- Email: [email protected]
Measurement of the absorption cross sections of SiCl4, SiCl3, SiCl2 and Cl at H Lyman-α wavelength
Chemical Physics Letters 561–562 (2013) 31–35
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Measurement of the absorption cross sections of SiCl4, SiCl3, SiCl2 and Cl at H Lyman-a wavelength R. Mével a,⇑, L. Catoire b, M. Fikri c, P. Roth c a
Graduate Aerospace Laboratories, California Institute of Technology 1200 E. California Blvd, Pasadena, CA 91125, USA Ecole Nationale Supérieure de Techniques Avancées ENSTA-ParisTech 828, boulevard des Maréchaux, 91762 Palaiseau Cedex, France c Institut für Verbrennung und Gasdynamik (IVG), Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47048, Germany b
a r t i c l e
i n f o
Article history: Received 3 December 2012 In final form 23 January 2013 Available online 31 January 2013
a b s t r a c t Atomic resonance absorption spectroscopy coupled with a shock tube is a powerful technique for studying high temperature dynamics of reactive systems. Presently, high temperature pyrolysis of SiCl4–Ar mixtures has been studied behind reflected shock waves. Using time-resolved absorption profiles at 121.6 nm and a detailed reaction model, the absorption cross sections of SiCl4 , SiCl3 , SiCl2 and Cl have been measured. Results agree well with available data for SiCl4 and constitute, to our knowledge, the first measurements for SiCl3 , SiCl2 and Cl at the Lyman-a wavelength. These data are relevant to silica particle production from SiCl4 -oxidant mixtures combustion synthesis. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Atomic resonance absorption spectroscopy (ARAS) coupled with a shock tube is a powerful technique for studying high temperature dynamics of reactive systems [1]. It has been widely used to determine elementary reaction rates for a variety of chemical systems including H2–O2 [2], silane-based mixtures [3–5] or tin [6], among others [7–9]. Because of its very high sensitivity, the ARAS technique allows working with highly diluted mixtures. However, experimental measurements of rate constants with a reasonable accuracy is complicated by two main limitations of the method, (i) the role of secondary reactions, and (ii) the lack of spectral selectivity in the UV region as compared to the IR region. Even by working with very diluted mixtures (up to a few ppm of reactants), chemical isolation still remains difficult to reach and the profiles obtained are due to the reaction of interest and other interfering secondary reactions. Furthermore, reactants, intermediates and products can potentially absorb at the working wavelength. Javoy et al. [10], Catoire et al. [11], and Mével et al. [12] previously underlined the importance of modeling these additional contributions for N2O–Ar, SiCl4–N2O and SiH4–N2O mixtures, respectively. Indeed, if these absorptions are not considered, rate constant values would be affected by quite large uncertainties. Especially for intermediates, these absorptions are generally neglected because the absorption coefficients needed are not known. In addition, the absorption coefficients may be temperature-dependent and even for reactants and products these dependencies are generally not established for wide temperature ranges. ⇑ Corresponding author. E-mail address: [email protected] (R. Mével). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.01.035
Silicon tetrachloride is widely employed to produce solid SiO2 particles [13] which are used to form protective and/or insulating layers on semi-conductors [14,15]. One of the synthesis method used consists of doping a hydrogen–oxygen flame with SiCl4 [16,17]. Whether a thermal or plasma chemical vapor deposition process or a flame synthesis process is used to perform the synthesis of solid SiO2, the chemical reaction mechanism involved is very complex. The large number of applications of semi-conductors and electronic components [18–21] demonstrate the continuous relevance of improving the accuracy of elementary reaction rate parameters through chemical kinetics studies. In the present study, a validated SiCl4–Ar chemical kinetics model is used to interpret ARAS experiments at 121.6 nm performed with mixtures containing up to 200 ppm SiCl4. Absorption coefficients and their temperature-dependency were extracted for molecular SiCl4, SiCl3 and SiCl2 radicals, and Cl atoms. 2. Experimental The experiments were carried out in a stainless steel pressure driven shock tube with an internal diameter of 79 mm. It is divided by a thin aluminium diaphragm, into two sections, a driver section of 3.5 m and a driven section of 5.7 m in length. The internal surface has been specially prepared for ultra high vacuum purposes. The driven section can be heated and pumped down to pressures below 5 106 Pa by a turbomolecular pump. Gas mixtures were prepared manometrically in a stainless steel UHV storage cylinder which could be baked out and evacuated using a separate turbomolecular pumping unit. The residual gases in all UHV devices were analyzed by quadrupole mass-spectrometers and were found to be practically free of hydrocarbons. Silicon tetrachloride and
