Chemical Vapor Deposition Of Pyrocarbon Research Articles

Fluidized bed chemical vapor deposition of pyrocarbon on various types of powders: Heat and mass transfer analyses and nanotexture characterization

SiC powders coated by pyrocarbon are a promising feedstock for the synthesis of Ceramic-matrix composites. Preparing such powders is possible using low pressure Fluidized-Bed Chemical Vapor Deposition (FB-CVD) with propane as a precursor at 1000 °C. A preliminary study on non-porous and porous alumina powders as substrates has been carried out. With the help of a numerical assessment of heat transfer in the reactor, it has been possible to optimize the setup and achieve excellent deposition yields (up to 100%), even inside very small pores (7 nm), and transpose the protocol to 200-nm diameter SiC powders. The microstructure and texture of the deposits were characterized by TEM, showing dependence to pore size due to a texture-induction effect by the substrate surface. A quantitative assessment of mass transfer indicated that the results obtained here agree with past studies on FB-CVD of other materials and on pyrocarbon CVD on flat substrates.

Fluidized Bed Chemical Vapor Deposition of Pyrocarbon on Various Types of Powders: Heat and Mass Transfer Analyses and Nanotexture Characterization

Fluidized Bed Chemical Vapor Deposition of Pyrocarbon on Various Types of Powders: Heat and Mass Transfer Analyses and Nanotexture Characterization

A molecular-level analysis of gas-phase reactions in chemical vapor deposition of carbon from methane using a detailed kinetic model

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 without any artificial adjustments. The mole fractions of hydrogen, acetylene, ethylene, and benzene increased with a decreasing growth rate as the residence time and the initial methane pressure increased. Sensitivity analysis and reaction paths were conducted to identify the crucial reaction steps and explain how they impact in this pyrolysis process. Results showed that methane pyrolysis had an incubation stage to form a necessary gas atmosphere for the pyrolysis to move forward and C3 species were the main direct source for benzene formation. These results should be useful to understand methane pyrolysis at a molecular level in CVD, as well as the relationship between the gas species and the pyrocarbon.

An interpretation of the hydrogen inhibiting effect on chemical vapor deposition of pyrocarbon

A detailed kinetic mechanism, based on elementary steps, modeling both pyrocarbon deposition and gas phase composition obtained by propane pyrolysis on carbon fibers was recently proposed. This model, based on heterogeneous and homogeneous elementary steps was efficient to predict pyrocarbon deposition rate and gas phase composition when the influence of temperature (1173–1323K), residence time (0.5–4s), carbon surface fibers, and reactor inlet mixture were studied. In this paper, we discuss experimental and modeling results of hydrogen inhibiting effects on pyrocarbon deposited and gas phase composition during propane/hydrogen pyrolysis (temperature 1273K, residence time 1s, surface fibers 3960cm−1). Simulated results are coherent with experimental data when the pyrocarbon deposition rate is studied. Compared to pure propane chemical vapor deposition, the mole fractions of C2 gas phase species are not affected by an increasing of hydrogen inlet, whereas pyrocarbon deposition rate and mole fractions of C3 and aromatic species decrease. These results also confirm, in accordance with the literature, that hydrogen inhibits highly the formation of pyrocarbon and aromatic species.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon VIII. Carbon deposition from methane at low pressures

Chemistry and kinetics of chemical vapor deposition of pyrocarbon VIII. Carbon deposition from methane at low pressures

Chemistry and kinetics of chemical vapor deposition of pyrocarbon: VIII. Carbon deposition from methane at low pressures

Carbon deposition from methane was studied at methane pressures from 5 to 30 kPa, temperatures from 1050 to 1125°C, total residence times of 1, 2 and 4 s, using substrates with surface area/volume ratios, [ A S/ V R], of 0.79, 1.6, 3.2 and 7.4 mm −1. The deposition rates were determined as a function of the substrate length using substrates composed of 10 slices. Surface-related deposition rates are not constant, but decrease with increasing [ A S/ V R] ratio. Increasing methane pressure leads to various states of saturation adsorption; they are ascribed to different major carbon forming species. The rate-limiting step results from dissociation of the carbon–hydrogen surface complex, C ∞(H); the corresponding activation energy is in the range of 445±10 kJ/mol. The results at low pressures confirm those obtained at 100 kPa using argon as diluent gas.

Monitoring chemical vapour deposition of pyrocarbon by electron spin resonance

Pyrolytic carbon films were deposited on boron nitride substrates during plug-flow of a methane/argon gas mixture through a hot-wall tubular reactor at 100 kPa and 1100°C. Of particular interest is the time resolved development of chemical reactions within the free reactor volume, which is projected to the formed structures along the reactor axis. Spatially resolved electron spin resonance (ESR) is used to characterize the deposited films. The width and asymmetry of the ESR line varies monotonously along the reactor axis by more than one order of magnitude within 25 mm.

