Academic literature on the topic 'Crystallization Kinetics - Glasses'

Nanocrystallization of anatase phase was established in BaO-TiO2-B2O3 glass system. Crystallization kinetics of anatase phase in these glasses were investigated using non-isothermal differential scanning calorimetry (DSC) at three different heating rates (10, 20 & 30 K/min). Scanning Electron Microscopy (SEM) carried out on heat treated (at 920 K) glasses confirmed bulk nucleation and three-dimensional growth. Johnson-Mehl-Avarami model could not be applied for this system suggesting considerable overlap of the nucleation and growth involving complex transformation process. However, modified Kissinger and Ozawa models were used to calculate the effective activation energy associated with anatase crystallization. The kinetic exponent n was found to be temperature dependent indicating the change in the crystallization mechanism. This was attributed to the high entropy fusion of anatase phase, fast crystallization rate and nano dimension of the anatase phase.

In this study, Se85Te15−xAgx (x = 3, 6, 9 and 12) chalcogenide glasses were examined for their structure, crystallization kinetics, and physical characteristics. The kinetics of crystallization in these glasses were studied using various methods. By using the melt quenching process, Se85Te15−xAgx bulk alloys were created. The amorphous nature of the alloys was confirmed using High Resolution X-Ray Diffraction (HRXRD). The crystallization kinetics of the Se85Te15−xAgx glasses were studied using non-isothermal differential scanning calorimetry (DSC) measurements at heating speeds of 5, 10, 15, 20 and 25 K/min. The different characteristic temperatures, including the glass transition (Tg) and on-set crystallization (Tc) temperatures, have been determined from a variety of DSC thermograms. Using the Kissinger and Moynihan techniques, the activation energies of the glass transition (ΔEt) were computed.

Time-resolved x-ray scattering has been used to study the kinetics of polymorphic crystallization of amorphous metals on time-scales varying from minutes to milliseconds. Using a high flux synchrotron beamline and fast position-sensitive detector system, simultaneous wide and small angle scattering patterns from the transforming material have been acquired in situ, with time-resolution as short as 3ms. A fast pyrometric temperature controller has been developed to change and regulate the sample temperature with microsecond response. Two alloys have been identified in which crystallization proceeds by thermally activated nucleation and growth of a single equilibrium phase at all observable transformation rates. Crystal volume fractions as small as 10$ sp{-4}$ can be measured and the results are well explained by a simple nucleation and growth model. There is no evidence for a nucleation transient. Small angle scattering from such alloys reveals an increase in intensity prior to crystallization followed by a rise commensurate with the crystallization itself. In all other studied alloys however, as the transformation rate is increased, the crystallization mechanism changes such that transient crystal phases are formed en route to the equilibrium structure. Two new metastable phases have been crystallographically identified. At high equilibrium rates, the kinetically favored phases are those which require the least atomic rearrangement from the glassy state.

The formation of metastable crystalline phases in lithium disilicate glass has been a subject of controversy for decades. Here, one aspect of this problem relating to the stability of these non-equilibrium phases when glasses are heated for extended time periods in the nucleation regime is addressed. The results of a systematic experimental investigation on the persistence of metastable phases and the factors that may influence the appearance of such phases, e.g., water content, impurities, glass composition, and glass preparation procedure are presented. Growth rates of lithium disilicate crystals in lithium disilicate glass are measured as a function water concentration in the glass and of temperature in the deeply undercooled regime. The growth rate data obtained in this work are combined with data reported in the literature and used to assess the applicability of standard models of crystal growth for the description of experimental results over a very broad temperature range. The reduced growth rate versus undercooling graph is found to consist of three regimes. For undercoolings less than 140°C, the reduced growth rate curve is suggestive of either 2-D surface nucleation or screw dislocation growth. For undercoolings greater than 400°C, the reduced growth rate plot suggests the operative crystal growth mechanism is 2-D surface nucleation, but detailed calculations cast doubt upon this conclusion. In the intermediate undercooling range, there appears to be some sort of transitional behavior for which none of the standard models appear to be applicable. Further, it is observed that small differences in the viscosity data employed can produce enormous differences in the predicted growth rates at larger undercoolings. Results of the kinetic analyses conducted herein seem to indicate that the nature of the kinetic rate coefficient used in the standard growth models may be incorrect. Nucleation rates of sodium metasilicate crystals in a sodium silicate glass of composition 43Na₂O57SiO₂ (mol%) are investigated using the development technique. The results of this study are compared with the nucleation rate results recently obtained for this composition using a novel DTA method. The two techniques are found to agree within experimental error.

L'objectiu de la tesi és estudiar la cinètica de les cristal·litzacions primàries en vidres metàl·lics mitjançant simulacions de tipus phase field. Una cristal·lització primària és una transició de fase sòlid-sòlid on la fase que cristal·litza (fase transformada o fase secundaria) té una composició química diferent de la fase precursora (fase no transformada o fase primària).
Les dades experimentals obtingudes a partir de l'estudi calorimètric de cristal·litzacions primàries s'analitzen generalment en el marc del model KJMA (Kolmogorov, Johnson & Mehl, Avrami). Aquest model proporciona l'evolució temporal de la fracció transformada basant-se en tres hipòtesis:
- Els nuclis de la fase secundaria estan distribuïts aleatòriament en tot l'espai.
- El creixement d'aquests nuclis és isotròpic.
- El creixement s'atura únicament per xoc directe (hard impingement).

