SiC Grains Research Articles

Diverse assemblage of presolar and solar system materials in anhydrous interplanetary dust particles: Coordinated NanoSIMS and TEM analyses

A coordinated TEM and NanoSIMS isotopic imaging study of microtome sections of three anhydrous interplanetary dust particles (IDPs) revealed a diverse collection of primitive materials having disparate origins and histories. Presolar silicate grains that likely originated in asymptotic giant branch (AGB) stars were present in each IDP at abundances ranging from 140 (+320/-120) ppm to 2000 (+4600/-1700) ppm. A unique compound presolar grain was identified that consisted of a crystalline spinel core and amorphous silicate mantle having heterogeneous Fe content. This compound grain traces the changing conditions in the circumstellar region during condensation and is the first identified presolar spinel in an IDP. A presolar SiC grain, also rare in IDPs, was found to be enriched in 13C, 14N, and 28Si, consistent with mainstream SiC that originated in ∼ solar metallicity AGB stars. We determine presolar spinel and presolar SiC abundances of 760 (+1700/-630) ppm and 190 (+440/-160) ppm, respectively, in the individual IDPs.Two elongate whisker-like enstatite grains and one platy enstatite were found to have near-terrestrial O isotopic compositions (δ18O = -17 – 18 ‰) and show chemical evidence of equilibrium condensation from a high temperature gas. Two highly 16O-rich silicates with near-solar O isotopic compositions (δ18O = -79 ‰ and -83 ‰) were also identified and may represent the primordial dust reservoir. These silicates were crystalline equilibrated aggregates. The wide range of isotopic compositions observed in these silicate grains suggests they condensed from isotopically diverse reservoirs in the protoplanetary disk in different locations and/or times. The 16O-rich grains likely condensed in the inner solar system and were subsequently transported to the outer solar system, while grains having terrestrial O isotopic compositions likely condensed from the gas phase in the terrestrial planet forming region or beyond.The IDPs showed bulk 15N enrichments (δ15N = 15 – 129 ‰) and contained 15N-rich hotspots up to 1150 ‰, consistent with the presence of molecular cloud material. IDPs U2015D21 and W7013E17 had bulk O isotopic compositions that were offset from the carbonaceous chondrite anhydrous minerals line by ∼ 10 ‰ to more 17O-rich compositions. This 17O enrichment cannot be explained by the observed abundance of 17O-rich presolar grains in these particles and the source remains unknown. IDP W7027E6 had an unusual isotopically heavy bulk O isotopic composition (δ17,18O = 40 ‰, Δ17O = 19 ‰). W7027E6 lacked hydrous phases and was therefore not likely altered by isotopically heavy primordial water. We propose that the high temperature mineral assemblage in W7027E6 condensed in the inner solar system from an 16O-poor reservoir that existed prior to O isotope homogenization in the early nebula and was subsequently transported to the outer solar system.

Preparation, microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al4SiC4 powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al4SiC4 and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si5+ ions at 650 °C using a fluence of 1.1 × 1016 ions/cm2, which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.

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.

Presolar O- and C-anomalous grains in unequilibrated ordinary chondrite matrices

Presolar grains are trace components in chondrite matrices. Their abundances and compositions have been systematically studied in carbonaceous chondrites but rarely in situ in other major chondrite classes. We have conducted a NanoSIMS isotopic search for presolar grains with O- and C-anomalous isotopic compositions in the matrices of the unequilibrated ordinary chondrites Semarkona (LL3.00), Meteorite Hills 00526 (L/LL3.05), and Northwest Africa 8276 (L3.00). The matrices of even the most primitive ordinary chondrites have been aqueously altered and/or thermally metamorphosed, destroying their presolar grain populations to varying extents. In addition to randomly placed isotope maps, we specifically targeted recently reported, relatively pristine Semarkona matrix areas to better explore the original inventory of presolar grains in this meteorite. In all samples, we found a total of 122 O-anomalous grains (silicates + oxides), 79 SiC grains, and 22 C-anomalous carbonaceous grains (organics, graphites). Average matrix-normalized abundances with 1σ uncertainties are 151-46+50 ppm O-anomalous grains, 53-12+14 ppm SiC grains and 56-14+19 ppm carbonaceous grains in Semarkona, 55-10+11 ppm (O-anom.), 22-4+5 ppm (SiC) and 3-1+2 ppm (carb.) in MET 00526 and 12-3+6 ppm (O-anom.), 15-5+7 ppm (SiC) and 1-1+3 ppm (carb.) in NWA 8276. In relatively pristine ordinary chondrites and in primitive carbonaceous and C-ungrouped chondrites, the O and C isotopic composition of presolar grains and their matrix-normalized abundances are similar, despite the likely differences in chondrite-formation time and nebular location. These results suggest a relatively homogenous distribution of presolar dust across major chondrite-forming reservoirs in the solar nebula. Secondary asteroidal processes are mainly responsible for differences in presolar grain abundances between and within chondrites, highlighting the need to identify and target the most pristine chondrite matrices for such studies.

