Graphite Spheres Research Articles

Does solid carbon burn in oxygen to give the gaseous intermediate CO or produce CO 2 directly? Some experiments in a hot bed of sand fluidized by air

Relatively large spheres of non-porous graphite (diam. 2–12 mm) have been burned in electrically heated beds of sand, fluidized by air. The rate of reaction of the carbon was derived from measurements of the concentrations of CO and CO 2 in the gases leaving the bed. The carbon particle's temperature was monitored continuously during its combustion using a very fine thermocouple inserted in the graphite sphere. The temperature of the burning particle rose whilst its diameter shrank. The observations can be interpreted in terms of CO being the only product of oxidation for graphite particles burning between 1000 and 1400 K in such a fluidized bed. The oxidation of CO is inhibited by the proximity of sand, which provides a large surface area for radicals (e.g. OH and HO 2) to recombine. Nevertheless, CO does burn to CO 2 close to the carbon at the higher temperatures studied (1400 K), but at ∼ 1100 K it mainly diffuses away before burning in say a rising air bubble. In this case less heat is fed back to the burning carbon. The use of tiny sand particles can give small air velocities over the carbon particle, so that convection of CO away from the carbon is reduced and the full enthalpy of combustion of carbon to CO 2 is donated to the carbon. It seems likely that, just as radicals are important in oxidising CO to CO 2, they are also involved in burning solid carbon to CO.

Statistical pyrometric sizing of particles in fluidised bed combustion

A theoretical basis is presented for the concept of statistical pyrometric particle sizing (SPPS) in fluidised bed combustion (FBC). The theory makes it possible to obtain the size distributions for observable particles within given temperature ranges, and thus a statistical estimate can be made of the dependence of the temperature of a fuel particle on its size during FBC. The SPPS method has been experimentally validated by comparison with an independent reference method in a miniature fluidised bed reactor. Graphite spheres were burned one by one and measurements were taken with a two-colour fiber-optic pyrometer. Pyrometric signals were obtained many times during each particle's combustion and SPPS was applied to the results. The generation of CO 2 was also recorded and the evolution of the particle's size with time was determined from the CO 2 mass balance, assuming shrinking core combustion. The average sizes determined by SPPS and also from the CO 2 mass balance were in good agreement for all temperature ranges. The elevation of the particle's temperature above that of the bed varied between 20 and about 200°C.

Effect of elongated graphite on mechanical properties of hot-rolled ductile iron

The mechanical properties of hot-rolled and then annealed ductile iron were evaluated. The deformation of this two-phase material and the effects of the elongated graphite spheres on the mechanical properties and the development of anisotropy of mechanical properties were studied. An attempt was made to describe the anisotropy of tensile strength in terms of the deformation of graphite spheres and of the root curvature, area fraction, and the interbridges that result in disproportional changes of stress concentration, loading capacity, and the tendency to break the interbridges and link the neighboring deformed graphite spheres.

On graphite and the 2175 A extinction profile

Classical electromagnetic theory is used to study the constraints placed on interstellar graphite grains by the observations of Fitzpatrick & Massa (who found large variations in FWHM γ of the bump with minimal, and uncorrelated, variations in the central wavelength λ 0 ). Accurate calculations using the discrete dipole approximation are used to test the accuracy of the commonly used 1/3-2/3 approximation for graphite spheres. We show that the 1/3-2/3 approximation is sufficiently accurate for use in studying variations in the extinction profile due to changes in graphite grain size or coatings on the grains. We investigate the effect on the 2175 A extinction profile of (1) changes in size distribution, (2) changes in grain shape, and (3) coatings of ice or other material

Reduction of the Reactivity of Water Ingress in Modular Pebble-Bed High-Temperature Reactors

In high-temperature reactors (HTRs), ingress of water introduces positive reactivity. Normally, this is controlled by the reactor itself, but in hypothetical situations, there could be a need for an active support by the control system. Calculational research identifies three reasons for the reactivity change caused by the water: (a) a negative contribution by the absorption of the hydrogen, (b) a positive contribution by the softening of the neutron energy spectrum, and (c) a reduction of the neutron leakage losses due to a shift in the neutron flux local distribution. By increasing the carbon/heavy metal ratio, the reactivity effect can be reduced to almost zero or even to negative values. In the modular pebble-bed HTR, this effect can be accomplished in a simple manner. By adding 25% of graphite spheres to the regular batches of feed fuel elements, the neutron spectrum effect is reduced, and the fractional absorption of hydrogen is increased; thus, the maximum excess reactivity is limited to 0.3%. The effect on economy and safety is negligible.

