MODELLING AND SIMULATION OF MATERIALS SYNTHESIS:
CHEMCIAL VAPOR DEPOSITION AND INFILTRATION OF
PYROLYTIC CARBON
Aijun Li, Koyo Norinaga and Olaf DeutschmannInstitute for Chemical Technology and Polymer Chemistry University of Karlsruhe, D- 76128 Karlsruhe, Germany
e-mail: deutschmann@ict.uni-karlsruhe.de Web page: http://www.dmann.de/uni
Key words: Chemical vapor infiltration, CVD, modeling, CFD, pyrolytic carbon
Abstract. Numerical simulation of materials synthesis based on detailed models for the chemical kinetics and transport processes is expected to support development and optimization of production processes. Exemplarily, chemical vapor deposition and infiltration of pyrolytic carbon for the production of carbon fiber reinforced carbon is studied by recently developed modeling approaches and computational tools. First, the development of a gas phase reaction mechanism of chemical vapor deposition (CVD) of carbon from unsaturated light hydrocarbons (CH4, C2H4, C2H2, and C3H6) is presented. The mechanism consisting of
757 reactions among 230 species is based on existing information on elementary reactions and evaluated by comparison of numerically predicted and experimentally determined product composition for more than 40 stable gas phase compounds in a CVD flow reactor. The reactor was operated at widely varying conditions: 800-1100 °C and 2-15 kPa. Experimentally observed pressure and temperature effects on the species profiles as function of residence time are well predicted. Second, a model and computer code is presented for the numerical simulation of chemical vapor infiltration (CVI) carbon for the production of carbon fiber reinforced carbon. The chemistry model is based on a multi-step reaction scheme for pyrocarbon deposition, derived from the elementary mechanism, and a hydrogen inhibition model of pyrocarbon growth. This chemical model is implemented in transient 2D simulations of chemical vapor infiltration. The coupled models for mass transport (convection and diffusion), chemical vapor deposition and surface growth, gas-phase and surface chemical reactions are numerically solved by a FEM approach. Three sets of experiments were exemplarily simulated with inlet flows of 20 kPa CH4, 20 kPa CH4 with 4 kPa H2, and
20 kPa CH4 with 10 kPa H2, all at a temperature of 1095°C. The continuous infiltration,
pyrolysis, and deposition of methane and its consecutively formed CxHy products lead to
1 INTRODUCTION
The numerical simulation of materials synthesis based on detailed models for the chemical kinetics and transport processes are expected to support development and optimization of production processes. Chemical reactors for materials synthesis are often characterized by interactions between chemical reactions in usually more than one phase, e.g. gas phase and solid phase, mass transport by convection and diffusion, heat transport in different phases, and the continuous variations of geometrical and physical properties such as structure of fluid region, surface to volume ratio, and porosity.
The Chemical Vapor Infiltration of hydrocarbon into carbon/carbon compositions has been of great scientific interest since its development in 1960s. Although it has been widely accepted that the interaction of homogeneous gas-phase reactions, heterogeneous surface reactions, and mass transport controls the texture and the densification mode of pyrocarbon, yet the process has not been understood in detail. Up to now, much effort has been made to reveal the essentials of CVI processes [1-2]. Isobaric and isothermal chemical vapor infiltration (CVI) of carbon is an accepted process in synthesizing C/C composite used in the aerospace industry (rocket and missile nozzles and brake disks). A predictive and reliable model for CVD and CVI of carbon is useful for the development and optimization of production processes for C/C composites.
In this contribution, we will discuss modeling approaches for the description of chemical vapor deposition (CVD) and infiltration (CVI) of carbon/carbon composites and computational tools for the numerical simulation of the chemical reactor for synthesizing these materials. The CVD study leads to a detailed reaction mechanism, in particular for the processes in the gas phase. A reduced version of that chemical model is then implemented in transient 2D simulations of a chemical vapor infiltration process. The model predictions are compared with experimentally derived data.
