No description available.

Eutectic Fe-C, Ni-C and Fe-C-Si alloys have been solidified directionally in an electron-beam floating zone apparatus under carefully controlled conditions and the resulting morphologies in these alloy systems have been studied as a function of rate. In addition, the transition from the stable iron-graphite (grey iron) to the metastable iron-iron carbide (white iron) mode has been observed and has been related to the iron-carbon/iron-iron carbide double phase diagram by means of undercooling measurements. Interlamellar spacing measurements carried out on both the graphitic and carbidic modes using material of the same composition and purity under identical experimental conditions showed a λaR-1/2 and a λaR-1/3 relationship in the iron-graphite and iron-iron carbide eutectic systems, respectively. / Thesis / Doctor of Philosophy (PhD)

No description available.

The largest cost in manufacturing PEM fuel cells for automotive applications is due to the bipolar plate. The current graphite material used for the bipolar plate is very brittle and difficult to machine to the rigorous specifications needed for fuel cell stacks. This paper introduces the development of a fuel cell test stand for simultaneous testing of six individual fuel cells. To establish a long-term performance evaluation, the fuel cells incorporate a baseline graphite material that undergoes testing in the fuel cell environment. The graphite is an industry standard material that should not corrode when subjected to continual testing. The baseline model will be used in development of novel composite materials that will be tested under the same conditions for comparison to the graphite. Furthermore, the new materials and applied manufacturing methods could reduce the overall cost of fuel cell stacks in the future. Funding for this project was generously donated by the Virginia Center for Innovative Technology and the National Science Foundation. / Master of Science

Graphite foams have high bulk thermal conductivity and low density, making them an excellent material for heat exchanger applications. This research focused on the characterization of graphite foams under various processing conditions (different foaming pressures and particle additions), specifically studying the effects of porosity on the thermal properties. The characterization of the foams included measuring cell sizes, percent open porosity, number of cells per square inch, bulk density, Archimedes density, compression strength, thermal conductivity, thermal resistance, and permeability. Several relationships between the structure and properties were established, and a recommendation for the processing conditions of graphite foams for the use in heat exchangers was determined. / Master of Science

Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This thesis presents a failure evaluation methodology for nuclear graphite components used in high-temperature gas reactors. The failure methodology is aimed at predicting the failure of real parts based on the mechanical testing results of material specimens. The method is a statistical failure methodology for calculating the probability of failure of graphite components, and has been developed and implemented numerically in conjunction with a finite element analysis. Therefore, it can be used on any geometry and load configuration that can be modelled using finite element analysis. The methodology is demonstrated by mechanical testing of NBG-18 nuclear grade graphite specimens with varying geometries under various loading conditions. Some tests were developed as an extension of the material characterisation, specifically engineered to assess the effect of stress concentrations on the failure of NBG-18 components. Two relevant statistical distribution functions, a normal distribution and a twoparameter Weibull distribution are fitted to the experimental material strength data for NBG-18 nuclear graphite. Furthermore, the experimental data are normalised for ease of comparison and combined into one representative data set. The combined data set passes a goodness-of-fit test which implies the mechanism of failure is similar between data sets. A three-parameter Weibull fit to the tensile strength data is only used in order to predict the failure of independent problems according to the statistical failure methodology. The analysis of the experimental results and a discussion of the accuracy of the failure prediction methodology are presented. The data is analysed at median failure load prediction as well as at lower probabilities of failure. This methodology is based on the existence of a “link volume”, a volume of material in a weakest link methodology defined in terms of two grouping criteria. The process for approximating the optimal size of a link volume required for the weakest link failure calculation in NBG-18 nuclear graphite is demonstrated. The influence of the two grouping criteria on the failure load prediction is evaluated. A detailed evaluation of the failure prediction for each test case is performed for all proposed link volumes. From the investigation, recommended link volumes for NBG-18 are given for an accurate or conservative failure prediction. Furthermore, failure prediction of a full-sized specimen test is designed to simulate the failure condition which would be encountered if the reactor is evaluated independently. Three specimens are tested and evaluated against the predicted failure. Failure of the full-size component is predicted realistically but conservatively. The predicted failure using link volume values for the test rig design is 20% conservative. The methodology is based on the Weibull weakest link method which is inherently volume dependent. Consequently, the conservatism shows that the methodology has volume dependency as experienced in the classic Weibull theory but to a far lesser extent. / AFRIKAANSE OPSOMMING: Hierdie tesis beskryf ‘n metode wat gebruik kan word om falings in kern grafiet komponente te voorspel. Hierdie komponente word in hoë temperatuur gas reaktore gebruik. Die falings metodologie beoog om die falings van regte komponente te voorspel wat gebaseer is op meganiese toets resultate van materiaal monsters. Dit is ‘n statistiese falings metodologie wat die waarskynlikheid van faling vir grafiet komponente bereken. Die metode is numeries ontwikkel en geïmplementeer deur middel van die eindige element metode, dus kan die metodologie toegepas word op enige geometrie en belastingsgeval wat dan gemodelleer kan word deur gebruik te maak van eindige element metodes. Die metodologie word gedemonstreer deur gebruik te maak van NBG-18 kern grafiet toets monsters. Sommige van hierdie toetse is ontwikkel as ‘n uitbreiding van die materiaal karakterisering wat spesifiek ontwerp is om die effek van die spannings konsentrasies op die faling van die NBG-18 komponente te evalueer. Twee relevante statistiese verspreiding funksies word gekoppel aan die eksperimentele sterkte data van die NBG-18 kern grafiet, naamlik ‘n normale verspreiding en ‘n twee-parameter Weibull verspreiding. Die data stelle word ook genormaliseer vir gemak van vergelyking en gekombineer in een verteenwoordigende data stel. Die gekombineerde data stel slaag ‘n korrelasie toets wat impliseer dat die meganisme van faling soortgelyk is tussen die data stelle. ‘n Drie-parameter Weibull korrelasie op die trek toets monsters word gebruik vir die statistiese falings metodologie. Die analise van die eksperimentele resultate sowel as ‘n bespreking van die akkuraatheid van die faling voorspelling metodologie word voorgelê. Die data word geanaliseer by gemiddelde faling voorspelling asook by laer voorspellings van falings. Hierdie metode is gebaseer op die bestaan van ‘n “ketting volume” wat die volume van ‘n materiaal wat gebruik word in die swakste ketting voorstel en koppel aan die metodologie. ‘n Metode vir die benadering van die ketting volume word voorgestel en daaropeenvolgend gebruik om die ketting volume te bereken vir NBG-18. ‘n Gedetailleerde evaluasie van die falings voorspelling vir elke toets geval word uitgevoer vir die voorgestelde ketting volumes. Gebaseer op hierdie ondersoek is voorgestelde ketting volumes vir NBG-18 gegee vir beide akkurate en konserwatiewe falings voorspellings. Verder was ‘n volgrootte strukturele toets ontwikkel om dieselfde falings omstandighede te simuleer wat verwag is gedurende normale werking van die reaktor. Drie monsters word getoets en geëvalueer teen die voorspelde faling vir beide die berekende ketting volume groottes. Faling van die volgrootte komponente word realisties asook konserwatief voorspel. Die voorpselling is 20% konserwatief. Die metodologie is gebaseer op die Weibull metode wat inherent volume afhanklik is; gevolglik dui die konserwatisme aan dat die metodologie oor volume afhanklikheid beskik soos ondervind word in die klassieke Weibull teorie, maar tot ‘n baie kleiner mate.