炭素複合材料の耐火性能と電磁波遮へい性能

Graphite and phenol-formaldehyde resin spheres (GPS) were used in the manufacture of fire resistive and electromagnetic shield composites. The effects of specific gravity, thickness and phenol-for-maldehyde resin content of GPS on the fire resistivity and electromagnetic shield of composites were discussed. The carbonizing temperature of the composites relative to the fire resistive and electromagnetic shielding properties was also discussed. The fire resistivity was tested by a burn-through method, by cutting with an oxygen-acetylene torch and by an oxygen index method in accordance with the Japanese Industrial Standards (JIS) K 7201. The electromagnetic shielding property was tested by a DUAL chamber method in accordance with ASTM ES7-83. The fire resistive and electromagnetic shielding properties of the composites were improved by increasing the specific gravity and thickness of the GPS composites as well as increasing the carbonizing temperature, and reducing the phenol-formaldehyde resin content.

The nature of the dust around R Coronae Borealis stars - Isolated amorphous carbon or graphite fractals?

Wright (1989) has proposed that the R CrB phenomenon is caused by the occurrence of graphite fractal grains, each composed of numerous, less than 15-nm-radius graphite spheres. It is shown that both the observational data and models of dust formation are more in accord with an earlier model by Hecht et al. (1984): that the R CrB type dust is composed of isolated small (less than 100-nm-radius) glassy or amorphous carbon grains. It cannot be ruled out that a significant number of fractal grains exist, provided that they have dielectric properties different from bulk graphite and closer to amorphous carbon, and that the individual grains in the fractals are below 1 nm in size. However, the existence of such fractal grains around R CrB stars might be ruled out by the lack of a 2400-A feature in the interstellar extinction curve. 30 refs.

Historical statistics of the United States: Colonial times to 1970

An understanding of the transport of fission products in High Temperature Gas-Cooled Reactors (HTGRs) is needed for operational safety as well as source term estimations. We have measured diffusion coefficients of Cs in IG-110 by using the release method, wherein we infused small graphite spheres with Cs and measured the release rates using ICP-MS. Diffusion behavior was investigated in the temperature range of 1100–1300 K. We have obtained: DCs=(1.0×10-7m2/s)exp-1.1×105J/molRT and, compared our results with those available in the literature.

Using SN 1987A light echoes to determine mass loss from the progenitor

We have monitored the many light echoes seen near SN 1987A to find how the SN progenitor influenced its preexplosion environment. There is a smooth distribution of light dimming from radii 3"-10", consistent with an echo from the mass lost at a constant rate from a red supergiant atmosphere. This flow terminates in two shocks (the smaller, bow-shaped echo and the oval-shaped echo described herein), the larger containing 12 times the mass of the flow. The total mass in all these structures (assuming spherical symmetry) is 4.5 M_sun_, assuming M_gas_/M_dust_ = 1000 and a grain model favored by our data (50 nm radius graphite spheres). The progenitor needed ~ 10^6^ yr to create these structures, assuming a constant mass loss at velocity I5 km s^-1^. The dust in this region is small-grained (mostly radii < 70 nm) and isotropically scattering (g < 0.2). Interaction between the progenitor blue supergiant and red supergiant winds is complex, but are likely contained within a 1.5 pc diameter, roughly spherical structure. We have also discovered a fourth star in the Sanduleak - 69^deg^202 system, 2" southwest of the SN (P.A. = 243^deg^).

