Conventional Graphites Research Articles

Microstructural characterization of the CGB graphite grade from the molten salt reactor experiment

In the 1960s, the Molten Salt Reactor Experiment demonstrated the feasibility of molten salt reactors for civil applications. The Molten Salt Reactor Experiment used a nuclear graphite grade known as CGB in the core as the fast neutron moderator. For application in this purpose, CGB graphite underwent additional impregnation steps that are not typically used in conventional nuclear graphites. These additional impregnations significantly lowered the permeability and reduced the size of pore openings in CGB graphite, for the purpose of inhibiting molten salt ingression into the graphite. Although several publications report the mechanical properties and irradiation response of CGB graphite, little information has been published about the microstructure or sealant used for this graphite grade. Here the results of multiple advanced microscopy techniques are presented with the objective to investigate the unique microstructure and sealing technology of legacy material from the Molten Salt Reactor Experiment so it can be potentially readopted for modern reactors. The microstructural characterization shows that some pores were fully sealed by the impregnation whereas other regions contained foam-like materials that partially filled the void content. This manuscript also investigates and documents the nature and possible source of the sealant material. The pore sealing methods used for CGB are discussed and compared with other common technologies used to reduce the impregnation of molten salts into graphite components. Knowledge of CGB graphite's microstructure can enable development of new pore infiltration technologies to improve in operando behavior advancing the next generation of molten salt reactors.

Nanoburl Graphites.

A critical challenge for the application of graphite is low strength, which originates from the easy cleavage of graphite (0002) planes. Inspired by the burl strengthening mechanism observed in tree trunks, nanodiamond particles converted into graphite onions are used as "nanoburls" embedded in graphite (0002) lattice planes to eliminate the graphite (0002) plane cleavage of bulk graphites prepared by spark plasma sintering from graphite powders. Covalent bonds are built between carbon atoms by sp3 hybridization at the interface between the graphite onions and flakes, which triggers an electron redistribution to form positive/negative charge domains within. Thus, pairs of pseudo-Schottky junctions are created by the hybridization, which further enhances the bonding between the graphite onions and flakes. With these bonding mechanisms, and with voids between the graphite powders filled in by the volume expansion associated with the change of nanodiamonds to the graphite onions, the loose compaction of graphite powder becomes consolidated at 1700°C. The proposed nanoburl mechanism shows its potential and bestows the nanoburl graphites with strength five times that of conventional graphites prepared from graphite powders. The concept of nanoburl strengthening can be important in the microstructural design and property enhancement of other layered materials.

Hexagonal platelet graphite and its application in Li-ion batteries

Highly crystalline graphite powder was prepared from the low temperature (800–1000 °C) graphitization of renewable materials using a magnesium catalyst. The magnesium catalyst was easily removed, resulting in a highly pure graphite product. The graphite has an interesting microstructure, consisting of mainly hexagonal platelets (up to 10 μm), and some graphene particles. This unique microstructure results in a high specific surface area. However, the graphite has a high density and a degree of graphitization higher than some conventional artificial graphites made at 2850 °C. When cycled in Li half-cells, the graphite displays good cycling stability and high reversible capacity (up to 370 mAh g−1). After carbon coating, the surface area of the graphite is greatly reduced, and its coulombic efficiency is greatly improved, resulting in performance that is comparable to commercial high surface area battery synthetic graphites. Graphite made by low temperature Mg catalyzation of renewable plant derived precursors represents a new direction for research in the development of primary synthetic graphites and the application of sustainable materials. We anticipate that Mg catalyzed graphites could have interesting applications in batteries, as catalyst supports or in other fields.

Nanoporous Walls on Macroporous Foam: Rational Design of Electrodes to Push Areal Pseudocapacitance

Supercapacitors, also known as electrochemical capacitors, are considered the most promising energy storage devices owing to their high power densities and long lifespan. [ 3–5 ] The fast charge and discharge capability make supercapacitors favorable for applications in hybrid vehicles, portable electronics, and backup energy systems. [ 6–10 ] Carbonaceous materials, including carbon nanotubes and graphene, are being widely studied as alternatives to conventional graphites. [ 2 , 11–14 ] However, carbon-based materials usually show low energy density as they store charges electrostatically at their surfaces, so they have intrinsically low specifi c areal capacitance ( C a ) in the range of 10 − 40 μ F cm − 2 . Transition metal oxides/hydroxides store charges with surface faradaic (redox) reactions, which enable higher energy density compared to carbon. Metal oxides/hydroxides such as MnO 2 , NiO, Ni(OH) 2 , CoO x and their compounds have recently come into focus in the design of high-energy-density charge-storage materials. [ 15–27 ] Despite these efforts, practical energy storage applications still require higher specifi c capacitance. One way out is to design nanometer-scale electrode materials with very large surface areas and structural stability. In this context, porous nanostructures are of great interest because they can reduce ionic and electronic diffusion distance and provide large electrode/electrolyte contact area. For example, porous Ni and Au electrodes as current collectors have recently been reported, which signifi cantly improve the specifi c capacitance when covered with the pseudoactive material MnO 2 . [ 28 , 29 ] Also, nanoporous graphene electrodes with ∼ 4 nm pores drastically enhance the specifi c capacitance up to 166 F g − 1 . [ 30 ] For metal oxides,

