Density Graphite Research Articles

Long-Cycle and High-Burnup Fuel Assembly for the VHTR

For a prismatic VHTR fuel assembly, a physics study has been performed to maximize the fuel performance in terms of the cycle length and the discharge burnup for a given fuel enrichment. The relationship between the fuel performance and the fuel configurations has been investigated in terms of the TRISO packing fraction, diameter of the fuel kernel, fuel management, and moderating power of the fuel block. Both a typical low-enrichment uranium fuel (LEU) and a fuel made of transuranics (TRU) from LWR spent fuel are considered in this paper. It is shown that in order to obtain a long refueling cycle and a high burnup at the same time, the fuel loading needs to be increased together with the moderating power of the fuel block. Three ways are considered for a higher moderation of the fuel block: a larger pitch of the coolant hole pattern, an extra graphite thickness in the fuel block, and a higher graphite density. The impact of the increased pitch on the fuel temperature is also evaluated with a thermal analysis code. We have shown that long refueling cycles and high burnups can be achieved simultaneously for both LEU and TRU fuels.

석탄계 피치를 이용한 고밀도 흑연 제조

Graphite has hexagonal closed packing structure with two bonding characteristics; (1) van der waals bonding between c axis, and (2) covalent bonding in the a and b axis. The weak van der waals bonds cause self-lubricant property, and the strong covalent bonds cause excellent electric and thermal conductivity. Furthermore, graphite is chemically very inert because of the material composed of only carbon elements. Thus, graphite is very useful for mechanical sealing materials. However, Graphite have porous microstructure because starting materials of graphite produce many volatile during the manufacturing processes. This causes low density of graphite, which is unsuitable for the mechanical sealing materials. Thus, further impregnation process is generally needed to enhance the graphite density. In this work, high density graphite is prepared with the principle of densification when coke and pitch binder, prepared from thermal treatment of coal tar pitch, become dehydrogenation during graphitization or carbonization.

Laser induced modification of surface structures

The results on surface modification of materials of different structures; morphology, grain sizes, density and porosity by exposure to nanosecond laser light are given. Laser induced changes in their surface characteristics are presented. Surface layers of Si 3N 4, SiC dense ceramics and BN graphite and turbostratic pressed powders are studied by scanning electron microscopy to reveal the new nanostructures (nanowires or nanotubes) and new morphologies. A pronounced evolution in structure and grain size of BN graphite powders was demonstrated in laser processing layers.

Molecular Dynamics Simulation Study of the Structure of Poly(ethylene oxide) Brushes on Nonpolar Surfaces in Aqueous Solution

The structure of poly(ethylene oxide) (PEO, M(w) = 526) brushes of various grafting density (sigma) on nonpolar graphite and hydrophobic (oily) surfaces in aqueous solution has been studied using atomistic molecular dynamics simulations. Additionally, the influence of PEO-surface interactions on the brush structure was investigated by systematically reducing the strength of the (dispersion) attraction between PEO and the surfaces. PEO chains were found to adsorb strongly to the graphite surface due primarily to the relative strength of dispersion interactions between PEO and the atomically dense graphite compared to those between water and graphite. For the oily surface, PEO-surface and water-surface dispersion interactions are much weaker, greatly reducing the energetic driving force for PEO adsorption. This reduction is mediated to some extent by a hydrophobic driving force for PEO adsorption on the oily surface. Reduction in the strength of PEO-surface attraction results in reduced adsorption of PEO for both surfaces, with the effect being much greater for the graphite surface where the strong PEO-surface dispersion interactions dominate. At high grafting density (sigma approximately 1/R(g)(2)), the PEO density profiles exhibited classical brush behavior and were largely independent of the strength of the PEO-surface interaction. With decreasing grafting density (sigma < 1/R(g)(2)), coverage of the surface by PEO requires an increasingly large fraction of PEO segments resulting in a strong dependence of the PEO density profile on the nature of the PEO-surface interaction.

Low temperature, ion-enhanced, implanted photoresist removal

We investigate the synergistic effects of a remote rf oxygen plasma source operated simultaneously with rf bias at low temperature and find that the photoresist removal rates substantially exceed those observed with either plasma source alone. When applied to high dose ion-implanted photoresist, optical emission curves show a transition between the hydrogen-devoid crust, where oxygen radical consumption is reduced, and the underlying bulk photoresist, where oxygen radical consumption is substantial. Differences between the reactant and product optical emission traces are indicative of complex mechanisms during the crust removal, which are likely related to crust inhomogeneities observed from scanning electron microscopy (SEM) analysis. Crust removal times are as short as 13–16 s, crust removal rates are 1–1.3μm∕min or about half the removal rate of unimplanted photoresist, and crust density estimates are similar to the density of graphite. Because photoresist removal rates are comparable to conventional high temperature downstream ashers, a single step low temperature dual-mode plasma process is able to remove both the crust and bulk photoresist in less than 40 s.

