Ablation mechanism of TaC coating fabricated by chemical vapor deposition on carbon-carbon composites

Ablation characteristics and mechanism at high temperature for TaC coatings on carbon-carbon composites were investigated by ablation experiments with low power laser and oxyacetylene flame. The results show that the TaC coating is decomposed at the initial stage of laser ablation in atmosphere, and free carbon diffused to the surface, then oxidized to the melt including carbon, oxygen and tantalum. With the increase of ablation time, the melt is oxidized to low valent tantalum-oxide and Ta2O5 is formed finally. During the melt cooling, needle-like crystals of Ta2O5 are precipitated. Between the melt and TaC coating, there exists a diffusion transition layer with thickness of 1-2 μm. The transition layer consists of fine crystals and pores including carbon, oxygen and tantalum. The oxyacetylene flame ablation at 2 300 ℃ results in the rapid oxidation of TaC and formation of protective liquid films of tantalum oxide on the coating surface, where the liquid film can fill up the cracks and cover the coating. In such case, the oxidation mechanism of TaC is converted to the oxygen solution and diffusion control mechanism.

Microstructure and isothermal oxidation of 3Al_2O_3·2SiO_2/SiC coating on high and low density carbon-carbon composites

Carbon-carbon composite （C/C） materials are prone to severe oxidation and volatilization problems. To address these issues, mullite （3Al2O3.2SiO2）/silicon carbide （SIC） coatings were deposited on C/C composite substrates characterized into high and low densities. The coatings were applied by a two-step approach： pack cementation and silica sol based slurry coating processes. The relationship between the microstructure of 3Al2O3·2SiO2/SiC coatings and C/C substrates during isothermal oxidation cycle at 1 500 ℃ was investigated using X-ray diffractometer （XRD） and scanning electron microscope （SEM） mounted with energy dispersive spectrometer （EDS）. The results indicate that the density of the substrates has a marked effect on the coatings. Dense, thick and well-bonded coatings are obtained in the high density substrate. After 106 h of isothermal oxidation, the high density substrate with 3Al2O3-2SiO2/SiC coating offers effective protection as compared to low density substrate suffering recession.

Change in the Pore Structure of Carbon-Carbon Composites during Successive Stages of High-Pressure Impregnation and Heat Treatment

Pore structure of C/C （Carbon-Carbon） composite after several stages of pitch impregnation under the high pressure and heat treatment was investigated by means of low temperature nitrogen adsorption and the standard contact porosimetry. Total pore volume, pore size distribution and specific surface area were calculated for samples of composite after several successive stages of treatment. The radius of pores presented in the material changes from 1 nm to 90 tam. Total pore volume and specific surface area both decrease after successive stages of pitch impregnation under the pressure, whereas heat treatment up to 1,750 ℃ and 2,000 ℃ leads to creation of some porous space and pore volume expansion. The bulk porosity of C/C composite comes down from 33.7% to 13.7% after the serial stages of treatment and the specific surface area is reduced by half compared to the initial material.

A FACILE SYNTHESIS OF 2-TRIFLUOROMETHYLPYRROLE VIA 1,3-DIPOLAR CYCLOADDITION REACTION OF 2-TRIFLUOROMETHYLOXAZOLONE AND THE ACTIVATED CARBON-CARBON MULTIPLE BOND

The 1.3-dipolar cycloaddition reaction of 2-trifluoromethyl- oxazolone and the activated carbon-carbon multiple bond was studied and gave a convenient way to synthesize 2-trifluoromethylpyrrole derivatives.

