Activation energy 3 1/2 Ea Ea 1/T Figure 5.3 Characteristic Arrhenius plot:k versus T-with k=koexp (-Ea/kT),involving a reacting gas and a porous medium(from Hedden and Wicke,1957). determined by the kinetics of the chemical reactions with an apparent activation energy(Ea) and the concentration gradient across the porous solid is negligible.At intermediary temperatures,within regime II the mass diffusion through the pores influences the rate of conversion which should correspond to about half-value of Ea.When the temperature is raised the diffusion factor becomes comparable with the rate reactions leading to an inter- nal concentration gradient.Then in regime III at high temperatures the rate deposition becomes almost independant;the diffusion of gases through the stagnant boundary layer which always exists in a laminar flow(Carlsson,1985),controls the process.In this last case the carbon infiltration inside the pores is not effective;a deposition rate of about a few microns per hour is usually observed.This is the main limitation for the usual making of these C/C composites as we will consider it in the following sections. 3 CVI processes and efficiency The infiltration and the deposition of pyrocarbons in different porous substrates have been largely investigated starting from the classical isothermal and isobaric process.Its major drawback is the very low infiltration rate related to the diffusion constants.Many develop- ments have been published to improve this situation in this multiparameter technique.As presented in Fig.5.4,for the system responses we will analyze the matrix characteristics in relation with the following two major requirements,infiltration homogeneity and microstructure control of the matrix. 3.1 The process parameters The numerous external constraints acting at different scales are divided into three different classes as summarized in Fig.5.4: (i)Geometrical and Energetical-considering the sources of heat and their distribution inside the reactor and the preform.The heating method,resistive inductive or radiative,is associated with either hot wall or cold wall reactors.This is a basic difference which ©2003 Taylor&Francis
determined by the kinetics of the chemical reactions with an apparent activation energy (Ea) and the concentration gradient across the porous solid is negligible. At intermediary temperatures, within regime II the mass diffusion through the pores influences the rate of conversion which should correspond to about half-value of Ea. When the temperature is raised the diffusion factor becomes comparable with the rate reactions leading to an internal concentration gradient. Then in regime III at high temperatures the rate deposition becomes almost independant; the diffusion of gases through the stagnant boundary layer which always exists in a laminar flow (Carlsson, 1985), controls the process. In this last case the carbon infiltration inside the pores is not effective; a deposition rate of about a few microns per hour is usually observed. This is the main limitation for the usual making of these C/C composites as we will consider it in the following sections. 3 CVI processes and efficiency The infiltration and the deposition of pyrocarbons in different porous substrates have been largely investigated starting from the classical isothermal and isobaric process. Its major drawback is the very low infiltration rate related to the diffusion constants. Many developments have been published to improve this situation in this multiparameter technique. As presented in Fig. 5.4, for the system responses we will analyze the matrix characteristics in relation with the following two major requirements, infiltration homogeneity and microstructure control of the matrix. 3.1 The process parameters The numerous external constraints acting at different scales are divided into three different classes as summarized in Fig. 5.4: (i) Geometrical and Energetical – considering the sources of heat and their distribution inside the reactor and the preform. The heating method, resistive inductive or radiative, is associated with either hot wall or cold wall reactors. This is a basic difference which Figure 5.3 Characteristic Arrhenius plot: k versus T 1 with k k0 · exp (Ea/kT ), involving a reacting gas and a porous medium (from Hedden and Wicke, 1957). ES III II I ~ 1/2 E = Ea a ~ 0 1/T Log k Activation energy © 2003 Taylor & Francis