En la cristal·lizació de vidres metàl·lics s'ha observat experimentalment un alentiment de la cinètica respecte del comportament calculat emprant la citada cinètica KJMA. Aquest alentiment s'explica a la literatura en base a que en aquest tipus de transformacions, controlades per difusió, la interacció entre els cristalls no és directa sinó que es produeix a través dels perfils de concentració (soft impingement) i, a més, l'evolució d'aquests perfils de concentració causa canvis en la concentració de la matriu amorfa, estabilitzant la i per tant fent que la nucleació de nous cristalls esdevingui no aleatòria. Diversos autors han proposat modificacions del model KJMA per tal d'intentar superar aquestes limitacions, basats bé en consideracions geomètriques, bé en aproximacions de camp mitjà. A pesar de tot, cap d'aquests models és capaç d'explicar satisfactòriament la cinètica observada en cristal·litzacions primàries. L'objectiu d'aquest treball ha estat la simulació realista de la cinètica de les transformacions primàries per trobar una explicació consistent a les diferències observades entre les dades experimentals i els models teòrics disponibles.
Per tal de poder descriure de forma realista el procés de cristal·lització primària s'ha d'estudiar el procés de nucleació i creixement de la fase secundaria alhora que es resol l'equació de difusió en la fase primària. En aquest treball s'ha emprat un model de simulació phase field que permet estudiar aquest sistema introduint una nova variable lligada al camp de concentració que pren dos valors diferents segons es tracti de fase transformada o no transformada. Amb aquest tipus de models també es poden introduir diferents protocols de nucleació i per tant estudiar independentment els efectes de la nucleació en la cinètica. D'aquesta manera s'han realitzat simulacions en 2 i 3 dimensions de cristal·litzacions primàries amb diferents graus de fracció transformada final). Els resultats de les simulacions s'ha comparat amb el model KJMA i, contra el que es preveia, s'ha obtingut un bon acord entre les fraccions transformades del model KJMA i de les simulacions. Donat que el model KJMA no reprodueix satisfactòriament el comportament experimental d'aquest resultat es dedueix que ni el soft impingement ni la nucleació no aleatòria son les responsables de l'alentiment de la cinètica obtingut en cristal·litzacions primàries.
Per tal de trobar una explicació físicament convincent del comportament observat experimentalment s'ha aprofundit en l'estudi teòric de les cristali·litzaciones primàries, incloent-hi l'efecte dels canvis composicionals que tenen lloc en la matriu a mesura que la transformació es produeix. Aquest fet, tot i ser conegut a la bibliografia, ha estat sistemàticament ignorat en l'elaboració de models cinètics. En concret, s'ha fet palès que canvis en la composició química de la fase primària han d'afectar de forma radical a la viscositat, que varia fortament a prop de la transició vitrea, i han de produir canvis en les propietats de transport atòmic. Això s'ha modelat a través de l'assumpció d'un coeficient de difusió depenent de la concentració, en base a la relació modificada d'Stokes-Einstein entre la viscositat i el coeficient de difusió. Les simulacions phase-field amb un coeficient de difusió d'aquest tipus donen lloc a una cinètica més lenta i que mostra un acord excel·lent amb la cinètica experimentalment observada en cristal·litzacions primàries de vidres metàl·lics. Per tant, les simulacions phase field confirmen que la cinètica de les cristal·litzacions primàries està controlada fonamentalment pel canvi en les propietats de transport atòmic, mentre que els efectes de soft impingement i nucleació no aleatoria, tot i estar presents, son secundaris.
El objetivo de la tesi es estudiar la cinética de las cristalizaciones primarias en vidrios metálicos mediante simulaciones de tipo phase field. Una cristalización primaria es una transición de fase sólido-sólido donde la fase que cristaliza (fase transformada o fase secundaria) tiene una composición química diferente a la fase precursora (fase no transformada o fase primaria).
Los datos experimentales obtenidos a partir del estudio calorimétrico de cristalizaciones primarias se analizan generalmente en el marco del modelo KJMA (Kolmogorov, Johnson & Mehl, Avrami). Este modelo proporciona la evolución temporal de la fracción transformada basándose en tres hipótesis:
- Los núcleos de la fase secundaria están distribuidos aleatoriamente en todo el espacio
- El crecimiento de estos núcleos es isotrópico
- El crecimiento se detiene únicamente por choque directo (hard impingement).

