Modelling of chemical vapour deposition of carbon based on detailed surface chemistry

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1 Modelling of chemicl vpour deposition of cron sed on detiled surfce chemistry Aijun Li *, Koyo Noring, Günter Schoch, Sven Lichtenerg, Olf Deutschmnn Institut für Technische Chemie und Polymerchemie, Universität Krlsruhe(T), D-73 Krlsruhe, Germny Center for Advnced Reserch of Energy Conversion Mterils, oido University, 88 Spporo, Jpn * Corresponding uthor. E-mil: li@ict.uni-rlsruhe.de Astrct. Modelling of pyrolytic cron deposition from ethylene s function of pressure ws performed t vrious tempertures nd two different surfce re/volume rtios. A surfce rection mechnism ws proposed, consisting of three elementry rection steps for ech hydrocron species. Stedy-stte cron deposition profiles s function of prtil pressures were used to estimte the corresponding dsorption equilirium constnts nd elementry cron deposition rtes from mjor intermedite species of ethylene pyrolysis, while the mesured cron deposition rtes of hydrocron/hydrogen mixtures were employed to clculte the dsorption equilirium constnt of hydrogen. Bsed on the optimized inetic dt of surfce elementry rections, simultions of pyrolytic cron deposition from ethylene were crried out with the plug flow model of the monolith sustrte, respectively. Decresing surfce re to volume rtio fvours the sturted dsorption of precursor t decresing prtil pressures. The effects of hydrogen, temperture nd prtil pressure on cron deposition rtes were predicted, which grees well with erlier pulished experimentl results.. Introduction Pyrolytic cron is one of the most importnt cron mterils, especilly for cron fier-reinforced cron (C/C) composites in the erospce industry,. Isoric nd isotherml chemicl vpor infiltrtion/deposition (CVI/CVD), eing the most ccepted process for the synthesis of C/C composites, hs een investigted for more thn hlf century. From the point of view of modeling, lot of effort hs een devoted to the rection mechnism develop including homogeneous gs-phse rections of pyrolysis of light hydrocrons nd heterogeneous surfce rections of solid cron grow. The generl lumped mechnism of cron deposition ws proposed y üttinger nd co-worers 3, however detiled gs-phse rection mechnism hs to e figured out in order to give deep insight into deposition process for given cron precursor ecuse of the decomposition complexity of vrious hydrocrons. The generl rection mechnism of cron deposition mentioned ove elieves tht () the gs-phse involved in cron CVD gets mtured with progressive pyrolysis of the precursor; () those mjor hydrocron species formed consequently in the gs-phse ct s secondry precursors of cron with different ctivity; (3) hydrogen plys n importnt role in cron deposition, preventing dsorption of hydrocron molecules on the deposition surfce. Noring proposed detiled gs-phse rection mechnism for cron deposition from light hydrocrons nd vlidted the mechnism sed on pyrolysis experiments of C-C3 species 4. On the other hnd, lthough Li et l 5 proposed method for the development of surfce method, ll simultions of CVD/CVI of cron deposition re lmost performed y use of overll surfce rection mechnisms nowdys. In the present wor, methodology for developing detiled surfce rection mechnism of cron deposition is proposed nd corresponding inetic dt re tried to wor out using experimentl dt pulished erly,7.. Modelling Approch.. Mjor Species. The gs-phse rection mechnism nlysis of ethylene pyrolysis is performed t pressure of P, temperture of C nd residence time of s, using the mechnism proposed in [4]. The detiled rection mechnism includes 7 gseous species nd 887 reversile elementry rections. Fig. illustrtes the rection rte flow of 3 mjor species. Those numers shown in Fig. represent the reltive consumption rtes of these mjor species. Compred with the results in Fig. of [4], it implies tht mjor species re lwys similr under conditions of low pressure CVD of ethylene. Topic T A. Li 7

