Graphite Stack Research Articles

Interaction between graphite oxide and Congo red in aqueous media

Interaction between graphite oxide and Congo red, a diazo dye containing NH 2 and SO 3 functional groups, was studied using UV–vis, FTIR, capillary electrophoresis and potentiometric methods. UV–vis experiments managed to elicit a colorful set of reactions, which were dependent on the sonication of solution. It was established that the molecules of Congo red after sonication are able to penetrate through the hydration shell and extract the protons attached to the functional groups on the surface of graphite oxide. Interaction between the graphite oxide particles and Congo red molecules is suggested to stabilize the aqueous suspensions of single-layer graphite oxide sheets. Potentiometric titration data show that the Congo red molecules adsorbed on the surface of graphite oxide are able to prevent the penetration of OH − ions to reach the acidic functional groups inside the graphite oxide stacks. Electrostatic repulsion among these functional groups is quite high. Ionization of the graphite oxide colloidal particles with adsorbed Congo red molecules leads to the significant changes in their conformation.

Enhanced Electron Transfer Rates by AC Voltammetry for Ferrocenes Attached to the End of Embedded Carbon Nanofiber Nanoelectrode Arrays.

The effect of the interior structure of carbon nanomaterials on their electrochemical properties is not well understood. We report here the electron transfer rate (ETR) of ferrocene (Fc) molecules covalently attached to the exposed end of carbon nanofibers (CNFs) in an embedded nanoelectrode array. The ETR in normal DC voltammetry was found to be limited by the conical graphitic stacking structure interior of CNFs. AC voltammetry, however, can cope with this intrinsic materials property and provide over 100 times higher ETR, likely by a new capacitive pathway. This provides a new method for high-performance electroanalysis using CNF nanoelectrodes.

Mesoporous carbon nitride–silica composites by a combined sol–gel/thermal condensation approach and their application as photocatalysts

Mesoporous carbon nitrides, silicas and their composites have been prepared by a combined sol–gel and thermal condensation approach. Precursors for the carbon nitride (cyanamide) and silica (TEOS) are mixed and condensed simultaneously. After condensation and heat treatment it is observed that the carbon nitride and silica formed highly interpenetrating mesophases which leads either to the formation of mesoporous carbon nitride or silica after selective removal of one of the phases. Importantly, the carbon nitride preserves its graphitic stacking even in the spatial confinement introduced by the surrounding silica phase. As both precursors are liquids this approach allows convenient shaping into thin and thick films or monoliths of mesoporous carbon nitrides. Enhanced photocatalytic activity is observed for the production of hydrogen from water when these mesoporous carbon nitrides are applied as photocatalyst in comparison to the bulk, but also to other mesoporous carbon nitrides, prepared by the reported two-step, hard templating approach.

Reproducible Synthesis of Magnetic Carbon through Ion Implantation

Comparative study of magnetic properties of highly oriented pyrolytic graphite irradiated with different particles suggests that ions (H+, He+, C+) with the energy in the range of hundreds keV yield larger values of induced magnetization compared to the protons with the energy of several MeV. These values increase with ion fluence but abruptly decrease when the concentration of interstitial defects becomes so large that the graphite stacking sequence is collapsed.

Method based on isotope ratio mass spectrometry for evaluation of carbon activation in the reactor graphite

The general idea of this work is to introduce an evaluation method to restore the irradiation parameters of graphite or other carbonaceous materials using experimental and modelling results of 13C generation in the irradiated material. The method is based on coupling of stable isotope ratio mass spectrometry and computer modelling of the reactor core to evaluate the realistic characteristics of the reactor core such as the neutron fluence in any position of the reactor graphite stack or other graphite constructions. The generation of carbon isotopes 13C and 14C in the irradiated graphite of the RBMK-1500 reactor has been estimated by modelling of the reactor core with computer codes MCNPX and CINDER90. Good agreement of simulated and measured Δ 13C/ 12C values in graphite of the central part of the reactor core indicates that the neutron flux (1.40 × 10 14 n/cm 2 s) is modelled accurately in the graphite sleeve of the fuel channel. The simulated activity of 14C is compared with the one measured by the β spectrometry technique. Results indicate that production of 14C from 14N in the RBMK-1500 reactor is considerable and has to be taken into account in order to make proper evaluation of 14C activity. Measured 14C specific activity values correspond to 15 ± 4 ppm impurity of 14N in graphite samples from the RBMK-1500 reactor core.

