Large SiC Grains Research Articles

Nucleation and growth of SiC at the interface between molten Si and graphite

In the present work, the evolution of the SiC layer formed at the interface between liquid silicon and solid carbon is studied using a diffusion couple configuration. Reaction conditions were isothermal, with a temperature of 1450 °C maintained from 2 min to 4 h. The rapid heating and cooling of the Si–C diffusion couple specimens were achieved using a Pulse-Electric Current Sintering system. Crystallographic, compositional, and phase distribution data obtained after different reaction times were used to develop a two-stage model for SiC growth at the interface between molten Si and C. Initially, the formation of SiC at the interface is governed by diffusion of C into the molten Si and dissolution/reprecipitation of formed SiC nuclei. These nuclei further grow into larger SiC grains at the Si–C interface and this initial stage is successfully modeled using the Johnson-Mehl-Avrami-Kolmogorov model. Once a continuous SiC layer forms at the Si–C interface, the growth of SiC is controlled by the diffusion of C through the SiC layer, which can be modeled using a power rate law. However, the nature of this diffusion is difficult to determine with certainty since the rate laws for both grain boundary and bulk diffusion fit experimental data equally well.

Effects of the Ar flow rate on the composition, structure, and properties of near-stoichiometric SiC fibres that were annealed at 1600°C

SiC fibres are widely used in the aerospace and nuclear fields due to their unique high-temperature resistance. Maintaining excellent high-temperature resistance is necessary for the application of fibres. This study involved processing nearly stoichiometric SiC fibres in Ar at 1600 °C for 1 h. The effects of the Ar flow rate on the composition, structure, and properties of fibres after high-temperature treatment were investigated by elemental analysis, SEM, EDS, TEM, XRD, and Raman spectroscopy. The results showed that the nearly stoichiometric SiC fibres did not undergo noticeable pyrolysis or active oxidation at a low Ar flow rate (<0.2 L/min) with a weight loss rate of less than 1% and that the tensile strength retention rate was as high as 86%. As the Ar flow rate was gradually increased above 0.2 L/min, an active oxidation reaction occurred, and the reaction products of SiO and CO were deposited on the fibre surfaces to form large SiC grains. The fibres gradually evolved into a skin-care structure, and the mechanical properties degraded rapidly. A high-temperature inert gas environment with a low flow rate is very important for preserving the mechanical properties of near-stoichiometric SiC fibres at high temperatures.

Formation of composite SiC-C coatings on graphite via annealing Si-melt in CO

A method for the synthesis of silicon carbide–graphite composite coatings is proposed and described. The method is based on the two simultaneous chemical reactions involving graphite. One of these reactions consists of the chemical interaction of a silicon molten mass with carbon monoxide on the surface of the graphite at a temperature slightly exceeding the melting point of silicon. It transforms Si into gaseous silicon monoxide (SiO) and SiC. Simultaneously, a second reaction involves the SiO in the transformation of the graphite itself into SiC. A coating with a thickness exceeding 1 mm is formed during the synthesis. The resulting coating has high mechanical strength and hardness. Samples of the composite coating were investigated via scanning electron microscopy, energy-dispersive spectroscopy , Raman spectroscopy , XRD, and nanoindentation . The coating has a complicated structure. It consists of a dense overlayer of SiC and has a branched structure consisting of dendrite-like SiC crystals extending into the depth of the graphite. The dendrites are interspersed with large (up to 20 μm) single-crystalline SiC grains consisting of predominantly cubic polytypes. The coating leads to significant hardening of the material: the composite has a hardness of 28 GPa, which is ~254 times higher than the hardness of the original graphite surface. • A method is proposed for the formation of protective SiC coatings on graphite. • The method consists of annealing of graphite with Si melt in presence of CO. • Thick, up to 1 mm coatings are being formed in 20 min at low T (1450C).

