Graphite Grains Research Articles

Surface chemical deposition coating of graphite particles

Surface chemical deposition coating was used to form single core-shell graphite grains with medium coal tar pitch as raw material.Scanning electron microscopy and particle size measurement showed that the surface of the graphite grains was obviously changed and the coating layers were formed first on the edges and corners of a graphite sheet and then extended to the whole sheet surface.X-ray diffraction,infrared spectroscopy and differential scanning calorimetry showed that the coating layers were mesophase pitch.

HTR Fuel Waste Management: TRISO separation and acid-graphite intercalation compounds preparation

Considering the need to reduce waste production and greenhouse emissions and still keeping high energy efficiency, various 4th generation nuclear energy systems have been proposed. As far as graphite-moderated reactors are concerned (future high temperature fast or thermal reactors), one of the key issues is the large volumes of irradiated graphite encountered. With the objective to reduce volume of waste in the HTR concept, it is very important to be able to separate the fuel from low level activity graphite representing a large volume. The separated TRISO particles can then be reprocessed for waste separation or disposed off in geological repository. In addition, preparation of acid-GICs from the separated graphite may constitute a way to recycle this waste. We used HTR-type compact fuel with ZrO 2 TRISO particles to test two separation methods: low (H 2SO 4 + H 2O 2) and high (H 2SO 4 + HNO 3) temperature acid treatments. In both cases the TRISO separation was complete but some TRISO layers oxidized at high temperature. At low temperature, the desegregation of graphite grains is facilitated by intercalation of sulfuric acid between the graphene layers. The acid-GIC obtained consists of pure phases of high quality suggesting their potential industrial recycling.

Fir continuum and line emissions from interstellar medium in galaxies

A self-consistent method has been evolved to infer physical parameters like density, radiation field and abundances using line and continuum radiations as diagnostics. For that purpose, we first calculate the temperatures of graphite and silicate grains using the model of Li and Draine (Astrophys. J. 554:778, 2001) by solving self-consistently the energy balance for G 0 (1–104) times the radiation field following Weingartner and Draine (Astrophys. J. Suppl. Ser. 134:263, 2001). Consequently, infrared emission fluxes are also obtained. To keep it simple, this is presented in the empirical form of parameters T D and wavelength. The same model of the grain is adopted for photoelectric heating of gas using the formalism of Weingartner and Draine (Astrophys. J. Suppl. Ser. 134:263, 2001) (hereafter referred to as WD) and Bakes and Tielens (Astrophys. J. 427:822, 1994) (hereafter referred to as BT) for radiation field cited above in the range (6<hν≤13.6 eV). Temperature and abundances are determined using our own code for PDR very similar to cloudy code. All the possible sources of heating and cooling are considered for setting up the thermal balance. For the gas phase abundances that vary with depth in the cloud due to dust, self- and mutual shielding, chemical balance is solved. Most of the photoionization, photodissociation or chemical reaction rates are taken from UMIST database. We present an analysis of the cooling lines of singly ionized carbon [CII] at 158 μm and neutral oxygen [OI], at 63 μm and far infrared (FIR) continuum for a variety of star forming galaxies. Method of analysis of observational data is different from that of Malhotra et al. (Astrophys. J. 561:766, 2001). The radiation field G 0, density N h and abundance of carbon are obtained through best fit of observed and calculated intensities for lines and continuum radiations.

ANOMALOUS SILICATE DUST EMISSION IN THE TYPE 1 LINER NUCLEUS OF M81

We report the detection and successful modeling of the unusual 9.7\mum Si--O stretching silicate emission feature in the type 1 (i.e. face-on) LINER nucleus of M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we determine the feature in the central 230 pc of M81 to be in strong emission, with a peak at ~10.5\mum. This feature is strikingly different in character from the absorption feature of the galactic interstellar medium, and from the silicate absorption or weak emission features typical of galaxies with active star formation. We successfully model the high signal-to-noise ratio IRS spectra with porous silicate dust using laboratory-acquired mineral spectra. We find that the most probable fit uses micron-sized, porous grains of amorphous silicate and graphite. In addition to silicate dust, there is weak PAH emission present (particularly at 11.3\mum, arising from the C--H out-of-plane bending vibration of relatively large PAHs of ~500--1000 C atoms) whose character reflects the low-excitation AGN environment, with some evidence that small PAHs of ~100--200 C atoms (responsible for the 7.7\mum C--C stretching band) in the immediate vicinity of the nucleus have been preferentially destroyed. (abstract continues)

