Isotropic Graphite Research Articles

高温ガス炉用黒鉛材料の応力勾配下における破壊靭性

Fracture behavior of nuclear grade graphites was investigated under tensile-compressive stress distribution or stress gradient in front of inherent flaws in graphite materials, using the tension-bending bimodal strength test machine.Tensile, three and four point bending tests and fracture toughness test were carried out with notched specimens (width×thickness×gage length=20mm×10mm×60mm, notch length=0-5.2mm) made of fine grade isotropic graphite, IG-110 and coarse grained quasi-isotropic graphite, PGX, and the following conclusions were derived;(1) The following relationship between notch length, a and fracture strength, σf was well expressed as;σf=Aa-1/2, where A: constant.(2) Fracture toughness estimated by ASTM standard decreases as ligament ratio, a/W decrease in the range of a/W<0.05.(3) The highest fracture resistivity was observed in 3 point bending test and the lowest one was in tensile test.(4) Fracture toughness defined by on-set point of fracture can be evaluated in unique in these strength tests.

Mechanism of chemical sputtering of graphite under high flux deuterium bombardment

Chemical sputtering of graphite materials (isotropic graphite and carbon fiber composite) was studied by irradiation of 5 keV D 3 + beam with a flux up to 4×10 21 m −2 s −1 , which is more than one order magnitude higher than previous low flux beam experiments (< 10 20 m −2 s −1) . The chemical sputtering yield was obtained from measurements of the released methane signal with a quadrupole mass analyser. It was found that the methane yield at peak temperatures is almost independent of flux from 5×10 20 to 4×10 21 m −2 s −1 . Peak temperatures range between 900 and 1000 K, which is higher than those of the previous low flux experiments (<900 K, <10 20 m −2 s −1) . By comparing our experimental results with calculation results based on Roth's model, the annealing effect of radiation damage to prevent methyl group formation appears to be unimportant.

Qualitative study of beta silicon carbide residual stress by Raman spectroscopy

Silicon carbide was grown by chemical vapor deposition on isotropic pyrolytic graphite substrates using methyltrichlorosilane as the source gas and hydrogen as the carrier gas. Raman microscopy was used to qualitatively determine the residual stress existing in chemical vapor deposition (CVD) beta silicon carbide. The results showed that the residual stress of CVD beta silicon carbide not only depends on the deposition temperature but also on the other reaction parameters, such as precursor concentration and carrier gas flow rate. Raman microscopy study results were tried to compare with those of the wafer central deflection method which was reported previously, and explained using scanning electron microscopy (SEM) photographs and transmission electron microscopy (TEM) observations. The residual stress of CVD beta silicon carbide is always compressive and can be relaxed by recovery and recrystallization processes because intrinsic stress is the predominant stress in the films.

00/02634 Pulverization and corrosion studies of bare and cobalt-encapsulated metal hydride electrodes

Carbon nanotubes (CNTs) are incorporated into mesocarbon microbead (MCMB)-derived isotropic graphite to improve their mechanical properties. CNTs are homogenously distributed on the MCMB surface by acid-treatment and mechanical mixing. The composites are prepared by cold isostatic pressing, carbonization, and graphitization. The mechanical properties and isotropy ratios of the CNT/MCMB composites are determined by four-point bend tests and thermal expansion measurements, respectively. The addition of CNTs improves the flexural strength by ca. 20%, while keeps a low isotropy ratio. CNTs dispersed on particle interfaces improve the interfacial strength, this reinforcing mechanism is confirmed by a fracture mode analysis with scanning electron microscope.

Effects of hydrogen molecular desorption on the heat load on plasmafacing materials

Measurements of the heat load on C (isotropic graphite) and W targets wereperformed in linear steady plasma facility MAP-II (materials and plasma),which produces a low-energy hydrogen plasma 10-20 eV with an electron densityof (1.0-5.0)×1017 m-3. From the measurements of the heatremoved by the target coolant, heat flow to the target and Balmer intensitiesnear the targets we found that the heat load was larger on the C target than Wbecause of higher molecular hydrogen desorption. The population densities ofvibrational levels in the d 3Πu state of molecular hydrogenshifted to higher levels near the C target in the spectroscopy of the Fulcherband. From these results, it was found that molecular hydrogen desorptionplays an important role in the heat load on plasma facing materials and itdepends on the materials, which have different characteristics of hydrogenmolecular desorption and energy states of desorbed hydrogen.