2 DEVELOPMENT OF A DETAILED KINETIC MODEL OF GAS PHASE REACTIONS
In the CVD of carbon, a great variety of hydrocarbons and hydrocarbon radicals are formed by gas phase reactions, and any of these species has a potential to chemisorb or physisorb at the growing pyrolytic carbon surface and thus to form pyrolytic carbon. We developed a detailed chemical kinetic model which simulates gas phase reactions in CVD of carbon from unsaturated light hydrocarbons. The experimental results obtained in our previous work [3] on analysis of gas phase compounds in CVD of carbon from ethylene, acetylene, and propylene are used for model validation. Comparisons between the computed mole fractions and the experimental mole fractions are presented for more than 30 products including hydrogen and hydrocarbons ranging from methane to coronene.
2.1 Modeling approach
757 reactions, which is given elsewhere [4]. The process of the model development is roughly given in the followings: First, hydrocarbon reactions are extracted from the mechanism of aromatics formation in acetylene and ethylene flames reported by Wang and Frenklach [5]. This mechanism is very comprehensive and covers formation of polycyclic aromatic hydrocarbons up to pyrene but does not involve formations of hydrocarbons with odd numbers of carbon atoms such as toluene and indene. This was supplemented by the mechanism complied by Marinov et al. [6]. The detailed reactions on the C3 and C4 species are included from the mechanism reported by Hidaka et al. [7] and Tsang [8]. The formation mechanisms of the larger polycyclic aromatic hydrocarbons (PAH) up to coronene by Richter et al. [9] were also added to account for those larger species found in the reactor product stream. The simulations were conducted by using several tools of the software packages DETCHEM [10] and the code HOMREA [11]. The results discussed below were achieved by the computation of time-dependent homogeneous reaction systems. The program input includes forward reaction rate parameters and thermodynamic polynomials for all the participating species in addition to temperature, pressure, and concentrations of the reactants. The program calculates the rate constant of the backward reaction for every reaction given. The model was validated and adjusted by comparing the computation results with CVD experimental results of various hydrocarbons. Sensitivity analyses, which identify elementary reactions with major influences on the overall process, were extensively performed to improve the model predictability.
2.2 Model evaluation
The kinetic model was evaluated by comparison of the numerically predicted product composition with the one measured by gas chromatography analysis in a CVD flow reactor. The experimental procedure is described and a further discussion is given in references [3] and [4], respectively.
C2H4 CH4 C3H6 H2 C6H6 HCCCH3 1,3-C4H6 C2H2 H2CCCH2 1.0E-03 1.0E-02 1.0E-01 1.0E+00 0 0.2 0.4 0.6 0.8 H2 C2H4 1,3-C4H6 C6H6 C2H2 CH4 1.0E-03 1.0E-02 1.0E-01 1.0E+00 0 0.2 0.4 0.6 0.8 1 H2 C2H4 1,3-C4H6 C6H6 C2H2 CH4 1.0E-03 1.0E-02 1.0E-01 1.0E+00 0 0.2 0.4 0.6 0.8 1 C2H2 H2 C2H4 C6H6 C4H4 CH4 1.0E-03 1.0E-02 1.0E-01 1.0E+00 0 0.4 0.8 1.2 1.6 C2H2 H2 C2H4 C6H6 C4H4 CH4 1.0E-03 1.0E-02 1.0E-01 1.0E+00 0 0.4 0.8 1.2 1.6 mo le f ra c ti ons, -time, s (a) (b) (c)
Figure 1: Comparison of model predictions (lines) with experimental mole fraction profiles (symbols) of major
species during pyrolysis of ethylene (a), acetylene (b) and propylene (c) in a flow reactor at 900 °C and 8 kPa.
