Thick Graphite Research Articles

Improved Image Quality and Beam Stability for a High Contrast Imaging Beam Line Used for Megavoltage Cone-beam CT

Purpose/Objective(s): To determine preferred system components and settings to improve image quality and beam stability of an imaging beam line which utilizes the beam of a clinical linear accelerator (linac) incident on a low-Z target used for megavoltage cone-beam CT (MV CBCT). Materials/Methods: Image quality is determined from MV CBCT using the EMMA phantom. This phantom contains a low contrast module consisting of 2 cm thick water equivalent with cylindrical inserts of variable contrast and sizes. The diameters of the cylinders are 3 mm, 5 mm, 7 mm, 10 mm, and 20 mm. There are four groups of inserts with variable densities of 1%, 3%, 5%, and 7% relative to water. The following system components and settings were varied to improve the imaging beam line: beam energy, target material, cranio-caudal image length and the detector buildup plate. The 6 MV treatment beam is stable. Lower energies, desired for improved image quality, push the linac toward a less stable beam. The 6 MV treatment beam with tungsten target and stainless steel flattener was used as well as unflattened beams generated from a 1.3 cm thick industrial diamond target at 4 and 6 MV and a 1.3 cm thick graphite target at 4 MV only. The graphite target thickness, limited by available physical space, was not thick enough to stop primary electron leakage at 6 MV. The diamond target is much higher density and absorbs the primary electron beam at 6 MV. Flat panel buildup plates of different thicknesses of copper, aluminum, brass, and no plate, were assessed with a standard CT phantom. The effect of scatter reduction on image quality was assessed at field lengths of 2 cm, 5 cm, 15 cm, 27.4 cm. The phantom used was a head-size water cylinder with tissue equivalent materials from the CT phantom placed in it. All images are taken with a dose of 10 cGy delivered at the depth of maximum dose at the isocenter. Results: For the tungsten target at 6 MV, 5% inserts down to 3 cm diameter are resolved. With both the graphite and the diamond target at 4 MV, 3% inserts are visible at all sizes. A small difference was observed between the diamond target at 4 MV and 6 MV, indicating that image quality is affected more by target Z than beam energy. Different buildup plates did not play a significant role in soft tissue contrast. In the scatter reduction study, only a weak dependence of contrast to noise ratio on field length was observed. Improved spatial resolution was observed with the graphite and diamond targets compared to the tungsten target. Evaluation of 2D projections shows an increase in f50 value from 0.5 lp/mm to 0.6 lp/mm. This improvement may be attributed to the absence of the flattening filter, which acts as a scatter source. This improvement did not directly translate to CBCT images due to limited number of projections used and the resolution of reconstructed volume (256x256x270). Conclusions: A diamond target offers the possibility of running MV CBCT at a more stable energy of 5-6 MV with image quality comparable to graphite at 4 MV. A narrow field and lower-Z (or no) build-up plate separately yield a modest improvement in image quality. Patient imaging is scheduled for the summer, 2007. It is anticipated that MV CBCT images taken for the same patient with the treatment and imaging beams will be available to compare and show at the meeting.

Long-Cycle and High-Burnup Fuel Assembly for the VHTR

For a prismatic VHTR fuel assembly, a physics study has been performed to maximize the fuel performance in terms of the cycle length and the discharge burnup for a given fuel enrichment. The relationship between the fuel performance and the fuel configurations has been investigated in terms of the TRISO packing fraction, diameter of the fuel kernel, fuel management, and moderating power of the fuel block. Both a typical low-enrichment uranium fuel (LEU) and a fuel made of transuranics (TRU) from LWR spent fuel are considered in this paper. It is shown that in order to obtain a long refueling cycle and a high burnup at the same time, the fuel loading needs to be increased together with the moderating power of the fuel block. Three ways are considered for a higher moderation of the fuel block: a larger pitch of the coolant hole pattern, an extra graphite thickness in the fuel block, and a higher graphite density. The impact of the increased pitch on the fuel temperature is also evaluated with a thermal analysis code. We have shown that long refueling cycles and high burnups can be achieved simultaneously for both LEU and TRU fuels.