Determination of Neutron Spectra in a Graphite Sphere for Fusion Reactor Studies

AbstractCalculated and experimental results for the neutron spectra at different radii in a graphite sphere irradiated with 14.1 MeV neutrons were shown to be in satisfactory agreement over the energy range 14.1 to 1.8 MeV neutrons. A group of curves were constructed which gives the radius of a graphite sphere shield required to attenuate the neutron intensity to a certain value. The data set used in the present work, with carbon‐12 cross section, is shown to be useful for spherical calculations.

The role of peripheral percolation in detachment of fines from burning carbon particles

Batch combustion of graphite spheres and char particles made by devolatilization of a South African coal has been carried out on a grate and in a fluidized bed. 0.04 m ID combustors equipped with a two-exit head have been used. Overall amount of carbon fines formed during a combustion test as well as time-resolved curves of carbon fines generation rates were obtained with this apparatus. Temperatures were varied within ranges 660 to 820°C and 400 to 550°C for graphite and South African char, respectively. Optical microscopy and computer aided image analysis were used to measure sizes of carbon fine particles (up to 250μm) collected during combustion. A 2-D percolation model, based on Monte Carlo simulations for a square lattice, has been developed to obtain qualitative trends on the influence of peripheral percolation in actual combustion of carbon particles. The main feature, of the model is that non-zero gradients of porosity are considered across the particle. Increasing such gradients can be interpreted as the change of carbon combustion rate from chemical kinetics towards diffusional control due to increases of operating temperature. Along this line, theoretical numbers and sizes of percolated clusters and experimental losses and sizes of carbon fines correspond to each other. All these variables decrease as porosity gradients increase. Percolation is the only source of fines generation in combustion on grate and, as a consequence, when conversion rate is controlled by external diffusion, fractional carbon loss due to fine generation is very small, namely, less than 1%. Experimental results are in agreement with these expectations. Conversion rate control in practical fluidized combustion is also dominated by external diffusion, so that, even in this case, the loss of carbon should be that small. Actual loss, more than one order of magnitude larger, is due to attrition by abrasion of particles which by far overtakes peripheral percolation in producing carbon fines.

Electric potential of nonspherical interstellar grains

The equilibrium electric potential of graphite and silicate spheres and infinite cylinders is calculated for the conditions characteristic of diffuse clouds, the interstellar medium, and H II regions around hot stars. Collisions of the dust grains with electrons and ions are taken into account together with photoelectric emission. The dependence of the potential on the grain parameters (shape, size, chemical composition, and velocity) and also on the gas parameters is briefly discussed. It is found that the potential of nonspherical grains can differ appreciably from that of spherical ones only in diffuse clouds (however, their ratio is usually less than 10).

Neutron Physical Investigations on the Shutdown Effect of Small Boronated Absorbing Spheres for Pebble-Bed High-Temperature Gas-Cooled Reactors

An emergency shutdown system for high-temperature gas-cooled pebble-bed reactors is proposed in addition to the common absorber rod shutdown system. This system is based on the strongly absorbing effect of small boronated graphite spheres (called KLAK), which trickle in case of emergency by gravity from the top reflector into the reactor core. The inner reflector of the Siemens-Argonaut reactor was substituted by an assembly of spherical Arbeitsgemeinschaft Versuchsreaktor fuel elements, and the shutdown effect was examined by installing well-defined KLAK nests inside this assembly. The purpose was to develop and prove a calculational procedure for determining criticality values for assemblies of large fuel spheres and small absorbing spheres.

Elastic analysis of deformation near a spherical carbon particle embedded in iron

The deformation around a graphite sphere in an iron matrix which is subjected to a uniform far-field strain is analyzed. Unlike an isotropic inclusion, the stress within the sphere is not constant but rather is confined to a boundary layer near the outer surface of sphere. The stress field caused by differential thermal contraction is considered.

The burning of graphite spheres with gas-phase equilibrium

The method of activation energy asymptotics is used to develop a simple but accurate description of the history of a cold graphite sphere suddenly immersed in a hot oxidizing ambient under conditions such that chemical equilibrium in the gas phase prevails. The conditions for the applicability of the approximate description are obtained as part of the analysis. The results are compared with direct numerical solutions of the describing equations.

