Effects of density and heat treatment of C/C preforms on microstructure and mechanical properties of C/C–SiC composites

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C–SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C–SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C–SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C–SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon–carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C–SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.     

Microstructure and ablation mechanism of C/C–SiC composites

C/C–SiC composites were prepared by molten infiltration of silicon powders, using porous C/C composites as frameworks. The porosities of the C/C–SiC composites were about 0.89–2.8 vol%, which is denser than traditional C/C composites. The ablation properties were tested using an oxyacetylene torch. Three annular regions were present on the ablation surface. With increasing pyrocarbon fraction, a white ceramic oxide layer formed from the boundary to the center of the surface. The ablation experimental results also showed that the linear and mass ablation rates of the composites decreased with increasing carbon fraction. Linear SiO₂ whiskers of diameter 800 nm and length approximately 3 μm were formed near the boundaries of the ablation surfaces of the C/C–SiC composites produced with low-porosity C/C frameworks. The ablation mechanism of the C/C–SiC composites is discussed, based on a heterogeneous ablation reaction model and a supersaturation assumption.     

Friction and wear properties of C/C–SiC braking composites

Two series of C/C–SiC composites were fabricated via precursor infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) using porous C/C composites with different original densities as preforms, respectively. The tribological characteristics of C/C–SiC braking composites were investigated by means of MM-1000 type of friction testing machine. The friction and wear behaviors of the two series of composites were compared and the factors that influence the friction and wear properties of C/C–SiC composites were discussed. Results show that the friction and wear properties relate close-knit to the content of SiC and porosity. As the original preform density increasing, the content of SiC and porosity decrease, and then the friction coefficient increases obviously, the braking time and the wear rate both decrease. Preparation techniques play an important role in the tribological properties of C/C–SiC composites. Compared with PIP process, the samples from CVI have a little higher friction coefficient, shorter braking time and higher wear rate.     

New Trends and Future Opportunities in the Enzymatic Formation of C−C,C−N,and C−O bonds

Organic chemistry provides society with fundamental products we use daily. Concerns about the impact that the chemical industry has over the environment is propelling major changes in the way we manufacture chemicals. Biocatalysis offers an alternative to other synthetic approaches as it employs enzymes, Nature's catalysts, to carry out chemical transformations. Enzymes are biodegradable, come from renewable sources, operate under mild reaction conditions, and display high selectivities in the processes they catalyse. As a highly multidisciplinary field, biocatalysis benefits from advances in different areas, and developments in the fields of molecular biology, bioinformatics, and chemical engineering have accelerated the extension of the range of available transformations (E. L. Bell et al., Nat. Rev. Meth. Prim. 2021 , 1, 1–21). Recently, we surveyed advances in the expansion of the scope of biocatalysis via enzyme discovery and protein engineering (J. R. Marshall et al., Tetrahedron 2021 , 82, 131926). Herein, we focus on novel enzymes currently available to the broad synthetic community for the construction of new C−C, C−N and C−O bonds, with the purpose of providing the non-specialist with new and alternative tools for chiral and sustainable chemical synthesis.     

Experimental and numerical studies of the lower flammability limit of mixtures of C1–C5 hydrocarbons with air

The lower concentration limit of flammability of hydrocarbon-air mixtures has been studied experimentally and by numerical simulation. Simulation using a detailed mechanism of chemical reactions has shown that calculations results are in good agreement with experimental data on the effect of water vapor on the lean concentration limit of flammability of hydrocarbon mixtures with air. The presence of water vapor at low concentrations in the mixture does not affect the lower concentration limit of flammability, but, at the same time, significantly changes the flame propagation velocity. Key words: concentration limits of flammability, opposed-jet premixed flame, hydrocarbons.     

Microstructure and mechanical properties of WC–C nanocomposite films

WC–C nanocomposite film was prepared by using a hybrid deposition system of r.f.-PACVD and DC magnetron sputtering. W concentration in the film was varied from 5.2 to 42 at.% by changing the CH₄ fraction of the mixture sputtering gas of Ar and CH₄. Hardness, residual compressive stress and electrical resistivity were characterized as a function of W concentration. Raman spectroscopy, XRD and high resolution TEM were employed to analyze the structural change in the film for various W concentrations. In the present W concentration range, the film was composed of nano-sized WC particles of diameter less than 5 nm and hydrogenated amorphous carbon matrix. Content of the WC particles increased with increasing W concentration. However, the mechanical properties of the film increased only when the W concentration was higher than 13 at.%. Structural analysis and electrical conductance measurements evidently showed that the increase in hardness and residual stress occurred as the WC particles were in contact with each other in the amorphous carbon matrix.     