Dose evaluation of boron neutron capture synovectomy using the THOR epithermal neutron beam: a feasibility study

Rheumatoid arthritis is one of the most common epidemic diseases in the world. For some patients, the treatment with steroids or nonsteroidal anti-inflammatory drugs is not effective, thus necessitating physical removal of the inflamed synovium. Alternative approaches other than surgery will provide appropriate disease control and improve the patient's quality of life. In this research, we evaluated the feasibility of conducting boron neutron capture synovectomy (BNCS) with the Tsing Hua open-pool reactor (THOR) as a neutron source. Monte Carlo simulations were performed with arthritic joint models and uncertainties were within 5%. The collimator, reflector and boron concentration were optimized to reduce the treatment time and normal tissue doses. For the knee joint, polyethylene with 40%-enriched Li2CO3 was used as the collimator material, and a rear reflector of 15 cm thick graphite and side reflector of 10 cm thick graphite were chosen. The optimized treatment time was 5.4 min for the parallel-opposed irradiation. For the finger joint, polymethyl methacrylate was used as the reflector material. The treatment time can be reduced to 3.1 min, while skin and bone doses can be effectively reduced by approximately 9% compared with treatment using the graphite reflector. We conclude that using THOR as a treatment modality for BNCS could be a feasible alternative in clinical practice.

Long-Cycle and High-Burnup Fuel Assembly for the VHTR

For a prismatic VHTR fuel assembly, a physics study has been performed to maximize the fuel performance in terms of the cycle length and the discharge burnup for a given fuel enrichment. The relationship between the fuel performance and the fuel configurations has been investigated in terms of the TRISO packing fraction, diameter of the fuel kernel, fuel management, and moderating power of the fuel block. Both a typical low-enrichment uranium fuel (LEU) and a fuel made of transuranics (TRU) from LWR spent fuel are considered in this paper. It is shown that in order to obtain a long refueling cycle and a high burnup at the same time, the fuel loading needs to be increased together with the moderating power of the fuel block. Three ways are considered for a higher moderation of the fuel block: a larger pitch of the coolant hole pattern, an extra graphite thickness in the fuel block, and a higher graphite density. The impact of the increased pitch on the fuel temperature is also evaluated with a thermal analysis code. We have shown that long refueling cycles and high burnups can be achieved simultaneously for both LEU and TRU fuels.

Effect of sulfur on the synthesis and modification of carbon nanostructures

Sulfur-free and sulfur-containing carbon phases have been synthesized from methane using a carbonyl iron catalyst and modified by melting with sulfur. The particle size distribution measurements show that the particle radii are shifted to smaller values after the reaction with sulfur. The content of sulfur in the modified carbon fractions depends on the particle size. In AFM images of some fractions, thin platelets of graphite (thickness of ca. 10 nm) have been seen. XRD analysis points on the aggregation of the carbon particles, which occurs at once with the sedimentational fractionation. Taking into account a tendency to increase the sulfur content inverse proportional to the particle size in the fractions, one can suggest that the sulfur is more likely to react with the finest ingredients of synthesized carbon phase.

Application of graphite-based sacrificial layers for fabrication of LTCC (low temperature co-fired ceramic) membranes and micro-channels

Fabrication of sensors and micro-fluidic structures from low temperature co-fired ceramic (LTCC) sheets is a growing interest in the micro-packaging community. Such devices usually have inner cavities, whose production is quite complicated. The most elegant method to build such structures so far achieved is by a fugitive phase that is introduced into the multilayer and removed during firing. This paper, therefore, is aimed to introduce the graphite-based sacrificial paste developed for this purpose, and it is constructed in two sections: (i) selection of paste and determination of LTCC open-porosity elimination temperature, and (ii) fabrication and characterization of pressure sensitive LTCC membranes. In the former section, it is shown that increased heating rates (and decreasing tape thickness) shift the open porosity elimination temperature of LTCC by 20 °C, which is small compared to the shift of graphite oxidation temperature (about 100 °C). In the latter section, three parameters affecting the balance between the graphite oxidation and LTCC sintering are studied: heating rate, graphite phase thickness and width of the membrane inlet/outlet channels. As expected, larger heating rates and narrow inlet/outlet channels are found to hinder the oxidation of graphite and evacuation of the resulting products, which results in swollen membranes. Large graphite thickness, through the increased channel height, results in lower swelling in spite of the larger amount of graphite to be oxidized. Membranes with low swelling are found to exhibit excellent pressure sensing characteristics, whereas those with high swelling display hysteretic behavior.

