Silicon Carbide Grains Research Articles

SILICON CARBIDE GRAINS OF TYPE C PROVIDE EVIDENCE FOR THE PRODUCTION OF THE UNSTABLE ISOTOPE 32 Si IN SUPERNOVAE

Carbon-rich grains are observed to condense in the ejecta of recent core-collapse supernovae, within a year after the explosion. Silicon carbide grains of type X are C-rich grains with isotpic signatures of explosive supernova nucleosynthesis have been found in primitive meteorites. Much rarer silicon carbide grains of type C are a special sub-group of SiC grains from supernovae. They show peculiar abundance signatures for Si and S, isotopically heavy Si and isotopically light S, which appear to to be in disagreement with model predictions. We propose that C grains are formed mostly from C-rich stellar material exposed to lower SN shock temperatures than the more common type X grains. In this scenario, extreme $^{32}$S enrichments observed in C grains may be explained by the presence of short-lived $^{32}$Si ($\tau$$_{1/2}$ = 153 years) in the ejecta, produced by neutron capture processes starting from the stable Si isotopes. No mixing from deeper Si-rich material and/or fractionation of Si from S due to molecular chemistry is needed to explain the $^{32}$S enrichments. The abundance of $^{32}$Si in the grains can provide constraints on the neutron density reached during the supernova explosion in the C-rich He shell material. The impact of the large uncertainty of the neutron capture cross sections in the $^{32}$Si region is discussed.

EUROPIUM s -PROCESS SIGNATURE AT CLOSE-TO-SOLAR METALLICITY IN STARDUST SiC GRAINS FROM ASYMPTOTIC GIANT BRANCH STARS

Individual mainstream stardust silicon carbide (SiC) grains and a SiC-enriched bulk sample from the Murchison carbonaceous meteorite have been analyzed by the Sensitive High Resolution Ion Microprobe - Reverse Geometry (SHRIMP-RG) for Eu isotopes. The mainstream grains are believed to have condensed in the outflows of 1.5 to 3 Msun carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived from our measurements are compared with previous astronomical observations of carbon-enhanced metal-poor (CEMP) stars enriched in elements made by slow neutron captures (the s-process). Despite the difference in metallicity between the parent stars of the grains and the metal-poor stars, the fr(151Eu) values derived from our measurements agree well with fr(151Eu) values derived from astronomical observations. We have also compared the SiC data with theoretical predictions of the evolution of Eu isotopic ratios in the envelope of AGB stars. Because of the low Eu abundances in the SiC grains, the fr(151Eu) values derived from our measurements show large uncertainties, being in most cases larger than the difference between solar and predicted fr(151Eu) values. The SiC aggregate yields a fr(151Eu) value within the range observed in the single grains and provides a more precise result (fr(151Eu) = 0.54 +/- 0.03, 95 percent conf.), but is approximately 12 percent higher than current s-process predictions. The AGB models can match the SiC data if we use an improved formalism to evaluate the contribution of excited nuclear states in the calculation of the 151Sm(n,g) stellar reaction rate.

Ba isotopic compositions in stardust SiC grains from the Murchison meteorite: Insights into the stellar origins of large SiC grains

