Chemical vapor infiltration of carbon fiber felt: optimization of densification and carbon microstructure

A carbon fiber felt with a fiber volume fraction of 7.1% was infiltrated at temperatures of 1070 and 1095 °C and methane pressures from 5 to 30 kPa to confirm the inside-outside densification derived from model studies with capillaries 1 mm in diameter. Bulk densities and residual open porosities were determined as a function of infiltration depth at various heights of the felt. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Inside-outside densification was demonstrated up to the maximum pressure of 30 kPa at 1070 °C and up to 13.5 kPa at 1095 °C, leading to bulk densities above 1.9 g/cm³. A pure, high-textured carbon matrix is formed in the pressure range from 9.5 to 11 kPa at 1095 °C. At lower and higher methane pressures and lower temperature, a less textured carbon is formed. The results are based on the growth mechanism of carbon deposition. They strongly support recent conclusions that high-textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic hydrocarbons to small linear hydrocarbons, preferentially ethine. This model is called the particle-filler model. Aromatic hydrocarbons are the molecular particles and small linear hydrocarbons are the molecular filler, necessary to generate fully condensed planar structures.     

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.     

Influence of the deposition parameters on the texture of pyrolytic carbon layers deposited on planar substrates

Pyrolytic carbon layers were deposited from methane on planar substrates (pyrolytic boron nitride) at various residence times, methane pressures and deposition temperatures. The depositions were performed in a cavity oriented perpendicular to the gas flow. The small surface area/reactor volume ratio of the reactor geometry allows depositions in the growth and nucleation mechanism. Transmission electron microscopy was applied to study the texture and microstructure of the carbon layers. A texture transition from medium- to high-textured pyrolytic carbon occurs as a result of increasing residence times, methane pressures and temperatures. Improved textures are generally correlated with increasing deposition rates, which are not necessarily constant during long-term depositions. Lower textures are observed in the vicinity of the substrate interface that are attributed to the influence of the substrate morphology and microstructure.     

Deposition rates during the early stages of pyrolytic carbon deposition in a hot-wall reactor and the development of texture

Pyrolytic carbon layers were deposited from methane/oxygen/argon mixtures on planar substrates (silicon wafers) at a total pressure of 100 kPa, a maximum gas residence time of 2 s and a temperature of 1100 °C. The depositions were performed in a hot-wall reactor with the substrate oriented parallel to the gas flow. Particular attention was paid to factors that influence the reproducibility of the deposited layers. Scanning and transmission electron microscopy were applied to study the thickness profiles and the texture of the carbon layers. The surface topography was investigated by atomic force microscopy. For pyrolytic carbon deposited without oxygen, an alteration from medium- to high-textured carbon is observed with increasing residence time. Islands are observed on the surface of the layer whose size increases with the texture. For pyrolytic carbon deposited with 3% oxygen, lower deposition rates were obtained and a strong modification of the texture is found compared to gas mixtures without oxygen.     

Kinetics of surface reactions in carbon deposition from light hydrocarbons

Carbon deposition from ethene, ethine and propene as a function of pressure was studied at various temperatures and two different surface area/volume ratios. Deposition rates as a function of pressure of all hydrocarbons indicate Langmuir-Hinshelwood kinetics which suggests that the deposition process is controlled by the heterogeneous surface reactions (growth mechanism). These kinetics are favored at decreasing reactivity (C₃H₆>C₂H₂>C₂H₄), decreasing temperature and residence time as well as increasing surface area/volume ratio. A linear rate increase at high pressures suggests that carbon is additionally or preferentially deposited by aromatic condensation reactions between polycyclic aromatic hydrocarbons large enough to be physisorbed or condensed on the substrate surface (nucleation mechanism). The results completely agree with earlier results obtained with methane.     

Chemistry and kinetics of chemical vapor deposition of pyrocarbon: VI. influence of temperature using methane as a carbon source

The chemical vapor deposition of carbon from methane was investigated at an ambient pressure of about 100 kPa, a methane partial pressure of 10 kPa and temperatures ranging from 1050–1125°C. Carbon deposition rates and compositions of the gas phase as a function of residence time have been determined using a substrate with a surface area/reactor volume ratio of 40 cm⁻¹. Increasing temperatures lead to strongly increasing deposition rates, decreasing partial pressures of ethane and increasing partial pressures of ethene, ethine and benzene. The overall activation energy of carbon deposition, determined from the initial deposition rates at a residence time versus zero amounts to 446 kJ/mol as compared to 431.5, 448 and 452.5 kJ/mol reported in earlier papers. Two possible rate-limiting steps are discussed, namely dissociation of methane, which is favored in the earlier papers, and dissociation of carbon–hydrogen surface complexes.     