Dust changes in Sakurai’s Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 μm-sized melilite grains

ABSTRACT 6–14 μm Spitzer spectra obtained at 6 epochs between 2005 April and 2008 October are used to determine temporal changes in dust features associated with Sakurai’s Object (V4334 Sgr), a low mass post-AGB star that has been forming dust in an eruptive event since 1996. The obscured carbon-rich photosphere is surrounded by a 40-milliarcsec torus and 32 arcsec PN. An initially rapid mid-infrared flux decrease stalled after 2008 April 21. Optically thin emission due to nanometre-sized SiC grains reached a minimum in 2007 October, increased rapidly between 2008 April 21–30 and more slowly to 2008 October. 6.3-μm absorption due to PAHs increased throughout. 20 μm-sized SiC grains might have contributed to the 6–7 μm absorption after 2007 May. Mass estimates based on the optically thick emission agree with those in the absorption features if the large SiC grains formed before 1999 May and PAHs formed in 1999 April–June. Estimated masses of PAH and large-SiC grains in 2008 October, were 3 × 10−9 M⊙ and 10−8 M⊙, respectively. Some of the submicron-sized silicates responsible for a weak 10 μm absorption feature are probably located within the PN because the optical depth decreased between 2007 October and 2008 October. 6.9-μm absorption assigned to ∼10 μm-sized crystalline melilite silicates increased between 2005 April and 2008 October. Abundance and spectroscopic constraints are satisfied if $\lesssim$2.8 per cent of the submicron-sized silicates coagulated to form melilites. This figure is similar to the abundance of melilite-bearing calcium–aluminium-rich inclusions in chondritic meteorites.

Beneficial effect of low BN additive on densification and mechanical properties of hot-pressed ZrB2–SiC composites

The incorporation of 1 wt% hexagonal BN (hBN) into ZrB2–30 vol% SiC could noticeably better the fracture toughness, hardness, and consolidation behavior of this composite. This research intended to scrutinize the effects of various amounts of hBN (0–5 wt%) on different characteristics of ZrB2–SiC materials. The hot-pressing method under 10 MPa at 1900 °C for 120 min was employed to sinter all designed specimens. Afterward, the as-sintered samples were characterized using X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Vickers technique. The hBN addition up to 1 wt% improved relative density, leading to a near fully dense sample; however, the incorporation of 5 wt% of such an additive led to a composite containing more than 5% remaining porosity. The highest Vickers hardness of 23.8 GPa and fracture toughness of 5.7 MPa.m1/2 were secured for the sample introduced by only 1 wt% hBN. Ultimately, breaking large SiC grains, crack bridging, crack deflection, crack branching, and crack arresting were introduced as the chief toughening mechanisms in the ZrB2–SiC–hBN system.

TEM study on the morphology and interface microstructure of C/C-SiC composites fabricated by liquid infiltration

Carbon fiber reinforced carbon and silicon carbide dual matrix composites (C/C-SiC) have been widely used in aerospace field thanks to their superior high temperature properties. However, the interfacial microstructure of the composites is still not thoroughly understood in details. In this study, we focus on the morphology of the reaction-formed SiC and the interfacial microstructure between carbon and SiC in the C/C-SiC composites fabricated by liquid silicon infiltration performed at 1700 °C. The fine microstructure of the composites was investigated systematically by using transmission electron microscopy. The results show that two main types of interface are formed in the composites: (i) the fine-grained SiC is formed adjacent to the pyrolytic carbon, followed by a layer of coarse-grained SiC; and (ii) some extremely large SiC grains are formed directly on the pyrolytic carbon, and different pyrolytic carbon coatings can be connected by these large SiC grains. Additionally, some subgrains with grain size of a few tens of nanometers or even smaller can be found near the pyrolytic carbon or carbon fibers. The formation mechanism of the SiC/carbon interface is also discussed based on the fine microstructure observed.

Effects of aluminum, boron and carbon additives on microstructure of porous silicon carbide ceramics and their properties

Abstract Porous SiC ceramics with in-situ plate-like grain growth were fabricated by pressureless-sintering using Al, B and C additives, and the effects of Al, B and C additives on the microstructure of porous SiC ceramics and their properties were investigated. Whereas the porous SiC ceramics sintered with B-C additives mostly consisted of fine SiC grains without grain growth, Al additives with B-C system enhanced plate-like grain growth of SiC in the porous SiC ceramics. Notably, the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives consisted of plate-like SiC grains of around 10-20 μm, and the plate-like SiC grains stuck out of the walls of pores and the sintered surface in the porous SiC ceramics. XRD results suggested that the plate-like SiC grains formed in the porous SiC ceramics sintered with Al-B-C additives mainly consisted from 6H (α-SiC) phase. Compressive strength and thermal conductivity of the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives at room temperature were 50 MPa and 10 W/m·K, respectively. From these results, it is concluded that high performance porous SiC ceramics with unique morphology of entangled large plate-like SiC grains were successfully achieved based on in-situ grain growth.