We report barium isotopic measurements in 12 large (7–58μm) stardust silicon carbide grains recovered from the Murchison carbonaceous chondrite. The C-, N-, and Si-isotopic compositions indicate that all 12 grains belong to the mainstream population and, as such, are interpreted to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. Barium isotopic analyses were carried out on the Sensitive High Resolution Ion Microprobe – Reverse Geometry (SHRIMP-RG) with combined high mass resolution and energy filtering to eliminate isobaric interferences from molecular ions. Contrary to previous measurements in small (<5μm) mainstream grains, the analyzed large SiC grains do not show the classical s-process enrichment, having near solar Ba isotopic compositions. While contamination with solar material is a common explanation for the lack of large isotopic anomalies in stardust SiC grains, particularly for these large grains which have low trace element abundances, our results are consistent with previous observations that Ba isotopic ratios are dependent on grain size. We have compared the SiC data with theoretical predictions of the evolution of Ba isotopic ratios in the envelopes of low-mass AGB stars with a range of stellar masses and metallicities. The Ba isotopic measurements obtained for large SiC grains from the LS+LU fractions are consistent with grain condensation in the envelope of very low-mass AGB stars (1.25M⊙) with close-to-solar metallicity, which suggests that conditions for growth of large SiC might be more favorable in very low-mass AGB stars during the early C-rich stages of AGB evolution or in stable structures around AGB stars whose evolution was cut short due to binary interaction, before the AGB envelope had already been largely enriched with the products of s-process nucleosynthesis.

PRODUCTION OF CARBON-RICH PRESOLAR GRAINS FROM MASSIVE STARS

About a year after core collapse supernova, dust starts to condense in the ejecta. In meteorites, a fraction of C-rich presolar grains (e.g., silicon carbide (SiC) grains of Type-X and low density graphites) are identified as relics of these events, according to the anomalous isotopic abundances. Several features of these abundances remain unexplained and challenge the understanding of core-collapse supernovae explosions and nucleosynthesis. We show, for the first time, that most of the measured C-rich grain abundances can be accounted for in the C-rich material from explosive He burning in core-collapse supernovae with high shock velocities and consequent high temperatures. The inefficiency of the $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction relative to the rest of the $\alpha$-capture chain at $T > 3.5\times10^8 \mathrm{K}$ causes the deepest He-shell material to be carbon rich and silicon rich, and depleted in oxygen. The isotopic ratio predictions in part of this material, defined here as the C/Si zone, are in agreement with the grain data. The high-temperature explosive conditions that our models reach at the bottom of the He shell, can also be representative of the nucleosynthesis in hypernovae or in the high-temperature tail of a distribution of conditions in asymmetric supernovae. Finally, our predictions are consistent with the observation of large $^{44}$Ca/$^{40}$Ca observed in the grains. This is due to the production of $^{44}$Ti together with $^{40}$Ca in the C/Si zone, and/or to the strong depletion of $^{40}$Ca by neutron captures.

Combustion synthesis of high purity SiC powder by radio-frequency heating

Abstract High purity silicon carbide (SiC) powder was produced via combustion synthesis by radio frequency (RF) heating. The reaction described here was involved in four stages: preheating, initial combustion reaction, product formation and cooling. Significantly, the initial combustion reaction started from a low ignition temperature (Ti) in the range of 1160 °C–1200 °C, subsequently the reaction proceeded under RF heating. The C/Si ratio, reaction temperature and reaction time had a large influence on the resultant SiC powder. Particularly, based on the comparison of the ignition temperature and the reaction degree under different C/Si ratios, it was firstly proposed that the C/Si ratio with 1:1 was not the optimal condition for the initial combustion reaction. Moreover, with the increase of reaction temperature and the reaction time, the phase transitions of SiC from 3C to 6H and from 6H to 15R were found, respectively. The SiC grains grow with the reaction time due to the successive reaction processes including formation, decomposition, fusion and reformation.

AGB Stars Migration fom the Central of the Milky Way Galaxy to Our Sun’s Birthplace and Its Relation with Silicon Carbide Grains

Silicon carbide grains found in meteorites are peculiar in their age and isotopic ratios; they formed before the Sun was born, and their isotopic signature indicates that they come from a different galactic region. This work aims to seek a possible paradigm for such richness and peculiarity through Monte Carlo simulation of scattering of Asymptotic Giant Branch (AGB) stars off molecular cloud. Such approach randomly generates AGB stars in regions close to the Galaxy bulge and examines possibility of migration to outer regions by scattering off molecular cloud. A successful explanation to this problem will influence how we think nuclides were formed and then distributed in the Galaxy and will shed new light unto the age and the chemical evolution of the Milky Way Galaxy. Thus, it is important that we know where do they come from and how do they end up in our backyard?

