《纺织复合材料》课程参考文献（Fibers and Composites）05 CHEMICAL VAPOR INFILTRATION PROCESSES OF CARBON MATERIALS

5 CHEMICAL VAPOR INFILTRATION PROCESSES OF CARBON MATERIALS P Delhaes 1 Introduction Monolithic carbons as well as thin films or carbon-carbon composites are prepared from pyrolysis and consequent carbonization of an initial organic phase.Compared to bulk pieces these composites present a noncatastrophic failure under severe experimental constraints: this is the basic reason for their broad applications (Delmonte,1981).Before going into the details of the different processes and the associated composites manufacturing it is note- worthy to remind some basic concepts on carbon deposition.Indeed the structures and prop- erties of a deposited carbon or pyrocarbon are related to the pyrolysis conditions which control the mechanisms of carbon formation.The matrix precursor can be either liquids or gases;in the first case the process is labelled as the impregnation technique starting usually from fluid resins or pitches(Rand,1993).The second process is the cracking of a hydrocar- bon gas at high temperature under an inert atmosphere.The associated coating process on hot bulk surfaces is called the chemical vapor deposition(CVD)and alternatively on porous substrates the chemical vapor infiltration(CVD)technique. In this chapter,we will examine mainly the thermal decomposition of an hydrocarbon gas, the most versatile deposition process,which is widely used to make carbon-carbon com- posites;starting from a porous preform made of shopped or continuous carbon fibers, arranged in different bundles arrays,which are usually ex polyacrilonitrile (PAN)type. These composites are realized by filling all the voids of these preforms with an adapted fiber-matrix interaction (Inagaki,2000). This presentation will start with a comparison between the CVD and CVI processes including the historical aspects which have been introduced a long time ago. Both techniques are based on the competition between the chemical reactions and the physical transport properties as discussed in Section 2.Then in Section 3 we present the different processes starting from pioneering works in the sixties until the more recent improved processes.Several new approaches have been developed these last years to opti- mize the infiltration processes and to understand them keeping in mind that the two key points are to speed up the deposition efficiency and to ameliorate the deposit quality of the matrix. In Section 4,the approach will deal with the study of the formation of pyrocarbons,their different microstructures described at different length scales and their growing stability. ©2003 Taylor&Francis

5 CHEMICAL VAPOR INFILTRATION PROCESSES OF CARBON MATERIALS P. Delhaès 1 Introduction Monolithic carbons as well as thin films or carbon–carbon composites are prepared from pyrolysis and consequent carbonization of an initial organic phase. Compared to bulk pieces these composites present a noncatastrophic failure under severe experimental constraints: this is the basic reason for their broad applications (Delmonte, 1981). Before going into the details of the different processes and the associated composites manufacturing it is note￾worthy to remind some basic concepts on carbon deposition. Indeed the structures and prop￾erties of a deposited carbon or pyrocarbon are related to the pyrolysis conditions which control the mechanisms of carbon formation. The matrix precursor can be either liquids or gases; in the first case the process is labelled as the impregnation technique starting usually from fluid resins or pitches (Rand, 1993). The second process is the cracking of a hydrocar￾bon gas at high temperature under an inert atmosphere. The associated coating process on hot bulk surfaces is called the chemical vapor deposition (CVD) and alternatively on porous substrates the chemical vapor infiltration (CVI) technique. In this chapter, we will examine mainly the thermal decomposition of an hydrocarbon gas, the most versatile deposition process, which is widely used to make carbon–carbon com￾posites; starting from a porous preform made of shopped or continuous carbon fibers, arranged in different bundles arrays, which are usually ex polyacrilonitrile (PAN) type. These composites are realized by filling all the voids of these preforms with an adapted fiber–matrix interaction (Inagaki, 2000). This presentation will start with a comparison between the CVD and CVI processes including the historical aspects which have been introduced a long time ago. Both techniques are based on the competition between the chemical reactions and the physical transport properties as discussed in Section 2. Then in Section 3 we present the different processes starting from pioneering works in the sixties until the more recent improved processes. Several new approaches have been developed these last years to opti￾mize the infiltration processes and to understand them keeping in mind that the two key points are to speed up the deposition efficiency and to ameliorate the deposit quality of the matrix. In Section 4, the approach will deal with the study of the formation of pyrocarbons, their different microstructures described at different length scales and their growing stability. © 2003 Taylor & Francis