Nanometer-scale distribution of field emission current from the arc-prepared carbon thin film

In order to clarify the mechanism of low-macroscopic-field (LMF) electron emission from carbon related materials, atomistic properties including local tunneling barrier height (LBH) and field emission (FE) current distributions of the arc-prepared carbon thin film have been measured by using scanning tunneling microscopy. From the LBH images, it is found that the carbon thin film surface consists of nanometer-scale graphite grains whose crystallinities and electronic properties are different although the local work function of the film is rather homogeneous. The obtained FE images show that the FE currents are varied more than one order of magnitude, depending on the grains, and that the FE current is higher at the edge of the individual grains. The authors cannot find any specific emission sites that give extremely higher emission currents, indicating that the specific emission sites such as high-aspect-ratio nanoprotrusions or atomistic defects are not responsible to the LMF electron emission from carbon related materials.

THE FORMATION OF GRAPHITE WHISKERS IN THE PRIMITIVE SOLAR NEBULA

It has been suggested that carbonaceous grains are efficiently destroyed in the interstellar medium and must either reform in situ at very low pressures and temperatures or in an alternative environment more conducive to grain growth. Graphite whiskers have been discovered associated with high-temperature phases in meteorites such as calcium aluminum inclusions and chondrules, and it has been suggested that the expulsion of such material from proto stellar nebulae could significantly affect the optical properties of the average interstellar grain population. We have experimentally studied the potential for Fischer-Tropsch and Haber-Bosch type reactions to produce organic materials in protostellar systems from the abundant H2, CO, and N2 reacting on the surfaces of available silicate grains. When graphite grains are repeatedly exposed to H2, CO, and N2 at 875 K abundant graphite whiskers are observed to form on or from the surfaces of the graphite grains. In a dense, turbulent nebula, such extended whiskers are very likely to be broken off, and fragments could be ejected either in polar jets or by photon pressure after transport to the outer reaches of the nebula.

Molecular dynamics modelling of nanocarbon cluster properties under conditions close to HE detonation

We use molecular dynamics for modelling properties of carbon nanoclusters. The size of modelled carbon nanoclusters is below 5 nm, which is typical of detonation di- amond nanoclusters. We have found their structural changes at P = 0 to be as follows: Dia- mond → Diamond core + GL-surface → sandwich-type graphite → Graphite-like liquid. In smaller clusters the transformations start at a lower temperature. Adaptive Template Analysis (ATA) was used to determine the structures. We studied evaporation properties at temperatures above 5000 K. For clusters of several thousands of atoms, the simple depen- dence kvap ∼ e −T0/T /N 1/3 (T0 is constant) is quite good. It has been found out that densities of saturated vapour for clusters containing from 4000 to 8000 atoms are very close at T = 5000 K. The structure of nanoclusters was studied at nonzero pressures set by an argon environ- ment. Calculated results suggest that the patterns for different temperatures are qualita- tively similar for three pressures under study (20, 25 and 30 GPa). At T = 1000-1500 K, the initial diamond core is preserved and a thin disordered GL layer is present on the sur- face. At T = 2000-5000 K, graphite grains form in the sample and a thin layer of liquid is present on its surface. The sample is amorphous at 5500 K and 6000 K. The prevalence of the graphite phase at these pressures seems to come from the absence of long-range interaction in REBO-2002.

