Ind. Eng. Chem. Res. 1997, 36, 5537-5540
5537
BHCl2 Formation during Chemical Vapor Deposition of Boron in a Dual-Impinging Jet Reactor N. A. Sezgi, T. Dogˇ u,* and H. O 2. O 2 zbelge Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey
Chemical vapor deposition (CVD) of boron from BCl3 and H2 was investigated in a dual-impinging jet reactor which was connected to an FT-IR spectrometer for on-line chemical analysis of the reactor outlet stream. Formation of the intermediate, BHCl2, during CVD of boron on a hot tungsten substrate was experimentally verified. Boron deposition started at substrate temperatures of around 750 °C and showed a significant deposition rate increase with an increase in temperature. At a surface temperature of 1350 °C, fractional conversion of BCl3 to B and BHCl2 was found to be around 0.6 and 0.2, respectively. At temperatures less than 1100 °C, fractional conversion of BCl3 to BHCl2 was found to be higher than the fractional conversion to boron. Formation of BHCl2 occurs in the gas phase even at temperatures lower than 750 °C. Contribution of surface reactions to the formation of BHCl2 increases with an increase in temperature. Introduction Boron fibers are frequently used in the production of reinforced composites. Due to their hardness, light weight, high tensile strength, and high corrosion resistance, boron has found applications in the production of refractory coatings and electronic components as well as in the production of fiber-reinforced polymeric and metal matrix composites. Production of boron fibers by the chemical vapor deposition (CVD) technique typically involves the reaction of BCl3 (or BBr3) with hydrogen over a hot solid surface (usually tungsten) (DeBolt, 1982; Peticorps et al., 1988). Thermal decomposition of diborane (Brotherton, 1970) and drawing of fibers from molten boron are some of the other techniques that may be used in boron fiber production. In a CVD reactor, a cold reactant gas mixture flows parallel to a filament-heated substrate, resulting in boron deposition of the surface. In such a CVD reactor, molecular and thermal diffusion of BCl3 to the tungsten substrate play important roles on the deposition rate (Carlton and Oxley, 1967; Vandenbulcke, 1983; Jenkinson and Pollard, 1984; Scholtz and Hlavacek, 1990; Scholtz et al., 1990, 1991a,b). In the work of Jenkinson and Pollard (1984) it was illustrated that contribution of temperature gradients to mass transfer (thermal diffusion) may significantly change the deposition rate of boron in a parallel flow reactor. Such transport limitations were minimized in impinging jet CVD experiments (Vandenbulcke and Vuillard, 1976, 1977; Vandenbulcke, 1985). In the work of Kassemi et al. (1993) mathematical analysis of fluid flow and heat transfer was presented for a vertical fiber growth reactor. In most of the previous studies on CVD of boron, the deposition rate was determined either by gravimetric techniques (Sekine et al., 1989; Carlton et al., 1970) or by measurement of the thickness of the deposit using a scanning electron microscope (Jansson et al., 1989; Gruber, 1970; Vandenbulcke and Vuillard, 1976, 1977; Scholtz et al., 1991a). The composition of the product gas was not experimentally measured in any of these studies. Thermodynamics of the system favors the formation of an intermediate product (BHCl2) during CVD of boron. Carlton et al. (1970) proposed a kinetic * Author to whom correspondence should be addressed. S0888-5885(97)00225-X CCC: $14.00
model assuming that BHCl2 is in equilibrium with the reactant species in the vapor phase. Considering the formation of BHCl2 and following a mass-transferequilibrium model where the interfacial gaseous phase is in equilibrium with the solid, Vandenbulcke and Vuillard (1977) and Vandenbulcke (1985) predicted deposition rates of boron by CVD. However, formation of BHCl2 was not included in the models proposed more recently by Scholtz et al. (1990, 1991a,b). In the recent experimental study of Haupfear and Schmidt (1994) no evidence of an intermediate product was indicated during CVD of boron from BBr3 and H2. In a number of these previous publications, modeling studies were presented for the hydrodynamics, heat transfer, diffusion, and reaction within the boundary layer adjacent to the growing surface. In some of these studies a local equilibrium assumption was made for the formation of BHCl2 in the gas phase. However, there is no published work in which experimental evidence was reported for the formation of the intermediate species (BHCl2) by the on-line measurement of the CVD reactor exit gas composition. None of the previous studies reported quantitative information about the gas-phase composition or fractional conversions of BCl3 to boron and BHCl2. The major contribution of this communication is the use of FT-IR to analyze the composition of inlet and outlet streams to verify the existence of intermediate species and also to obtain quantitative information about the relative values of conversions of BCl3 to boron and BHCl2. These data should be useful in future modeling and kinetic studies. Experimental Section In this study a novel dual-impinging jet quartz reactor was constructed and used to investigate CVD of boron in a system where diffusion limitations were minimized. In this system, a gaseous mixture of hydrogen and BCl3 flows through two orifices and impinges at right angles on both sides of the reaction surface (tungsten filament at 2 mm wide and 0.015 mm thick) (Figure 1). The reaction surface was at equal distances (0.5 cm) from both orifices. The tungsten filament was held between two electrodes and heated to the desired temperature (750-1350 °C) by applying dc power (Figure 2). These electrodes are connected to a dc power supply through specially designed glass-metal joints. The upper elec© 1997 American Chemical Society
5538 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Figure 1. Schematic diagram of the reaction system.