They are functions of different external parameters that we will analyze to establish a rela- tionship between the deposition mechanisms and the structural and physical properties of the matrix.These pyrocarbons will be defined as a subclass of non-crystalline graphitic carbons (Delhaes,2000).In Section 5 some basic concepts regarding a few infiltration models are pre- sented;our approach starting with thermodynamics equilibrium at the first level is relevant to out-of-equilibrium modeling.The main emphasis will be about the competition and the inter- actions between the chemical kinetics and the transport coefficients in a porous medium. Dynamic modeling of CVI processes result from these theoretical works. Finally the physical properties and the numerous applications of carbon-carbon compos- ites are summarized.They are related to their peculiar characteristics-light weight associ- ated with high mechanical strength including stiffness,wear,toughness,and thermal shock resistance. 2 General background on CVD and CVI processes 2.1 Definitions and outlines The different types of allotropic forms for crystalline as well as non-crystalline carbons are essentially functions of two basic parameters.They are the nature and phase of the precur- sor and the experimental method selected to deliver the process energy respectively (Delhaes and Carmona,1981).Indeed the final product,except hexagonal graphite,is in a metastable thermodynamic state which needs to be defined.Two main classes of precursors exist,either pure solid carbons or various gas and liquid hydrocarbons containing possibly hetero-elements as oxygen,nitrogen or even halogens.In the first case,this is a physical process as produced by thermal or laser beam evaporation,ion-plasma techniques as elec- trical discharges or sputtering techniques.Starting from hydrocarbon precursors rather com- plex chemical processes are ocurring including the pyrolysis and carbonization steps to obtain a pyrocarbon coating.In that situation the supplementary energy is also afforded by similar techniques but the control of the complex and numerous chemical reactions is essen- tial.This is the natural approach for CVI processes that we are developing in this chapter. Following a chronological approach it is necessary to remind that the main point is to con- trol both the homogeneous gas phase reactions and the heterogeneous ones at the solid-gas interface,and thereafter the deposition mechanism on a selected substrate.Many studies have been accomplished to understand the mechanism of carbon formation from different gaseous organic compounds (Tesner,1984)and to establish a relationship between the deposition conditions and the structures of pyrocarbons (Bokros,1969).Before going into the details of the physical process which can be defined as a balance between chemistry and hydrodynamics we will introduce the basic chemical mechanisms which have been established with some emphasis on the progress gained during these last years. 2.2 Chemistry of carbon formation We summarize both the mechanisms and kinetics of the overall reactions.In general the pre- cursors are alcanes,usually methane or propane,but also non-saturated hydrocarbons(such as acetylene or propylene)or aromatic molecules (benzene and derivatives)which are decomposed at a lower temperature than the alcanes.From all the experimental studies it results that a general trend can be drawn to furnish an overall reaction modeling which consider two main classes (Huttinger,1998). ©2003 Taylor&Francis

