Graphite Grains Research Articles

The potential of the dust grains of comet halley

We calculate the electrostatic potential of silicate, graphite and carbon submicronic grains in the coma of comet P/Halley located at 1 AU from the Sun using the “orbit-limited” solution. The physical properties of the materials as well as the plasma parameters of the subsolar cometosheath are taken into account. We find that the potential lies between −3 and +4 volts in all cases, under likely conditions. It could be negative, between 0 and −10 volts, if the plasma would be a little more dense than usually admitted, as suggested by recent measures.

On graphite and the 2175 A extinction profile

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

Interstellar grains in meteorites

Primitive meteorites contain grains of silicon carbide, graphite and diamond formed outside the Solar System and probably before its birth. The isotopic compositions of these grains provide a record of stellar nucleosynthesis and of condensation processes near carbon stars; the fact of their survival places constraints on conditions in the solar nebula and early Solar System. The search is now on for other surviving stellar condensates, such as nitrides and oxides.

Ion induced structures and electrical conduction in implanted polymer films

A conducting-grain picture is presented, based on the idea that the ion-polymer interaction process can be simply described as a rapid dissipation of the thermal energy converted from the kinetic energy in the ion track region. Thermal dissipation and efficiency through electronic ionization and nuclear collisions are quite different. A critical transient temperature, determined by the critical energy deposition density, is necessary to activate a thermodynamic relaxation process along the ion track creating conducting graphitic grains. Our high resolution temperature-dependent dc conductivity data reveal a two component conductivity that depends on both one-dimensional variable-range hopping (VRH) and three-dimensional VRH. The relative importance of 3-D VRH conductivity over the entire conductivity seems to depend on the relative rate of the ion energy loss through electronic ionization processes.

Dust radiation in active galactic nuclei – I. Spherical distribution

We model the infrared-ultraviolet spectrum assuming that the intrinsic radiation due to an accretion disc and a spherical power-law component is reprocessed by an optically thin dust cloud surrounding the nucleus. We find that in order to explain the infrared data for bright AGN we have to assume that only graphite grains are present. This means that the dereddening of optical/ultraviolet spectra should not be done using an extinction curve for graphite/silicate standard interstellar composition. However, further studies are necessary to determine the location of the dust along our line of sight

UV spectra of graphite microparticles produced by laser vaporization

Abstract Submicronic graphitic particles produced by laser vaporization of a bulk graphite target and sublimated on LiF substrates are investigated. Transmission Electron Microscopy analysis denotes the presence of particles ranging from small clusters to submicronic length scale. Optical absorption and reflectance spectra in the wavelength range from 100 to 2000 nm show that the main contribution to the observed optical features comes from surface plasmon resonance of π-electrons and from interband transitions of σ-electrons of the graphite grains. Interferential effects of light due to the granular structure of the films are also present in transmission spectra. Energy shift of the spectral position of the plasmon resonance is observed either in transmission or in reflectance spectra. This shift is due to a size effect of the graphite particles.

Laboratory study of carbon particles and clusters

To investigate the role of pure carbon in interstellar absorption, optical spectroscopy on laboratory-produced carbon grains, C60 Buckminsterfullerene and small carbon clusters has been carried out. The results obtained so far are: (a) The UV absorption of small (diameter ca. 10 nm) carbon grains depends strongly on the atomic structure of the grain material. A fit of the laboratory data to the interstellar 217 nm feature by graphitic grains seems to be possible provided some serious experimental problems can be overcome. (b) There is so far no clear-cut coincidence between the measured spectra of neutral and ionised C60 and the known interstellar bands in the UV and IR. (c) The spectra of carbon clusters (C5–ca. C11) trapped in cryogenic matrices show great similarities to the pattern of diffuse interstellar bands in the visible range.

