R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 0.5s 15 0123 distance (mm) ng. 2. Densification of a model pore(60 x 2000 um2 in cross section; 20 mm in length) open at both ends by P-CVI from propane at 050%C and P=l kPa:(a)pyrocarbon thickness profiles along the pore length for tR=0.5, 5 and 60 s, (b)pore filling ratio as a function of Ig, according to Ref. [13]. fR=60 s. These features could tentatively be ex- deposit might be low molecular weight hydrogenated plained as follows. When residence time is very radicals, yielding a poorly organized pyrocarbon short, the maturation process is very limited and (SL/DL). When fg is raised, the maturation process the species in the gas phase responsible for the is more significant and the pyrocarbon highly anisotropic(RL). TEM data show that the size of the carbon layers is relatively large, i.e. 1.9 nm as a mean at pore center(with some layers as large as 4 Ingesting an echanism from gaseous species still limited in size(to diffuse far from pore entrance before reacting). Finally long residence times, the increase in hydrogen con- centration at pore center, might again favor the formation of hydrogenated species of relatively small molecular weights, yielding pyrocarbon of lower ani sotropy(Ae=16). Further, the anisotropy and oxi dation resistance of pyrocarbon can be improved by doping the gas phase with a boron-bearing gaseous 0.5s species [18] 0 1 2 34 5 6 78 9 10 3.2. P-Cl as a way to tailor multilayered inter- depth(mm) Anisotropy of pyrocarbon deposited along a 60-um model ore(with rectangular cross section) from propane at T=950/C and P=3 kPa for different residence times (Ig=0.5, 10 and 60 From a mechanical standpoint, anisotropic pyr s). The anisotropy is expressed in terms of extinction angle, A rbon is probably the best FM interphase for non- oxide CMCs [2-5]. Unfortunately, it is oxidation544 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) Ž 2 Fig. 2. Densification of a model pore 60=2000 mm in cross section; 20 mm in length open at both ends by P-CVI from propane at . Ts10508C and Ps1 kPa: a pyrocarbon thickness profiles along the pore length for Ž . t s0.5, 5 and 60 s, b pore filling ratio as a Ž . R function of t , according to Ref. 13 . w x R t s60 s. These features could tentatively be ex- R plained as follows. When residence time is very short, the maturation process is very limited and the species in the gas phase responsible for the Fig. 3. Anisotropy of pyrocarbon deposited along a 60-mm model pore with rectangular cross section from propane at Ž . Ts9508C and Ps3 kPa for different residence times ŽtRs0.5, 10 and 60 s . The anisotropy is expressed in terms of extinction angle, . Ae w x 14 . deposit might be low molecular weight hydrogenated radicals, yielding a poorly organized pyrocarbon Ž . SLrDL . When t is raised, the maturation process R is more significant and the pyrocarbon highly anisotropic RL . TEM data show that the size of the Ž . carbon layers is relatively large, i.e. 1.9 nm as a mean at pore center with some layers as large as 4 Ž nm , suggesting an in situ lateral growth mechanism . from gaseous species still limited in size to diffuse Ž far from pore entrance before reacting . Finally, for . long residence times, the increase in hydrogen con￾centration at pore center, might again favor the formation of hydrogenated species of relatively small molecular weights, yielding pyrocarbon of lower ani￾sotropy Ž . A s168 . Further, the anisotropy and oxi- e dation resistance of pyrocarbon can be improved by doping the gas phase with a boron-bearing gaseous species 18 . w x 3.2. P-CVI as a way to tailor multilayered inter￾phases From a mechanical standpoint, anisotropic pyro￾carbon is probably the best FM interphase for non￾oxide CMCs 2–5 . Unfortunately, it is oxidation w x