Chemical vapor infiltration of pyrocarbon from methane pyrolysis: … · 2019. 4. 2. · methane pressure/argon pressure at 1:9. The temperature varied from 1,323 to 1,398 K. The

mater.scichina.com link.springer.com Published online 24 December 2018 | https://doi.org/10.1007/s40843-018-9379-7Sci China Mater 2019, 62(6): 840–852

Chemical vapor infiltration of pyrocarbon frommethane pyrolysis: kinetic modeling with textureformationChunxia Hu1, Hejun Li1*, Shouyang Zhang1, Wei Li1 and Ni Li2*

ABSTRACT A complete mechanism of methane pyrolysis isproposed for chemical vapor infiltration of pyrocarbon withdifferent textures, which contains a detailed homogeneousmechanism for gas reactions and a lumped heterogeneousmechanism for pyrocarbon deposition. This model is easilyapplied to simulate gas compositions and pyrocarbon de-position in a vertical hot-wall flow reactor in the temperaturerange of 1,323–1,398 K without any adjusting parameters andpresents better results than previous mechanisms. Resultshave shown that the consumption of methane and the pro-duction of hydrogen are well enhanced due to pyrocarbondeposition. Pyrocarbon deposition prevents the continuouslyincreasing of acetylene composition and leads to the reductionin the mole fraction of benzene at long residence times in thegas phase. The carbon growth with active sites on the surface isthe controlling mechanism of pyrocarbon deposition. C1species is the precursor of pyrocarbon deposition at 1,323 K,and the primary source over the whole temperature range. Astemperature increases, gas phase becomes more mature anddepositions from acetylene, benzene and polyaromatic hy-drocarbons become more prevalent. A general pyrocarbonformation mechanism is derived with the specific precursorsand illustrates that the maturation of gas compositions isbeneficial to forming planar structures with hexagonal ringsor pentagon-heptagon pairs, namely, high textured pyr-ocarbon. The results are in well agreement with experiments.

Keywords: kinetic modeling, pyrolytic carbon, texture, chemicalvapor infiltration, methane

INTRODUCTIONCarbon materials are now experiencing booming devel-opment in various applications [1–3]. Aimed at wellmechanical performance at high temperatures, carbon

composites are developed with carbon or ceramic fibers,an interlayer coating (such as pyrocarbon) on the fibers,and a matrix of carbon or ceramic. The compositescombine the advantages of fibers and matrixes and fulfillthe demand in aerospace industry, for example, carbonfiber reinforced pyrocarbon (C/C) composites used asairplane brakes [4]. New properties have been revealed astime goes by and promote wide applications of thecomposites. The well biocompatibility and susceptibilityunder corrosion atmosphere and favorable mechanicalproperties equip C/C composites as a novel biomaterial[5,6]. Carbon fiber reinforced multilayered pyrocarbon-silica matrix composites show good flexural strength,fracture toughness, electrical conductivity and electro-magnetic interference shielding performance, which arevery promising in eliminating electromagnetic inter-ference for the protection of sensitive devices and en-vironments [7,8].

In order to fabricate the composites, a well-knowntechnology is chemical vapor infiltration (CVI), consist-ing of gas pyrolysis producing various intermediates andsolid matrix deposition on the fiber surfaces. The gaseousprecursors could be hydrocarbons for pyrocarbon de-position or chlorosilanes for silicon deposition. With thetechnology, no damage is made on fibers and the matrixshows fine properties. Meanwhile, a high bondingstrength of fiber/matrix is produced, which is very im-portant for the composite applications. The process is acombination of chemical reactions, gas flow, masstransfer and heat transfer. Thus, with different para-meters, the matrix, for example, pyrocarbon, is formedwith different nanotextures, which are low, medium andhigh textured pyrocarbons distinguished by the orienta-tion angles [9] and produce a significant impact on the

1 State Key Laboratory of Solidification Processing, C/C Composites Research Center, Northwestern Polytechnical University, Xi’an 710072, China2 Department of Mechanical Engineering, California State University, Los Angeles, CA 90032, USA* Corresponding authors (emails: [email protected] (Li H); [email protected] (Li N))

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composite properties [10]. To manufacture compositeswith pyrocarbon of a desired texture, a better under-standing of the pyrocarbon formation mechanism in CVIprocess is in an urgent demand.

Hüttinger et al. [11,12] investigated the influence ofCVI conditions (temperatures, pressures, surface area/volume ratios) on pyrocarbon deposition from methanepyrolysis by experiments, resulting a pure high-texturedcarbon matrix at 1,095°C and a lower textured carbon atlower temperatures. The formation mechanism for pyro-carbon in different textures was discussed based on thearomatic growth [13]. The qualitative analysis shows clearinsight of the forceful influence on pyrocarbon depositionand formation by CVI parameters, while little attentionwas paid to the nature of methane pyrolysis. Later, detailsof hydrocarbon pyrolysis in the gas phase during CVIwere studied. Norinaga et al. [14,15] analyzed the gasphase compounds from ethylene, acetylene and propylenepyrolysis in CVI of pyrocarbon with a honeycombmonolith as the preform in a vertical flow reactor. Adetailed kinetic model with 227 species and 827 ele-mentary reactions was developed. Nevertheless, althoughthe mole fractions of gas intermediates were well re-produced by the mechanism, the model actually ignoredthe pyrocarbon deposition on the preform. Devin-Ziegleret al. [16–18] studied the pyrolysis of propane for CVI ofpyrocarbon by experiments and simulations in a jetstirred reactor. The special reactor, although not widelyused in composite preparations in industry, has the ad-vantage of providing the uniform gas composition in thewhole volume. A detailed kinetic mechanism of 608 re-actions with 172 species was proposed and able to re-produce the molar fractions of major gas products up topyrene (C16H10). Lumped heterogeneous reactions forpyrocarbon deposition were calculated based on the ex-perimental species concentrations and pyrocarbon de-position rates, declaring that smooth laminar pyrocarbonwas quantitatively formed with small unsaturated species(ethylene and acetylene). Regretfully, no further discus-sion was presented regarding the formation mechanismof pyrocarbon in different textures.