Development of Novel SiC/CBC (Ceramic Bonded Carbon) with 3D Micro-network Structure

A new SiC coating on carbon particles has been developed using SiO powders as the Si source by a chemical vapor reaction. Uniform SiC coating was obtained on individual particle with the thickness of 10 nm to 10 μm. The SiC coated carbon particles sintered by PECS exhibited unique microstructures with a 3D micro-network structure consisting of graphite grains covered with thin SiC grain boundaries. The sintered products at 2000°C showed a high bending strength of 150 MPa.

THE INFLUENCE OF IRON POWDER AND DISINTEGRATED STEEL SCRAP ADDITIVES ON THE SOLIDIFICATION OF CAST IRON

The study proves that by introducing the iron powder and disintegrated steel scrap to low-sulphur cast iron still before the inoculation carried out with a conventional graphitising inoculant, the mechanical properties similar to those obtained during the inoculation treatment carried out on cast iron with the recommended high sulphur content are achieved. The said operation increases the number of crystallisation nuclei for dendrites of the primary austenite. In this case, the iron particles act as substrates for the nucleation of primary austenite gp due to a similar crystallographic behaviour of the regular face centered cubic lattice The more numerous are the dendrites of primary austenite, the less free space is available in the interdendritic spaces for the formation of graphite eutectic grains, which makes the structure more refined (more eutectic grains) and the mechanical properties higher.

Solid carbon paste-based amperometric sensor with electropolymerized film of 2-amino-5-mercapto-1,3,4-thiadiazole

An amperometric sensor with attractive sensing behavior was prepared by using solid carbon paste as substrate for electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole (AMT). Morphology, polymerization mechanism and charge-transfer performance of the poly-AMT (PAMT) film on the composite substrate surface were investigated by using SEM, in situ UV–vis spectroelectrochemistry and electrochemical impedance spectrum methods. The results indicate that the PAMT film covered the wax portion prior to the graphite grains on the substrate surface, and the resulting sensor showed remarkable decrease in charge-transfer resistance with the increase in the amount of deposited PAMT. At the physiological pH the sensor gave sensitive and well separated voltammetric signals for two bioflavonoid isomers with the same electro-active moiety, luteolin and fisetin. The sensor achieved amperometric sensitivities of 1.41 nA nM−1 cm−2 to luteolin and 0.648 nA nM−1 cm−2 to fisetin in a linear range of 10–700 nM, with the lowest detection limits of 3 nM luteolin and 5 nM fisetin, respectively. The study demonstrates that the solid carbon paste is a superior substrate material for PAMT film sensor.

Frequency and size distribution dependence of visible and infrared extinction for astronomical silicate and graphite grains

In a recent paper, the frequency and size distribution dependence of extinction spectra for astronomical silicate and graphite grains was analyzed in the context of MRN type interstellar dust models in the far ultraviolet and ultraviolet regions. These grains were taken to be homogeneous spheres following a power law size distribution. In the present work we extend the analysis further to cover the visible as well as the infrared part of the electromagnetic spectrum. The analytic formulas presented here along with those given in the earlier paper would enable one to evaluate extinction for these grains within a wider wavelength range 1000–22,500 Å and analyze the observational interstellar extinction data in far greater details.

Ne ISOTOPES IN INDIVIDUAL PRESOLAR GRAPHITE GRAINS FROM THE MURCHISON METEORITE TOGETHER WITH He, C, O, Mg-Al ISOTOPIC ANALYSES AS TRACERS OF THEIR ORIGINS