Studies on singular electrochemical performances at low potential of new type fibrous nanocarbon

Fibrous nanocarbons were synthesized at low temperature by catalytic decomposition of hydrocarbons and carbon monoxide. They were then characterized by micro-structural and electrochemical techniques. It was found that the as-prepared nanocarbons could be classified into platelet, herringbone and tubular morphologies. The highest tap densities (70–90%) were observed for platelet morphologies. These were comparable with conventional graphite, and correlated with the high degree of graphitization that is associated with fast reduction in a H 2/He flow. The electrochemical discharge profiles of the platelet morphologies were similar to that of material graphitized at >2400 °C. Such materials may be acceptable alternatives to conventional synthetic graphites.

Charge Storage Effects in Alkaline Cathodes Containing Fluorinated Graphite

Enhanced charge storage effects are explored in aqueous alkaline cathodes containing fluorinated graphite (FG), when used to replace conventional carbons as the conductor in the cathode. Electrodes formed to accentuate this effect display approximately twice the expected cathode capacity. Discharge of alkaline Zn anode AAA cells prepared with cathodes containing 85-90% graphite and 10-15% of different cathode salts approach the expected theoretical mAh capacities of 406 313 308 292 NiOOH, and 231 However, equivalent cells, but formed with 85-90% FG and 10-15% of the respective cathode materials, discharge to approximately double this capacity per gram of AgO, NiOOH, BaFeO, or This anomaly is related to an observed intrinsic alkaline cathode capacity in FGs which does not occur in conventional graphites or carbon blacks. The FG polymers are observed to simultaneously maintain two roles in the cathode composite; functioning both as a conductive matrix, and also adding intrinsic capacity to the cathode. © 2001 The Electrochemical Society. All rights reserved.

Structural graphite for high-temperature gas-cooled reactors

A new type of nuclear graphite has been developed, consisting of noncalcined petroleum coke with coal-tar pitch binder. This material has strength properties that are about twice those of conventional graphites. The irradiation resistance of the new graphite is good over the temperature range of 800 ° to 1400 °C to a fluence of 1.3 × exp 26 n/m 2(0.18 MeV).

Surface area and porosity of unmodified graphitic matrices A3-27 and A3-3 (1950) and oxidized matrix A3-3 (1950)

Graphitic matrices consist of graphite grains and ungraphitized binder carbon and are used as structural material in the spherical fuel elements of high-temperature, helium-cooled nuclear reactors (HTR). Like conventional nuclear graphites, matrix materials are porous, with an open and closed porosity of 11 vol% and 14 vol%. The generation of new pores and the opening of existing pores due to corrosion by gaseous coolant impurities alters the diffusion kinetics of the fission product cesium and the retention of tritium. Therefore, the oxidation-induced changes of the porosity of two matrix varieties were studied. The pore volumes accessible to xylene at 300 K and the nitrogen adsorption and desorption isotherms at 77 K were measured. The surface area was determined by the BET (Brunauer-Emmett-Teller) procedure and the α s -method. As standards for the α s -plots, a recently proposed isotherm for ungraphitized carbon and the isotherm for the original bulk matrix was used. The α s -plots are indicative of the appearance of micropores and a large increase in the surface area in mesopores due to oxidation. At low burnoff, the enlargement of the accessible pore volume is mainly caused by the opening of closed pores. Assuming a cylindrical pore shape, the mean radius of pores <0.1 μm was estimated and was found to be independent of the degree of oxidation. Based on the apparent BET surface area and the surface area in mesopores, r̄ BET = 16 Å and r ̄ α = 26 A ̊ was obtained.

The behavior of graphite under neutron irradiation

The environment in which graphite is expected to be exposed in high power fusion reactors will impose a dose of ≂50 dpa, temperatures in the range 600–1200 °C, cyclic temperatures, and intense surface interaction with the plasma. A review is presented of the irradiation behavior of various types of graphite under these conditions, and we demonstrate that to have long life components it will probably be necessary to accept very large dimensional changes. The use of conventional graphites will give components of relatively short life due to the generation of porosity by internal strains. Consideration is given to the effects of helium generation in the lattice and surface erosion as contributory factors.