The aim of this work is to propose a complete me-chanism of methane pyrolysis for pyrocarbon formationwith different textures in CVI process. We choose me-thane as the gas precursor in this work, for the reason thatit is the ideal hydrocarbon for various reasons [19] andquite easy to be acquired with low cost in industry. In thesecond section of this paper, experimental methods toobtain gas compositions and deposition rates of pyro-carbon in CVI were listed in general. In the third section,

the kinetic model for methane pyrolysis with pyrocarbondeposition during CVI was described. Comparisons be-tween the experimental and simulation results of molefractions of gas intermediates and pyrocarbon depositionrates were shown in the fourth section. And the pre-cursors to deposit pyrocarbon in methane pyrolysis wereclarified. In the last section, the formation mechanism ofpyrocarbon with different textures was analyzed based onthe resulting pyrocarbon precursors.

EXPERIMENTAL METHODStudies of methane pyrolysis and pyrocarbon depositionhave begun previously and the detailed description ofexperimental setups has been presented [20–22]. Hereonly the essential features are recalled. The pyrolysis wascarried out in a vertical hot-wall flow reactor with analumina-ceramic-tube deposition area of 22 mm in dia-meter and 20 mm in length. The deposition area is con-nected with the gas inlet by a narrow tube of 8 mm indiameter and a conical nozzle of 60° in angle to avoid thepre-decomposition of methane before arriving at thedeposition area and to generate a plug-flow gas flow[22,23]. Methane pyrolysis in an empty deposition area isconsidered as pure gas-phase reactions, since the pyro-carbon deposition could be neglected considering a smallarea/volume ratio of 0.28 mm−1. For studying pyrocarbondeposition, a honeycomb structure made of cordierite islocated in the deposition area to improve the area/volumeratio. The honeycomb structure has 62 square shapedchannels per cm2. The cross-section of each channel isaround 1.1 mm2 and the wall thickness amounts to0.18 mm. Pre-deposition of the honeycomb structure wasdone for nearly 20 h in order to fill the pore entrance onthe surface and to get rid of its porous structure [21]. Anydiffusion influence on pyrocarbon deposition and for-mation is excluded as the porous structure in this pre-form is simple long straight pores. Fig. 1 presents theschematic of the reactor and the cross-section of a pre-form.

The experiments were performed at a residence time of1 s and a total pressure of 100 kPa with an initial ratio ofmethane pressure/argon pressure at 1:9. The temperaturevaried from 1,323 to 1,398 K. The deposition rate wasderived from the mass increase of the honeycombstructure related with the surface area. A Poropak-Rcolumn and a CP Sil 5 CB column were used to separatethe gaseous and liquid products in methane pyrolysis.The gaseous products were analyzed with a Siemens Si-chromat 1 gas chromatograph equipped with a gas split, aTCD- and a FID-detector. The liquid products were

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dissolved in acetone and analyzed with a Siemens Si-chromat 1–4 gas chromatograph equipped with a FID-detector [22,23].

MODELING OF PYROCARBONDEPOSITION

Homogeneous reactionsMethane pyrolysis produces various hydrocarbons andradicals, ranging from light species, such as hydrogen ormethyl, to polyaromatic species, such as pyrene (C16H10).In order to reproduce their experimental concentrations,a detailed mechanism with gas elementary reactions hadbeen developed by Hu et al. [24]. The mechanism iscomposed of 909 reversible elementary reactions and 241radicals and molecules. The reactions, including homo-lytic dissociation reactions, radical recombination reac-tions, radical/molecular disproportionation reactions, β-scission reactions, radical addition reactions, hydrogenabstraction reactions and isomerization reactions, areprimarily collected from various literatures. The primaryreaction mechanism of C1−C4 species in methane pyr-olysis is derived from the reaction scheme obtained byBecker et al. [20,23,25]. Reactions of C3−C4 species inHidaka’s mechanism [26] and Tsang’s mechanism [27]are added additionally. C5−C9 reactions are those pro-posed by Marinov et al. [28] related to the combustionmechanism of methane and ethane premixed flames.Reactions with large polyaromatic hydrocarbons are col-lected from mechanisms proposed by Richter et al. [29] orNorinaga et al. [30]. The kinetic parameters of reactionsand thermodynamic parameters of species are inheritedfrom the related references. Pressure effects on pressure-dependent reactions involving small species are describedby Troё parameters [31]. The mechanism has been ver-ified to be able to simulate the species concentrationswhen pyrocarbon deposition can be ignored [24].