The influence of B4C and MgB2 additions on the behavior of MgO–C bricks

Carbon-containing refractory materials have received great attention over the last years due to their importance in the steelmaking process. The oxidation of carbon present in refractory materials at temperatures above 500 °C is usually accompanied by the decrease of their mechanical strength and chemical resistance. Aiming to improve the oxidation resistance of carbon-oxide refractories, the use of materials known as antioxidants has been extensively studied. In this work we evaluated the performance of MgB₂ and B₄C antioxidants when incorporated into MgO–C bricks. We observed that the co-addition of metallic antioxidants and B₄C or MgB₂ leads to refractory bricks with enhanced hot modulus of rupture and resistance against oxidation and slag corrosion. However, the excessive addition of these antioxidants could impair the performance of the obtained bricks. Thus, when determining the optimum concentration of MgB₂ and B₄C to be added into MgO–C refractories, one must take into consideration this behavior.     

Preparation and Characterization of C10–C14 Alkyl Cellulose Ester Sulfate Surfactant

The aim of this work is to synthesize surfactants based on cellulose with different molecular weights. Raw cotton cellulose was tailored into cellulose segments with different molecular weights by a hydrothermal process, then the average degree of polymerization (DP) was determined by viscosimetry and the molecular weight distribution was estimated by gel permeation chromatography. The C₁₀–C₁₄ alkyl cellulose ester sulfate surfactants were prepared by hydrophilic sulfonation and hydrophobic esterification. The surface tension of the surfactants solution was obtained by the Wilhelmy plate method. Results showed that the cellulose segments presented a broader distribution compared with the raw material. The critical micelle concentration (CMC) value decreased from 1.08 to 0.86 wt% as the hydrophobic chain length was increased from 10 to 14. The CMC values of cellulose surfactants with C₁₄-acyl chloride hydrophobization decreased from 1.32 to 0.86 wt% as the DP was decreased from 2,700 to 296.     

Effect of nanocarbon sources on microstructure and mechanical properties of MgO–C refractories

A study of microstructural evolution, mechanical and thermo-mechanical properties of MgO–C refractories, based on graphite oxide nanosheets (GONs), carbon nanotubes (CNTs) and carbon black (CB), was carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending and thermal shock tests. Meanwhile, these results were compared to the conventional MgO–C refractory containing 10 wt% flaky graphite prepared under the same conditions. The results showed that higher cold modulus of rupture was obtained for the composition containing GONs, and the composition containing CNTs exhibited larger displacement after coking at 1000 °C and 1400 °C. Also, the addition of nanocarbons led to an improvement of the thermal shock resistance; in particular, both compositions containing CNTs and CB had higher residual strength ratio, approaching the thermal shock resistance of the reference composition containing 10 wt% flaky graphite, as it was associated with the presence of nanocarbons and in-situ formation of ceramic phases in the matrix.     

Characterization and properties of iron-incorporated gismondine prepared at 80°C

Iron-incorporated zeolites were successfully synthesized at a low temperature such as 80°C by choosing appropriate starting materials and characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES), wide-angle X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and magnetic susceptibility. ICP-AES showed␣that Fe component can be readily incorporated␣up to a maximum extent of Fe substitution, Fe/(Fe + Al) × 100 = 22.7%. XRD measurements suggested that the zeolites obtained have a crystal structure of gismondine type. The characterizations identified that the Fe component present in the products is all incorporated into the zeolite framework. The ammonia and water desorption profiles were compared for Fe-free and 22.7% Fe-zeolites ion-exchanged for NH₄⁺ by means of TG-MS and DSC. The ammonia desorption peak temperatures considerably shifted toward lower temperatures by the introduction of Fe, suggesting decreased solid acidity. DSC thermograms of the as-synthesized gismondines revealed that they do not contain free water (i.e., water not coordinated to cations) in the pores irrespective of the Fe content. The enhanced catalytic reactivity of the Fe-incorporated gismondines was also confirmed from the decomposition of hydrogen peroxide. An apparent activation energy of 43 kJ mol⁻¹ was obtained independent of the Fe contents in zeolites. This value was much lower than 70 kJ mol⁻¹ for the same reaction in the homogeneous solution containing iron alum as a reference sample.     