Low‐Cost Melt Formed Siliconized Silicon Carbide Radiant Tube Materials

This paper describes a new low‐cost, melt infiltration method for manufacturing silicon carbide–silicon (SiC─Si) and graphite–silicon (G─Si) materials for radiant tube applications, and the physical and mechanical characteristics of these materials. Characterization of the microstructures and various physical and mechanical properties of three SiC─Si and one G─Si materials which were produced by this newly developed processing method are discussed in detail. The C‐ring and O‐ring specimens tested in compression showed that at all test temperatures (room to 1350°C), the SiC─Si materials are substantially stronger than the G─Si material, which actually contained 8.5 vol% SiC due to reaction of the graphite with the molten Si during processing. The strengths as determined by the C‐ring and O‐ring tests for each of the materials were similar, thus indicating that the inherent flaws on the inside and outside surfaces of the tubes are similar in severity. Since these materials showed significant deformation when tested at and above 1000°C, the strength of the G─Si material and one of the SiC─Si materials was also measured at room and at elevated temperatures, using a tensile test procedure. Results of the tensile tests illustrate that, due to stress redistribution at 1000°C and above, the C‐ring and O‐ring data overestimate the actual fracture strengths of these materials. The fracture toughness of the G─Si material measured at room temperature and 1000°C was much less than that of the SiC─Si materials. This behavior was explained by the presence of low‐strength graphite grains, which did not inhibit crack propagation as effectively as the SiC grains.

Dust reverberation - A model for the infrared variations of Fairall 9

view Abstract Citations (74) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dust Reverberation: A Model for the Infrared Variations of Fairall 9 Barvainis, Richard Abstract Over a period of 10 yr starting in 1978, the ultraviolet flux from the quasar Fairall 9 was observed by Clavel, Wamsteker, & Glass to have varied by over a factor of 30. The infrared varied as well, with light curves very similar in shape but evincing a delay of about 1 yr relative to the UV. Clavel et al. suggested that the data were consistent with infrared reradiation of the UV light by dust. In this paper a detailed model is described in which dust heated by the central optical/UV source is responsible for the infrared emission from Fairall 9. Fits to the light curves at the four measured infrared wavelengths (the J, H, K, and L bands) are presented, which match both the spectral and temporal behavior of the infrared light. Good fits are obtained if the emitting grains are located in clouds where the grains are able to either survive episodes of high UV flux, or reform on a rapid time scale (~months) following such episodes. A detailed treatment is given of the emission from clouds of significant UV and infrared optical depth, illuminated on one face by a central UV continuum source. Using typical grain emissivity laws, inferred sublimation temperatures for the grains lie in the range 1300-2000 K, depending on the assumed opacity of the clouds. This range is consistent with either graphite or silicate grains, or some combination of the two. Because of the nature of the variations in Fairall 9, the model is able to distinguish whether the dust lies in a spherical or disklike geometry. The radial distribution can be constrained, however, and it appears that clouds directly exposed to the UV exist at distances ranging from <~ 0.3 lt-yr to about 1.3 lt-yr. The variable part of the optical/UV continuum appears to have a flat spectrum extending into the near- infrared, suggesting an optically thin emission mechanism rather than emission from an optically thick accretion disk. Publication: The Astrophysical Journal Pub Date: December 1992 DOI: 10.1086/172012 Bibcode: 1992ApJ...400..502B Keywords: Active Galactic Nuclei; Cosmic Dust; Infrared Radiation; Interstellar Extinction; Light Curve; Quasars; Accretion Disks; Emission Spectra; Ultraviolet Astronomy; Astrophysics; INFRARED: GALAXIES; GALAXIES: QUASARS: INDIVIDUAL NAME: FAIRALL 9; ISM: DUST; EXTINCTION full text sources ADS | data products SIMBAD (3) NED (1) MAST (1)