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R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35
Argon used in the present study are highly pure (P99.999% for SiCl4 and P99.9999% for Ar). Silicon tetrachloride, which is a liquid at STP conditions, was injected and evaporated in a stainless steel vessel. The shock tube is equipped with diagnostics for atomic resonance absorption spectroscopy (ARAS) consisting of a microwave excited discharge lamp (microwave EMS Mikrotron 200 Mark III), the optical absorption path in the shock tube (equal to the diameter of the tube), a 1 m vacuum UV monochromator (McPherson 225 GCA) and a solar blind photomultiplier (EMR 542 G-08-12). The lamp was operated with a flowing gas mixture of 1 mol% H2 in He maintained at a pressure of about 600 Pa and a microwave power of about 50 W. It should be noted that the emission line shape, which depends on the optical depth of the plasma and unexcited layers, and the spectral response of the monochromator are not precisely known. Moreover, unknown parameters, such as the broadening coefficients, have to be estimated in order to calculate the absorption line shape. For all the aforementioned reasons, comparison of the measured absorption cross section with theoretical prediction is very uncertain and was not attempted in the present study. In most ARAS studies, a calibration procedure is employed rather than theoretical calculations in order to link the light absorption and the species concentration [3,6,22]. Nevertheless, the determination of cross sections in different experimental conditions with different apparatus are generally consistent. Many examples can be cited: (i) for silane, the value measured by Mével et al. [12] at 130.5 nm agrees with the values reported by Itoh et al. [23], Cooper et al. [24] and Suto and Lee [25], (ii) for nitrous oxide, the value obtained by Javoy et al. [10] at 130.5 nm is in good agreement with the values measured by Just [26], and (iii) for the vinyl radical, data measured by Hunzinker et al. [27] at 310 K by using photochemical modulation spectroscopy at 423.2 nm are consistent with data reported by Pibel et al. [28] at 298 K by using cavity ring-down spectroscopy at 423 nm and by Ismail et al. [29] at 293 K by using initial absorption of iodine radical from C2H5I at 423.2 nm.
Figure 1. Comparison between experimental and calculated (solid lines) Cl atom profiles during the thermal decomposition of SiCl4 in argon. Experimental data were obtained at k = 134.7 nm [36].
3. Computational procedure and results Previous shock-tube experiments using SiCl4–Ar mixtures [30,31] revealed strong absorption at the Lyman-a wavelength, k=121.6 nm. At first glance this represents a drawback for the study of the elementary kinetics of SiCl4 by using the ARAS technique. However, this allows for the study of the thermal decomposition of SiCl4 in a direct way, as well as the direct determination of rSiCl4 at Lyman-a wavelength for temperatures at which no decomposition is observed. Nevertheless, it is tenable that radicals (SiCl3, SiCl2, SiCl) and atoms (Si, Cl) formed during SiCl4 thermal decomposition also absorb, and the fact that no apparent absorption change is visibly observable does not mean that SiCl4 does not decompose. For instance, if SiCl3 has the same ri as SiCl4, the absorption will remain constant although a significant amount of SiCl4 has disappeared. It is therefore almost impossible to deduce directly from the present experiments the absorption coefficients ri , except as specified just above for SiCl4 for the lowest temperatures studied (roughly 1400 K). Nevertheless, an indirect approach is possible and these ri can be deduced from the absorption signals interpreted in terms of a validated detailed chemical kinetics model. A SiCl4–Ar chemical kinetics model has been built using available data from the literature [32–35]. It is given as a Supplemental material. It consists of 10 reactions and 8 species, including Ar. The model validation is demonstrated in Figures 1 and 2 against the formation of Cl and Si atoms during the thermal decomposition of SiCl4 highly diluted in Ar for a wide temperature range and pressures around 150 kPa [36]. The mixtures studied are reported in
Figure 2. Comparison between experimental and calculated (solid lines) Si atom profiles during the thermal decomposition of SiCl4 in argon. Experimental data were obtained at k = 251.6 nm [36].