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.

Chemistry and kinetics of chemical vapour deposition of pyrocarbon: VII. Confirmation of the influence of the substrate surface area/reactor volume ratio

Carbon deposition from a methane–hydrogen mixture (pCH4=17.5 kPa, pH2=2.5 kPa) was studied at an ambient pressure of about 100 kPa and a temperature of 1100°C, using deposition arrangements with surface area/reactor volume ratios, [AS/VR], of 10, 20, 40 and 80 cm−1. Steady-state deposition rates and corresponding compositions of the gas phase as a function of residence were determined. The deposition rates in mol/h increase with increasing [AS/VR] ratio at all investigated residence times up to 1 s. However, surface-related deposition rates in mol/m2h decreased. As the same results have been obtained in a preceding study using pure methane at a partial pressure of 10 kPa, it has been confirmed that all the kinetics can be determined by changing the [AS/VR] ratio.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon — IV pyrocarbon deposition from methane in the low temperature regime

Pyrocarbon deposition from methane was studied at ambient pressure and 1100 °C using a vertical hot-wall deposition reactor; the methane initial partial pressure was varied up to 75 kPa and the residence time up to 1 second. The influence of hydrogen partial pressure was studied at constant methane partial pressure. Steady-state pyrocarbon deposition rates and corresponding compositions of the gas-phase were determined. Reaction models of homogeneous gas-phase and heterogeneous pyrocarbon deposition reactions were derived and used for simulation of pyrocarbon deposition kinetics influenced by residence time, initial partial pressures of methane and hydrogen, and concentrations of active sites. The importance of active sites and their blocking by hydrogen is discussed with special emphasis.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon — III pyrocarbon deposition from propylene and benzene in the low temperature regime

Pyrocarbon deposition from propylene and benzene was studied at ambient pressure and 1000 °C using a vertical hot-wall reactor. Initial partial pressures of the hydrocarbons and residence time were varied; the influence of hydrogen in the feed gas was additionally studied. Gaseous and liquid reaction products were analyzed by on-line gas chromatography. The results are discussed in terms of homogeneous gas-phase and heterogeneous pyrocarbon deposition reactions. Reactions models have been derived and used for simulation of pyrocarbon deposition rates and their inhibition by hydrogen. Pyrocarbon deposition from propylene is shown to be controlled by complex pyrolysis reactions in the gas-phase, whereas benzene forms pyrocarbon directly i.e. without significant formation of intermediate products in the gas-phase.

Chemistry and kinetics of chemical vapor deposition of pyrocarbon — V influence of reactor volume/deposition surface area ratio

Pyrocarbon deposition from methane was studied at ambient pressure and a temperature of 1100 °C using a vertical hot-wall reactor with a honeycomb structure as substrate. Compared to the deposition tube used in previous studies this substrate exhibits a 15-fold increase in surface area. The methane initial partial pressure was varied up to 75 kPa, and the residence time up to 1 second. The results are compared with those obtained with the tube. The surface area is shown to decisively influence the chemistry and kinetics of deposition reactions and thus also the major growth species of pyrocarbon. With residence time increasing from zero to one second the ratio of surface-related pyrocarbon deposition rates obtained with tube and honeycomb structure vary by a factor of about 18. With increasing initial partial pressure, deposition rates tend to approach a limiting value indicating saturation adsorption. Pyrocarbon deposition rates as a function of residence time and initial partial pressure have been simulated considering complex homogeneous and heterogeneous reactions.

Chemistry and kinetics of chemical vapour deposition of pyrocarbon: I. Fundamentals of kinetics and chemical reaction engineering

Open scientific problems of chemical vapour deposition of pyrocarbon are discussed on the basis of brief historical and literature reviews. Some thermodynamic considerations are followed by detailed analyses of reaction pathways in hydrocarbon pyrolysis. Reaction schemes are derived, and corresponding differential equations have been solved to show the influence of (i) initial partial pressure of the hydrocarbon and (ii) residence time on the rate of pyrocarbon deposition in general as well as carbon deposition profiles along the reactor axis. Finally, some suggestions are presented for model-based analyses of pyrocarbon deposition. In the following parts of this paper, these fundamentals will be applied for an as good as possible analysis and simulation of pyrocarbon deposition kinetics. It will be the goal to correlate pyrocarbon structures not with the deposition conditions, but with the dominating growth species.

Chemical vapour deposition of pyro-carbon, SiC, TiC, TiN, Si and Ta on different types of carbon fibres

Chemical vapour deposition of pyro-carbon, SiC, TiC, TiN, Si and Ta on different types of carbon fibres