SiC Grains Research Articles

Effects of dopants on electrical, thermal, and mechanical properties of porous SiC ceramics

Nitrogen-, vanadium-, aluminum-, boron-, and scandium-doped porous SiC ceramics were fabricated to investigate the effects of dopants on electrical, thermal, and mechanical properties of porous SiC ceramics. The electrical resistivity of a nitrogen-doped porous SiC ceramic was 2.1 × 10−1 Ω cm four orders of magnitude lower than that of an undoped porous SiC ceramic (1.2 × 103 Ω cm). A B-doped porous SiC ceramic exhibited the highest thermal conductivity (16.6 W/(m K)) and flexural strength (25.9 MPa), whereas a Sc-doped porous SiC ceramic exhibited the lowest thermal conductivity (7.7 W/(m K)) and flexural strength (10.5 MPa) among the doped porous SiC ceramics. The electrical resistivity was strongly influenced by the doping, whereas the thermal and mechanical properties were dependent primarily on the necking area between SiC grains. The results suggest that the electrical conductivities of porous SiC ceramics can be successfully tuned independently of the thermal conductivity by a suitable doping.

S-processing in AGB Stars Revisited. III. Neutron Captures from MHD Mixing at Different Metallicities and Observational Constraints

Abstract We present post-process neutron-capture computations for Asymptotic Giant Branch (AGB) stars of 1.5–3 M ⊙ and metallicities −1.3 ≤ [Fe/H] ≤ 0.1. The reference stellar models are computed with the FRANEC code, using the Schwarzschild's criterion for convection; our motivations for this choice are outlined. We assume that MHD processes induce the penetration of protons below the convective boundary, when the Third Dredge Up occurs. There, the 13C n-source can subsequently operate, merging its effects with those of the 22Ne(α, n)25Mg reaction, activated at the temperature peaks characterizing AGB stages. This work has three main scopes. (i) We provide a grid of abundance yields, as produced through our MHD mixing scheme, which are uniformly sampled in mass and metallicity. From this, we deduce that the solar s-process distribution, as well as the abundances in recent stellar populations, can be accounted for, without the need of the extra primary-like contributions suggested in the past. (ii) We formulate analytic expressions for the mass of the 13C-pockets generated to allow easy verification of our findings. (iii) We compare our results with observations of evolved stars and with isotopic ratios in presolar SiC grains, also noticing how some flux tubes should survive turbulent disruption, carrying C-rich materials into the winds even when the envelope is O-rich. This wind phase is approximated through the G-component of AGB s-processing. We conclude that MHD-induced mixing is adequate to drive slow n-capture phenomena accounting for observations; our prescriptions should permit its inclusion into current stellar evolutionary codes.

Cluster Analysis of Presolar Silicon Carbide Grains: Evaluation of Their Classification and Astrophysical Implications.

Cluster analysis of presolar silicon carbide grains based on literature data for 12C/13C, 14N/15N, δ 30Si/28Si, and δ 29Si/28Si including or not inferred initial 26Al/27Al data, reveals nine clusters agreeing with previously defined grain types but also highlighting new divisions. Mainstream grains reside in three clusters probably representing different parent star metallicities. One of these clusters has a compact core, with a narrow range of composition, pointing to an enhanced production of SiC grains in asymptotic giant branch (AGB) stars with a narrow range of masses and metallicities. The addition of 26Al/27Al data highlights a cluster of mainstream grains, enriched in 15N and 26Al, which cannot be explained by current AGB models. We defined two AB grain clusters, one with 15N and 26Al excesses, and the other with 14N and smaller 26Al excesses, in agreement with recent studies. Their definition does not use the solar N isotopic ratio as a divider, and the contour of the 26Al-rich AB cluster identified in this study is in better agreement with core-collapse supernova models. We also found a cluster with a mixture of putative nova and AB grains, which may have formed in supernova or nova environments. X grains make up two clusters, having either strongly correlated Si isotopic ratios or deviating from the 2/3 slope line in the Si 3-isotope plot. Finally, most Y and Z grains are jointly clustered, suggesting that the previous use of 12C/13C = 100 as a divider for Y grains was arbitrary. Our results show that cluster analysis is a powerful tool to interpret the data in light of stellar evolution and nucleosynthesis modeling and highlight the need of more multi-element isotopic data for better classification.