Heterogeneous reactionsPyrocarbon is assumed to be deposited by heterogeneousreactions between species and solid surface, as shown inthe following:

m nC H C(B)+2H ,m nC(S)

2 (I)

where C(S) is the active site on the surface and C(B) is thecarbon atom newly-grown on the surface. No reversiblereactions are mentioned, assuming that once pyrocarbonis deposited, it will not react to produce gas species again.Here three types of deposition reactions are considered:(1) Reactions with CHx(x≤4) formed by methane dehy-drogenation. Simulation studies [32] based on densityfunctional theory have shown that the highest energybarrier for methane dehydrogenation is close to the ac-tivation energy of carbon deposition with methane pyr-olysis and experimental results [20,22,33] have shownthat the deposition rate at zero residence time is largerthan zero, both indicating that pyrocarbon could be de-posited quickly with methane or its dehydrogenationproducts, i.e., CHx(x≤4). CH3 has been successfully con-sidered as a main reactant in deposition simulations withelementary surface reactions [34–36]. Meanwhile, theconcentration of methane with or without pyrocarbondeposition is quite different, which will be compared inquantity in the next section.

(2) Reactions with small species formed in methanepyrolysis, for example, C2 species. These species are withthe second largest concentrations in the gas phase.Ethylene with a double bond and acetylene with a triplebond are highly active and have similar bonds with thosein pyrocarbon, which seems to be involved in reactions ofpyrocarbon deposition. Previous research has underlinedthat acetylene addition is an important way to producepyrocarbon in ethylene, acetylene and propane pyrolysis[18,23].

(3) Reactions with large species formed in methanepyrolysis, such as C6H6. This group of species is inter-esting for their comparable structures with that of pyro-carbon, although their concentrations are much lowerthan those of small species. Besides, benzene has beenconsidered as the significant “particle” in the popular“particle filler theory” [13] for pyrocarbon deposition.

The kinetic parameters of heterogeneous reactions arecalculated from the experimental deposition rates ofpyrocarbon. For Reaction (I), the deposition rate(mol m−2 h−1) is expressed as

r mk= [C(S)][C H ], (1)m n

where k is the rate constant of bimolecular reactions

Figure 1 Schematic of the reactor and the cross-section of a preform.



(m3 mol−1 h−1), [CmHn] is the species concentration(mol m−3) without pyrocarbon deposition and [C(S)] isthe active site density on the surface, expressed inmol m−2. Suppose that the rate constant follows theArrhenius law in the temperature range studied (1,323–1,398 K). The activation energies and Arrhenius factors ofdeposition reactions are estimated by plotting the loga-rithms of the reaction rate constants vs. the inverse of thetemperatures. Aimed at getting the reaction rate constant,the experimental deposition rate is used as an input inEquation (1). [CmHn] is calculated by homogeneous re-actions, indicating the species concentration before pyro-carbon deposition. Since [C(S)] is usually considered tobe constant [34–36] and multiplying the reaction rateconstant by a constant has no influence on its trend,[C(S)] is combined with the reaction rate constant toavoid the complexity and inaccuracy of describing thepyrocarbon surface, leading to

r mk= [C H ] (2)m nwith kʹ=k[C(S)] named as the “apparent” reaction rateconstant. Henceforth the reaction rate constants of het-erogeneous reactions are replaced by the “apparent” re-action rate constants unless otherwise specified.

With this simplification, the heterogeneous reactionscan be treated as “apparent” homogeneous gas reactionsand added into the homogeneous reaction mechanismwith their Arrhenius parameters. As the pyrocarbon de-position rate is quite small, the appearance of “gaseous”pyrocarbon will have little influence on the total pressurein the reactor. The “apparent” homogeneous reactionmechanism is used in the same way as the purelyhomogeneous reaction mechanism. Species concentra-tions and pyrocarbon deposited are given in mole frac-tions of the total “gas” phase. The concentration of majorspecies and the deposition rate can be calculated as

X PR T[C H ] =

×× , (3)m n

C Hm n

rX P VR T S=

× ×× × × , (4)

C(B)

where XC Hm n and XC(B) are simulated mole fraction ofspecies CmHn and pyrocarbon C(B), P is the total pres-sure, V is the reactor volume, R is the gas constant, T isthe reacting temperature, S is the surface area of the re-actor, and τ is the residence time.

Reactor flow modelAccording to the experimental setup, the deposition areain the reactor is tubular and the gas flow is a plug flow.

The plug flow in a tubular area gets rid of the transversediffusion in the axial direction and assumes the perfectmixing in the radical direction, which can be easily de-scribed by one-dimensional equations. Hence, under theexperimental conditions, the equations for this “one-dimensional” chemical reacting flow are the first-orderordinary differential equations (ODEs) with the axialcoordinate variable, satisfying:the mass continuity equation

u Ax Aux uA x A s M

dd +

dd +

dd = ; (5)k

K

k kL=1

g

the species conservation equation

uA Yx Y A s M

M A s A

dd +

= ( + ); (6)

kk

k

K

k k

k k k

L=1

L

g

the energy equation

uA hYx

C Tx uux

h Y u A s M

A Q A s M h

dd

+ dd +dd

+ + 12

= ; (7)

