R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 and 320 X 2000 um2)machined in purified graphite, conventional CVI [1]. It also depends upon an easily (ii)single SiC fibers(Nicalon NLM-202 or Hi-Nica- adjustable parameter, the residence time, fg.Further, lon fibers, 12-15 um in diameter, from Nippon tg also controls the maturation of the gas phase, i.e arbon, Tokyo), attached to rectangular SiC holder, the formation of intermediate species, and, hence for P-CVD experiments, (i single carbon (T300 the microtexture of the pyrocarbon deposited. The from Toray, Tokyo) or SiC (Nicalon or Hi-Nicalon) influence of T, P and Ig on pyrocarbon deposition fiber tows, also attached to carbon or SiC holders, was studied through experiments performed on the for the preparation of model ' minicompositesand model pores finally, (iv) real carbon or SiC fiber preforms dis- The thickness profile of pyrocarbon deposited chitecture with an initial open porosity of 602%er ar- from propane along the 60-um pore(drawn for a playing a two(or pseudo-two)-dimensional fibe time corresponding to pore entrance blocking) de- Carbon was deposited from C,H, hydrocarbons pends on fg(Fig 2a). The related pore filling ratio 8 ainly propane but methane, propene or benzene, (calculated by integrating the thickness profile along have also been used for the purpose of comparison) the pore length and dividing the result by half the at P=1-10 kPa, T=900-1100@C and Ig=0.2-60 initial pore volume) goes through a minimum for s. Silicon carbide was deposited from MTS-H, at intermediate tg values(Fig 2b). The highest pore 900-1000C and IR=1-10 S In filling is achieved for the lowest residence times, i.e order to form smooth deposits, i.e. deposits with a when the gas phase undergoes little maturation, py nanocrystalline microstructure, SiC in multilayered rocarbon being formed from small species, which ceramics, was codeposited with 3% carbon(to penetrate easily the pore. Conversely, for intermedi prevent SiC grain growth), with a=H2/MTS ate IR values, aromatic species are thought to be formed in the gas phase, which are preferentially precursor, at P=2-5 kPa, T=900-1000C, IR adsorbed on the pore wall near the pore entrance, 0. 1-5 s and B=NH3/BCl3=1. 2. For the deposi- resulting in an early pore sealing and a low filling tion of multilayers, temperature was maintained con- ratio. For high tg values corresponding to an impor stant and the thickness of the layers controlled tant gas phase maturation, the amount of aromatic through the residence times and the numbers and species is thought to decrease (gas phase reactant durations of pressure pulses depletion and hydrogen concentration increase)to he deposits were characterized according to vari- the benefit of smaller species diffusing readily along ous techniques including, electron probe microanaly the pore and yielding again a relatively high pore sis(EPMA), Auger electron spectroscopy (AES), filling. Pore filling ratio increases as P decreases,a X-ray diffraction (XRD), transmission and scanning pressure of about I kPa being necessary to com electron microscopies(TEM and SEM). The anisot- pletely fill the 60-um pore at T=950C for t=10 ropy of the pyrocarbon was assessed by optical S Increasing T favors pore entrance plugging partic- microscopy in polarized light and the so-called ex- ularly when the pore size is small. An optimum tinction angle, Ae, measured. Pyrocarbon was re- temperature seems to be about 950C. Finally, the smooth laminar (SL) for 120<A<18, dark lami- ness profile and filling ratio, when propane(or nar(DL)for 4<A< 12 and, finally, isotropic(d propylene) is replaced by methane [13] when a<4°[14 The anisotropy of pyrocarbon deposited along a pore can be controlled by playing with the T, P and tR parameters, I.e. with the maturation of the gas 3. Results and discussion 3.1. P-CV as a way to control pyrocarbon microt and P=3 kPa, pyrocarbon is deposited along the ture pore: (i) with a homogeneous microtexture, either poorly anisotropic (SL/DL; A=12%) or highly The PyC densification of a pore by P-cvi de- anisotropic (RL, Ae=20%), for IR=0.5 and 10 S, pends on P and T, as previously reported, for respectively, or (ii) with an anisotropy gradient foR.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) 543 2 and 320=2000 mm machined in purified graphite, . Ž. Ž ii single SiC fibers Nicalon NLM-202 or Hi-Nica￾lon fibers, f12–15 mm in diameter, from Nippon Carbon, Tokyo , attached to rectangular SiC holder, . for P-CVD experiments, iii single carbon T300 Ž. Ž from Toray, Tokyo or SiC Nicalon or Hi-Nicalon .