Saw damage as an etch mask for the acidic texturization of multicrystalline silicon wafers

The surface of multicrystalline silicon solar cells are etched by mixtures of HF, HNO3 and H2SiF6 in order to remove saw damage caused by wafer slicing, as well as to create a water surface topography that provides a low reflectance for incident light, otherwise known as the texture. Topographically analyzing wafer surfaces before and after etching has revealed that the saw damage controls the texturized wafer surface's final topography. The first key factor is the dimension and magnitude of the plastic stress field introduced by indenting SiC grains into the wafer surface during the wafering process. The second key factor is that lattice-stressed silicon is etched at a higher rate than unstressed bulk silicon. At the wire entrance, side sharp and large SiC grains create the deepest indention pits, and therefore the deepest of the water surface stress fields. The lattice-disturbed silicon inside these pits is etched at a higher rate compared to the pit's side walls, which are uniformly attacked across the wafer area. Consequentially, existing pits deepen, and these areas generate the wafer's lowest reflectivity. At the wire exit side, a higher number of smaller and rounder SiC particles indent the surface and create more numerous and shallower indention pits compared to the wire entrance side. The resulting stress field is less deep, so less silicon is removed from inside of these pits during etching compared to the wire entrance side. This yields to a wafer surface region consisting of shallowly etched pits and higher reflectance. It is concluded that the saw damage acts like an etch mask in the texturization of multicrystalline silicon wafers.

Revisiting the lifetime estimate of large presolar grains in the interstellar medium

Some very large (>0.1μm) presolar grains are sampled in meteorites. We reconsider the lifetime of very large grains (VLGs) in the interstellar medium focusing on interstellar shattering caused by turbulence-induced large velocity dispersions. This path has never been noted as a dominant mechanism of destruction. We show that, if interstellar shattering is the main mechanism of destruction of VLGs, their lifetime is estimated to be ≳108yr; in particular, very large SiC grains can survive for ~1Gyr. The lifetimes obtained for VLGs are comparable to the longest residence time derived for some presolar grains based on the cosmic-ray exposure time. However, most presolar SiC grains show residence times significantly shorter than 1Gyr, which may indicate that there is a more efficient mechanism than shattering in destroying VLGs, or that VLGs have larger velocity dispersions than 10kms−1. We also argue that the enhanced lifetime of SiC relative to graphite can be the reason why we find SiC among μm-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite.

Formation mechanism of large SiC grains on SiC fiber surfaces during heat treatment

Three low oxygen-containing SiC fibers with different surface compositions were heat-treated at high temperature in argon, and large SiC particles were observed on the surfaces of SiC fibers with oxygen-containing surface layers. The SiC particles were identified as single crystal β-SiC. The growth process, formation mechanism of these large SiC grains and their influence on fiber strength were investigated. Nano-sized SiC particles were precipitated from the decomposition of SiCxOy in the fiber surface that acted as nuclei, and these nuclei grew gradually to form large grains because of the gaseous reaction of SiO and CO. The formation of large SiC grains was related closely to the SiC fiber surface composition, which affected SiC nucleation and SiO diffusion from the core to the fiber surface. The large grains influenced the fiber strength degradation significantly and should be avoided to retain high fiber strength.

Hardness and toughness of hot pressed ZrB2–SiC composites consolidated under relatively low pressure

ZrB2-based composites, containing 15, 20, 25, and 30 vol% SiC, have been prepared by hot pressing at temperatures of 1700, 1850 and 2000°C for 30min under a relatively low pressure of 10MPa. Densification and mechanical properties of ZrB2–SiC composites have been investigated. Fully dense ZrB2–30 vol% SiC composite with a relative density of 99.8% is obtained at 2000°C. The highest values of Vickers hardness (21.3 GPa) and fracture toughness (4.7MPam1/2) belong to this sample. Vickers hardness increases exponentially as the relative density of composite increases. A simplified equation was developed for the Vickers hardness of the investigated ZrB2–SiC composites as a function of relative density and SiC content. Microstructural investigation by means of optical and scanning electron microscopy shows that by addition of SiC particles, some toughening mechanisms such as crack deflection, crack branching, microcracking, crack bridging, break of large SiC grains, and crack arresting by porosity are appeared.