En la cristalización de vidrios metálicos se ha observado experimentalmente un retardo de la cinética respecto del comportamiento calculado usando la cinética KJMA. Este retardo se explica en la literatura en base a que en este tipo de transformaciones, controladas por difusión, la interacción entre los cristales no es directa sino que se produce a través de los perfiles de concentración (soft impingement) y, además, la evolución de estos perfiles de concentración causa cambios en la concentración de la matriz amorfa, estabilizándola y por tanto haciendo que la nucleación de nuevos cristales sea no aleatoria. Varios autores han propuesto modificaciones del modelo KJMA para intentar superar estas limitaciones, basados bien en consideraciones geométricas, bien en aproximaciones de campo medio. A pesar de todo, ninguno de estos modelos es capaz de explicar satisfactoriamente la cinética observada en cristalizaciones primarias. El objetivo de este trabajo ha sido la simulación realista de la cinética de las transformaciones primarias para hallar una explicación consistente a las diferencias entre los datos experimentales y los modelos teóricos disponibles.
Para describir de manera realista el proceso de cristalización primaria se tiene que estudiar el proceso de nucleación y crecimiento de la fase secundaria a la vez que se resuelve la ecuación de difusión en la fase primaria. En este trabajo se ha usado un modelo de simulación phase-field que permite estudiar este sistema introduciendo una nueva variable ligada al campo de concentración que toma dos valores diferentes según se trate de fase transformada o no transformada. Con este tipo de modelos también se pueden introducir diferentes protocolos de nucleación y por tanto estudiar independientemente los efectos de la nucleación en la cinética. De esta manera se han realizado simulaciones en 2 y 3 dimensiones de cristalizaciones primarias con diferentes grados de fracción transformada final. Los resultados de la simulaciones se han comparado con el modelo KJMA y, en contra de lo que se preveía, se ha obtenido un buen acuerdo entre las fracciones transformadas del modelo KJMA y de las simulaciones. Dado que el modelo KJMA no reproduce satisfactoriamente el comportamiento experimental, de este resultado se deduce que ni el soft impingement ni la nucleación no aleatoria son las responsables del retardo en la cinética obtenido en cristalizaciones primarias.
Para encontrar una explicación físicamente convincente del comportamiento observado experimentalmente se ha profundizado en el estudio teórico de las cristalizaciones primarias, incluyendo el efecto de los cambios composicionales que tienen lugar en la matriz a medida que la transformación se produce. Este hecho, aún y ser conocido en la bibliografía, ha sido sistemáticamente ignorado en la elaboración de modelos cinéticos. En concreto, se ha hecho patente que cambios en la composición química de la fase primaria tienen que afectar de forma radical a la viscosidad, que varía fuertemente cerca de la transición vítrea, y tienen que producirse cambios en las propiedades de transporte atómico. Esto se ha modelado a través de la asunción de un coeficiente de difusión dependiente de la concentración, en base a la relación de Stokes-Einstein modificada entre la viscosidad y el coeficiente de difusión. Las simulaciones phsae-field con un coeficiente de difusión de este tipo dan lugar a una cinética más lenta y que muestra un acuerdo excelente con la cinética experimentalmente observada en cristalizaciones primarias de vidrios metálicos. Por tanto, las simulaciones phase-field confirman que la cinética de las cristalizaciones primarias está controlada fundamentalmente por los cambios en las propiedades de transporte atómico, mientras que los efectos de soft-impingement y nucleación no aleatoria, aún y estar presentes, son secundarios.
The aim of this thesis is to study the kinetics of primary crystallization in metallic glasses by means of phase-field simulations. A primary crystallization is a solid-solid phase transformation where the crystallized phase (transformed phase or secondary phase) has a chemical composition different than the precursor phase (untransformed phase or primary phase).
Experimental data from calorimetric studies of primary crystallization are usually studied in the framework of the KJMA model (Kolmogorov, Johnson & Mehl, Avrami). This model yields the temporal evolution of the transformed fraction on the basis of three main assumptions:
- A random distribution of particle nuclei of the secondary phase
- The growth of these nuclei is isotropic
- The growth is only halted by direct collisions (hard impingement).

In the crystallization of metallic glasses, a slowing down of the kinetics respect the behavior calculated with the KJMA kinetics has been observed. This delay is explained in the literature by the fact that in this kind of transformations, that are diffusion controlled, the interaction between the crystals is not direct but through the concentration profiles (soft impingement) and moreover, the evolution of these profiles causes changes in the concentration of the amorphous matrix, stabilizing it and thus, the nucleation of new nuclei become non random. Several authors had proposed modifications to the KJMA model to try to overcome these limitations, based either on geometrical considerations or in mean field approaches. However, none of these models is able to explain the observed kinetics in primary crystallizations. The aim of this work has been the realistic simulation of the kinetics of primary crystallization to find a explanation to the differences between the experimental data and the available theoretical models.
In order to describe in a realistic way the process of a primary crystallization, the nucleation and growth process of the secondary phase has to be studied at the same time that the diffusion equation is solved in the primary phase. In this work, it has been used a phase field model for the simulations that allows to study this system introducing a new variable, coupled to the concentration field, that takes two different values in each of the existing phases. With these kinds of models, different nucleation protocols can also be introduced and thus, independently study the effects of the nucleation in the kinetics. Therefore, 2 and 3 dimensional simulations of primary crystallization have been performed with several degrees of final transformed fraction. The simulation results have been compared with the KJMA model and, unexpectedly, a good agreement between the simulations and the KJMA model has been obtained. As the KJMA model does not reproduce satisfactorily the experimental behavior, from this result can be deduced that neither the soft impingement nor the non random nucleation are the responsible of the slowing down observed in the kinetics of primary crystallization.
In order to find a physical convincing explanation of the observed experimental behavior, the theoretical study of primary crystallization has been extended, including the effects of the compositional changes that take place in the matrix as the transformation proceed. This fact, notwithstanding being known in the literature, has been systematically ignored in the development of the kinetics models. In particular, it has become clear that changes in the chemical composition of the primary phase have to radically affect the viscosity, that strongly varies near the glass transition, and some changes in the atomic transport properties must occur. This has been modeled through the assumption of a compositional dependent diffusion coefficient, on the basis of a modified Stokes-Einstein relation between viscosity and diffusion coefficient. Phase field simulations with a diffusion coefficient of this type yield a slower kinetics and show an excellent agreement with the kinetics experimentally observed in primary crystallization of metallic glasses. Thus, phase field simulations confirm that the kinetics of primary crystallization is fundamentally controlled by the changes in the atomic transport properties, while the soft impingement and non random effects, although being present, are secondary.