Reevaluation of the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reaction rates

The competing $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg and $^{22}$Ne($\alpha,n$)$^{25}$Mg reactions control the production of neutrons for the weak $s$-process in massive and AGB stars. In both systems, the ratio between the corresponding reaction rates strongly impacts the total neutron budget and strongly influences the final nucleosynthesis. The $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg and $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction rates was re-evaluated by using newly available information on $^{26}$Mg given by various recent experimental studies. Evaluations of The evaluated $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg reaction rate remains substantially similar to that of Longland {\it et al.} but, including recent results from Texas A\&M, the $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction rate is lower at a range of astrophysically important temperatures. Stellar models computed with NEWTON and MESA predict decreased production of the weak branch $s$-process due to the decreased efficiency of $^{22}$Ne as a neutron source. Using the new reaction rates in the MESA model results in $^{96}$Zr/$^{94}$Zr and $^{135}$Ba/$^{136}$Ba ratios in much better agreement with the measured ratios from presolar SiC grains.

Presolar stardust in highly pristine CM chondrites Asuka 12169 and Asuka 12236.

We report a NanoSIMS search for presolar grains in the CM chondrites Asuka (A) 12169 and A12236. We found 90 presolar O‐rich grains and 25 SiC grains in A12169, giving matrix‐normalized abundances of 275 (+55/−50, 1σ) ppm or, excluding an unusually large grain, 236 (+37/−34) ppm for O‐rich grains and 62 (+15/−12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs indicate abundances of 58 (+18/−12) and 20 (+12/−8) ppm, respectively. The SiC abundances are in the typical range of primitive chondrites. The abundance of presolar O‐rich grains in A12169 is essentially identical to that in CO3.0 Dominion Range 08006, higher than in any other chondrites, while in A12236, it is higher than found in other CMs. These abundances provide further strong support that A12169 and A12236 are the least‐altered CMs as indicated by petrographic investigations. The similar abundances, isotopic distributions, silicate/oxide ratios, and grain sizes of the presolar O‐rich grains found here to those of presolar grains in highly primitive CO, CR, and ungrouped carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a similar population of presolar oxides and silicates in their matrices to those accreted by the parent bodies of the other CC groups. The lower abundances and larger grain sizes seen in some other CMs are thus most likely a result of parent‐body alteration and not heterogeneity in nebular precursors. Presolar silicates are unlikely to be present in high abundances in returned samples from asteroids Ryugu and Bennu since remote‐sensing data indicate that they have experienced substantial aqueous alteration.

Characterization of fusion welded ceramics in the SiC-ZrB2-ZrC system

Various SiC-ZrB2-ZrC ceramics were joined by fusion welding to determine the maximum silicon carbide content that could be joined. Commercial powders were hot pressed, machined, and preheated to 1450 °C before joining with a tungsten inert gas welding torch at 160–200 A. Resulting welds were cross-sectioned and analyzed to determine which compositions were weldable and to characterize microstructural evolution in welded samples. As compositions approached the ternary eutectic, the welds had smaller SiC grains and exhibited better weldability. Penetration depth of welds was controlled by a combination of current input and welding speed. The ternary eutectic in the system was found at 36.9 ± 1.3 vol% SiC, 42.7 ± 1.5 vol% ZrB2, and 20.4 ± 1.9 vol% ZrC and its melting temperature was 2330 ± 23 °C. A ternary phase diagram for the SiC-ZrB2-ZrC was constructed and proposed via microstructural analysis of arc melted pellets on binary joins between each binary eutectic and the ternary eutectic in the system.