Figure 3. FT-IR spectra of reactor inlet (a) and outlet (b) streams.
Figure 2. Dual-impinging jet CVD reactor.
trode is fixed, while the lower electrode dips into a mercury pool connected to the dc power supply. With this arrangement thermal expansion of the filament was accommodated and the reaction surface remained taut by the weight of the electrode. The temperature of the reaction surface was measured using an optical pyrometer. In order to keep the temperature constant, the applied voltage was continuously regulated. The flow rates of hydrogen and BCl3 gases were independently controlled by flow control valves, and these gases were mixed before the reactor. The inlet temperature of this gas mixture was 25 °C, while the temperature of the reactor outlet stream was found to be around 50 °C during the experiments. Helium gas was used for purging the system before and after each run. An FT-
IR spectrometer (Midac M1800) connected to the reactor outlet allowed on-line qualitative and quantitative chemical analysis of this stream. Considering the toxicity of BCl3, the reactor effluent stream was washed in two washing bottles (one filled with water and the other with ethyl alcohol) connected in series. The whole system was built in a continuously vented chamber. The results reported in this paper were obtained with reactant gas mixtures of 1.08-2.14% BCl3 in hydrogen at atmospheric pressure. The total flow rate of the inlet gas mixture was adjusted to 2.8 cm3‚s-1. The stream was divided and entered the reactor from two orifices. Results and Discussion Typical FT-IR spectra obtained from the reactant gas (BCl3 + H2) and the reactor effluent stream are shown in Figure 3. The BCl3 peaks are observed between 900 and 1000 cm-1 in the reactant and product streams. In the product gas, the typical group of peaks from HCl are observed between 2700 and 3050 cm-1. In addition to the peaks corresponding to BCl3 and HCl, some peaks are observed at 850-910, 1050-1150, and also 2625-
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5539
2700 cm-1. These peaks are assigned to BHCl2 since this spectrum is consistent with the infrared spectrum of BHCl2 reported in the earlier work of Myers and Putnam (1963). Myers and Putnam reported the same wavenumbers for the IR absorption peaks of BHCl2. This result provides experimental evidence for formation of BHCl2 during CVD of boron from BCl3 and H2. Within the detection limits of the instrument, the FTIR spectrum obtained from the reactor effluent stream does not indicate any other intermediate species besides BHCl2. From these results it is concluded that two independent overall reactions should be considered for the description of this process.
3 BCl3(g) + H2(g) w B(s) + 3HCl(g) 2
Figure 4. Typical CVD reactor effluet stream composition at a substrate temperature of 915 °C (inlet composition: 2.14% BCl3 in hydrogen).
BCl3(g) + H2(g) w BHCl2(g) + HCl(g) The following relations are used for the evaluation of fractional conversion of BCl3 to boron (x1) and to BHCl2 (x2) from the experimental data of product and reactant steam compositions.
yHCl )
yBCl3 )
(3x1 + x2)y°BCl3 1 1 + y°BCl3x1 2
(1 - x1 - x2)y°BCl3 1 1 + y°BCl3x1 2
yBHCl2 )
y°BCl3x2 1 1 + y°BCl3x1 2
(1)
(2)
Figure 5. Temperature dependence of fractional conversion of BCl3 to B (x1) and to BHCl2 (x2) at temperatures higher than 1000 °C (set I: 1.08% BCl3 in hydrogen).