Radiation damage of graphite in fission and fusion reactor systems

There are several compelling reasons to consider graphite use in fusion reactors. The plasma tolerance for impurities is much higher for the low atomic number ( Z=6) carbon materials, so it is useful as a plasma interface material. Its high thermal shock resistance provides unique limiter design options and its low neutron capture cross section combined with very low activation can provide a fusion reactor system with much alleviated maintenance, waste disposal, environmental and safety problems. Its applicability as a neutron reflector applies to fusion as well as fission systems. However, the nature of the radiation damage to the graphite lattice places some fundamental limits on its useful lifetime in fusion as well as fission reactors. This creates design and economic problems primarily. Increasing the crystalline perfection of artificial graphites is suggested as one method of reducing the crystallite damage. The life expectance for the isotropic conventional graphites will in each case depend on the reactor component for which it will be used and on its design considerations. Based on neutron damage and related dimensional changes it is estimated graphite will be tenable to about 3 × 10 22 n/cm 2 (EDN) at 400°C, 0.6 × 10 22 n/cm 2 (EDN) at 1000°C and 1.4 × 10 22 n/cm 2 (EDN) at 1400°C. There are no data above 1400°C on which to speculate. A dose of 2 × 10 22 n/cm 2 may be accumulated in times ranging from as short as a few months in the first wall region of high power density designs to the fusion plant lifetime (30 years) in the neutron reflector region behind the blanket.

Effect of Hot-pressing Pressure on Some Properties of Polycrystalline Graphite made from Coke Powder with addition of Boron Carbide

Polycrystalline graphites were made from a calcined pitch coke powder with boron carbide (B4C) additive in amounts varying from0to16.7wt% by hot-pressing at 2200°C under various pressures from 20 to 400 kg/cm2. The effect of hot-pressing pressure was investigated on some properties such as density, bending strength, electrical resistivity, graphitization degree and oxidation behaviour of the hot-pressed solids.It was found that the density and bending strength of the hot-pressed solids increased remarkably with increasing hot-pressing pressure. The obtained values were, however, depended on the content of B4C additive;that is, in the cases of4.8wt% and16.7wt% B4C additions, the bending strength was about900kg/cm2 and about1400kg/cm2 respectively for solids made at a maximum pressure of400kg/cm2.Electrical resistivity decreased with the increase of hot-pressing pressure and was abou 0.5-0.6×10-3 ohm-cm for all the hot-pressed solids with B4C additive made at400kg/cm2.The d (002) spacings of the hot-pressed solids expanded slightly and three dimensional disordering of graphite structure occurred with the increase of hot-pressing pressure.Their graphitization degree, however, progressed much more than those for the solids without B4C additive. The degradation behaviour of graphite structure is thought to be due to boron diffusion and its interstitial occupation between layers in addition to substitutional positions.All the hot-pressed solids made with B4C additive showed excellent resistance against air oxidation and H2SO4-HNO3acid solution, compared with those of solids without B4C additive and some conventional graphites.

Graphite and boron carbide composites made by hot-pressing

Composites consisting of graphite and boron carbide were made by hot-pressing mixed powders of coke carbon and boron carbide. The change of relative density, mechanical strength and electrical resistivity of the composites and the X-ray parameters of coke carbon were investigated with increase of boron carbide content and hot-pressing temperature. From these experiments, it was found that boron carbide powder has a remarkable effect on sintering and graphitization of coke carbon powder above the hot-pressing temperature of 2000° C. At 2200° C, electrical resistivity of the composite and d(002) spacing of coke carbon once showed minimum values at about 5 to 10 wt% boron carbide and then increased. The strength of the composite increased with increase of boron carbide content. It was considered that some boron from boron carbide began to diffuse substitutionally into the graphite structure above 2000° C and densification and graphitization were promoted with the diffusion of boron. Improvements could be made to the mechanical strength, density, oxidation resistance and manufacturing methods by comparing with the properties and processes of conventional graphites.

Irradiation behavior of poco graphites

Results are reported for irradiation testing of a high density (1.8 g/cm 3) and low density (1.5 g/cm 3) grade of Poco graphite at temperatures from 400 to 1400°C up to maximum exposures of 14 × 10 21 n/cm 2. The graphites behave isotropically under irradiation and show improved dimensional stability over conventional nuclear graphites. An attempt is made to relate the unusual irradiation behavior of the material to its microstructure.

High-temperature mass spectrometry—I. Free vaporization studies of graphites

Some of the differences in the published data on the free vaporization of conventional (ATJ, ZTA, UT-6) and pyrolytic graphites have been resolved. Relative ion intensities, apparent rates of vaporization, and activation energies of vaporization have been measured for carbon species C 1 through C 5, for conventional graphites in the temperature range 2800°–3000°K, and for pyrolytic graphites in the temperature range 2600°–3260°K. Differences in the free evaporation behavior of the two types of graphites are discussed. For the first time, evidences for carbon evaporation from the new ‘white’ allotropic form of carbon are presented; some of the anomalous vaporization behavior observed with both types of graphites is attributed to the vaporization of carbon from this new allotropie form of carbon. Plausible explanations are presented to account for the differences in activation energies of vaporization for C 4 and C 5 molecules for pyrolytic and conventional graphites. For conventional graphites at temperatures above about 2950° ± 50° K, the rates of vaporization of carbon species were found to increase abruptly, and the total mass-loss appears to occur by vaporization of carbon and ejection of crystallites.

