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As três famílias de ligas de maior importância comercial para a indústria de fundição apresentam a formação de eutéticos do tipo facetado/não-facetado durante a solidificação: os ferros fundidos com grafita (eutético austenita/grafita), os ferros fundidos brancos (eutético austenita/carboneto) e as ligas Al-Si (eutético alfa/silício). No caso dos ferros fundidos brancos, a morfologia dos carbonetos eutéticos exerce influência decisiva sobre o comportamento mecânico e a resistência à degradação (em função de solicitações como abrasão ou ciclos térmicos e/ou mecânicos) das ligas resistentes a desgaste pertencentes ao sistema Fe-C-X (em que X representa um ou mais dos elementos Cr, V, Mo, W e, mais raramente, Nb). Investigou-se o efeito do potencial nucleante do banho metálico sobre o eutético de ligas do sistema Fe-C-V, e conseqüentemente sobre a morfologia do carboneto. As alterações de morfologia do eutético austentita/carboneto VC em função da adição de alumínio ou titânio foram avaliadas por meio de microscopias óptica e eletrônica de varredura. Observou-se que a adição de alumínio ou titânio, em ligas com teores residuais de oxigênio ou nitrogênio, promoveu alteração do eutético VC de irregular para fibroso (no caso da adição de alumínio) e para divorciado (no caso da adição de titânio). Conclui-se que a presença de partículas nucleantes induziu a nucleação seletiva da fase facetada (carboneto VC) a temperaturas mais elevadas, diminuindo o grau de cooperação durante o crescimento do eutético. / The three most commercially important families of alloys for the foundry industry show the formation of faceted/non-faceted eutectics type during solidification: cast irons with graphite (austenite/graphite eutectic), white cast irons (austenite/carbide eutectic) and Al-Si alloys (alfa/silicon eutectic). In the case of white cast irons, the eutectic carbide morphology has decisive influence on mechanical behavior and on degradation resistance (due to solicitation such as abrasion or thermal and/or mechanical cycling) of wearing resistant alloys of the Fe-C-X system (where X stands for one or more of the following elements: Cr, V, Mo, W and, seldom, Nb). The metallic bath nucleant potential on the eutectic of the Fe-C-V system and, thus, on the carbide morphology, were investigated. It was evaluated the nucleant potential for aluminum and titanium addition to the bath. The morphologies changes for the austenite/carbide eutectic were evaluated by optical and scanning electronic microscopy. It was observed that aluminum or titanium addition in alloys with residual content of oxygen or nitrogen promoted VC eutectic morphology change from irregular to fibrous (when aluminum is added) and to divorced (when titanium is added). It is concluded that the presence of nucleant agents in the metallic bath induced the preferred nucleation of the faceted phase at higher temperatures, decreasing the cooperation during the eutectic growth.

Desde a apresentação do primeiro modelo da transição de fase de primeira ordem no início do século XX, acumulou-se muito conhecimento a respeito da cinética deste fenômeno. A Teoria Clássica da Nucleação (TCN) calcula o tamanho do núcleo crítico maximizando a diferença na energia livre de Gibbs entre as fases atual e nova, separadas por uma superfície de tensão bem definida. Técnicas experimentais modernas indicam que a TCN calcula a taxa de nucleação com acurácia razoável somente em estreita faixa de temperatura, indicando deficiências no modelo. Correções na teoria melhorou sua dependência com a temperatura, mas os resultados encontrados são superiores aos medidos experimentalmente em cerca de quatro ordens de grandeza. Diferentes abordagens têm surgido, como a Teoria Adiabática da Nucleação e a Teoria Entrópica da Nucleação (TEW). Ainda que aquela traga em seu escopo teórico parte do modelo clássico, como a tensão superficial, elege a maximização da entropia como a força motriz da transição de fase. O mesmo fez a TEN, porém sem especificar uma superfície de tensão. A primeira aplicação desta teoria foi na condensação do vapor de água. Mediante uma teoria da flutuação, no seu aspecto mais simplificado, foi determinado o tamanho do núcleo critico. Os resultados obtidos são menores do que os experimentais em cerca de três ordens de grandeza, devido a excessiva simplicidade do modelo empregado. Entretanto, verifica-se que a dependência do tamanho do núcleo com a supersaturação assemelha-se aos dados experimentais. Os resultados indicam que a abordagem entrópica é promissora. / In Classical Nucleation Theory (CNT) the size of the critical nucleous is obtained by maximizing the difference in bulk Gibbs free energy between the new and the evolving phase separated by a surface having a well defined tension, in relation to the nucleous size. CNT, albeit the introduction of numerous correction terms, did not compare well with experimental data by many orders of magnitude. Other theories have been proposed as the Adiabatic and Entropy Theones, ANT and ENT, respectively. While ANT rests on some CNT assumptions as the existence of a well defined surface separating the new and the old phase, considers the entropy as the driving force of phase transition instead of Gibbs Free Energy. On the other hand, ENT only rests on the assunption of the entropy as the driving force without considering a surface tension. This theory is applied to the study of water vapor condensation, by calculating the size of the critical nucleous with the aid small amplitude fluctuation theory. The results compare well with experimental values, being off by about 3 orders of magnitude but its dependence with supersaturation agrees well with experiment. The results are satisfactory and may be improved by refining the theory.

Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Sc.D.

En partant d'une problématique industrielle correspondant à la naissance de fissures lors de l'excavation d'un tunnel dans un matériau argileux, nous proposons de nous intéresser à un sujet plus large qui est celui de la nucléation d'une fissure de dimension macroscopique. Ainsi, des résultats généraux et théoriques sont établis tout au long de ce mémoire. Ces derniers sont toutefois utilisés pour proposer une interprétation aux faciès de fissuration observés lors de l'excavation de galeries de stockage à grandeur profondeur. Dans un premier temps, un modèle géométrique idéalisé pour des fissures est justifié. Cette étude préalable permet à la fois d'acter la nature fermée des fissures susceptibles de nucléer ainsi que d'utiliser le critère mixte en statuant sur le bienfondé d'une étude bidimensionnelle équivalente. En précisant la notion de longueur de nucléation stable et en s'inspirant des outils classiques de la mécanique linéaire de la rupture, nous établissons dans un second temps notre propre critère de nucléation. Pour ce faire, nous proposons, grâce à des modélisations micromécaniques, de revisiter l'approche thermodynamique usuelle dans un cadre adiabatique afin d'être davantage en accord avec la brutalité du phénomène étudié. Par la suite, nous mettons aux points différentes méthodes permettant d'utiliser le critère suggéré. Enfin, le critère est mis en œuvre dans le cadre de la problématique industrielle initiale pour essayer de donner une justification à l'anisotropie des géométries des fissures observées, et ce, en prenant en compte les différentes directions de forage de l'ouvrage. Dans un deuxième temps, nous proposons de nous intéresser, de manière plus prospective, aux problématiques liées aux modèles locaux d'endommagement. Une attention particulière est accordée à la notion de stabilité des états d'équilibre. Ainsi, une adaptation du critère de Hill est proposée et des différences quantitatives sont soulignées lorsqu'un cadre adiabatique, nous paraissant plus justifié, est pris en compte. Enfin, le problème de la localisation de l'endommagement dans un problème unidimensionnel est abordé / Starting from an industrial issue that is cracks onset when excavating a tunnel, this work aims at giving new insights into a more general problematic which is the initiation of macroscopic cracks. Thus, general and theoretical results are established. Nevertheless, they are applied in order to give some explanations to the excavation-induced fractures observed around the deep geological repository. To begin with, an idealised geometrical model is detailed and justified. Thanks to this preliminary work, we establish that the cracks that should be taken into account are closed ones. In addition we show that it's possible for small cracks length to work on an equivalent bidimensionnal problem. This last result allows us to apply the mixed criteria. After giving the definition of a stable crack initiation length we define our own criteria to predict cracks onset. In order to do so and in order to be more in adequacy with the caracter brutal of a crack initiation, we offer through a micromecanic modelisation to deploy the usual thermodynamic approach in an adiabatic context. Different methods to compute the key quantity which is the incremental energy released rate are then built. Finally, the criteria is applied to give some justifications to the anisotropic geometry of the excavation-induced fractures. In a second part of this work, we focuse on the problematics tied to the local damage models. For instance, the notion of stability for an equilibrium state is discussed. Hill's stability critera is adapted to damage problems. Once again, it seems that an adiabatic context is more suited and the differences implied are emphasized. To conclude, we offer to investigate the localisation issue in one dimensionnal problems

Aerosols are ubiquitous throughout Earth's atmosphere and their size, chem- ical composition, and concentration cause varying degrees of impact on climate and human health. Atmospheric aerosols can affect climate by their varied interactions with incoming solar radiation and their role in cloud formation and microphysics. Nucleation of fresh particles plays a significant role in the number of boundary layer cloud condensation nuclei (CCN). Elevated sulfuric acid concentration from power production has long been shown to contribute to new particle formation, but is not present in all instances where nucleation is observed. A third component has long been hypothesized and different studies in different locations have shown evidence of either ammonia, amines, or organics acting in conjunction with sulfuric acid and water to initiate new particle formation under certain meteorological conditions. While atmospheric nucleation has been examined world-wide in many urban as well