They are functions of different external parameters that we will analyze to establish a rela￾tionship between the deposition mechanisms and the structural and physical properties of the matrix. These pyrocarbons will be defined as a subclass of non-crystalline graphitic carbons (Delhaès, 2000). In Section 5 some basic concepts regarding a few infiltration models are pre￾sented; our approach starting with thermodynamics equilibrium at the first level is relevant to out-of-equilibrium modeling. The main emphasis will be about the competition and the inter￾actions between the chemical kinetics and the transport coefficients in a porous medium. Dynamic modeling of CVI processes result from these theoretical works. Finally the physical properties and the numerous applications of carbon–carbon compos￾ites are summarized. They are related to their peculiar characteristics – light weight associ￾ated with high mechanical strength including stiffness, wear, toughness, and thermal shock resistance. 2 General background on CVD and CVI processes 2.1 Definitions and outlines The different types of allotropic forms for crystalline as well as non-crystalline carbons are essentially functions of two basic parameters. They are the nature and phase of the precur￾sor and the experimental method selected to deliver the process energy respectively (Delhaès and Carmona, 1981). Indeed the final product, except hexagonal graphite, is in a metastable thermodynamic state which needs to be defined. Two main classes of precursors exist, either pure solid carbons or various gas and liquid hydrocarbons containing possibly hetero-elements as oxygen, nitrogen or even halogens. In the first case, this is a physical process as produced by thermal or laser beam evaporation, ion-plasma techniques as elec￾trical discharges or sputtering techniques. Starting from hydrocarbon precursors rather com￾plex chemical processes are ocurring including the pyrolysis and carbonization steps to obtain a pyrocarbon coating. In that situation the supplementary energy is also afforded by similar techniques but the control of the complex and numerous chemical reactions is essen￾tial. This is the natural approach for CVI processes that we are developing in this chapter. Following a chronological approach it is necessary to remind that the main point is to con￾trol both the homogeneous gas phase reactions and the heterogeneous ones at the solid–gas interface, and thereafter the deposition mechanism on a selected substrate. Many studies have been accomplished to understand the mechanism of carbon formation from different gaseous organic compounds (Tesner, 1984) and to establish a relationship between the deposition conditions and the structures of pyrocarbons (Bokros, 1969). Before going into the details of the physical process which can be defined as a balance between chemistry and hydrodynamics we will introduce the basic chemical mechanisms which have been established with some emphasis on the progress gained during these last years. 2.2 Chemistry of carbon formation We summarize both the mechanisms and kinetics of the overall reactions. In general the pre￾cursors are alcanes, usually methane or propane, but also non-saturated hydrocarbons (such as acetylene or propylene) or aromatic molecules (benzene and derivatives) which are decomposed at a lower temperature than the alcanes. From all the experimental studies it results that a general trend can be drawn to furnish an overall reaction modeling which consider two main classes (Hüttinger, 1998). © 2003 Taylor & Francis

Homogeneous gas phase reactions The radical processes are favorized,the primary and secondary mechanisms give birth to free radicals which are recombined immediately.In a first step non-saturated species(as allenes,propyne,butadiene)are formed.Following the formation of these aliphatic compounds different cyclization processes occur;there are mainly the C3 and C cyclization ways and the HACA (H abstraction and C2H2 addition) mechanisms(Frenklach,1996).Aromatics and polyaromatics(HAP)are formed for a longer reaction time (Lucas and Marchand,1990).This is the so-called maturation effect involving the aromatic route which depends on the residence time of the molecules in the free reacting volume(Ferron et al.,1999).In situ spectroscopic analysis have been carried out to follow both the mechanisms and kinetics of these complex reactions(Chen and Back,1979; Ferron et al.,1997).They give valuable information but no definite results for the complete understanding of the pyrocarbon deposition.Indeed the active species are essentially free radicals which are instable (life-time about 0.01 sec).The stable molecules which are side products,not directly transformed in solid carbon,are preferentially detected by these dif- ferent analytical techniques as mass spectroscopy or gas chromatography. Heterogeneous surface reactions They are fundamental for the formation rate of pyrocarbons,starting from the nucleation and the growing processes on a given substrate. These interfacial reactions,associated with sticking coefficients and surface migrations, specific for each species,have been investigated for a long time (Tesner,1984);they are very difficult to analyze and therefore to control.One attempt has been to consider Langmuir-Hinshelwood model for the kinetics of surface reactions(Benziger and Huttinger, 1999a,b).In particular,the influence of catalysts as transition metals and additive reacting gas are predominant factors.For example,hydrogen but also oxygen or chlorine gas play a role for both the deposition mechanism and the type of deposit. Starting from the initial works (Diefendorf,1960;Tesner,1984)many studies have been done to understand the intrinsic mechanisms but with a rather small insight for the details of the chemical roads which remain a current challenge. In conclusion,without taking account of the specific nature of the pristine gases,a sum- mary of all the possible generic radical type reactions is given on the schema presented in Fig.5.1 (Huttinger,1998).The chemistry inside the homogeneous gas phase leads to the aliphatic formation(C2)as ethylene or acetylene,then the aromatic route (C6)of benzene type species,and the formation of polyaromatics,then droplets in the gas phase (homoge- neous nucleation)when the residence time is long enough(Grisdale,1953). This is in qualitative agreement with the thermodynamic calculations at thermal equilib- rium concerning acetylenic and aromatic species carried out with thermodynamic softwares Increasing residence time() Homogeneous [C2] IC6] [c6]n Gas phase Hydrocarbon k, aliphatic polyaromatic Nucleation Soots reactions (ex:CH4) species aromatic species aggregates in vapor phase ks Heterogeneous Surface PyroC.(A) PyroC.(B) PyroC.(C) SL? RL? 7 reactions Deposition of pyrolytic carbons Figure 5.I Simplified reaction scheme for pyrocarbon deposition;the rate constants are labelled(Ki) (adapted from Huttinger,1998). ©2003 Taylor&Francis

