others, but the hard sphere system which does not have any well defined potential energy landscape also exhibits similar slow dynamics in the high density limit. Thus it is not clear how to treat the hard sphere systems within the same energy landscape formalism. Dasgupta et al. [10, 11, 12, 13, 14, 15] showed that one can explain the slow dynamics of these hard core systems in term of a free energy landscape picture. They and other researchers showed that these system have many aperiodic local minima in its free energy landscape, with free energy lower than that of the liquid. Using the Ramkrishnan-Yussouff free energy functional, we have performed multi parameter variational minimizations to map out the detailed density distribution of glassy free energy minima. We found that the distribution of the widths of local density peaks at glassy minima is spatially heterogeneous. By performing hard sphere event driven molecular dynamics simulation, we show that there exists strong correlation between these density inhomogeneity and the local Debye-Waller factor which provides a measure of the dynamic heterogeneity observed in simulations. This result unifies the system of hard core particles with the other soft core particles in terms of a landscapebased description of dynamic heterogeneity. In chapter 5, we extend the same free energy analysis to a polydisperse system and show that there is a critical polydispersity beyond which the crystal state is not stable and glassy states are thermodynamically stable. We also found a reentrant behaviour in the liquid-solid phase transition within this free-energy based formalism. These results are in qualitative agreement with experimental observations for colloidal systems.

Les matériaux moléculaires amorphes, aussi appelés verres moléculaires, sont constitués de molécules organiques de petite taille capables de s’organiser de façon désordonnée. En plus de présenter certaines des propriétés analogues à celles des polymères, ils offrent des avantages supplémentaires, puisqu’ils sont des espèces isomoléculaires dont la synthèse, la purification et la mise en œuvre sont facilitées par leur viscosité relativement faible. Toutefois, la préparation souvent exigeante de ces matériaux et leur durée de vie utile limitée par leur tendance à relaxer vers l’état cristallin demeurent des obstacles à leur utilisation pour certaines applications, e.g. opto-électronique, nanolithographie, pharmaceutique. Le développement de stratégies visant à faciliter la préparation de la phase vitreuse et éviter sa cristallisation est donc essentiel à la conception de matériaux moléculaires amorphes fonctionnels. L’objectif principal de cette thèse est d’établir des relations entre la structure moléculaire des verres moléculaires et leurs propriétés. Pour y arriver, différentes librairies de composés modèles, des dérivés analogues de triazine ayant démontré une excellente capacité à former une phase vitreuse, sont utilisées pour i) déterminer l’influence de la nature et de la position des groupements sur la triazine; ii) explorer l’influence des liaisons hydrogène sur les propriétés des verres lorsque leur structure comporte des groupements fonctionnels reconnus pour faciliter la cristallisation et lorsque leurs conditions de préparation se rapprochent de celles employées en industrie et iii) exploiter la phase amorphe afin d’étudier la photosensibilité des azobenzènes (azo) en vue d’optimiser leur utilisation dans des applications. Tout d’abord, l’influence des différents groupes substituants sur la triazine (groupements de tête, auxiliaires et liants) sur la capacité des composés à former une phase vitreuse (GFA), sur sa stabilité cinétique (GS) et sur sa température de transition vitreuse (Tg) est étudiée. Un système de classification des composés développé à partir de mesures de calorimétrie différentielle à balayage (DSC) et des mesures de spectroscopie infrarouge (IR) à température variable combinées à des analyses chimiométriques facilitent la rationalisation des rôles joués par chaque groupe. L’impact des liaisons hydrogène (H), de la barrière énergétique de rotation et de l’encombrement stérique des groupements est ainsi déterminé, permettant de conclure que le groupe de tête est le plus influent et que la présence de liaisons H n’est pas essentielle au GFA mais qu’elle est importante pour obtenir une Tg élevée. Ensuite, l’influence des liaisons H sur les propriétés des verres se rapprochant de ceux exploités dans l’industrie est explorée. Des mesures de spectroscopie IR à température variable, de DSC et de résolution de structures cristallines ont permis de conclure que les liaisons H réussissent à nuire à la cristallisation des composés et ce, même s’ils sont simultanément fonctionnalisés avec des motifs qui favorisent la cristallisation (empilements π-π entre dérivés stilbènes fluorés et non fluorés). De plus, trois composés analogues fonctionnalisés avec un groupement de tête possédant une capacité décroissante à établir des liaisons H (donneur, accepteur, aucune) ont été déposés en phase vapeur (PVD), une technique employée entre autres dans l’industrie opto-électronique pour évaluer leur capacité à former des verres ultrastables. Les films ainsi préparés présentent tous des propriétés similaires à celles des verres ultrastables précédemment étudiés, telles qu’une plus grande densité et anisotropie, et sont tous plus stables que ceux préparés par refroidissement à partir de l’état liquide. Toutefois, le verre formé du composé avec un groupement de tête donneur de liaisons H est moins stable que les autres d’au