Vapor Deposition Of Pyrolytic Carbon Research Articles

Enhancing inward and outward mass transport during chemical vapor deposition of pyrolytic carbon for better synthesis of ZTC

Zeolite-templated carbon (ZTC) can be prepared by chemical vapor deposition (CVD), which involves the coupled mass transport and chemical reaction of carbon precursors. Combining theoretical and experimental approaches, this work demonstrated that reducing the particle size of the template enhanced both the inward and outward mass transport during zeolite-templated CVD, which, in turn, improved the structural properties of the resulting ZTC. Specifically, the inward diffusion of the carbon precursor was modelled quantitatively by fitting the deposition mass of pyrolytic carbon measured in zeolite templates of different particle sizes (d = 4, 0.8 and 0.002 mm). This model revealed that the inward diffusion of the carbon precursor was enhanced in smaller zeolite particles, allowing more uniform deposition of pyrolytic carbon inside the zeolite and better replication of the zeolite's pore structure in the resulting ZTC. For the outward mass transport, semi-quantitative analysis was conducted on the intermediate components that were responsible for the external deposition of pyrolytic carbon. In particular, the concentration of the intermediate components was predicted to increase with the particle size of the zeolite. This prediction was confirmed by the experimental characterization results, where ZTC samples prepared with larger zeolite particles showed thicker external layers of quasi-crystalline graphitic carbon.

Radial growth kinetics of epitaxial pyrolytic carbon layers deposited on carbon nanotube surfaces by non-catalytic pyrolysis

Chemical vapor deposition of pyrolytic carbon (PyC) onto carbon nanotube (CNT) surfaces by hydrocarbon pyrolysis can thicken the nanotube diameter until the desirable size. Vapor-induced thickening of CNTs may require no metal catalysts because the CNT-templated growth can easily convert PyC deposits into epitaxial graphene shells. However, little work has been done on the kinetics of the vapor-induced CNT thickening process during the non-catalytic pyrolysis. In this study, we discuss the growth mechanism of epitaxial PyC layers on CNT surfaces achieved by the low-pressure (≤1.5 kPa) pyrolysis of ethylene, which is distinct from previous studies using much higher gas pressures (4–100 kPa) for the PyC deposition. The average thickness of deposited PyC layers is calculated by analyzing the transmission electron microscopy images of PyC-coated CNTs. The crystallinity of PyC-coated CNTs is evaluated by Raman spectroscopy and X-ray diffraction. The thickness of PyC layers on CNT surfaces can be adjusted by simply changing the growth time under typical pyrolysis parameters (800 °C-1.5 kPa of ethylene). We find a two-stage nonlinear behavior to describe the growth kinetics of PyC layers, indicating that the radial growth rate of PyC layers can be influenced by the number of surface active sites. This study reveals the self-assembly mechanism of graphitic layers on the CNT template, which is expected to promote the diameter-controlled synthesis of CNTs.

Pyrolytic carbon infiltrated nanoporous alumina reducing contact resistance of aluminum/carbon interface

We fabricated a highly conductive pyrolytic carbon layer that penetrates into nanoporous alumina channels and that strongly adheres to an aluminum (Al) current collector in order to reduce the contact resistance at the interface between a carbon active material and an Al current collector. The fabrication method includes the following processes: (i) Al anodization, (ii) alumina etching, (iii) nickel seed layer deposition on the alumina surface, and (iv) chemical vapor deposition of pyrolytic carbon grown by the catalytic nickel layer. From SEM and EDX analyses, we confirmed that the pyrolytic carbon could be uniformly placed into an alumina nanoporous template on the Al substrate. Based on measurements of the transverse resistance and electrochemical capacitor tests, the fabricated composite layer of pyrolytic carbon and nanoporous alumina significantly reduced contact resistance of Al/carbon interface due to the highly conductive pyrolytic carbon formed through the nanoporous alumina on the Al surface.

Modeling of Chemical Vapor Deposition of Pyrolytic Carbon in a Gas-Spouted Bed Reactor

Spouted beds are used in chemical vapor deposition as they provide good solid mixing, thereby enhancing high mass and heat transfer rates. In the present study, investigations on the kinetics of a coating process by chemical vapor deposition were carried out in a spouted-bed reactor. Pyrolytic carbon coatings were studied by thermal cracking of acetylene mixed with carrier gas argon at 1350 °C. A theoretical model was proposed based on the mass balance of the precursor in the spout and annulus zones at isothermal condition, and the model equations were solved using MATLAB (version 7.1) software. The effect of the operating gas velocity, initial static bed height, and acetylene concentration on the coating rate was examined experimentally and also by simulation results. The simulated results agreed well with the experimental results. Model equations used for simulation were utilized to evaluate the reaction rate constant and radial gas diffusion coefficient and then to study the effect of spout diameter, operating gas velocity, and precursor concentration on the overall conversion of the precursor.