Processing parameters 2.Hydrodynamical -Residence time 1.Geometrical and energetical yT°P 3.Chemical -Reactor type and size -Preform as porous substrate Q T ps -Hydrocarbon precursor phase,nature -Heat source flow rate°，temperature T,pressure P CVI Processes 4.Composite characterizations 5.Insitu experiments 6.Modeling deposition/infiltration rates T and P maps Volume/surface chemical reactions structural and textural characteristics IR spectroscopy versus (optical and electronic microscopies) Mass spectrometry Mass and heat transfers Gas chromatography System responses Figure 5.4 Summary of the parameters and system responses in CVI processes. involves for hot wall technique isothermal and isobaric conditions whereas thermal gradi- ents (Lieberman et al.,1975)or pressure gradients and forced flows (Lackey and Starr, 1991)exist in the cold wall approach.This one can be also combined with either laser or DC and RF plasma uses (Lachter et al.,1985;Levesque et al.,1989). The preform to densify is also crucial through its nature,orientation,and volumic frac- tion of the carbon fibers (Delhaes et al.,1984);its position and volume occupation inside the furnace are noteworthy. (ii)Hydrodynamical-the flow regime inside the reactor is related to the nature of the pre- cursor fluid but also the size and the shape of the reactor;usually a low value of the Rayleigh number characterizes this laminar flow.The precursors are in a gaseous phase at different pressures;under isothermal conditions a laminar flow is expected and the residence time (see definition,Fig.5.4)is the key parameter.However a forced flow will conduct to a quite different behavior as already demonstrated (Vaidyaraman et al.,1996). (iii)Chemical-the nature of the precursor is important even if the generic reactions are recognized(Fig.5.1).For example natural gas i.e.methane,is the most stable hydrocarbon and the associated decomposition conditions will be specific compared to the other precur- sors.Besides liquid precursors,as cyclohexane and aromatic derivatives have been also used in a new fast densification technique that we will describe later(David et al.,1995). A complementary approach concerns the system responses as presented in Fig 5.3:the first ones are the material requirements,essentially the type of carbon matrix,the deposition rates,and the overall carbon yield.Its quality has to be optimized with the highest final den- sity and a well defined type of microstructure.The classical "black box"approach which concerns only ex situ relevant parameters (Loll et al.,1977)has been recently improved. Both experimental and theoretical approaches have been developed.In situ observations by FTIR in-line mass spectroscopy or gas chromatography,have deepened the gas chemistry (Chen and Back,1979;Ferron et al.,1999)and global modeling of engineering techniques are in constant progress(Ofori and Sotirchos,1997).They will contribute in the future to the overall process control. ©2003 Taylor&Francis
involves for hot wall technique isothermal and isobaric conditions whereas thermal gradients (Lieberman et al., 1975) or pressure gradients and forced flows (Lackey and Starr, 1991) exist in the cold wall approach. This one can be also combined with either laser or DC and RF plasma uses (Lachter et al., 1985; Levesque et al., 1989). The preform to densify is also crucial through its nature, orientation, and volumic fraction of the carbon fibers (Delhaès et al., 1984); its position and volume occupation inside the furnace are noteworthy. (ii) Hydrodynamical – the flow regime inside the reactor is related to the nature of the precursor fluid but also the size and the shape of the reactor; usually a low value of the Rayleigh number characterizes this laminar flow. The precursors are in a gaseous phase at different pressures; under isothermal conditions a laminar flow is expected and the residence time (see definition, Fig. 5.4) is the key parameter. However a forced flow will conduct to a quite different behavior as already demonstrated (Vaidyaraman et al., 1996). (iii) Chemical – the nature of the precursor is important even if the generic reactions are recognized (Fig. 5.1). For example natural gas i.e. methane, is the most stable hydrocarbon and the associated decomposition conditions will be specific compared to the other precursors. Besides liquid precursors, as cyclohexane and aromatic derivatives have been also used in a new fast densification technique that we will describe later (David et al., 1995). A complementary approach concerns the system responses as presented in Fig 5.3: the first ones are the material requirements, essentially the type of carbon matrix, the deposition rates, and the overall carbon yield. Its quality has to be optimized with the highest final density and a well defined type of microstructure. The classical “black box” approach which concerns only ex situ relevant parameters (Loll et al., 1977) has been recently improved. Both experimental and theoretical approaches have been developed. In situ observations by FTIR in-line mass spectroscopy or gas chromatography, have deepened the gas chemistry (Chen and Back, 1979; Ferron et al., 1999) and global modeling of engineering techniques are in constant progress (Ofori and Sotirchos, 1997). They will contribute in the future to the overall process control. Figure 5.4 Summary of the parameters and system responses in CVI processes. 