Secondary particle yield and energy release data from intranuclear-cascade-evaporation model calculations of high energy (20–1100 MeV) neutron interaction with elements of shielding and biological importance

Secondary particle (neutron, proton, pion and heavy ion) yields and energy release data for 20, 50, 100, 300, 500, 800 and 1100 MeV neutron collisions with H, C, N, O, Al, Si, Ca, Fe and Pb have been calculated using the intranuclear-cascade-evaporation model. The low-energy limit is discussed and compared with the ENDF/B-IV neutron data. The mean elastic recoil energies have also been estimated by means of the Ranft formula for angular distribution. The multigroup response functions (kerma factors and production cross-sections) have been obtained and applied calculating the energy deposition and particle yields in the deep-penetrated graphite and iron spheres. The results for energy deposition of the ANISN neutron transport code using a high energy cross-section library and the derived kermas are about 10% overestimated relative to the thick-target results from the high energy particle transport code HETC. In the neutron transport discrete ordinates calculation the secondary production is underestimated when comparing with the HETC code.

The role of cavities and mantles in the ultraviolet extinction peak of graphite spheres with particular reference to a possibly discovered C60 structure

A computer routine using rigorous Guttler theory was developed to consider effects of concentric cavities and organic mantles within and around graphite spheres, with a view to assessing the stability of the extinction peak near 4.60 μm−1. The newly discovered stable carbon molecule of 60 atoms gives an absorption band at ∼4.60 μm−1 which closely agrees with the observed; moreover, it leads to the observed extinction rise in the far UV and can possibly explain the fine structure around the 5.5–7 μm−1 dip, while its own structure deals successfully with the main objections to previous carbon-graphite models. The role of the electric vector component perpendicular to the basal plane of the crystal in achieving the fit is stressed.

Use of the ZZFEWG 1/31-1F data set for spherical calculations

Measurements of the fast neutron spectra in a graphite sphere irradiated with 14.1 MeV neutrons were compared with spectra calculated using the ANISN code with ZZFEWG 1/31-1F data set. This set contains a wide variety of elements suitable for both shielding and fusion blanket studies. The measured data were obtained by means of the NE-213 liquid organic scintillator detector. The intensity of neutron output was monitored by the associated particle method using an NE-102A plastic scintillator. The agreement between measurements and calculations indicate that the cross section set is satisfactory for studying the graphite shield experiments over the energy range 14.1–1.8 MeV. Also it would be possible to carry out a study on fusion reactor blankets using spherical geometry.

Diffusion of Fission Products in Matrix Graphite for VHTR Fuel Compacts

Diffusion coefficients of Ce, Sr and Ba in the matrix graphite for VHTR fuel compact have been determined by measuring the radial profiles of 141Ce, 89Sr and 140Ba concentrations in slightly irradiated graphite spheres which have U02 kernels at the center. Temperature dependences of the diffusion coefficients of Sr and Ba have been determined between 1,175 and 1,375°C, and they are expressed as where T is the absolute temperature and R the gas constant (8.314 J/mol·K). The diffusion coefficient of Ce in the above temperature range was rather higher than those of Sr or Ba, and the magnitudes of these coefficients were expressed in following order; DCe>DSr>DBa. The effect of matrix graphite density on diffusion of the fission products was studied, but difference was not found clearly in the diffusion coefficients of Ba which had been obtained in both of low and high density matrices at 1,375°C.

Application of x-ray fluorescence spectrometry for identification of unirradiated pebble-bed reactor fuel element

This paper deals with the investigation of the potential application of x-ray-fluorescence spectrometry for the identification of unirradiated fuel elements for the pebble-bed reactor. Measurements were carried out on graphite spheres impregnated with different chemical elements at various concentrations. The intensities of the K χ lines of these trace elements, as a function ofconcentration, were measured in order to establish the optimal concentrations needed for fuel-element identification, and the required sensitivity of the spectrometer employed. These measurements have shown that x-ray-fluorescence methods can be employed with advantage for identifying peeble-bed-reactor fuel elements. There are enough suitable chemical elements and a wide range of concentrations of each element, which may be used for this purpose.