Reactive sintering and thermodynamics of Ti3SiC2/SiC composites

The synthesis of Ti3SiC2/SiC composites was studied by testing 3TiC+2Si or 3TiC+2.2Si + xAl (x = 0 or 0.2) as sintering mixtures. In order to reveal the mechanism of reactive sintering of Ti3SiC2/SiC composites, the mixtures were sintered either by pressureless sintering or by sintering under pressure between 1200 °C and 1500 °C during 15min. The results showed that the nucleation of SiC grains occurs first at the surface of the TiC grains, limiting therefore the progression of the conversion at 1200 °C and 1300 °C. Silicon defects are the main limitation of the synthesis of Ti3SiC2/SiC composites because of volatilization and molten silicon losses. The addition of aluminum promotes the conversion by reacting with oxygen and resulting in the formation of Al2O3 at the grain boundaries. The optimum experimental parameters for the synthesis of Ti3SiC2/SiC composites were determined to be a TiC/Si/Al molar ratio of 3/2.2/0.2 maintained at 1500 °C for 15min.

Ultrafast low-temperature near-seamless joining of Cf/SiC using a sacrificial Pr3Si2C2 filler via electric current field-assisted sintering technique

A novel layered structure material, Pr3Si2C2, was synthesized at a low temperature of 850 °C using a molten salt approach for the first time, and subsequently used as the joining filler for carbon fibers reinforced SiC composites (Cf/SiC). A robust near-seamless Cf/SiC joint was successfully obtained at 1509 °C (Ti) for 30 s, while an ultrafast heating rate of 6000 °C/min was applied via electric field-assisted sintering technology. The near-seamless joining process was attributed to the newly precipitated SiC grains, which were densified well with the Cf/SiC matrix by liquid-assisted sintering. The liquid phase was in-situ formed by the eutectic reaction between Pr3Si2C2 and SiC. The shear strength of the near-seamless joint obtained at 1509 °C for 30 s was 17.6 ± 3.0 MPa. The failure occurred in the Cf/SiC matrix. The formation of near-seamless Cf/SiC joints dismisses the issues related to thermal mismatch between Cf/SiC matrices and traditional joining fillers.

Mechanical and tribological properties of SiC-GNPs composites prepared by oscillatory pressure sintering

SiC-based ceramics, as structural parts and wear-resistant parts, are required to possess outstanding mechanical properties and wear resistant. Herein, to prepare SiC-based ceramics with desirable properties, the effects of graphene nanoplatlets (GNPs) content on the densification, microstructure, mechanical and tribological properties of SiC-GNPs composites by using an oscillatory pressure sintering (OPS) process were investigated. The addition of GNPs could inhibit the growth of SiC grains, and the resultant mechanisms such as crack bridging, deflection, and GNPs pull-out also promoted the strengthening and toughening of such composites. The flexural strength and fracture toughness of the SiC-1.0 wt% GNPs composites attained the maximum values of 515 MPa (∼27% increase) and 7.11 MPa·m1/2 (∼26% increase), respectively. In addition, the friction behavior of SiC-GNPs composites was analyzed by selecting GCr15 steel ball as friction side under drying friction conditions. Compared with pure SiC ceramics, the friction coefficient and wear rate of SiC-1.0 wt% GNPs ceramics were as low as 0.65 (reduced by ∼25%) and 5.17 × 10−7 mm3·N−1·m−1 (reduced by ∼41%), respectively. This phenomenon can mainly be attributed to the lubricating film formed by GNPs during sliding and excellent mechanical properties.

Effect of low activation rare-earth oxides on sintering of β-SiC

The feasibility of sintering β-SiC at 1850 ℃ with using low-activation rare-earth oxides as sintering additives was studied. Thermodynamic calculation suggested that all five rare-earth oxides (Y2O3, Sc2O3, CeO2, La2O3, and Pr6O11) were suitable sintering additives. Comprehensive microstructure characterization further revealed that Pr6O11 was the most effective sintering additive as it suffered the least influence from the content effect. A mechanism based on liquid phase sintering was proposed based on microstructure observations. The radiation stability of sintered SiC was investigated as well. The intergranular phases (rare-earth oxides and silicates) and SiC grains remained after being irradiated to 17 dpa at 750 ℃.