Biomass valorization via electrocatalytic carbon–carbon bond cleavage

Renewable electrocatalytic upgrading of biomass feedstocks into valuable chemicals is one of the promising strategies to relieve the pressure of traditional energy-based systems.Through electrocatalytic carbon–carbon bond cleavage of high selectivity,various functionalized molecules,such as organic acids,amides,esters,and nitriles,have great potential to be accessed from biomass.However,it has merely received finite concerns and interests in the biorefinery.This review first showcases the research progress on the electrocatalytic conversion of lipid/sugar-and lignin-derived molecules(e.g.,glycerol,mesoerythritol,xylose,glucose,1-phenylethanol,and cyclohexanol)into organic acids via specific carbon–carbon bond scission processes,with focus on disclosing reaction mechanisms,recognizing actual active species,and collecting feasible modification strategies.For the guidance of further extensive studies on biomass valorization,organic transformations via a variety of reactions,including decarboxylation,ring-opening,rearrangement,reductive hydrogenation,and carboxylation,are also disclosed for the construction of similar carbon skeletons/scaffolds.The remaining challenges,prospective applications,and future objectives in terms of biomass conversion are also proposed.This review is expected to provide references to develop renewed electrocatalytic carbon–carbon bond cleavage transformation paths/strategies for biomass upgrading.

Prediction on Carbon/Carbon Composites Ablative Performance by Artificial Neutral Net

A preliminary estimation of ablation property for carbon-carbon composites by artificial neutral net （ANN） method was presented. It was found that the carbon-carbon composites＇ density, degree of graphitization and the sort of matrix are the key controlling factors for its ablative performance. Then, a brief fuzzy mathematical relationship was established between these factors and ablative performance. Through experiments, the performance of the ANN was evaluated, which was used in the ablative performance prediction of C/C composites. When the training set, the structure and the training parameter of the net change, the best match ratio of these parameters was achieved. Based on the match ratio, this paper forecasts and evaluates the carbon-carbon ablation performance. Through experiences, the ablative performance prediction of carbon-carbon using ANN can achieve the line ablation rate, which satisfies the need of precision of practical engineering fields.

TEMPERATURE EFFECT ON OXIDATION PROTECTION LIFE OF MULTI-LAYER COATING FOR C/C COMPOSITES

A multi-layer coating for carbon-carbon composites with a Si-W outer layer,a SiC barrier layer and a SiC transition layer is prepared by the combination of siliconiza-tion, CVD and liquid reaction method. The effect of temperature on the oxidation protec-tion life from 1600 to 17OOC is investigated. The thickness of the oxide film on thecoating surface is measured through SEM and EMPA. A concept of critical oxide filmthickness and a critical oxidation protection time is put forward, and, based on it, the re-lation of the critical time with temperature is analyzed. The re8ults show that the relationof the critical oxidation protection time with temperature found by the experiments is fullyidentical with that calcu1ated based on the critical thickness- The physical significance andthe effect factors of the critical oxide film thickness are discussed.

Double SiC Oxidation Protective Coating on C/C Composites Prepared by Spark Plasma Sintering

Easy oxidation of carbon limits applications of carbon-carbon composites in an oxygen-containing environment. In this study, a two-layer SiC coating was prepared on carbon-carbon composites by a Spark plasma sintering technology at 1350°C for 1 min. The coating was mainly composed of β-SiC and Si and well bonded with the substrate. The double SiC coating could effectively protect the C/C composites from oxidation at 1600°C for 120 h, and the corresponding weight loss was only 2.62%.

Clean production of lactic acid by selective carbon-carbon bond cleavage of biomass feedstock over a novel carbon-bismuth oxychloride photocatalyst

The use of functional materials such as carbon-bismuth oxyhalides in integrated photorefineries for the clean production of fine chemicals requires restructuring.A facile biomass-assisted solvothermal fabrication of carbon/bismuth oxychloride nanocomposites(C/BiOCl)was achieved at various temperatures.Compared with BiOCl and C/BiOCl-120,C/BiOCl-180 exhibited higher crystallinity,wider visible light absorption,and a faster migration/separation rate of photoinduced carriers.For the selective C–C bond cleavage of biomass-based feedstocks photocatalyzed by C/BiOCl-180,the xylose conversion and lactic acid yield were 100%and 92.5%,respectively.C/BiOCl-180 efficiently converted different biomass-based monosaccharides to lactic acid,and the efficiency of pentoses was higher than that of hexoses.Moreover,lactic acid synthesis was favored by all active radicals including superoxide ion(·O_(2)^(−)),holes(h^(+)),hydroxyl radical(·OH),and singlet oxygen(^(1)O_(2)),with·O_(2)^(−)playing a key role.The fabricated photocatalyst was stable,economical,and recyclable.The use of biomass-derived monosaccharides for the clean production of lactic acid via the C/BiOCl-180 photocatalyst has opened new research horizons for the investigation and application of C–C bond cleavage in biomass-based feedstocks.