A One-Step, Solvothermal Reduction Method for Producing Reduced Graphene Oxide Dispersions in Organic Solvents

Refluxing graphene oxide (GO) in N-methyl-2-pyrrolidinone (NMP) results in deoxygenation and reduction to yield a stable colloidal dispersion. The solvothermal reduction is accompanied by a color change from light brown to black. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images of the product confirm the presence of single sheets of the solvothermally reduced graphene oxide (SRGO). X-ray photoelectron spectroscopy (XPS) of SRGO indicates a significant increase in intensity of the C=C bond character, while the oxygen content decreases markedly after the reduction is complete. X-ray diffraction analysis of SRGO shows a single broad peak at 26.24 degrees 2theta (3.4 A), confirming the presence of graphitic stacking of reduced sheets. SRGO sheets are redispersible in a variety of organic solvents, which may hold promise as an acceptor material for bulk heterojunction photovoltaic cells, or electromagnetic interference shielding applications.

A new estimation method for the degree of graphitization for random layer lattices

The so-called P 1 structural parameter is well-known as an index for estimating the degree of graphitization, which was proposed by Houska and Warren. The parameter denotes the probability of finding a graphitic stacking relation between the adjacent carbon layer planes and was generally estimated by an inverse Fourier transform based on their theory. However, in their method, the Fourier integration range is too narrow to evaluate the Fourier coefficients precisely and to discuss in detail the P 1 values obtained from them. In the present study, a newly developed estimation method for the degree of graphitization with higher accuracy using the least square refinement method was presented and the obtained results were compared with those obtained using the conventional inverse Fourier method. The developed program for the analysis will be released in near future.

Development and degradation of graphitic microtexture in carbon nanospheres under a morphologically restrained condition

Graphitic microtexture in carbon nanospheres (CNSs, average diameter = 500–1000 nm) formed by a heat-treatment between 2000 °C and 2900 °C was investigated with XRD, Raman spectrum, SEM and TEM. By the heat-treatment up to 2000 °C, CNS particles in any size were polyhedronized. In the microtexture of polyhedronized CNS particles thus obtained, graphitic stacking structure of aromatic layers was well developed in the polyhedron face regions, while an aggregation of column-like crystallite was confirmed in the polyhedron ridge region. It was also shown that some ridges of CNSs with the average diameter over 700 nm were collapsed by the heat-treatment at 2900 °C. These collapses include an exfoliation of aromatic layers in which the torn layers have hairpin-like microtexture at their ends, and a crack of layer stacks with adjacent layer stacks. According to a direct examination with TEM, it was clarified that a ratio of ridges with either type of damage to a total number of ridges is obviously higher in CNS particles with larger diameter. This phenomenon was discussed in relation to the high crystallinity in the polyhedron face regions and the aggregation of structural defects in the ridge regions.

Breaking AB stacking order in graphite oxide: ab initio approach

Different bulk structures of graphite oxide were systematically investigated using density functional theory (DFT). Our model consisted of a hexagonal in-plane structure of graphene with hydroxyl and epoxide groups, and different oxidation levels and water content. The graphitic AB stacking order was stable in anhydrous graphite oxide, independent of oxidation levels. The hydrogen bonding interaction of layers became weaker as the oxidation level increased to the saturation limit. When water molecules were present in highly oxidized graphite oxide, the AB stacking order was broken due to entropic disorder. The interlayer distances increased with the oxidation level: the interlayer distance was 5.1 A for low oxidation graphite oxide and 5.8 A for high oxidation graphite oxide. The calculated interlayer distance of hydrated graphite oxide was 7.3 A, which is in excellent agreement with experimental observations.