Transport of Atomic Hydrogen through Graphite and Its Reaction with Azoaromatic Compounds

Graphite is a major non-iron component in commercial iron granules that are typically used for groundwater remediation. Recent studies suggest graphite inclusions in commercial iron may serve as both adsorption and reaction sites for nitrogenous pollutants such as nitroaromatics, nitrate esters, and heterocyclic nitramines. In this study, we investigated graphite-mediated reduction of azoaromatic compounds with elemental iron in dialysis cells, where azo compounds and iron were physically separated by graphite foil. Both the nonpolar azobenzene and the water-soluble orange G were reduced to aniline, suggesting that exposed graphite in granular iron may mediate reduction of both polar and nonpolar compounds. Orange G reduction was zero-order and commenced after a long initial lag. Both the lag time and the zero-order rate constant varied with graphite thickness, consistent with the explanation that orange G reduction was limited by atomic hydrogen, which was formed via anaerobic iron corrosion and spilled over to graphite. Involvement of atomic hydrogen was confirmed by detection of deuterated aniline when iron was placed in a D2O-based buffer. Our results indicate that atomic hydrogen is mobile in graphite at room temperature, is reactive toward azoaromatic compounds, and may be consumed during transport in graphite.

Measurement of neutron spectra produced in the forward direction from thick graphite, Al, Fe and Pb targets bombarded by 350 MeV protons

Neutron energy spectra at 0-degree produced from stopping-length graphite, Al, Fe and Pb targets bombarded by 350 MeV protons were measured at the neutron TOF course at RCNP of Osaka University. The experiments were performed by the time-of-flight technique with the flight path length of 11.4 and 95 m, and neutron energy spectra were obtained in the energy range from 10 MeV upto the maximum energy 350 MeV. Monte Carlo calculations by MCNPX, PHITS and MARS15 were performed to compare the obtained experimental data, and these simulation results at 0-degree generally underestimated the experimental data for all targets in the energy range above 20 MeV.

Clamp Load Loss due to Elastic Interaction and Gasket Creep Relaxation in Bolted Joints

An experimental study is presented in order to determine the clamp load loss due to elastic interaction and gasket creep relaxation in bolted joints. Studied parameters include the gasket material and thickness, bolt spacing, tightening sequence, fastener grip length, and level of the fastener preload. The joint is composed of two steel flanges and a gasket made of styrene butadiene rubber or flexible graphite. The flanges are fastened together using M12x1.75 Class 10.9 fasteners. Force washers are used to monitor bolt tensions in real time. Four different gasket thicknesses of styrene butadiene rubber (1/16, 1/8, 3/16, and 1/4 in.) and two thicknesses of flexible graphite (1/16 and 1/8 in.) are considered. For the same bolt circle of the flange, the bolt spacing is varied by using a different number of bolts; spacing that corresponds to using three, five, or seven bolts is considered in this study. The effect of the tightening strategy is studied by using sequential, star, or simultaneous tightening patterns. Bolt tightening is accomplished by using either an electric digital torque wrench with various control options or by using a production-size multiple spindle fastening system that is capable of simultaneous tightening of all fasteners. Experimental data is presented and analyzed, in order to study the effect of the various parameters on the clamp load loss due to the combined effect of elastic interaction and gasket creep relaxation at room temperature.