Homogeneous gas phase reactions The radical processes are favorized, the primary and secondary mechanisms give birth to free radicals which are recombined immediately. In a first step non-saturated species (as allenes, propyne, butadiene) are formed. Following the formation of these aliphatic compounds different cyclization processes occur; there are mainly the C3 and C4 cyclization ways and the HACA (H abstraction and C2H2 addition) mechanisms (Frenklach, 1996). Aromatics and polyaromatics (HAP) are formed for a longer reaction time (Lucas and Marchand, 1990). This is the so-called maturation effect involving the aromatic route which depends on the residence time of the molecules in the free reacting volume (Ferron et al., 1999). In situ spectroscopic analysis have been carried out to follow both the mechanisms and kinetics of these complex reactions (Chen and Back, 1979; Ferron et al., 1997). They give valuable information but no definite results for the complete understanding of the pyrocarbon deposition. Indeed the active species are essentially free radicals which are instable (life-time about 0.01 sec). The stable molecules which are side products, not directly transformed in solid carbon, are preferentially detected by these dif￾ferent analytical techniques as mass spectroscopy or gas chromatography. Heterogeneous surface reactions They are fundamental for the formation rate of pyrocarbons, starting from the nucleation and the growing processes on a given substrate. These interfacial reactions, associated with sticking coefficients and surface migrations, specific for each species, have been investigated for a long time (Tesner, 1984); they are very difficult to analyze and therefore to control. One attempt has been to consider Langmuir–Hinshelwood model for the kinetics of surface reactions (Benziger and Hüttinger, 1999a,b). In particular, the influence of catalysts as transition metals and additive reacting gas are predominant factors. For example, hydrogen but also oxygen or chlorine gas play a role for both the deposition mechanism and the type of deposit. Starting from the initial works (Diefendorf, 1960; Tesner, 1984) many studies have been done to understand the intrinsic mechanisms but with a rather small insight for the details of the chemical roads which remain a current challenge. In conclusion, without taking account of the specific nature of the pristine gases, a sum￾mary of all the possible generic radical type reactions is given on the schema presented in Fig. 5.1 (Hüttinger, 1998). The chemistry inside the homogeneous gas phase leads to the aliphatic formation (C2) as ethylene or acetylene, then the aromatic route (C6) of benzene type species, and the formation of polyaromatics, then droplets in the gas phase (homoge￾neous nucleation) when the residence time is long enough (Grisdale, 1953). This is in qualitative agreement with the thermodynamic calculations at thermal equilib￾rium concerning acetylenic and aromatic species carried out with thermodynamic softwares [C6] aromatic species k1 Hydrocarbon (ex: CH4) [C2] aliphatic species k2 k3 Nucleation in vapor phase Soots PyroC. (A) SL ? PyroC. (B) RL ? PyroC. (C) ? k 5 k6 Deposition of pyrolytic carbons [C6]n polyaromatic aggregates Increasing residence time () k4 Homogeneous Gas phase reactions Heterogeneous Surface reactions Figure 5.1 Simplified reaction scheme for pyrocarbon deposition; the rate constants are labelled (Ki) (adapted from Hüttinger, 1998). © 2003 Taylor & Francis