Combustion of Solid Carbon with High Density and Carbon/Carbon-Composite in the Stagnation Flow Field

Carbon combustion has been studied in the stagnation flow field, which can be specified uniquely by the velocity gradient. Use has been made of a rod of high-density graphite and/or carbon/carbon-composite as a test specimen. It has succeeded in conducting combustion experiments under conditions with high velocity gradients, up to 6000 s-1. It is observed even for the high-density graphite that the combustion rate increases monotonically and reaches the diffusion-limited value with increasing surface temperature at high velocity gradients, while there appears a discontinuous change in the combustion rate, related to the establishment of CO flame, at relatively low velocity gradients. It is also observed that the combustion rate is nearly independent of graphite density, which can even hold for the carbon/carbon-composite. In addition, a discontinuous change in the combustion rate can even be observed for the carbon/carbon-composite when the velocity gradient is relatively low. As for the effect of airflow temperature, it is found that the combustion rate in high-temperature airflow is suppressed, compared to that in room-temperature airflow, because of a reduced mass transfer rate through the thickened boundary layer, when the velocity gradient is the same. Theoretical works have also been conducted and it has been found that the “Frozen” mode without CO flame and the “Flame sheet” mode with infinitely fast gas-phase reaction can fairly represent combustion behavior before and after the establishment of the CO flame, respectively, as far as the trend and the approximate magnitude are concerned. Since the high-density graphite and/or carbon/carbon-composite are expected to be used as heat-shielding structural materials in various aerospace applications, knowledge for their combustion mechanisms is considered to give useful information in preventing their oxidation at high temperatures.

Closed, heated reaction chamber design for dynamic high-temperature x-ray-diffraction analyses of gas/solid displacement reactions

A closed, x-ray transparent chamber for containing a hot reactive gas (generated from an internal condensed source) has been designed and evaluated for use in dynamic x-ray-diffraction analysis of a gas/solid displacement reaction. The chamber consisted of a square-bottom base and lid machined from dense pyrolytic graphite. The base contained a flat pedestal, upon which SiO2 microshells (the reactant oxide) were placed, raised above adjacent cavities holding Mg flakes (the condensed precursor to the reactive gas). Upon heating to 650 °C, the Mg evaporated and reacted with the SiO2 inside the sealed chamber. By passing incident and diffracted x rays through the vertical side walls of the chamber and by blocking undesired graphite-diffracted x rays with platinum, the Mg(g)∕SiO2(s) displacement reaction could be tracked with time. This is the first use of dynamic high-temperature x-ray diffraction analysis to monitor the progress of a displacement reaction involving a reactant gas that was generated and conf...

Columnar diamond film coverage of vacuum arc erosion canyons on graphite

Vacuum arcs are generated on dense graphite materials with enhanced local plasma density leading to strong improvement in arc mobility and stability. Observations of cross sections of the arc canyon erosion trace reveals erosion craters covered with a porous columnar film having a diamond structure. Scanning electron microscope (SEM) images of this growth structure are presented at various resolutions.

The Effect of the Graphitizing Heat Treatment and Boron Content on Boron Distribution in High Carbon Steel

It is convenient to analyze the distribution of boron in high carbon steel with boron, simply using neutron-induced radiography with a neutron fluency of 1.9 x 1013 [cm-2]. It was revealed by the neutron-induced radiography that the distribution of boron was dependent on boron contents, graphitizing temperature and time. The density of boron track increased with increasing boron contents. But the density of the boron track and graphite in high carbon steel graphitized at 700°C is higher than that of high carbon steel graphitized at 750°C. The density of graphite in high carbon steel also depends upon the content of boron and the graphitizing temperature. The shape of the boron track was changed from sphere to rod type when annealed at 800°C, in steel containing 50ppm of boron, due to different phases of boride. The distribution of boron segregation or boronrich precipitates in high carbon steel was well documented with a neutron-induced radiography, but the direct relationship between graphite and boron was not clarified by it. Furthermore, the analysis of electron probe X-ray microanalyzer (EPMA) also showed that the high amount of boron coexisted with carbon in graphite in high carbon steel.

A future for nuclear energy: pebble bed reactors

Pebble Bed Reactors could allow nuclear plants to support the goal of reducing global climate change in an energy hungry world. They are small, modular, inherently safe, use a demonstrated nuclear technology and can be competitive with fossil fuels. Pebble bed reactors are helium cooled reactors that use small tennis ball size fuel balls consisting of only 9 grams of uranium per pebble to provide a low power density reactor. The low power density and large graphite core provide inherent safety features such that the peak temperature reached even under the complete loss of coolant accident without any active emergency core cooling system is significantly below the temperature that the fuel melts. This feature should enhance public confidence in this nuclear technology. With advanced modularity principles, it is expected that this type of design and assembly could lower the cost of new nuclear plants removing a major impediment to deployment.