Ne isotopes measured in individual presolar graphite grains, solid samples of extinct stars preserved in primitive meteorites, provide information on the type of stellar sources of the grains and on nucleosynthetic mixing and ion-trapping processes which were operating. We present Ne and He isotope analyses of single presolar graphite grains from the KFB1 density fraction extracted from the carbonaceous chondrite Murchison. In addition, we measured isotopes of C, O, and Mg-Al with the NanoSIMS ion microprobe to better constrain the origin of the grains. Eleven out of 51 presolar graphite grains contain nucleosynthetic {sup 22}Ne above our detection limit. This fraction of {sup 22}Ne-rich grains is similar to the one reported by Nichols et al. although we have a lower {sup 22}Ne detection limit. We detected rare He-shell {sup 20}Ne in one {sup 22}Ne-rich grain and obtained the {sup 20}Ne/{sup 22}Ne ratio (0.03 {+-} 0.02) of the He-shell of an Asymptotic Giant Branch (AGB) star with 1.5-2 M {sub sun} and subsolar metallicity. We also detected {sup 4}He in this grain, while in the other grains, which originally acquired He, He-loss seems to be significant. We found unequivocal evidence for radiogenic {sup 22}Ne (Ne-R) in another graphite grain, which likelymore » condensed in a core-collapse supernova and which incorporated live radioactive {sup 22}Na (t {sub 1/2} = 2.6 yr). For the other grains, a clear assignment to a stellar source is more difficult to make. Putative stellar sources are supernovae, AGB stars, born-again AGB stars, J-type carbon stars, and CO novae.« less

Wear mechanisms and tool life management of WC–Co drills during dry high speed drilling of woven carbon fibre composites

Woven carbon fibre composites are extensively used in aerospace, automotive and civil applications. Optimization of high speed drilling of composites is an issue of great economical impact. Tool wear and part quality are central to the definition of the objective function and constraints of the optimization scheme. This paper presents an experimental investigation of the wear mechanisms of tungsten carbide (WC) drills during dry high speed drilling of quasi-isotropic woven graphite fibre epoxy composites. Tool wear was evaluated at spindle speeds of up to 15,000 rpm using a standard two flute drill. The paper examines the nonlinear behavior of this tribo-system and the interdependence of the wear process and cutting forces in relation to surface damage of the system components. It was found that chipping and abrasion were the main mechanisms controlling the deterioration of WC drill. The two friction regimes, the lightly and heavily loaded, were found to dictate the increase in forces, delamination of composite and surface roughness. The aggressive rubbing by fractured graphite fibres and WC grains against the soft epoxy matrix caused high temperature rise and consequently enhanced flank wear. During the primary and secondary wear stages, wear on the flank face of main cutting edges was found to be dominant, while adhesion of carbon was found to occur along with abrasion in the tertiary zone. Tool life results revealed the increase in the delamination and surface roughness with transition from the primary to tertiary wear regime. The correlation between tool wear, delamination damage and surface roughness was established. Finally it was concluded that a tool replacement strategy could be devised by monitoring the cutting forces.

Crust and mantle contributions to gold-forming process at the Daping deposit, Ailaoshan gold belt, Yunnan, China