Influence of the surface area/volume ratio on the chemistry of carbon deposition from methane

The chemistry of carbon deposition from methane as a function of methane pressure was studied at a temperature of 1100 °C and surface area/volume ratios of 0.8 and 3.2 mm⁻¹ by analysis of both gaseous and condensing, i.e. aromatic reaction products. Conversion of methane as well as the yields of the hydrocarbons formed increase with increasing pressure. The surface area/volume ratio has a significant influence on the formation of aromatic hydrocarbons showing much higher yields at the lower ratio. This result, expected from preceding studies of deposition rates, confirms that a change of this ratio leads to a change of the deposition chemistry of carbon.     

Influence of total pressure on the microstructures and growth mechanism of ZrC coatings prepared by chemical vapor deposition from the Zr-Br2-C3H6-H2-Ar system

Zirconium carbide (ZrC) coatings were deposited on graphite substrates by chemical vapor deposition from the Zr-Br₂-C₃H₆-H₂-Ar system. The influence of total pressure on the growth of ZrC was investigated in the range of 5–60 kPa. As the total pressure increased, the deposition rate increased evidently, and the preferential orientation of ZrC coatings changed from the (200) plane to the (220) plane. The growth mechanism changed from a mass transport reaction to a surface reaction at the total pressure of 20–40 kPa. At the total pressure below 20 kPa, the deposition was dominated by crystal growth, so the coatings were composed of well-faceted pyramidal-shaped crystals growing along the <001> direction. At the total pressure above 60 kPa, the growth of ZrC coatings was controlled by the nucleation mechanism, so the coatings were cluster-like crystals rapidly growing along the <110> direction. In addition, low pressure was conducive to the formation of near-stoichiometric ZrC without free carbon. These variations of ZrC coatings can mainly be attributed to gas supersaturation and remarkably changed transport diffusion coefficients with increasing total pressure.     

The effect of hydrogen pressure and aromatic structure on methane yields from the hydropyrolysis of aromatics

The chemistry of the formation of methane from the hydrogasification of naphthalene, substituted naphthalenes and toluene has been investigated using a flow tube. Temperatures were varied between 650–1050 °C (depending on the aromatic) and pressures ranged over 0.5–2 MPa. The results show that methane yields increase with increasing hydrogen pressure. For naphthalene the methane yield increases linearly with temperature for a given pressure. Methyl substituents are lost from aromatic rings, in a reaction which is insensitive to hydrogen pressure, to form 1 mole of methane and the parent aromatic. At these hydrogen pressures the phenolic group in 1-naphthol is removed predominately as H₂O to form naphthalene and the methane yields from this species parallel those from naphthalene. Analyses of the condensed products demonstrate that increased hydrogen pressure results in a reduction in the amounts of high molecular weight condensation products resulting in increased yields of methane.     

Dependence of pyrocarbon microstructure on the substrate and annealing during the initial stage of chemical vapor deposition

Electron microscopic and electron spectroscopic techniques were applied to study interface properties, microstructure and texture of pyrolytic carbon obtained after short-time chemical vapor deposition (CVD) on planar Si substrates. The pyrolytic carbon was obtained in a hot-wall reactor from methane at a total pressure of 20 kPa and temperature of 1100 °C. Only short depositions between 2.5 and 240 min were performed. The carbon deposition starts with the nucleation of isolated islands. The increase of residence and deposition time leads to the formation of a continuous layer by larger island sizes and higher island densities, a transition from rough to smooth surfaces and formation of pores on smooth surfaces. An increased deposition rate during the first 15 min is observed which is correlated with a granular morphology of the carbon layer. Using BN-covered Si wafers with a surface roughness on a 100 nm scale reduces the texture degree in the vicinity of the interface and strengthens adhesion of the pyrolytic carbon compared to the smooth Si substrate. The texture of high-textured pyrolytic carbon is improved significantly by annealing at 1100 °C.     

