Isotropic Graphite Research Articles

International Comparison on Thermal-Diffusivity Measurements for Iron and Isotropic Graphite Using the Laser Flash Method in CCT-WG9

The first international pilot study of thermal-diffusivity measurements using the laser flash (LF) method was organized by the working group 9 (WG9) of the Consultative Committee for Thermometry (CCT) of the Bureau International des Poids et Mesures (BIPM). Four National Metrology Institutes (NMIs) participated in this comparison. Thermal-diffusivity measurements on the Armco iron and the isotropic graphite IG-110 were carried out from room temperature to about 1200 K. The sample sets consist of five disk-shaped specimens of 10 mm in diameter and (1.0, 1.4, 2.0, 2.8, and 4.0) mm in thickness, each cut from the same block of material. These sample sets were specifically prepared for the comparison and sent to the participants. In the pilot comparison, the thermal diffusivity of each sample was estimated using the LF method with a specific extrapolating procedure. This procedure has the advantage of determining the inherent thermal diffusivity of the material. The extrapolated value in a plot of measured apparent thermal-diffusivity values versus the amplitude of the output signal corresponding to the temperature rise during each measurement is defined as the inherent thermal diffusivity. The overall results showed good agreement between independent laboratories, measurement equipment, and specimen thicknesses. The thermal diffusivities of the materials were determined using our measured results. A quantitative evaluation of the variability of the data obtained by the participants has been done, by evaluating the deviations from the reference value, the Z-value, and the En-number. Some data showed a large deviation from the reference value. It was concluded that these are caused by an insufficient time response of the measurement equipment and some difficulties with changing the pulsed heating energy. The effect of the thermal expansion on the thermal diffusivity was checked. It was found that the thermal-expansion effect was very small and negligible in this case.

Kinetic studies on the changes in structural and physical properties of isotropic graphite during graphitization

Kinetic studies on the changes in structural and physical properties of isotropic graphite during graphitization

Raman Study of the Relationship between Microstructure and Physical Properties of Isotropic Graphite

The degree of graphitization, crystallite size, fracture surface and pore morphology of three kinds of isotropic graphite from different companies were characterized by Raman spectroscopy and scanning electron microscopy (SEM). The relationship between physical properties and microstructure was studied.The results showed that the raman values of (ID/IG) of three samples were about from 0.32 to 0.47. The flexural strength and electrical resistivity were directly proportional to the value of (ID/IG) , while the thermal conductivity was inversely proportional to that value.

Study on the Mechanism of Diamond Wear in Precision Cutting of Isotropic Pyrolytic Graphite

Isotropic pyrolytic graphite is a kind of graphite material. Because of its excellent performance, it is gradually used in aero-engine turbine shaft seals. But this material is a kind of brittle material, and it wears seriously during the cutting process. Through analysis of the process of cutting this material with PCD insert, the main wear region and the wear pattern at different cutting time were obtained. Using the scanning electron microscopy to observe the wear regions, the wear mechanism of PCD was obtained. At the first stage of the wear mechanisms were mainly mechanical wear and abrasive wear, the main wear regions were rake face and the cutting edge. With the increase of cutting distance, about 32 min later, both the depth of the crater on the rake face and the depth of micro grooves on the flank face were increased. Abrasive wear on the cutting face and the micro grooves wear on the flank face were the main wear pattern at the stable wear stage. At rapid wear stage, cutting edge was transformed from micro chipping into tool tipping. This caused by erosion wear .Experimental results indicated that the wear pattern was typical mechanical wear, abrasive wear, cutting edge erosion wear and possible adhesive wear.

PS34 微粒等方性黒鉛の破壊靱性に及ぼす切欠き先端形状の影響

In order to investigate the effect of notch-tip geometry on the fracture toughness of fine-grained isotropic graphite ETU-10, three-point-bending fracture toughness tests have been performed by using two types of single-edge notched beam specimens with different notch angles of approximately 15° and 30°. In this test, the load-displacement curve was measured by using the load cell and the laser displacement sensor. The values of the fracture toughness were calculated based on linear elastic fracture mechanics by using the maximum force which could be obtained by the load-displacement curve. As a result, the values of the fracture toughness of the specimens with the notch angles of approximately 15° and 30° were 0.933 MPa m^<1/2> and 0.926 MPa m^<1/2>, respectively. This suggested that the values of the fracture toughness were not affected between the notch angles. Further work has been ongoing to discuss the effect of the notch-tip geometry such as angle and root radius of notch in terms of finite element analysis.

