Graphite Grains Research Articles

Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains

We present a method for calculating the infrared emission from a population of dust grains heated by starlight, including very small grains for which stochastic heating by starlight photons results in high transients. Because state-to-state transition rates are generally unavailable for complex molecules, we consider model PAH, graphitic, and silicate grains with realistic vibrational mode spectra and realistic radiative properties. The vibrational density of states is used in a statistical-mechanical description of the emission process. Unlike previous treatments, our approach fully incorporates multiphoton heating effects, important for large grains or strong radiation fields. We discuss how the temperature of the grain is related to its vibrational energy. By comparing with an exact statistical calculation of the emission process, we determine the conditions under which the thermal and the continuous cooling approximations can be used to calculate the emission spectrum. We present results for the infrared emission spectra of PAH grains of various sizes heated by starlight. We show how the relative strengths of the 6.2, 7.7, and 11.3um features depend on grain size, starlight spectrum and intensity, and grain charging conditions. We show results for grains in the cold neutral medium, warm ionized medium, and representative conditions in photodissociation regions. Our model results are compared to observed ratios of emission features for reflection nebulae and photodissociation regions, the Milky Way, normal spiral galaxies, and starburst galaxies.

XPS, Raman spectroscopy, X‐ray diffraction, specular X‐ray reflectivity, transmission electron microscopy and elastic recoil detection analysis of emissive carbon film characterization

AbstractFor a better understanding of the physical and electronic properties of emissive carbon films, one of the best ways is to compare the results obtained with several surface and structural analysis techniques. In this article, different types of carbon film depositions for developing large flat panel displays by field emission displays are analysed and the results are correlated with their emissivity. Pulse laser ablation films, high‐temperature plasma‐enhanced chemical vapour deposition (PECVD) films and low‐temperature PECVD films are characterized by XPS, Raman spectroscopy, X‐ray diffraction (XRD), specular X‐ray reflectivity, transmission electron microscopy (TEM) and elastic recoil detection analysis (ERDA). The analyses lead us to conclude that the sp2/sp3 ratio is not a crucial parameter for carbon film emissivity. Crystalline structure seems more important. The presence of graphite grains is essential for good and uniform emission. Combination of XPS, TEM, XRD, Raman spectroscopy and ERDA is necessary for the study of carbon film emission. Copyright © 2001 John Wiley & Sons, Ltd.

Consequences of size-effect and spectral inhomogenity on Raman and photoluminescence spectra of a-C:H

The size of crystals in the nanometer regime limits the precision of their location in the inverse space, and hence, the precision of the physical quantities, which are related to the inverse space. This process can be described as multiplication (masking) of the infinite chain with a pulse function, which represents the size of the crystallite. This multiplication leads to a convolution in the inverse space. The broadening below some sizes is larger than the homogeneous spectral broadening in the given crystal. This extra broadening is spectrally inhomogeneous, which is spectacularly demonstrated by the ‘hole burning’ in non-crystalline condensed matter. The visible photoluminescence of hydrogenated amorphous carbon shows this inhomogeneously broadened feature. The Raman spectrum of graphitic grains should also be interpreted on this basis. The normal Raman D band broadens as a consequence of the broadening of the phonon modes and should be narrower than the G band, which, being a resonant Raman process, is broadened by the separate broadening of the π and the π* energy levels, and the further broadening of the phonon mode, which also broadened the G band. The broader G band is a generally accepted experimental fact.

The effect of postgrowth ion irradiation on the microstructure and the interface properties of amorphous carbon films on silicon

We studied the effect of postgrowth low energy (1.5 keV) Ar+ ion irradiation of various amorphous carbon (a-C) films with different microstructure and bonding. Detailed x-ray diffraction was used to identify the existing phases in thin (30–200 nm) a-C films, deposited on Si and subjected to Ar+ ion irradiation and to study the mechanisms of formation of diamond, lonsdaleite (diamond-2H), silicon carbide, and graphite grains. The relative content of each crystalline phase, the grain size, and distribution depend strongly on the microstructure of the as-grown films that is affected by the bias voltage applied during deposition of the films. Ion bombardment promoted the formation of graphite throughout the films and diamond and SiC grains close to the a-C/Si interface. Diamond and SiC nucleation at the a-C/Si interface area occurs beyond the penetration depth of the Ar+ ions and therefore is not attributed to a direct, ballistic ion–solid interaction but rather to a mechanism of charge transfer through conductivity channels. The latter is also considered and discussed in terms of other features of the as-grown a-C films such as density, voids content, electrical resistivity, and surface and interface morphology.