2 . Surfce Rection Mechnism. In the present wor, surfce rection mechnism is proposed for ech mjor species, which consists of three elementry rection steps. Consider two gseous species system s follows: c C c C ( s) θ, σ y + + (s) ( s), θ e, σ e θ, σ y (s) θ e, σ e C ( s), θ, σ y x C + y ( s), θ, σ K K () () (3) Where c i represent the concentrtions of hydrogen or hydrocron ner the surfce, K i the rection equilirium constnt defined y the corresponding forwrd nd cwrd rection rte constnts i nd -i, the deposition rte constnt, θ i the surfce coverge of empty surfce sites(e) or dsortes, σ the numer of sites occupied y ech dsorte species. A If Г defines the density of cron surfce sites (mol m - ) nd first order rections re ssumed, then ciσi θi, θi Γ r c θ Γ θ Γ θγ r3 σ e θγ r c θeγ σ Assuming tht cron deposition is the rection rte determining step nd other rections re t the equilirium stte, cron deposition r is s follows xγ yk c r xr3 y + K c + yk c The ove deposition rection rte revels tht competition of dsorption/desorption equilirium etween hydrogen nd hydrocron molecules ffects cron deposition rte; if hydrogen concentrtion vnishes, then Γ y + (9) r x xk c where it is cler tht the new prmeter Г/r (s) is liner with the reciprocl of hydrocron concentrtion given temperture. For cron deposition from ethylene, x nd y re nd 4, respectively, nd the site density Г of prrolytic cron surfce is.e-5 mol m -. The reltion of Г/r nd /c is shown in Fig. The cron deposition rtes re strcted from the symptote of experimentl dt with respect to vrious hydrocron pressures mesured for the lower prt of the tuulr CVD rector pulished in figure 4 of []. The reltion of nd K with the reciprocl of temperture is deduced s shown in Fig. nd with n ctive energy of 98 J mol - represents strong endothermic rection, while the equilirium constnt K with stndrd molr rection free energy chnge of 95 J mol - mes out tht ethylene dsorption/desorption is strong endothermic rection. The pprent ctive energy of the overll cron deposition rection from ethylene is 55 J mol - reported in [8]. Therefore the stndrd molr rection free energy chnge of hydrogen dsorption/desorption is out 38 J mol - ( ). In the present wor, CVD experimentl dt of other mjor species re ville t only C. ydrogen dsorption/desorption equilirium constnt K t C is strcted with the sme pproch using the help of hydrogen inhiition experimentl dt of [5, 7], s following: Γ + yk c K + c () r xk c K c Topic T x A. Li 7 (8) (4) (5) () (7)

3 3. Results nd Discussion 3.. Effect of Processing Prmeters. According to the surfce rection mechnism proposed ove, sturted dsorption phenomen re expected for hydrocrons on pyrolytic cron surfce, resulting in mximl deposition rte from the originl hydrocron species. Cron deposition profile species re modeled t temperture C using rections (-3) nd no gs-phse rections re ten into ccount in order to investigte deposition rte from only the originl inlet hydrocron species. Simultion results re presented in Fig.3, in which ethylene, propylene nd,3-utdine re dsored in first order rection while cetylene is dsored in third order rection. The criticl pressure corresponding to sturted dsorption of the originl hydrogen decreses with incresing size of hydrocron molecule, therefore experimentl investigtion of sturted dsorption nd deposition for hydrocrons hs to e performed t low pressures. At condition of C, it should e less thn P. The inhiition effect of hydrogen on cron deposition is reported in [7]. In the present wor, the inhiition effect is presented with competition dsorption etween hydrogen nd hydrocrons. Fig.3 shows the inhiition effect with respect to hydrogen prtil pressures t conditions of hydrocron prtil pressure of P, totl pressure of r nd temperture of C. Fig.4 presents the effect of temperture on cron deposition, implying tht lower temperture fvors homogeneous deposition profile. Fig.4 gives the stedy-stte surfce relted deposition rtes s function of sustrte length t temperture of C with respect to vrious surfce re to volume rtios. It suggests tht lower surfce re is eneficil to the formtion of romtic species which show higher deposition rtes thn liner hydrocrons. 3.. Plug Model. Anlysis of processing prmeters ws performed sed on the plug model which is onedimensionl gs-phse model plus surfce rections. Cron deposition from propylene ws simulted t conditions of P, s nd C with sufce re of 3. mm -. Fig.5 shows the predicted deposition profile nd Fig.5 presents the distriution of some mjor species. Concentrtion of propylene decreses very quicly nd hydrogen, methne, ethylene nd cetylene dominte in the gs-phse. The contriution of cron deposition from romtic species is very less thn tht from liner hydrocrons. A reltive homogeneous deposition profile is predicted with only little it high rte in the medil of the rector, which grees with the experimentl result. 4. Conclusion Modelling of pyrolytic cron deposition from ethylene s function of pressure ws performed t vrious tempertures sed on three-step surfce rection mechnism. Kinetic dt were evluted using stedy-stte cron deposition experimentl dt. Bsed on the optimized inetic dt of surfce elementry rections, simultions of pyrolytic cron deposition from ethylene were crried out with the plug flow model of the monolith sustrte, respectively. Decresing surfce re to volume rtio fvours the sturted dsorption of precursor t lower prtil pressures. The inhiition effect of hydrogen on cron deposition rtes were predicted, which grees well with erlier pulished experimentl results. Acnowledgements The uthors grtefully cnowledge support of DFG through the grnt Pyrolytic cron from the gs phse: from elementry rections to process simultions Topic T A. Li 7

