This dissertation focuses on the investigation of the microstructures of two nuclear grade matrix graphites. These graphites were intended for use in the core components of a high temperature test reactor (HTTR) of the pebble bed modular reactor (PBMR) design. The graphites were provided in the form of fuel spheres and a reflector block. The techniques used in the analysis of the materials include fracturing, etching, scanning electron microscopy (SEM), nano-indentation, x-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructures of the materials were characterized successfully. The fuel sphere material consisted of a high concentration of curved graphite flakes and grains in contact with turbostratic matrix graphite. The well graphitized flakes and grains were polycrystalline in nature. Delamination cracks were prevalent in the graphite crystallites. There was no significant difference in the microstructures of the center, interior and surface regions of the fuel sphere material. No evidence of amorphous carbon or resin residues was found. The reflector material consisted of a low concentration of graphite crystallites embedded within turbostratic matrix graphite. Delamination cracks were observed within the graphite crystallites, and many cavities were present in the material. TEM observation also revealed the presence of diamond crystallites. It was concluded that the fuel sphere graphite was most probably suitable for use as is, provided that the material also possessed other required properties for use in a HTTR. The reflector material however was considered to be unsuitable for use in a HTTR. It was thus suggested that the reflector material could be made more suitable by sufficient graphitization of the turbostratic graphite which formed the bulk of the material.

This thesis investigates the use of bromine intercalation of graphite as a method to simulate and investigate irradiation damage. In particular this study investigates the effects of intercalation on dimensional change on the macro and micro scales and how these changes combine to affect Young’s modulus. Highly Orientated Pyrolytic Graphite has been used to gather data as a close approximation to single crystal graphite. Three different grades of polycrystalline nuclear graphite have been used to investigate the effect of different microstructure on intercalation and subsequent property changes. The graphites have been characterized by optical microscopy, pycnometry and x-ray powder diffraction and texture measurements. A number of bespoke rigs were designed and manufactured to carry out sorption, tomography and laser vibrometry experiments.The results indicate that the rate of dimensional change for polycrystalline graphites is significantly lower than for single crystal graphites. Modelling of dimensional change suggests that the difference in expansion is due to closure of porosity. Closer investigation of the dimensional change within the microstructure shows that the majority of the dimensional change is driven by expansion of filler particles.The young’s modulus results show an initial increase in modulus followed by a decrease, which corresponds with empirical evidence for irradiated samples. It is postulated that the initial increase in modulus is due to crystal expansion and that the subsequent decrease is due to crack growth. After experimentation some samples show significant cracking which would appear to support this assertion.

The aim of this work has been to apply the techniques of non-destructive testing and evaluation to the graphite fuel channel bricks which form the core of an Advanced Gas-Cooled reactor. Two modes of graphite degradation have been studied: subsurface cracks originating from the keyway corners of the bricks and the reduction in material density caused by radiolytic oxidation. This work has focused on electromagnetic inspection techniques. Brick cracking has been studied using a multi-frequency eddy current technique with the aim of determining quantitative information. In order to accurately control the crack dimensions this work has used radially machined slots as an analogue. Two sensor geometries were studied and it was determined that slots of at least 10 mm through-wall extent could be located. A novel, empirical method of determining the slot size is presented using a brick machined with a series of reference slots. Machined slots originating from a keyway could be sized to within 2 mm using this method. A parametric 3D finite element study was also carried out on this problem. These simulations could distinguish the location of the slots and had some sensitivity to their size, however, the model was found to be overly sensitive to the specific mesh used. Two new contributions to the inverse problem are presented. The first is a minor extension to the usual adjoint problem in which one system now contains a gradiometer. The second is a proposed solution to the ambiguous nature of the inner product required by the sensitivity formulation. This solution has been validated with finite element modelling. Density reduction has been studied via its relationship with electrical conductivity using a technique based on impedance spectroscopy. An inverse eddy current problem has been solved using the regularised Gauss-Newton method to determine the conductivity of the brick over its cross section. The associated forward problem has been solved using the finite element method on a simplified geometry. Tikhonov regularisation has been employed to overcome the ill-posed nature of the inverse problem. This method has been applied to a range of sample and sensor geometries and found to produce excellent results from laboratory data provided the finite element model is well calibrated. Bore or surface conductivity values can be reproduced to better than 1% with the accuracy reducing with distance from the sensor. The sensitivity of the algorithm to the regularisation parameter has been studied using the L-curve method and the effect of two regularisation operators has also been examined. A new method of choosing the regularisation parameter a priori is proposed and tested. Data taken during reactor outages produces physically realistic profiles although the results appear off-set from electrical resistivity values measured using the four-point method. The focus of future work should be to remove this effect which will likely require improvements to the forward model.