水-酒精混合蒸气的管外凝结

在不同蒸气压力,相同蒸气流速条件下,完成了不同酒精浓度的混合蒸气在不同管径的竖直管外凝结换热实验。凝结换热特性曲线显示了相似的特性：随着酒精浓度的增加,凝结传热系数显著下降;随着表面过冷度的增加,凝结传热系数显示出有峰值的非线性特点。在相同条件下,半径为5 mm管外的凝结传热系数峰值出现在较大过冷度范围内,且峰值高于在半径为10 mm管外的凝结传热系数峰值。当蒸气压力为84. 52 kPa,流速为2 m·s^-1时,酒精浓度为1%的混合蒸气在半径为5 mm竖直管外凝结传热系数最高达150 kW·m^-2·K-¹,约为水蒸气的8倍。此外,根据记录的凝结形态,珠状凝结出现在很广的浓度以及过冷度范围内。     

Consideration of reaction mechanisms leading to pyrolytic carbon of different textures

A distinction between a growth and a nucleation mechanism is not sufficient to draw direct conclusions in relation to the texture of pyrolytic carbon. This is determined by the carbon formation mechanisms, which are analogous or at least similar to the mechanisms of aromatic growth. The latter mechanisms are reviewed in the first part of the paper with special consideration of structural chemical aspects. The relevance of the individual mechanisms is analyzed in the second part based on experimentally determined reaction products. Most important mechanisms are aryl-aryl combination, intramolecular dehydrocyclization and ethine addition reactions. The influence of mechanisms concerning an inhibition of the formation of five-membered rings and a transformation of five- into six-membered rings is difficult to estimate. The results indicate that a high textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic to small linear hydrocarbons (ethine). This model is called the particle-filler model (aromatic hydrocarbons: molecular particles; ethine: molecular filler).     

Analysis of products of high-temperature pyrolysis of various hydrocarbons

Ethane, ethylene, acetylene, propane and neopentane have been pyrolyzed at 1173 K, and methane at 1372 K in a flow system, and the volatile pyrolysis products analyzed. Eleven aromatic hydrocarbons, containing 14 or fewer carbon atoms, accounted for 98 + % of the liquid products recovered in each case. Benzene was the main product, followed by naphthalene. No compounds with branched chains or multiple substituents were present, and compounds containing even numbers of carbons comprised 93–99% of each mixture. Acetylene was a major component of the gaseous effluent from each of the initial hydrocarbons. The effect of temperature on the composition of the gaseous effluent during pyrolysis of methane, ethane and ethylene was determined. Carbon film deposition from methane commenced at about 1273 K; from ethane at 1015 K and from ethylene at 1100 K, in each instance coinciding with the appearance of acetylene in the effluent. As the temperature was raised, at first the increase in the rate of carbon deposition closely followed the increase in the concentration of acetylene in the effluent. It is proposed that acetylene may be a common factor in the pyrolysis of aliphatic hydrocarbons, perhaps acting as the precursor of both surface carbon and aromatic hydrocarbons by a process of head-to-tail linkage of two-carbon units at active surface sites to form chains that then undergo dehydrogenation to carbon or cyclization and desorption as aromatic species.     

Densification of a 2D carbon fiber preform by isothermal, isobaric CVI: Kinetics and carbon microstructure

Chemical vapor infiltration of a 2D carbon fiber preform with a 0/0/90/90° fiber architecture and a fiber volume fraction of 22.5% was investigated as a function of methane pressure at various temperatures as well as a function of infiltration time at constant pressure. Inside-outside densification was obtained at the most attractive temperature of 1095 °C up to 29 kPa resulting in a maximum bulk density of 1.84 g cm⁻³ and a matrix density of 2.17 g cm⁻³, which corresponds to high-textured carbon. Texture formation can be perfectly explained with the earlier proposed particle-filler model. Studies at increasing infiltration times suggest a recrystallization of carbon deposited in the early stages of the infiltration.     

Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition

An ultrathin sheet-like carbon nanostructure, carbon nanosheet, has been effectively synthesized with CH₄ diluted in H₂ by an inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Nanosheets were obtained without catalyst over a wide range of deposition conditions and on a variety of substrates, including metals, semiconductors and insulators. Scanning electron microscopy shows that the sheet-like structures stand on edge on the substrate and have corrugated surfaces. The sheets are 1 nm or less in thickness and have a defective graphite structure. Raman spectra show typical carbon features with D and G peaks at 1350 and 1580 cm⁻¹, respectively. The intensity ratio of these two peaks, I(D)/I(G), increases with methane concentration or substrate temperature, indicating that the crystallinity of the nanosheets decreases. Infrared and thermal desorption spectroscopies reveal hydrogen incorporation into the carbon nanosheets.     