Interstellar dust from the Milky Way to the Magellanic Clouds

view Abstract Citations (917) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Dust from the Milky Way to the Magellanic Clouds Pei, Yichuan C. Abstract Interstellar dust in the Magellanic Clouds, with a weak or nearly absent 2175 A extinction feature, may be of interest in studies of galaxies in early stages of chemical evolution. To this inquiry, we extend the graphite-silicate grain model, introduced by Mathis, Rumpl, & Nordsieck and developed by Draine & Lee, from the Milky Way to the Magellanic Clouds. We find that the empirical extinction curves in the Large and Small Magellanic Clouds can be reproduced by adjusting only the relative abundances of graphite and silicate grains, while leaving all other model properties fixed to those appropriate for the Galactic extinction curve. Using the graphite-silicate models, we calculate the absorption and scattering optical depths, the mass-density ratio of interstellar dust to neutral hydrogen, and the Kramers-Kronig relation for all three galaxies. We also present a fitting function for the three extinction curves, valid not only over the observed range of wavelengths but also over the full range as predicted by the models. All the quantities we derived here are independent of the dust-to-gas ratios in the Milky Way and the Magellanic Clouds, and can be applied to other galaxies if they contain Galactic or Magellanic-type dust. Publication: The Astrophysical Journal Pub Date: August 1992 DOI: 10.1086/171637 Bibcode: 1992ApJ...395..130P Keywords: Cosmic Dust; Intergalactic Media; Interstellar Extinction; Interstellar Matter; Magellanic Clouds; Milky Way Galaxy; Chemical Evolution; Far Ultraviolet Radiation; Kramers-Kronig Formula; Astrophysics; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: INTERSTELLAR MATTER; GALAXIES: MAGELLANIC CLOUDS; ISM: DUST; EXTINCTION full text sources ADS | data products SIMBAD (3) NED (2)

Rate constants for thermal cesium ion desorption on clean and graphite-covered iridium studied by laser-induced desorption

Desorption of cesium ions from clean and graphite-covered polycrystalline iridium was studied at surface temperatures of 600–910 K. Cs was adsorbed on the surface and the population decay due to thermal desorption was followed to obtain desorption rate constants. The surface population of Cs at different times was probed by laser-induced desorption, which gave a signal of laser-induced desorbed ions proportional to the surface concentration. With this method ionic rate constants in the range 10−5−10−1s−1 were obtained. The desorption energy of Cs+ on Ir was found to be 2.07±0.16 eV, in agreement with previous experimental results. On graphite-covered Ir three parallel desorption modes were found, all with a desorption energy of 1.8 eV, but with frequency factors of 7 × 108, 9 × 109 and 2 × 1011 s−1, respectively. The highest frequency factor corresponds to desorption from a state with a desorption energy of 1.8 eV on the graphite layer. The two lower factors are due to Cs at graphite grain bounderies, and between the graphite layer and the Ir surface. Diffusion of these populations to the state on top of the graphite layer, followed by desorption, is concluded to be the mechanism behind the observed results.

Effect of Fillet Rolling at the Root of the Tooth on Bending Fatigue Strength of Spur Gears. ADI, S45C Gears.

In order to obtain higher bending fatigue strength of gear teeth required to cope with severe driving conditions of gears, shot peening is widely used. It is a very effective method to improve the bending fatigue strength as is well known, but it also roughers the tooth surface, resulting in lowered tooth surface strength. To overcome this weak point, fillet rolling at the root of the tooth is proposed. The surfaces at the roots of S45C gears and austempered ductile iron (ADI) gears are markedly work hardened by this method. Bending fatigue strength of fillet-rolled gears is investigated compared with that of unrolled gears. Bending fatigue strength of S45C gears is improved by up to 33%, but that of ADI gears is lowered by about 9% due to microcracks connecting neighboring graphite grains just below the surface of the tooth face, produced in fillet rolling. Bending fatigue strength of ADI gears, where a 3-mm-thick layer is removed from both side edges by turning, is improved by about 16%.