The coefficient of thermal expansion of nuclear graphite with increasing thermal oxidation

Two grades of nuclear graphite, an isotropic graphite (GCMB) and an anisotropic graphite (PGA), were thermally oxidized to high weight losses. Average values of the coefficient of thermal expansion (CTE) in the temperature range 20-600 °C for GCMB and PGA in the parallel direction were unaffected by thermal oxidation up to 60% and 50% weight loss, respectively. The average CTE values for the PGA samples in the perpendicular direction were also unaffected by oxidation in the range 10-50% weight loss, although a slight increase in the CTE in the early stages of oxidation was observed. A new model based upon a continuous network of material, able to transmit thermal strains, which persists after oxidation, was proposed to explain the insensitivity of the CTE to oxidation. The transmission of thermal strains in the continuous network model was considered as a percolation phenomenon. Application of percolation theory to the effect of thermal oxidation on electrical conductivity of oxidized GCMB graphite suggests that the percolation threshold occurs at very high levels of oxidation that are in excess of 95% weight loss.

Microstructural study of residual stress in chemically vapor deposited β-SiC

Silicon carbide was chemically vapor deposited (CVD) on isotropic pyrolytic graphite substrates by using methyltrichlorosilane as the source gas and hydrogen as the carrier gas. Transmission electron microscopy was used to observe microstructural defects in CVD β-SiC caused by residual stresses. Microstructures resulting from recovery and annealing effects during the deposition processes were also observed and are discussed. The magnitude of residual stress between deposition temperatures of 1400 and 1600°C decreased from about 2000 to about 250 MPa, estimated by the wafer central deflection method.

カーボンダストの重水素保持特性

In a nuclear fusion device, plasma facing materials (PFMs) such as graphite are bombarded by extremely high heat and particle fluxes during an abnormal discharge called as disruption. As a result of the disruption, the PFMs are eroded and deposited on the wall. In the case of graphite plasma facing material, carbon dusts are produced. Flaky carbon dust was produced from isotropic graphite by electron beam evaporation. The absorption properties of deuterium in carbon dust were investigated by changing an absorption temperature (773-1173 K), time (10 min-24 hr) and deuterium gas pressure (1-100 Pa). The amount of adsorbed deuterium was evaluated by using thermal desorption spectroscopy. Deuterium was desorbed from the carbon dust at the temperature higher than 873 K in form of HD and D2. At the absorption temperature of 973 K and the pressure of 100 Pa, the amount of deuterium absorbed in the dust saturated within 5 hr. This saturation level was one or two orders of magnitude larger than that of isotropic graphite. The heat of solution for deuterium was 0.27 eV which was almost the same as that of isotropic graphite.

Effect of Compressive Prestress on Young's Modulus and Thermal Conductivity of Carbon Materials

Effect of compressive prestress on the correlation between thermal conductivity and Young's modulus of carbon materials, fine-grained isotropic graphite IG-430U and a felt type C/C composite CX-2002U, was examined. Young's modulus and thermal conductivity decreased due to compressive prestress. The change in thermal conductivity of carbon materials was related to that of Young's modulus via sound velocity. The decrease in thermal conductivity of IG-430U graphite due to prestress was mostly correlated with that in Young's modulus. Also, decrease in thermal conductivity of C/C composite showed the similar tendency as the case for the graphite.

Isotropic carbon and graphite fibers from chemically modified coal-tar pitch

A precursor for a general purpose carbon fiber was prepared from coal tar pitch (CP) modified with 10 % p-benzoquinone (BQ) at 380 ‡C for 3 hours. Such a modification raised the softening of the pitch from 85 ‡C to 271 ‡C at a yield of 43 %. The modified pitch was spun smoothly at a rate of 480 m/min into a fiber of 20 Μm diameter. The fiber was stepwise stabilized at 236 ‡C (5 ‡C/min) and 312 ‡C (1 ‡C/min) for 3 hours at each temperature. Successively,carbonization and graphitization were performed at 1,000 ‡C and 2,400 ‡C, respectively, for one hour. Both the carbonized and graphitized fibers exhibited tensile strength of 570 MPa. The structural parameters of carbon and graphite fibers were their orientation values of 56.2 and 58.1 %, relatively low Lc(002) of 11.24 and 25 a, and large interlayer spacing (d002) of 3.86 and 3.49 a, respectively.