Numerical simulation of isothermal chemical vapor infiltration process in fabrication of carbon-carbon composites by finite element method

The chemical vapor infiltration process in fabrication of carbon-carbon composites is highly inefficient and requires long processing time. These limitations add considerably to the cost of fabrication and restrict the application of this material. Efforts have been made to study the CVI process in fabrication of carbon-carbon composites by computer simulation and predict the process parameters, density, porosity, etc. According to the characteristics of CVI process, the basis principle of FEM and mass transport, the finite element model has been established. Incremental finite element equations and the elemental stiffness matrices have been derived for the first time. The finite element program developed by the authors has been used to simulate the ICVI process in fabrication of carbon-carbon composites. Computer color display of simulated results can express the densification and distributions of density and porosity in preform clearly. The influence of process parameters on the densification of preform has been analyzed. The numerically simulated and experimental results give a good agreement.

Highly Efficient Oxidative Cleavage of Carbon-Carbon Double Bond over meso-Tetraphenyl Cobalt Porphyrin Catalyst in the Presence of Molecular Oxygen

Highly efficient and selective carbon-carbon double bond aerobic cleavage of olefins catalyzed by metallopor- phyrins was investigated, and carbonyl compounds and epoxide were produced as the main products. CoTPP （co- balt meso-tetraphenyl porphyrin） showed excellent activity for the oxidative cleavage of carbon-carbon double bond by using styrene as model compound, in which the TOF （turnover frequency） and selectivity toward benzaldehyde was obtained with 2×10^4h-1 and 86%,respectively.

Micro-pleating in carbon-carbon composites under a cyclic load

The mechanical behavior of three-dimensional integral braided carbon-carbon composites (C/C) has been explored under unconventional fatigue loading, and a series of special experimental phenomena were observed. In this present paper, one of these phenomena, the micro silk-like pleating of the pyrocarbon matrix, is discussed. With thermodynamic analysis, it is shown that small areas of high-temperature contribute to the micro-pleating formation. Moreover, slight friction between pyrocarbon laminates has been proven to produce the small high-temperature areas during cyclic tensile loading.

Transition-Metal-Catalyzed Carbon-Carbon Bond Activation in Asymmetric Synthesis

Catalytic asymmetric transformations involving carbon-carbon bond cleavages open intriguing strategies for the synthesis of chiral complex molecules.The transient organometallic species mainly generate from the oxidative addition of cyclic compounds with low valence metals or from β-carbon elimination of tert-alkoxyl metals.This overview covers the recent advances in transition-metal-catalyzed carbon-carbon bond activation in asymmet-ric synthesis.

Copper-Catalyzed Aerobic Oxidative Cleavage of Unstrained Carbon-Carbon Bonds of 1,1-Disubstituted Alkenes with Sulfonyl Hydrazides

Main observation and conclusion Alkoxy radical-mediated carbon-carbon bond cleavages have emerged as a powerful strategy to complement traditional ionic-type transformations.However,carbon-carbon cleavage reaction triggered by alkoxy radical intermediate derived from the combination of alkyl radical and dioxygen,is scarce and underdeveloped.Herein,we report alkoxy radical,which was generated from alkyl radical and dioxygen,mediated selective cleavage of unstrained carbon-carbon bond for the oxysulfonylation of 1,1-disubstituted alkenes,providing facile access to a variety of valuableβ-keto sulfones.Mechanistic experiments indicated alkoxy radical intermediate that underwent subsequent regioselectiveβ-scission might be involved in the reaction and preliminary computational studies were conducted to provide a detailed explanation on the regioselectivity of the C-C bond cleavage.Notably,the strategy was successfully applied for constructing uneasily obtained architecturally intriguing molecules.