The cores of early UK graphite moderated research and production nuclear fission reactors operated at temperatures below 150°C. Due to this low temperature their core graphite contains significant amounts of stored (Wigner) energy that may be released by heating the graphite above the irradiation temperature. This exothermic behavior has lead to a number of decommissioning issues which are related to long term "safe-storage", reactor core dismantling, graphite waste packaging and the final disposal of this irradiated graphite waste. The release of stored energy can be modeled using kinetic models. These models rely on empirical data obtained either from graphite samples irradiated in Material Test Reactors (MTR) or data obtained from small samples obtained from the reactors themselves. Data from these experiments is used to derive activation energies and characteristic functions used in kinetic models. This present research involved the development of an understanding of the different grades of graphite, relating the accumulation of stored energy to reactor irradiation history and an investigation of historic stored energy data. The release of stored energy under various conditions applicable to decommissioning has been conducted using thermal analysis techniques such as Differential Scanning Calorimetry (DSC). Kinetic models were developed, validated and applied, suitable for the study of stored energy release in irradiated graphite components. A potentially valid method was developed, for determining the stored energy content of graphite components and the kinetics of energy release. Another parameter investigated in this study was dedicated in the simulation of irradiation damage using ion irradiation. Ion bombardment of small graphite samples is a convenient method of simulating fast neutron irradiation damage. In order to gain confidence that irradiation damage due to ion irradiation is a good model for neutron irradiation damage the properties and microstructure of various grades of ion irradiated nuclear graphite were also investigated. Raman Spectroscopy was employed to compare the effects of ion bombardment with the reported effects of neutron irradiation on the content of the defects. The changes of the of defect content with thermal annealing of the ion irradiated graphite have been compared with the annealing of neutron irradiated nuclear graphite.