k

K

kk

p

k

K

k kk

K

k k

k K

K

k k k

=1

=1

2L

=1

L e L= b

f

b

g

g g

the momentum equation

A Px uAux

Fx uA s M

dd +

dd +

dd + = 0. (8)k

K

k kL=1

g

In the above equations, x represents the axial co-ordinate, ρ is the mass density, u is the axial velocity, Kg isthe number of gas species, AL is the surface area per unitlength, A is the cross-section area of the reactor, sk is themolar production rate of species k by all surface reactions,Mk is the molecular weight of species k, k is the molarproduction rate by gas reactions, Yk is the mass fraction ofspecies k, hk is the specific enthalpy of species k, Cp is themean heat capacity per unit mass of gases, T is the ab-solute gas temperature, Qe is the heat flux from the sur-rounding to the tube wall, Kb is the number of surfacespecies, P is the absolute pressure, F is the drag forceexerted on the gases by the tube wall, which is expressedin terms of a friction factor f:

Fx A u f

dd =

12 . (9)L

2

Since constant temperature and pressure are used in thecalculations to simulate the isothermal isobaric experi-



mental condition, the energy equation is provided onlyfor the sake of completeness and not solved in the si-mulations. Furthermore, all terms containing parametersrelated with surface species vanish for the surface reac-tions being treated as the “apparent” homogeneous gasreactions. The direct-solution method in DASPK [37] isemployed to solve the resulting system of ODEs, whichadopts backwards-differencing methods for the integra-tion of different species.

COMPARISON BETWEEN SIMULATIONSAND EXPERIMENTSIn the following part, we will discuss pyrocarbon de-posited from the dehydrogenation products CHx(x≤4), thesmall species acetylene and the large species benzene. Si-mulation results of gas mole fractions and pyrocarbondeposition rates are compared with the experimental datapublished by Hüttinger et al. [22,33]. The conclusion ofthese analyses will lead to consideration of a combineddeposition mechanism with various species, which deliversbetter simulation results than previous heterogeneousmechanisms by Tang [36], Lacroix [35] and Li [38].

CHx(x≤4) depositionThe pyrocarbon deposition with CHx(x≤4) and its kineticparameters calculated based on the Arrhenius law are asfollowing:

xCH C(B) + 2H ,x x( 4C(S)

2

A=1.98×108, E=226.89 kJ mol−1. (II)The plot for parameter determinations is illustrated in

Fig. S1a in Supplementary information. The logarithmicrate constant of CHx(x≤4) deposition has a good linearrelationship with the inverse of temperature, with anadjusted R2 (adj. R2) of 0.997. Fig. 2 depicts the pyr-ocarbon deposition rates at the residence time of 1 smeasured in experiments (symbols) and simulations(lines). The comparison shows that CHx(x≤4) can quanti-tatively be the precursor of pyrocarbon only at 1,323 K.When temperature increases, the experimental datashows a clear exponential increase while the simulationdeposition rate by CHx(x≤4) increases linearly. This gives asign that more species are able to deposit on the surface athigh temperatures. Fig. 3 shows comparisons of gas phasemole fractions between experimental (symbols) and si-mulation results (lines). The dash lines and solid lines arethe simulation results without or with pyrocarbon de-position, respectively. The low concentrations of acet-ylene and benzene in the gas phase at 1,323 K, as well asthe small diffusion coefficients [19], limit the reactions

between species and deposition surface, illustrating thequite well consistency between the CHx(x≤4) depositionrate and the experimental result. With the increasingtemperature, mole fractions of methane decrease, whilethose of the other main species show opposite trends,indicating the increasing maturation of gas species andthe enhanced possibility of more gas species to be in-volved in the pyrocarbon deposition. The considerationof pyrocarbon deposited from CHx(x≤4) makes a moreconsistent modeling of gas concentrations. An obviousadditional consumption of methane and a clearly in-creasing production of hydrogen are observed with thereaction of pyrocarbon deposition. The underestimationof methane mole fraction at low temperatures and longresidence times is due to the inaccuracy of methane gasphase reaction rates, as mole fractions of methane si-mulated with homogeneous reactions are similar withthose measured in experiments with pyrocarbon deposi-tion. Slight decreases in mole fractions of C2 species oc-cur, especially at long residence times. A better agreementis displayed for benzene mole fractions when pyrocarbondeposition is considered, except at 1,323 K. This has beennoticed even when no pyrocarbon deposition is involvedin the simulation, illustrating that the deviation is in-hibited from the inaccuracy of pure homogeneous reac-tions at 1,323 K.

Acetylene depositionThe reaction of acetylene deposition and related kineticparameters are listed below:

C H 2C(B) + H ,2 2C(S)

2

A=3.82×103, E=96.26 kJ mol−1. (III)

Figure 2 Comparison of pyrocarbon deposition rates at 1 s betweenexperimental data and simulation results from CHx(x≤4) deposition atdifferent temperatures. Symbols represent experimental results, whilethe line is simulation results.