Brittle destruction of carbon based materials under severe thermal loads

To evaluate the dust formation under severe surface heat loads, 120 keV electron beam pulses have been applied to different specimens of carbon based materials, especially fine and coarse grain graphites, CFCs, and pyrolytic carbon. The incident heat load was up to 1.5 GW m −2 for pulse durations of 0.1–2 s. The eroded material was collected by catcher probes or within a volumetric trap. The evaluation of the probe and trap surfaces by means of high resolution scanning electron microscopy (SEM) and image analysis gave information on the size distribution of the eroded particles. A fraction of the particles detected from heated fine grain graphite had sizes of the order of the graphite grains itself (several ten microns), and a considerable amount of material was deposited on the probes in form of sub-μm particles. The structure of these particles was examined by TEM. Furthermore, the morphology of electron beam generated carbon dust particles was compared to carbon fragments collected in tokamak facilities.

Crystallography and structural evolution of cubic boron nitride films during bias sputter deposition

The mechanism and the crystallography of the growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrates by radio-frequency bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy. Particular attention has been paid to the atomic structure of graphitic (sp 2-bonded) BN and grain boundaries in the c-BN films. The c-BN films grow in the sequence of amorphous boron nitride (a-BN), turbostratic boron nitride (t-BN) and c-BN layers, similar to previous results. The sp 2-bonded BN material consists of small regions, 1–5 nm thick, forming in a layered manner parallel to the substrate surface. Each region consists of parallel lamellae in both the hexagonal and rhombohedral (h-BN and r-BN, respectively) configurations. Three orientation relationships between h-BN and r-BN phases were determined: OR-1: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (0001) h-BN‖(0001) r-BN; OR-2: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 1 ̄ ) r-BN ; OR-3: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 2) r-BN . The r-BN crystallites within the sp 2-bonded BN grow preferentially in such a way that the (01 1 ̄ 2) r-BN ‖(01 1 ̄ 1 ̄ ) r−BN plane is almost parallel to the grain boundary. Twinning about both the basal planes and the {01 1 ̄ 1} r-BN planes is common within r-BN phase. The c-BN crystallites adjacent to the graphitic BN layer are highly twinned, the {111} c-BN twin planes being parallel to the basal planes of the sp 2-bonded BN. The c-BN phase nucleates on the oriented graphitic BN layer in a semicoherent manner and obeys specific orientation relationships with the hexagonal and rhombohedral phases, namely: OR-I: [ 2 ̄ 110] r-BN ‖[110] c-BN OR-II: [ 2 ̄ 110] r-BN ‖[ 1 ̄ 1 ̄ 0] c-BN (01 1 ̄ 1 ̄ ) r-BN ‖( 1 ̄ 11) c-BN ( interface plane) (01 1 ̄ 2) r-BN ‖(1 1 ̄ 1 ̄ ) c-BN ( interface plane) (0001) r-BN ≈‖(1 1 ̄ 1) c-BN (0001) r-BN ≈‖(1 1 ̄ 1) c-BN They are related by a rotation of 180° about their common axis, [0001] r-BN ‖[1 1 ̄ 1] c-BN , [2 1 ̄ 1 ̄ 0] h-BN ‖[110] c-BN (01 1 ̄ 2) h-BN ‖(001) c-BN (0001) h-BN ‖(1 1 ̄ 1) c-BN In the mainly c-BN region of the films, twins are present about more than one of the sets of {111} c-BN planes. The intrinsic microstructure of the c-BN films and the crystallography between the cubic and graphitic BN phases show that the structure of cubic boron nitride is directly related to the structure of the precursor phases. The atomic structure of an interface depends on the orientation relationship between adjacent c-BN grains and the boundary inclination. The grain boundaries consist of twin boundaries when two adjacent grains are oriented close to the [110] c-BN zone axis and the boundary plane is parallel to the {111} c-BN close-packed planes of both grains. However, a thin layer, 1–2 nm, of sp 2-bonded BN forms between the c-BN grains when the boundary plane inclines a few degrees from the {111} c-BN planes of the adjacent grains.

