Silicon Carbide Grains Research Articles

Further experiments on collisional tribocharging of cosmic grains

We experimentally investigated tribocharging in bouncing dust grain collisions under conditions which are relevant for the early solar nebula. We used silica, diamond, graphite, and silicon carbide grains and cm-sized targets of aluminum, iron, and graphite. We varied particle size around 1 µm, collision velocity between 38 to 106 m s −1 , and target temperature between 240 K and 360 K. Generally, the grains acquire something in the order of hundreds or thousands of negative ele- mentary charges, which is at the upper end of the range previously discussed in astrophysical research. Mainly, a temperature difference caused by the dissipation of collisional energy or by initial temperatures determines the amount of charge transfer. It suggests a thermally diffusive charge transfer process of negative charge carriers. Nevertheless, also other processes of sec- ondary importance were effective. Material-dependent contact charging is ruled out as the dominant charge transfer process in our experiments. For most particle-target combinations, the charge transfer can be described by an empirical function of collision energy which we propose for future astrophysical applications, especially for applications on the sticking efficiency in the preplanetary dust aggregation process, on the particle motion in the solar nebula, and on the possibility of nebular lightning.

New results of presolar-grain studies and constraints on nucleosynthesis and stellar evolution

A new type of secondary ion mass spectrometer, the NanoSIMS, allows the elemental and isotopic analysis of smaller samples than could be measured previously. This instrument has been successfully applied to studies of new types of presolar grains in primitive meteorites and interplanetary dust particles (IDPs). Examples include O- and Mg-isotopic measurements of sub-μm spinel grains, the discovery of presolar silicates in IDPs and meteorites, and the Mg- and Ti-isotopic analysis of rare classes of presolar silicon carbide grains.

Detection of Silicon Nitride Particles in Extreme Carbon Stars

On the basis of spectra taken by the Infrared Space Observatory and laboratory measurements, we present the first spectroscopic evidence for the presence of silicon nitride (Si3N4) grains in the circumstellar shells of a number of extreme carbon stars. Laboratory measurements of α, β, and amorphous Si3N4 are compared with spectra of the extreme carbon stars IRAS 21318+5631 and AFGL 2477. Both stars show an absorption feature near the well-known 11 μm band of silicon carbide (SiC) but peaking at and extending to considerably shorter wavelengths. We demonstrate that the position and the band profile of the observed features coincide well with the main features of laboratory Si3N4 spectra, especially in the case of IRAS 21318+5631. While this band alternatively may be explained by SiC grains plus interstellar silicate absorption, in the way worked out by Speck and coworkers, there is further strong evidence for Si3N4 grains in IRAS 21318+5631 and possible evidence for Si3N4 grains in AFGL 2477 provided by weaker absorption features in the wavelength region longward of the main feature.

Highly oriented diamond growth on Si xGe 1− x (100) thin films

Highly oriented (100) diamond films have been successfully grown on Si x Ge 1− x (100) thin films by bias enhanced nucleation (BEN) in microwave plasma chemical vapor deposition (MPCVD) system. Raman spectra show the 1332 cm −1 peak which proves the formation of diamond. Diamond nucleation density on Si x Ge 1− x substrate estimated by scanning electron microscopy is higher than 10 9 cm −2. The interface between diamond and Si x Ge 1− x substrate was characterized by transmission electron microscopy (TEM). About 20 nm decrease in thickness of the Si x Ge 1− x film was observed after bias enhanced nucleation step. TEM shows the existence of silicon carbide and heteroepitaxial diamond grains grown on Si x Ge 1− x substrate. Characterization from high-resolution TEM on the specimen of short time deposition reveals that a number of epitaxial diamond grains were directly nucleated on Si x Ge 1− x with {111} interplanar spacing ratio of diamond and Si x Ge 1− x of 2:3. The diamond nucleation is found to be preferred on the ridge position of the rough substrate surface. Diamond {100} facets were quickly developed in the early stage of growth.