The plot of parameter determinations is presented inFig. S1b (red solid line) in Supplementary information.The adj. R2 of the fitting line is 0.827 because lnk at1,323 K is higher than the expected value according tovariations of the other points. Fig. 4 depicts the com-parison of pyrocarbon deposition rates at 1 s betweensimulations and experiments. Acetylene deposition ratesfrom simulations show a slight exponential increase withthe increase of temperature, while the values are muchsmaller than the experimental results within the tem-perature range considered. We could expect that acet-ylene may be one of the precursors for pyrocarbondeposition. Fig. S2 presents the mole fractions of majorgases computed with homogenous and heterogeneousreactions (solid lines) and with pure gas-phase mechan-isms (dash lines), and the counterparts from experiments(symbols).The involvement of pyrocarbon deposition byacetylene in the simulation mechanism leads to reducedmole fractions of C2 species and C6 species, which aremore comparable to experimental results (shown inFig. S2). Calculated mole fractions of C2H2 show similarvariation trends with the experimental results, althoughthe absolute values are larger than those from experi-ments. The hydrogen mole fractions at different tem-peratures are not influenced by the involvement of theheterogeneous reaction, because of the low C/H ratio inacetylene. A slight increase of methane mole fraction is

shown in pyrocarbon deposition process, indicating thatthe presence of pyrocarbon deposition has influenced thereaction pathways, for example, some hindered routes ofmethane consumption under this situation.

Except acetylene, ethylene and ethane are also smallspecies, which show large mole fractions in methanepyrolysis for pyrocarbon deposition and may have a greatinfluence on pyrocarbon deposition. Similar analyseshave been done with all C2 species by taking the de-

Figure 3 Mole fraction variations of gas species at different residence times and different temperatures. Symbols: experimental results; dash lines:simulation results with pure homogeneous reactions and solid lines: simulation results with CHx(x≤4) deposition reaction.

Figure 4 Comparison of pyrocarbon deposition rates at 1 s betweenexperimental data and simulation results from acetylene deposition atdifferent temperatures. Symbols represent experimental results, whilethe line is simulation results.



position rate as

r k k

k

= 2 [C H ] + 2 [C H ]

+2 [C H ]. (10)C H 2 2 C H 2 4

C H 2 6

2 2 2 4

2 6

However, the simulation results (deposition rates andmole fractions of gas species) are almost the same withthose from only acetylene deposition in the mechanism.This is expected as acetylene has the largest mole fractionamong the three species and is the most active based onthe reactivity of C2 species with aromatic hydrocarbons[39]. Therefore, for the simple and easy application, onlyacetylene deposition is presented and discussed here.

Benzene depositionThe reaction of benzene deposition and its kinetic para-meters are:

C H 6C(B) + 3H ,6 6C(S)

2

A=3.24×104, E=111.15 kJ mol−1. (IV)The determination plot of these parameters is presented

in Fig. S1c (red solid line) in Supplementary information.It should be mentioned that although a line is fitted, itsadj. R2 is negative (−0.44) because of an “abnormal” valueof lnk at 1,323 K. Considering the inaccuracy of benzenemole fraction at 1,323 K obtained with the homogeneoussimulation, the experimental data has been used to re-place the simulated mole fraction and no improvement isobserved, implying that benzene deposition is not fol-lowing Arrhenius law in the whole temperature range of1,323–1,398 K. Yet, the simulations with Reaction (IV)have shown something new of reproducing experimentalphenomena. Fig. 5 gives the comparison between theexperimental and simulated deposition rates of pyro-carbon at 1 s from benzene at different temperatures. Thesimulated results present a more clearly exponential in-crease than those from methane or acetylene deposition,giving a positive suggestion for the involvement of ben-zene deposition at high temperatures. The smaller de-position rates by benzene at all temperatures show thatthere are other species that can be precursors for carbondeposition. The simulated deposition rate at 1,323 K isquite large and breaks the decreasing trend with the de-creasing temperature, which is expected as a result of thenegative adj. R2.

The benzene mole fractions with benzene depositionare presented in Fig. 6. Symbols are the experimental datawhile solid and dash lines are simulation results with orwithout the heterogeneous reaction. Mole fractions ofother main species are left out, as there is no differencebetween the simulation results with and without benzene

deposition in the simulation mechanism. This is reason-able, since benzene is a minor product and is producedafter some pre-reaction time (0.1–0.4 s depending on thereaction temperature). The decrease of the benzene molefraction at long residence times is simulated for the firsttime, confirming that benzene is one of the precursors forpyrocarbon deposition.

Only one aromatic hydrocarbon, benzene, is con-sidered, whereas several kinds of polyaromatic hydro-carbons are produced in methane pyrolysis. The otherpolyaromatic hydrocarbons for pyrocarbon depositionare left out for the mole fraction of each polyaromatichydrocarbon being quite small, varying from 10−5 to 10−18

in the homogeneous simulations. The quantitative ex-perimental data of each polyaromatic hydrocarbon is stillbeing tested, since these small values vary significantly in

Figure 5 Comparison of pyrocarbon deposition rates at 1 s betweenexperimental data and simulation results from benzene deposition atdifferent temperatures. Symbols represent experimental results, whilethe line is simulation results.

Figure 6 Mole fraction variations of benzene at different residencetimes and different temperatures. Symbols: experimental results; dashlines: simulation results with pure homogeneous reactions and solidlines: simulation results with benzene deposition reaction.



several analyses at the same experimental situation withan uncertainty of ±32% [15]. As an even stronger ex-ponential increase in pyrocarbon deposition rates is ob-served from the experiments at high temperatures,polyaromatic hydrocarbons are likely to be involved inpyrocarbon deposition at high temperatures. Furtherdiscussion about pyrocarbon deposition by polyaromatichydrocarbons will be given in the following section.