Infrared Properties of Molecular Cirrus. II. Cloud‐to‐Cloud Variations in Graphite and Polycyclic Aromatic Hydrocarbon Content

We have conducted a photometric survey of translucent molecular clouds in the four IRAS wave bands. We find notable cloud-to-cloud variations in mid-IR emission, with the ratio I12/I25 varying by up to 1 order of magnitude and often above unity. Because the clouds in our sample are nearby and translucent, the cloud heating is well constrained, so that the observed differences in infrared emission must represent differences in grain content. We show from first principles that a dust model containing only two components, Mathis-Rumpl-Nordsieck (MRN) grains in thermal equilibrium and very small grains undergoing stochastic heating, can never produce I12/I25 ≥ 1 under the conditions that prevail in the cloudy interstellar medium. We fit the clouds in this sample with a three-component model which contains continuum emission from both MRN grains and very small graphite grains, plus emission features from a mixture of polycyclic aromatic hydrocarbon (PAH). The cloud-to-cloud variations in mid-IR emission require significant differences in their composition of small grains and PAH molecules. Models which describe the formation and evolution of these particles in the interstellar medium must explain the presence of these variations in a cloud sample which is located in the solar neighborhood.

Dust properties in NGC 6611

An investigation on dust properties in the NW portion of NGC 6611 has been conducted polarimetrically to test the existence of abnormal extinction in that section of the open cluster in an independent way. As previously suggested for the η Carinae nebula by Tapia et al. (1988b) and subsequently confirmed by Marraco et al. (1993), the canonical relation between and (the wavelength of maximun interstellar polarization) is not valid for stars belonging to dusty H ii regions, as is the case for the observed portion of M 16. This may arise mainly from the presence of silicate grains of a slightly larger size than the standard ISM and also from a considerable increase in mean graphite grain size, according to previous results from Chini & Wargau (1990). About 50% of the observed stars in NGC 6611 present indications of intrinsic polarization in their measurements; a similar percentage was found in IC 2944 (Vega et al. 1994), another young open cluster in close relation with an H ii region.

Presolar dust grains from meteorites and their stellar sources

Primitive meteorites contain small concentrations (ppb to ppm) of presolar dust grains that have survived largely unaltered the processes that led to the formation of the solar system. Minerals identified to date include diamond, silicon carbide (SiC), graphite, silicon nitride (Si3N4), corundum (Al2O3), spinel (MgAl2O4), and possibly titanium oxide (TiO2). These grains exhibit large isotopic anomalies indicative of a stellar origin. Variations in the isotopic ratios of the major elements and of many trace elements contained in the grains range over more than 4 orders of magnitude. The presolar dust grains preserve memories of both nucleosynthesis in the parent stars and galactic chemical evolution. Most of the silicon carbide and corundum grains formed in the winds of red giant and asymptotic giant branch stars. Most graphite grains, some SiC and corundum grains, and all silicon nitride grains originated most likely in supernova ejecta. A few SiC and graphite grains appear to have a nova origin. The origin of the diamonds is still unknown but at least a small fraction apparently comes from supernovae. Diamonds are only 2 nm in size. The other types of presolar grains are larger and range from ≈ 0.2 to 20 μm. These sizes are larger than those inferred for dust in the interstellar medium but are comparable to the sizes of interstellar dust in the heliosphere identified by the Gallileo and Ulysses spacecraft missions.

Titanium carbide nanocrystals in circumstellar environments.

Meteorites contain micrometer-sized graphite grains with embedded titanium carbide grains. Although isotopic analysis identifies asymptotic giant branch stars as the birth sites of these grains, there is no direct observational identification of these grains in astronomical sources. We report that infrared wavelength spectra of gas-phase titanium carbide nanocrystals derived in the laboratory show a prominent feature at a wavelength of 20.1 micrometers, which compares well to a similar feature in observed spectra of postasymptotic giant branch stars. It is concluded that titanium carbide forms during a short (approximately 100 years) phase of catastrophic mass loss (>0.001 solar masses per year) in dying, low-mass stars.

Selection effects on interstellar dust in heliosphere

Detailed studies of the selection effects at work on interstellar dust grains during their entry into the heliosphere are important deducing the conditions of grains in the very local interstellar medium. In particular we study the influence of Lorentz and radiation pressure forces on the interstellar dust flux. Lorentz forces acting on grains selectively deplete smaller sized grains from their original flow near the heliopause and in the heliosphere, irrespective of dust composition. If interstellar grains in the very local interstellar medium are composite fluffy agglomerates as proposed for grains in the diffuse interstellar medium, then the radiation pressure force is sufficiently small that the size distribution and material composition can be retrieved from in situ measurements in the solar system. Although grains that consist only of silicate are scarcely influenced by radiation pressure, the flux of bare graphite grains is expected to decrease by radiation pressure in contrast to the constant flux derived from in situ measurements of grains in the heliosphere. It is unlikely that interstellar grains are destroyed by field emission or sputtering, even though the grains pass through shocks to enter the heliosphere, and by mutual collisions or collisions with interplanetary dust particles.

