Density Graphite Research Articles

Molten salt corrosion of high density graphite and partially stabilized zirconia coated high density graphite in molten LiCl–KCl salt

Corrosion studies were performed on uncoated high density graphite and plasma sprayed partially stabilized zirconia (PSZ) coated high density graphite with NiCrAlY bond coat in molten LiCl–KCl eutectic salt at 600°C for periods of 250h, 1000h and 2000h under inert argon atmosphere. High density graphite showed weight loss while PSZ coated high density graphite showed weight gain. There is no significant attack and degradation of top PSZ coating in molten salt, however microcracks were observed at the bond coat-substrate interface after 2000h of exposure. PSZ coated high density graphite exhibited excellent corrosion resistance in molten LiCl–KCl salt due to chemical stability and absence of phase transformation as confirmed from scanning electron microscopy, X-ray diffraction and laser Raman studies, however adhesion of the coating has to be improved.

A Monte Carlo model of the Varian IGRT couch top for RapidArc QA

The objectives of this study are to evaluate the effect of couch attenuation on quality assurance (QA) results and to present a couch top model for Monte Carlo (MC) dose calculation for RapidArc treatments. The IGRT couch top is modelled in Eclipse as a thin skin of higher density material with a homogeneous fill of foam of lower density and attenuation. The IGRT couch structure consists of two longitudinal sections referred to as thick and thin. The Hounsfield Unit (HU) characterization of the couch structure was determined using a cylindrical phantom by comparing ion chamber measurements with the dose predicted by the treatment planning system (TPS). The optimal set of HU for the inside of the couch and the surface shell was found to be respectively −960 and −700 HU in agreement with Vanetti et al (2009 Phys. Med. Biol. 54 N157–66). For each plan, the final dose calculation was performed with the thin, thick and without the couch top. Dose differences up to 2.6% were observed with TPS calculated doses not including the couch and up to 3.4% with MC not including the couch and were found to be treatment specific. A MC couch top model was created based on the TPS geometrical model. The carbon fibre couch top skin was modelled using carbon graphite; the density was adjusted until good agreement with experimental data was observed, while the density of the foam inside was kept constant. The accuracy of the couch top model was evaluated by comparison with ion chamber measurements and TPS calculated dose combined with a 3D gamma analysis. Similar to the TPS case, a single graphite density can be used for both the thin and thick MC couch top models. Results showed good agreement with ion chamber measurements (within 1.2%) and with TPS (within 1%). For each plan, over 95% of the points passed the 3D gamma test.

COMPARATIVE STUDY OF ELECTRONIC PROPERTIES OF GRAPHITE AND HEXAGONAL BORON NITRIDE (h-BN) USING PSEUDOPOTENTIAL PLANE WAVE METHOD

We studied various ground state properties, e.g. cohesive energy, exfoliation energy, equilibrium lattice constants, elastic constant (C33), compressibility, band structure, density of states and charge density of Graphite, Graphene and Hexagonal Boron Nitride (h- BN ) using pseudopotential plane wave method. Most of the calculated physical quantities of graphite are found to be close to those of h- BN and these are in good agreement with available experimental data.

The Thermal Conductivity Simulation Calculation of Polyacryonitrile-Based Carbon Fiber/Pitch-Based Carbon/Carbon Composites

The thermal conductivity of polyacryonitrile-based carbon fiber/pitch-based carbon/carbon composites was simulated by the finite element analysis. After the density, thermal capacity and thermal conductivity of graphite and carbon fiber were given for known parameters, the geometric grid figure) was generated after the Gambit software processed the unit geometric model grids and the thermal conductivities of carbon/carbon composites were simulated by the Fluent software.

Preparation of LaNi<sub>5</sub> by FFC Cambridge Process

A new technique was proposed for preparation of LaNi5 powder by direct electrolytic deoxidation of solid La2O3–NiO mixture oxides in molten CaCl2–NaCl. Oxides pellets was sintered and used as cathode. High – density graphite was used as anode. Electrolysis was performed. Current change during electrolysis was recorded. XRD was used to analyse the composition of products. SEM was used to analysis surface and cross section of pellets before sintering, after sintering and after electrolysis. The preparation mechanism and the influences of sintering temperature and cell voltage on electrolytic deoxidation was studied. Results show that the optimum conditions to prepare LaNi5 are：NiO-La2O3（Ni:La=5:1）pellets are formed at 30MPa, sintered at 850°C for 5h, electrolyzed under 3.2V in CaCl2–NaCl melt at 850°C for more than 12 hours.