Chemistry and kinetics of chemical vapor deposition of pyrolytic carbon from ethanol

Synthesis of pyrolytic carbon as a matrix for carbon fiber reinforced carbon composites by chemical vapor infiltration (CVI) is studied experimentally and numerically using the oxygen-containing precursor ethanol. The effects of residence time on microstructure and deposition rate of pyrolytic carbon are investigated. A short residence time is found to favor the formation of high-textured pyrolytic carbon. The evolutions of microstructure and deposition rate of pyrolytic carbon are compared with those of carbon deposited from methane. Compared to methane, ethanol exhibits a much higher deposition rate of pyrolytic carbon with similar microstructures. Pyrolysis of ethanol is modeled using a two-dimensional flow model coupled with a detailed gas-phase reaction mechanism involving 261 species taking part in 1177 reversible reactions. Reaction rate analysis reveals that C 3-hydrocarbons are the most important intermediate species contributing to the maturation of gas-phase composition. A comparison of the kinetic predictions with equilibrium calculations demonstrates that the CVD reactor applied is operated far away from equilibrium.

Accurate Coupled Cluster Calculations of the Reaction Barrier Heights of Two CH3• + CH4 Reactions

We have computed barrier heights of 71.8 +/- 2.0 and 216.4 +/- 2.0 kJ mol(-1) for the reactions CH4 + CH3* --> CH3* + CH4 and CH4 + CH3* --> H* + C2H6, respectively, using explicitly correlated coupled cluster theory with singles and doubles combined with standard coupled cluster theory with up to connected quadruple excitations. Transition-state theory has been used to compute the respective reaction rate constants in the temperature interval of 250-1500 K. The computed rates for the reaction to ethane are orders of magnitude slower than those used in the mechanism of Norinaga and Deutschmann (Ind. Eng. Chem. Res. 2007, 46, 3547.) for the modeling of the chemical vapor deposition of pyrolytic carbon.

Diffuse interface method for simulation of the chemical vapor deposition of pyrolytic carbon: Aspects of the mathematical formulation

AbstractThe present work is inspired by Anderson et al. (Phys. D Nonlinear Phenom. 2000; 135(1–2):175–194) and Noll (http://www.math.cmu.edu/wn0g/noll) and falls in the conceptual line of the Ginzburg–Landau class of first‐order phase‐transition models based on the concept of phase‐field parameter. Trying to keep the exposition as much general as possible, we develop below a thermodynamically consistent rationalization of the physical process of (anisotropic) deposition of pyrolytic carbon from a gas phase. The derivation line we follow is well established in the field of the modern continuum physics. From the covariance of the first principle of thermodynamics, the second Newton's law and the Liouville's theorem with respect to the one‐dimensional Lie groups of transformations, the balance laws for the temperature, linear momentum and density are formulated. This system of partial differential equations is comprehended further by the constitutive laws for the phase field, the stress, and the heat and entropy fluxes obtained in a form consistent with the Clausius–Duhem understanding of the second law. The result referred to as a local, strongly coupled initial boundary value problem of chemical vapor deposition (IBVP‐CVD) constitutes the general mathematical description of the CVD process. The weak form of isotropic IBVP‐CVD is then derived and discretized by means of the discontinuous Galerkin method. At the end of the paper, we also derive the weak formulations for the local lifting operators that provide the stabilization mechanism for the discontinuous Galerkin discretization scheme. Copyright © 2008 John Wiley & Sons, Ltd.

Detailed Kinetic Modeling of Gas-Phase Reactions in the Chemical Vapor Deposition of Carbon from Light Hydrocarbons

The chemical kinetics of the pyrolysis of the hydrocarbons ethylene, acetylene, and propylene are modeled in detail under conditions relevant to the chemical vapor deposition of pyrolytic carbon. A mechanism that consisted of 227 species and 827 reactions (most of which are reversible) is developed and computed using a software package designed for computing time-dependent homogeneous reaction systems. Experimental results used for model validation are obtained using a vertical flow reactor at 900 °C, pressures of 2−15 kPa, and residence times of up to 1.6 s. The products are analyzed using on-line and off-line gas chromatography. Computational and experimental results are compared for more than 30 products, including hydrogen, small hydrocarbons (ranging from methane to C4 species), and aromatic hydrocarbons (ranging from benzene to coronene). The resulting reaction model predicts the profiles of the major pyrolysis products (mole fractions of >10-2) of the three hydrocarbons, as a function of both resid...