In Situ Deposition of Multiphase Carbon Composite Coating on SiC Grains Through Thermal Plasma Dissociation of SiC

A multiphase carbon coating was deposited on silicon carbide grains in situ by dissociation of the carbide in an inert gas plasma produced by a carbon arc. Characterisation of the composite coating by X-ray diffraction, SEM, EDX analysis, microRaman spectroscopy and microhardness tests has revealed that different phases of carbon such as graphite, diamond-like carbon, lonsdaleite and diamond grow simultaneously in the plasma process. The carbon coating is likely to find various applications in industry. SE/508

Extinct Technetium in Silicon Carbide Stardust Grains: Implications for Stellar Nucleosynthesis

The isotopic composition of ruthenium (Ru) in individual presolar silicon carbide (SiC) stardust grains bears the signature of s-process nucleosynthesis in asymptotic giant branch stars, plus an anomaly in 99Ru that is explained by the in situ decay of technetium isotope 99Tc in the grains. This finding, coupled with the observation of Tc spectral lines in certain stars, shows that the majority of presolar SiC grains come from low-mass asymptotic giant branch stars, and that the amount of 99Tc produced in such stars is insufficient to have left a detectable 99Ru anomaly in early solar system materials.

Processing of Presolar Grains around Post–Asymptotic Giant Branch Stars: Silicon Carbide as the Carrier of the 21 Micron Feature

Some proto–planetary nebulae (PPNs) exhibit an enigmatic feature in their infrared spectra at � 21 m. This feature is not seen in the spectra of either the precursors to PPNs, the asymptotic giant branch (AGB) stars, or the successors of PPNs, ‘‘normal’’ planetary nebulae (PNs). However, the 21 m feature has been seen in the spectra of PNs with Wolf-Rayet central stars. Therefore, the carrier of this feature is unlikely to be a transient species that only exists in the PPN phase. This feature has been attributed to various molecular and solid-state species, none of which satisfy all constraints, although titanium carbide (TiC) and polycyclic aromatic hydrocarbons (PAHs) have seemed the most viable. We present new laboratory data for silicon carbide (SiC) and show that it has a spectral feature that is a good candidate for the carrier of the 21 m feature. The SiC spectral feature appears at approximately the same wavelength (depending on the polytype/grain size) and has the same asymmetric profile as the observed astronomical feature. We suggest that processing and cooling of the SiC grains known to exist around carbon-rich AGB stars are responsible for the emergence of the enigmatic 21 m feature. The emergence of this feature in the spectra of post-AGB stars demonstrates the processing of dust due to the changing physical environments around evolving stars.

プラズマアークランプを用いた瞬間溶融による炭化珪素への高融点金属被覆

Tungsten (W) and molybdenum (Mo) were coated on silicon carbide (SiC) as refractory armor using high power plasma arc lamp at powers up to 23.5 MW/m2. Both W powder and Mo powder were melted and formed coating layers on SiC. The effect of pretreatment (vapor deposition of titanium (Ti), W and Mo and annealing) and sample heating conditions on microstructure of the coating and coating/substrate interface were investigated. The microstructure was observed by scanning electron microscopy (SEM) and optical microscopy (OM). A strong W coating was successfully formed. Tungsten vapor deposition and pre-heating at 5.2 MW/m2 made for a refractory layer containing no cracks which propagated into the SiC substrate. This layer was formed without the thick reaction layers (WC and W5Si3) reported in previous studies. Moreover the thinner interface transition layer and armor avoid coefficient of thermal expansion (CTE) mismatch driven failure previously reported. For this study, small WC grains were observed adjacent to interface. Silicon carbide grains and W5Si3 grains were observed within W coating. By contrast, Mo was not formed as well as W due to larger CTE mismatch than that for W and SiC.