Combination depositionAs discussed above, we can conclude that at the lowtemperature (1,323 K), pyrocarbon deposition is fromCHx(x≤4) and as temperature increases (1,348–1,398 K), acombination of CHx(x≤4), acetylene and aromatic hydro-carbons produces pyrocarbon. Based on this rule, theArrhenius parameters of acetylene and benzene deposi-tion are recalculated (as shown in Fig. S1b and c (dashblue line) in Supplementary information):

C H 2C(B) + H ,2 2C(S)

2

A=1.69×105, E=139.80 kJ mol−1; (III’)

C H 6C(B) + 3H ,6 6C(S)

2

A=1.33×107, E=180.21 kJ mol−1. (IV’)

The adj. R2 has been improved to be over 0.9 for bothreactions without the data at 1,323 K. Polyaromatic hy-drocarbons with more than six carbon atoms are taken asa whole in pyrocarbon deposition to avoid the inaccuracyintroduced by several small values. The kinetic para-meters for the deposition from polyaromatic hydro-carbons are analogized from those of benzene deposition,because of the similar structures among aromatic hy-drocarbons. According to the results of homogeneoussimulations, mole fractions of polyaromatic hydrocarbonsare normalized to C6 and the total value is even smallerthan that of benzene at 1,323 and 1,348 K and in the sameorder at 1,373 and 1,398 K. Therefore, pyrocarbon de-positions from polyaromatic hydrocarbons are supposedto happen at >1,348 K with a mature gas phase.

Based on the above heterogeneous reactions, pyro-carbon depositions are analyzed along with the homo-geneous reactions. The simulated pyrocarbon depositionrates at 1 s from each separate species are presented incolumns with different fill patterns in Fig. 7. Comparedwith experimental data, the pyrocarbon deposition ratesare reproduced very well with the largest deviation of 12%at 1,398 K. This deviation indicates the improved im-portance of polyaromatic hydrocarbons in pyrocarbondeposition. With the increase of the temperature, gascompositions become more mature and pyrocarbon de-

posited from aromatic hydrocarbons increases ex-ponentially and becomes the dominated part. Moreaccuracy information about large polyaromatic hydro-carbons is required for a better agreement, which will berevealed in the future work. The present simulation de-position rates have also been compared with simulationresults from heterogeneous mechanisms with elementaryreactions by Tang [36] and Lacroix [35], as shown inFig. 7. The mechanism from Tang generates a clear ex-ponential increase of deposition rates with the increase oftemperature, although it underestimates the values overthe whole temperature range. The mechanism fromLacroix predicts the deposition rates quite well at lowtemperatures while gives lower values at high tempera-tures. These indicate that the hypothesis of elementaryreactions still needs improvements and more polyaro-matic hydrocarbons should be involved in pyrocarbondeposition at high temperatures. What should be men-tioned is that simulations with elementary surface reac-tions include much more variables to be solved than thepresent one and are usually hard to converge. Simulationsof deposition rates with lump heterogeneous reactionsfrom Li [38] have been done as well, leading to quitesmall values (

the present mechanism and compared with the experi-mental data and simulation results of previous mechan-isms, as shown in Fig. 8. Symbols represent theexperimental data, while solid lines and dash dot lines arethe present and previous simulation results with pyro-carbon deposition. The mole fractions of main gases fromthe previous heterogeneous mechanisms have no appar-ent difference with the simulation results based on thehomogeneous mechanism and deviate from the experi-mental results. Good agreements are shown between theexperimental and present simulation results. No over-consumption of the precursors for pyrocarbon depositionis observed when all the heterogeneous reactions are in-volved without adjusting the kinetic parameters. Themole fraction of acetylene is almost constant at long re-sidence times for each temperature. The reductions ofbenzene at long residence times and high temperaturesare correctly reproduced by present simulations. Theunderestimation of methane and benzene mole fractionsat low temperatures is due to the inaccurate kineticparameters of homogeneous reactions in methane pyr-olysis, which has been analyzed previously in the singlespecies deposition.

GLOBAL MODEL FOR PYROCARBONFORMATION WITH DIFFERENTTEXTURESIn the previous section, precursors for pyrocarbon de-

position have been identified and discussed. With theincrease of the temperature, more gas intermediates areproduced in methane pyrolysis and the precursors forpyrocarbon deposition vary from a single reactant speciesto a combination of small species and aromatic hydro-carbons. The mechanism of pyrocarbon deposition isfirstly discussed based on these precursors. It should benoted that deposition mechanism and formation me-chanism of pyrocarbon are distinguished from eachother, although in most discussions they are used equally.Pyrocarbon deposition mechanism only focuses on sur-face reactions between the gas and the surface, whichresults more carbon atoms on the surface or to be con-sidered as part of the solid [40]. On the other hand,pyrocarbon formation mechanism is the chemistry ofcarbon formation, describing the formation of differenttextures [13].

