Mosaics Domains 10um 10μm (d) Mosaics Domains 5um 15μm Figure 7.7 Pitch-based unidirectional C-C composites exhibiting different (a)-(b)optical textures; and (c)-(d)SEM fracture surfaces.(See Color Plate V.) was observed for those containing basic-nitrogen compounds.However,the presence of oxygen functional groups (phenolic,carbonyl,etc.)has a negative effect on the optical texture (Menendez et al.,1997),generating small size textures and even destroying pitch graphitizability when occurring in high concentrations.Primary QI,present in coal-tar pitches,also yield optical textures of a small size(Fig.7.7).These differences are related to the thermal reactivity of the compounds and the viscosity of the pyrolysis system which addresses the development of mesophase and finally the carbon microstructure.Moreover, for a given pitch the optical texture of the resultant carbon can be modified by variations in the carbonization conditions(heating rate,use of pressure). When dealing with C-C composites,the presence and characteristics of the fiber through fiber-matrix interfacial effects also yield preferential orientations of the matrix with respect to the fiber (Appleyard et al.,1995)as shown in Fig.7.7. The wide range of optical textures which can be generated is of special relevance to the properties of the C-C composites.Large-size optical textures are currently associated in coke with high electrical conductivity,while small sizes are associated with improved mechanical strength.However,recent studies have shown that domain-like structures(from petroleum pitches)can also give high strength carbon materials(Perez et al.,2000).This suggests that other factors related to packing or interlaminar bonding/interactions also play a role.But it is not only the microstructure of the matrix,but also the strength of the fiber-matrix bonding that is critical in the behavior of the C-C composite under mechani- cal loading,and this can be influenced by the optical texture of the matrix.Matrices with a dominant mosaic texture exhibited stronger bonding with AS4k fibers than those with domains.Whereas the former resulted in a pure brittle failure mode(Fig.7.7c),the latter ©2003 Taylor&Franciswas observed for those containing basic-nitrogen compounds. However, the presence of oxygen functional groups (phenolic, carbonyl, etc.) has a negative effect on the optical texture (Menéndez et al., 1997), generating small size textures and even destroying pitch graphitizability when occurring in high concentrations. Primary QI, present in coal-tar pitches, also yield optical textures of a small size (Fig. 7.7). These differences are related to the thermal reactivity of the compounds and the viscosity of the pyrolysis system which addresses the development of mesophase and finally the carbon microstructure. Moreover, for a given pitch the optical texture of the resultant carbon can be modified by variations in the carbonization conditions (heating rate, use of pressure). When dealing with C–C composites, the presence and characteristics of the fiber through fiber–matrix interfacial effects also yield preferential orientations of the matrix with respect to the fiber (Appleyard et al., 1995) as shown in Fig. 7.7. The wide range of optical textures which can be generated is of special relevance to the properties of the C–C composites. Large-size optical textures are currently associated in coke with high electrical conductivity, while small sizes are associated with improved mechanical strength. However, recent studies have shown that domain-like structures (from petroleum pitches) can also give high strength carbon materials (Pérez et al., 2000). This suggests that other factors related to packing or interlaminar bonding/interactions also play a role. But it is not only the microstructure of the matrix, but also the strength of the fiber–matrix bonding that is critical in the behavior of the C–C composite under mechani￾cal loading, and this can be influenced by the optical texture of the matrix. Matrices with a dominant mosaic texture exhibited stronger bonding with AS4k fibers than those with domains. Whereas the former resulted in a pure brittle failure mode (Fig. 7.7c), the latter Figure 7.7 Pitch-based unidirectional C–C composites exhibiting different (a)–(b) optical textures; and (c)–(d) SEM fracture surfaces. (See Color Plate V.) 10 µm 10 µm Mosaics Domains Mosaics Domains 5 µm 15 µm (a) (c) (d) (b) © 2003 Taylor & Francis