Thermal analysis for graphitization and ablation depths of diamond films

In the present study, a thick diamond film deposited on a 6″ silicon wafer was planarized by an ArF excimer laser. In order to predict the graphitization thickness formed in the diamond film by a laser beam moving with a sliding velocity as well as a fluence, the general solution of the three-dimensional temperature distribution is successfully developed to be a function of the fluence and pulse duration of the laser beam. A reported model developed for the graphitization probability of diamond film is adopted in the present study to determine the graphitization thickness if the temperature solutions are substituted into this model. The ablation thickness in the graphitization layer can be determined so long as the local temperature in the specimen is higher than the ablation temperature of the graphitization layer. Then, the thickness of graphite residual on the working surface after ablation can be obtained by the subtraction of the ablation thickness from the graphitization thickness. This graphite thickness is proved to be very close to the value obtained from scratching tests. This implies that the temperature solutions predicted by the present model are trustworthy. The effect of the scanning velocity is insignificant to the surface temperature if the pulse duration time is much shorter than the time period between two adjacent laser pulses.

Formation and role of graphite and nickel silicide in nickel based ohmic contacts to n-type silicon carbide

Nickel-based contacts, deposited on 4H-SiC C-face substrates, were annealed at temperatures ranging from 800to1040°C and the phase composition of the contact layers analyzed by x-ray diffraction techniques. Ni2Si was identified as the dominant phase for annealing temperatures exceeding 925°C, with further increases in concentration with increasing temperature. At the highest annealing temperature of 1040°C, a 40nm thick nanocrystalline graphite film at the Ni2Si–SiC interface was discovered and its presence confirmed by Raman spectroscopy. The roles of the Ni2Si and graphite films in the formation of ohmic contacts were determined by their subsequent exclusion from the contact composition. Following deposition and annealing, the Ni2Si and graphite layers were etched away selectively and replaced with new metal films deposited at room temperature and without any annealing. Measurement of the current-voltage characteristics revealed that the ohmic nature of the contacts was preserved after removal of the Ni2Si and the graphite layers. It is concluded that the main reason for the conversion of Schottky to ohmic contacts during high-temperature annealing is a change of the electrical properties of the underlying SiC rather than being attributable to the presence of nickel silicides or graphite. It is proposed that a solid-state reaction between nickel and silicon carbide, similar to catalytic graphitization of carbon, may take place during Ni–SiC contact annealing at the temperature of 1040°C. This process may result in the creation of sufficient carbon vacancies in the near-interface region of the SiC to allow increased electron transport through the Schottky barrier.

Synthesis and characterization of metal-filled carbon nanotubes by microwave plasma chemical vapor deposition

A new type of palladium-based metal-filled carbon nanotubes (MF-CNTs) was fabricated and their characteristics were investigated. The MF-CNTs were grown on Pd/SiO 2/Si substrates and Pd/Mo mesh by a microwave plasma-enhanced chemical vapor deposition method using CH 4 and H 2 gasses with the technique of bias-enhanced growth method. The structure and composition of Pd-based MF-CNTs were thoroughly investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. TEM and EDS analyses showed that MF-CNTs were composed of thick cylindrical graphite walls, and filled with Pd metal inside their roots. Based on TEM characterizations, we proposed a fruitful description for the encapsulating mechanisms.

Deposition and characterization of graphite-black coating for absolute pyroelectric detectors

We describe the process of depositing graphite-black on thin pyroelectric detector substrates and compare this with previous work on gold-black coating, documented in the literature. Spectrophotometric measurements indicate that graphite-coating reflectance, at normal incidence, varies by less than 5% from 250 to 2500 nm. The electrical characterization, indicates that graphite coating can be used for the radiant-electrical calibration of pyroelectric detectors. Measurements with electrical heating made on graphite resistance, indicate that the permissible absolute power that could measure the pyroelectric detector could measure, is a function of the graphite thickness.

A new SPD process for spheroidal cast iron

A new severe plastic deformation (SPD) process for cast iron named cylinder covered compression (CCC) is developed by the present authors. In CCC process, specimens are embedded in a steel cylinder and hot-compressed. Then the compressed material is cut into pieces, machined out of surface layers, stacked, embedded in a cylinder and hot-compressed again. By CCC process, spheroidal cast iron has been successfully hot compressed up to 99.2% reduction in height. The shape ratio of deformed graphite, β, increases as the amount of reduction increases up to 80%, after which β changes with no relationship of the reduction. In case of 80% deformed specimens, a lamellar structure of graphite and metal matrix forms. Further deformation leads to the decrease of the thickness of graphite and the fragmentation of graphite.