Multiple pulse-heating experiments with different current to determine total emissivity, heat capacity, and electrical resistivity of electrically conductive materials at high temperatures

A modified pulse-heating method is proposed to improve the accuracy of measurement of the hemispherical total emissivity, specific heat capacity, and electrical resistivity of electrically conductive materials at high temperatures. The proposed method is based on the analysis of a series of rapid resistive self-heating experiments on a sample heated at different temperature rates. The method is used to measure the three properties of the IG-110 grade of isotropic graphite at temperatures from 850 to 1800 K. The problem of the extrinsic heating-rate effect, which reduces the accuracy of the measurements, is successfully mitigated by compensating for the generally neglected experimental error associated with the electrical measurands (current and voltage). The results obtained by the proposed method can be validated by the linearity of measured quantities used in the property determinations. The results are in reasonably good agreement with previously published data, which demonstrate the suitability of the proposed method, in particular, to the resistivity and total emissivity measurements. An interesting result is the existence of a minimum in the emissivity of the isotropic graphite at around 1120 K, consistent with the electrical resistivity results.

等方性黒鉛の黒鉛化過程における構造，物性変化の速度論的検討

等方性黒鉛の黒鉛化過程における構造，物性変化の速度論的検討

Characteristics of dissimilar laser-brazed joints of isotropic graphite to WC–Co alloy

Abstract The effect of Ti serving as an activator in a eutectic Ag–Cu alloy filler metal in dissimilar laser-brazed joints of isotropic graphite and a WC–Co alloy on the joint strength and the interface structure of the joint is investigated in this study. To evaluate the joint characteristics, the Ti content in the filler metal was increased from 0 to 2.8 mass%. The laser brazing was carried out by irradiating a laser beam selectively on the WC–Co alloy plate in Ar atmosphere. The threshold content of Ti required to join isotropic graphite to WC–Co alloy was 0.4 mass%. The shear strength at the brazed joint increased rapidly with increasing Ti content up to 1.7 mass%, and a higher Ti content was found to be likely to saturate the shear strength to a constant value of about 14 MPa. The isotropic graphite blocks also fractured at this content. The concentration of Ti observed at the interface between isotropic graphite and the filler metal indicates the formation of an intermetallic layer of TiC.

Prediction and Measurement of Thermal Transport Across Interfaces Between Isotropic Solids and Graphitic Materials

Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate.

In situ measurement of hydrogen isotope retention using a high heat flux plasma generator with ion beam analysis

We present Plasma Surface Dynamics with Ion Beam Analysis (PS-DIBA), a device that makes it possible to carry out in-situ ion beam analysis of deuterium retention under high-density plasma irradiation, relevant to divertor plasma conditions. The device is equipped with a newly developed novel dc plasma source, which can generate high-density plasma (>1018 m−3) with an electron temperature of around 5 eV in steady state. The dynamic behavior of deuterium retention in ITER R&D tungsten exposed to deuterium plasma was observed by using nuclear reaction analysis in PS-DIBA. The results indicate that deuterium retention strongly depends on the surface temperature of the tungsten sample during plasma exposure. After plasma termination, the retained deuterium in tungsten decreases faster than in isotropic graphite.

Tritium Absorption of CO-Deposited Carbon Films, Graphite and Polycrystalline Tungsten

Tritium retention in plasma facing materials is a primary issue for ITER and next step fusion devices, since it greatly affects its safety and operational schedule. In the ITER, carbon and tungsten are used as divertor materials. In the present study, co-deposited carbon film, tungsten and isotropic graphite were exposed to tritium gas, and then the amount of absorbed tritium was investigated. During the tritium exposure, the partial pressure of tritium gas was kept at 10 Pa. The sample temperature was kept a constant in the range from RT to 573 K. The amounts of absorbed tritium were evaluated by ²-ray-induced X-ray spectrometry (BIXS). The amounts of absorbed tritium in co-deposited carbon films were one or two orders of magnitude larger than that of polycrystalline tungsten and isotropic graphite. The amount of absorbed tritium for co-deposited carbon film with a high volume density (1.53 g/cm3) was several times larger than that of the film with a low volume density (1.13 g/cm3). The amount of absorbed tritium increased with the temperature. These results indicate that co-deposited carbon films can absorb much larger amount of tritium than tungsten and graphite, and carbon film density affects the amount of absorbed tritium.