L’électrode négative des batteries lithium-ion est communément en graphite. Bien qu’ayant une capacité spécifique intéressante, le vieillissement, la cinétique d’intercalation et le transport du lithium à la fois dans le matériau actif et les porosités de l’électrode limitent son fonctionnement optimal et homogène. Dans ce travail de thèse, les mécanismes à l’origine de ces limites sont explicités grâce à un modèle multi-physique de type électrode poreuse.Une étude de sensibilité des paramètres du modèle a montré l’importance des paramètres liés à la cinétique d’intercalation et au transport du lithium en phase solide et liquide. L’exploitation du modèle, validé expérimentalement, montre que, lors du fonctionnement de l’électrode, les apparences d’hétérogénéité de lithiation sont corrélées à la forme particulière du potentiel d’équilibre du graphite vis-à-vis de son taux de lithiation. La modélisation de la distribution de taille des particules, amplifie grandement ces hétérogénéités et dégrade fortement la performance globale de l’électrode. En première approche, une mesure operando de la distribution des états de lithiation confirme l’aspect hétérogène du fonctionnement de l’électrode.Les données des performances en cyclages et en calendaire de cellules graphite-NMC ont permis de construire différents modèles de vieillissement de l’électrode. La croissance de la couche de passivation (SEI) peut expliquer à elle seule la perte de lithium cyclable. Les hétérogénéités de SEI obtenues par le modèle sont négligeables en l’état. Les gains de capacités et les pertes brutales sont expliqués respectivement par des mécanismes de dissolution de SEI et de formation de lithium-plating. / Negative electrodes of lithium-ion batteries are mainly based on graphite, because of their good electrochemical properties. Unfortunately, intercalation kinetics, aging phenomena and lithium transport through active material and electrode porosity decay the optimal and homogeneous operations of this electrode. Origins of these limits are investigated in this work thanks to a porous electrode model.A sensitivity study indicates that preponderant model parameters are related to the kinetics and lithium transport in solid and liquid phases. The model is experimentally validated at a cell scale and predicts the appearances of lithium heterogeneities during the graphite lithiation. They are correlated to the staged shape of the graphite equilibrium potential. Modeling additional inhomogeneity sources, especially particle distribution, amplifies these heterogeneities and decrease drastically cell performance. In a first approach, an operando measure of the local lithiation state confirms this heterogeneity aspect during operations.In a second part, data of cycled and calendar aged graphite-NMC cell validates different aging models. The growth of the passive layer on the graphite surface (SEI) explains the cyclable lithium loss on its own. SEI heterogeneities are negligible in the porous model as opposition to experimental finding. Capacity recoveries and sudden loss are explained respectively via a SEI dissolution mechanism and lithium-plating correlated to the degradation of the electrode transport properties.

The entry part introduce into the problematic of rework processes by molding injection tools for thermoplastic. Aim is necessity of change current rework process usage by electro discharge machining (EDM) technology.The theoretical part of the thesis describes the physical essence of EDM technology, the system of production of cutting electrodes and related issues. The practical part of work compare the process of production of copper and graphite electrodes with focus on production time and costs. The production parameters of the EDM process were analyzed and the wear of copper and graphite electrodes during the production process was compared. Main result of the thesis is an evaluation of the research activity, interpretation of the results of this activity and a proposal of measures to shorten the production time of the cutting electrodes and the time of the EDM machining by these electrodes.