Non-contact characterisation of conductivity gradient in isotropic polycrystalline graphite using inductance spectroscopy measurements

The two main graphite ageing processes in advanced gas-cooled reactors (AGRs) are fast neutron damage and radiolytic graphite oxidation. These processes change the properties of the graphite core and lead to a reduction in the graphite density, ie a weight loss. Electrical conductivity measurement of the graphite material is among the possible methods for estimating the weight loss and hence provides a measure of the graphite ageing. The inductance spectroscopy technique using eddy current sensors has been implemented in a wide variety of NDT applications, since it enables the possibility of extracting the electrical parameters variation from eddy current measurements and examining more fully the internal material structure. In this paper we present a non-contact eddy current method for determining non-destructively the electrical conductivity gradient through 100 mm-thick graphite sections using inductive spectroscopic measurements collected by a gradiometer. The method employs the commercial finite element software COMSOL to generate the data from a model of a real profile distribution. This data is translated to the profile conductivity of the graphite test object via the solution of an inverse problem. The linear Tikhonov regularisation method and the non-linear regularised Gauss Newton technique are employed in the solution of the inverse problem. During the optimisation, the forward problem is evaluated using COMSOL. Numerical optimisation tests have been carried out for different cases of step conductivity profiles. Initial results using simulated data show that representative estimates of the conductivity profile of the modelled graphite sections have been obtained.

Microstructure and thermophysical properties of graphite foam/glass composites

Mesophase pitch based graphite foams with different thermal properties and cell structures were infiltrated with glass by pressureless infiltration to prepare potential alternative composites for cooling electronics. Microstructure, thermal diffusivity and coefficient of thermal expansion (CTE) of the obtained composites were investigated. It was demonstrated that there was excellent wettability of the graphite foam by molten glass, and the foam framework was retained well after infiltration, which could facilitate good heat transfer throughout the composites. The highest thermal diffusivity of the composites reached 202.80 mm 2/s with a density of 3.81 g/cm 3. And its CTE value was 4.53 ppm/K, much lower than the corresponding calculated result (7.46 ppm/K) based on a simple “rule of mixtures” without considering the space limitations of the graphite foams. Thus, the mechanical interlocking within the space limitations of the graphite network played a crucial role in limiting the thermal expansion of the glass. The CTEs of the graphite foam/glass composites varied from 4.53 to 7.40 ppm/K depending on the graphite foam density which varied from 0.82 to 0.48 g/cm 3. The CTEs were a good match to those of semiconductor chips and packaging materials.

Characteristics of a fluidized bed electrode for a direct carbon fuel cell anode

The characteristics of a fluidized bed electrode applied as a direct carbon fuel cell anode, which has an inner diameter of 35mm and height of 520mm and employed bamboo-based activated carbon (BB-AC) as a feedstock, are vigorously studied under various experimental conditions. The optimal performance of the fluidized bed electrode direct carbon fuel cell (FEBDCFC) anode with the BB-AC as a fuel is obtained under the following conditions with a limiting current density of 95.9mAcm−2: reaction temperature, 923K; N2 flow rate, 385mlmin−1; O2/CO2 flow rate, 10/20mlmin−1; nickel particle content, 30g; and a cylindrically curved nickel plate as a current collector. Under the same optimal conditions, the limiting current density of the FEBDCFC anode with oak wood-based activated carbon and activated carbon fiber as the fuel is determined to be 94.5 and 88.4mAcm−2, which is lower than that determined for BB-AC as the fuel. Comparatively, the limiting current density for graphite, which is utilized as the carbon fuel for this fuel cell system, could not be unequivocally determined because no plateau of the limiting current density against the overpotential is observed.

Research on Removal of Sulfide Inclusion in Steel Surface by Molten Salt Electrolysis

Steel including sulfide inclusion was used as cathode, while high density graphite rod was used as anode. Sulfide inclusion in steel surface was removed by molten salt electrolysis in molten CaCl2 at 850 under the stationary voltage. The influences of the voltage and electrolytic time on the process of reaction were investigated. The distributions of the sulfide inclusion were investigated by SEM. The desulfurated ratio was investigated by quantitative metallographic analysis, Cyclic voltammetry was performed to characterize reactions of cathode. The results show that the voltage and the time of electrolysis were in direct proportion with desulfurated ratio. The higher voltage and the longer electrolysis time is, the greater the desulfurated ratio is.

