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.     

Effect of Ti （C,N） on Properties of Low-carbon MgO- C Bricks

The effect of Ti （ C, N） on properties of low-carbon MgO - C bricks was investigated. The phase composition and the microstructure of the matrix of low-carbon MgO - C brick containing Ti （ C, N） were studied by XRD and SEM analysis together with EDS. The results showed that Ti （ C, N） distributed in the matrix of lowcarbon MgO - C brick uniformly after being treated at 1 600 ~C for 3 h in coke powder bed, and Ti （C, N） and MgO formed a solid solution. After the treatment at 1 600 ℃ for 3 h in coke powder bed, the bulk density and cold crushing strength of low-carbon MgO - C brick with Ti （ C, N） decreased, and the apparent porosity and linear change rate of specimens increased. The oxidation resistance of low-carbon MgO - C brick with Ti（ C, N） was superior to that of low-carbon MgO - C brick with no additives, but inferior to that of low-car- bon MgO - C brick with Al powder. The slag resistance of the specimen with Ti （ C, N） was excellent as well.     

Shock-wave synthesis of diamonds from fullerenes C60−C100

Shock-wave synthesis of diamond from C₆₀–C₁₀₀-fullerene powder was first accomplished by using the explosive compaction technique with plane wave loading in the pressure range of 24–40 GPa. The compacts of various initial composition comprised diamond, FCC C₆₀-fullerite, graphite, and amorphous carbon. The largest diamonds of 0.1–1.0 m were obtained under shock loading of pellets consisting of copper powder with 5 wt. % fullerite at 24 and 38 GPa, and pellets consisting of copper powder with 10 wt. % fullerite at 40 GPa. The end product consists of diamond without intermediate diamondlike phases such as n-diamond and hexagonal diamond (lonsdaleite).Central Machine-Building Technology Research Institute, 109088 Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 131–138, March–April, 1995.     

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.     

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.     

Effects of Cα -Oxidation in the Fungal Metabolism of Lignin

C_α-Oxidation (benzyl alcohol oxidation) is a prominent reaction in the degradation of lignin by white-rot fungi. This study showed that such oxidation markedly retards metabolism of a nonphenolic β-O-4 model compound, 1-(3-methoxy-4-ethoxyphenyl)-2-(o-methoxyphenoxy)propane-1,3-diol, by cultures of Phanerochaete chrysosporium Burds. Surprisingly, however, selective chemical C_α -oxidation of spruce lignins enhanced their depolymerization by the cultures. Thus the decrease in intrinsic degradability of substructures is more than compensated by another effect of C_α-oxidation in lignin. One possibility is that the oxidation increases the accessibility of the lignin to enzymes by decreasing its steric complexity. This study also revealed that the β-O-4 model, like lignin in wood, is degraded in part via C_α-oxidation by P. chrysosporium. Reduction of the α-carbonyl groups thus formed does not occur. Addition of L-glutamate to ligninolytic cultures completely suppresses their competence to degrade the model compound, as it does their ability to oxidize lignin to CO₂. This result strengthens past evidence indicating that substructure models are metabolized by the same enzyme system as lignin.     

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.     

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.     

Enhanced Production of C2–C4 Olefins Directly from Synthesis Gas

An attempt made for the selective production of C₂–C₄ olefins directly from the synthesis gas (CO + H₂) has led to the development of a dual catalyst system having a Fischer–Tropsch (K/Fe–Cu/AlOx) catalyst and cracking (H-ZSM-5) catalyst operate in consecutive dual reactors. The flow rate (space velocity) and H₂/CO molar ratio of the feed have been optimized for achieving higher CO conversions and olefin selectivities. The selectivity to C₂–C₄ olefins is further enhanced by optimizing the reaction temperature in the second reactor (cracking), where the product exhibited 51% selectivity to C₂–C₄ hydrocarbons rich in olefins (77%) with a stable time-on-stream performance in a studied period of 100 h.     

Research of the Activity of Catalysts on the Base of H3PW12O40 in Partial Oxidative Conversion of C3–C4 alkanes

Catalysts from heteropoly acid H₃PW₁₂O₄₀ and its Cs, Na, Ba, Pb, Ca, Cd, Cr, Mn, V, La salts supported on clinoptilolite, alumosilicate are highly active in oxidative conversion of propane–butane (OCPB) mixture and formation of C₂–C₄ olefins, oxygen-containing compounds at temperatures T = 100–800 °C. Optimum yields of ethylene and propylene are achieved on heteropoly acid its Cs and Cr salts. The processes of oxidative dehydrogenation (ODPB) and cracking are concurrent in formation of olefins. High activity is caused by dispersity of supported catalysts (XRD, IRS) both formation of crystal hydrates and an amorphous phase of heteropoly acid in a condition of interaction with the carrier.     