(3)
In this analysis calibration factors of BCl3 and HCl, which were evaluated for different characteristic peaks of these species, were used. Mole fraction of BHCl2 in the product steam was then determined, together with x1 and x2 values from eqs 1-3. Boron deposition rate and selectivity may then be evaluated (Sezgi, 1996) from
RB ) ARB,s ) F0y°BCl3x1 SB )
RB x1 ) RB + RBHCl2 x1 + x2
(4) (5)
In some experiments the boron deposition rate was also determined from the weight change of the filament during the reaction. Comparison of fractional conversion of BCl3 to boron (x1) evaluated from gas compositions and from the weight change of the filament agreed well. As a result of this analysis, calibration factors of the characteristic peaks of BHCl2 were also determined. Experimental results indicate that the system reaches steady state in a short time period and that the effluent gas composition does not indicate a significant fluctuation with time during the CVD process (Sezgi et al., 1996). A typical set of experimental data showing the composition of reactor effluent stream (at 915 °C) is shown in Figure 4. Two sets experimental data obtained in the temperature ranges of 1025-1350 °C (with a reactant gas composition of 1.08% BCl3 in hydrogen) and 780-1125 °C (with a reactant gas composition of 2.14% BCl3 in
Figure 6. Temperature dependence of fractional conversion of BCl3 to B (x1) and to BHCl2 (x2) at low temperatures (set II: 2.14% BCl3 in hydrogen).
hydrogen) are shown in Figures 5 and 6, respectively. No significant effect of gas composition was observed in the temperature dependence of conversions (Sezgi, 1996) over the range of gas mixture compositions of these two sets of experiments. As shown in these figures, fractional conversion of BCl3 to boron (x1) increases with an increase of temperature. In experiments conducted over 1000 °C, x1 increases from 0.29 to about 0.60 as the temperature increases from 1025 to 1350 °C (Figure 5). On the other hand, no CVD of boron is observed below 750 °C in this dual-impinging jet reactor (Figure 6). This result indicates that surface reactions do not occur at significant rates at low temperatures. However, a significant fraction of BCl3 is converted to BHCl2 even at temperatures as low as 780 °C.
5540 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Conversion of BCl3 to BHCl2 (x2) shows an increasing trend in the temperature range of 780-850 °C with a maximum value of 0.3 at 850 °C. At this temperature, conversion of BCl3 to B (x1) is only about 0.1. With further increase of temperature, x1 shows a significant increase while the conversion to BHCl2 decreases. Below 1100 °C x2 values are higher than x1 values. At higher temperatures CVD of boron (x1) is much more significant than the formation of BHCl2 (x2). These results indicate the formation of BHCl2 in the gas phase at temperatures as low as 780 °C. Some preliminary experimental results obtained by Sezgi (1996) in a tubular reactor had indicated that formation of BHCl2 took place even at temperatures as low as 350 °C. This is additional evidence of formation of BHCl2 in the gas phase at low temperatures. At high temperatures BHCl2 formation through surface reactions, with a possible mechanism as suggested by Jansson et al. (1989), might also be significant. Results obtained at high temperatures (above 1000 °C) show a slight decreasing trend of x2 in parallel with a significant increase of x1 with an increase of temperature (Figure 5). At 1350 °C, the overall conversion of BCl3 reaches values around 0.8 where conversion of BCl3 to B is about 0.6. The relative sensitivities of the boron deposition and BHCl2 formation rates to temperature determine the observed temperature dependence of x1 and x2. A possible contribution of BHCl2 to the boron deposition rate via a surface reaction might also be an explanation for the slight decreasing trend of x2 with temperature. As mentioned above the temperature of the gases leaving the impinging jet reactor is around 50 °C. At such low temperatures formation of BHCl2 is not likely. Equilibrium values of x2 are close to zero for temperatures lower than 250 °C. This illustrates that the formation of BHCl2 takes place within the thin thermal boundary layer adjacent to the hot surface and also on the surface depending upon the surface temperature. Concluding Remarks In this work, it was experimentally verified that the intermediate species BHCl2 forms during CVD of boron. It was also shown that significant qualitative and quantitative