A three-dimensional network model describing a non-linear composite material

A three-dimensional network model for performing non-linear time-dependent simulations of the electrical characteristics related to a composite material is presented. The considered compounds are represented by a cubic lattice and consist of conducting particles distributed in an insulating matrix. Earlier studies of the non-linear characteristics of silicon carbide (SiC) grains and of the linear frequency-dependent electrical properties of composites are combined and extended. The calculations are compared to measurements on ethylene–propylene–diene monomer rubber filled with angular SiC grains. The field-dependent conductivity measured for the unconsolidated SiC powder is used as input to the simulations. The model can manage the conductivity difference of seven decades between the constituents and the strong exponential non-linearity of the conducting particles. The network calculations replicate the experimental characteristic at high filler concentrations, where direct ‘face’ contacts between the filler grains dominate the behaviour. At lower concentrations, it is shown that indirect ‘edge’ contacts involving the polymer control the current transport also in the non-linear high field range. The general effective conductivity describing an edge connection in the linear case is no longer appropriate. Non-linear mechanisms in the polymer and the conducting grains within a field enhanced limited region around the contact need to be represented by an equivalent circuit element with a case-dependent resulting expression.

Polytype distribution of circumstellar silicon carbide

Silicon carbide (SiC) is a particularly interesting species of presolar grain because it is known to form on the order of a hundred different polytypes in the laboratory, and the formation of a particular polytype is sensitive to growth conditions. Astronomical evidence for the formation of SiC in expanding circumstellar atmospheres of asymptotic giant branch (AGB) carbon stars is provided by infrared (IR) studies. However, identification of the crystallographic structure of SiC from IR spectra is controversial. Since 95% of the presolar SiC isolated from meteorites formed around carbon stars, a determination of the structure of presolar SiC is, to first order, a direct determination of the structure of circumstellar SiC. We therefore determined the polytype distribution of presolar SiC from the Murchison CM2 carbonaceous meteorite using analytical and high-resolution transmission electron microscopy (TEM). High-resolution lattice images and electron diffrac- tion of 508 individual SiC grains demonstrate that only two polytypes are present, the cubic 3C (-SiC) polytype (79.4% of population by number) and the hexagonal 2H (-SiC) polytype (2.7%). Intergrowths of these two polytypes are relatively abundant (17.1%). No other polytypes were found. A small population of one-dimensionally disordered SiC grains (0.9%), whose high density of stacking faults precluded classifica- tion as any polytype, was also observed. The presolar origin of 2H -SiC is unambiguously established by tens-of-nanometers-resolution secondary ion mass spectroscopy (NanoSIMS). Isotopic maps of a TEM- characterized 2H -SiC grain exhibit non-solar isotopic compositions of 12 C/ 13 C 64 4 and 14 N/ 15 N 575 24. These measurements are consistent with mainstream presolar SiC thought to originate in the expanding atmospheres of AGB carbon stars. Equilibrium condensation calculations together with inferred mineral condensation sequences predict relatively low SiC condensation temperatures in carbon stars. The laboratory observed condensation temperatures of 2H and 3C SiC are generally the lowest of all SiC polytypes and fall within the predictions of the equilibrium calculations. These points account for the occurrence of only 2H and 3C polytypes of SiC in circumstellar outflows. The 2H and 3C SiC polytypes presumably condense at different radii (i.e., temperatures) in the expanding stellar atmospheres of AGB carbon stars. Copyright © 2003 Elsevier Ltd

A NanoSIMS study of Si- and Ca-Ti-isotopic compositions of presolar silicon carbide grains from supernovae