ASYMPTOTIC GIANT BRANCH STARS MIGRATION FROM THE CENTRAL OF THE MILKY WAY GALAXY TO OUR SUN’S BIRTHPLACE AND ITS RELATION WITH SILICON CARBIDE GRAINS

Silicon carbide grains found in meteorites are peculiar in their age and isotopic ratios; they formed before the Sun was born and their isotopic signature indicates that they come from a different galactic region. This study aims to seek a possible paradigm for such richness and peculiarity through Monte Carlo simulation of scattering of Asymptotic Giant Branch (AGB) stars off molecular cloud. Such approach randomly generates AGB stars in regions close to the Galaxy bulge and examines possibility of migration to outer regions by scattering off molecular cloud. A successful explanation to this problem will influence how we think nuclides were formed and then distributed in the Galaxy and will shed new light unto the age and the chemical evolution of the Milky Way Galaxy. Thus, it is important that we know where do they come from and how do they end up in our backyard?

Composition, structure, and dielectric properties of SiC-AlN ceramic materials

We have studied the structure, relative dielectric permittivity (ɛ), and dielectric loss tangent (tanδ) of SiC-AlN ceramic materials. The results demonstrate that both ɛ and tanδ are anomalously high in the composition range 30–50 wt % AlN at low frequencies (0.1 kHz). We show that the increase in ɛ may be due to a barrier effect on silicon carbide and aluminum nitride grain boundaries and to migration polarization.

Investigations on hydrobromination of silicon in the presence of silicon carbide abrasives as a purification route of kerf loss waste

Hydrobromination of three types of silicons (reagent grade silicon (RG-Si), metallurgical grade silicon (MG-Si), and kerf loss silicon (KL-Si)) has been investigated in a flow reactor with and without abrasive grains of silicon carbide (SiC) under various conditions. All the silicons reacted with HBr to form bromosilanes around 400°C, irrespective of the presence or absence of SiC. The reactivity of each Si sample was evaluated with respect to the conversion of HBr, while the purity of products was analyzed after hydrolyzing them to silicon dioxide. Metal impurities such as Al, Fe, and Ni in the starting materials were dramatically reduced in the hydrolyzed products. The addition of SiC did not cause any increase in impurities. Rather, it resulted in an improvement of the reaction kinetics in a temperature region of 350–400°C, probably as a result of better heat transfer in the reactant owing to the high thermal diffusivity of SiC. This demonstrates that the bromination reaction is effective to extract pure silicon from kerf loss waste even where SiC is present.

Fabrication and characterization of fully ceramic microencapsulated fuels

The current generation of fully ceramic microencapsulated fuels, consisting of Tristructural Isotropic fuel particles embedded in a silicon carbide matrix, is fabricated by hot pressing. Matrix powder feedstock is comprised of alumina–yttria additives thoroughly mixed with silicon carbide nanopowder using polyethyleneimine as a dispersing agent. Fuel compacts are fabricated by hot pressing the powder–fuel particle mixture at a temperature of 1800–1900°C using compaction pressures of 10–20MPa. Detailed microstructural characterization of the final fuel compacts shows that oxide additives are limited in extent and are distributed uniformly at silicon carbide grain boundaries, at triple joints between silicon carbide grains, and at the fuel particle–matrix interface.

Instrumentation and modeling of high-pressure roller crusher for silicon carbide production

This paper describes the modeling, simulation, and the instrumentation of a high-pressure roller crusher (HPRC) for the production of silicon carbide grains. The study is to make a model and then simulate a high-pressure roller crusher. An HPRC is an important part in the production of silicon carbide, where the grains are crushed into powder form and then sieved into specified sizes based on their usage. The selection of sensors for the HPRC is also considered in this study. This paper presents a model based on Johanson's theory for roller compactors by considering the delay process. The simulation result is done using Matlab tools and Simulink to verify the approach proposed in the paper. Conclusions are drawn at the end of the paper.