Deposition and characterization of graphite-black coating for absolute pyroelectric detectors

We describe the process of depositing graphite-black on thin pyroelectric detector substrates and compare this with previous work on gold-black coating, documented in the literature. Spectrophotometric measurements indicate that graphite-coating reflectance, at normal incidence, varies by less than 5% from 250 to 2500 nm. The electrical characterization, indicates that graphite coating can be used for the radiant-electrical calibration of pyroelectric detectors. Measurements with electrical heating made on graphite resistance, indicate that the permissible absolute power that could measure the pyroelectric detector could measure, is a function of the graphite thickness.

A new SPD process for spheroidal cast iron

A new severe plastic deformation (SPD) process for cast iron named cylinder covered compression (CCC) is developed by the present authors. In CCC process, specimens are embedded in a steel cylinder and hot-compressed. Then the compressed material is cut into pieces, machined out of surface layers, stacked, embedded in a cylinder and hot-compressed again. By CCC process, spheroidal cast iron has been successfully hot compressed up to 99.2% reduction in height. The shape ratio of deformed graphite, β, increases as the amount of reduction increases up to 80%, after which β changes with no relationship of the reduction. In case of 80% deformed specimens, a lamellar structure of graphite and metal matrix forms. Further deformation leads to the decrease of the thickness of graphite and the fragmentation of graphite.

Synthesis of corn-shape carbon nanofibers on Si and Mo substrates by bias-enhanced microwave plasma chemical vapor deposition

A new type of corn-shape carbon nanofibers (CCNFs) with metal-free tips was fabricated and their characteristics were investigated. The CCNFs were grown on Ni/SiO 2/Si and Ni/Mo mesh substrates by a microwave plasma-enhanced chemical-vapor deposition method using CH 4 and H 2 gases with the technique of bias enhanced growth method. Transmission electron microscopy and selected area electron diffraction show that CCNFs were composed of thick cylindrical pure graphite walls, and had nanometer-sized tips with roots. The tips’ apex angles of CCNFs were dependent on the substrate temperature, as the cone angles of 20, 39 and 60°.

Magnetism of Pt0.67Fe0.33Alloy

PtFe alloys show a rich variety of magnetism with changing of Fe concentration and atomic configuration due to the competitive ferromagnetic and antiferromagnetic spin correlations. The PtFe alloys containing 18–40 at.% of Fe show the cubic Cu3Au type atomic order for which corners and face centers of fcc unit cell are occupied by Fe and Pt atoms, respectively and excess Fe atoms occupy the face center position. Vinokurova et al. studied the magnetism of Pt1 XFeX alloys with 0:3 < X < 0:35 and reported that the alloys show a ferromagnetic–antiferromagnetic (F–A) phase transition below TC. 3) The motivation of the present work is to investigate the magnetic phase transition of Pt1 XFeX alloys with these Fe concentration range. In this letter, we report the susceptibility and neutron diffraction data for a Pt0:67Fe0:33 alloy single crystal and propose a different model from the previous authors for the magnetism of Pt1 XFeX alloys with relevant Fe concentration range. A Pt0:67Fe0:33 alloy single crystal was grown by Bridgman method in a furnace with a carbon electrode under the Ar gas atmosphere. The SQUID system at Materials Characterization Central Laboratory in Waseda University was used for the magnetic susceptibility measurement. Both of zero field cooled (ZFC) and field cooled (FC) processes were measured at the temperature range between 5K and 250K in the magnetic field of 500Oe. Neutron scattering experiments were performed using T1-1 triple axis spectrometer installed in a thermal guide of JRR-3M, JAERI, Tokai. Incident neutrons with wavelength of 0.246 nm were used together with a thick pyrolitic graphite filter to eliminate the higher order contaminations. Magnetic susceptibility data are shown in Fig. 1. Ferromagnetic response appears at around 200K, but decreases below 100K. Since the FC data do not trace the same path with the ZFC data below 100K, the alloy shows a re-entrant spinglass like behavior. These data are consistent with those of the previous authors. Neutron scattering measurements were carried out for two different states of the specimen. At first, magnetic scattering measurements were performed for the as-grown sample. After the measurements, the specimen was annealed at 1000 C for 24 h to develop the atomic order. Then, the similar measurements were performed using the annealed specimen. Temperature variations of the ferromagnetic peak intensities studied at 1 0 0 and 1 1 0 and that of the antiferromagnetic peak at 1/2 0 0 for the as-grown sample are given in Fig. 2. The ferromagnetic peaks 1 0 0 and 1 1 0 appear at around 200K and the intensities increase with decreasing temperature down to 100K, then decrease below this temperature. The antiferromagnetic peak 1/2 0 0 appears at 120K. The antiferromagnetic peak intensity monotonously increases with decreasing temperature. The experimental data for the annealed specimen obtained by the similar measurements are given in Fig. 3. Temperature variations of the magnetic peaks are qualitatively the same, but the antiferromagnetic peak intensity for the annealed sample is almost twice of that for the as-grown sample, despite that the ferromagnetic peak intensities are almost the same. Since the line widths of these magnetic peaks were the same for the samples before and after annealing, these data 0 0.005 0.01 0.015 0.02