O PVC é um dos principais polímeros termoplásticos da atualidade graças à sua grande versatilidade. Entretanto, uma de suas características é a baixa estabilidade térmica, sendo necessário o emprego de aditivos em sua estrutura. Uma das maiores preocupações mundiais é a segurança, sendo que muitos produtos levam proteção anti-chama em sua composição. Entre os vários agentes retardadores de chama a grafite aparece como um material promissor e pouco estudado. No presente trabalho foi estudada uma série de grafites com propriedades anti-chama oriundas de óxido de grafite \"flake\". O óxido de grafite foi submetido a vários tratamentos térmicos em atmosfera para a obtenção de grafites expandidas. O comportamento anti-chama dessas grafites foi testado adicionando-se 20% em peso no PVC. Membranas desses compósitos foram preparadas utilizando-se um \"doctor blade\". Os compósitos de grafites produzidas foram caracterizados por microscopia de varredura eletrônica e análise térmica. Isotermas de adsorção/dessorção de N2 também foram coletadas para determinação de área superficial específica e distribuição de poros. A grafite com melhores propriedades anti-chama foi aquela obtida em temperatura mais elevada, 900oC, ou seja, a grafite com mais elevada área superficial específica e com maior capacidade de produção de fuligem. / PVC is one of the most important thermoplastic polymers because of its versatility. However, like other polymers, for many technological uses, there is a need to enhance its thermal stability. Security is a global concern, so, the use of some fire retardants is required in polymer composites. Expanded graphite appears as a cleaner type of fire retardants, not well known yet. In the present work, expanded graphite obtained from the graphite oxide was studied. It were prepared PVC composite-membranes with 20 weight % of expanded graphite employing Doctor Blade. The samples were characterized by scanning electron microscope, thermogravimetric analysis and N2 adsorption/desorption isotherms were also collected for the determination of specific surface area and porous distribuition. The results show that the best fire retardant behaviour was achieved by the expanded graphite at 900oC, the graphite with the higher specific surface area and with the largest capacity of soot production.

L’exfoliation en phase liquide du graphite est un procédé simple susceptible de produire du graphène à faible coût. Ces dernières années, de nombreuses équipes ont exploité la cavitation acoustique et la cavitation hydrodynamique comme moyen d’exfoliation. La cavitation acoustique ne peut traiter qu’une quantité limitée de fluide et génère des défauts sur la structure du graphène,tandis que la cavitation hydrodynamique dans une solution en écoulement n’agit que localement pendant une durée très brève. Les équipes de recherche utilisant ce dernier procédé compensent cette brièveté en imposant à la solution chargée en graphite des différences de pression très fortes, et utilisent alors des infrastructures macroscopiques lourdes pour lesquelles il est difficile de distinguer le rôle du cisaillement de celui de la cavitation. Nous avons cherché à développer un nouveau procédé d’exfoliation basé sur l’utilisation de microsystèmes fluidiques capables de générer un écoulementcavitant avec un débit supérieur à 10 L/h pour une différence de pression modérée n’excédant pas 10 bar. Une nouvelle génération de laboratoires ‘sur puce’ a ainsi été imaginée et réalisée, permettant de traiter des solutions surfactées chargées en microparticules de graphite. Il est apparu que laconcentration solide et la durée de traitement sont des paramètres cruciaux pour l’efficacité du procédé. Par rapport à un écoulement monophasique laminaire microfluidique, l’écoulement cavitant produit plus de produits exfoliés et de graphène, avec un rendement de l’ordre de 6%. Ceci indique que l’implosion des bulles et la turbulence favorisent également les interactions entre particules. Ce procédé d’exfoliation microfluidique, qui ne nécessite une puissance que de quelques Watts, permet d’envisager à terme une production économe et écologique de graphène en suspension. / Liquid phase exfoliation of graphite is a simple and low-cost process, that is likely to produce graphene. The last few years, many researchers have used acoustic or hydrodynamic cavitation as an exfoliating tool. Acoustic cavitation is limited to low volumes and defects are present on the graphenesheets ; hydrodynamic cavitation inside a flowing solution acts briefly. So, people are using big reactors running with high pressure drops, and it is difficult from a fundamental point of view to know the physical role of shear rate versus cavitation, in the exfoliation process. We have tried to develop a new process funded on hydrodynamic cavitation ’on a chip’, with flow rates above 10 L/h and pressure drop below 10 bar. A new generation of ’labs on a chip’ has been designed and performed, processing with aqueous surfactant graphite solutions. The solid concentration and the duration of the process have proved to be key parameters. Cavitating microflows have exhibited a better efficiency (up to ~6%) than laminar liquid microflows, for the production of graphene flakes. Collapsing bubbles and turbulence are also likely to enhance particles interactions. Such a microfluidic process, which requires an hydraulic power of a few Watt, makes possible a further low-cost and green production of graphene sheets.