The Daping gold deposit contains one of the largest resources in the Ailaoshan gold belt, the most economically significant Cenozoic gold belt in China. Most previous researchers considered Daping to be a reworked type gold deposit, and the ore-forming fluid to have been a mixture of magmatic and meteoric water. Ore-forming materials were considered to be derived mainly from the diorite host rocks or metamorphic basement by water-rock interaction. Detailed observation and in-situ laser Raman analyses discovered a large amount of fine-grained, highly-crystallized graphite in the auriferous sulfide quartz veins with obvious O peaks and very weak D peaks in the laser Raman spectrum, which suggests that those graphite grains were formed under granulite facies conditions. Isotope analyses show that the ( 87Sr/ 86Sr) 0 and εNd (0) of the Daping scheelite, the earliest precipitated ore mineral in the paragenesis, vary between 0.7088 and 0.7112 and − 8.43 and − 6.20, respectively, and were projected in the lower crust field on a ( 87Sr/ 86Sr) 0 versus εNd (0) diagram. Noble gases isotopic compositions of fluid inclusions in Daping scheelites were performed by a high vacuum gas mass spectrum, and the results show that the 3He/ 4He ratios are (0.988–1.424) × 10 − 6 with an average of 1.205 × 10 − 6 , corresponding to R/Ra values of 0.706–1.018, with an average of 0.898. The 40Ar/ 36Ar ratios are between 1801.8 and 2663.8, much higher than that of the air (295.5); 20Ne/ 22Ne and 21Ne/ 22Ne are 9.600 to 11.56 and 0.028 to 0.0467, respectively, and the respective 134Xe/ 132Xe and 136Xe/ 132Xe are 0.394 to 0.692 and 0.301 to 0.462, demonstrating that ore-forming fluids and materials of the Daping mine derived mainly from the transitional zone between the lower crust and upper mantle. The δD compositions of fluid inclusions in Daping gold deposit are − 60.0‰ to − 85‰, with an average of − 74.5‰, whereas δ 18O H2O are 2.39‰ to 7.59‰, averaging 5.68‰, indicating that the ore-forming fluids consist predominantly of metamorphic fluid, with contributions from mantle-derived primary magmatic fluids. The δ 13C compositions of CO 2 in fluid inclusions of Daping gold deposit lie between − 3‰ to − 6.5‰, suggesting that most of the CO 2 in the ore-forming fluids was mantle-derived, and part came from the lower crust in the Ailaoshan gold belt. In addition, primitive mantle normalized platinum group elements (PGE) patterns for the auriferous ores are similar to that of the Cenozoic lamprophyre dykes in the gold deposit, but quite different from that of the diorite host rocks, also implying that the ore-forming materials in the Daping deposit derived mainly from the lower crust or even the upper mantle. The new data imply that crust–mantle interaction may have played an important role in the mineralization of the Daping deposit. At around 33 Ma, the middle and lower crust in the Daping area suffered high-temperature and high-pressure metamorphism because of ductile deformation and upwelling upper mantle magmas. The ore-forming fluids enriched in CO 2, 3He, 20Ne and 130Xe, generated by granulite facies metamorphism and baking by upwelling upper mantle magmas, were transported to the middle to upper crust along the ductile shear zone along with highly crystallized graphite grains, and finally precipitated auriferous sulfide-quartz veins in brittle structures because of declining temperature, pressure, and subsequent boiling. Daping is a typical ductile shear zone controlled orogenic gold deposit.

POLARIZED FAR-INFRARED AND SUBMILLIMETER EMISSION FROM INTERSTELLAR DUST

Polarized far-infrared (FIR) and submillimeter emission is calculated for models of nonspherical dust grains that are constrained to reproduce the observed wavelength-dependent extinction and polarization of starlight. For emission from regions where the magnetic field is perpendicular to the line of sight, the FIR emission is expected to have substantial linear polarization at wavelengths λ ≳ 100 μm, but the degree of linear polarization, and its variation with wavelength, is model dependent. Models in which the starlight polarization is produced by both amorphous silicate and graphite grains have linear polarizations between 6% and 10% at λ>100 μm, but for some models in which only silicate grains are spheroidal, the linear polarization increases from about 3% at 100 μm to about 15% at 1 mm. We briefly discuss the implications of these results for the removal of the polarized dust emission from maps of the cosmic microwave background, as well as the possibility of discriminating among interstellar dust models based on observations of FIR and submillimeter linear polarization.

A study of frequency and size distribution dependence of extinction for astronomical silicate and graphite grains

It is generally agreed that interstellar dust grains consist of two main components, namely, silicates and graphites. Some models, like MRN model, assume these grains to be homogeneous spheres following a power-law size distribution. This paper presents, in the framework of Mie theory, a parametrization of extinction spectrum curves of the silicates and the graphites separately in terms of frequency and the minimum and maximum of sizes in the distribution. Analytic expressions in ultraviolet and far-ultraviolet are presented for both types of grains. The values of maximum and minimum sizes for which these equations are valid have been identified. These equations can be useful in a number of situations involving silicate and graphite grains.

