Influence of Isothermal CVD and CV Conditions on the Deposition Kinetics and Structure of BN 1193 5E8 040 0.35 r=14x106ky 045 F do 22 6 0 f-7x10- kgs 180 nm 0.40 150200250:00350400 Position from the fiber surface Fig. 12. of the greatest coherent domain and the interlayer Fig. 13. Evolution of the greatest coherent domain and the interlayer nickness. as a function of the mass flow rate for the 6.2 specimen referer taken in the middle of the fibrous preforms( the preform for the protected-ICVI process. Le and La are shadd n in the distance in the bn thickness. as a function of the locatio ess. Le and La are shaded gray and white, respec- f=7×10kg/s) NH3 2N+EH2 K Total supersaturation of the gas phase at the gas/solid inter- face could be defined, as a function of the actual local partial ressures in the gaseous reactants and product, by using the The supersaturation in boron (2B)and nitrogen(2N)can be obtained when the actual partial pressures are compared to the boron and nitrogen partial pressures that are in equilibrium with solid BN at a given temperature(pB and pN) (pHcu'PBd (3) Thermodynamic calculations showed that the yield of reac- tion(1)was always very high at 700oC (99%, with respect to the reactant BCla or NHa, whichever had the lowest partial ressure).30 Therefore, one also could say that the del 阳唱唱 from equilibrium or supersaturation is high (or low) and that of completion of the reactants conversion is high(or low ). In the same manner. supersaturation with respect to the (10) elements of the bn solid can be defined if we assume a local uilibrium state in the supersaturated gas phase at the gas/ ace. which allows determination of the actual boron BCL,+3H,2B+3HCI bEN=K (12) K'PBCI (PH )3/2 where K= KBKN. These relations are obviously also related to he degree of completion of the reactants conversion. With regard to the CVD process and the structure of thewith Kp = ~pHCl eq ! 3 pBCl3 eq pNH3 eq (2) Total supersaturation of the gas phase at the gas/solid inter￾face could be defined, as a function of the actual local partial pressures in the gaseous reactants and product, by using the relation34 St = pBCl3 pNH3 ~pHCl eq ! 3 ~pHCl! 3 pBCl3 eq pNH3 eq (3) Thermodynamic calculations showed that the yield of reac￾tion (1) was always very high at 700°C (>99%, with respect to the reactant BCl3 or NH3, whichever had the lowest partial pressure).30 Therefore, one also could say that the departure from equilibrium or supersaturation is high (or low) and that the degree of completion of the reactants conversion is high (or low). In the same manner, supersaturation with respect to the elements of the BN solid can be defined if we assume a local equilibrium state in the supersaturated gas phase at the gas/ solid interface, which allows determination of the actual boron and nitrogen partial pressures: BCl3 + 3 2 H2 →← B ` 3HCl (4) with pB = K8 p pBCl3 ~pH2 ! 3/2 ~pHCl! 3 (5) and NH3 →← N + 3 2 H2 (6) with pN = K9 p pNH3 ~pH2 ! 3/2 (7) The supersaturation in boron (SB) and nitrogen (SN) can be obtained when the actual partial pressures are compared to the boron and nitrogen partial pressures that are in equilibrium with solid BN at a given temperature (p° B and p° N): SB = pB p° B = KBpBCl3 ~pH2 ! 3/2 ~pHCl! 3 (8) KB = K8 p p° B (9) SN = pN p° N = KNpNH3 ~pH2 ! 3/2 (10) KN = K9 p p° N (11) and SBSN = K pBCl3 pNH3 ~pHCl! 3 (12) where K 4 KBKN. These relations are obviously also related to the degree of completion of the reactants conversion. With regard to the CVD process and the structure of the deposits, high supersaturations and low temperatures are gen￾Fig. 12. Evolution of the greatest coherent domain and the interlayer distance in the BN thickness, as a function of the mass flow rate for the protected-ICVI process. Lc and La are shaded gray and white, respec￾tively. Samples were taken in the middle of the fibrous preforms (the 6.2 specimen reference). Fig. 13. Evolution of the greatest coherent domain and the interlayer distance in the BN thickness, as a function of the location in the preform for the protected-ICVI process. Lc and La are shaded gray and white, respectively (T 4 700°C, P 4 1.33 kPa, NH3/BCl3 4 1, H2/BCl3 4 1, and f 4 7 × 10−6 kg/s). May 1999 Influence of Isothermal CVD and CVI Conditions on the Deposition Kinetics and Structure of BN 1193