Abnormal grain growth in iron-containing SiC fibers

Understanding the role of sintering aids during microstructure evolution is critical to the manufacture of densified SiC fibers. A variety of TEM characterization techniques are combined to investigate grain growth behavior in iron-doped SiC fibers. Ultra-large SiC grains in micron size, as the self-assembly of nano sub-grains into a similar orientation, were consistently discovered at the surface and indicative of abnormal grain growth. The growth front consisted of polycrystalline nanograins wetted by iron-rich particles, where several sub-grains were found to unify their (111) planes with a misorientation angle less than 10°, indicating grain rotation at the growth front. It is proposed that iron-rich particles form a quasi-liquid interfacial phase during sintering, which facilitates coherent attachment of grains and results in fast grain growth using neighboring irregular-shaped nanograins as building blocks. The imperfect ordered coalescence of nanograins introduces structural heterogeneities, including low angle grain boundaries and porosities.

Investigation of Microstructure and Nanoindentation Hardness of C+ & He+ Irradiated Nanocrystal SiC Coatings during Annealing and Corrosion

The microstructure and nanoindentation hardness of unirradiated, irradiated, annealed and corroded SiC coatings were characterized. Irradiation of 400 keV C+ and 200 keV He+ with approximately 10 dpa did not cause obvious amorphous transformation to nanocrystal SiC coatings and induced helium bubbles with 2–3 nm dimension distributed uniformly in the SiC matrix. High temperature annealing resulted in the transformation of SiC nanocrystals into columnar crystals in the irradiated region. Line-shaped bubble bands formed at the columnar crystal boundaries and their stacking fault planes and made the formation of microcracks of hundreds of nanometers in length. Meanwhile, some isolated helium bubbles distributed in SiC grains still maintained a size of 2–3 nm, despite annealing at 1200 °C for 5 h. The SiC coating showed excellent corrosion resistance under high-temperature, high-pressure water. The weight of the sample decreased with the increase of corrosion time. The nanoindentation hardness and the elastic modulus increased significantly with C+ and He+ irradiation, while their values decreased with high-temperature annealing. An increase in the annealing temperature led to an increased reduction in the values. Corrosion caused the decrease of nanoindentation hardness and the elastic modulus in the whole test depth range, whether the samples were irradiated or unirradiated.

Experimental investigation of damage formation and material removal in ultrasonic assisted grinding of RBSiC

Ultrasonic assisted grinding (UAG) has been considered as a prominent processing method of the reaction bonded silicon carbide (RBSiC). To improve the knowledge of UAG process, both conventional grinding (CG) and UAG were used to process the RBSiC for in-depth investigation. Grinding forces, surface topographies, and subsurface damages during CG and UAG were compared. Furtherly, the ground surface was analyzed on aspects of both topographical characteristics and material removal mechanism. The results indicated that the removal of material is mainly achieved by the intersections of cracks initiated from both big SiC particles and mixture area of silicon matrix and small SiC grains. The crack propagation during UAG was more intensified due to the ultrasonic impact, which results in higher efficiency of machining RBSiC.

Mass and metallicity distribution of parent AGB stars of presolar SiC

The vast majority (≳90%) of presolar SiC grains identified in primitive meteorites are relics of ancient asymptotic giant branch (AGB) stars, whose ejecta were incorporated into the Solar System during its formation. Detailed characterization of these ancient stardust grains has revealed valuable information on mixing processes in AGB interiors in great detail. However, the mass and metallicity distribution of their parent stars still remains ambiguous, although such information is crucial to investigating the slow neutron-capture process, whose efficiency depends on mass and metallicity. Using a well-known Milky Way chemo-dynamical model, we followed the evolution of the AGB stars that polluted the Solar System at 4.57 Gyr ago and weighted the stars based on their SiC dust productions. We find that presolar SiC in the Solar System predominantly originated from AGB stars with M ∼ 2 M⊙ and Z ∼ Z⊙. Our finding well explains the grain-size distribution of presolar SiC identified in situ in primitive meteorites. Moreover, it provides complementary results to very recent papers that characterized parent stars of presolar SiC.

