The regeneration of cones produced by the n-like growth features gives the smooth aspect to the laminar together with its lower anisotropy and density (Bourrat et al.,2002). 3.3.2 Regenerative rough laminar pyrocarbons (REL) In most of the processes another self-generation of cones occurs,based on a different mechanism.Contrarily to smooth laminar,the cones are generated by small layer defects: a angles measured by TEM darkfield are weak:15<<18(Fig.8.11b).The defects in concern lies at nanometric scale,as lattice defects for example.It is assumed that the layer growth mechanism change for a "lateral"mechanism and thereafter the deposit acquires a higher capability to transmit defects.This is referred to as"covering efficiency"of layers (Bourrat et al.,2002). REL pyrocarbon is formed in different processes that have been alternatively developed: e.g.pulse-CVI(Dupel et al.,1997)or related to the mother molecule,e.g.toluene(Bourrat et al.,2002)or from boron-doped process(Tombrel and Rappeneau,1965;Jacques et al., 1997).In these processes the layer diameter,L2,is systematically larger while the density keeps high.Layers are larger,highly densely packed but paradoxically the coherent lengths are smaller(e.g.pulse-CVI in Fig.8.8b).It can be assumed that the growth mechanism is mixed:layers grow following the"atom by atom"(or small species)after diffusion onto the surface,with speculative forms as phenyl radical or monocyclic aromatic in the case of toluene.The transition is not precisely known at that time. 3.3.3 Regenerative features of very high temperature laminar pyrocarbons Far from CVI conditions,deposits obtained at very high temperature exhibit the superposi- tion of primary cones and regenerative cones within the deposit(Fig.8.7b).Thus polished surface of deposit get a fractal appearance known as "cauliflower-like"texture of high temperature pyrocarbon(Fig.8.7a).The vanishing of previous cones does not occur because the size of the defects are very small and in the same time the hexagonal lattice is perfect and supple enough to transmit any defect at long distance without fading.Goma et al. (1985)have shown that layer diameters are indeed very large (Fig.8.7c).It is probably resulting from a lateral growth mechanism.The covering effect is much higher;the cones are very sharp related to very small defects in the lattice,and transmitted on long distance. 3.4 Secondary cones generated by gas-phase nucleated particles As pointed out very soon by Tesner(1984),blacks and pyrocarbon growth have to be con- sidered as competitive mechanisms but in a given domain of high pressure/temperature.This competition is well documented in the case of granular pyrocarbons for which the nucleation and growth of solid particles in the gas phase is a key step in their growth.These particles by depositing on the growing surface are responsible for the generation of cones(Fig.8.5a). The different processes,as fluidized bed or static CVD,offer many different combinations. A very wide and open transition occurs,just based on the size of the gas phase particles (improperly called soot),their density and the accessible surface,i.e.surface/volume ratio different on fluidized bed or static surface.All these combinations give rise to the many different granular types reported in the literature. ©2003 Taylor&FrancisThe regeneration of cones produced by the -like growth features gives the smooth aspect to the laminar together with its lower anisotropy and density (Bourrat et al., 2002). 3.3.2 Regenerative rough laminar pyrocarbons (REL) In most of the processes another self-generation of cones occurs, based on a different mechanism. Contrarily to smooth laminar, the cones are generated by small layer defects:  angles measured by TEM darkfield are weak: l5 18 (Fig. 8.11b). The defects in concern lies at nanometric scale, as lattice defects for example. It is assumed that the layer growth mechanism change for a “lateral” mechanism and thereafter the deposit acquires a higher capability to transmit defects. This is referred to as “covering efficiency” of layers (Bourrat et al., 2002). REL pyrocarbon is formed in different processes that have been alternatively developed: e.g. pulse-CVI (Dupel et al., 1997) or related to the mother molecule, e.g. toluene (Bourrat et al., 2002) or from boron-doped process (Tombrel and Rappeneau, 1965; Jacques et al., 1997). In these processes the layer diameter, L2, is systematically larger while the density keeps high. Layers are larger, highly densely packed but paradoxically the coherent lengths are smaller (e.g. pulse-CVI in Fig. 8.8b). It can be assumed that the growth mechanism is mixed: layers grow following the “atom by atom” (or small species) after diffusion onto the surface, with speculative forms as phenyl radical or monocyclic aromatic in the case of toluene. The transition is not precisely known at that time. 3.3.3 Regenerative features of very high temperature laminar pyrocarbons Far from CVI conditions, deposits obtained at very high temperature exhibit the superposi￾tion of primary cones and regenerative cones within the deposit (Fig. 8.7b). Thus polished surface of deposit get a fractal appearance known as “cauliflower-like” texture of high temperature pyrocarbon (Fig. 8.7a). The vanishing of previous cones does not occur because the size of the defects are very small and in the same time the hexagonal lattice is perfect and supple enough to transmit any defect at long distance without fading. Goma et al. (1985) have shown that layer diameters are indeed very large (Fig. 8.7c). It is probably resulting from a lateral growth mechanism. The covering effect is much higher; the cones are very sharp related to very small defects in the lattice, and transmitted on long distance. 3.4 Secondary cones generated by gas-phase nucleated particles As pointed out very soon by Tesner (1984), blacks and pyrocarbon growth have to be con￾sidered as competitive mechanisms but in a given domain of high pressure/temperature. This competition is well documented in the case of granular pyrocarbons for which the nucleation and growth of solid particles in the gas phase is a key step in their growth. These particles by depositing on the growing surface are responsible for the generation of cones (Fig. 8.5a). The different processes, as fluidized bed or static CVD, offer many different combinations. A very wide and open transition occurs, just based on the size of the gas phase particles (improperly called soot), their density and the accessible surface, i.e. surface/volume ratio different on fluidized bed or static surface. All these combinations give rise to the many different granular types reported in the literature. © 2003 Taylor & Francis