Quantum Study of Laser-Induced Initial Activation of Graphite-to-Diamond Conversion

Recently (Science 2009, 325, 181), femtosecond-resolved electron energy loss spectroscopy (FEELS) was used to map the structural changes of graphite upon laser irradiation, revealing the change from sp(2) to sp(3), i.e., diamond-like, hybridization. With a laser excitation energy of 2.39 eV and a fluence of 1.5 mJ/cm(2), the most pronounced changes were observed in approximately 180 fs, a time similar to the temporal resolution of the technique. The presence of the laser field turns the electronic wavefunction into a wavepacket whose quantum dynamics governs the onset of the structural rearrangement. Density functional theory calculations with a quantum propagator that include the laser field show that the charge density of graphite expands between the layers in an ultrafast process of the order of approximately 10 fs. Calculations as a function of the field/fluence further show different values of the maximum bond order reached at the stationary state. The experimentally used value is at the crossover between two regimes. It is tempting to associate the second regime with the electron organization necessary to achieve ablation or melting. The application of the model demonstrates its potential for examining the dynamical nature of the charge density and chemical bonding as it forms.

The interplay between structure and orbitals in the chemical bonding of graphite

Recently, the dynamics of atomic distances and orbital charge in graphite has been investigated by ultrafast electron diffraction, electron energy loss spectroscopy and transient optical absorption. A subtle interplay between structural motions and electronic degrees of freedom was found responsible for several peculiar behaviors like coherent phonon emission, photoinduced diamond formation, and graphene ablation. Here, we calculate ab initio the charge density of graphite, and observe its evolution during the above-mentioned structural distortions. The modifications of the electron energy loss and the optical spectra during c-axis compression and expansion are calculated and found in agreement with recent experiments.

Re-evaluation of the product of (W/e)air and the graphite to air stopping-power ratio for 60Co air kerma standards

Experiments which determine the product of (W/e)air, the average energy deposited per coulomb of charge of one sign released by an electron coming to rest in dry air, and , the Spencer–Attix mean restricted mass collision stopping-power ratio for graphite to air, in a 60Co or 137Cs beam are reanalysed. Correction factors, e.g., to account for gaps about a calorimeter core or perturbations due to a cavity's presence, are calculated using the EGSnrc Monte Carlo code system and these generally decrease the value of (W/e)air for each experiment. Stopping-power ratios are calculated for different choices of density correction and average excitation energy (I-value) for graphite. To calculate an average value (W/e)air for the BIPM air kerma standard, each experimental result is multiplied by the ratio /. While individual values of are sensitive to the I-values and density corrections assumed, this ratio varies by less than 0.1% for different choices. Hence, the product (W/e)air is relatively insensitive to these choices. The weighted mean of the updated data is (W/e)air J C−1 ± 0.2%, suggesting that the accepted value of 33.97 J C−1 ± 0.1% is 0.8% too high. This has implications for primary 60Co air kerma standards worldwide and potentially for the choice of graphite I-value and density correction for the calculation of the graphite stopping power, as well as the value of (W/e)air.

Effect of Residence Time of Graphitisation on Thermal Conductivity of Molded Graphite

The effect of residence time of graphitisation on thermal conductivity of molded graphite has been examined. The examination has been conducted by varying residence time of graphitisation of molded carbon with petroleum coke as raw material and coal tar pitch. Graphitisation has been conducted by heating molded graphite at 2500 °C in argon atmosphere with residention time of 10, 30 and 90 minutes. Graphitisation degree, density, shrinking mass and porosity of molded graphite were examined and so was its thermal conductivity. The result showed that the decrease of porosity and the increase of graphitisation degree due to the increasing of residention time of graphitisation will increase the thermal conductivity of graphite. Molded graphite graphitisized with residence time for 90 minutes residention time gave thermal conductivity of 2.134 Watt/mK and graphitization degree 0.718.

The electrical resistivity of liquid carbon under rapid heating of dense isotropic graphite in different media

Experiments are considered which involve pulsed heating of graphite (in several microseconds) and registering of electrical resistance of liquid carbon. Samples of MF-307 dense (2 g/cm3) isotropic graphite with a cross section of ∼0.3 × 0.3 mm and 10–15 mm long are heated in water or in thick-walled (outside diameter D ∼ 10 mm, inside diameter d ∼ 0.5 mm) sapphire capillary tubes. It is confirmed that the heating in water at atmospheric pressure does not enable one to obtain and investigate liquid carbon: at best, only the beginning of the liquid state region is attained. The heating in sapphire tubes causes the emergence of pulsed pressure (up to ten kilobars) after expanding graphite comes against the tube wall. This growing pressure (within several microseconds) enables one to investigate the liquid state of carbon in a confined volume. The isochoric heating provided the possibility of measuring the electrical resistivity of liquid carbon at high specific energies (up to ∼32 kJ/g) and high pressures; such measurements are quite expensive in the case of stationary investigations.