Presolar Stardust in Asteroid Ryugu

We have conducted a NanoSIMS-based search for presolar material in samples recently returned from C-type asteroid Ryugu as part of JAXA's Hayabusa2 mission. We report the detection of all major presolar grain types with O- and C-anomalous isotopic compositions typically identified in carbonaceous chondrite meteorites: 1 silicate, 1 oxide, 1 O-anomalous supernova grain of ambiguous phase, 38 SiC, and 16 carbonaceous grains. At least two of the carbonaceous grains are presolar graphites, whereas several grains with moderate C isotopic anomalies are probably organics. The presolar silicate was located in a clast with a less altered lithology than the typical extensively aqueously altered Ryugu matrix. The matrix-normalized presolar grain abundances in Ryugu are ppm for O-anomalous grains, ppm for SiC grains, and ppm for carbonaceous grains. Ryugu is isotopically and petrologically similar to carbonaceous Ivuna-type (CI) chondrites. To compare the in situ presolar grain abundances of Ryugu with CI chondrites, we also mapped Ivuna and Orgueil samples and found a total of 15 SiC grains and 6 carbonaceous grains. No O-anomalous grains were detected. The matrix-normalized presolar grain abundances in the CI chondrites are similar to those in Ryugu: ppm SiC and ppm carbonaceous grains. Thus, our results provide further evidence in support of the Ryugu–CI connection. They also reveal intriguing hints of small-scale heterogeneities in the Ryugu samples, such as locally distinct degrees of alteration that allowed the preservation of delicate presolar material.

Abundant presolar silicates of the CM chondrite Asuka 12169: Implications for the thermal and aqueous alteration of the CM parent body

The solar system could be separated into two zones based on the isotopic dichotomy between non-carbonaceous and carbonaceous groups, with the latter likely accreted in the outer solar system. Among carbonaceous groups, the CM chondrite contains high abundances of organic carbon and water. They have undergone aqueous alteration, thermal metamorphism and brecciation to different degrees (e.g., Rubin et al., 2007; Tonui et al., 2014; Zolensky et al., 1997), which contributed to erasing most of the solar nebular records. Asuka 12169 was reported as the most primitive CM chondrite based on petrological and geochemical results, with little aqueous alteration (Kimura et al., 2020). In this paper, we report a survey of presolar grains in the fine-grained matrix and the accretionary rims of chondrules and CAIs in this meteorite, based on NanoSIMS mapping of C-, O-, and Si-isotopes. A total of 158 presolar grains were identified, including 119 silicates/oxides (208 ± 20 ppm), 38 SiC (73 ± 12 ppm) and 1 carbonaceous grain (2-2+5 ppm). These abundances are within the maximum abundance ranges of primitive chondrites (80–280 ppm for O-rich grains and 10–180 ppm for C-rich grains). In comparison with most CM chondrites (<40 ppm), Asuka 12169 is uniquely rich in presolar silicates (185 ± 18 ppm), with a high presolar silicate/oxide ratio of ∼8, therefore providing robust evidence for little aqueous alteration. The high abundances of presolar SiC and silicates in Asuka 12169 clearly show its pristine properties regarding both thermal and aqueous alteration. Group 1, 2, 3 and 4 subtypes of presolar O-rich grains account for 84%, 2.5%, 0.8% and 12.6%, respectively. One O-rich grain shows a high enhancement in 17O/16O and a subsolar 18O/16O ratio (17O/16O = 6.45 ± 0.09 × 10−3 and 18O/16O = 1.90 ± 0.02 × 10−3), indicating a stellar origin in binary star systems or novae. Most identified presolar SiC are mainstream grains of AGB origins. One with 28Si-excess is classified as an X grain, suggesting a supernova origin. There are two SiC grains that have 12C/13C < 10 but close-to-solar Si isotopic ratios, and are therefore classified as AB type. The pristine features of Asuka 12169 suggest that it was probably located in the outermost few kilometers of the CM asteroid, where temperature was high enough for sublimation of water ice under vacuum, but where no aqueous alteration occurred, and where the depth was enough for lithification. The high abundances of various types of presolar grains, together with the petrographic information of Asuka 12169, provide crucial constrains on the original properties and subsequent evolution of the CM asteroids.