Effets d'irradiations neutroniques dans des materiaux composites graphite-pyrocarbone obtenus par craquage thermique de gaz naturel

Preparation and main characteristics of composite graphite/pyrocarbon bodies designated “BB5G” and obtained from thermal cracking of natural gas at about 1000°C, under normal pressure, are briefly reviewed. Dimensional changes induced by neutron irradiation in these materials were measured. Irradiation temperatures were approximately 60°C, 350°C and 450°C. Fast neutron doses were respectively about 4 × 10 20 for the lowest temperature, and 1.1–1.3 × 10 21 for the two higher ones. At 60°C, expansion was observed (3 . 0 per cent for the maximum dose received) but a shrinkage for 350 and 450°C irradiations (0 . 7 per cent to 1 . 0 per cent). Comparing the behaviour of “BB5G” with that of conventional nuclear graphites, it seems that the use of the former material should be feasible for structural parts in nuclear reactors. Finally, some remarks are made concerning the bulk behaviour of such composite bodies.

Modifications du spectre de pores de graphites artificiels densifies par craquage de gaz naturel. Etude au moyen d'un nouveau porosimetre a absorption γ

A brief description is given of a new mercury porosimeter, using γ-beam absorption. This apparatus allows measurements of local heterogeneities in pore size distribution. It has been used to study the changes in pore size distribution after deposition of a pyrocarbon inside of the pores of conventional nuclear graphites. The deposits were obtained by thermal decomposition of natural gas, under normal pressure, at temperature around 1000°C. The influence of main parameters such as the gas flow, hydrogen content and initial pore size distribution was studied. Such modification of pore size distribution seems to be an interesting way of reducing the radiolytic corrosion of graphites in CO 2.

Non-destructive techniques for the quality control of erosion resistant graphites for solid propellant rocket motors

Erosion and shock resistant conventional graphites are used as nozzle throat materials for solid propellant rocket motors. Motors with small nozzle throat diameters are particularly sensitive to variations in graphite quality affecting erosion resistance. Evidence from firing tests on a variety of graphites shows that bulk density, open porosity and flexural strength all play their part in determining erosion resistance whilst not necessarily being interrelated. To meet the high degree of reliability required of rocket motors quality control of individual nozzle components is necessary. Non-destructive techniques have been developed for the determination of the above properties in finish machined nozzle shapes, particularly the determination of flexural strength by measuring resonance frequency.

Graphitic Materials Hot‐Worked with a Dispersed Liquid Carbide: Thermal and Electrical Conductivity

Graphitic materials produced by hot‐working with a dispersed liquid‐carbide phase (termed HWLC graphites) have high thermal and electrical conductivities compared with hot‐worked or conventional graphites prepared without additives. Thermal diffusivity and electrical conductivity were measured at room temperature on two HWLC materials containing either molybdenum carbide or zirconium carbide. This process of liquid‐phase sintering with compressive deformation imposes a strong preferred orientation on the predominantly graphitic material, and the resulting thermal conductivity in the preferred direction of the layer planes is greater than the thermal conductivity of copper or silver. Thus, liquid‐phase stress sintering induces in graphitic materials an essentially new microstructure that provides a means of realizing the inherent high thermal conductivity of the graphite layer plane in a massive body.

HWLC graphites prepared by hot-working extruded stock

Extruded graphite stock containing molybdenum metal has been investigated as a starting material for the process of hot-working with a dispersed liquid carbide (the HWLC process). The extruded stock is easily deformed in compression along the extrusion axis with little risk of fracture providing that the hot-working stresses are kept below 3000 psi until the isotropic state is passed. Measurements have been made of chemical composition, density, compressive and flexural strengths, thermal and electrical conductivities, and thermal expansitivity for a series of HWLC-Mo materials hot-worked to various strain levels up to 65 per cent true strain. These properties indicate that a wide range of preferred orientations is attainable, the isotropic state for the extruded material used being attained at a strain level of 20 per cent. The properties of the HWLC-Mo graphites which have been strained only to the isotropic state show only modest improvements over the properties of conventional graphites. At higher levels of hot-working strain, the characteristic dense, uniform microstructure of the HWLG graphites is imposed on the extruded stock. Thus, the properties of material prepared from extruded stock approach those of materials prepared from molded stock provided that the extruded stock is hot-worked to a sufficiently higher strain level.