moins un ordre de grandeur, suggérant que les liaisons H limitent le niveau de stabilité atteignable par PVD. Finalement, un verre à base de triazine fonctionnalisé avec un groupement azo est employé pour étudier d’un point de vue moléculaire les perturbations provoquées par la photoisomérisation de l’azo. Grâce à une nouvelle méthode de spectroscopie IR, il est possible d’observer un gradient d’environnement moléculaire le long de la molécule lors de la photoisomérisation, permettant de soutenir certaines hypothèses relatives au déplacement macroscopique de la matière qui en résulte. Les mélanges de verres à base de triazine servent aussi de plateforme idéale pour découpler l’influence de la Tg et du contenu en azo sur la photo-orientation de l’azo, mais aussi sur la cinétique d’écriture et l’efficacité des réseaux de diffraction (SRG). Ce travail permet ainsi de déterminer une zone optimale de Tg pour l’inscription de SRG. Ces nouvelles connaissances mèneront à la conception plus rationnelle de nouveaux verres moléculaires, pouvant s’étendre à d’autres matériaux amorphes. / Amorphous molecular materials, also known as molecular glasses, are small organic molecules capable of being organized in a disordered manner. In addition to sharing some of the useful properties of polymers, they offer additional advantages because they are isomolecular species for which synthesis, purification and processing are facilitated by a relatively low viscosity. However, the usually demanding preparation conditions of these materials and their limited functional lifetime due to their tendency to relax to the crystalline state remain obstacles to their use for certain applications, e.g. opto-electronics, nanolithography, pharmaceuticals. The development of strategies to facilitate the preparation of the vitreous phase and avoid its crystallization is therefore essential for the design of functional amorphous molecular materials. The main objective of this thesis is to establish relationships between the molecular structure of molecular glasses and their properties. To achieve it, various libraries of model compounds, analogues of triazine derivatives that have demonstrated excellent glass-forming ability, are used to i) determine the influence of the nature and the position of the groups on the triazine; ii) explore the influence of hydrogen (H) bonds on the properties of glasses when their structure includes functional groups known to facilitate crystallization and when their preparation conditions are similar to those used in industry; and iii) exploit the amorphous phase in order to study the photoresponsiveness of azobenzenes (azo) in order to optimize their use in different applications. The influence of the various substituent groups on the triazine (headgroup, ancillary and linkers) on the glass-forming ability (GFA), the kinetic glass stability (GS) and the glass transition temperature (Tg) of the compounds is first studied. A classification system based on differential scanning calorimetry (DSC) and variable temperature infrared spectroscopy (IR) measurements combined to chemometrics analyses facilitate the rationalization of the roles played by each group. The impact of the H-bonds, the energy of the rotation barrier, and the steric hindrance of the groups is determined, leading to the conclusion that the headgroup is the most influential group and that the presence of H-bonds is not essential to the GFA, but important to obtain a high Tg. The influence of the H-bonds on the properties of glasses approaching those exploited in industry is then explored. Variable temperature IR spectroscopy measurements, DSC studies, and single crystal structure resolution have led to the conclusion that H-bonds impede the crystallization of the compounds even though they are simultaneously functionalized with moieties that promote crystallization (π-π stacking between fluorinated and non-fluorinated stilbene groups). In addition, three similar compounds functionalized with a headgroup presenting a decreasing capability to establish H-bonds (donor, acceptor, none) were vapor-deposited (PVD), a technique used, among others, in the opto-electronic industry, to evaluate their capability to form ultrastable glasses. These PVD glasses all show properties that are similar to those previously reported for ultrastable glasses, including higher density and anisotropy, and are all more kinetically stable than glasses prepared by cooling from the viscous state. However, the PVD glasses prepared with a H-bond donor headgroup are less stable than the others by at least an order of magnitude, suggesting that H-bonds limit the level of kinetic stability achievable by PVD. Finally, a triazine molecular glass functionalized with an azo group is used to study, from a molecular point of view, the perturbations caused by the photoisomerization of the azo. A new IR spectroscopy method was developed to observe a molecular environment gradient along the molecule during photoisomerization, making it possible to support certain hypotheses concerning the resulting macroscopic transport of the material. Triazine-based molecular glass blends are also used as an ideal platform for decoupling the influence of Tg and azo content on the azo photo-orientation, but also on the inscription kinetics and the diffraction efficiency of surface relief gratings (SRGs). This work enables the determination of an optimal Tg range for the inscription of SRGs. Altogether, these new insights will lead to a more rational design of new molecular glasses, which can extend to other amorphous molecular materials.