1.0E-04 1.0E-03 1.0E-02 1.0E-01 0 0.2 0.4 0.6 0.8 1.0E-04 1.0E-03 1.0E-02 1.0E-01 0 0.2 0.4 0.6 0.8 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 0 0.2 0.4 0.6 0.8 mole fra c tions, -time, s + +
Figure 2: Comparison of model predictions (lines) with experimental mole fraction profiles (symbols) of minor
Mole fractions of minor compounds (< 10-2) found in gas phase at 900 °C and 8 kPa are shown as a function of residence time in Figure 2 for propylene pyrolysis. The minor compounds are classified into C3, C4 hydrocarbons (left), benzene substitutes (middle) and compounds with two rings and acenaphthylene (right). Although gaps between the experiments and the computations are still found in some cases, in particular the acenaphthylene profile, generally the mechanism keeps its capability in predicting mole fractions of the minor species with mole fractions ranging from 10-5 to 10-2.
Predictive performances of the present mechanism at varying pressures are demonstrated in Figure 3 for ethylene pyrolysis. Generally, the pressure effects on the consumptions of the source hydrocarbons as well as the formations of the major products are modeled very well. An increase in pressure enhances the consumptions of source hydrcarbons as well as the formations of the products. This simple rule is not valid for the acetylene formations in ethylene pyrolysis. The acetylene mole fraction increases with increasing pressure at short residence time up to 0.2 s whereas it shows maxima at 8 kPa at longer residence times. Acetylene consumption becomes dominant at highest pressure of 15 kPa. This rather complicated experimentally observed trend is correctly traced by the computations.
1.0E-02 1.0E-01 1.0E+00 0.0 0.2 0.4 0.6 0.8 1.0 1.0E-03 1.0E-02 1.0E-01 0.0 0.2 0.4 0.6 0.8 1.0 1.0E-03 1.0E-02 1.0E-01 0.0 0.2 0.4 0.6 0.8 1.0 1.0E-01 1.0E+00 0.0 0.2 0.4 0.6 0.8 1.0 1.0E-04 1.0E-03 1.0E-02 1.0E-01 0.0 0.2 0.4 0.6 0.8 1.0 1.0E-04 1.0E-03 1.0E-02 1.0E-01 0.0 0.2 0.4 0.6 0.8 1.0 2 kPa 4 kPa 8 kPa 15 kPa 2 kPa 4 kPa 8 kPa 15 kPa C2H4 time, s mole f ra c tions, - H2 CH4 C2H2 1,3-C4H6 C6H6
Figure 3: Comparison of model predictions (lines) with experimental mole fraction profiles (symbols) of major
3 COUPLING OF KINETICS AND COMPUTATIONAL FLUID DYBNAMICS
As shown above, the chemistry inside the homogeneous gas phase leads to aliphatic formation, ethylene and acetylene, and, for the aromatic route, benzene-type species and the formation of PAHs, while heterogeneous surface chemistry results in various textured pyrolytic carbons from different gaseous organic compounds in CVD. The very detailed model derived in the first stage of this study can now be used to develop a more global picture of the chemical processes, which leads to a mechanism that still keeps the numerical simulation of transient 2D CVI processes tractable. It should be noted that every additional chemical species would introduce an additional partial differential transport equation to be solved.
Therefore, a reduced multi-step homogeneous reaction model is integrated with a heterogeneous surface reaction model of hydrogen inhibition for pyrocarbon deposition, as shown in Fig.4. All gas-phase kinetic data (k1-k4) are from the multi-step “parallel-consecutive” model [12] and surface reaction kinetic data (k5-k8) are obtained and adjusted via a function of the [H2]/[CxHy] mole ratio by fitting CVD experimental results of Hüttinger et al. [13-14].
Figure 4: The multi-step deposition model with consideration of the hydrogen inhibition.
The principle difference of CVI to CVD is the huge and continuously varying surface area with progressive densification of porous substrates. In the present work, models of carbon felts with randomly distributed, non-overlapping fibers were generated and then the evolution of surface area per volume was calculated numerically.