1. Geometrical and energetical – Reactor type and size – Preform as porous substrate – Heat source flow rate QØ, temperature T, pressure P 3. Chemical – Hydrocarbon precursor phase, nature CVI Processes 4. Composite characterizations deposition/infiltration rates structural and textural characteristics (optical and electronic microscopies) 6. Modeling Volume/surface chemical reactions versus Mass and heat transfers Q T P° = 2. Hydrodynamical – Residence time Processing parameters Vt T° P System responses 5. Insitu experiments T and P maps IR spectroscopy Mass spectrometry Gas chromatography © 2003 Taylor & Francis
(a) (b) 00000 Sus- Ind ol 'Gas 'Gas Isothermic-isobaric Thermal gradient (c) (d) <P, Sus ● Ind 8 Lig Gas Pressure gradient "film boiling" (Sus:susceptor (int./ext.)-Ind:induction coil) (Lig:liquid precursor-Gas:gas inlet) (☑porous substrate) Figure 5.5 Sketches of the basic infiltration techniques(adapted from Kotlensky,1973) 3.2 Outline of the principal methods The basic infiltration techniques are schematically drawn in Fig.5.5.As recently under- lined by Golecki the various infiltration methods are at different stages of maturity and understanding (Golecki,1997).The isothermal and isobaric CVI,the oldest"hot wall"tech- nique,is still widely used both in laboratories and industry.Its main advantage is a good parameter control,in particular for large furnaces where a large number of complex pre- forms can be densified together.As already indicated a good matrix quality with a selected microstructure and a low residual porosity is obtained (Lackey and Starr,1991).The main drawback is a quite long processing time,sometimes larger than 500h with a very slow rate of deposit associated with a very low overall precursor efficiency,a few percent only with the natural gas.New routes to develop rapid infiltration techniques have been explored to increase the process efficiency.We present them,giving some interesting examples: (i)Derived from the isothermal process,three ways have been explored:the catalytic CVI using transition metals for increasing the rate deposition(McAllister and Wolf,1993),the plasma enhanced CVI(Levesque et al.,1989),and the pulsed flow where a cyclic evacua- tion of the reaction chamber and a back filling with reagents is done (Dupel et al.,1994). These approaches appear more interesting for the basic understanding of the infiltration mechanisms than to get an economical and technical gain. ©2003 Taylor&Francis
3.2 Outline of the principal methods The basic infiltration techniques are schematically drawn in Fig. 5.5. As recently underlined by Golecki the various infiltration methods are at different stages of maturity and understanding (Golecki, 1997). The isothermal and isobaric CVI, the oldest “hot wall” technique, is still widely used both in laboratories and industry. Its main advantage is a good parameter control, in particular for large furnaces where a large number of complex preforms can be densified together. As already indicated a good matrix quality with a selected microstructure and a low residual porosity is obtained (Lackey and Starr, 1991). The main drawback is a quite long processing time, sometimes larger than 500 h with a very slow rate of deposit associated with a very low overall precursor efficiency, a few percent only with the natural gas. New routes to develop rapid infiltration techniques have been explored to increase the process efficiency. We present them, giving some interesting examples: (i) Derived from the isothermal process, three ways have been explored: the catalytic CVI using transition metals for increasing the rate deposition (McAllister and Wolf, 1993), the plasma enhanced CVI (Levesque et al., 1989), and the pulsed flow where a cyclic evacuation of the reaction chamber and a back filling with reagents is done (Dupel et al., 1994). These approaches appear more interesting for the basic understanding of the infiltration mechanisms than to get an economical and technical gain. Figure 5.5 Sketches of the basic infiltration techniques (adapted from Kotlensky, 1973). ( porous substrate) “film boiling” Liq Isothermic–isobaric Gas T2 < T1 Thermal gradient Gas P2 (< P1) P1 Pressure gradient Gas Ind Ind Sus Sus (Sus: susceptor (int./ext.) – Ind: induction coil) (Liq: liquid precursor – Gas: gas inlet) (a) (b) (c) (d) © 2003 Taylor & Francis