We report results from NanoSIMS isotopic measurements on 37 presolar silicon carbide grains of type X which are believed to have formed in the ejecta of supernova explosions. Isotopic data were obtained for Si and Ca-Ti (all grains), C and N (two grains), and Ti (one grain). All X grains exhibit large enrichments in 28Si (up to 5× solar), in agreement with previously studied X grains. On a scale of 200 nm, the Si-isotopic ratios do not vary by more than the analytical uncertainties of several percent in all but one X grain. This implies that most X grains formed from well-mixed regions in supernova ejecta. X grain M9-68-3 is characterized by two regions with distinct Si- and Ti-isotopic signatures which may either represent two distinct grains or overgrowth of matter from two different mixtures in the supernova ejecta. Most of the Ca in the X grains is most likely contamination as indicated by close to normal 42Ca/ 40Ca ratios. Seven X grains show enhanced 44Ca/ 40Ca ratios of up to 6× the solar ratio. Spatial distributions of 44Ca excesses and Ti are positively correlated, giving strong support to the view that excesses in 44Ca are due to the decay of radioactive 44Ti. Inferred initial 44Ti/ 48Ti ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic 44Ca is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic 44Ca and 48Ti as well as in 44Ti/ 48Ti, pointing to presence of a small Ti-rich subgrain or heterogeneous loss of Ca and Ti after grain formation. This grain has a unique Si-isotopic composition with 30Si/ 29Si = 2.2× the solar ratio and C- and N-isotopic compositions as typically observed in X grains.

Large-scale fabrication of lightweight Si/SiC ceramic composite optical mirror

Silicon carbide relative to other more traditional materials offers some excellent characteristics including high stiffness, high toughness, low toxicity, low thermal distortion and potential cost and schedule advantages. These properties make silicon carbide very attractive for a variety of applications in precision optical structures, especially when considering space-borne application. In this paper, lightweight silicon carbide mirror was prepared for the large pointer mirror or the primary mirror of the high resolution camera. Reaction-bonded silicon carbide was used to fabricate the 250-mm diameter circular mirror with 30-mm thickness and 1.72-kg weight and the 120-mm hexagonal mirror with 15-mm thickness and 0.23-kg weight. Open back honeycomb lightweight structure was produced to gain 35.1 and 24.3 kg/m 2 areal density, respectively. Polishing technique different from optical glass polishing was introduced to achieve less than 0.3 λ at 632.8-nm p– v surface error and less than 5-nm rms surface roughness. The stress and displacement under factual condition were estimated using finite element analysis method. The reaction-bonded silicon carbide was a ceramic matrix of free silicon containing a bimodal distribution of silicon carbide grains. The green body of silicon carbide grains and free carbon was fabricated by slip casting. Inserts were used to form the complex lightweight structure. During silicon vapor infiltration process, silicon reacted with carbon to form new silicon carbide phase and the pores of green body disappeared. The resulting material was single phase alpha silicon carbide. The results indicated that reaction-bonded silicon carbide can be used as optical mirror substrates. Currently, experiments are under way to fabricate a large-scale lightweight Si/SiC optical mirrors with larger than 50-cm diameter.

Comparative optical investigations of sintered and monocrystalline black and green silicon carbide (SiC)

Crystalline SiC appears in many different polytypes of cubic, hexagonal, and rhombohedral structures. These polytypes are colorless transparent or exhibit various colors evoked by doping with different elements. Dense sintered S-SiC (solid-state sintered) and LPS-SiC (liquid-phase sintered) were known in black color only, but recently a new liquid-phase sintering process was developed to achieve green LPS-SiC as well. Whereas in S-SiC the polycrystalline grains are homogeneously doped with 0.2 wt% boron, in the LPS-types the SiC grains contain up to 1.2 wt% Al, 0.3 wt% N and 0.1% O having a structure comprising a SiC(Al,N,O) mixed crystal shell and a pure SiC core. The difference in color of polycrystalline SiC bodies seems to result from small amounts of carbon in the sintered specimens (0.2–0.5 wt% C). Green sintered LPS-SiC is obtained, after free carbon has largely been removed by a suitable oxidation process prior to sintering. To get information on the various types of sintered SiC, the optical extinction and absorption spectra of black and green sintered SiC and green Acheson-SiC single crystals were quantitatively measured in the spectral range between about 1.4 and 4.1 eV. While the absorption coefficients of the single crystals vary between about 50 and 200 cm −1, the extinction coefficients of the sintered materials are between 2000 and 7000 cm −1. Nevertheless the absorption bands in the more or less transparent region of the green and black materials can easily be attributed to one another. Hence, the reason for these absorption processes must be assumed to be the same. In the same way, position and slope of the absorption edges are correlated amongst green or black SiC, irrespective of, whether the material is single crystal or sintered.

Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

The strontium, zirconium, molybdenum, and barium isotopic compositions predicted in the mass-losing envelopes of asymptotic giant branch (AGB) stars of solar metallicity and mass 1.5, 3, and 5 M☉ are discussed and compared with recent measurements in single presolar silicon carbide (SiC) grains from the Murchison meteorite. Heavy-element nucleosynthesis via the s-process occurs in the helium intershell, the region between the helium-burning and hydrogen-burning shells, producing heavy elements beyond iron. After a limited number of thermal runaways of the helium shell (thermal pulses), at the quenching of each instability, the convective envelope penetrates into the top layers of the helium intershell (third dredge-up), mixing newly synthesized 12C and s-process material to the stellar surface. Eventually, the envelope becomes carbon-rich (C ≥ O), a necessary condition for SiC grains to condense. In the helium intershell, neutrons are released by (α, n) reactions on 13C and 22Ne during interpulse phases and the thermal pulses, respectively. A 13C pocket is assumed to form in a tiny region in the top layers of the helium intershell by injection of a small amount of protons from the envelope during each third dredge-up episode. This 13C then burns radiately during the interpulse phase. The average neutron density produced is low, but of long duration, so the total neutron exposure is high. We have explored a large range of possible 13C abundances in the pocket. In low-mass AGB stars (1.5 M☉ ≤ M ≤ 4 M☉), a second small burst of neutrons is released by marginal 22Ne burning in the thermal pulse. The neutron density reaches quite a high peak value but is of short duration, so the neutron exposure is low. In intermediate-mass AGB stars (4 M☉ < M ≤ 8 M☉), the 22Ne neutron source is more efficiently activated. The neutron capture process has been followed with a postprocessing code that considers all relevant nuclei from 4He to 210Po. The predicted isotopic compositions of strontium, zirconium, molybdenum, and barium in the envelopes of low-mass AGB stars of solar metallicity are in agreement with the isotopic ratios measured in individual presolar SiC grains, whereas predictions for intermediate-mass stars exclude them as the sources of these grains. A multiplicity of low-mass AGB stars with metallicity around solar, having different masses and experiencing different neutron exposures, are required to account for the measured spread in heavy-element isotopic compositions among single presolar SiC grains. The range of neutron exposures corresponds, on average, to a lower mean neutron exposure than that required to reproduce the s-process main component in the solar system.

Three-dimensional impedance networks for modelling frequency dependent electrical properties of composite materials

A three-dimensional impedance network model for describing the frequency dependent electrical properties of composite materials has been developed. The considered compounds consist of conducting particles dispersed in an insulating matrix and the ratio between the conductivities of the two phases is around seven decades. The character of the contacts between the particles will have a critical effect on the electrical characteristics. Two different types of contacts, represented by edge and face connections, are taken into account. A sharp edge connection gives an extremely localized field enhancement. Instead of trying to resolve this rapidly diverging electric field, an equivalent circuit element that reproduces the overall behaviour of the edge contact was introduced. The distribution of the filler grains also affects the electrical properties. The large agglomerates that appear in random distributions are not representative of the well-dispersed real materials. Several methods to create a more even than random dispersion of the conducting particles were therefore utilized. Simulations were performed and compared to measurements on ethylene–propylene–diene monomer rubber filled with silicon carbide grains. The calculations reproduce the general characteristics of the experimental results.

Micron diamond composites with nanocrystalline silicon carbide bonding

Diamond composites with nanocrystalline cubic silicon carbide bonding were sintered from diamond/amorphous silicon mixtures under high pressure and high temperature (p = 5 GPa and temperatures up to 1673 K). Differential scanning calorimetry, ex situ x-ray, and Raman spectroscopy investigations showed that amorphous silicon partially transformed into nanocrystalline silicon at 873 K under 5 GPa. This was followed by the formation of nanocrystalline silicon carbide from the reaction between the silicon and diamond after silicon melting. Refinement of the x-ray diffraction patterns of composites with the Rietveld method revealed that considerable microstrain (0.3–0.5%) remained within the nanocrystalline silicon carbide grains. Small strain (0.1–0.2%) was observed in the compacted diamonds, but after the reaction they became almost strain free (&lt;0.1%). Enhanced fracture toughness was obtained for hybrid composites compared to liquid-infiltrated composites.