This thesis discusses in general terms the theory and application of time-differential perturbed angular correlation gamma-ray spectroscopy (TDPAC) to the study of solid state physics. The technique yields valuable information on the electric field gradients experienced by radionuclides which have been substituted for nonradioactive isotopes or inserted as impurities into various inorganic compounds. The indium halides are examined in a series of experiments. The first applications of this technique to the study of graphite intercalation compounds are discussed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Multiple papers have been published which attempt to predict the thermal conductivity or thermal diffusivity of graphite “nanofluids” 1–6. In each of the papers empirical methods (with no consideration of quantum mechanical principles or a structural reference) are employed in an attempt to understand and predict the heat transfer characteristics of a nanofluid. However, the results of each of these papers vary considerably. The primary reason for this may relate to the inability to construct a representative material model (based on the chemical structure), that can accurately predict the thermal enhancement properties based on the intercalated adsorption of a fluid with a noticeable heat capacity 3. This project has strived to simulate the interaction of (nano-scale) graphite particles “dispersed” in water (at the structural level of effective surface “wetting”). The ultimate purpose is to enhance the heat conduction capacity. The strategy was to initially focus on the structural properties of the graphite powder, followed by incremental exposure to water molecules to achieve a representative model. The procedure followed includes these experimental steps: a) Molecular resolution porosimetry (i.e. BET) experiments, to determine the graphene “platelet” surface area to correlate with the minimum crystallite size (where a single graphite crystal is made up of multiple unit cells) of the graphite powder samples. b) Powder X-ray diffraction (PXRD) analyses of the graphite powder samples each supplied by different manufacturers in order to determine their respective crystallographic structures. c) Infrared (IR) and Raman vibrational spectra characterisation of all of the graphite powder samples for further structure confirmation. d) Thermo-gravimetric analysis (TGA) of graphite powder and water mixture samples, to try and determine the point at which the bulk water has separated and evaporated away from the graphite powder/water mixture, resulting in a minimum layer of water adsorbed on the graphite surface and inter-/intra-particle graphite spaces. e) Differential scanning calorimetry (DSC) of the “dry” and “surface-wetted” graphite samples to determine their specific heat capacities. f) Laser flash analysis (LFA) of the “dry” and “surface-wetted” graphite samples to determine their thermal diffusivity and thermal conductivity. g) The computer simulated analysis of the graphite powder exposed to water by means of computational modelling, to elucidate the various conformational approaches of water onto the graphite surface and the associated thermodynamic behaviour of water molecules ad/absorbed at the graphite surface. Data from the TGA measurements allowed for the determination of the appropriate amount of water required in order to not only experimentally prepare graphite “surface-wetted” samples, but also to determine the effective amount of absorbed water to be considered in the computational models. For experimental verification, both dry and wet graphite samples should then be used in a laser flash analysis (LFA), in order to elucidate the effect the interfacial layer of water has on the thermal properties of graphite. A computerised model of a single graphite crystal exposed to water was created using the MedeA (v. 2.14) modelling software. The conformational behaviour and energy states of a cluster of water molecules on the graphite surface were then analysed by using the VASP 5.3 software (based on a periodic solid-state model approach with boundary conditions), to determine the energetics, atomic structure and graphite surface “wetting” characteristics, at the atomistic level. The results of the computerised model were correlated to the physical experiments and to previously published figures. Only once a clear understanding of the way in which water molecules interact with the graphite surfaces has been obtained, can further investigation follow to resolve the effect that exposure of larger graphite surfaces to polar solvents (such as water and lubricants) will have on the heat conductance of such ensembles. This scope of further work will constitute a PhD study. / Dissertation (MEng)--University of Pretoria, 2015. / tm2015 / Mechanical and Aeronautical Engineering / MEng / Unrestricted

The fire behaviour of linear low-density polyethylene composites containing 10 wt.% of different carbon-based fillers was studied. Cone calorimeter tests conducted at a heat flux of 35 kW_m_2 showed that the expandable graphite sample reduced the peak heat release rate by about 50 % while the flake graphite increased the ignition time by about 80 %. Pyrolysis combustion flow calorimetry results were practically identical for all composites. This reveals shortcomings of this bench-scale flammability test method when the flame retardancy mechanisms relies on either the development of physical barrier layer at the surface of the burning sample or on reflecting the incident heat flux. Similarly, it was found that laser pyrolysis-thermogravimetric analysis generated outcomes that did not correlate with the cone calorimeter results at all. In particular, the composite based on expandable graphite performed poorly. The likely explanation is that the aspect ratios of the small samples were such that the barrier effects on which this system relies, was negated by edge effects. / Dissertation (MSc)--University of Pretoria, 2018. / Chemical Engineering / MSc / Unrestricted