Laboratory studies of circumstellar and interstellar materials

When the most primitive types of meteorite (chondrites) and interplanetary dust particles (IDPs) formed in the solar protoplanetary disk (solar nebula), all of them incorporated at least two types of presolar material: interstellar organic matter and circumstellar grains. The organic matter probably formed in the protosolar molecular cloud. To date, 10 types of circumstellar grain have been found and their isotopic compositions show that they formed around RGB/AGB stars, supernovae and, possibly, novae and WR stars. It has been proposed that interstellar silicates are also present in IDPs, but the evidence for this is less compelling. The range of isotopic compositions exhibited by the circumstellar grains require a minimum of 35–40 stellar sources, suggesting that they are a fairly representative sample of circumstellar dust production in the Galaxy. In general, the relative abundances of circumstellar grains from different sources are similar to what is expected from astronomical estimates provided that the contribution from supernovae is relatively small. The one exception is graphite grains from AGB stars that are highly depleted. Collisional erosion, radiation damage and sputtering are all expected to affect grains in the ISM on relatively short timescales. Yet, for reasons that are unclear, examination of the circumstellar grains has found little evidence for any of these processes. The circumstellar grains also provide information about grain nucleation and growth in stellar winds. For instance, only graphite grains from AGB stars seem to have formed by heterogeneous nucleation. Graphite and SiC grains from AGB stars as large as 10–20 µm have been found in meteorites, probably requiring regions of high gas density rather than the usual assumption of uniformly expanding outflows. However, few graphite and SiC grains have similar C isotopic compositions, suggesting that they tend to form at different stages of AGB evolution.

SODIUM-22 FROM SUPERNOVAE: A METEORITE CONNECTION

There are several meteoritic noble gas components whose isotopic abundances are distinctly different from the solar abundance. One of these components, Ne-E(L), is a 22Ne-rich component carried by presolar graphite. Neon-22 is dominantly from the radiogenic decay of 22Na (T 1/2 = 2.6 a), but it also contains 22Ne produced by 14N(α, γ)18F(e +ν)18O(α, γ)22Ne. The proportions of 22Ne from the two different origins vary with graphite density. Many low-density graphite grains (1.65-1.72 g cm–3) exhibit 18O and 28Si excesses, signatures of supernova origin. We revisited noble gas data on bulk (=aggregates) and single-grain analyses of presolar graphite to better understand the origin of Ne-E(L). We conclude that essentially all 22Ne in low-density graphite grains originated from 22Na produced in the O/Ne zone in supernovae. The initial presence of 22Na in these grains indicates that grains formed within a few years after the supernova explosion. The absence of implanted 22Ne can be explained if the grains and the gas in the ejecta became decoupled when they encountered the reverse shock and sputtering eroded the grains in the outer H-rich zone and in the interstellar medium.

Complex texture and structure of shocked quartz mineral with graphite grains

Complex texture and structure of shocked quartz mineral with graphite grains

New Stellar Sources for High‐Density, Presolar Graphite Grains

WepresentC,N,O,Si,Al-Mg,K,Ca,andTiisotopicanalyses ofsevenhigh-density(ORG1f, � � 2:02 2:04 gcm � 3 ) graphite grains from Orgueil with 12 C/ 13 C ratios smaller than 20. The presence of 44Ti in three of these grains indicates an origin in Type II supernovae (SNe). The 13 C excesses in these SNe grains, however, remain enigmatic. The remaining grains have extremely large Ca and Ti isotopic anomalies, some of which are much larger than those predicted for envelopes of asymptotic giant branch (AGB) stars. These anomalies in conjunction with low 12 C/ 13 C ratios can only be explained by pure nucleosynthetic He-shell components of AGB stars. Born-again AGB stars that experience a late He flash are able to explain the low 12 C/ 13 C ratios of some of the grains along with the presence of extreme enrichments in the Ca and Ti isotopes. This study indicates that high-density graphite grains havemultiple stellar sources: SNe and born-again AGB stars, in addition to the previously established low-metallicity AGB stars. Subject headingg dust, extinction — meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB — supernovae: general