A free boundary problem describing reaction–diffusion problems in chemical vapor infiltration of pyrolytic carbon

In this paper we consider chemical vapor deposition of pyrolytic carbon from methane in hot wall reactors. Especially, we deal with the interaction of homogeneous gas-phase and heterogeneous surface reactions. The resulting mathematical model is composed of a system of reaction–diffusion equations in a corner domain supplied with the Gibbs–Thomson law, which describes the movement of the free boundary, arising from the carbon deposition. We prove a short time existence and uniqueness result in Hölder spaces. We achieve this by contraction arguments and transforming the Gibbs–Thomson law to local coordinates to obtain a nonlinear parabolic equation on a manifold.

Chemistry and kinetics of chemical vapor deposition of pyrolytic carbon from methane

The paper summarizes various recent results on chemical vapor deposition of carbon from methane in the low temperature regime around 1100 °C. The following topics are discussed: gas phase reactions, surface/deposition reactions, interactions of gas phase and surface/deposition reactions, hydrogen inhibition, influence of temperature. Based on the results a simplified reaction scheme is presented which was successfully used to simulate the influence of temperature on carbon deposition. This simulation as well as the influence of the substrate surface area/reactor volume-ratio (3rd parameter of CVD) are treated in other papers of the conference.

CVD in Hot Wall Reactors—The Interaction Between Homogeneous Gas-Phase and Heterogeneous Surface Reactions

The paper is concerned with chemical vapor deposition in hot wall reactors and in particular with the interaction of homogeneous gas-phase and heterogeneous surface reactions. Based on model considerations it is shown that this interaction has a tremendous influence on the deposition chemistry and kinetics in all cases in which the precursor gas undergoes complex gas-phase reactions. The decisive parameter determining the interaction is given by the ratio of free volume of the deposition space and size of the surface area of the substrate; it is termed the “third parameter” of chemical vapor deposition. Predictions from the model are experimentally confirmed using results on chemical vapor deposition of pyrolytic carbon from methane. The importance of this “third parameter” of chemical vapor deposition in investigations of kinetics and for technical processes of chemical vapor deposition and infiltration is discussed in the conclusion.

Mass Spectrometric Study of the Gas Phase During Chemical Vapor Deposition of Pyrolytic Carbon

The pyrolysis of CH 4 , of C 3 H 8 and of the mixture 20C 3 H 8 /80CH 4 was investigated in a hot wall CVD reactor by mass spectrometry. Experiments were conducted as a function of temperature and of surface-to-volume (S/V) ratio of the substrate. The main by-products of the pyrolysis were identified and reaction mechanisms were proposed taking into account homogenous decomposition/formation reactions. Cyclic species are possibly involved in the formation mechanism of pyrocarbon.

Chemical vapor deposition of pyrolytic carbon on polished substrates

Pyrolytic carbon thin (4-100 nm) films were obtained from methane in a hot wall reactor on optically polished inert substrates by varying the deposition time and temperature. They were characterized by all modes of TEM. They are composed in majority of lamellar pyrocarbon whose thickness and disorder increases with increasing temperature. Isotropic carbon islands are also observed at the upper surface of the film

Chemical vapor deposition of pyrolytic carbon on carbon substrates: I: Effect of substrate surface characteristics on the kinetics of deposition

Chemical vapor deposition (CVD) of pyrolytic carbon on 25 different carbon substrates has been investigated at 1273–1338 K, using a mixture of 10% methane in argon flowing at 150 cc/min (STP). The substrates covered a wide spectrum of carbons (porous/nonporous, graphitic/nongraphitic, and small/large surface areas). The results indicate that there are at least four different categories of carbon substrates. The first category includes nonporous, graphitized, or carbonized substrates having a small surface area. The deposition rate increases with surface area. The second category includes microporous carbons with a small external surface area. The early deposition of pyrolytic carbon blocks the micropores, and the reaction continues on the external surface in a manner similar to the first category. The third category includes nonporous, graphitized, and carbonized substrates having a large surface area. The deposition rate on the graphitized substrates is smaller than the carbonized substrates provided that their initial surface area is the same. With carbonized substrates, the total and active surface areas are contributing differently to the CVD rate. However, the contribution of active surface area (ASA) is less pronounced than that of the total surface area (TSA). The fourth category includes activated graphitized carbons. The activation process enhances the ASA and develops microporosity. The newly developed ASA is apparently located inside the micropores since the deposition rate drops substantially after the commencement of reaction. When the micropores are blocked, the entrances to internal ASA are blocked and the reaction continues on the external surface in a manner similar to that of the third category.