Sa conductivité électronique ou encore sa transparence optique sont autant de propriétés physico-chimiques singulières du graphène qui expliquent le nombre accru de méthodes d’exfoliation de précurseurs graphitiques développées pour l’obtention de ce matériau. Pour palier à l’utilisation d’un oxyde de graphite/graphène caractérisé par une chimie de surface mal maitrisée, des graphites fluorés, de cristallinité mais aussi de concentration en fluor variables, ont été préparés par fluoration de graphite sous fluor moléculaire pur après optimisation des paramètres. Les précurseurs, que ce soit par fluoration dynamique ou statique, ainsi obtenus ont été caractérisés finement : diffraction des rayons X, spectroscopies IR et Raman et leur texture sondée par Microscopie Electronique à Balayage et à Transmission. Suite à cela, trois méthodes d’exfoliation ont été mises en place, basées sur des mécanismes différents : i) une exfoliation par choc thermique, déjà connue mais dont les mécanismes de décomposition ont été affinés dans cette étude, ii) une exfoliation en voie liquide, avec l’utilisation pour la première fois d’un graphite fluoré pour la synthèse de graphène fluoré multi feuillets par voie électrochimique pulsée, et enfin iii) une méthode originale, peu conventionnelle, basée sur l’interaction laser femtoseconde/graphite hautement fluoré pour induire des mécanismes de réduction contrôlée, et surtout d’exfoliation de la matrice. Ces méthodes ont permis de mettre en évidence l’intérêt de la présence de fluor dans la course actuelle pour la synthèse de graphène, et ont montré l’obtention de matériaux graphéniques,possédant une fonction résiduelle fluorée intéressante pour certaines applications. / Its electronic conductivity or its optical transparency are unequaled physicochemicalproperties of graphene which explain the increased number of exfoliation methods based ongraphitic precursors to obtain this material. To overcome the use of a graphite/graphene oxidecharacterized by a poorly controlled surface chemistry, graphite fluorides, with variablecrystallinity and also fluorine concentration, were prepared by fluorination of graphite under puremolecular fluorine atmosphere after optimization of the process parameters. The obtainedprecursors, whether by dynamic or static fluorination, were characterized : X-Ray diffraction, FTIRand Raman spectroscopies for the structure, and their texture probed by Scanning andTransmission Electron Microscopy. After that, three methods of exfoliation were developed, basedon different mechanisms: i) a thermal shock, already known but decomposition mechanisms wererefined in this study, ii) an exfoliation within liquid medium by pulsed electrochemical treatment,using for the first time a fluorinated graphite for the synthesis of few-layered fluorinated grapheneand finally iii) an unconventional method, based on the interaction between femtosecond laser andhighly fluorinated graphite to induce mechanisms like controlled reduction, and especially for thisstudy exfoliation of the matrix. These methods have permit to highlight the interest of fluorine inthe current race for the synthesis of graphene, and have shown the production of graphenematerials, having an interesting fluorinated residual functionalization for some applications.

A graphite foam was developed at Oak Ridge National Laboratory (ORNL) by Dr. James Klett and license was granted to POCO Graphite, Inc. to manufacture and market the product as PocoFoam™. Unlike many processes currently used to manufacture carbon foams, this process yields a highly graphitic structure and overcomes many limitations, such as oxidation stabilization, that are routinely encountered in the development of carbon foam materials. The structure, thermal properties, electrical resistivity, isotropy, and density uniformity of PocoFoam™ were evaluated. These properties and characteristics of PocoFoam™ are compared with natural and synthetic graphite in order to show that, albeit similar, it is unique. Thermal diffusivity and thermal conductivity were derived from Fourier's energy equation. It was determined that PocoFoam™ has the equivalent thermal conductivity of metals routinely used as heat sinks and that thermal diffusivity is as much as four times greater than pure copper and pure aluminum. SEM and XRD results indicate that PocoFoam™ has a high degree of crystalline alignment and near theoretical d spacing that is more typical of natural flake graphite than synthetic graphite. PocoFoam™ is anisotropic, indicating an isotropy factor of 0.5, and may yield higher thermal conductivity at cryogenic temperatures than is observed in polycrystalline graphite.

>Magister Scientiae - MSc / This project focuses on the development of simple, highly sensitive, accurate, and low cost electrochemical sensors based on the modification of pencil graphite electrodes by the electrochemical reduction of graphene oxide-metal salts as nanocomposites (ERGO-MC-PGE; MC = Sb or Au nanocomposite). The electrochemical sensors ERGO-Sb-PGE and ERGO-Au-PGE were used in the determination of paracetamol (PC) in pharmaceutical formulations using adsorptive stripping differential pulse voltammetry. The GO was prepared from graphite via a modified Hummers’ method and characterized by FTIR and Raman spectroscopy to confirm the presence of oxygen functional groups in the conjugated carbon-based structure whilst, changes in crystalline structure was observed after XRD analysis of graphite and GO. / 2023-10-07