Effect of B4 C on Oxidation Resistance of Low-Carbon MgO-C Materials

B4C was added into the low-carbon MgO-C materials in order to improve the oxidation resistance. The results show adding 0. 3wt% B4C can get the best oxidation resistance and adding 0. 2 wt% B4C can get the highest hot modulus of rupture. Altogether, adding 0. 2wt% B,C into the low-carbon MgO-C materials can get better oxidation resistance and hot strength.     

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.     

Crossover Equation of State for Selected Hydrocarbons (C4–C7)

The organic Rankine cycle (ORC) has attracted attention for waste heat recovery and renewable energy systems. An accurate prediction for thermodynamic properties of working fluids is of great importance for cycle performance evaluations and system design. Particularly, hydrocarbons are promising for their good performance and low global warming potentials. Moreover, the thermal efficiency of the ORC is higher when the evaporation temperature is closer to the critical temperature, which makes the properties in the critical region rather important. Recent research has shown that using mixture as working fluid can achieve better temperature matches. Therefore, an equation of state (EoS) that can be extended to mixture calculations is more attractive. Specific EoS for selected hydrocarbons is precise, but very complex. Cubic EoSs, such as widely used Peng–Robinson EoS and Soave–Redlich–Kwong (SRK) EoS, fail to accurately predict liquid densities over wide pressure ranges or pressure–density–temperature (pρT) properties in the near-critical region. This work combines the volume translation approach and the crossover method to provide better prediction for thermodynamic properties in the critical region and in regions far from the critical point. A crossover volume translation SRK EoS is developed and used for n-butane, i-butane, n-pentane, i-pentane, n-hexane, i-hexane and n-heptane. The volume translation term is set as a constant to ensure the accuracy of the saturated liquid density at low reduced temperatures. Then, the crossover method is introduced into the volume translation EoS to improve the predictions of thermodynamic properties in the critical region. Six crossover parameters are used, which are constants or functions of acentric factor and critical parameters. Therefore, none of the parameters in the crossover volume translation SRK EoS is adjustable, which makes the crossover EoS totally predictive and easily extend to mixtures. Comparisons show that the crossover EoS is in much better agreement with experimental data than the original SRK EoS.     

The Development of Refractories Made in China for C.C. in Baogang

In recent years great progress has been made in developing refractories to replace som imported ones for C.C. in Baogang.Mean-while,many new products have also bee devel-oped to meet the demand of C.C. production in Baogang on the basis of Chinese conditions ,This paper presents the refractories products and their quality,application for C.C. steel ladles,refining devices,tundish and compares them with those imported ones.     

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.     

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.     

Properties of MgO–Fe–C refractories as linings of vanadium-extraction converter

For the purpose to extend the service life of MgO–C bricks used as linings of vanadium-extraction converters, MgO–Fe–C bricks with different carbon content were designed and the properties of this novel refractory were investigated by comparing to the traditional MgO–C bricks. The results showed that the poor service life of MgO–C bricks was due to the poor sinterability of the oxidized layer at 1400 °C, whereas the oxidized layer of MgO–Fe–C brick was well sintered due to the oxidation of Fe particles in the oxidized layer and formation of MgO–FeO_ss in air atmosphere. Excellent oxidation resistance and corrosion resistance against vanadium containing slag were also obtained due to the increase of compactness of oxidized layer and concentration of FeO in the oxidized layer compared to MgO–C bricks, and it is considered that MgO–Fe–C brick is a favorable substitute of MgO–C refractory to be used as linings of vanadium-extraction converters.     

Effect of Additives on Properties of MgO—ZrO2—C Material

The paper describes the effect of additives Al,Si,SiC and A4C on the expanson of MgO-ZrO2-C material after being coked.Theresults indicate that Al and Si were oxi-dized to form Al2O3 and SiO2 respectively,and then re-acted with CaZrO3 or stabilizer in c-ZrO2 to form calcium aluminate,spinel(MA),dicalcium silicate(C2S) and forsterite (M2S) ,Meznwhile,α-C2S ras transformed to γ-C2S and c-ZrO2 to m-ZrO2 when temperature changed.All the above reactions resulted in the decrease of the amount of Al4C3 and SiC and the increase in bulk volume,which caused the stucture of MgO-ZrO2-C material de-stroyed.Hence,contrary to the MgO-C material,when adding Al,and Si,the MgO-ZrO2-C material would be structurally deteriorated after heat-treatment and its strength and corrosion resistance decreased.     

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.