Sintering and mechanical behavior of SiC and WC co-added TiC-based composites densified by hot-pressing

In this investigation, the individual and synergetic influences of the SiC whiskers and WC particles on the various features of TiC were studied. Fully dense specimens were secured when 40 vol% SiC additive was added to the TiC and TiC-WC systems. The nucleation and growth of the new SiC grains were found as the two main mechanisms positively affecting the sinterability of TiC. Although WC particles were completely dissolved into the TiC matrix in the TiC-WC system, they were entrapped in the newly grown SiC grains in the TiC-WC-SiC system, contributing to strengthening of the specimen. The highest values of flexural strength (590 MPa) and Vickers hardness (2525 HV0.5 kg) were attained for the sample codoped with SiC whiskers and WC particles.

Neutron Capture on the s -Process Branching Point Tm171 via Time-of-Flight and Activation

The neutron capture cross sections of several unstable nuclides acting as branching points in the s process are crucial for stellar nucleosynthesis studies. The unstable ^{171}Tm (t_{1/2}=1.92 yr) is part of the branching around mass A∼170 but its neutron capture cross section as a function of the neutron energy is not known to date. In this work, following the production for the first time of more than 5mg of ^{171}Tm at the high-flux reactor Institut Laue-Langevin in France, a sample was produced at the Paul Scherrer Institute in Switzerland. Two complementary experiments were carried out at the neutron time-of-flight facility (n_TOF) at CERN in Switzerland and at the SARAF liquid lithium target facility at Soreq Nuclear Research Center in Israel by time of flight and activation, respectively. The result of the time-of-flight experiment consists of the first ever set of resonance parameters and the corresponding average resonance parameters, allowing us to make an estimation of the Maxwellian-averaged cross sections (MACS) by extrapolation. The activation measurement provides a direct and more precise measurement of the MACS at 30keV: 384(40)mb, with which the estimation from the n_TOF data agree at the limit of 1 standard deviation. This value is 2.6 times lower than the JEFF-3.3 and ENDF/B-VIII evaluations, 25% lower than that of the Bao etal. compilation, and 1.6 times larger than the value recommended in the KADoNiS (v1) database, based on the only previous experiment. Our result affects the nucleosynthesis at the A∼170 branching, namely, the ^{171}Yb abundance increases in the material lost by asymptotic giant branch stars, providing a better match to the available pre-solar SiC grain measurements compared to the calculations based on the current JEFF-3.3 model-based evaluation.

Isotopic Abundances in Presolar SiC Grains accounted by s-Processing from MHD-induced Mixing in low mass AGB stars

In the past, the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the so-called Asymptotic Giant Branch (or AGB) could not be explained by self-consistent models, forcing researchers to extensive parameterisations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding to the formation of 13C and then the activation of the 13C(α,n)16O reaction. In the last decade, some physically-based mixing mechanisms have been considered to solve this problem. Nowadays, a big step forward in s-process studies would be to understand what is among the suggested ones the physical model better accounts for the observational constrains. In this paper we analyse a model where the 13C forms as a feedback of MHD processes in the stellar plasma. We compare results of nucleosynthesis models for low mass AGB stars (M<3M©), developed from the MHD scenario, with the record of isotopic abundance ratios of s-elements in presolar SiC grains, which were shown to offer precise constraints on the 13C reservoir. We find that n-captures driven by magnetically-induced mixing can well account for the SiC data and that this is due to the fact that our 13C distribution fullfills the above constraints rather accurately. We show comparisons between model predictions and measurements for isotopes of Sr, Zr, Ba, Mo and Ru as representative examples of light and heavy s-elements.