Professor Wanci Shen and his research activity in carbon materials

Professor Wanci Shen was born in 1940 in Hubei Province.He studied metallic materials in Tsinghua University from 1957 to 1963.After that,he became a faculty member in Tsinghua University. He was one of the earliest Chinese scientists to investigate natural graphite and its processing,graphite intercalation compounds,flexible graphite,isotropic graphite and graphite materials as electrodes for batteries.He was awarded the National Invention Award in 1993 and the China Building Materials Award in 2006.He has published more than 100 academic papers.Prof.Shen was awarded the Carbon Outstanding Achievement award of China in 2011 by the Selection Committee for the Carbon Outstanding Achievement award.

Measurements of cross sections and charged pion spectra in proton-carbon interactions at 31 GeV/c

Interaction cross sections and charged pion spectra in p+C interactions at 31 GeV/c were measured with the large acceptance NA61/SHINE spectrometer at the CERN SPS. These data are required to improve predictions of the neutrino flux for the T2K long baseline neutrino oscillation experiment in Japan. A set of data collected during the first NA61/SHINE run in 2007 with an isotropic graphite target with a thickness of 4% of a nuclear interaction length was used for the analysis. The measured p+C inelastic and production cross sections are 257.2 +- 1.9 +- 8.9 mb and 229.3 +- 1.9 +- 9.0 mb, respectively. Inclusive production cross sections for negatively and positively charged pions are presented as a function of laboratory momentum in 10 intervals of the laboratory polar angle covering the range from 0 up to 420 mrad. The spectra are compared with predictions of several hadron production models.

Temperature effects in high fluence ion modification of HOPG

The ion-induced processes of HOPG (UPV-1T) modification under high-fluence (10 18–10 19 ion/cm 2) 30 keV Ar + and Ne + ion irradiation have been studied from room temperature till 400 °C. The temperature dependences of ion–induced electron emission yield have been found, in contrary to isotropic graphite MPG-8, the essentially different for HOPG between heating and following cooling measurements. For Ar + irradiation only in the 150–300 °C temperature range anomalously large depth of ion-modified surface layer with developed morphology have been found. The results are discussed in frame of the Ehrlich–Schwoebel instability.

Neutral Beam Armor for NSTX Upgrade

The National Spherical Torus Experiment (NSTX) is a low aspect ratio, spherical torus (ST) configuration device which is located at Princeton Plasma Physics Laboratory (PPPL). This device is presently being upgraded to enhance its operations by adding a second Neutral Beamline (NBL). This change will nearly double the power available to the plasma but necessitate improvements to other design aspects of NSTX. Included in these upgrades are the relocation and upgrade of the NSTX Neutral Beam Armor to capture both sets of beamline source profiles while maintaining the same level of vacuum vessel wall protection.In order to minimize the space required to accomplish this, it has been proposed to relocate and reuse the existing armor array, improving the design so that two overlapping sets of beam profiles both fit completely. This beamline overlap could possibly cause the armor tiles to experience higher heat fluxes which translate into higher internal mechanical stresses. This would be mitigated by changing the isotropic graphite (ATJ) tiles in the overlap areas to a rugged 3D carbon-fiber composite (CFC) material, capable of handling thermally-induced stresses. Additional benefits to this recycling design proposal include opportunities to reduce project cost, increase diagnostic port access, and improve an awkward and difficult mounting scheme.

Study on the Critical Cutting Thickness and the Precision Cutting Experiments of Isotropic Pyrolyric Graphite

Isotropic pyrolyric graphite (IPG) is a new kind of brittle material, it not only has the general advantages of ordinal carbonaceous materials such as high temperature resistance, lubrication and abrasion resistance, but also has the advantages of impermeability and machinability that carbon/carbon composite doesn’t have. So it can be used for sealing the aeronautics and astronautics engines turbine shaft and the ethylene high-temperature equipment. The mechanism of this material removal during the precision cutting was analyzed by using the theory of strain gradient. The critical cutting thickness of IPG was calculated for the first time. Furthermore, the cutting process parameters such as cutting depth and feed rate which corresponding to the scale of brittle-ductile transition deformation of IPG was calculated. The prediction model of surface roughness in precision cutting of IPG was developed based on the Genetic algorithm. Using the surface roughness prediction model, the study investigates the influence of the cutting speed, the feed rate and the cutting depth on surface roughness in precision turning process was researched.