First‐principle study of electronic structures of multi‐layer graphenes

AbstractEnergetic evaluation of possible stacking structures of monolayer, bi‐layer and tri‐layer graphene crystals has been performed using the density‐functional theory under the local density approximation AA stacking of carbon hexagonal monolayer and graphite with AB stacking was also discussed for the reference. It was shown that multi‐layer graphenes are direct band‐gap semiconductors, and the width of their band gaps of graphene crystals can be adjusted by changing the number of graphene layers. The property of band structures of multi‐layer graphenes suggests the potential application for solar cells. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Functionalized graphene sheet—Poly(vinylidene fluoride) conductive nanocomposites

AbstractPVDF nanocomposites based on functionalized graphene sheets, FGS prepared from graphite oxide, and exfoliated graphite, EG, were prepared by solution processing and compression molding. FGS remains well dispersed in the PVDF composites as evidenced by the lack of the characteristic graphite reflection in the composites. Although the α‐phase of PVDF is seen in the EG‐based composites, a mixture of α‐ and β‐phases is present in the FGS analogs. SEM and TEM imaging show smooth fractured surfaces with oriented platelets of graphite stacks and obvious debonding from the matrix in the EG‐PVDF composites. In contrast, the FGS‐PVDF composites show a wrinkled topography of relatively thin graphene sheets bonded well to the matrix. Storage modulus of the composites was increased with FGS and EG concentration. A lower percolation threshold (2 wt %) was obtained for FGS‐PVDF composites compared to EG‐PVDF composites (above 5 wt %). Lastly, the FGS‐PVDF composites show an unusual resistance/temperature behavior. The resistance decreases with temperature, indicating an NTC behavior, whereas EG‐PVDF composites show a PTC behavior (e.g., the resistance increases with temperature). We attribute the NTC behavior of the FGS based composites to the higher aspect ratio of FGS which leads to contact resistance predominating over tunneling resistance. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 888–897, 2009

Production and advantages of carbon-coated graphite for the anode of lithium ion rechargeable batteries

Carbon coating of substrate graphite powder was carried out by carbonizing the mixture with poly(vinyl alcohol) at 600~1500°C. From the measurement of anode performance on the carbon-coated graphite prepared from 21 graphite substrates, crystallite thickness with three-dimensional regular stacking and BET surface area of the substrate graphite were used as parameters for the selection of the substrate graphite. The production scale of carbon-coated graphite powders was successfully increased up to about 1000 kg. Carbon-coated graphite was shown to give a smaller irreversible capacity for the first charge/discharge cycle than graphite without a carbon coating and could be used in an electrolyte solution containing a small amount of propylene carbonate (PC), of which anode performance was almost comparable with those in the solutions without PC. With a high discharge rate of about 10 mA/cm2, carbon-coated graphite gave better anode performance, larger discharge and smaller irreversible capacities, than the graphite without a carbon coating.

Anomalous Angular Dependence of the Dynamic Structure Factor near Bragg Reflections: Graphite

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q, omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

Protection of nuclear graphite toward fluoride molten salt by glassy carbon deposit

Molten salt reactor represents one of the promising future Generation IV nuclear reactors families where the fuel, a liquid molten fluoride salt, is circulating through the graphite reactor core. The interactions between nuclear graphite and fluoride molten salt and also the graphite surface protection were investigated in this paper by powder X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy coupled with X-ray microanalysis. Nuclear graphite discs were covered by two kinds of protection deposit: a glassy carbon coating and a double coating of pyrolitic carbon/glassy carbon. Different behaviours have been highlighted according to the presence and the nature of the coated protection film. Intercalation of molten salt between the graphite layers did not occur. Nevertheless the molten salt adhered more or less to the surface of the graphite disc, filled more or less the graphite surface porosity and perturbed more or less the graphite stacking order at the disc surface. The behaviour of unprotected graphite was far to be satisfactory after two days of immersion of graphite in molten salt at 500 °C. The best protection of the graphite disc surface, with the maximum of inertness towards molten salt, has been obtained with the double coating of pyrolitic carbon/glassy carbon.