as remote forested locations, it has not been studied extensively in the non-forested Midwestern United States, where sulfuric acid from coal-fired power plants and ammonia from agricultural activity are prevalent. For this doctoral dissertation work, instruments were designed, built, and tested for the purpose of investigating the concentration, size distribution, and volatility of atmospheric aerosols in non-forested rural continental environments in the Midwestern United States. An impact assessment of the University of Iowa Power Plant on air quality in Johnson County, IA highlights the ability to field test the emission ratios of fine particulates emissions to other gaseous emissions. Analysis on 20 years of climatically relevant aerosol properties in the rural Midwestern location of Bondville, IL reveals enhancement of particle number in the Spring and Fall seasons. Bondville is also the location of a three-year aerosol vertical profiling field campaign, where ultra-fine particles were found to be enhanced in the planetary boundary layer. The long standing records are compared with current full aerosol size distribution particle measurements for a period of ∼ 10 months in Bondville, where the seasonality of high particle number concentrations are verified and attributed to nucleation. Nucleation is observed to varying degrees in all seasons at this location, but is most prevalent and intense in the Spring and Fall months under otherwise clean atmospheric conditions. This work paves the way for a more in depth examination of the volatility of fine particle matter during nucleation and the development of a Midwestern chemical nucleation model to investigate numerous nucleation conditions and mechanisms. This work will contribute important information to the atmospheric science community on the process controlling the particle number size distribution in the region.

This Thesis considers peculiarities of nanoparticle formation from the gas in different systems. The main role of the surface condensation in the nanoparticle growth in metal flames was established through a series of experiments and was described by the developed model. The stagnation of the post-nucleation nanoparticle growth was experimentally revealed and theoretically explained. The influence of generation conditions on the post-processing nanoparticle properties was examined. The non-isothermal approach to correct the homogeneous nucleation theory was developed. The results of this work can be summarized in 3 categories: (1) Nanoparticle formation in metal flames. In this work, it was demonstrated that the surface condensation is a main process responsible for nanooxides growth during metal combustion. It was shown that the rate of this condensation growth is consistent with the exponential law, which could lead to the formation of the lognormal particle size distribution in the system, where the Brownian coagulation is suppressed. The post-nucleation stagnation of the nanoparticle growth was found. The particle overheating was suggested as a cause of the growth stagnation. The found stagnation leads to the accumulation of the supercritical clusters in the system generating nanoparticles. The role of these supercritical clusters in the nanoparticle agglomeration was considered. (2) Study of properties of nanoparticles generated in different metal flames. The light absorption, photoluminescence and magnetic properties of nanoparticles produced in different metal flames were examined. The significant broadening of the absorption edge was found in nanooxides produced by direct metal combustion. This broadening allowed one to excite the unforeseen photoluminescence from these nanoparticles. The significant light absorption in the visible light found in the titania particles produced by metal combustion allows one to consider these particles as a prospective photocatalyst. The unusual optical properties revealed were related to the extreme conditions of the nanoparticle formation, namely, to high energy release (about 5 eV per condensing molecule). The stabilization of spinel structure was found in iron oxide particles synthesized by iron combustion. It allowed one to produce nanoparticles with magnetization close to the high-limit value of the bulk. (3) Approach to correct the homogeneous nucleation theory. The existing homogeneous nucleation theory implies that nucleation occurs at isothermal conditions, i.e. subcritical clusters have the same temperature as the ambient gas does. However, the theory overestimates the actual nucleation rate and underestimates the critical cluster size. It is understandable that due to release of the latent heat of condensation, the cluster temperature in the nucleating system should be higher than the environment temperature. In this work, it was suggested the method to account for the cluster overheating during nucleation. It was demonstrated that the consistent description of the detailed balance in the nucleating system may allow one to evaluate magnitudes of overestimation of the actual nucleation rate and underestimation of the number of molecules in the critical cluster, usually obtained by the isothermal nucleation theory. The numerical estimates are in good agreement with the wellknown experimental results. The implications of the results to generation of atmospheric aerosols were discussed.

