Influence of lsothermal CVd and CV Conditions on the Deposition Kinetics and Structure of Bn 0.30 0.15兰 1.0 051015202530354045555 Total now rate (I0"*kg/s) Fig 3. Influence of the total mass flow rate on the relative weight o deposit(Am/m)on the bulk substrate via CVd and in the internal and ternal portions of the fibrous preform via pre 1.33 kPa, NH / BCI,= 1, and H/BCI,= 1) ematic of the diffusion es with external convective noted as“l”) and pure diffusion( denoted as“2”) to the fibrous rent domain along the c-axis of hexagonal bn. The greatest herent domain is the domain with the area is defined as La x le). For the interlayer distance, a mini- mum of five measurements was made on each image. the between the lowest and the in the figures. as well as rsion. Well crystallized graphite was used as a to measure the interlayer distance. The maximal error, from the procedures that have been used for our data acquisition, image treatments and final measurements. was -0.03 nm 0510520253344555560 Total now rate (10 x kg/s) IIL. Results Variation of the uniformities with the total mass flow rate on Deposition Kinetics bulk substrate U and in the preforms(■)LU,●)IU,and(★) protected-ICVI The influence of the deposition temperature on the do tion rate in the internal and external portions of the preform (CVI)and on the graphite rings(CVD)already has been re especially slightly greater uniformities are obtained by using ported for equal partial pressures of BCl3, NH3, and H, and a he protected-ICVI process, in comparison to classical ICvI total mass flow rate of 7 x 106 kg/s 30, 31 The deposition rates (see Figs. 4 and 5) in the preform are dependent exponentially on the inverse of High flow rates lead to deposition conditions where the CVd process is only controlled by the surface kinetics; thus, the they stabilize and even decrease in the internal portion, thus CVD deposition mechanism can be studied and the deposition indicating a mass-transfer limitation, which leads to poor in conditions in the ICVI process can be compared. The nil order filtration uniformity (IU 65% and LU = 45% at 800C. with respect to argon, allows both the total flow rate and the instead of 80% and 70%, respectively, at 700oC). Such a mass transfer limitation was not observed in CVD. However, the sure of a species was changed, to determine the apparent re- following study was performed at 700oC to preserve fairly action order with respect to this species. 0 Under these condi- good deposit uniformities via ICVI tions, the residence time of the gaseous phase within the reactor The influence of the mass-flow -rate variations on the cvd was maintained constant. That point could be important if some deposition rate on the graphite rings shows a transition in the homogeneous intermediate reaction occurs. On the other hand process from a mass-transfer and chemical-kinetics control to a a constant total pressure allows restriction of the variations pure chemical kinetic limitation(Fig. 3). At flow rates greater the diffusion conditions to the compositional effect to the kinetics at the surface of the bulk substrates is shown either by product HCl, was observed to be zero. 30, 3I Moreover, HCI has a weak influence on the bn deposition rate in the preform; this high value(98%)of the longitudinal uniformity(Fig. 4). The result is certainly due to similar mass-transfer conditions of the protected-ICVI process seems to be al ways governed by a cor reactant species(the diffusion coefficient of argon and HCI are bination of mass transfer and chemical kinetics, because of the not very different). The experimental results that concern the high specific surface area of the preform, which adsorbs a large reactants NH3 and BCl3 permit the determination of two dis- amount of reactants(Fig. 3). a slightly lower growth rate and tinct domains in either case, as shown, for example, in Fig. 6.herent domain along the c-axis of hexagonal BN. The greatest coherent domain is the domain with the greatest surface (the area is defined as La × Lc). For the interlayer distance, a mini￾mum of five measurements was made on each image. The average between the lowest and the greatest interlayer value is depicted in the figures, as well as the dispersion. Well￾crystallized graphite was used as a standard, to measure the interlayer distance. The maximal error, from the procedures that have been used for our data acquisition, image treatments, and final measurements, was ∼0.03 nm. III. Results (1) Deposition Kinetics The influence of the deposition temperature on the deposi￾tion rate in the internal and external portions of the preform (ICVI) and on the graphite rings (CVD) already has been re￾ported for equal partial pressures of BCl3, NH3, and H2 and a total mass flow rate of 7 × 10−6 kg/s.30,31 The deposition rates in the preform are dependent exponentially on the inverse of temperature in the 500°–700°C range. At higher temperatures, they stabilize and even decrease in the internal portion, thus indicating a mass-transfer limitation, which leads to poor in￾filtration uniformity (IU 4 65% and LU 4 45% at 800°C, instead of 80% and 70%, respectively, at 700°C). Such a mass￾transfer limitation was not observed in CVD. However, the following study was performed at 700°C to preserve fairly good deposit uniformities via ICVI. The influence of the mass-flow-rate variations on the CVD deposition rate on the graphite rings shows a transition in the process from a mass-transfer and chemical-kinetics control to a pure chemical kinetic limitation (Fig. 3). At flow rates greater than ∼25 × 10−6 kg/s, the main control by the heterogeneous kinetics at the surface of the bulk substrates is shown either by the negligible influence of the total flow rate or by the very high value (98%) of the longitudinal uniformity (Fig. 4). The protected-ICVI process seems to be always governed by a com￾bination of mass transfer and chemical kinetics, because of the high specific surface area of the preform, which adsorbs a large amount of reactants (Fig. 3). A slightly lower growth rate and especially slightly greater uniformities are obtained by using the protected-ICVI process, in comparison to classical ICVI (see Figs. 4 and 5). High flow rates lead to deposition conditions where the CVD process is only controlled by the surface kinetics; thus, the CVD deposition mechanism can be studied and the deposition conditions in the ICVI process can be compared. The nil order, with respect to argon, allows both the total flow rate and the total pressure to be maintained constant while the partial pres￾sure of a species was changed, to determine the apparent re￾action order with respect to this species.30 Under these condi￾tions, the residence time of the gaseous phase within the reactor was maintained constant. That point could be important if some homogeneous intermediate reaction occurs. On the other hand, a constant total pressure allows restriction of the variations of the diffusion conditions to the compositional effect. The apparent reaction order, with respect to the gaseous product HCl, was observed to be zero.30,31 Moreover, HCl has a weak influence on the BN deposition rate in the preform; this result is certainly due to similar mass-transfer conditions of the reactant species (the diffusion coefficient of argon and HCl are not very different). The experimental results that concern the reactants NH3 and BCl3 permit the determination of two dis￾tinct domains in either case, as shown, for example, in Fig. 6, Fig. 2. Schematic of the diffusion cages with external convective flow (denoted as “1”) and pure diffusion (denoted as “2”) to the fibrous preform. Fig. 3. Influence of the total mass flow rate on the relative weight of deposit (Dm/m) on the bulk substrate via CVD and in the internal and external portions of the fibrous preform via protected-ICVI (T 4 700°C, P 4 1.33 kPa, NH3/BCl3 4 1, and H2/BCl3 4 1). Fig. 4. Variation of the uniformities with the total mass flow rate on (m) the bulk substrate U and in the preforms (j) LU, (d) IU, and (.) TU for protected-ICVI. May 1999 Influence of Isothermal CVD and CVI Conditions on the Deposition Kinetics and Structure of BN 1189