4 99 C C 9 3,3-Butdiene Butyne propine ,-Butdiene C C C cyclopentdiene dicetylene styrene C C indene 57 iphenyl enzene toluene 3 85 phenylcetylene 5 98 phennthrene enzylenzene Figure : Determintion of mjor intermedite gs-phse species sed on rection rte nlysis of ethylene pyrolysis t conditions of C, P nd s. Those numers represent reltive consumption rte of species. The detiled gs-phsed mechnism of [4] is employed. Г/r c (s) y x y.88x y.45x +.9 ln(),ln(k). -. y -739x y -48x K y.578x /c E-4 7.4E-4 7.8E-4 8.E-4 8.E-4 9.E-4 /T Figure : The reltion etween Г/r nd the reciprocl of ethylene concentrtion (); determintion of inetic dt for cron deposition from ethylene (). Topic T A. Li 7

5 Deposition rte /mol m - h C_C4 C_C C_C3 C_C4 Deposition rte /mol m - h -. C_C4 C_C C_C3 C_C Pressure /P ydrogen Pressure /P Figure 3 : Inhiition effect with respect to hydrogen prtil pressures t conditions of hydrocron prtil pressure of P, totl pressure of r nd temperture of C.. Deposition rte /mol m - h C 95 C 9 C Pressure /P Deposition rte /mol m - h E+.E-.E- 3.E- 4.E- l /m 3./mm.8/mm /mm Figure 4 : Effect of temperture on cron deposition from with respect to ethylene pressures t conditions of C (); stedy-stte surfce relted deposition rtes s function of sustrte length t temperture of C with respect to vrious surfce re to volume rtios (). Molr frction / Deposition rte/mol m - h -.3. X XC4 XC XC4 XAC34 XPC34 XC3. XC4 XC XAC XAC3 XC98 XA3 Experiment Plug model l/m l/m Figure 5 : Cron deposition from propylene predicted t conditions of P, s nd C with surfce re of 3./mm (); Distriution of some mjor species hydrogen, methne, ethylene nd cetylene etc.(). References [] G. Svge. Cron-Cron Composite; Chpmnn nd ll: London, 993. [] I. Goleci. In World of Cron Fiers nd Composites; P. Delhes, Ed.; Tylor nd Frncis: London nd New Yor, 3. [3] A. Becer, K. üttinger, Cron. 3 (998) 3-4. [4] K. Noring, O. Deutschmnn. Ind. Eng. Chem. Res., 4 (7), [5] K. Noring, K. J. uttinge. Cron. 4 (3) [] A. Li, O. Deutschmnn. Chem. Eng. Sci. (7) [7] A. Becer, Z. u, K.J. uèttinger. Fuel 79 () [8] P. Tesner, Chemistry nd Physics of Cron, 9 (984), 5-. Topic T A. Li 7

Rates of chemical reactions

Rates of chemical reactions Rtes of chemicl rections Mesuring rtes of chemicl rections Experimentl mesuring progress of the rection Monitoring pressure in the rection involving gses 2 NO( g) 4 NO ( g) + O ( g) 2 5 2 2 n(1 α) 2αn