Theoretical Estimate of the Half-life for the Radioactive 134Cs and 135Cs in Astrophysical Scenarios

We analyze the CsBa and Cs Ba β − decays, which are crucial production channels for Ba isotopes in asymptotic giant branch (AGB) stars. We calculate, starting from relativistic quantum mechanics, the effects of multichannel scattering onto weak decays, including nuclear and electronic excited states (ESs) populated above ≃10 keV, for both parent and daughter nuclei. We find increases in the half-lives for T > 108 K (by more than a factor of 3 for 134Cs) as compared to previous works based on systematics. We also discuss our method in view of these previous calculations. An important impact on half-lives comes from nuclear ES decays, while including electronic temperatures yields further increases of about 20% at energies of 10–30 keV, typical of AGB stars of moderate mass (M ≲ 8 M ⊙). Despite properly considering these effects, the new rates remain sensitively lower than the Takahashi & Yokoi values, implying longer half-lives at least above 8–9 keV. Our rate predictions are in substantial accord with recent results based on the shell model, and strongly modify branching ratios along the s-process path previously adopted. With our new rate, nucleosynthesis models well account for the isotopic admixtures of Ba in presolar SiC grains and in the Sun.

Preparation of Ti3SiC2 MAX phase from Ti, TiC, and SiC by SPS

Ti3SiC2 MAX phase was prepared from Ti, TiC, and SiC using spark plasma sintering. Three sintering temperatures, namely 1450, 1550, and 1600 °C were employed. The effect of the atmosphere, dwell time, and SiC grain size were studied. The prepared samples were subjected to XRD, Vickers hardness, and SEM analyses. Phase composition of the samples revealed that a maximum MAX phase content of ∼90 wt% is achieved at the sintering temperature of 1550 °C. TiC (∼9 wt%) and small amount of SiC were identified as secondary phases. Values of the Vickers hardness exhibited a steady decrease with an increasing sintering temperature from 704 HV at 1450 °C to 544 HV at 1600 °C due to the reduction of the TiC content in the sample. The porosity of the samples with the highest Ti3SiC2 content reached 5%. The coefficient of thermal expansion fell into the interval from 9.8 to 11 × 10−6 K−1.

The effects of annealing atmosphere and intrinsic component on high temperature evolution behaviors of SiC fibers

In this work, the second (F1) and third (F2) generation SiC fibers were treated at different temperature under vacuum and Ar atmosphere to analyze the influence of fiber composition and atmosphere on the high temperature evolution of SiC fibers. The results reveal that SiC decomposition and graphene formation could occur in SiC fibers over 1600 °C under vacuum or 2000 °C under Ar atmosphere. The mechanical properties of both F1 and F2 fibers are remarkably degraded accompanied by the SiC decomposition. The formed graphene size is related to the SiC grain size before decomposition, while the increment of temperature will promote SiC grain growth and SiC decomposition simultaneously. Under Ar atmosphere, the Si sublimation rate is slow so that SiC grains can grow fully, therefore, the converted graphene crystallite size is much larger than that under vacuum. SiC decomposition rate is related to the excess carbon and pores in the fibers. The inhibition effect of carbon micro-coating around SiC grains formed by excess carbon and promotion effect of pores formed by SiCxOy decomposition co-exist in SiC fibers. This work reveals that effective inhibition of Si sublimation is of significance for the stability of SiC fibers under ultra-high temperature.

TEM analyses of in situ presolar grains from unequilibrated ordinary chondrite LL3.0 Semarkona

We investigated six presolar grains from very primitive regions of the matrix in the unequilibrated ordinary chondrite Semarkona with transmission electron microscopy (TEM). These grains include one SiC, one oxide (Mg-Al spinel), and four silicates. This is the first TEM investigation of presolar grains within an ordinary chondrite host (in situ) and the first TEM study to report on any presolar silicates (in or ex situ) from an ordinary chondrite. Structural and elemental compositional studies of presolar grains located within their meteorite hosts have the potential to provide information on conditions and processes throughout the grains’ histories.Our analyses show that the SiC and spinel grains are stoichiometric and well crystallized. In contrast, the majority of the silicate grains are non-stoichiometric and poorly crystallized. These findings are consistent with previous TEM studies of presolar grains from interplanetary dust particles and chondritic meteorites. The individual silicates have Mg#’s ranging from 15 to 98. Internal compositional heterogeneities were observed in several grains, including Al in the SiC, Mg and Al in the spinel, and Mg, Si, Al, and/or Cr in two silicates. We interpret the poorly crystalline nature, non-stoichiometry, more Fe- rather than Mg-rich compositions, and/or compositional heterogeneities as features of the formation by condensation under non-equilibrium conditions.Evidence for parent body alteration includes rims with mobile elements (S or Fe) on the SiC grain and one silicate grain. Other features characteristic of secondary processing in the interstellar medium, the solar nebula, and/or on parent bodies, were not observed or are better explained by processes operating in circumstellar envelopes. In general, there was very little overprinting of primary features of the presolar grains by secondary processes (e.g., ion irradiation, grain-grain collisions, thermal metamorphism, aqueous alteration). This finding underlines the need for additional TEM studies of presolar grains located in the primitive matrix regions of Semarkona, to address gaps in our knowledge of presolar grain populations accreted to ordinary chondrites.