Deuterium retention of DIII-D DiMES sample

The deuterium retention property of B 4C converted graphite and isotropic graphite exposed to DIII-D deuterium plasma was examined by using a technique of thermal desorption spectroscopy. Major outgassing species were HD, D 2 and CD 4 in both the graphite and the B 4C. In the case of the graphite, the ratios of deuterium desorbed in the forms of HD, D 2 and CD 4 to the total desorption amount of deuterium were 40%, 27% and 33%, respectively. In the case of the B 4C, which was covered by carbon due to redeposition, these ratios were similar to those of the graphite. In a thermal desorption spectrum of deuterium, three desorption peaks appeared in both the graphite and the B 4C covered by the redeposition layer. At low temperature region, the desorption rate of deuterium for the B 4C covered by the redeposition layer was larger than that of the graphite. From two dimensional distribution of deuterium retention, it was seen that the retained amount at the electron drift side was quite large. The amount at the ion drift side and the edge of inward major radius was also observed to be large. The average retained amount of the graphite was almost the same as that of the B 4C covered by the redeposition layer.

Formation Mechanism of SiC Gradient in Carbon Materials

High-density isotropic graphite substrates with and without a notch produced at the surface were contactedwith metallic silicon and heated at 1350 and 1450°C. The contribution of open pores in the substrates for the formationof SiC-gradient was discussed. At 1350°C, no SiC-gradient was observed at the surface of the substrate without thenotch, but the gradient in the substrate was formed at 1450°C. On the substrates with notch, fibers were formed on thenotch wall of the substrate heated at 1350°C. By heating at 1450°C, however, SiC was detected at the portion beyondthe nose of notch. In present study, it was expected that small amount of 02 are enclosed in the open pores and in thenotch. Based on the expected reactions among silicon, carbon and oxygen, the formation of SiC-gradient in thesubstrate at 1450°C was found to be possibly formed due to the reaction between carbon on the wall of pores and SiOgas through the open pores from the surface of the substrate.

Distribution of Ionized Carbon during Simulated Plasma Disruption for the Isotropic Graphite Target.

The behavior of ionized carbon during the simulated plasma disruption is investigated with the Magneto-Plasma-Dynamic (MPD) Arc Jet. The temporal and spatial distributions of the ionized carbon were measured by emission spectroscopy. Distributions of CII and CIII were obtained. For the isotropic graphite target, the emission intensity increased as the target is exposed by the heat flux from plasma. Two consecutive peaks of intensity were observed at the point near the target surface. A simple model of redeposition and surface roughness could explain these phenomena.

Biaxial Fatigue Strength of a Fine-Grained Isotropic Graphite for HTTR

Biaxial fatigue tests on an isotropic graphite IG-11 used in the HTTR were done in the stress regimes of tension-tension and compression-tension at room temperature to evaluate its fatigue strength at the biaxial state. Biaxial fatigue tests at 1,273K in a vacuum were also done in the latter stress regime. Applied stress levels in the longitudinal direction ranged from 75 to 90% of the mean static strength, and in the transverse direction, from 53 to 74% of the mean strength. It was indicated from the tests that (1) the difference in the fatigue strength between the uniaxial and biaxial was observed when the applied stress levels were smaller than about 90% of the mean strength, (2) the biaxial fatigue strength was found to be larger than that for the uniaxial if the biaxial applied stresses were normalized to the mean biaxial static strength, (3) the biaxial fatigue strength at 1,273K was equal to or larger than that at room temperature, (4) the evaluation of the integrity of graphite components in the HTTR, from the aspect of the biaxial fatigue, was believed to be conservative on the basis of the uniaxial fatigue strength and the biaxial static strength.

Effects of prestresses on mechanical properties of isotropic graphite materials

Graphite materials which are used for plasma facing components and other components are subjected to stresses due to the high heat flux from the fusion plasma. Some mechanical properties of graphite materials can change due to the prestresses. The property changes should be considered for the design of the plasma facing components. The purpose of this study is to examine the effects of prestresses on the mechanical properties of isotropic graphite materials. Compressive prestresses were applied to two kinds of isotropic fine-grained graphites (IG-430 and IG-11) at 298 K (both), 1873 K (IG-11), 2273 K (IG-11) and 2283 K (IG-430). As a result, the decrease in Young's modulus for IG-430 due to high-temperature prestressing was 56% which was much larger than the 6.4% that was due to prestressing at 298 K. The results for IG-11 were the same as those for IG-430 graphite. This finding was considered to be due primarily to a difference in degree of the preferred orientation of crystallites in the graphite on the basis of the Bacon anisotropy factor (BAF) obtained from X-ray diffraction measurement of the prestressed specimens. Furthermore, high-temperature compressive prestressing produced an increase in the strength of the isotropic graphite, although room temperature prestressing produced no such effect. The results obtained here suggest that the isotropic graphite which is subjected to high-temperature compressive stresses can become anisotropic in service.