FORMATION OF SiC GRAINS IN PULSATION-ENHANCED DUST-DRIVEN WIND AROUND CARBON-RICH ASYMPTOTIC GIANT BRANCH STARS

We investigate the formation of silicon carbide (SiC) grains in the framework of dust-driven wind around pulsating carbon-rich Asymptotic Giant Branch (C-rich AGB) stars in order to reveal not only the amount but also the size distribution. Two cases are considered for the nucleation process; one is the LTE case where the vibration temperature of SiC clusters $T_{\rm v}$ is equal to the gas temperature as usual, and another is the non-LTE case in which $T_{\rm v}$ is assumed to be the same as the temperature of small SiC grains. The results of hydrodynamical calculations for a model with stellar parameters of mass $M_{\ast}$=1.0 $M_{\odot}$, luminosity $L_{\ast}$=10$^{4}$ $L_{\odot}$, effective temperature $T_{\rm eff}$=2600 K, C/O ratio=1.4, and pulsation period $P$=650 days show the followings: In the LTE case, SiC grains condense in accelerated outflowing gas after the formation of carbon grains and the resulting averaged mass ratio of SiC to carbon grains of $\sim$ 10$^{-8}$ is too small to reproduce the value of 0.01-0.3 inferred from the radiative transfer models. On the other hand, in the non-LTE case, the formation region of SiC grains is inner than and/or almost identical to that of carbon grains due to the so-called inverse greenhouse effect. The mass ratio of SiC to carbon grains averaged at the outer boundary ranges from 0.098 to 0.23 for the sticking probability $\alpha_{\rm s}$=0.1-1.0. The size distributions with the peak at $\sim$ 0.2-0.3 $\rm{\mu}$m in radius cover the range of size derived from the analysis of presolar SiC grains. Thus the difference between temperatures of small cluster and gas plays a crucial role in the formation process of SiC grains around C-rich AGB stars, and this aspect should be explored for the formation process of dust grains in astrophysical environments.

SULFUR MOLECULE CHEMISTRY IN SUPERNOVA EJECTA RECORDED BY SILICON CARBIDE STARDUST

We studied about 3400 presolar silicon carbide (SiC) grains from the Murchison CM2 meteorite for C- and Si-isotopic compositions. Among these grains we identified 7 unusual or type C SiC (U/C) grains, characterized by isotopically heavy Si, and 36 supernova type X SiC grains, characterized by isotopically light Si. Selected U/C and X grains were also measured for S-, Mg-Al-, and Ca-Ti-isotopic compositions. We show that the U/C grains incorporated radioactive 44Ti, which is evidence that they formed in the ejecta of Type II supernova (SNII) explosions. Abundances of radioactive 26Al and 44Ti are compatible with those observed in X grains. U/C and X grains carry light S with enrichments in 32S of up to a factor of 2.7. The combination of heavy Si and light S observed in U/C grains is not consistent with abundance predictions of simple supernova models. The isotope data suggest preferential trapping of S from the innermost supernova zones, the production site of radioactive 44Ti, by the growing silicon carbide particles. A way to achieve this is by sulfur molecule chemistry in the still unmixed ejecta. This confirms model predictions of molecule formation in SNII ejecta and shows that sulfur molecule chemistry operates in the harsh and hot environments of stellar explosions.