Dust Formation in the Wind of AGB Stars—The Effects of Mass, Metallicity and Gas-Dust Drift

Dust production in the wind of stars evolving through the asymptotic giant branch is investigated by using a stationary wind model, applied to results from stellar evolution modelling. Results regarding 1–8M⊙ stars of metallicities Z=0.014 (solar) and Z=2×10−3 are compared, to infer the role played by stellar mass and chemical composition on the dust formation process. We find a dichotomy in mass: stars of (initial) mass below ∼3M⊙ produce silicates and alumina dust before they become carbon stars, then carbonaceous dust; the higher mass counterparts produce only silicates and alumina dust, in quantities that scale with metallicity. The presence of drifts with average drift velocities ∼5 Km/s leads to higher dust formation rates owing to the higher growth rates of the dust grains of the different species. However, no significant changes are found in the overall optical depths, because the higher rate of dust formations favours a fast expansion of the wind, that prevents further significant production of dust. As far as oxygen-rich stars are concerned, the presence of drifts makes the main dust component to change from olivine to pyroxene. The release of the assumption that the number density of the seed particles is independent of the dust species considered affects dust formation in the wind of carbon stars: a factor 10 reduction in the density of the seeds of SiC leads to bigger sized SiC grains, and partly inhibits the formation of solid carbon, since the wind is accelerated and the densities in the carbon formation zone are smaller. No substantial differences are found in the winds of oxygen-rich stars.

Low‐temperature Pr3Si2C2‐assisted liquid‐phase sintering of SiC with improved thermal conductivity

AbstractA novel Pr3Si2C2 additive was uniformly coated on SiC particles using a molten‐salt method to fabricate a high‐density SiC ceramics via liquid‐phase spark plasma sintering at a relatively low temperature (1400°C). According to the calculated Pr–Si–C‐phase diagram, the liquid phase was formed at ∼1217°C, which effectively improved the sintering rate of SiC by the solution–reprecipitation process. When the sintering temperature increased from 1400 to 1600°C, the thermal conductivity of SiC increased from 84 to 126 W/(m K), as a consequence of the grain growth. However, an increasing amount of the sintering additive increased the interfacial thermal resistance, resulting in a decrease of thermal conductivity of the materials. The highest thermal conductivity of 141 W/(m K) was obtained for the material having the largest SiC grains and an optimized amount of the additive at the grain boundaries and triple junctions. The proposed Pr3Si2C2‐assisted liquid‐phase sintering of SiC can be potentially used for the fabrication of SiC‐based ceramic composites, where a low sintering temperature would inhibit the grain growth of SiC fibers.

Microstructure and properties of porous SiC ceramics with AlN–RE2O3 (RE = Sc, Y, Lu) additives

The intrinsic microstructure and crystalline phases of porous SiC ceramics with 5 vol% AlN–RE2O3 (RE = Sc, Y, Lu) additives were characterized by high-resolution transmission microscopy with energy-dispersive spectroscopy and X-ray diffraction. The homophase (SiC/SiC) and heterophase (SiC/junction) boundaries were found to be clean; that is, amorphous films were not observed in the specimens. In addition, ScN, YN, and LuN were formed as secondary phases. The flexural strength and thermal conductivity of the ceramics were successfully tuned using different additive compositions. The flexural strength of the ceramics improved by a factor of ~3, from 11.7 MPa for the specimen containing Y2O3 to 34.2 MPa for that containing Sc2O3, owing to the formation of a wide necking area between SiC grains. For the same reason, the thermal conductivity improved by ~56%, from 9.2 W·m−1·K−1 for the specimen containing Lu2O3 to 14.4 W·m−1·K−1 for that containing Sc2O3.