Ž . fiber tows, also attached to carbon or SiC holders, for the preparation of model ‘minicomposites’ and, finally, iv real carbon or SiC fiber preforms dis- Ž . playing a two or pseudo-two -dimensional fiber ar- Ž . chitecture with an initial open porosity of 60%. Carbon was deposited from C H hydrocarbons x y Žmainly propane but methane, propene or benzene, have also been used for the purpose of comparison. at Ps1–10 kPa, Ts900–11008C and tRs0.2–60 s. Silicon carbide was deposited from MTS–H at 2 Ps1–5 kPa, Ts900–10008C and tRs1–10 s. In order to form smooth deposits, i.e. deposits with a nanocrystalline microstructure, SiC in multilayered ceramics, was codeposited with f3% carbon toŽ prevent SiC grain growth , i.e. with . asH2rMTS -1. Finally, BN was deposited from BCl –NH 3 3 precursor, at Ps2–5 kPa, Ts900–10008C, tRs 0.1–5 s and bsNH rBCl s1.2. For the deposi- 3 3 tion of multilayers, temperature was maintained con￾stant and the thickness of the layers controlled through the residence times and the numbers and durations of pressure pulses. The deposits were characterized according to vari￾ous techniques including, electron probe microanaly￾sis EPMA , Auger electron spectroscopy AES , Ž . Ž. X-ray diffraction XRD , transmission and scanning Ž . electron microscopies TEM and SEM . The anisot- Ž . ropy of the pyrocarbon was assessed by optical microscopy in polarized light and the so-called ex￾tinction angle, A , measured. Pyrocarbon was re- e ferred to as rough laminar RL when Ž . A )188, e smooth laminar SL for 12 Ž . 8-A -188, dark lami- e nar DL for 4 Ž . Ž. 8-A -128 and, finally, isotropic I e when A -48 w x 14 . e 3. Results and discussion 3.1. P-CVI as a way to control pyrocarbon microtex￾ture The PyC densification of a pore by P-CVI de￾pends on P and T, as previously reported, for conventional CVI 1 . It also depends upon an easily w x adjustable parameter, the residence time, t . Further, R t also controls the maturation of the gas phase, i.e. R the formation of intermediate species, and, hence, the microtexture of the pyrocarbon deposited. The influence of T, P and tR on pyrocarbon deposition was studied through experiments performed on the model pores. The thickness profile of pyrocarbon deposited from propane along the 60-mm pore drawn for a Ž time corresponding to pore entrance blocking de- . pends on tR Ž . Fig. 2a . The related pore filling ratio d Žcalculated by integrating the thickness profile along the pore length and dividing the result by half the initial pore volume goes through a minimum for . intermediate t values Fig. 2b . The highest pore Ž . R filling is achieved for the lowest residence times, i.e. when the gas phase undergoes little maturation, py￾rocarbon being formed from small species, which penetrate easily the pore. Conversely, for intermedi￾ate t values, aromatic species are thought to be R formed in the gas phase, which are preferentially adsorbed on the pore wall near the pore entrance, resulting in an early pore sealing and a low filling ratio. For high t values corresponding to an impor- R tant gas phase maturation, the amount of aromatic species is thought to decrease gas phase reactant Ž depletion and hydrogen concentration increase to . the benefit of smaller species diffusing readily along the pore and yielding again a relatively high pore filling. Pore filling ratio increases as P decreases, a pressure of about 1 kPa being necessary to com￾pletely fill the 60-mm pore at Ts9508C for tRs10 s. Increasing T favors pore entrance plugging partic￾ularly when the pore size is small. An optimum temperature seems to be about 9508C. Finally, the quality of the infiltration is better, in terms of thick￾ness profile and filling ratio, when propane or Ž propylene is replaced by methane 13 . . w x The anisotropy of pyrocarbon deposited along a pore can be controlled by playing with the T, P and t parameters, i.e. with the maturation of the gas R phase during the holding step Fig. 3 . At Ž . Ts9508C and Ps3 kPa, pyrocarbon is deposited along the pore: i with a homogeneous microtexture, either Ž . poorly anisotropic SL Ž . rDL; Aes128 or highly anisotropic RL; Ž . A s208 , for t s0.5 and 10 s, e R respectively, or ii with an anisotropy gradient for Ž