An Original Way to Investigate Silver Migration Through Silicon Carbide Coating in TRISO Particles

Extensive release of the metastable silver nuclide Ag110m from fully intact tristructural‐isotropic (TRISO) particles raises concerns over safety of advanced nuclear reactors. In this study, we propose a new model to interpret the silver migration mechanism in SiC based on experimental observations from both Ag/SiC composite pellets and TRISO particles with an entrapped silver layer. For the Ag/SiC composite pellets heat treated at 1450°C, silicon was detected in the silver phase, amorphous carbon was found, and new β‐SiC had formed at the Ag/SiC interface. The results indicate that Ag in its liquid state reacts with SiC by forming a Ag–Si alloy. Carbon precipitates as a second phase or reacts with the Ag–Si alloy to form new SiC. Results from the heat‐treated TRISO particles trapping Ag show that Ag penetrates through the SiC layer and is present in either “finger‐shape” or “wedge shape” at the SiC grain boundaries. Ag was also found inside abnormally large SiC grains at the trailing edge between Ag and SiC, indicating the recrystallization of SiC. A dissolution‐reaction model was proposed to explain Ag migration through SiC, and this model is supported by thermodynamic calculations.

Microstructure and Thermal Conductivity of Silicon Carbide with Yttria and Scandia

A fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 vol% Y2O3–Sc2O3 additives. The ceramic had a bimodal microstructure consisting of large and small equiaxed SiC grains. Observation with high‐resolution transmission electron microscopy (HRTEM) showed two kinds of homophase (SiC/SiC) boundaries, that is crystallized and clean boundaries, and a fully crystallized junction phase. The thermal conductivity of the SiC ceramic was 234 W (m·K)−1 at room temperature. The high thermal conductivity was attributed to a clean SiC lattice and good contiguity between SiC grains.

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.

Al-B-C 조제 β-SiC의 스파크 플라즈마 소결에 미치는 α-SiC seed 첨가 영향: 미세 구조 변화

The unique features of spark plasma sintering process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. <TEX>$\beta$</TEX>-SiC powder with 0, 2, 6, 10 wt% of <TEX>$\alpha$</TEX>-SiC particles (seeds) and 4 wt% of Al-B-C (sintering aids) were spark plasma sintered at <TEX>$1700-1850^{\circ}C$</TEX> for 10 min. The heating rate, applied pressure and sintering atmosphere were kept at <TEX>$100^{\circ}C/min$</TEX>, 40 MPa and a flowing Ar gas (500 CC/min). Microstructural development of SiC as function of seed content and temperature during spark plasma sintering was investigated quantitatively and statistically using image analysis. Quantitative image analyses on the sintered SiC ceramics were conducted on the grain size, aspect ratio and grain size distribution of SiC. The microstructure of SiC sintered up to <TEX>$1700^{\circ}C$</TEX> consisted of equiaxed grains. In contrast, the growth of large elongated SiC grains in small matrix grains was shown in sintered bodies at <TEX>$1750^{\circ}C$</TEX> and the plate-like grains interlocking microstructure had been developed by increasing sintering temperature. The introduction of <TEX>$\alpha$</TEX>-SiC seeds into <TEX>$\beta$</TEX>-SiC accelerated the grain growth of elongated grains during sintering, resulting in the plate-like grains interlocking microstructure. In the <TEX>$\alpha$</TEX>-SiC seeds added in <TEX>$\beta$</TEX>-SiC, the rate of grain growth decreased with <TEX>$\alpha$</TEX>-SiC seed content, however, bulk density and aspect ratio of grains in sintered body increased.

Effect of the specimen size on strength of Si 3N 4 + SiC composite

Bending and contact strength of a recently developed carbon derived Si 3N 4 + SiC micro/nanocomposite have been investigated in four-point and opposite sphere modes using specimens with two different sizes of 3 mm × 4 mm × 45 mm and 1 mm × 2 mm × 20 mm. The fracture origins during bending test were clusters of pores and large SiC grains with different size, shape and location, which resulted in relatively low strength (675 MPa and 832 MPa) and Weibull moduli (6.4 and 8.6). The fracture origins in specimens tested in contact mode were cone cracks, originated and arised during the loading, and their similar size and shape, together with the preexisting stress field resulted in high strength (1167 MPa and 1997 MPa) and relative high Weibull moduli (17 and 15). In agreement with the theory, bending test specimens with smaller effective volume exhibited higher strength. In contact test, interaction of cone cracks occured in the small specimens at lower load as in larger one, which resulted in the lower strength of small specimens.