Analysis of BDBA in RBMK-1500 reactor with long-term loss of heat removal from the core

The Ignalina nuclear power plant (NPP) is a twin-unit with two RBMK-1500, graphite moderated, boiling water, multichannel reactors. The accident management guidelines for beyond design basis accidents (BDBAs) are in a stage of preparation at Ignalina NPP. The most challenging event from BDBAs is the unavailability of water sources for heat removal from fuel channels (FCs). Due to specific design of RBMK, there are a few possibilities for heat removal from reactor core by non-regular means: depressurisation of reactor cooling system (RCS) (if pressure in cooling circuit is high) and supply of water into cooling system from low pressure water sources, removal of heat from graphite stack by reactor gas circuit, removal of heat from reactor core using cooling circuit of control and protection system channels, etc. The possibility to remove the heat using cooling circuit of control and protection system channels looks very attractive, because the channels with control rods are cooled with water supplied by the system totally independent from the reactor cooling system. The heat from fuel channels, where heat is generated, through graphite columns is transferred in radial direction to cooled channels with control rods. Therefore, the heat removal from RBMK-1500 reactor core using control rods cooling circuit can be used as non-regular mean for reactor cool-down in case of BDBAs with loss of long-term heat removal from the core. This article presents the analysis of large loss of coolant accident (LOCA) with emergency core cooling system failure and station blackout cases without any operator intervention and when the cooling of reactor control rods is restored. For the station blackout case the other non-regular measures (depressurisation of reactor cooling system, water injection from hydro-accumulators, deaerators and city water system) are evaluated. The analysis was performed using RELAP5 Mod3.2, RELAP/SCDAPSIM and RELAP5-3D codes. The use of three codes allows to investigate all aspects of accident with long-term loss of heat removal from the core: RELAP5 code allows to perform thermal-hydraulic analysis using detailed RBMK-1500 model; RELAP/SCDAPSIM code is necessary for modelling of core damage progression; RELAP5-3D code with special “multidimensional heat conduction” model allows to model heat transfer from hot fuel channels through graphite column in radial direction. Results of the analysis have shown, that the heat removal from control rods cooling circuit allows to slowdown effectively the core heat-up process.

Graphite combustion in the Unit 4 reactor during the active phase of the Chernobyl NPP accident: A probable scenario

A probable scenario of combustion of graphite from the graphite stack of the core of the Unit 4 reactor at Chernobyl NPP during the accident of April 26, 1986, was discussed. The deciding role in combustion was for the first time attributed to the sectional ring bushings intended for imporving the heat transfer from the heat carrier in the process channel to graphite of the stack. The assumptions made were supported by appropriate calculations.

Specifics of RBMK core cooling in beyond design basis accidents

The most dangerous beyond design basis accidents for RBMK reactors, leading to the worst consequences, are related to the loss of long-term heat removal from the core. Due to a specific design of RBMK, there are a few possibilities for heat removal from reactor core by non-regular means: removal of heat from graphite stack by reactor gas circuit, removal of heat from reactor core using control rods cooling circuit, depressurisation of reactor cooling system, supply of water into cooling system from low pressure water sources, etc. This paper presents the analysis of such heat removal by employing RELAP5, RELAP5-3D and RELAP/SCDAPSIM codes. The analysis was performed for Ignalina nuclear power plant with RBMK-1500 reactor. The analysis of result shows that the restoration of water supply into control rod channels enables to remove 10–30 MW of the generated heat from the reactor core. This amount of removed heat is comparable with reactor decay heat in long-term period and allows to slowdown the core heat-up process. However, the injection of water to reactor cooling system is considered as main strategy, which should be considered in RBMK-1500 accident management procedure.