Accelerated oxidation and microstructure evolution of SiC in the presence of NaF

Oxidation behaviors of SiC in the presence of some impurities are very important for the performance of SiC used in high temperature environments such as aerospace vehicles and nuclear reactors. In this study, the effects of NaF on the oxidation behaviors of SiC powder with grain sizes from 0.3~2 μm were investigated. NaF with contents of 0.2~10 wt.% was added to the 6H-SiC powder (~ 1 μm) and the mixtures were oxidized at 500~800 °C for 30 min in air atmosphere. Compared with pure SiC, enhanced oxidation was observed with the NaF addition as low as 0.2 wt.%. When the content of NaF was increased to 2.0%, the weight gain ratio of SiC reached 23.0% at the oxidation temperature of 700 °C, which was only 0.6% for pure SiC powder. In the TG analysis, the onset oxidation temperature for the SiC-2.0%NaF sample was significantly decreased to 570 °C. The weight gain was increased with the decrease of SiC grain size. The intensive oxidation was confirmed by SEM, TEM, XRD and XPS characterizations. The formation of free carbon during the oxidation process was demonstrated by XRD and Raman spectroscopy. New polytype of SiC of 15R and defects were formed in the 6H-SiC matrix after oxidation. The results showed that NaF could greatly accelerate the oxidation of SiC and played a very important role in the microstructure evolution of SiC. Possible mechanism for the effects of NaF on the oxidation behaviors of SiC was discussed.

Modeling the formation of the 13C neutron source in AGB stars

A major source of uncertainty in AGB models is the partial-mixing process of hydrogen, required for the formation of the so-called 13C pocket. Among the attempts to derive a self-consistent treatment of this physical process, there are 2D and 3D simulations of magnetic buoyancy. The 13C pocket resulting from mixing induced by magnetic buoyancy extends over a region larger than those so far assumed, showing an almost flat 13C distribution and a negligible amount of 14N. Recently, it has been proved to be a good candidate to match the records of isotopic abundance ratios of s-elements in presolar SiC grains. However, up to date such a magnetic mixing has been applied in post-process calculations only, being never implemented in a stellar evolutionary code. Here we present new stellar models, performed with the 1-d hydrostatic FUNS evolutionary code, which include magnetic buoyancy. We comment the resulting s-process distributions and show preliminary comparisons to spectroscopic observations and pre-solar grains measurements.

Effect of Al contained in polymer‐derived SiC crystals on creating stable crystal grain boundaries

AbstractSi‐Al‐C‐O materials are well‐known as a good precursor material for obtaining a dense, excellent heat‐resistant SiC polycrystalline materials. Although it has been found that the aluminum contained in Si‐Al‐C‐O materials plays an important role in obtaining dense SiC materials, its actual behavior during heat‐treatment processes has not been investigated. In this study, we investigated the behavior of aluminum contained in the SiC crystals, during the densification of SiC crystals produced from the Si‐Al‐C‐O material. A part of the aluminum contained in the SiC crystals plays an important role in creating a thermodynamically stable grain boundary by the diffusional transportation of aluminum to the SiC grain boundaries at temperatures above 1700°C. Subsequently, most of the aluminum disappeared during the heat treatment at higher temperatures (~1900°C); as a consequence, a dense SiC solid solution with uniformly distributed residual aluminum (0.15wt%) was formed without a secondary phase at the grain boundary.

Self lubrication of pressureless sintered SiC ceramics

The self lubrication of SiC ceramics prepared by pressureless sintering was investigated by annealing at different temperatures in air. It was found that low coefficient of friction can be obtained for SiC ceramics by annealing, and the coefficient of friction of SiC ceramics decreased with the increase of annealing temperature. The SiC ceramics with annealing at 1500 °C revealed the best self lubricating property, with the coefficient of friction of 0.4, while the coefficient of friction of SiC ceramics without annealing was as high as 0.7. The SiC ceramics with annealing at 1000 °C can show self lubrication over a long sliding distance without increasing wear. The formation of α-cristobalite on the annealed SiC ceramics can further lubricate SiC ceramics and shorten run in period. Self lubrication mechanism of SiC ceramics with annealing is considered that on the one hand, the formation of silicon oxide layers after annealing can reduce the number of SiC grains directly exposed to the sliding interface in the early stage of the sliding, decreasing the number of hard SiC wear particles formed on the sliding interface; and on the other hand, silicon oxide wear particles generated by the wear of the silicon oxide layers formed by annealing can be used as solid lubricant.