Coke formation during metal dusting of iron in CO‐H2‐H2O gas with high CO content

AbstractIron carburisation and coke formation during metal dusting of iron have been investigated in the gas mixture of 75%CO‐24.8%H2‐0.2%H2O at 600°C and 700°C. In all cases, cementite is formed at the surface, together with a coke layer on the top. In the coke layer, two morphologies of graphite are identified: compact bulk graphite with a uniform thickness and a columnar structure, and filamentous carbon with iron‐containing phases at the tip or along its length. The examination of coke formation in different stages of reaction at 700°C reveals that the coke contains two layers. The inner layer is composed of filaments, while the outer layer consists of the compact columnar graphite. After 2 h reaction the top compact graphite layer has suffered a serious deformation and has formed fractures because of the growth of catalytic filamentous carbon underneath. These filaments grow outside from these fractures and finally cover the whole surface after 4 h reaction. At 600°C, however, the coke contains a thick bulk graphite layer and non‐uniformly distributed filaments on the top. The bulk graphite layer is composed of many graphite columns which are loosely piled and are vertical to the surface. Each graphite column consists of many fine graphite fibres in parallel with the columnar axis. Filaments grow outside preferably from the gaps among these graphite columns and along the grinding scratches. TEM analysis of the coke detects very convoluted filaments with iron‐containing particles at the tip or along their length. XRD and TEM analyses show that these particles are Fe3C rather than metallic iron.

DEMS study of gas evolution at thick graphite electrodes for lithium-ion batteries: the effect of γ-butyrolactone

Differential electrochemical mass spectrometry (DEMS) was used to study the reductive decomposition of an electrolyte based on ethylene carbonate/dimethyl carbonate (EC/DMC), as well as the formation of a solid electrolyte interphase (SEI) in this electrolyte, at thick (75–100 μm) porous graphite composite electrodes. A number of graphite electrodes differing in their electrochemical lithium intercalation properties were investigated in potential-sweep experiments. They proved to be similar with respect to the evolution of ethylene and hydrogen gas during the first two charge/discharge cycles. Due to an incomplete coulombic conversion, a high irreversible capacity, as well as slow diffusion kinetics and an enhanced ohmic resistance of the electrodes, SEI formation on these thick electrodes was not yet complete after the first charge/discharge cycle. Undesired gas evolution can be reduced by adding γ-butyrolactone (GBL) as an electrolyte co-solvent. The amount of ethylene and hydrogen gas evolved decreases with increasing percentages of GBL in an EC/DMC electrolyte, indicating that the SEI layer is built up from GBL rather than from EC decomposition products.