Evidence of Graphitic AB Stacking Order of Graphite Oxides

Graphite oxide (GO) samples were prepared by a simplified Brodie method. Hydroxyl, epoxide, carboxyl, and some alkyl functional groups are present in the GO, as identified by solid-state 13C NMR, Fourier-transform infrared spectroscopy, and X-ray photoemission spectroscopy. Starting with pyrolytic graphite (interlayer separation 3.36 A), the average interlayer distance after 1 h of reaction, as determined by X-ray diffraction, increased to 5.62 A and then increased with further oxidation to 7.37 A after 24 h. A smaller signal in 13C CPMAS NMR compared to that in 13C NMR suggests that carboxyl and alkyl groups are at the edges of the flakes of graphite oxide. Other aspects of the chemical bonding were assessed from the NMR and XPS data and are discussed. AB stacking of the layers in the GO was inferred from an electron diffraction study. The elemental composition of GO prepared using this simplified Brodie method is further discussed.

The individual channel monitoring (ICM) proposal to improve the safety performance of RBMK

A comprehensive study has been completed in relation to the accident analysis of RBMK with key results documented in three companion papers in the present journal volume. The study also aimed at the comparison between selected accident analysis topics in Light Water Reactors (LWR) and RBMK. The relevance of the Fuel Channel Blockage (FCB) event within the area of accident analysis was confirmed. Owing to the probability of occurrence, also connected with the large number of channels, estimated in the order of 10 −2/reactor/year, the FCB is actually part of the Design Basis Accidents (DBA) for the RBMK. In case the FCB is part of the DBA, a noticeable difference occurs in results from safety evaluations between LWR and RBMK: in the latter case, a DBA causes a loss of integrity for the pressure barrier and, though to a limited extent, damaged fuel overpasses such a barrier. The FCB event also causes contamination in reactor cavity and in selected parts of the overall confinement, damage in various graphite blocks and, even excluding the MPTR risk owing to the findings from the second companion paper in this journal volume, mechanical loads on neighbouring fuel channels, graphite stacks and reactor tank that need extensive examination before reactor restart. Therefore, a proposal has been formulated for performing the feasibility analysis for the design of a system denominated ICM (Individual Channel Monitoring). The goal of the ICM is the early detection of the FCB event and the triggering of scram in such a way to prevent pressure tube damage and, definitely, over-passing of the pressure barrier by molten or damaged fuel during a DBA situation. The ICM is based upon the signals of pressure-drop (or flow-rate) and fluid temperature transducers installed in the bottom and the upper parts of the fuel channels, respectively. The performed study shows that steam superheating at fuel channel outlet occurs early after the blockage event and the related temperature signal can be used to cause scram. The availability of sophisticate computational tools including the detailed neutron kinetic model for each core channel, made possible the preliminary conclusion of the study.

Energetic evaluation of possible stacking structures of Li-intercalation in graphite using a first-principle pseudopotential calculation

Energetic evaluation of possible stacking structures of Lithium–graphite intercalation compounds (Li–GICs) has been performed using the density-functional theory under the local density approximation with or without the generalized gradient correction. The calculated formation energies of Li–GICs well reproduced the stability of the structures and their lattice parameters. That is, the calculated formation energies (Δ E) are negative for the equilibrium phases of LiC 6, LiC 12 and LiC 18 with αA, αAA and αAAA stacking where one-third of the carbon hexagons have corresponding Li atoms. In contrast to that, Δ E of hypothetical phases of LiC 2, LiC 4, LiC 6 and LiC 8 with different in-plane ratios of carbon-hexagon to Li are positive. Hypothetical LiC 4 with the Aα′B stacking has also a positive Δ E, but the equilibrium LiC 18 with αABA stacking has a negative Δ E. AA stacking of carbon hexagonal monolayer and graphite with AB stacking are also discussed for the reference state of these Li–GICs.