Free Growth of 4H-SiC by Sublimation Method

4H-SiC crystals of cylindrical shape 25 mm and 45 mm in diameter have been grown by the Modified Lely method in a graphite crucible with a guide. The crystals had not mechanical contacts with the walls of the crucible and no formation of polycrystalline SiC during the growth to decrease the stresses. This was confirmed by the facets observed at the edges of the crystals. Numerical simulations of the growth process have been performed. Introduction Silicon carbide is a wide band gap semiconductor with unique properties that make it suitable for application in high frequency, high temperature and high power devices. These applications prompted increased interest in growth of high quality, large diameter single crystals with defect reduction. The objective of this paper is the study of 4H-SiC growth by the Modified Lely method without polycrystalline rim and mechanical contact with graphite walls of the crucible in order to decrease the stresses. Besides, the aim was to obtain crystals with a cylindrical shape and flat top surface to minimize the number of low grain boundaries. Numerical modeling has been used to optimize the design of the crucible. Experimental details 4H-SiC crystals , 25 and 45 mm in diameter, have been grown by the Modified Lely method over the 2000-2050°C temperature range (measured by the top pyrometer : 1 in Fig. 2) in argon [1] (Fig.1). An experimental set-up with RF heating and graphite crucible was used. The crucible (Fig.2) was wrapped in a graphite felt for thermal insulation, and the whole assembly was placed inside a water cooled quartz reactor. Under these experimental conditions we obtained 4H single crystals with a thickness of 5-10 mm. The growth rate was 200-300 μm/h. An increase of the growth rate resulted in 6H-SiC growth. The SiC source powder was loaded both between a dense graphite crucible and a thin walled inner graphite cylinder and inside of the inner cylinder. With this configuration it is possible to reduce the influence of the leakage of Si and the reaction of Si from the main central source with the walls in order to maintain an excess of Si over the seed. To eliminate the influence of the periphery we cut out the seed after gluing to the graphite holder and avoided 6H polycrystalline SiC growth at the periphery. 4H-SiC wafers of 25 and 45 mm in diameter were used as seeds. The ingots were grown on the C-face of seeds which are 8° off-axis. The distance between the powder source and the seed was 15-20 mm. The growth process consisted of : (a) annealing in vacuum at a temperature lower than 1000°C, (b) heating of the crucible up to growth temperature at a high argon pressure, (c) decrease of the pressure (3-5 torr) and growth at low argon pressure. Fig. 2 : Schem right-half par Numerical d Intense effor profiles in th especially rad without a gr minimize the in growth ca Knudsen equ total mass flu Here γi is th equilibrium p ) 25 mm (thickness 5mm Fig.1. Photographs of “as-grown” 4H-SiC atic representation of the Fig. 3 : Compar t of the reactor with guide in the growth ca etails t has been focused on the crucible design to optim e growth area [1-7]. Electromagnetodynamics must iative heat transfer within the growth cavity [1]. The aphite guide (3 on Fig. 2) was optimized by heat radial temperature gradient (Fig. 3). In order to consi vity, we have used a model based on the heterogeneo ations (Eq. 1) to relate the partial pressures of the spec x i R : ( ) RT 2 M P P R i eq i i i i S i π α − γ = (i=Si,SiC2,Si2C) e sticking coefficient and αi is the evaporation c ressures eq i P obey to mass action law. 2605 K without guide radial gradient : axial gradient : 2.Graphite crucible 4. Graphite foam 5. SiC seed 6. SiC powder 7. Induction coils 3. Graphite guide 1.Top pyrometer measure 45 mm (thickness 10mm)

Mechanical properties of monolithic compressed expanded graphite-based adsorbents related to the temperature of pyrolysis and activation, studied with ultrasound

Compressed expanded graphite (CEG) blocks produced from expanded graphite (EG) of apparent density of 2.6 kg m −3 and surface area of 20–40 m 2/g, provided by Carbone-Lorraine Group (France), were applied to the preparation of microporous adsorbents. About 100 cubic CEG blocks were prepared, with a porosity in the range 87.7–99.3%. Thirty CEG blocks of the same porosity equal to 94.32% were impregnated with polyfurfuryl alcohol and varnish, pyrolyzed next at the temperatures of 400, 450, 500 or 550 °C, and finally activated at 900 and 950 °C. The effect of temperature of both the pyrolysis and activation, as well as the effect of the type of applied impregnating agent on the elastic properties of the resultant adsorbents, are discussed. It was found that the stiff impregnating agent can destroy the weak, sensitive network of the highly porous CEG if its concentration exceeds a certain threshold value, characteristic of a given CEG porosity. A high temperature of pyrolysis can increase the cross-linking degree of the polymer, resulting in a higher stiffness of the adsorbents. A clear difference of properties between samples impregnated with polyfurfuryl alcohol and varnish was found as a result of the different wettability of these impregnating agents and different ability to create additional cross-links.