for methane (Lieberman and Pierson,1974)and other hydrocarbons(Diefendorf,1969)and confirmed by experiments in closed systems.The first authors defined an equilibrium ratio R=C2H2/CH function of P and T,which is relevant for further correlation to the type of deposited pyrocarbon(see Section 4). 2.3 Deposition mechanisms It concerns essentially the condensation of gases on a solid surface,i.e.the nucleus and growing mechanisms.The basic thermodynamic approach for a hemispheric germ indicates that the Gibbs enthalpy for heterogeneous nucleation is always lower than for the homoge- neous one,depending on the contact angle and the wetting conditions on a given substrate (Adamson,1976). The experimental consequence is that a pyrocarbon should be deposited at a lower T and (or)P than predicted by the classical homogeneous nucleation which gives rise to the formation of carbon blacks(Donnet and Voet,1976).This is an interfacial property depend- ing on both the nature of the gas phase and the solid surface.It has been shown that the sub- strate plays a role for the first pyrocarbon layers from the physical(roughness,curvature and surface energy)and chemical (nucleation sites)point of view.In particular it has been demonstrated that the active sites on a graphene surface and the plane edges are the prefer- ential nucleation sites which control the first step for the kinetic deposition(Hoffman et al., 1985).In this chemisorption process the main parameter is the active surface area (ASA) and its percentage with the total area(Ismail and Hoffman,1991).More recently it has been evidenced by scanning tunneling microscopy (STM)on a pyrographite that the initial deposit is sensitive to the residence time parameter and a sort of wetting transition is detected associated with a morphological change indicating the influence of the gas com- position on the adsorption process(Bouchard et al.,2001).This ideal substrate which pres- ents a low surface energy is strongly modified for non-crystalline carbons as fibers;the density of active sites is much more higher and controls the pyrocarbon deposit (Ismail and Hoffman,1991)and therefore the interfacial strength between fibers and matrix. Other planar substrates have been used for electron microscopy works as,for example, alumina(Desprez and Oberlin,1997;Soutric,1998)or boron nitride(De Pauw et al.,2003); apparently it does not change strongly the kinetics on the growing process after deposit of the first layers.This growing process has also been examined by several authors(Kaae,1985). Two types of cone structures are usually recognized when the nucleus are created only on the substrate or regenerated at different levels (Diefendorf,1960).These various features include the structure of nodules under different substrate conditions as presented in Fig.5.2 (Coffin,1964).As developed in classical CVD models(Bryant,1977)the morphology of pyrocarbons with their cone growing structure,and through their density and porosity,are functions of the deposit conditions.In particular when the coating temperature is as high as 1,500C a less dense pyrocarbon is obtained which is considered as a non-graphitable carbon (Ford and Bilsby,1976). 2.4 Comparison between standard CVD and CVI processes In a standard technique,i.e.under isothermal and isobaric conditions the main difference between CVD and CVI concerns the type of substrate.As pointed out during the early stud- ies(Kotlensky,1973)there are very strong similarities between the pyrocarbon deposition ©2003 Taylor&Francis