Microstructure and mechanical properties of carbon-precursor-added B4C and B4C−SiC ceramics subjected to pressureless sintering

Organic-carbon-precursor-added B₄C and B₄C–SiC ceramics were subjected to pressureless sintering at various temperatures. The carbon precursor increased the densification of the B₄C and B₄C–SiC ceramics sintered at 2200 °C to 95.6 % and 99.1 % theoretical density (T.D.), respectively. The pyrolytic carbon content of the B₄C–SiC composite decreased with increasing SiC content. The graphitization degree of pyrolytic carbon decreased slightly with the amount of carbon precursor and content of SiC. The 95 wt. % B₄C–5 wt. % SiC composite added with 7.5 wt. % carbon precursor and sintered at 2200 °C outperformed the other B₄C–SiC composites, and its sintered density, flexural strength, Young’s modulus, and microhardness were 98.6 % T.D., 879 MPa, 415 GPa, and 28.5 GPa, respectively. These values were higher than those of composites prepared via pressureless sintering and comparable to those of composites fabricated via hot pressing and/or using metal or oxide additives.     

A vibrational study of the activation sequence of C–H and C–C bonds of isobutene and 1-butene on Mo(110) and (4 × 4)-C/Mo(110) surfaces

The thermal decomposition pathways of isobutene and 1-butene on both Mo(110) and 4 × 4-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C–H and C–C bonds. On clean Mo(110), the CH₂ group of isobutene decomposes upon heating to 150 K, producing either a /-bonded isobutenylidene [(CH₃)₂CCH] species or a 1,1-di-/-bonded isobutenyl [(CH₃)₂CC] species. Upon further heating, extensive C–H bond scission occurs to form hydrocarbon fragments which do not contain CH₃ or CH₂ groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH₃)₂HCC] by 300 K, which subsequently decomposes via C–C bond scission to generate surface methyl groups. The different activation sequence of the C–H and C–C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.     

Study of molding structures and physicomechanical properties of carbon-ceramic composite materials of the SiC–C system

The microstructure of composite materials of the composition SiC–C is analyzed. It is established that they are a separate group of materials containing a ceramic matrix. The ceramic matrix experiences tensile stresses, as a result of which within the composite material a traditional internal stress field is distorted. The ceramic matrix increases strength at carbon phase boundaries of the composite material, and it reduces porosity. An excess of ceramic material reduces strength and thermal stress resistance. Requirements are provided for porosity of the structure that govern the optimum field of material composition.     

Microstructure and oxidation resistant of Si–NbSi2 coating on Nb substrate at 800°C and 1000°C

The Si–NbSi₂ composite coating with a smooth surface was successfully prepared on Nb substrate by hot dip silicon-plating (HDS) technology. The composite coating is composed of Si outer layer, NbSi₂ interlayer and Nb₅Si₃ interfacial layer. And the average surface roughness (RSa) and specific surface area growth rate (Sdr) are only 0.275 μm and 2.85%, respectively. The cyclic oxidation test shows that the Si–NbSi₂ composite coating has a very excellent oxidative resistance after oxidation at 800 °C for different times. After oxidation for 40 h, the Δm/S and oxide layer thickness of the coating are only 3.72 mg/cm² and 8 μm, respectively. After oxidation at 1000 °C for 20 h, the coating surface is almost completely covered by a dense SiO₂ layer, the Δm/S and oxide layer thickness of the coating are 7.28 mg/cm² and 15 μm, respectively. The Si–NbSi₂ composite coating presents good self-healing ability and excellent oxidation resistance, which can significantly prolong the service life of bare Nb in oxidation environment.     

Effects of the molecular weight and CC functionality of poly 1-hexene on the properties of poly 1-hexene-based high impact polystyrene

Here we are aimed to unravel the effects of CC functionality and molecular weight of the rubber on the final properties of poly1-hexene-based high impact polystyrenes (HIPS). In this regard, various HIPS samples were synthesized by free radical polymerization of styrene in the presence of different weight fractions of various poly1-hexene-based impact modifiers including: (i) high molecular weight poly1-hexene (PHex), (ii) low molecular weight poly1-hexene (Olig), and (iii) 1-hexene/1,5-hexadiene copolymer (Copolym). Results showed that by increasing CC functionality from PHex to Oligm and Copoly, the degree of grafting increases which has its influence on the mechanical, thermal and morphological perspectives of the synthesized HIPSs. Besides CC unsaturation degree, the effect of rubber molecular weight on the final HIPS properties was studied as well. According to the results, molecular weight has significant effect on the final HIPS performance, too. Finally, our obtained results suggest new HIPS/Copolym sample as the one with the highest mechanical and thermal properties which is comparable well with commercial HIPS/polybutadiene grades. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47169.     