Erosion of Carbon-Based Materials Under High-Fluence Heavy-Ion Irradiation

Carbon-based materials are widely used in aerospace industry due to their strength, heat shielding properties, and a number of other specific characteristics [1]. To trace the radiation damage influence on the erosion regularities the sputtering yields Y of highly-oriented pyrolytic graphite (HOPG), polycrystalline graphites and one-dimensional composite material under high-fluence (10 – 10 ion/cm) 30 keV N2 and Ar ions at the incidence angles from θ = 0 (normal incidence) to θ = 80, developed morphology and crystalline phase state from low temperatures (T ~ −180C) till elevated ones (T ~ 400C) have been studied. At normal and near normal ion incidence Reflection High Energy Electron Diffraction has shown that diffraction patterns correspond to a high degree of disorder (diffuse halo) at the temperatures less the defect annealing temperature Ta. At elevated temperatures (T > Ta) the patterns have been found similar to those for the non-irradiated surfaces. It has been found that Ta is easy determined by a step-like behavior of a temperature dependence of ion-electron emission yield γ. This γ-jump has been explained by the dependence of secondary electron path length λ on changes in lattice structure (transition from strongly disordered surface layer under ion irradiation at T Ta) [2]. The Y(θ) dependences measured at T Ta, due to the ion-induced diffusion processes, causing three-fold suppression of sputtering in comparison with irradiation at T < Ta. The Ar irradiation at the room temperature results to strong sputtering suppressing in comparison with the isotropic polycrystalline graphites at the same conditions.

Thermal and electrical conduction properties of vertically aligned carbon nanotubes produced by water-assisted chemical vapor deposition

We present both thermal and electrical conduction properties of vertically aligned carbon nanotubes (VACNTs), synthesized by the water-assisted chemical vapor deposition method using Fe–Ti–O nanoparticles as catalyst. Thermal diffusivity and electrical resistance of VACNTs have been measured by the laser flash method and direct-current four-terminal method, respectively. The VACNTs are found to have thermal diffusivities of the same order as isotropic graphite and the electrical characteristics of semiconductors. The electrical resistance shows a T −1/4 temperature dependence, which implies that the conduction of electrons is dominated by 3D Mott variable range hopping.

Characterization of Surface Modifications of Plasma-Facing Components in LHD

The Large Helical Device (LHD) has been equipped with movable- and fixed-type material probe systems. Characterization studies of surface modifications on plasma-facing components (PFCs) have been actively progressing by using these probes. After exposure of the PFCs to the plasma, various kinds of surface analysis were conducted. The first walls and divertor tiles of LHD are made of stainless steel and isotropic graphite (IG-430U, Toyo Tanso Co., Ltd.), respectively. They are frequently exposed not only to high-power pulsed main discharges but also to wall-conditioning processes such as glow discharge cleaning (GDC). Thus, the surfaces of the PFCs are drastically changed due to sputtering erosion, impurity deposition, and melting damage. Graphite divertor tiles are eroded primarily during the main discharges; the eroded carbon migrates and deposits on the first-wall surfaces, particularly near the divertor array. First walls are eroded mainly during GDC, which significantly changes the condition of the PFCs. During the main discharges, the majority of incidence particles to the first wall are energetic neutrals (CX neutrals) generated by charge-exchange collisions. Studies of the material damage caused by CX neutrals also have been done. In this paper, the characteristics of surface modifications of PFCs by means of material probe experiments and subsequent surface analysis are summarized.

Specific surface area, shape, and size of fine carbon filler particles

We have studied the specific surface area, shape, and size of fine particles of isotropic and needle cokes and isotropic graphite. The results demonstrate that, as the average particle size is reduced from 1000 to 1.5 μm, the BET specific surface area of the powders increases from 0.1 to 100 m2/g, exceeding the geometric surface area of the particles by a factor of 40–60. The particle size distributions of the coke and graphite powders produced under identical milling conditions are similar to one another and can be represented by the Rosin-Rammler equation. The anisometry of the powder particles is higher at a stronger microtexture of the starting material and decreases with decreasing particle size.