for methane (Lieberman and Pierson, 1974) and other hydrocarbons (Diefendorf, 1969) and confirmed by experiments in closed systems. The first authors defined an equilibrium ratio R  C2H2/C6H6 function of P and T, which is relevant for further correlation to the type of deposited pyrocarbon (see Section 4). 2.3 Deposition mechanisms It concerns essentially the condensation of gases on a solid surface, i.e. the nucleus and growing mechanisms. The basic thermodynamic approach for a hemispheric germ indicates that the Gibbs enthalpy for heterogeneous nucleation is always lower than for the homoge￾neous one, depending on the contact angle and the wetting conditions on a given substrate (Adamson, 1976). The experimental consequence is that a pyrocarbon should be deposited at a lower T and (or) P than predicted by the classical homogeneous nucleation which gives rise to the formation of carbon blacks (Donnet and Voet, 1976). This is an interfacial property depend￾ing on both the nature of the gas phase and the solid surface. It has been shown that the sub￾strate plays a role for the first pyrocarbon layers from the physical (roughness, curvature and surface energy) and chemical (nucleation sites) point of view. In particular it has been demonstrated that the active sites on a graphene surface and the plane edges are the prefer￾ential nucleation sites which control the first step for the kinetic deposition (Hoffman et al., 1985). In this chemisorption process the main parameter is the active surface area (ASA) and its percentage with the total area (Ismail and Hoffman, 1991). More recently it has been evidenced by scanning tunneling microscopy (STM) on a pyrographite that the initial deposit is sensitive to the residence time parameter and a sort of wetting transition is detected associated with a morphological change indicating the influence of the gas com￾position on the adsorption process (Bouchard et al., 2001). This ideal substrate which pres￾ents a low surface energy is strongly modified for non-crystalline carbons as fibers; the density of active sites is much more higher and controls the pyrocarbon deposit (Ismail and Hoffman, 1991) and therefore the interfacial strength between fibers and matrix. Other planar substrates have been used for electron microscopy works as, for example, alumina (Desprez and Oberlin, 1997; Soutric, 1998) or boron nitride (De Pauw et al., 2003); apparently it does not change strongly the kinetics on the growing process after deposit of the first layers. This growing process has also been examined by several authors (Kaae, 1985). Two types of cone structures are usually recognized when the nucleus are created only on the substrate or regenerated at different levels (Diefendorf, 1960). These various features include the structure of nodules under different substrate conditions as presented in Fig. 5.2 (Coffin, 1964). As developed in classical CVD models (Bryant, 1977) the morphology of pyrocarbons with their cone growing structure, and through their density and porosity, are functions of the deposit conditions. In particular when the coating temperature is as high as 1,500 C a less dense pyrocarbon is obtained which is considered as a non-graphitable carbon (Ford and Bilsby, 1976). 2.4 Comparison between standard CVD and CVI processes In a standard technique, i.e. under isothermal and isobaric conditions the main difference between CVD and CVI concerns the type of substrate. As pointed out during the early stud￾ies (Kotlensky, 1973) there are very strong similarities between the pyrocarbon deposition © 2003 Taylor & Francis

女女女女b古 Figure 5.2 Cross sections of a nodule of pyrocarbon with the associated model (from Coffin,1964): (a)nodule formed from surface asperity:(b)nodule formed from a foreign particle. on a static substrate or a dynamic bulk surface,the so-called usual fluid bed technique,and the carbon growth inside a porous substrate.The phenomenological competition between the mass and heat transport phenomena and the overall chemical reactions is the basic point to consider.We can enounce two statements which are fundamental in this presentation: (i)The overall chemistry by itself is mainly related to the local conditions in particular for the surface reactions.For a given precursor,they are supposed very similar in the gas phase for both CVD and CVI processes.However with porous preforms an increase of the surface to volume ratio (S/V)is a novel parameter to take into account(Huttinger,1998). As shown on the simplified schema presented Fig.5.1,the deposits will result from the competition between the rate constants of the homogeneous and heterogeneous reactions. (ii)The transport phenomena related to the hydrodynamics of fluids and to the energetic supply would be quite different.They are submitted to different scale factors related to the piece and reactor sizes which are sometimes very different(as in classical CVD)or compa- rable:they have to be examined for each type of process(see Section 3);on a general way the kinetic deposition of pyrocarbons can be represented by an Arrhenius plot which describes the general heterogeneous reactions of gases with porous solids (Hedden and Wicke,1957).As presented in Fig.5.3 three different regimes are recognized;they are related by a different rate limiting step.In regime I,at low temperatures the deposition rate (k)is ©2003 Taylor&Francis