Elastic properties of monolithic porous blocks of compressed expanded graphite related to their specific surface area and pore diameter

Highly porous monolithic blocks of compressed expanded graphite (CEG) with various apparent densities ranging from 0.026 to 0.276 g cm −3, produced of initial expanded graphite of an apparent density of 4 g l −1, were characterized using a gas helium pycnometer, a mercury porosimetry and physical adsorption of N 2 gas at 77 K. The dynamic elastic module of CEG cubes were determined by means of measurements of the velocity of the longitudinal ultrasonic wave (frequency of 100 kHz) propagated along three mutually orthogonal directions: parallel and perpendicular to the bedding plane (or the uniaxial compression direction). CEG was found to be characterized by macropores and mesopores of various diameters. The most probable sizes of pores are 10, 2.4, and 0.9 μm, 4 and 2.5 nm. The elastic properties were related to the porous structure of CEG—represented by the volume of open and closed pores, specific surface area S BET and pore diameter.

Corn-shape carbon nanofibers with dense graphite synthesized by microwave plasma-enhanced chemical vapor deposition

Corn-shape carbon nanofibers (CCNFs) with metal-free tips have been synthesized by a microwave plasma-enhanced chemical-vapor deposition method using CH4 and H2 gasses. The CCNFs were grown on Ni/SiO2/Si and Ni/Mo mesh substrates using a bias-enhanced growth method, and they were analyzed by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The cones are composed of cylindrical pure graphite sheets, and have nanometer-sized tips and roots. The tips’ apex angles of CCNFs have cone angles of 20°, 39°, and 60° depending on the growth conditions such as substrate temperature.

α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

AbstractThe formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H2‐H2O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe3C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.

Neutronics and thermal effects of graphite foams in the performance of nuclear energy systems

Procedures to produce light graphite foam (∼0.5 g/cm 3) that exhibits heat conductivities similar to full density graphite have been developed at ORNL. The consequent substantial reduction in the thermal inertia might have a significant impact in standard designs of graphite system and it could make possible new concepts. We discuss two possible applications: (a) a modular, zero burnup reactivity swing, reactor and (b) the pebble bed accelerator-driven transmutator.

Study of the Mg-Nd alloy obtained by electrolysis in molten oxifluoride media

Mg-Nd alloys have been produced by electrolysis of the molten mixture LiF-NdF3-MgF2 using Nd2(CO3)3 and MgF2 as raw materials. An electrolysis cell was designed having the anode made of super dense graphite and the cathode made of molybdenum metal. The quasi-binary system (NdF3-LiF)eutectic-MgF2 was investigated and the liquidus line was determined using thermo-differential analysis. The solubility of Nd2(CO3)3 in the LiF-NdF3-MgF2 system was investigated by the carbothermal technique.

Mathematical simulation of proton polarimeters with carbon-like analyzing target

The program of the mathematical simulation of the different polarimeters of protons with carbon-like analyzers, working in the 1–5 GeV region of energies, is created. Using the POLO2P version of program the optimization of main parameters of analyzing filter has been carried out. The characteristics of the “focal plane” polarimeters with low dense carbon, polyethylene and graphite (single or double) targets are compared. Optimal parameters of filter and maximal achievable value of polarimeter efficiency was calculated.

The graphite core–valence–valence Auger spectrum

The graphite core–valence–valence (CVV) Auger spectrum is analysed, within a one-particle approximation, by comparison to the X-ray excited valence band (VB) photoemission spectrum, whose structure is well understood in terms of the graphite density of states (DOS). The aim is to identify the electron states originating the components into which the CVV spectrum is resolved by double differentiation. Contributions assigned to self-folds of VB photoemission features and hence due to transitions coupling electrons from the same DOS feature, dominate the spectrum away from the middle region, while folds between different features are important in the middle region. A single particle approximation proves to be adequate to account for the low binding energy, mostly p-like, part of the spectrum. It brakes down however in the high binding energy, mainly s-like, region where spectral distortion is ascribed to a nonnegligible (≈2 eV) Coulomb repulsion between final state holes of s-symmetry.

Evaluation of Pore Structure of Exfoliated Graphite by Mercury Porosimeter

Pore structure of exfoliated graphite. which is responsible for sorption of various liquids, was evaluated by a mercury porosimeter. By using of a U-shape dilatometer, large pores, which seemed to correspond to the spaces among particles, were measured. The pore volume measured by this U-dilatometer was a little less than the value calculated from the theoretical density of graphite. It is worthwhile to be pointed out. however, that the pore structure determined by this U-dilatometer is closer to the real one than that by normal type dilatometer.