Modeling CVD synthesis of carbon nanotubes: Nanoparticle formation from ferrocene

Catalyst nanoparticles play an important role in the synthesis of carbon nanotubes. In this paper, we present a two-equation model that can predict the formation process of iron nanoparticles from ferrocene fed into a CVD reactor. The model, combined with an axisymmetric two-dimensional computational fluid dynamics (CFD) simulation, includes the mechanism of nucleation and surface growth of an iron particle and bi-particle collision. The model predicts that the diameter of a particle will increase with an increase in the reaction temperature or the radial distance from the center of the reactor. Iron particles may deposit on the reactor wall; our model predicts that the thickness of the layer consisting of deposited iron particles will decrease with an increase in the axial distance from the entrance. The first prediction was validated by experimental observations reported by other researchers. In addition to the CFD simulation, a dimensional analysis was conducted to find pi-numbers that govern the process of particle formation; three pi-numbers were identified. Furthermore, one-dimensional governing equations were obtained under the assumptions of constant diffusion coefficient and collision frequency function, and solutions for particle diameter were obtained in qualitative agreement with the earlier CFD simulations.     

Diameter control of multiwalled carbon nanotubes using experimental strategies

Multiwalled carbon nanotubes (MWNTs) were synthesized using a chemical vapor deposition floating feed method in a vertical reactor. Effects of the preparation variables on the average diameter of carbon nanotubes were systematically examined using the fractional factorial design (FFD), path of the steepest ascent, and central composite design (CCD) coupled with the response surface methodology. From the FFD study, the main and interactive effects of reaction temperature, methane flow rate, and chamber pressure were concluded to be the key factors influencing the diameter of MWNTs. Two empirical models, representing the dependence of the diameter of carbon nanotubes at the vicinities around maximum (420 nm) and minimum (15 nm) on the reaction temperature and methane flow rate, were constructed in two independent CCD studies. These models, shown as contour diagrams, indicated that the diameter of carbon nanotubes generally increased with increasing reaction temperature and methane flow rate. Based on both models, the diameter of MWNTs from 15 to 420 nm can be controlled precisely by using a continuous CVD fabrication method.     

Reaction rate constant of methane clathrate formation

Particle size distribution measurements were performed during the growth stage of methane hydrate formation in a semi-batch stirred tank crystallizer. Experiments were carried out at temperatures between 275.1 and 279.2 K and pressures ranging from 3873 to 5593 kPa. The reaction rate constant of methane hydrate formation was determined using the model of Bergeron and Servio (AIChE J 2008;54:2964). The experimental reaction rate constant was found to increase with temperature, following an Arrhenius-type relationship, from 8.3 × 10⁻⁸ m/s to 6.15 × 10⁻⁷ m/s over the 4° range investigated, resulting in an activation energy of 323 kJ/mol. An increase in pressure of approximately 600 kPa did not have any effect on the reaction rate constant. Population balances, based on the measured critical nuclei diameter and that predicted by homogeneous nucleation theory, were also used for comparison purposes. The initial number of hydrate particles was calculated using the mole fraction of methane in the bulk liquid phase and compared to that predicted by an energy balance.     

Analysis of gas phase compounds in chemical vapor deposition of carbon from light hydrocarbons

Product distributions in the pyrolysis of ethylene, acetylene, and propylene are studied to obtain an experimental database for a detailed kinetic modeling of gas phase reactions in chemical vapor deposition of carbon from these light hydrocarbons. Experiments were performed with a vertical flow reactor at 900 °C and pressures from 2 to 15 kPa. Gas phase components were analyzed by both on-line and off-line gas chromatography. More than 40 compounds from hydrogen to coronene were identified and quantitatively determined as a function of the residence time varied up to 1.6 s. Product recoveries were generally more than 90%. Analysis of the kinetics of the conversion of the hydrocarbons resulted in global reaction orders of 1.2 (ethylene), 2.7 (acetylene), and 1.5 (propylene). First order dehydrogenation reactions and third order trimerization reactions leading to benzene are decisive reactions for ethylene and acetylene, respectively. Conversion of propylene should also be based on two simultaneous reactions, a first order dissociation reaction, and second order reactions such as bimolecular reaction of propylene resulting an allyl and a propyl radical. These insights should be useful to develop a reaction mechanism based on elementary reactions in forthcoming studies.