According to the present knowledge, the deposition ofpyrocarbon has two mechanisms: a growth mechanismand a nucleation mechanism [40]. The growth mechan-ism expresses that carbon grows by the chemisorption ofgas intermediates at the active sites (mainly at the edge ofexisting layers). This implies that an active site is essentialfor pyrocarbon deposition. Yet, no size limitation for thegas intermediates is set, which signifies that either smalllinear species or aromatic species can be involved. In thisstudy, the gases are CHx(x≤4), acetylene, benzene andpolyaromatic hydrocarbons. The nucleation mechanism

Figure 8 Mole fraction variations of gas species at different residence times and different temperatures. Symbols: experimental results; dash dot lines:simulation results with previous heterogeneous elementary reactions and solid lines: simulation results with present “apparent” homogeneous reactions.



is the physisorption of large polyaromatic hydrocarbonson the surface, developing new carbon layers. This me-chanism is favored when large polyaromatic hydro-carbons are in large mole fractions and no active sites areavailable. The mechanism has been ignored when estab-lishing the heterogeneous reactions of pyrocarbon de-position in the previous section. According to thedeposition rates in this work, the mechanism should havehappened at 1,398 K, evidenced from the deviation be-tween the simulation result and the experimental data. Asimilar result had been concluded in previous experi-ments [12]. As no accurate data of each large polyaro-matic hydrocarbons are presented in this work andphysisorption seems to be less important in the carbondeposition, the nucleation mechanism will not be dis-cussed further.

In the growth mechanism, active sites on the surfaceedges are mainly arm-chair sites and zig-zag sites [13].The growth of pyrocarbon layers should be relevant to themechanism of aromatic growth in the gas phase, despitethe different transition state and much stronger effects ofsteric factors. Dominant surface reactions are hydrogenabstraction-carbon addition, followed by dehydrocycli-zation. Fig. 9 depicts schematics of pyrocarbon depositionin atoms with carbon addition from C1 (CHx(x≤4)), C2(acetylene), C6 (aromatic hydrocarbon) and their com-binations. Aromatic hydrocarbon depositions in atomsare represented by that of benzene according to their si-milar structures for the sake of conciseness. For zig-zagsites, three additional carbons are needed to form a newhexagonal ring, which can be C1 (Fig. 9b) or a combi-nation of C1 and C2 (Fig. 9d). The addition of C6 on zig-zag sites produces a biphenyl type bond first and thendehydrocyclizes to form a pentagon ring (Fig. 9e). Pen-tagon rings can also be formed by additions of C1(Fig. 9a) or C2 (Fig. 9c). Small molecule depositions be-side biphenyl type bonds within C6 deposition lead topentagon rings (Fig. 9h), hexagonal rings (Fig. 9f) orheptagon rings (Fig. 9g) by dehydrocyclization or iso-merization. For arm-chair sites, a new hexagonal ring can

be produced by a single kind of species, as shown inFig. 9i, k or m. The arm-chair site could close rapidly withan addition of C1, forming a pentagon ring (Fig. 9j).Combinations of different species condense on arm-chairsites forming new structures, for example, a pentagonring by C1 and C6 deposition (Fig. 9o), a hexagonal ringby C2 and C6 deposition (Fig. 9n), or a heptagon ring anda heptagon ring by C1 and C6 deposition (Fig. 9p). Notethat the above discussion with Fig. 9 only gives a lumpconsideration of pyrocarbon deposition with differentspecies and does not mean the real deposition process,which is certainly much more complicated.

Repeating the pyrocarbon deposition in atoms, carbonlayers are formed. Fig. 10 shows schematics of pyro-carbon formation in layers. The hexagonal rings form aperfect structure with a highly oriented planar layer(Fig. 10a), while the individual formations of pentagon orheptagon rings bring defects in the carbon layers anddestroy the planar structure, as shown in Fig. 10b and c.

Figure 9 Schematics of pyrocarbon deposition in atoms with carbon addition from C1 (CHx(x≤4)), C2 (acetylene), C6 (aromatic hydrocarbon) and theircombinations. (a–h) new atoms on zig-zag sites and (i–p) new atoms on arm-chair sites.

Figure 10 Schematics of pyrocarbon formation in layers. (a) a highly-oriented layer with hexagonal rings, (b) a disordered layer with pentagonrings, (c) a disordered layer with heptagon rings, and (d) a highly-oriented layer with pentagon-heptagon pairs.



The pair formations of pentagon and heptagon rings alsolead to a highly-oriented carbon layer (Fig. 10d), which hasbeen observed in regenerative laminar pyrocarbon withhigh texture [41,42]. Based on the model, several conclu-sions are obtained with respect to the pyrocarbon forma-tion with different textures. At 1,323 K, very limited waysof pyrocarbon deposition of pentagon and hexagonal ringsare available as C1 is the only precursor, leading to a lowdeposition rate and a low textured pyrocarbon. When thetemperature increases, the maturation of gases increasesand acetylene, benzene and polyaromatic hydrocarbonsjoin the pyrocarbon deposition process. The texture ofdeposited pyrocarbon increases with the increasing degreeof gas maturation. Hexagonal rings can be produced byadding one species or a combination of several species anddisorders from pentagon rings or heptagon rings could beeliminated by making a pair of them, showing that highlyoriented planar layers with larger deposition rates are ea-sier to be formed. These are supported by the experimentalresults. An increasing of orientation angles in pyrocarbonhas been observed with the increase of the temperature[12]. Extinction angle tests have shown that pyrocarbonformed at 1,343 K is in medium texture while that at1,368 K is in high texture [11].