INTERSTELLAR RESIDENCE TIMES OF PRESOLAR SiC DUST GRAINS FROM THE MURCHISON CARBONACEOUS METEORITE

The time span between the formation of presolar grains in stellar outflows and their incorporation into early solar-system solids is poorly constrained. Knowledge of this time span is essential for a better understanding of the processing of grains in the interstellar medium (ISM) and formation processes of the solar system. Here, we report interstellar residence times of ∼3–1100 Myr for large (∼5–50 μm) presolar SiC grains, based on their content of He and Ne produced by Galactic cosmic rays. A majority of these grains have interstellar residence times on the order of a few tens up to less than 200 Myr, considerably shorter than theoretical estimates of interstellar dust lifetimes (∼500 Myr), but long enough to require formation of the majority of the grains before that of the presolar molecular cloud. The age distribution may be explained by starburst activity 1–2 billion years prior to the birth of the Sun. Our findings provide essential "ground truth" to constrain models of interstellar dust lifetimes and destructive processes in the ISM.

Mechanical properties of carbon-derived Si3N4+SiC micro/nano-composite

In this study the mechanical properties of a recently developed carbon-derived Si3N4+SiC micro/nano-composite have been investigated with the aim to study the influence of the SiC addition on the room-temperature strength and high-temperature creep behavior. The bending strength values are in the interval from σ0 = 675 MPa to σ0 = 832 MPa with the Weibull modulus of 6.4 and 8.6 The contact strength is changing from σ0R = 1997 MPa to σ0R = 1167 with mR = 17 and mR = 15. Fracture origins in the specimens tested in bending are clusters of pores and large SiC grains and in the specimens tested in contact mode cone cracks. The composite exhibits high creep resistance thanks to the changed chemical composition and viscosity of the intergranular phase comparing to the monolithic Si3N4 and due to the interlocked silicon nitride grains by the intergranularly located SiC nanoparticles.

Bending and Contact Strength of a Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt;+SiC Micro/Nano Composite

Bending and contact strength of a carbon derived in-situ reinforced Si3N4-SiC micro/nanocomposite have been investigated. Four-point bending strength was measured using specimens with different effective volume and Weibull statistical analysis has been used for characterization of the strength values in the form of characteristic strength and Weibull modulus. The characteristic strength values of the investigated composite were σ0 = 675 MPa and σ0 = 832 MPa with the Weibull modulus of 6,4 and 8,6 for the specimens with effective volume 15,9 mm3 and 1,28 mm3 respectively. Contact strength of the investigated material was carried out using sphere on sphere and roller on roller methods and the obtained results are σ0S = 1997 MPa, σ0R = 1242 MPa, and mS = 17,1 mR = 6. Fracture origins in the specimens with effective volume of 15.9 mm3 tested in four-point bending were surface and volume located technological defects with dimensions from 10 μm to 180 μm, mainly in the form of clusters of pores and large SiC grains. Fracture mirror sizes were measured and the mirror constant was calculated. Fracture origins in the specimens with effective volume of 1.28 mm3 tested in four-point bending were surface (subsurface) defects with the same type as for the specimens with a higher effective volume. The fracture during the contact strength test „roller/roller“ has been initiated under the surface of the specimens and during the contact strength test „sphere/sphere“ by creation and growth of the cone cracks to critical size.

Fractographic Montage for a Si3N4–SiC Nanocomposite

A silicon nitride–silicon carbide nanocomposite has been prepared by an in situ method that utilizes C+SiO2 carbo‐thermal reduction during the sintering process. The materials consist of a silicon nitride matrix, with an average grain size of 140 nm, and inter‐ and intragranular SiC particles with sizes of approximately 250 and 45 nm, respectively. The four‐point bending strength and its distribution were investigated. The fracture origins were identified and characterized using fractographic methods, and a fractographic montage of the Weibull plot and fracture origins was constructed. The fracture origins were subsurface and volume located processing defects with sizes from 5 to 460 μm, mainly in the form of clusters of pores, together with clusters of large SiC grains.