(Bulk) metallic glasses ((B)MGs) are known to exhibit the highest yield strength of any metallic material (up to 5GPa), and show an elastic strain at ambient conditions, which is about ten times larger than that of crystalline materials. Despite these intriguing mechanical properties, BMGs are not used as structural materials in service, so far. The major obstacle is their inherent brittleness, which results from severe strain localization in so-called shear bands. MGs fail due to formation and propagation of shear bands. A very effective way to attenuate the brittle behaviour is to incorporate crystals into the glass. The resulting BMG composites exhibit high strength as well as plasticity. Cu-Zr-Al-based BMG composites are special to that effect, since they combine high strength, plasticity and work-hardening. They are comprised of the glass and shape-memory B2 CuZr crystals, which can undergo a deformation-induced martensitic transformation. The work-hardening originates from the martensitic transformation and overcompensates the work-softening of the glass. The extent of the plasticity of BMG composites depends on the volume fraction, size and particularly on the distribution of the B2 CuZr crystals. Nowadays, it is very difficult, if not impossible to prepare BMG composites with uniformly distributed crystals in a reproducible manner by melt-quenching, which is the standard preparation method. Flash-annealing of BMGs represents a new approach to overcome this deficiency in the preparation of BMG composites and is the topic of the current thesis. Cu46Zr46Al8 and Cu44Zr44Al8Hf2Co2 BMGs were flash-annealed and afterwards investigated in terms of phase formation, crystallization kinetics and mechanical properties. Flash-annealing is a process, which is characterized by the rapid heating of BMGs to predefined temperatures followed by instantaneous quenching. A temperature-controlled device was succesfully developed and built. The Cu-Zr-Al-based BMGs can be heated at rates ranging between 16 K/s and about 200 K/s to temperatues above their melting point. Rapid heating is followed by immediate quenching where cooling rates of the order of 1000 K/s are achieved. As a BMG is flash-annealed, it passes the glass-transition temperature, Tg, and transforms to a supercooled liquid. Further heating leads to its crystallization and the respective temperature, the crystallization temperature, Tx, divides the flash-annealing of BMGs into two regimes: (1) sub-Tx-annealing and (2) crystallization. The structure of the glass exhibits free volume enhanced regions (FERs) and quenched-in nuclei. Flash-annealing affects both heterogeneities and hence the structural state of the glass. FERs appear to be small nanoscale regions and they can serve as initiation sites for shear bands. Flash-annealing of Cu-Zr-Al-based BMGs to temperatures below Tg leads to structural relaxation, the annihilation of FERs and the BMG embrittles. In contrast, the BMG rejuvenates, when flash-annealed to temperatures of the supercooled liquid region (SLR). Rejuvenation is associated with the creation of FERs. Compared to the as-cast state, rejuvenated BMGs show an improved plasticity, due to a proliferation of shear bands, which are the carrier of plasticity in MGs. Flash-annealing enables to probe the influence of the free volume in bulk samples on their mechanical properties, which could not be studied, yet. In addition, B2 CuZr nanocrystals precipitate during the deformation of flash-annealed Cu44Zr44Al8Hf2Co2 BMGs. Deformation-induced nanocrystallization does not occur for the present as-cast BMGs. Flash-annealing appears to stimulate the growth of quenched-in nuclei, which are subcritical in size and can also dissolve, once the BMG is heated to temperatures in the SLR. Rejuvenation represents a disordering process, whereas the growth of quenched-in nuclei is associated with ordering. There is a competition between both processes during flash-annealing. The ordering seems to lead to a “B2-like” clustering of the medium range of Cu44Zr44Al8Hf2Co2 BMGs with increasing heating duration. So far, there does not exist another method to manipulate the MRO of BMGs. If Cu44Zr44Al8Hf2Co2 BMGs are flash-annealed to temperatures near Tx, most likely compressive resiudal stresses develop near the surface, which is cooled faster than the interior of the BMG specimen. They hinder the propagation of shear bands and increase the plasticity of flash-annealed BMGs in addition to rejuvenation and deformation-induced nanocrystallization. If BMGs are heated to temperatures above Tx, they start to crystallize. Depending on the exact temperature to which the BMG is flash-annealed and subsequently quenched, one can induce controlled partial crystallization. Consequently, BMG composites can be prepared. Both Cu-Zr-Al-based BMGs are flash-annealed at various heating rates to study the phase formation as a function of the heating rate. In addition, Tg and Tx are identified for each heating rate, so that a continuous heating transformation diagram is constructed for both glass-forming compositions. An increasing heating rate kinetically constrains the crystallization process, which changes from eutectic (Cu10Zr7 and CuZr2) to polymorphic (B2 CuZr). If the Cu-Zr-Al-based BMGs are heated above a critical heating rate, exclusively B2CuZr crystals precipitate, which are metastable at these temperatures. Thus, flash-annealing of Cu46Zr46Al8 and Cu44Zr44Al8Hf2Co2 BMGs followed by quenching enables the preparation of B2 CuZr BMG composites. The B2 precipitates are small, high in number and uniformly distributed when compared to conventional BMG composites prepared by melt-quenching. Such composite microstructures allow the direct observation of crystal sizes and numbers, so that crystallization kinetics of deeply supercooled liquids can be studied as they are flash-annealed. The nucleation kinetics of devitrified metallic glass significantly diverge from the steady-state and at high heating rates above 90 K/s transient nucleation effects become evident. This transient nucleation phenomenon is studied experimentally for the first time in the current thesis. Once supercritical nuclei are present, they begin to grow. The crystallization temperature, which depends on the heating rate, determines the crystal growth rate. At a later stage of crystallization a thermal front traverses the BMG specimen. In levitation experiments, this thermal front is taken as the solid-liquid interface and its velocity as the steady-state crystal growth rate. However, the thermal front observed during flash-annealing, propagates through the specimen about a magnitude faster than is known from solidification experiments of levitated supercooled liquids. As microstructural investigations show, crystals are present in the whole specimen, that means far ahead of the thermal front. Therefore, it does not represent the solid-liquid interface and results from the collective growth of crystals in confined volumes. This phenomenon originates from the high density of crystals and becomes evident during the heating of metallic glass. It could be only observed for the first time in the current thesis due to the high temporal resolution of the high-speed camera used. The heating rate and temperature to which the BMG is flash-annealed determine the nucleation rate and the time for growth, respectively. The size and number of B2 CuZr crystals can be deliberately varied. Thus mechanical properties of B2 CuZr BMG composites can be studied as a function of the volume fraction and average distance of B2 particles. Cu44Zr44Al8Hf2Co2 BMG specimens were flash-annealed at a lower and higher heating rate (35 K/s and 180 K/s) to different temperatures above Tx and subsequently subjected to uniaxial compression. BMG composites prepared at higher temperatures show a lower yield strength and larger plastic strain due to the higher crystalline volume fraction. They not only exhibit plasticity in uniaxial compression, but also ductility in tension as a preliminary experiment demonstrates. Furthermore, nanocrystals precipitate in the amorphous matrix of BMG composites during deformation. They grow deformation-induced from quenched-in nuclei, which are stimulated during flash-annealing. In essence, flash-annealing of BMGs is capable of giving insight into most fundamental scientific questions. It provides a deeper understanding of how annealing affects the structural state of metallic glasses. The number and size of structural heterogeneities can be adjusted to prepare BMGs with improved plasticity. Furthermore, crystallization kinetics of liquids can be studied as they are rapidly heated. Transient nucleation effects arise during rapid heating of BMGs and they cannot be described using the steady-state nucleation rate. Therefore, an effective nucleation rate was introduced. Besides, the flash-annealing process rises the application potential of BMGs. The microstructure of BMG composites comprised of uniformly distributed crystals and the glass, can be reliably tailored. Thus, flash-annealing constitutes a novel method to design the mechanical properties of BMG composites in a reproducible manner for the first time. BMG composites, which exhibit high strength, large plasticitiy and as in the case of B2 CuZr BMG composites as well work-hardening behaviour, can be prepared, so that the intrinsic brittleness of monolithic BMGs is effectively overcome.