Characterization of nitride coatings on high density graphite deposited by magnetron sputtering

Cathode deposit consolidation operation after electrorefining of spent metallic fuels of fast breeder reactors involves melting of U and Pu at 1300 °C after distillation of occluded chloride salt and Cd, in graphite crucibles. Nitride coatings possessing high hardness, melting point and thermodynamic stability against reactive materials and molten LiCl–KCl salts have greater potential for coating the graphite crucibles. In the present study nanocrystalline TiN, ZrN and Ti–Si–N coatings were deposited on high density graphite disc and crucible samples by DC/RF magnetron sputtering. The coated samples were characterized by SEM, GIXRD and AFM. The results indicated that coatings with uniform thickness of 3 to 6 µm were deposited on high density graphite which adheres well to the substrate. The surface morphology of TiN, ZrN and Ti–Si–N coatings examined by SEM and AFM showed the presence of spherical nanoparticles of nitrides getting agglomerated into clusters. Characterisation of nitride coated crucibles was carried out before and after uranium melting by induction heating to simulate cathode processor crucible conditions. TiN and Ti–Si–N coating appears to offer better stability, ease of ingot release and coating adhesion. The paper highlights the results of the present investigation.

Design of the 50 kW neutron converter for SPIRAL2 facility

SPIRAL2 is a facility for the study of fundamental nuclear physics and multidisciplinary research. SPIRAL2 represents a major advance for research on exotic nuclei. The radioactive ion beam (RIB) production system is comprised of a neutron converter, a target and an ion source. This paper is dedicated to the designing of the 50 kW neutron converter for the SPIRAL2 facility. Among the different variants of the neutron converter, the one based on a rotating solid disk seems quite attractive due to its safety, ease in production and relatively low cost. Dense graphite used as the converter's material allows the production of high-intensity neutron flux and, at the same time, the heat removal from the converter by means of radiation cooling. Thermo-mechanical simulations performed in order to determine the basic geometry and physical characteristics of the neutron production target for SPIRAL2 facility, to define the appropriate beam power distribution, and to predict the target behaviour under the deuteron beam of nominal parameters (40 MeV, 1.2 mA, 50 kW) are presented. To study the main physical and mechanical properties and serviceability under operating conditions, several kinds of graphite have been analyzed and tested. The paper reports the results of such measurements. Radiation damage is the most important issue for the application of graphite as neutron converter. It is well known that the thermal conductivity of the neutron-irradiated graphite is reduced by a factor of 10 from the initial value after irradiation. Difference in volume expansions between the matrix and the fiber results in serious damage of neutron-irradiated C/C composites. Calculations showed that at high temperature the effect of neutron radiation is not so critical and that the change in thermal conductivity does not prevent the use of graphite as neutron converter.

Influence of Synthesis Conditions and Heat Treatment on the Structure of Ti3SiC2-Derived Carbons

We investigate the characterization and adsorption modeling of Ti3SiC2-derived carbons (Ti3SiC2-DCs) prepared by chlorination at three temperatures (600, 800, and 1000 degrees C) as well as samples heat-treated at 1100 degrees C for one and three day periods. The modeling exploits our recent finite Wall thickness model-based density functional theory approach (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532), Utilizing experimental adsorption isotherms OF Ar at 87 K for the characterization. In general, characterization results show that the carbon wall structure of carbide-derived carbons (CDCs) is precursor-inherited, and Ti3SiC2-DCs have predominantly graphitic walls whose helium density is close to the true density of graphite (2.27 g/cm(3)), while SiC-derived carbons (SiC-DCs) have predominantly diamond-like walls whose helium density approaches the true density of diamond (3.52 g/cm(3)). This precursor-inherited character is also seen to give rise to two distinct evolutions of the Pore Structure of Ti3SiC2-DCs and SiC-DCs under chlorination temperature or post-heat-treatment conditions. In Particular, both pore enlargement and opening are observed for Ti3SiC2-DCs, but only pore opening, especially in the region of smallest pores (<5 angstrom), is found for SiC-DCs. For adsorption modeling results, the ordering of pore network accessibility, Ar at 87 K < CH4 at (313K and 333K) < CO2 at or above 273 K, has been specifically found for the Ti3SiC2-DC samples prepared by chlorination at 800 degrees C, providing high CO2/CH4 selectivity by the carbon. Heat treatment of this sample at 1100 degrees C for a 1 day period gives rise to significant improvement of pore network accessibility but subtle change in pore bodies. Finally, shrinkage is found for all investigated CDCs and the I day heat-treated sample, but swelling is found for the 3 day heat-treated carbon sample, under high-pressure CO2 adsorption conditions.