Grooving on SiC Ceramics Surface by Reaction with Solid Metal (Part 2)

Chemical reaction between SiC ceramics and Ni or Fe foil has been used for making grooves on the sliding face of SiC ceramics. To accomplish the grooving process, the grooved face of SiC ceramics should be crack free and smooth. The reaction product layer between SiC ceramics and Ni foil was composed of deposited graphite, δ-Ni2Si and Ni3Si2. Generation of cracks in SiC ceramics was not dependent on the product species, but dependent on the size of deposited graphite. Larger graphite grains more deteriorated reaction layers, and generation of cracks in SiC ceramics was prevented. Reaction between SiC ceramics and Fe foil yielded graphite, Fe5Si3 and Fe3Si. Extended reaction vanished Fe3Si and generated no cracks in SiC ceramics. The thickness of SiC ceramics consumed was calculated from the thickness of Ni or Fe foil and the product species and was shown to coincide with the observed value.

Modeling of plasma hydrogen isotope behavior in porous materials (graphites/carbon—carbon composites)

This paper describes the physical and the theoretical fundamentals of the model TIPO (Tritium Inventory in Porous Materials) developed primarily to analyze tritium retention in graphites and porous carbon-carbon composites proposed for plasma-facing applications in next-step fusion devices. A key improvement of the model, compared with those already available in literature, consists of a better characterization of the processes in the complex bulk microstructure typical of porous graphites. The model treats both the near-surface and the bulk regions, and the main emphasis is on the mechanisms of retention beyond the implantation range such as migration of hydrogen isotopes along the interconnected pores, chemisorption on the pore surfaces, and diffusion across the graphite grains. The governing equations are formulated together with the main assumptions adopted to develop the computational algorithm. To assess its predictive capabilities and to identify the ranges of potential applicability, the model has been calibrated against fundamental experimental results and applied to make predictions of the tritium inventory in the armors of plasma-facing components of NET/ITER fusion devices. The results of these analyses are herein presented and discussed.

Cometary constraints on the planet forming environment

Molecular, elemental and isotopic abundances of comets provide sensitive diagnostics for models of the primitive solar nebula. New measurements of the N 2 NH and NH 2 abundances in comets together with the in situ Giotto mass spectrometer and dust analyzer data provide new constraints for models of the comet forming environment in the solar nebula. An inventory of nitrogen-containing species in comet Halley indicates that NH 3 and CN are the dominant N carriers observed in the coma gas. The elemental nitrogen abundance in the gas component of the coma is found to be depleted by a factor ∼75 relative to the solar photosphere. Combined with the Giotto dust analyzer results for the coma dust component, we find for comet Halley N gas + dust ∼ 1 6 the solar value. The measurement of the CN carbon isotope ratio from the bulk coma gas and dust in comet Halley indicates a significantly lower value, 12 C 13 C = 65 ± 9 than the solar system value of 89 ± 2. Because the dominant CN carrier species in comets remains unidentified, it is not yet possible to attribute the low isotope ratio predominantly to the bulk gas or dust components. The large chemical and isotopic inhomogeneities discovered in the Halley dust particles on 1μ scales are indicative of preserved circumstellar grains which survived processing in the interstellar clouds, and may be related to the presolar silicon carbide, diamond and graphite grains recently discovered in carbonaceous chondrites. Less than 0.1% of the bulk mass in the primitive meteorites studied consists of these cosmically important grains. A larger mass fraction (∼5%) of chemically heterogeneous organic grains is found in the nucleus of comet Halley. The isotopic anomalies discovered in the PUMA 1 Giotto data in comet Halley are probably also attributable to preserved circumstellar grains. Thus the extent of grain processing in the interstellar environment is much less than predicted by interstellar grain models, and a significant fraction of comet nuclei (∼5%) may be in the form of preserved circumstellar matter. Comet nuclei probably formed in much more benign environments than primitive meteorites.