Glassy phases play an important role in our daily life and in many industries such as the food, pharmaceutical, and construction and are responsible for certain vital mechanisms in living species. Whereas crystals are solid phases that show periodicity of the constituent atoms or molecules, glasses are disordered solids that lack long-range positional order but behave mechanically like solids. Chapter 1 of the current thesis presents an introduction to the characteristics and dynamics of glassy phases. How they are derived from the liquid phase, and how they transform into the crystalline solid phase, thermodynamically more stable. The temperature at which a liquid transforms to the amorphous (glassy) phase is called the glass transition temperature, Tg. Along this thesis the relaxation dynamics of prilocaine (PLC) and stiripentol (STP), and the isothermal crystallization process of the latter have been experimentally studied. Both PLC and STP are drugs used in medical applications mainly as anesthesia and for the treatment of epilepsy, respectively. The studied materials have been analyzed by Broadband Dielectric Spectroscopy (BDS), Differential Scanning Calorimetry, X-Ray diffraction, Raman and I.R spectroscopy and confocal microscopy. The physical principles of BDS, the main experimental tool employed, are presented in Chapter 2. The details of the experimental set-ups are stated in Chapter 3. In the case of a pharmaceutical product, being able to control and foresee the aggregation phase and dissolution rate of the substance is vital. Many drugs are poorly soluble in water and thus, in biological media. The glass state of a drug is a non-equilibrium state that presents higher free energy than the crystal. This implies, that a glassy drug dissolves more rapidly and can be absorbed in larger amounts. Nevertheless, the higher free energy of glassy phases represents at the same time a major problem for shelf-life, since metastable phases are prone to spontaneously transforming into the stable crystalline state. This is a major problem, since wrong dosage or agglomeration of a drug could render it useless or toxic for the human body. Understanding the glass and crystallization dynamics of drugs, and their interaction with water is key to develop more efficient products. Water is the universal biological solvent. For most materials the addition of water leads to a decrease in viscosity, or equivalently, an increase of molecular mobility, resulting in a lower glass transition temperature Tg (the higher the water content the lower the Tg). This is referred to as the plasticizing effect of water. Chapter 4 presents a detailed analysis of both pure and hydrated prilocaine. Results show that the addition of water to PLC leads to the formation of PLC-water complexes, possibly water-bridged monomers or dimers that increase Tg. This antiplasticizing effect of water on the molecular mobility of a simple glass former represents a significant exception to the alleged universality of water as drug plasticizer. The physico-chemical origins of this behavior have been confirmed by studying the effect of confinement of the pure and hydrated drug in the pores of a nonporous structure (Chapter 5). In the case of STP, not only the glassy dynamics were studied, but also the crystallization process (Chapter 6). A sublinear correlation between the characteristic crystal-growth time and the relaxation time of the cooperative relaxation dynamics of stiripentol was found. This correlation was observed also in other substances, which suggests that it is a general correlation at temperatures above Tg. This may allow predicting a substance's crystallization time as a function of temperature. The results of this thesis provide valuable insight into the kinetics and relaxation dynamics, as well as the phase stability, of both studied drugs that could be general to other amorphous drugs. Global conclusions are outlined in Chapter 7.
Las fases vítreas son importantes en la vida diaria, en industrias como la alimentaria, farmacéutica y construcción, y son responsables de mecanismos vitales en organismos vivos. Mientras que los cristales son fases sólidas que muestran periodicidad en sus átomos o moléculas constituyentes, los vidrios son sólidos desordenados que carecen de orden posicional de largo alcance pero que se comportan mecánicamente como sólidos. El cap. 1 introduce las características y dinámica de las fases vítreas. Cómo se derivan de la fase líquida y cómo se transforman en la fase sólida cristalina, termodinámicamente más estable. La temperatura a la que un líquido se transforma en la fase amorfa (vítrea) se denomina temperatura de transición vítrea, Tg. En esta tesis se estudió experimentalmente la dinámica de relajación de prilocaína (PLC) y estiripentol (STP), y el proceso de cristalización isotérmica del último. Ambas sustancias son fármacos utilizados en aplicaciones médicas principalmente como anestesia y para el tratamiento de la epilepsia, respectivamente. Los materiales estudiados han sido analizados por Espectroscopia Dieléctrica de Banda Ancha (BDS), Calorimetría de Barrido Diferencial, Difracción de Rayos X, espectroscopía Raman e I.R y microscopía confocal. Los principios físicos de BDS, la principal herramienta experimental empleada, se presentan en el Cap. 2 y las configuraciones experimentales en el Cap. 3. En productos farmacéuticos es vital controlar y prever la fase de agregación y velocidad de disolución de la sustancia. Muchos medicamentos son poco solubles en agua y, por lo tanto, en medios biológicos. El estado vítreo es un estado de no equilibrio que presenta una mayor energía libre que el cristal. Consecuentemente, un medicamento vítreo se disuelve más rápidamente y puede absorberse mejor. Sin embargo, la mayor energía libre de las fases vítreas representa al mismo tiempo un problema importante para su vida útil, ya que las fases metaestables son propensas a transformarse espontáneamente en el estado cristalino estable. Ésto es un problema importante, ya que la dosificación incorrecta o la aglomeración de un medicamento pueden volverlo inútil o tóxico para el cuerpo humano. Comprender la dinámica vítrea y la cristalización de las drogas, y su interacción con el agua es clave para desarrollar productos más eficientes. El agua es el solvente biológico universal. En la mayoría de los materiales el agregado de agua conduce a una disminución de la viscosidad o a un aumento de la movilidad molecular, dando una Tg más baja (más agua, menos Tg). Esto se conoce como el efecto plastificante del agua. El Cap. 4 presenta un análisis detallado de la PLC pura e hidratada, mostrando que la adición de agua a PLC conduce a la formación de complejos PLC-agua, posiblemente monómeros o dímeros conectados por agua que aumentan la Tg. Este efecto antiplastificante del agua sobre la movilidad molecular de un simple formador de vidrio representa una excepción significativa a la supuesta universalidad del agua como plastificante de fármacos. Los orígenes físico-químicos de este comportamiento se han confirmado al estudiar el efecto del confinamiento del fármaco puro e hidratado en los poros de una estructura no porosa (Cap. 5). En el caso de STP se estudió la dinámica vítrea y el proceso de cristalización (Cap. 6). Se encontró una correlación sublineal entre el tiempo característico de crecimiento del cristal y el tiempo de relajación de la dinámica cooperativa de relajación del STP y en otras sustancias, sugiriendo una correlación general a temperaturas superiores a Tg. Esto podría permitir predecir el tiempo de cristalización de una sustancia en función de la temperatura. Los resultados de esta tesis proporcionan información valiosa sobre la dinámica de relajación y cinética, así como la estabilidad de fase que podrían ser generales para otros fármacos