Improved ablation resistance of 3D-Cf/SiBCN composites with (PyC/SiC)3 multi-layers as interphase

Ablation behavior of 3D-Cf/SiBCN composites with (PyC/SiC)3 multi-layers as interphase was investigated in plasma ablation flame at a heat flux of 4.02 MW m−2. During ablation, amorphous SiBCN matrix is firstly oxidized and forms a borosilicate glass, in which B2O3 is volatilized as the ablation going on. At the ablation center with higher temperature, SiC and Si3N4 grains are precipitated from the amorphous SiBCN matrix. And then, these SiC and Si3N4 grains are oxidized, forming a certain viscous silica rich protective coating on the sample surface. Moreover, it is revealed that the SiC layers in (PyC/SiC)3 multi-layers interphase can protect the carbon fibers from ablation damage effectively. Compared with the Cf/(PyC)SiBCN composites, much improved ablation resistance is achieved for the Cf/(PyC/SiC)3SiBCN composites. The linear and mass recession rates of the Cf/(PyC/SiC)3SiBCN composites are as low as 1.7 ± 0.4 μm s−1 and 0.0427 ± 0.0053 mg mm−2s−1, which are ∼43 % and ∼21 % lower than those of Cf/(PyC)SiBCN.

Evaluation of the N13(α,p)O16 thermonuclear reaction rate and its impact on the isotopic composition of supernova grains

It has been suggested that hydrogen ingestion into the helium shell of massive stars could lead to high $^{13}$C and $^{15}$N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high $^{13}$C and $^{15}$N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context $^{13}$N($\alpha$,p)$^{16}$O the reaction plays an important role since it is in competition with $^{13}$N $\beta^+$-decay to $^{13}$C. The $^{13}$N($\alpha$,p)$^{16}$O reaction rate used in stellar evolution calculations comes from the CF88 compilation with very scarce information on the origin of this rate. The goal of this work is to provide a recommended $^{13}$N($\alpha$,p)$^{16}$O reaction rate, based on available experimental data. Unbound nuclear states in the $^{17}$F compound nucleus were studied using the spectroscopic information of the analog states in $^{17}$O nucleus that were measured at the Alto facility using the $^{13}$C($^7$Li,t)$^{17}$O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended $^{13}$N($\alpha$,p)$^{16}$O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present $^{13}$N($\alpha$,p)$^{16}$O reaction rate is found to be within a factor of two of the previous evaluation, with a typical uncertainty of a factor 2-3. The source of this uncertainty comes from the three resonances at $E_r^{c.m.} = 221$, 741 and 959 keV. This new error estimation translates to an overall uncertainty in the $^{13}$C production of a factor of 50. The main source of uncertainty on the re-evaluated $^{13}$N($\alpha$,p)$^{16}$O reaction rate currently comes from the uncertain alpha-width of relevant $^{17}$F states.

Microstructure and mechanical property of (TiB2‐SiC) agglomerate‐toughened B4C‐TiB2‐SiC composites

AbstractB4C‐TiB2‐SiC composites toughened by (TiB2‐SiC) agglomerates were prepared via reactive hot pressing with B4C and TiSi2 as raw materials. Phase composition, microstructure, and mechanical properties of the fabricated composites were investigated. The function of (TiB2‐SiC) agglomerates was analyzed, and the strengthening and toughening mechanism were also discussed. Results indicated that some of the in situ formed TiB2 and SiC were interlocked to form special (TiB2‐SiC) agglomerates in the matrix. The good comprehensive performances of 510 MPa flexural strength, 5.84 MPa·m1/2 fracture toughness, and 31.93 GPa hardness were obtained in the composites fabricated with 30 wt% TiSi2. The in situ introduced fine TiB2 and SiC grains refined the grains of B4C due to the pinning effect, which enhanced the strength. The special (TiB2‐SiC) agglomerates and the existing toughening phenomena such as crack deflection, branching, and microcrack regions induced by the mismatch of thermal expansion coefficients, had cumulative effects on improving the fracture toughness.