CONCLUSIONA new model of methane pyrolysis for CVI of pyrocarbonis presented in this paper, which contains a detailedhomogeneous reaction mechanism describing the gasphase reactions with elementary reactions and a lumpedheterogeneous reaction mechanism with lumped surfacereactions for pyrocarbon deposition. Mole fractions ofmain gas intermediates and deposition rates of pyro-carbon in the temperature range of 1,323–1,398 K with aflow reactor are reproduced well without any adjustingparameters by the model. The precursors for pyrocarbondeposition at different temperatures alter: C1 species at1,323 K, a combination of C1, acetylene and benzene at1,348 K and extra contributions from large polyaromatichydrocarbons at 1,373 and 1,398 K. C1 species is theprinciple source of pyrocarbon deposition in all tem-peratures, which has been proved by its large con-centration in the gas phase and large kinetic parametersof the deposition reaction. As a result of pyrocarbondeposition, mole fractions of consumed methane andproduced hydrogen increase noticeably. Acetylene de-position prevents the boost of its mole fractions at longresidence times. Pyrocarbon deposited from benzene andlarge polyaromatic hydrocarbons shows more propor-tions at high temperatures, illustrating the decreasing

mole fractions at long residence times in the gas phase.The growth mechanism, namely, carbon additions at theactive sites, is the main mechanism for pyrocarbon de-position. Based on the precursors of pyrocarbon deposi-tion, formation mechanisms of pyrocarbon with differenttextures reveal that pyrocarbon with a high texture ismore likely to be formed by hexagonal rings and penta-gon-heptagon pairs in a mature gas phase. Otherwise,lower textured pyrocarbon is formed with defects inducedby pentagon rings or heptagon rings. The results havebeen confirmed by experiments.

There remains complementary work to strengthen theaccurate quantity and general applicability of the con-clusions in this work. Gas concentrations of large poly-aromatic hydrocarbons in methane pyrolysis are essentialto improve the prediction of pyrocarbon deposition ratesat high temperatures. Information of the deposition sur-face would help identify the density of active sites inreality. Encouraging results from experiments, for ex-ample, atomic resolution imaging by high resolutiontransmission electron microscopy [43], are expected toreveal the heterogeneous steps in the real process ofpyrocarbon formation.

Received 1 October 2018; accepted 29 November 2018;published online 24 December 2018

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Acknowledgements This work was supported by the National NaturalScience Foundation of China (51521061 and 51472203), the “111”Project (B08040), and the Research Fund of State Key Laboratory ofSolidification Processing (NWPU), China (142-TZ-2016).

Author contributions Hu C and Li H designed the models; Hu Cperformed the modeling; Hu C, Zhang S and Li W contributed to thedata analysis. Hu C and Li N wrote the paper. All authors contributed tothe general discussion.

Conflict of interest The authors declare that they have no conflict ofinterest.

Supplementary information Supporting data are available in theonline version of the paper.



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Chunxia Hu is currently a PhD student at the School of Materials Science and Engineering, Northwestern PolytechnicalUniversity. Her research focuses on the manufacturing and properties of advanced carbon/carbon composites.

Hejun Li is a professor at the School of Materials Science and Engineering, Northwestern Polytechnical University. Hereceived his PhD degree from Harbin Institute of Technology in 1991. His current research interests include advancedcarbon/carbon composites, anti-oxidation coatings, paper based friction materials and nanomaterials.

Ni Li is now working in the College of Engineering, Computer Science, and Technology, Department of MechanicalEngineering, California State University, Los Angeles, USA. She received her PhD degree from the University of CentralFlorida, USA, in 2013. Her current research interests include dynamics and kinematics; optimization; control; instrument;and sensor design.

甲烷化学气相沉积不同织构热解碳的动力学分析胡春霞1, 李贺军1*, 张守阳1, 李伟1, 李霓2*

摘要本文建立了一种甲烷化学气相沉积不同织构热解碳的动力学模型. 该模型包括甲烷气相热解的详细基元反应及热解碳沉积的集总表面反应, 模拟了1323–1398 K立式热壁反应炉中热解碳沉积过程中不同气体的组分浓度及热解碳的沉积速率, 且模拟结果优于已有报道的模型、与实验结果吻合良好. 结果表明, 热解碳的主要沉积机理为碳源气体在表面活性位的生长机理, 其中C1物质为主要碳源气体. 随着温度升高, 甲烷热解混合气体趋于成熟, 乙炔、苯和多环芳香烃逐渐成为重要的碳源气体. 基于以上碳源气体的热解碳形成机理分析指出热解混合气体的成熟有利于形成由六元环及五元环-七元环组合而成的高定向平面结构, 即高织构热解碳, 与实验结论吻合良好.



Chemical vapor infiltration of pyrocarbon from methane pyrolysis: kinetic modeling with texture formation INTRODUCTIONEXPERIMENTAL METHODMODELING OF PYROCARBON DEPOSITIONHomogeneous reactionsHeterogeneous reactionsReactor flow model

COMPARISON BETWEEN SIMULATIONS AND EXPERIMENTSCH x(x≤4) depositionAcetylene depositionBenzene depositionCombination deposition

GLOBAL MODEL FOR PYROCARBON FORMATION WITH DIFFERENT TEXTURESCONCLUSION

Documents

Chemical vapor infiltration of pyrocarbon from methane pyrolysis: … · 2019. 4. 2. · methane pressure/argon pressure at 1:9. The temperature varied from 1,323 to 1,398 K. The