Electroerosion Resistance Performance of the Electroformed Cu-Graphite Composites

Cu—graphite composites, which combine with two kinds of commonly used electro-discharge machining (EDM) electrode materials, were prepared using the co-electroforming technique in an acid sulfate bath. To obtain an optimum combination of copper and graphite in the composites with an excellent electroerosion resistance performance, the effect of electrodeposition conditions, such as graphite bath content, current density, stirring rate, and electrolyte temperature, on the tool-electrode wear were investigated using micro-EDM experiments, and the functional relation of the tool-electrode wear ratio (t) and volume content percent of embedded graphite particles (s) in the composites was further developed. In comparison with the electroformed pure copper, the co-electroformed Cu-20.6vol% graphite EDM macro-scale electrode showed considerably better electroerosion resistance performance, mainly resulting from its optimal volume fraction ratio combination of copper with an excellent heat-transfer character, and graphite with an extremely high melting point, while the UV-LIGA electroformed Cu-20.6vol% graphite micro-feature EDM electrode exhibited much worse electroerosion resistance behavior, primarily owing to its rougher surfaces, lower strength, looser texture, and various deposition drawbacks.

Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses andquenching of phonon modes

The extremely high thermal conductivity of individual carbon nanotubes, predictedtheoretically and observed experimentally, has not yet been achieved for large nanotubeassemblies. Resistances at tube–tube interconnections and tube–electrode interfaces havebeen considered the main obstacles for effective electronic and heat transport. Here weshow that, even for infinitely long and perfect nanotubes with well-designed tube–electrodeinterfaces, excessive radial heat radiation from nanotube surfaces and quenching of phononmodes in large bundles are additional processes that substantially reduce thermal transportalong nanotubes. Equivalent circuit simulations and an experimental self-heating3ω technique were used to determine the peculiarities of anisotropic heat flow andthermal conductivity of single MWNTs, bundled MWNTs and aligned, free-standingMWNT sheets. The thermal conductivity of individual MWNTs grown by chemicalvapor deposition and normalized to the density of graphite is much lower (κMWNT = 600 ± 100 W m−1 K−1)than theoretically predicted. Coupling within MWNT bundles decreases this thermal conductivity to150 W m−1 K−1. Further decrease of the effective thermal conductivity in MWNT sheets to 50 W m−1 K−1 comes from tube–tube interconnections and sheet imperfections like dangling fiber ends,loops and misalignment of nanotubes. Optimal structures for enhancing thermalconductivity are discussed.

Decrease in the consumption of graphitized electrodes in arc furnaces during their forced cooling

The cost of electrodes during steelmaking is a substantial (up to 12.5%) expenditure part of the end product and is sometimes higher than the cost of the consumed electric power. Therefore, it is necessary to perform a set of measures aimed at decreasing the exploitation consumption of graphitized electrodes. In order to enhance the competitive ability of electric steel products, it is necessary to increase the strength and exploitation reliability of graphitized electrodes, which is controlled, to a great extent, by their technical characteristics. Russian industry produces (TU 48-12-52‐93) electrodes with a diameter from 75 to 555 mm operating at a current density from 10 to 25 A/cm 2 . The technical characteristics of electrodes for both ac and dc furnaces depend on the graphite density ( γ = 1.6‐2.2 g/cm 3 ) and porosity ( σ = 1.4‐2) × 10 5 ( Ω m) ‐1 ). The material density exerts a greater effect on the thermal stability of the electrode, and the porosity mostly affects its oxidation. In addition, the density and electrical conductivity of electrodes affect their electrical resistance and, hence, the electric losses during electric-power transmission. Under the formed economical conditions, the quality of graphitized electrodes is a determining factor; therefore, imported rather than Russian electrodes are most frequently used for the exploitation of furnaces, especially high-power furnaces. Although the imported electrodes are more expensive (by a factor of 1.5‐2.0) but have higher technical characteristics.