Orientador: Victor Ciro Solano Reynoso
Banca: Walter Katsumi Sakamoto
Banca: Silvio Rainho Teixeira
Resumo: Os vidros preparados para diversas aplicações têm um ponto em comum: a possibilidade de nuclear e cristalizar novas fases, quando preparados a partir de uma massa fundida ou através de um tratamento térmico acima da temperatura de transição vítrea. Neste trabalho são apresentadas de forma sucinta as formulações teóricas de formação de vidros incidindo principalmente nos processos de nucleação e crescimento de fases. O estudo da cristalização de vidros pode ser feito através de métodos cinéticos baseados na descrição teórica formulada por Johnson-Mehl- Avrami (JMA). Estes métodos descrevem os processos de nucleação e cristalização utilizando dados provenientes das curvas de DTA/DSC. Uma delas é aquela proposta por Kurajica, que determina os parâmetros cinéticos utilizando um modelo para a resolução da sobreposição dos picos de cristalização. Para a aplicação deste modelo, foram utilizados vidros teluretos, de composição 80TeO2 - 10Nb2O5 - 8Li2O - 2V2O5 (mol%) denominados TNLV, e vidros fosfatos, de composição 50P2O5 - 36Na2O - 10CdO - 4La2O3 (mol%), dopados com 20 e 60mg de CeO2 denominados PNCL20 e PNCL60. O estudo cinético teve início com a identificação das fases cristalinas formadas, utilizando a difratometria de raios X (DRX). Considerando a formação de três fases cristalinas para cada sistema vítreo, a equação proposta por Kurajica foi aplicada, utilizando o software Origin 7.0, para determinação dos parâmetros cinéticos, a partir dos ajustes dos dados de DSC. Os coeficientes de Avrami (n) determinados mostraram que para o sistema TNLV o crescimento se dá em três dimensões com mecanismos diferentes, enquanto que para os sistemas PNCL20 e PNCL60, o crescimento ocorre em três dimensões com reação de interface. Foi observado que... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The glasses prepared for various applications have one common point: the possibility of new phases nucleation and crystallization, when prepared from a melt or through a heat treatment above the glass transition temperature. In this work are presented briefly the theoretical formulations of formation of glass focusing mainly in the processes of phases nucleation and growth. The study of glass crystallization can be done by kinetic methods based on the theoretical description formulated by Johnson- Mehl-Avrami (JMA). These methods describe the processes of nucleation and crystallization using data from DTA/DSC curves. One model is that proposed by Kurajica which determine the kinetic parameters resolving overlapping peaks of crystallization. To apply this model were used a tellurite glass, with composition 80TeO2 - 10Nb2O5 - 8Li2O - 2V2O5 (mol%) denominated TNLV, and two phosphate glasses of composition 50P2O5 - 36Na2O - 10CdO - 4La2O3 (mol%) doped with 20 and 60mg of CeO2 denominated PNCL20 and PNCL60. The kinetic study started us with the identification of crystalline phase, using the X-ray diffraction (XRD). Considering that three crystalline phases are formed for each glassy system, the Kurajica equation was applied using the Origin 7.0 software, for determining the kinetic parameters. The calculated Avrami coefficients (n) shown that for the TNLV system the growth occurs in three dimensions with different mechanisms, while for the PNCL20 and PNCL60 systems the growth occurs in three dimensions with an interface reaction. It was observed that the PNCL20 system has higher activation energy that the PNCL60 system and XRD patterns showed that the characteristic peaks of the phases containing CeO2, become higher and thinner in the system PNCL60. These results show that the cerium may favors the glass crystallization.
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Os vidros preparados para diversas aplicações têm um ponto em comum: a possibilidade de nuclear e cristalizar novas fases, quando preparados a partir de uma massa fundida ou através de um tratamento térmico acima da temperatura de transição vítrea. Neste trabalho são apresentadas de forma sucinta as formulações teóricas de formação de vidros incidindo principalmente nos processos de nucleação e crescimento de fases. O estudo da cristalização de vidros pode ser feito através de métodos cinéticos baseados na descrição teórica formulada por Johnson-Mehl- Avrami (JMA). Estes métodos descrevem os processos de nucleação e cristalização utilizando dados provenientes das curvas de DTA/DSC. Uma delas é aquela proposta por Kurajica, que determina os parâmetros cinéticos utilizando um modelo para a resolução da sobreposição dos picos de cristalização. Para a aplicação deste modelo, foram utilizados vidros teluretos, de composição 80TeO2 - 10Nb2O5 - 8Li2O - 2V2O5 (mol%) denominados TNLV, e vidros fosfatos, de composição 50P2O5 - 36Na2O - 10CdO - 4La2O3 (mol%), dopados com 20 e 60mg de CeO2 denominados PNCL20 e PNCL60. O estudo cinético teve início com a identificação das fases cristalinas formadas, utilizando a difratometria de raios X (DRX). Considerando a formação de três fases cristalinas para cada sistema vítreo, a equação proposta por Kurajica foi aplicada, utilizando o software Origin 7.0, para determinação dos parâmetros cinéticos, a partir dos ajustes dos dados de DSC. Os coeficientes de Avrami (n) determinados mostraram que para o sistema TNLV o crescimento se dá em três dimensões com mecanismos diferentes, enquanto que para os sistemas PNCL20 e PNCL60, o crescimento ocorre em três dimensões com reação de interface. Foi observado que...
The glasses prepared for various applications have one common point: the possibility of new phases nucleation and crystallization, when prepared from a melt or through a heat treatment above the glass transition temperature. In this work are presented briefly the theoretical formulations of formation of glass focusing mainly in the processes of phases nucleation and growth. The study of glass crystallization can be done by kinetic methods based on the theoretical description formulated by Johnson- Mehl-Avrami (JMA). These methods describe the processes of nucleation and crystallization using data from DTA/DSC curves. One model is that proposed by Kurajica which determine the kinetic parameters resolving overlapping peaks of crystallization. To apply this model were used a tellurite glass, with composition 80TeO2 - 10Nb2O5 - 8Li2O - 2V2O5 (mol%) denominated TNLV, and two phosphate glasses of composition 50P2O5 - 36Na2O - 10CdO - 4La2O3 (mol%) doped with 20 and 60mg of CeO2 denominated PNCL20 and PNCL60. The kinetic study started us with the identification of crystalline phase, using the X-ray diffraction (XRD). Considering that three crystalline phases are formed for each glassy system, the Kurajica equation was applied using the Origin 7.0 software, for determining the kinetic parameters. The calculated Avrami coefficients (n) shown that for the TNLV system the growth occurs in three dimensions with different mechanisms, while for the PNCL20 and PNCL60 systems the growth occurs in three dimensions with an interface reaction. It was observed that the PNCL20 system has higher activation energy that the PNCL60 system and XRD patterns showed that the characteristic peaks of the phases containing CeO2, become higher and thinner in the system PNCL60. These results show that the cerium may favors the glass crystallization.