The CVI reactor used in the present work was described in detail in former papers [15]. Generally, the reactor can be divided into two parts: the free flow subdomain (narrow gap from the inlet to the outlet) and the porous subdomain (the felt). Fig.5 is the meshed
CH4 CH3 C2H4 C2H2 C6H6 C>6Hx k1 -H2 k2 -H2 k5 -H2 k4 -H2 k3 k6 -H2 k7 -H2 k8 -H2 C∞
Homogeneous gas phase reactions
geometrical model of the reactor and the carbon fiber felt. Obviously, mass transfer in free flow subdomain includes diffusion, convection and chemical reaction terms. Surface reactions in this subdomain can be neglected because it has very weak effect on gas-phase composition. Moverover, mass transfer by convection can be neglected inside the felt. Then 2D transient mass transfer equations of CVI are established, as shown in Equ.1, coupled with evolution equations of porosity and bulk density.
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 ( , , , , ) ( , , , , , ) ( , , , , , ) ( , , , , , ) ( U) 1, 2,...5 ( ) 1, 2,...5 v v C v i i i i i c c c c c eff i i i i S c c c c c C C S c c c c c C C S c c c c c c
D c c R i in free flow space
t c D c R i inside felt t M e R inside felt t M R inside t ρ ρ ∂ + ∇ ⋅ − ∇ + = = ∂ ∂ + ∇ ⋅ − ∇ = = ∂ ∂ = − ∂ ∂ = ∂ felt ⎧ ⎪ ⎪ ⎪ ⎪⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪⎩ [1]
Where ci (i=1, 2 … 5) denotes the concentration (mol m-3) of methane, ethylene, acetylene and benzene and hydrogen, respectively. Ri represents the chemical reaction terms (mol m-3 s -1
) in two different subdomains. U defines the velocity profiles of gas species in free flow space. Di and Dieff are the diffusivity (m2 s-1) in the free flow space and the effective diffusivity that depends on the tortuosity factor and equivalent radius of pores. e and ρ (kg m -3
) denote the porosity and bulk density of substrates. MC is the molar mass of carbon, and ρC
(kg m-3) the density of pyrolytic carbon which is a variable corresponding to different textured pyrolytic carbons [1].
4 RESULTS AND DISCUSSION
Figure 5: The meshed 2D symmetrical geometrical model of the reactor and the carbon felt.
Figure 6: The predicted density (g cm-3) distributions corresponding to 20 kPa CH4 (a), 20 kPa CH4 with 4 kPa
H2 (b), and 20 kPa CH4 with 10 kPa H2 (c).
a) b) c)
felt
Free flow space
As shown in Fig.7, simulation results corresponding to inlet flows of pure methane and additional 4 kPa H2 have good agreement with experimental results published by Zhang and Huettinger [6]. In the present work, an average density of pyrocarbon is adopted which can not distinguish various textured pyrocarbons and then leads to higher calculated bulk densities for the case of additional 10 kPa H2. As well known, pyrocarbon textures depend on the [C2Hn]/[C6Hm] ratio and hydrogen has strong inhibition effects on carbon deposition from hydrocarbons, especially from C6- species.
Figure 7: Bulk density distributions after 120h densification
The densification mode (from inside to outside) and pyrocarbon textures are attributed to both [H2]/[CxHy] ratio and [C2Hn]/[C6Hm] ratio. Additional hydrogen can be used to adjust the bulk density distributions and pyrocarbon textures, however too much hydrogen will result in dramatic debasement of the pyrocarbon textures and then leads to very low bulk densities. The measured values of carbon deposition rates in CVD experiments can be adopted for simulation of CVI processes only when the hydrogen inhibition is considered.
5 CONCLUSIONS
Reliable CFD simulations of chemical reactors for materials synthesis frequently call for the implementation of complex chemical reaction schemes. Exemplarily, this contribution presents new approaches for coupling detailed models for the chemical reactions in the gas-phase and on the surfaces for chemical vapor infiltration of light hydrocarbons for the production of carbon/carbon composites. The transient 2D simulation is now used for optimization of the reactor conditions.
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 0 2 4 6 8 10 12 14 16
Distance from the center, m m
B u lk de ns it y g c m -3
ACKNOWLEDGEMENTS
This research was performed in the Sonderforschungsbereich 551 “Carbon from the gas phase: elementary reactions, structures, materials”. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
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