Kinetics and mechanism of pressure reduction of nickel(II) with hydrogen from aqueous acetate solutions containing bismuth(III)

An investigation was made of the kinetics and mechanism of pressure reduction with hydrogen of aqueous nickel(II) from its acetate solution containing 30 g/dm 3 of Ni(II) and 0.1–1.5 g/dm 3 of Bi(III). Powdered graphite and tungsten carbide were used as catalysts. It was found that the stability of aqueous Bi(III) in reduced solutions was strongly limited when tungsten carbide was used. Reduction of aqueous nickel(II) from acetate solutions containing Bi(III) was found to be governed by the adsorption of bismuth species on the surface of the powdered catalyst. Bismuth(III) created substantial lengthening of the induction period of reduction of aqueous nickel(II) with hydrogen and the total inhibition of the process was observed at Bi(III) concentrations exceeding 2g/dm 3. A three-stage mechanism for the reduction of Ni(II) with hydrogen from its acetate solution containing Bi(III) and graphite powder was established involving adsorption of Bi(III) on the graphite surface, reduction of Bi(III) to metallic bismuth, and reduction of nickel(II) to metallic nickel on bismuth-coated graphite grains. Introduction of Bi(III) into feed nickel acetate solutions resulted in high quality composite powders with a compact and uniform nickel coating.

Diffusion constants of tritium in graphites and compensation effect

The diffusion constants of tritium and/or its isotopes for graphite so far reported scatter considerably in the Arrhenius plot diagram. This, however, has not been seriously considered, despite that the understanding of the scattering is indispensable to estimate the tritium inventory and permeation in/through the graphite first walls of D-T burning experimental deviees. We show in the present paper that there is regularity in the scattering: namely, the presence of a linear relation between the logarithms of the pre-exponential factors and the activation energies (the compensation effect). The temperature dependence of the diffusion constants and the compensation effect were analyzed by assuming the presence of three distinct diffusion channels in graphite: channels along the a- and c-axis in a graphite grain and along the grain boundary. Computational simulations could reproduce fairly well the temperature dependence of the experimental data and the compensation effect by assuming proper effective pre-exponential factors for the respective channels, where the activation energies for the respective channels were chosen from the observed values. Namely, the scattering of the diffusion constants and the compensation effect can be interpreted as a consequence of the multi-diffusion channels.

Size and density distribution of very small dust grains in the Barnard 5 cloud

The effects of the temperature fluctuations in small graphite grains on the energy spectrum and the IR surface brightness of an isolated dust cloud heated externally by the interstellar radiation field were investigated using a series of models based on a radiation transport computer code. This code treats self-consistently the thermal coupling between the transient heating of very small dust grains and the equilibrium heating of conventional large grains. The model results were compared with the IRAS observations of the Barnard 5 (B5) cloud, showing that the 25-micron emission of the cloud must be produced by small grains with a 6-10 A radius, which also contribute about 50 percent to the observed 12-micron emission. The remaining 12 micron flux may be produced by the polycyclic aromatic hydrocarbons. The 60-and 100-micron radiation is dominated by emission from large grains heated under equilibrium conditions.

Zodiacal emission. II - Dust near ecliptic

The location and brightness of the peak zodiacal emission are derived from IRAS observations of the diffuse infrared background radiation. A uniform ellipsoid geometrical model is fitted to the latitudes of peak brightness, from which the orientation of the surface of maximum dust density near the earth's orbit is derived. Deviations from the uniform ellipsoid indicate that this surface bends toward the invariable plane of the solar system beyond 2.5 AU from the sun. The variations of the peak zodiacal emission brightness with solar elongation and observation date allow two independent determinations of the gradient of the volume emissivity of interplanetary dust. Combined with theoretical calculations of the infrared emission from silicate and graphite grains heated by the sun, the gradient of the dust density is found to be n proportional to r exp (-1.1 + or - 0.2). Systematic brightness variations with ecliptic longitude reveal two potential asymmetries in the dust complex: excess emission following the earth in its orbit, and excess emission along lines of sight with ecliptic longitude lambda = 190 + or - 20 deg and solar lambda about 100 deg.

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

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