Weak Acetylene Adsorption Terminated Carbon-Carbon Coupling Kinetics on Silver Electrocatalysts

Owing to serious poison of downstream olefin polymerization catalysts from acetylene impurities,selective reduction of acetylene to ethylene is a pivotal process in petrochemical industry.However,during thermocatalytic and electrocatalytic acetylene semihydrogenation,acetylene C-C coupling inevitably occurs on current catalysts.The resultant oligomeric species(particularly long-chain hydrocarbons)block active sites and mass transportation,and eventually decrease catalytic activity and stability.In this work,we report Ag nanowires(NWs)as high-performance electrocatalysts for acetylene semihydrogenation,where the C-C coupling is unprecedentedly suppressed by weakening acetylene adsorption.In pure acetylene,1,3-butadiene Faradaic efficiency(FE)of Ag NWs is only 2.1%,which is far lower than 41.2%for Cu nanoparticles at−0.2 V versus reversible hydrsogen electrode.Ethylene partial current density of Ag NWs reaches 217 mA/cm^(2)at 0.85 V,which is considerably higher than those for state-of-the-art Cu-based electrocatalysts.Markedly,no 1,3-butadiene is produced on Ag NWs in a large two-electrode flow cell fed with crude ethylene containing 1 vol%acetylene,presenting thorough termination of acetylene C-C coupling.In situ electrochemical Raman spectroscopy and theoretical investigations reveal that weak acetylene adsorption on Ag surfaces is intrinsically responsible for prohibiting their oligomerization.This work will spark the rapid development of high-performance and stable electrocatalysts for reducing alkynes to olefins.

Nickel-Catalyzed Reductive Cross-Couplings:New Opportunities for Carbon-Carbon Bond Formations through Photochemistry and Electrochemistry

Metal-catalyzed cross-electrophile couplings have become a valuable tool for carbon-carbon bond formation.This minireview provides a comprehensive overview of the recent developments in the topical field of cross-electrophile couplings,provides explanations of the current state-of-the-art,and highlights new opportunities arising in the emerging fields of photoredox catalysis and electrochemistry.

Catalytic Asymmetric Carbon-Carbon Forming Reactions Catalyzed Chiral Schiff Base-Metal Complexes

1 Results In 1991, we disclosed the novel asymmetric catalysts prepared from chiral Schiff base and titanium alkoxide in the reaction of asymmetric silylcyanation of aldehydes (eq.1)[1]. Since our first report, chiral Schiff base-metal complex was proven to be efficient in a variety of asymmetric reactions. We reported the first example of enantioselective addition of diketene to aldehydes promoted by chiral Schiff base-titanium alkoxide complexes (eq.2)[2]. The products of this reaction have been cove...

Cobalt-catalyzed reductive alkynylation to construct C(sp)-C(sp^(3))and C(sp)-C(sp^(2))bonds

Transition-metal-catalyzed cross-electrophile coupling has emerged as a reliable method for constructing carbon–carbon bonds.Herein,we report a general method,cobalt-catalyzed reductive alkynylation,to construct C(sp)-C(sp^(3))and C(sp)-C(sp^(2))bonds.This presented reaction has a broad substrate scope,enabling the efficient cross-electrophile coupling between alkynyl bromides with alkyl halides and aryl or alkenyl(pseudo)halides.This presented reaction is conducted under mild conditions,tolerating many functional groups,thus suitable for the modification and synthesis of biologically active molecules.

Recent Advances in Organic Electrochemical C-H Functionalization

Organic electrochemistry has a rich history in organic synthesis and has been considered as a promising alternative to traditional chemical oxidants and reductants because it obviates the use of stoichiometric amount of dangerous and toxic reagents. In particular, the electrochemical C-H bonds functionalization is one of the most desirable approaches for the construction of carbon-carbon （C-C） and carbon-heteroatom （C-X） bonds. This review summarizes the substantial progress made in the last few years in C-H functionalization via organic electrochemistry. It is divided into sections on C-C, C-N, C-O, C-S, C-Halogen and C-P bond formation.