information could be obtained about the product distribution in such a CVD reactor using FTIR spectroscopy. Boron deposition was found to start at substrate temperatures over 750 °C, and a significant increase of deposition rate was observed with an increase in temperature. At low temperatures formation of BHCl2 is majorly due to gas phase reactions, while over 1000 °C the contribution of surface reactions to BHCl2 formation might be significant. Nomenclature A ) surface area of the substrate, m2 F0 ) molar flow rate of the reactant mixture, mol‚s-1 RB ) rate of boron deposition (extensive), mol‚s-1 RBHCl2 ) rate of formation of BHCl2 (extensive), mol‚s-1 RB,s ) boron deposition rate based on the unit surface area of the substrate, mol‚s-1‚m-2 SB ) selectivity defined by eq 5 T ) temperature x1 ) fractional conversion of BCl3 to B x2 ) fractional conversion of BCl3 to BHCl2
y°BCl3 ) mole fraction of BCl3 in the reactant stream yBCl3 ) mole fraction of BCl3 in the product stream
Literature Cited Brotherton, R. J. Progress in Boron Chemistry; Pergamon Press: Oxford, U.K., 1970. Carlton, H. E.; Oxley, J. H. Forced and Natural Convective Mass Transfer in Multicomponent Gaseous Mixtures. AIChE J. 1967, 13, 571. Carlton, H. E.; Oxley, J. H.; Hall, K. H.; Blocher, J. M. Kinetics of the Hydrogen Reduction of Boron Trichloride to Boron. In Proceedings of the 2nd International Conference on CVD; Blocher, J. M., Winters, J. C., Eds.; Electrochemical Society: Pennington, NJ, 1970; p 209. DeBolt, H. E. Boron and Other High Strength, High Modulus Low Density Filamentary Reinforcing Agents. Handbook of Composites; Lubin, G., Ed.; Van Nostrand Reinhold: New York, 1982. Gruber, P. E. On the Kinetics of Chemical Vapor Deposition. In Proceedings of the 2nd International Conference on CVD; Blocher, J. M., Winters, J. C., Eds.; Electrochemical Society: Pennington, NJ, 1970; p 25. Haupfear, E. A.; Schmidt, L. D. Kinetics of Boron Deposition from BBr3 + H2. Chem. Eng. Sci. 1994, 49, 2467. Jansson, U.; Boman, M.; Carlsson, J. O. Kinetics and Mechanism in CVD of Boron. J. Cryst. Growth 1989, 94, 171. Jenkinson, J. P.; Pollard, R. Thermal Diffusion Effects in Chemical Vapor Deposition Reactors. J. Electrochem. Soc. 1984, 131, 2911. Kassemi, M.; Go¨kogˇlu, S. A.; Panzarella, C. H.; Veitch, L. C. A Combined Heat-Transfer Analysis of a Single-Fiber CVD Reactor. AIChE J. 1993, 39, 1687. Myers, H. W.; Putnam, R. On the Properties of Monochlorodiborane. Inorg. Chem. 1963, 2 (3), 655. Petitcorps, Y. L.; Lahaye, M.; Pailler, R.; Naslain, R. Modern Boron and SiC CVD Filaments. Compos. Sci. Technol. 1988, 32, 31. Scholtz, J. H.; Hlavacek, V. CVD Reactors for the Coating of Fibrous Substrates Modeling and Numerical Simulation. J. Electrochem. Soc. 1990, 137, 3459. Scholtz, J. H.; Gatica, J. E.; Viljoen, H. J.; Revankar, V.; Hlavacek, V. CVD Reactors for the Synthesis of Inorganic Fibers. Modeling and Experimental Evaluation. Chem. Eng. Sci. 1990, 45, 2543. Scholtz, J. H.; Gatica, J. E.; Viljoen, H. J.; Hlavacek, V. Coating of Fibrous Substrates by CVD; Analysis of the Fiber Evolution. J. Cryst. Growth 1991a, 108, 190. Scholtz, J. H.; Gatica, J. E.; Viljoen, H. J.; Hlavacek, V. Analysis of Transport Phenomena in the Coating of Fibers by CVD. J. Phys. IV 1991b, 1, C2-135. Sekine, T.; Nakanishi, N.; Kato, E. Kinetics of Chemical Vapor Deposition of Boron Thin Film on Tungsten Substrate. J. Jpn. Inst. Met. 1989, 53 (7), 698. Sezgi, N. A. Kinetics of Boron Fiber Production. Ph.D. Thesis, Middle East Technical University, Ankara, Turkey, 1996. Sezgi, N. A.; Dogˇu, T.; O ¨ zbelge, H.O ¨ . Kinetic Studies for CVD of Boron in an Impining Jet Reactor. Proc. Fifth World Congr. Chem. Eng. 1996, 5, 148-153. Vandenbulcke, L. Theoretical Influence of the Chemical Vapor Deposition Processes on the Thickness and Structural Uniformity of the Deposits. Thin Solid Films 1983, 102, 149. Vandenbulcke, L. Theoretical and Experimantal Studies on the Chemical Vapor Deposition of Boron Carbide. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 568. Vandenbulcke, L.; Vuillard, G. Chemical Vapor Deposition of Amorphous Boron on Massive Substrates. J. Electrochem. Soc. 1976, 123, 278. Vandenbulcke, L.; Vuillard, G. Mass Transfer, Equilibrium and Kinetics in the Chemical Vapor Deposition of Boron from Impinging Jets. J. Electrochem. Soc. 1977, 124, 1931.
Received for review March 17, 1997 Revised manuscript received September 16, 1997 Accepted September 16, 1997X IE970225R
X Abstract published in Advance ACS Abstracts, October 15, 1997.