Microstructure and tribological behaviors of C/C–BN composites fabricated by chemical vapor infiltration

In this paper carbon fiber reinforced carbon–boron nitride binary matrix composites (C/C–BN) were prepared by chemical vapor infiltration (CVI). The infiltration of BN in the CVI process was controlled by the diffusion of BCl₃, and BN matrix was distributed homogeneously in the porous carbon fiber reinforced carbon matrix composites (C/C) due to the good infiltration ability of BN. The as-received C/C–BN composites were composed of 92 vol% C and 8 vol% BN. Both the friction coefficient and wear rate of C/C composites decreased significantly by the incorporation of BN. After heat-treated at 1600 °C, the interlayer spacing of CVI BN decreased to 3.36 Å, and CVI BN with high crystalline degree displayed the excellent lubricating effect, leading to the decrease of friction coefficient and wear rate. The improvement of the tribological properties also was partially attributed to the improved oxidation resistance and the formation of friction film by the incorporation of BN matrix.     

Influence of Ultra-Fine B4C and Nano-SiO2 on the Properties of Alumina-Graphite Refractories

The role of nano-SiO2 and ultra-fine boron carbide on the properties of alumina-graphite materials was investigated.The study showed that the ultra-fine boron carbide added modified the microstructure of residual carbon and promoted the chemical bond between residual carbon from phenolic resin and flake graphite.The carbon white could strengthen the residual carbon from phenolic resin.These two additives improved the mechanical properties of AG refractories at both room temperature and high temperature,and thermal shock resistance was improved noticeably.When the two additives were doped together,carbon white could retard the evaporation of B2O3.Thermal shock resistance was guaranteed with a smaller amount of ultrafine born carbide.     

Investigation of Some Physicochemical Properties of Mixtures of Morama Seed Oil with (C6–C9) n-Alkanes at 298.15 and Atmospheric Pressure

Densities ρ, ultrasonic speeds u and dynamic viscosities η, of mixtures of morama, Tylosema esculentum, seed oil with n‐hexane, n‐heptane, n‐octane and n‐nonane were determined over the entire composition range at 298.15 K and atmospheric pressure. Excess molar volumes, , excess molar free volumes , deviations in isentropic compressibility, Δκ_s, deviations in ultrasonic speed, Δu, deviations in viscosity, Δη, and the excess free energy of activation of viscous flow, ΔG*^E, were calculated therefrom and correlated by the Redlich–Kister equation for each of the [morama seed oil + (n‐hexane or n‐heptane or n‐octane or n‐nonane)] mixtures. The results have been discussed in terms of possible intermolecular interactions and structural effects.     

Thermally Induced Isomerization of Trilinolein and Trilinoelaidin at 250 °C: Analysis of Products by Gas Chromatography and Infrared Spectroscopy

The products formed by thermally induced isomerization of trilinolein and trilinoelaidin at 250 °C were studied by infrared spectroscopy and gas chromatography. The triglycerides of the 9c12c and 9t12t fatty acids, linoleic and linoelaidic acid respectively, were subjected to thermal treatment under nitrogen in glass. The products were removed at regular intervals and analysed by infrared spectroscopy using a single reflectance attenuated total internal reflectance crystal accessory. Trans-esterification of the products provided the corresponding fatty acid methyl esters which were studied by gas chromatography. The results show that the samples undergo decomposition and isomerization. Thermally induced 9c12c fatty acid (linoleic acid) molecules in the trilinolein molecules isomerize into 9c12t, 9t12c and 9t12t isomers. Thermally induced 9t12t fatty acid (linoelaidic acid) molecules in the trilinoelaidin molecules isomerize into 9c12t, 9t12c and 9c12c isomers. However, the concentration profiles are different for these two triglyceride samples. The rates of formation of isomers from linoleic acid are higher than the rates of formation of isomers from linoelaidic acid. In addition, these two fatty acids also isomerize into conjugated linoleic acids (CLAs). The profiles of the CLAs are identical in both cases.