(ii)Pressure gradient and forced flows:Several reactors have been built to control the gas hydrodynamics under isothermal conditions or with thermal gradient (Lackey and Starr,1991).In particular the forced flow-thermal gradient CVI process (see Fig.5.6) has been thoroughly developed (Vaidyaraman et al.,1995).The fabrication of valuable C/C composites with a matrix of uniform high thermal conductivity onto conventional size fibers is realized in a few hours under controlled parameters (Lewis et al., 1997). (iii)Strong thermal gradients under quasi isobaric conditions:this is the case of cold wall reactors with a graphite susceptor inside(see Fig.5.5d).The precursors are in a gaseous or liquid state;nevertheless in both situations there is a mobile reacting front on which the vapors decompose to produce the carbon deposit.Two main type of reactors have been real- ized with similar cylindrical geometries,the rapid vapor phase densification and the film boiling technique based on a liquid reservoir(Fig.5.7).Both techniques are very efficient, a single cycle of densification for a few hours as for the forced flow method is sufficient in the range.A high conversion efficiency is obtained,one order of magnitude higher than is classical processes i.e 20-50%,associated with a good quality of the final products (Golecki et al.,1995).To get a better insight on this type of process a small laboratory reac- tor equipped with an internal resistive heater has been built up(Rovillain et al.,2001)which can work with various liquid precursors.As shown in Fig.5.8 this process is based on a mobile reactive front with a steep densification profile which starts from the central part of the preform to the outside.This novel process has been widely investigated these last years concerning the chemical influence with halogen derivatives or iron catalytic effect(Okuno et al.,2001)and the hydrodynamical aspect with a mass barrier effect and the influence of high pressure reagents(Beaugrand,2000).The essential parameter appears to be the evolu- tive thermal gradient across the preform which controls both the high infiltration speed and the type of pyrocarbons. To conclude it should be mentioned that these industrial applications are covered by numerous patents;a comparison between these processes with their advantages and disad- vantages are presented in Chapter 6(Golecki,2003). Preform holder Preform Gas injector Punch Thermocouple Reagent Thermocouple Figure 5.6 Schematic of the preform and the reactor used in forced-CVI process (from Lewis etal,1996). ©2003 Taylor&Francis
(ii) Pressure gradient and forced flows: Several reactors have been built to control the gas hydrodynamics under isothermal conditions or with thermal gradient (Lackey and Starr, 1991). In particular the forced flow-thermal gradient CVI process (see Fig. 5.6) has been thoroughly developed (Vaidyaraman et al., 1995). The fabrication of valuable C/C composites with a matrix of uniform high thermal conductivity onto conventional size fibers is realized in a few hours under controlled parameters (Lewis et al., 1997). (iii) Strong thermal gradients under quasi isobaric conditions: this is the case of cold wall reactors with a graphite susceptor inside (see Fig. 5.5d). The precursors are in a gaseous or liquid state; nevertheless in both situations there is a mobile reacting front on which the vapors decompose to produce the carbon deposit. Two main type of reactors have been realized with similar cylindrical geometries, the rapid vapor phase densification and the film boiling technique based on a liquid reservoir (Fig. 5.7). Both techniques are very efficient, a single cycle of densification for a few hours as for the forced flow method is sufficient in the range. A high conversion efficiency is obtained, one order of magnitude higher than is classical processes i.e 20–50%, associated with a good quality of the final products (Golecki et al., 1995). To get a better insight on this type of process a small laboratory reactor equipped with an internal resistive heater has been built up (Rovillain et al., 2001) which can work with various liquid precursors. As shown in Fig. 5.8 this process is based on a mobile reactive front with a steep densification profile which starts from the central part of the preform to the outside. This novel process has been widely investigated these last years concerning the chemical influence with halogen derivatives or iron catalytic effect (Okuno et al., 2001) and the hydrodynamical aspect with a mass barrier effect and the influence of high pressure reagents (Beaugrand, 2000). The essential parameter appears to be the evolutive thermal gradient across the preform which controls both the high infiltration speed and the type of pyrocarbons. To conclude it should be mentioned that these industrial applications are covered by numerous patents; a comparison between these processes with their advantages and disadvantages are presented in Chapter 6 (Golecki, 2003). Figure 5.6 Schematic of the preform and the reactor used in forced-CVI process (from Lewis et al., 1996). Preform holder Preform Gas injector Punch Thermocouple Reagent Thermocouple © 2003 Taylor & Francis