Deflection of the local interstellar dust flow by solar radiation pressure.

Interstellar dust grains intercepted by the dust detectors on the Ulysses and Galileo spacecrafts at heliocentric distances from 2 to 4 astronomical units show a deficit of grains with masses from 1 x 10(-17) to 3 x 10(-16) kilograms relative to grains intercepted outside 4 astronomical units. To divert grains out of the 2- to 4-astronomical unit region, the solar radiation pressure must be 1.4 to 1.8 times the force of solar gravity. These figures are consistent with the optical properties of spherical or elongated grains that consist of astronomical silicates or organic refractory material. Pure graphite grains with diameters of 0.2 to 0.4 micrometer experience a solar radiation pressure force as much as twice the force of solar gravity.

Particle Shapes and the 2175 A Feature

The Galactic interstellar extinction curve exhibits a pronounced feature at 2175 ?. The most striking characteristic of this feature is the relatively fixed nature of its central wavelength, while the width of the feature may vary. Small graphite particles are often proposed as the carrier of this UV bump. However, graphite grains of a single size and shape cannot reproduce the observed width and peak characteristics simultaneously. Here, we investigate to what extent these characteristics may be reproduced by invoking a shape distribution of prolate and oblate graphite particles. The graphite constants calculated by Draine and Lee do not yield the narrowest 2175 ? features that have been observed. We have recalculated the dielectric function of astronomical graphite such that a spherical grain of radius 50 ? produces a feature with a peak at x0 = 4.6 ?m-1 and a width ? = 0.8 ?m-1. Using grains in the Rayleigh approximation, we have calculated extinction curves for combinations of prolate, spherical, and oblate grain shapes and successfully reproduce the stationary nature of the peak. In addition, such shape variations do give a standard deviation in the width of the feature of ~1/2 the observed standard deviation. However, the absolute values of the calculated widths fall short by ~10% compared with observed widths. The observed lack of correlation between peak position and width is not reproduced by the shape variations considered here.

Formation of vibrationally excited hydrogen molecules on a graphite surface

We study the recombination of hydrogen atoms on graphite grains in the limits of strong and weak interactions using a trajectory procedure. In the limit of strong gas–surface interaction dominated by chemisorption energies, the total reaction cross-section is 0.375 Å2 and the vibrational energies of H2(g) distribute between v=0 and 3. When the interaction is a Van der Waals type, σ is found to be as large as 17.4 Å2 and in this weak interaction limit, most of the reaction exothermicity is carried away by the vibrational motion of H2(g) producing strong excitation.

Temperature dependence of the passivation layer on graphite

The elevated temperature stability of the Solid Electrolyte Interface (SEI) formed on graphite during the first charge/discharge cycle has been investigated. This was done in order to determine its role in the high-temperature degradation process which occurs in a C/LiMn2O4 Li-ion cell. X-ray photoelectron spectroscopy (XPS) is used to probe the surface-layer growth and elemental composition of graphite electrodes exposed to different thermal treatment. The surfaces of cycled electrodes, when stored below 60°C, were seen to resemble closely those of unstored electrodes. An electrode stored at 60°C exhibited a significant increase in the amount of oxidized carbon and oxygen. Analysis by Ar sputtering suggests that a thick `macroscopic' layer coats the graphite electrode surface, but not the separate graphite grains; this is consistent with the observed rapid decrease in capacity and large cell-resistance on cycling such cells after storage. This capacity decrease was not observed for the cells stored at RT and 40°C.

New insights into carbon fluid chemistry and graphite precipitation: SIMS analysis of granulite facies graphite from Ponmudi, South India

Slow graphite carbon self-diffusion rates and a strong dependence of 13C/12C fractionations on fluid chemistry can be combined with secondary ion mass spectrometry (SIMS) analyses of 13C/12C of graphite to fingerprint graphite precipitation mechanisms and carbon chemistry of crustal fluids. The δ13C variations in single graphite grains in granulite facies rocks from Ponmudi, South India approach 10‰ and are interpreted to reflect three distinct generations of graphite formation and fluid infiltration, illustrating the potential of SIMS microdomain graphite analyses for deciphering crustal fluid influx histories. Our analyses provide insight into a longstanding debate in crustal metamorphism of South India, indicating that the fluid–rock interaction history included early, middle, and late interactions with C–O–H fluids.