The Influence of Material Type and Hardness on the Number of Embedded Abrasive Particles during Airborne-Particle Abrasion.

(1) Background: This paper aims to determine the influence of hardness on the number of abrasive material grains (SiC) embedded on the surface metal alloys and ZrO2 during abrasion. (2) Methods: Cylindrical samples were created: 315 made of Cr/Co, Ni/Cr or Ti, and 315 made of sintered ZrO2- 3TPZ-Y. These were divided into four groups (each n = 35 samples), and were treated with SiC grain sizes 50, 110, and 250 µm at pressures 0.2, 0.4, or 0.6 MPa. The samples were then observed in SEM to study SE and BSE. The surface coverage of abrasive material particles was determined by quantitative metallography. Five samples from each group were subjected to hardness measurements. The results were compared with three-factor variance analysis with using the post hoc Tukey test. (3) Results: The highest amount (40.06) of embedded abrasive was obtained for Ti alloy with a gradation of 250µm at a pressure of 0.6 MPa. The smallest amount of embedded grain (2.66) was obtained for ZrO2 for the same treatment parameters. (4) Conclusions: The amount of embedded abrasive particles depends on the type of treated material, gradation particles, and the amount of applied pressure. Harder treated materials are more resistant to grains of abrasive becoming embedded on surface.

Sakurai’s Object revisited: new laboratory data for carbonates and melilites suggest the carrier of 6.9-μm excess absorption is a carbonate

ABSTRACTWe present new room-temperature 1100–1800-cm−1 spectra of melilite silicates and 600–2000-cm−1 spectra of three randomly orientated fine-grained carbonates to determine the possible carrier(s) of a 6.9-μm absorption feature observed in a variety of dense astronomical environments, including young stellar objects and molecular clouds. We focus on the low-mass post-asymptotic giant branch star Sakurai’s Object, which has been forming substantial quantities of carbonaceous dust since an eruptive event in the 1990s. Large melilite grains cannot be responsible for the 6.9-μm absorption feature because the similarly shaped feature in the laboratory spectrum was produced by very low (0.1 per cent by mass) carbonate contamination, which was not detected at other wavelengths. Due to the high band strength of the 6.9-μm feature in carbonates, we conclude that carbonates carry the astronomical 6.9-μm feature. Replacement of melilite with carbonates in models of Sakurai’s Object improves fits to the 6–7-μm Spitzer spectra without significantly altering other conclusions of Bowey’s previous models except that there is no link between the feature and the abundance of melilite in meteorites. With magnesite (MgCO3), the abundance of 25-μm-sized SiC grains is increased by 10–50 per cent and better constrained. The mass of carbonate dust is similar to the mass of polycyclic aromatic hydrocarbon dust. Existing experiments suggest that carbonates are stable below 700 K; however, it is difficult to ascertain the applicability of these experiments to astronomical environments, and more studies are required.

Preparation of superhydrophobic-superoleophilic ZnO nanoflower@SiC composite ceramic membranes for water-in-oil emulsion separation

Superhydrophobic-superoleophilic (SHB-SOL) wettability has proved its superiority in simultaneously enhancing the permeability and selectivity of porous interfacial materials for water-in-oil (w/o) emulsion separation. Here, ZnO nanoflower (NF) modified SiC (ZnO NF@SiC) composite ceramic membranes are reported through a chemical bath deposition method. ZnO NFs with different sizes were grown onto SiC grains forming tunable micro-nano hierarchical structures on the membrane surfaces. After n-octyltriethoxysilane grafting, all the composite ceramic membranes exhibit outstanding SHB-SOL wettability (water contact angle > 150° and sliding angle SA < 10°). The ZnO NF@SiC membrane owning a middle-sized ZnO NF displays the highest SHB-SOL property. When used for w/o emulsion separation, the SHB-SOL ZnO NF@SiC membranes show significantly improved oil flux and water rejection compared with the pristine and the sole silane grafted SiC membranes. The water rejection of the optimal membrane for 1000 ppm water-in-hexane emulsion is ∼99% and the initial state oil flux is ∼1300 L·m−2·h−1 under a transmembrane pressure of 1 bar, which is relatively competitive among the reported hydrophobic ceramic membranes. The mechanisms of surface hierarchical structures, wetting behavior and separation performance are further revealed. This work may offer new insights into preparing SHB-SOL ceramic membranes for practical w/o emulsion separation.