A density functional theory (DFT) study was employed to explore the mechanism of the conversion of methane to benzene in chemical vapor infiltration (CVI) based on the concluded reaction pathways from C1-species to C6-species. The geometry optimization and vibrational frequency analysis of all the chemical species and transition states (TS) were performed with B3LYP along with a basis set of 6–311 +G(d, p), and Gaussian 09 software was used to perform the study. The rate constants were calculated by KiSThelP according to the conventional transition state theory (TST), and the Wigner method was applied to acquire the tunneling correction factors. Then the rate constants were fitted to the modified Arrhenius expression in the temperature range of 800–2000 K. As for the barrierless reactions calculated in this paper, the rate constants were selected from the relating references. Through the energetic and kinetic calculations, the most favorable reaction pathway for benzene formation from methane was determined, which were mainly made of the unimolecular dissociation. The conversion trend from C1-species to C4-species is mainly guided by a strong tendency to dehydrogenation and the pathways from C4-species to C6-species are all presumed to be able to produce C6H6 molecule.
Last. Cheng Zhang(NPU: Northwestern Polytechnical University)H-Index: 1
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Abstract To improve the long-term anti-oxidation performance of carbon/carbon (C/C) composites at high temperature, La2O3-modified ZrB2-SiC (ZSL) coating was prepared on SiC-coated C/C composites by pack cementation. The results indicated that significant improvement was achieved on the oxidation resistance of ZSL coated specimens compared with ZrB2-SiC (ZS) coating for SiC-coated C/C composites at 1500 °C and 1600 °C. The mass loss of the ZSL coated specimen was merely 0.6% after a static oxida...
Last. Xinfa Tian(NPU: Northwestern Polytechnical University)H-Index: 4
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To understand the effect of pyrocarbon (PyC) on the morphology stability of SiC nanowires at high temperatures (1800-2100°C), a PyC layer was prepared to wrap the 3C-SiC nanowire by chemical vapor deposition. The results showed that the existence and thickness of PyC layer played a crucial role in stabilizing the structures and morphology of SiC nanowires. The SiC nanowires without PyC layer transformed to platelet-shaped structures above 1800°C. The SiC nanowires with a ~1.5 μm PyC layer could ...
Last. Qiang Song(NPU: Northwestern Polytechnical University)H-Index: 26
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C/C composites with both high specific strength and excellent electromagnetic shielding efficiency are required for advanced space applications. For this purpose, SiC nanowires (SiCNWs) were introduced in C/C composites by electrophoretic deposition. The presence of SiCNWs in C/C induces the formation of a SiCNW/pyrocarbon coaxial structure, which refines the matrix and decreases the stress concentration of the matrix. The average flexural strength of C/C composites increases by ∼100% after addi...
Carbon fiber-reinforced carbon matrix composites (C/C composites) have excellent properties such as specific strength and specific elastic modulus, along with excellent fatigue resistance, thermal shock resistance, ablation resistance, frictional properties, lightweight, heat/thermal conductivity, and dimensional stability. C/C composites are the materials that exhibit almost no decline in mechanical properties, even at 2000–3000 °C, when protected from oxidation. A method of directly coating th...
Abstract The formation of polycyclic aromatic hydrocarbons (PAHs) in high temperature environments is an ongoing area of research due to the mismatch between the experimental and simulated concentration profiles of PAHs in several flames. In this work, the role of methyl radicals in the growth of PAHs is identified by developing a detailed reaction mechanism for the conversion of phenanthrene to pyrene by methyl radicals. The reaction energetics are obtained through quantum calculations using B3...
Last. Bingqing Wei(NPU: Northwestern Polytechnical University)H-Index: 96
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Materials with an ultralow density and ultrahigh electromagnetic-interference (EMI)-shielding performance are highly desirable in fields of aerospace, portable electronics, and so on. Theoretical work predicts that 3D carbon nanotube (CNT)/graphene hybrids are one of the most promising lightweight EMI shielding materials, owing to their unique nanostructures and extraordinary electronic properties. Herein, for the first time, a lightweight, flexible, and conductive CNT–multilayered graphene edge...
An experimental and modelling study of the pyrolysis and of the steam reforming of methane in mixtures representative of a gas produced from biomass gasification has been performed. The experimental study has been done in a plug flow reactor under a near atmospheric pressure (1.07 bar), for a residence time of 0.68 s and temperature ranging from 1200 up to 1800 K. Reactants and products were quantified on-line at the exit of the reactor by gas chromatography. The kinetic influence of the differe...
Abstract Despite progress in the understanding of how nanostructuring affects activity of heterogeneous catalysts, there are still fundamental questions to be answered. Do nanoparticle edges have the same catalytic properties as steps on single-crystal surfaces? How much does the increased structural flexibility of nanoparticles contribute to their activity? How do reaction barriers depend on the nanoparticle size? Herein we address these questions regarding an exemplary reaction sequence of com...
Last. Wei Li(NPU: Northwestern Polytechnical University)H-Index: 206
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This work describes a modeling study of methane pyrolysis in chemical vapor deposition (CVD). The model consists of a detailed chemical kinetic model, which includes 241 species and 909 gas-phase reactions for methane pyrolysis mechanism, and a plug-flow model, which describes the transport conditions in CVD. Reasonably good agreements were obtained between the simulation results and the experimental results of methane pyrolysis in CVD of pyrocarbon in a vertical hot-wall deposition reactor with...
Last. Lehua Qi(NPU: Northwestern Polytechnical University)H-Index: 26
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Abstract Methane is used as the carbon precursor for the fabrication of C/C composites by using chemical vapor infiltration. Demystify the heterogeneous reactions on the carbon surface during the pyrolysis of methane is important for us to understand the formation and growth of the pyrocarbon. In this paper, the dissociation mechanisms from CH4 to C2H2 on pristine graphene, N-doped graphene, and vacancy graphene during the chemical vapor infiltration process are systematically studied using the ...