Silicon nitride/silicon carbide nanocomposite obtained by nitridation of SiC: fabrication and high temperature mechanical properties

Abstract Si 3 N 4 /SiC nanocomposites have been successfully synthesized according to a novel technique consisting in the nitridation of silicon carbide raw materials followed by their sintering in the presence of appropriate sintering aids. Depending on the kind of additive used, the microstructure of the final nanocomposite can be tailored from elongated silicon nitride matrix grains with silicon carbide nanoparticles dispersed into them, through completely nano-sized composites consisting of equiaxed silicon carbide and silicon nitride grains. Thermomechanical properties are related to those microstructural features and to the secondary phase composition and its subsequent crystallisation.

Microstructural Changes in Liquid‐Phase‐Sintered Silicon Carbide during Creep in an Oxidizing Environment

The knowledge of the microstructural evolution during exposure to high temperatures is important to understanding the mechanisms responsible for the creep resistance of silicon carbide (SiC) ceramics. This includes not only the phase transformation of the SiC grains, but also the phase transformations of the oxynitride grain‐boundary phases. For this study, a series of SiC specimens were prepared with varying molar ratios of AlN‐Y2O3 additives. Increased creep resistance was observed in specimens with an additive system containing a 2:3 molar ratio or 60 mol% Y2O3. A continuous oxide layer of Y2Si2O7 formed at the surface during elevated temperature testing in air. No blistering or cracking was observed in this oxide coating. Further increase of the creep resistance was achieved by a post‐sintering nitrogen anneal.

Two percolation thresholds due to geometrical effects: experimental and simulated results

The electrical properties of a mixture of ethylene–propylene–diene monomer rubber and silicon carbide (SiC) have been measured as a function of filler concentration. It was found that mixtures containing angular SiC grains have a conductivity that displays not one, but two percolation thresholds. Different types of contacts between the conducting particles, being represented by edge and face connections, respectively, can explain the phenomenon. The two percolation thresholds are obtained at volume fractions of about 0.25 and 0.40, respectively. These values are higher than those predicted by theory, which can be explained by dispersion effects with only one phase being granular and the other being continuous. The value of the conductivity at the central plateau was found to be close to the geometric mean of the limiting conductivities at low and high concentrations. This is in good agreement with theory. With rounded SiC grains only one threshold is obtained, which is consistent with only one type of contact. The concentration dependence of the conductivity was simulated using a three-dimensional impedance network model that incorporates both edge and face contacts. The double-threshold behaviour also appears in the calculations. By dispersing the conducting particles more evenly than random, the thresholds are shifted towards higher concentrations as observed in the experiments.

Evidence for Extinct Vanadium-49 in Presolar Silicon Carbide Grains from Supernovae

We report Si, Ca, and Ti-V isotopic data for seven presolar silicon carbide grains of type X separated from the Murchison CM2 meteorite. The silicon carbide X grains most likely formed in the ejecta of Type II supernova explosions as indicated by large isotopic anomalies. The X grains of this study have 29Si/28Si and 30Si/28Si ratios of 0.56-0.75 and 0.41-0.62 times the solar value, respectively. Slight enrichments in 44Ca are observed, possibly due to the decay of radioactive 44Ti (t1/2 = 60 yr). While 46Ti/48Ti, 47Ti/48Ti, and 50Ti/48Ti ratios are close to solar, all X grains have enhanced 49Ti/48Ti ratios of up to 1.9 times the solar ratio. The 49Ti/48Ti ratios are positively correlated with 51V/48Ti ratios, suggestive of the incorporation of live 49V (t1/2 = 330 days) during silicon carbide condensation. This implies that the grains formed on a timescale of several months after supernova explosion.