Reaction of a Si(100) surface with a hot C 2H 4 beam

A clean Si(100) surface at 670°C in ultrahigh vacuum was irradiated with a C 2H 4 molecular beam produced from a nozzle at 900°C. The reaction products on the surface were investigated using X-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning electron microscopy. The central area of the surface irradiated with the beam was covered with a mixture of graphitic film and SiC grains. At the fringe of the central area, however, the SiC grains were the dominant products, which were epitaxially grown on the Si(100) surface. The density of the SiC grains decreased with the distance from the fringe. Compared with the results using the molecular beam produced from the nozzle at room temperature, it is concluded that the dissociation rate of the hot C 2H 4 molecules is higher than that of the cold molecules, resulting in a carbon film. The growth mechanisms of the graphitic and SiC products are discussed.

Effects of mixing on extinction by carbonaceous particles

Reported values for the absorption cross section of particulate carbon per unit mass range from under 4 to over 20 m2/g, and the intermediate value of 10 m2/g is used by many as a standard gram‐specific absorption cross section for atmospheric soot. In order to better understand the possible variations in absorption by atmospheric carbon, we reevaluated its optical properties in terms of the material composition and morphology of soot and the electrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to low‐density air/graphite volume mixtures are considered. The effects on extinction efficiency of aggregation and of internal mixing of carbon with sulfate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several homogeneous mixing rules (effective medium approximations), On the basis of our modeling of the optical properties of aggregates of graphitic carbon grains, we conclude that 10 m2/g may be over 50% too high in many cases, and we suggest that the mass absorption coefficient for the light‐absorbing carbon in diesel soot at a wavelength of 0.550 μm may often be less than 7 m2/g, although variations in optical constants and, especially, the specific gravity of the absorbing material make it difficult to assign a specific numerical value. Adhesion of carbon grains to sulfate droplet surfaces is expected to enhance their absorption by no more than about 30%. Soot randomly positioned within droplets, however, can display averaged absorption enhancement factors of about 2.5–4 for hosts with refractive indices ranging from 1.33–1.53, respectively, and radii ≳0.20 μm. Nonetheless, calculations indicate that for realistic dry particle populations, αa < 10 m2/g for graphitic carbon in the atmosphere unless (1) most of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 μm (which corresponds to a mass median diameter of 0.34 μm). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment when ascribing characteristic values for their absorption and scattering efficiencies.

Topography and structure of C60/C70+Ni film containing carbon nanotubes grown perpendicularly to the substrate

The structural changes occurring during the vacuum annealing, accompanied by Ni in C60/C70+Ni layers were studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), electron diffraction (ED) and Raman spectroscopy (RS). The layers were prepared by thermal deposition method and contained 2wt% of Ni. Obtained layers were annealed under the pressure of 10-5Torr at the temperatures of 770–820K. The initial layers showed mostly amorphous fullerenes structure what was confirmed by TEM, ED and Raman spectroscopy. The hillock-like topography of surface (hillock base diameter ∼750nm) was observed by AFM. No Ni or graphite crystalline structure were observed for initial layers. Annealed layers were composed of nanotubes with cylindrical shape, fullerenes and graphite grains what was confirmed by TEM and ED. In the Raman spectra, except of distorted C60 molecule vibrational band (broaden and shifted Ag(2) vibrational mode), the bands connected to vibrational mode of nanotubes (1565cm-1) and graphite (1585, 1620cm-1) were observed. AFM images exhibited the existence of hillocks (diameter in the base ∼300nm) surrounded by needle-like objects (diameter 20–30nm) protruding from the substrate.

Modeling gray iron solidification microstructure for prediction of mechanical properties

In the present paper the influence of different microstructure parameters on the tensile strength of gray iron is shown. It can be concluded that the volume fraction of primary solidified phase and the specific phase interface of free graphite have a considerable influence on the tensile strength. However, it is difficult to get a unique relationship between the microstructure and mechanical properties under different chemical compositions, nucleation and cooling conditions.A mathematical model is presented for the simulation of cooling and solidification of gray iron. This includes the heat conduction from the solidifying alloy into the mould and the solidification process. Here, the fraction of primary dendrites is calculated by the Scheil-Equation. During the eutectic crystallization, nucleation and grain growth processes are considered. The development of graphite lamellae morphology inside eutectic grains has been included into the model. Microstructure parameters, which have been calculated for different chemical compositions, nucleation and cooling conditions, are related to corresponding experimentally determined tensile strengths.