of failure(pseudoplastic behavior)leading to the relative movement of the two phases as the load is transferred to the fibers.However,composite from pitch with 11 wt%of QI(Fig.7.8a) failed in a catastrophic way (brittle behavior).In this case,the stronger fiber-matrix bonding increases the transfer of the load to the fiber,which in turn increases the overall strength of the composite,causing the composite to fail in a catastrophic-like tensile fracture(Fig.7.8a). As an improvement in the strength of the material is accompanied by an increase in its brittleness,possible ways to overcome this problem could be the use of matrix precursors prepared from two individual components in order to obtain a blend which gives on pyroly- sis a carbon with a combined optical texture,yielding both strength and capability of fracture propagation.The mechanical properties of the composite could also be improved by using the appropriate fiber,depending on the type of carbon matrix. 5 New developments in C-C composites Alternatives to the use of pressure and extended impregnation/carbonization cycles for preparing high density composites are the new developments in high carbon yield pitch- based matrix precursors or the application of oxidative stabilization treatments at the prepreg stage. Some pitches can be pre-treated to induce the polymerization of the components of the pitch in order to obtain high density C-C composites (in the range of 1.6-1.8gcm),with- out the need for applying further impregnation/carbonization cycles or reducing the number of those currently applied.Densities may be even higher than the above mentioned values when using blends of thermally treated pitch powder and phenolic resin as matrix precursor of unidirectional C-C composites (Tsushima et al.,1993).In this way,the use of pressure, which substantially increases the price of the composite,can be avoided.However,an increase in carbon yield is the result of an increased content in high molecular weight com- pounds,which may lead to greater viscosity.The new pitches can be expected to have a high beta resin (toluene insolubles/quinoline solubles)content.This entails large molecules that do not distil on carbonization,thereby giving rise to high carbon yields but a low enough viscosity for the penetration and wetting of the carbon substrate.The use of thermal treat- ment to remove volatiles and promote dehydrogenative polymerization reactions,either individually or combined with coking accelerators,such as sulfur(Fernandez et al.,1998; Oh and Park,1999)or AlCl3 (Mochida et al.,1985)has been tested mainly at laboratory scale.However,in recent years considerable attention has been given to air-blowing,because of its effectiveness and for economic reasons.The oxygen in the air acts as a polymerization promoter,increasing the molecular size of light compounds through dehydrogenative poly- merization reactions (Barr and Lewis,1978;Zeng et al.,1993;Fernandez et al.,1995),thus preventing their distillation and removal during the carbonization stage.The result is an increase in viscosity and a more disordered orientation of the lamellar aromatic molecules, limiting the growth and coalescence of mesophase,but still giving graphitizable or partially graphitizable carbon.With air-blown impregnating pitches(250C,18h)and AS4k carbon fibers,unidirectional composites of bulk densities up to 1.59-1.60 gcm3 were achieved without any further densification.Composites showed high flexural strength,but fiber-matrix bonding was too strong,leading to a brittle failure.A possible way to overcome this problem could be the use of untreated fibers which yield weaker fiber-matrix bonding. Other recent developments,designed to reduce costs by avoiding the pre-treatment step, include the direct oxidative stabilization of impregnated carbon preforms or pitch-based ©2003 Taylor&Francisof failure (pseudoplastic behavior) leading to the relative movement of the two phases as the load is transferred to the fibers. However, composite from pitch with 11wt% of QI (Fig. 7.8a) failed in a catastrophic way (brittle behavior). In this case, the stronger fiber– matrix bonding increases the transfer of the load to the fiber, which in turn increases the overall strength of the composite, causing the composite to fail in a catastrophic-like tensile fracture (Fig. 7.8a). As an improvement in the strength of the material is accompanied by an increase in its brittleness, possible ways to overcome this problem could be the use of matrix precursors prepared from two individual components in order to obtain a blend which gives on pyroly￾sis a carbon with a combined optical texture, yielding both strength and capability of fracture propagation. The mechanical properties of the composite could also be improved by using the appropriate fiber, depending on the type of carbon matrix. 5 New developments in C–C composites Alternatives to the use of pressure and extended impregnation/carbonization cycles for preparing high density composites are the new developments in high carbon yield pitch￾based matrix precursors or the application of oxidative stabilization treatments at the prepreg stage. Some pitches can be pre-treated to induce the polymerization of the components of the pitch in order to obtain high density C–C composites (in the range of 1.6–1.8 g cm3 ), with￾out the need for applying further impregnation/carbonization cycles or reducing the number of those currently applied. Densities may be even higher than the above mentioned values when using blends of thermally treated pitch powder and phenolic resin as matrix precursor of unidirectional C–C composites (Tsushima et al., 1993). In this way, the use of pressure, which substantially increases the price of the composite, can be avoided. However, an increase in carbon yield is the result of an increased content in high molecular weight com￾pounds, which may lead to greater viscosity. The new pitches can be expected to have a high beta resin (toluene insolubles/quinoline solubles) content. This entails large molecules that do not distil on carbonization, thereby giving rise to high carbon yields but a low enough viscosity for the penetration and wetting of the carbon substrate. The use of thermal treat￾ment to remove volatiles and promote dehydrogenative polymerization reactions, either individually or combined with coking accelerators, such as sulfur (Fernández et al., 1998; Oh and Park, 1999) or AlCl3 (Mochida et al., 1985) has been tested mainly at laboratory scale. However, in recent years considerable attention has been given to air-blowing, because of its effectiveness and for economic reasons. The oxygen in the air acts as a polymerization promoter, increasing the molecular size of light compounds through dehydrogenative poly￾merization reactions (Barr and Lewis, 1978; Zeng et al., 1993; Fernández et al., 1995), thus preventing their distillation and removal during the carbonization stage. The result is an increase in viscosity and a more disordered orientation of the lamellar aromatic molecules, limiting the growth and coalescence of mesophase, but still giving graphitizable or partially graphitizable carbon. With air-blown impregnating pitches (250 C, 18 h) and AS4k carbon fibers, unidirectional composites of bulk densities up to 1.59–1.60 g cm3 were achieved without any further densification. Composites showed high flexural strength, but fiber–matrix bonding was too strong, leading to a brittle failure. A possible way to overcome this problem could be the use of untreated fibers which yield weaker fiber–matrix bonding. Other recent developments, designed to reduce costs by avoiding the pre-treatment step, include the direct oxidative stabilization of impregnated carbon preforms or pitch-based © 2003 Taylor & Francis