on a static substrate or a dynamic bulk surface, the so-called usual fluid bed technique, and the carbon growth inside a porous substrate. The phenomenological competition between the mass and heat transport phenomena and the overall chemical reactions is the basic point to consider. We can enounce two statements which are fundamental in this presentation: (i) The overall chemistry by itself is mainly related to the local conditions in particular for the surface reactions. For a given precursor, they are supposed very similar in the gas phase for both CVD and CVI processes. However with porous preforms an increase of the surface to volume ratio (S/V) is a novel parameter to take into account (Hüttinger, 1998). As shown on the simplified schema presented Fig. 5.1, the deposits will result from the competition between the rate constants of the homogeneous and heterogeneous reactions. (ii) The transport phenomena related to the hydrodynamics of fluids and to the energetic supply would be quite different. They are submitted to different scale factors related to the piece and reactor sizes which are sometimes very different (as in classical CVD) or compa￾rable: they have to be examined for each type of process (see Section 3); on a general way the kinetic deposition of pyrocarbons can be represented by an Arrhenius plot which describes the general heterogeneous reactions of gases with porous solids (Hedden and Wicke, 1957). As presented in Fig. 5.3 three different regimes are recognized; they are related by a different rate limiting step. In regime I, at low temperatures the deposition rate (k) is Figure 5.2 Cross sections of a nodule of pyrocarbon with the associated model (from Coffin, 1964): (a) nodule formed from surface asperity; (b) nodule formed from a foreign particle. (a) (b) © 2003 Taylor & Francis

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 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 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 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 (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 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. 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 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 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 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. 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 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). 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

Gas outlet Condenser Filter Refrigerator Devesiculator N2 venting Porous sample Liquid precursor Susceptor 8 Inductor Thermocouples Figure 5.7 Boiling film CVI reactor with induction heating of an internal graphite susceptor immersed in the liquid precursor (from David et al.,1995). 4 Pyrocarbon microstructures For a long time different types of deposited pyrocarbons have been recognized and well characterized at various length scales.These microstructures are on the one hand dependant on the experimental parameters and techniques and on the other they present reproducible structural and physical characteristics that we will summarize.Indeed it has been observed that typical intrinsic microstructures which are obtained are basically the same for every CVD and CVI process.We will discuss both aspects in the following section. 4.1 The different types of pyrocarbons In a pioneering work Bokros has identified the experimental conditions favoring the depo- sitions in fluid beds of anisotropic types,i.e.laminar or granular-columnar,and isotropic carbon structures(Bokros,1969).Following Bokros'work,Lieberman and Pierson in the 2003 Taylor Francis

4 Pyrocarbon microstructures For a long time different types of deposited pyrocarbons have been recognized and well characterized at various length scales. These microstructures are on the one hand dependant on the experimental parameters and techniques and on the other they present reproducible structural and physical characteristics that we will summarize. Indeed it has been observed that typical intrinsic microstructures which are obtained are basically the same for every CVD and CVI process. We will discuss both aspects in the following section. 4.1 The different types of pyrocarbons In a pioneering work Bokros has identified the experimental conditions favoring the depo￾sitions in fluid beds of anisotropic types, i.e. laminar or granular-columnar, and isotropic carbon structures (Bokros, 1969). Following Bokros’ work, Lieberman and Pierson in the Figure 5.7 Boiling film CVI reactor with induction heating of an internal graphite susceptor immersed in the liquid precursor (from David et al., 1995). Condenser Gas outlet Filter Refrigerator Devesiculator N2 venting Liquid precursor Porous sample Susceptor Inductor Thermocouples © 2003 Taylor & Francis