Le but de ce travail est de comprendre l'influence des nanoparticules de silice sur les transitions physiques de matrices polymères de nature différente : l'alcool polyfurfurylique (PFA), le polytétrafluoroéthylène (PTFE) et le polydiméthylsiloxane (PDMS). Pour cela, les techniques d'analyse thermique conventionnelles (ATG, DSC, DMA) ont été couplées à des techniques atypiques (DSC multifréquence, FSC, UFSC).Dans le cas du PFA, les nanoparticules de silice ont entrainé une augmentation de la Tg ainsi qu'une amélioration des propriétés thermomécaniques. En outre, il a été démontré que la seule présence de silice suffit à favoriser les mécanismes de polymérisation. La cristallisation du PTFE à partir de l'état fondu a été étudiée pour la première fois sur une gamme de vitesse de refroidissement très large (jusqu'à 800 000 K.s-1). L'effet nucléant des nanoparticules de silice a également été mis en avant à faibles vitesses de refroidissement lors de l'étude de la cristallisation du PTFE chargé. Cependant, il s'est avéré qu'elle ralentit également la diffusion des chaines dans le milieu pour certaines vitesses. L'influence des nanoparticules de silice sur la transition vitreuse et la cristallisation du PDMS a finalement été étudiée. Les résultats ont montré que la silice n'induit pas d'effet significatif sur la transition vitreuse. D'autre part, la silice influence fortement la cinétique de cristallisation. Cet effet a été directement lié au fait que la silice favorise la nucléation sans influencer la diffusion des chaines.