CHARACTERIZATION OF PRESOLAR MATERIAL IN THE CR CHONDRITE NORTHWEST AFRICA 852

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains in the CR2 chondrite Northwest Africa (NWA) 852. Thirty-one O-anomalous grains were detected: 24 were identified as silicates ({approx}78 ppm); the remaining 7 are Al-rich oxides ({approx}38 ppm). NWA 852 is the first C2 chondrite containing O-anomalous presolar dust in concentrations comparable to other more primitive meteorites. Eight presolar SiC grains have been found, representing the highest abundance ({approx}160 ppm) observed so far in primitive meteorites. {sup 15}N-enriched matter is also present, although very heterogeneously distributed. Twenty-six of the O-anomalous grains are enriched in {sup 17}O, originating from the outflows of low-mass asymptotic giant branch (AGB) stars. We calculate a silicate/oxide abundance ratio of {approx}2, which indicates a higher degree of aqueous alteration than observed for other presolar-grain-rich meteorites. NWA 852 thus stands between the presolar-grain-rich CR3 chondrites (MET 00426, QUE 99177) and CR2 chondrites with low presolar grain abundances (Renazzo, NWA 530). We calculate an initial presolar silicate abundance of {approx}800 ppm for NWA 852, if silicate destruction by aqueous alteration is taken into account. Transmission electron microscope (TEM) investigation of one presolar Al-rich grain of an AGB star origin revealed that the grain mainly consistsmore » of a single crystal of hibonite with slightly varying orientations. A distinct subgrain (d < 100 nm) with a Ca/Ti ratio of {approx}1 is located in the central region, most likely indicating a perovskite-like phase. Our data suggest this phase to be a primary condensate and not an alteration product.« less

TUNGSTEN ISOTOPIC COMPOSITIONS IN STARDUST SiC GRAINS FROM THE MURCHISON METEORITE: CONSTRAINTS ON THEs-PROCESS IN THE Hf-Ta-W-Re-Os REGION

We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes 182,183,184,186W and 179,180Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (= solar) composition in the 182W/184W and 183W/184W ratios, with deficits in 182W and 183W with respect to 184W. The 186W/184W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar 182W/184W, 183W/184W, and 186W/184W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their 182W/184W, 183W/184W, and 179Hf/180Hf isotopic compositions, although a small adjustment in the s-process production of 183W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the 186W/184W ratios observed in the SiC grains, even when the current 185W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the 13C neutron source, the mass-loss law, the modeling of the third dredge-up, and the efficiency of the 22Ne neutron source) may affect current s-process predictions.

Alkali-bonded SiC based foams

Silicon carbide (SiC) foams were developed by using a low temperature process such as chemical consolidation that is suitable to replace the sintering step. An alkali aluminosilicates binder, also known as geopolymer, was used. It was prepared from metakaolin, as aluminosilicatic raw powder, and KOH/K 2 SiO 3 aqueous solution. The foaming agent was the metallic silicon present as impurity in SiC powders. Different grades of SiC were used as the main component (90 wt%) of the foams and the micro and macrostructures varied with the morphologies of the SiC raw powders. The surface of SiC grains participates to the geopolymeric process because of the dissolution of the silica layer into the alkaline solution. SiC foams were tested and characterized under oxidative atmospheres up to 1200 °C.

Effect of Meso to Micro Transition in Morphology Dependent Fracture of SiC Ceramics

Silicon carbide (SiC) is an important ceramic material usually found in polycrystalline form with grain boundary thickness ranging from a few nanometers to a few hundred nanometers and grains with multiple orientations with sizes of the order of few micrometers. The present work focuses on analyzing how the interplay between different orientations of SiC grains and different grain boundary thicknesses can be exploited for targeted improvement in the fracture resistance properties of SiC. Crack propagation simulations using the cohesive finite element method (CFEM) are performed on the finite element meshes developed on experimentally processed SiC morphologies. Analyses were performed at two different length scales: 300 μm × 60 μm (scale-1:Microscale) and 75 μm × 15 μm (scale-2:Mesoscale). Lower resolution microstructure at scale-1 does not explicitly consider the presence of grain boundaries (GBs). Higher resolution microstructure at scale-2 explicitly models GBs. Results indicate that the effect of change in grain orientation is on crack path only. The fracture resistance is not significantly affected. The presence of GBs may directly aid in strengthening a microstructure’s fracture resistance. However, indirectly it may weaken a microstructure by favoring the formation of microcracks. Significantly higher crack formation in grain interior while lower interfacial energy dissipation in comparison to interfaces indicates overall lower fracture strength of grain interiors in comparison to interfaces. If GBs are not accounted for, the second most influencing factor affecting fracture strength is the average grains size. Overall, it is mainly the GBs not the grain orientation distribution and grain size that significantly affects fracture strength.