Doctor of Philosophy(PhD) / Particle formation in RAFT-mediated emulsion polymerization has been studied using reaction calorimetry. By measuring the heat flow during controlled feed ab-initio emulsion polymerization in the presence of amphipathic RAFT agents, particle formation by self-assembly of these species could be observed. Two different monomer systems, i.e. styrene and n-butyl acrylate, and various degrees of hydrophobicity of the initial macro-RAFT agents have been studied and compared. The different macro-RAFT agents were synthesized by first forming a hydrophilic block of poly(acrylic acid) that would later on act as the electrosteric stabilizing group for the particles. Subsequently, different lengths of hydrophobic blocks were grown at the reactive end of the poly(acrylic acid) hydrophilic block via the RAFT-mediated controlled radical polymerization, either comprised of n-butyl acrylate or styrene. Two processes govern particle formation: adsorption of macro-RAFT agents onto growing particles and formation of new particles by initiation of micellar aggregates or by homogeneous nucleation. Competition between these processes could be observed when monomers with a relatively high (n-butyl acrylate) or low (styrene) propagation rate coefficient were used. A model describing particle formation has been developed and the results of model calculations are compared with experimental observations. Preliminary modeling results based on a set of reasonable physico-chemical parameters already showed good agreement with the experimental results. Most parameters used have been verified experimentally. The development of the molecular weight distribution of the macro-RAFT agents has been analyzed by different techniques. Quantification of the particle formation process by analytical techniques was difficult, but qualitative insights into the fundamental steps governing the nucleation process have been obtained. The amount of macro-RAFT agents initially involved in particle formation could be determined from the increase of molecular weight. The particle size distribution has been measured by capillary hydrodynamic fractionation, transmission electron microscopy and dynamic light scattering. From the data obtained from these particle-sizing techniques, the number of particles during the reaction could be monitored, leading to an accurate estimate for the particle formation time. Upon implementation of the experimental data obtained for the surface active macro-RAFT systems, the model demonstrated to be very sensitive towards the “headgroup” area of the macro-RAFT species. Three nucleation cases based on the initial surface activity of the macro-RAFT species in the aqueous phase are proposed to explain the deviations from the assumptions of the nucleation model. Even though the macro-RAFT species have a narrow molecular weight distribution, they are nevertheless made up of a distribution of block lengths of polystyrene upon a distribution of block lengths of poly(acrylic acid). The resulting differences in initial surface activity are the most probable reason for the observed differences between model calculations and experimental results for the nucleation time and particle size distribution of the final latex product. With the procedure described above, latexes have been synthesized without using conventional surfactants and the mechanisms involved in the particle formation for these systems have been elucidated. The results of this work enable production of latex systems with well defined molecular mass distributions and narrow particle size distributions. Furthermore, the technique based on the application of amphipathic RAFT agents is promising for the production of complex polymeric materials in emulsion polymerization on a technical scale.