4.2.2 From pitch to carbon matrix The processing of pitch-based composites involves a carbonization step(650-1,000C) which transforms the pitch into a graphitizable carbon,through a liquid crystal stage called mesophase.The conditions used are strongly dependent on the chemical composition and the rheological behavior of the pitch. All pitches,petroleum-and coal-based have in common the fact that their constituents are polycyclic aromatic compounds.But,their differences in molecular structure lead to differ- ent behaviors during carbonization(Perez et al.,2000),thereby influencing the properties of the resultant composite. In the initial stages of carbonization,the lightest compounds are released and the remain- ing polycyclic aromatic compounds polymerize and condense.The generation of gases,as a consequence of polymerization reactions,and the dimensional shrinkage that accompanies pitch carbonization,leads to the development of open and closed porosity in the composites. The improvement of the density and mechanical properties of the composites requires the elimination or reduction of this porosity by subsequent liquid impregnation (or CVD) followed by carbonization(Section 3 of this chapter). A knowledge of the temperatures at which all the physico-chemical changes involved in pitch carbonization occur is of great importance for the selection of an adequate precursor and optimum conditions for the preparation of a pitch-based C-C composite.The main fac- tors that must be borne in mind are the temperature of volatiles removal on pitch pyrolysis (Bermejo et al.,1994),viscosity/temperature history (Rand et al.,1989),and temperature interval between mesophase development and solidification,all of which can be monitored by thermogravimetric analysis,rheometry,and hot-stage microscopy,respectively.The information obtained is useful even if the experimental conditions used in the preparation of a C-C composite are rather different.For example,a reduction in the porosity (as deter- mined by optical microscopy)of undensified unidirectional C-C composites,prepared by a wet-winding procedure,from 12 to 4 vol%,was achieved by the adjustment of the opera- tional parameters (heating rate,molding temperature,duration of molding)to the character- istics of the binder pitch that was used as matrix precursor(Casal et al.,1998). Carbon yields of commercial coal-tar pitches are about 50 wt%under atmospheric con- ditions,but these can be substantially increased to 80 wt%by reducing the carbonization heating rate or by using pressure.Carbonization under high pressure (100 MPa)results in yields of 90 wt%.The use of a pressure of up to 207 MPa reduces the temperature associ- ated with thermal degradation and improves the carbon yield by reducing the loss of volatiles.McAllister and Lachman (1983)have shown that high pressure impregnation/ carbonization of multidirectional fiber preforms with pitch increases the yield and density of the final composite.After six cycles of pitch impregnation/carbonization under pressure, a composite of about 1.9 gcm3 was obtained. 4.2.3 The optical texture of the matrix The morphology,size,and orientation of the microcrystalline structures which constitute the optical texture of the carbon matrix can differ greatly depending on the composition of the pitch.As shown in Fig.7.7,they can vary from a very small size (<10um),mosaic-like structures,to large size (>60 um)domains (Marsh and Latham,1986).Pitches which contain compounds with a higher capacity of hydrogen transfer,i.e.hydroaromatics and naphthenics,tend to produce better ordered structures of a larger size.The same tendency ©2003 Taylor&Francis4.2.2 From pitch to carbon matrix The processing of pitch-based composites involves a carbonization step (650–1,000 C) which transforms the pitch into a graphitizable carbon, through a liquid crystal stage called mesophase. The conditions used are strongly dependent on the chemical composition and the rheological behavior of the pitch. All pitches, petroleum- and coal-based have in common the fact that their constituents are polycyclic aromatic compounds. But, their differences in molecular structure lead to differ￾ent behaviors during carbonization (Pérez et al., 2000), thereby influencing the properties of the resultant composite. In the initial stages of carbonization, the lightest compounds are released and the remain￾ing polycyclic aromatic compounds polymerize and condense. The generation of gases, as a consequence of polymerization reactions, and the dimensional shrinkage that accompanies pitch carbonization, leads to the development of open and closed porosity in the composites. The improvement of the density and mechanical properties of the composites requires the elimination or reduction of this porosity by subsequent liquid impregnation (or CVI) followed by carbonization (Section 3 of this chapter). A knowledge of the temperatures at which all the physico-chemical changes involved in pitch carbonization occur is of great importance for the selection of an adequate precursor and optimum conditions for the preparation of a pitch-based C–C composite. The main fac￾tors that must be borne in mind are the temperature of volatiles removal on pitch pyrolysis (Bermejo et al., 1994), viscosity/temperature history (Rand et al., 1989), and temperature interval between mesophase development and solidification, all of which can be monitored by thermogravimetric analysis, rheometry, and hot-stage microscopy, respectively. The information obtained is useful even if the experimental conditions used in the preparation of a C–C composite are rather different. For example, a reduction in the porosity (as deter￾mined by optical microscopy) of undensified unidirectional C–C composites, prepared by a wet-winding procedure, from 12 to 4 vol %, was achieved by the adjustment of the opera￾tional parameters (heating rate, molding temperature, duration of molding) to the character￾istics of the binder pitch that was used as matrix precursor (Casal et al., 1998). Carbon yields of commercial coal-tar pitches are about 50 wt % under atmospheric con￾ditions, but these can be substantially increased to 80 wt % by reducing the carbonization heating rate or by using pressure. Carbonization under high pressure (100MPa) results in yields of 90 wt %. The use of a pressure of up to 207MPa reduces the temperature associ￾ated with thermal degradation and improves the carbon yield by reducing the loss of volatiles. McAllister and Lachman (1983) have shown that high pressure impregnation/ carbonization of multidirectional fiber preforms with pitch increases the yield and density of the final composite. After six cycles of pitch impregnation/carbonization under pressure, a composite of about 1.9 g cm3 was obtained. 4.2.3 The optical texture of the matrix The morphology, size, and orientation of the microcrystalline structures which constitute the optical texture of the carbon matrix can differ greatly depending on the composition of the pitch. As shown in Fig. 7.7, they can vary from a very small size (10m), mosaic-like structures, to large size (60m) domains (Marsh and Latham, 1986). Pitches which contain compounds with a higher capacity of hydrogen transfer, i.e. hydroaromatics and naphthenics, tend to produce better ordered structures of a larger size. The same tendency © 2003 Taylor & Francis