Study of deposition patterns of plating layers in SiC/Cu composites by electro-brush plating

SiC reinforced copper composite coatings were prepared by electro-brush plating with micron-size silicon carbide (SiC) ranging from 1 to 5μm on pure copper sheet in this paper. The micro-structural characterizations of SiC/Cu composite coatings were performed by optical microscope and Scanning Electron Microscope (SEM) coupled with spectrometer, to study co-deposition mechanism of SiC/Cu. It was found that there were three different patterns of SiC deposition in plating layers during electro-brush plating process, i.e. the particles could deposit inside copper grains, in grain boundaries, or in holes of the surface. To investigate deposition mechanism of each pattern, size of SiC and copper grains was compared. By comparison of size of copper grains and hard particles, SiC were either wrapped in copper grains or deposited in grain boundaries. Moreover, electro-brush plating layers at different brush velocities and current densities were obtained respectively, to analyze the microstructure evolution of the composite coatings. The hardness of plating layers was measured. The results indicated at the current density of 3A/dm2, the SiC/Cu coating was compact with SiC content at a high level and the hardness reached a maximum.

Fabrication of Gradient Porous SiC Ceramics with Directional Pores without Templates

Gradient porous pure SiC ceramics with directional SiC crystals and pores were fabricated by the high-temperature recrystallization process. In this work, silicon carbide decomposed to produce the gas mixture. The flow of gas mixture along the axial direction in the porous green body induces the surface ablation, rearrangement, and recrystallization of silicon carbide grains, which leads to the formation of the aligned columnar silicon carbide crystals and tubular pores in the axial direction. The different rates of the decomposition and recrystallization of SiC in different parts of samples resulted in the formation of the gradient porosity and pore size along the axial section of sample. Si3N4 powder was used as a pore-former, which promotes the formation of the gradient and interconnectivity of pores. With increasing Si3N4 contents, the total porosity and weight loss increase while the pores size and bending strength decrease. The total porosity of samples is 38–45%, and their bending strength is 33–38 MPa.

HINTS FOR NEUTRINO-PROCESS BORON IN PRESOLAR SILICON CARBIDE GRAINS FROM SUPERNOVAE

We have studied more than 1000 presolar silicon carbide (SiC) grains from the Murchison CM2 chondrite for Cand Si-isotopic compositions. Twelve SiC X grains, characterized by strong enrichments in 28 Si and believed to originate from Type II Supernovae (SNeII), were also measured for Li- and B-isotopic compositions. None of these grains show resolvable isotope anomalies in Li or B. For the seven X grains without Li and B contributions from nearby or attached SiC grains of distinct origins we find on average 7 Li/ 6 Li = 11.83 ± 0.29 (solar system: 12.06) and 11 B/ 10 B = 4.68 ± 0.31 (solar system: 4.03). The average 7 Li/ 6 Li is compatible with the solar system ratio and the lithium in the X grains is likely largely dominated by contaminating Li of laboratory or meteoritic origin. Also, most of the boron in X grains appears to be contamination but the small 11 B excess of ∼16%, significant at the 2σ level, can be considered a hint for the presence of boron produced by the neutrino process in the parent SNeII. Despite this finding, a quantitative comparison of the B isotope and abundance data of X grains with model predictions reveals deficiencies in our current understanding of the details of B production in SNeII as well as on B chemistry and condensation in SNII ejecta.