Etude des premieres etapes de la cristallisation dans un verre du systeme sio::(2)-al::(2)o::(3)-li::(2)o(mgo) dans lequel de faibles quantites de titane et de zirconium sont introduites comme elements nucleants

The area of application of Second Harmonic Generation (SHG) technique is broadening now. Besides the search for new nonlinear crystals there are examples of SHG researches of equilibria of ferroelectric solid solutions and investigations of kinetics of noncetrosymmetric phases formation in the course of chemical reactions in mixtures and gels or during crystallization in glasses. In order to solve these problems on the basis of SHG data it became necessary to suggest a more unified approach to SHG measurements for the variety of non-centrosymmetric and ferroelectric substances. To provide standard conditions for SHG measurements, the materials prepared either by growth of crystals, sintering of ceramics, or by "wet" chemical synthesis should in all cases be reduced to thin powders. When powders with size partides (L) smaller than the maximum coherent length are used the intensity of second harmonic (I2ω) appears to be a good quantitative measure of nonlinear polarizability. In most oxides this is guaranteed if L < 3 mcm. Under this condition coherent length can be eliminated from the equation for the SH-intensity. The effect of the sample thickness on I2ω is eliminated by measuring the radiation from sample in the backward direction.

Abstract The deformation behavior and mechanical properties of a Cu54Zr22Ti18Ni6 bulk metallic glass during and after deposition by kinetic spraying were investigated. The bulk metallic glass feedstock particles were manufactured by inert gas atomization and were subsequently deposited onto mild steel substrates by means of kinetic spraying at different powder carrier gas temperatures [room temperature, 450°C (within the supercooled liquid region), and 550°C (above crystallization temperature)]. In addition, the phase compositions of the feedstock and as-sprayed BMG coatings were investigated using X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), and Transmitted Electron Microscopy (TEM). With an increase of the powder feed temperature, it was deduced that more intimate contact of the particles with the substrate was achieved which decreased the porosity of the resulting coating. However, crystallizations, which seemed to be induced by severe deformation and accumulated heat, were observed at localized regions in the coating. In addition, micro-hardness and bond strength were affected by the crystallization degree of the as-sprayed coatings