Temperature dependence of sputtering yield of carbon fiber-reinforced carbon composites with low energy and high flux deuterium ions

A super low energy ion source (SLEIS) was used to simulate the sputtering conditions at the divertor in next fusion experimental reactors such as ITER. One directional:1D-, two directional:2D-, three directional:3D-CFCs, isotropic graphite, and B 4C doped 1D-CFC were sputtered by SLEIS with high fluxes (1–2 × 10 20/m 2/s) of 200 eV D 3 + ions. Test samples were sputtered at temperatures of 200–800°C, where chemical sputtering dominates, with an incident angle of 30° and also at 500°C with incident angles of 0–60°. Sputtering yields were calculated from the weight loss of irradiated samples and the fluences. The sputtering yields of 1D-, 2D- and 3D-CFCs showed nearly the same temperature dependence as an isotropic graphite, though the yield of B 4C doped-1D CFCs was smaller than those of these CFCs. The angular dependence of the chemical sputtering yield is determined by the cosine value of the incident angle in each CFC, where the yield of B 4C doped-1D CFC is lower than those of MFC-1 (1D) and CX-2002U (2D).

ERD study of deuterium atoms implanted in edge-HOPG

The present study shows recent progress in the depth profiling of deuterium near the surface of basal oriented (BO) and edge oriented (EO) Highly Oriented Pyrolytic Graphite (HOPG) and the isotropic graphite, using the elastic recoil detection (ERD) analysis technique. The implantations were made at RT with 8 keV D + 2 ions to the fluences up to ∼10 22 D/m 2. The amounts of the trapped D atoms were measured as a function of fluence. It was shown in BO that the deuterium atoms lie in the depth range within 100 nm from the surface, agreeing with the depth profile computed using TRIM-code. However, the depth distribution for EO spreads over the depth beyond 700 nm. The depth profiles were also different from that in isotropic graphite. In this study, four types of EO classified according to mosaic spread have been investigated. The depth profiles obtained for EO samples were almost uniform, showing a constant concentration with depth. Careful determination of the saturation concentration yielded a value of 0.10–0.22 D atoms/C atoms.

Boron ion particles sputtered from boron films deposited on graphites

Boron films were deposited on pyrolytic and isotropic graphites by vapor deposition at different deposition rates. The particles sputtered from these films were measured directly by the QMS during 5 keV D 3 beam irradiation with a flux of 2 × 10 21 D/m 2 s. It was found that the sputtered particles were in the form of boron ion and neutral. The boron ion showed a different behavior from the neutral. The ion signal had a peak value at the beginning of each shot. It also depended on the conditions of the substrates and the deposition rate. These dependencies might be associated with the existence of oxygen on the surface.

Erosion and surface morphology of graphite materials under high flux beam irradiation

Surface morphology and its relation to the erosion mechanism of graphite materials (pyrolytic graphite (PG) and isotropic graphite) irradiated by high flux beams were studied. The irradiation was made by a 5 keV Ar beam with a maximum flux of 1.3 × 10 21 Ar/m 2 s with irradiation angles and fluences, and sample temperatures as experimental parameters. In the case of PG, the surface morphology was different between irradiation at RT (physical sputtering) and at elevated temperatures (1980 K, radiation enhanced sublimation (RES)). In addition, irradiation at oblique incidence (60°) tended to flatten the surface at RT, while at 1980 K the surface roughness developed to some extent. No clear flux dependence of the surface morphology was observed at 1980 K as long as the mean eroded depth was similar, though the RES yield was flux-dependent (decrease with flux). In the case of isotropic graphite, the distinctive surface morphology observed for PG was not found.

Thermal shock resistance of SiC compositionally graded C/C composites

Thermal shock resistance of SiC compositionally graded carbon fiber reinforced carbon composite (CFC) materials that are composed of a CVD SiC layer, a SiC/C graded layer and a CFC substrate was studied by thermal shock tests in air. Two types of CFCs were used: 1-D continuous fiber reinforced and 2-D felt reinforced composites. It was found that the SiC/C graded layer was to some extent effective to increase the resistance and that its effectiveness was larger when a difference in thermal expansion coefficient (CTE) between a substrate and β-SiC was smaller. The degree of an improvement of the thermal shock resistance for CFC substrates was lower than that for isotropic graphite substrate. The difference was attributed to the original cracks in CVD SiC layers on CFC substrates.