Table 1. A typical absorption signal at a wavelength of 121.6 nm, behind a reflected shock, is reported in Figure 3 together with the SiCl4, SiCl3, SiCl2 and Cl profiles computed using the detailed chemical kinetic model. From Figure 3 it is clear that SiCl4 decomposes thermally at about 1800 K although the absorption signal remains constant. This computational observation is experimentally
33
R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35 Table 1 Experimental conditions and SiCl4, SiCl3, SiCl2 and Cl absorption cross section values. XSiCl4 are ppm. Balance is Ar. N 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
XSiCl4 100 100 100 100 100 100 100 100 100 100 200 100 200 200 100 100
P5 (kPa)
T5 (K)
rSiCl4
rSiCl3
rSiCl2
rCl
(cm2)
(cm2)
(cm2)
(cm2)
144 153 155 154 142 139 120 121 145 136 128 136 135 143 137 124
1444 1546 1634 1826 2069 2082 2094 2137 2158 2158 2184 2253 2275 2310 2369 2415
9.96 1017 1.03 1016 9.40 1017 9.81 1017 9.98 1017 1.14 1016 1.17 1016 9.30 1017 6.89 1017 1.09 1016 6.22 1017 8.04 1017 5.61 1017 3.44 1017 2.52 1017 6.01 1017
– – 1.95 1016 1.54 1016 6.90 1017 7.75 1017 6.80 1017 1.08 1016 1.26 1016 1.05 1016 1.02 1016 – 8.70 1017 1.06 1016 1.49 1016 7.82 1017
– – – – – 1.55 1017 – – – 2.25 1017 – – 1.34 1017 4.40 1018 2.20 1018 2.35 1018
– – 2.87 1018 9.52 x1018 2.60 1017 2.13 1017 2.90 1017 2.62 1017 2.67 1017 1.91 1017 2.58 1017 2.62 1017 1.90 1017 2.18 1017 2.60 1017 2.53 1017
1600 K, absorption by Cl atoms is significant. Experiments were performed at temperature up to 2415 K from which the absorption cross sections for SiCl4, SiCl3, SiCl2 and Cl are extractable. Although the spectral resolution of the monochromator is 0.02 nm, Cl atoms seem able to absorb at the Lyman-a wavelength probably because of the pressure broadening and shift of the spectral lines. The temperature remains too low to extract the Lyman-a absorption crosssection of SiCl radicals. Because the signals were noisy, a Savitzky– Golay filter with a third-order polynomial and a span of 101 data points was used. Figure 4 shows an absorption signal that is filtered. The absorption coefficients ri are extracted from the filtered signals by using multi-linear regression according to the Lambert– Beer law:
lnð1 AðtÞÞ ¼ LðrSiCl4 ½SiCl4 ðtÞ þ rSiCl3 ½SiCl3 ðtÞ þ rSiCl2 ½SiCl2 ðtÞ þ rCl ½ClðtÞÞ
ð1Þ
where AðtÞ is the computed time-dependent absorption, ½SiClx ðtÞ represents the computed time-dependent concentration in SiClx species, ½ClðtÞ is the computed time-dependent concentration in
Figure 3. Absorption signal at k = 121.6 nm and SiCly and Cl computed profiles during SiCl4 pyrolysis. y = 2, 3 and 4.
confirmed by considering Figure 1 in which Cl atom formation is observed by using the ARAS technique at 134.7 nm during the thermal decomposition of SiCl4 at a temperature of about 1800 K. The same observation is reported by Catoire et al. [37] at around 1700 K by using the ARAS technique for chlorine atom at 134.7 nm for SiCl4–Ar mixtures which were 1000 times more concentrated than the mixture shown in Figure 1. The experiment reported in Figure 3 was performed with highly concentrated SiCl4 mixtures, therefore, depending on the temperature, it is likely that species other than SiCl4 and SiCl3 are responsible for the absorption at 121.6 nm: basically, for the lowest temperatures, up to about 1550 K, the absorption is mostly due to SiCl4, then in the 1600– 1800 K temperature range it is mostly due to SiCl4 and SiCl3. SiCl2 plays a more important role at temperatures above 2000 K. Above
Figure 4. Raw and filtered absorption signals at k = 121.6 nm. Conditions: XSiCl4 ¼ 100 ppm; T5 = 2369 K; P5 = 137 kPa.
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R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35
Cl-atoms, and L is the optical path length, in the current experiments, L is the diameter of the tube. Figure 5 compares the timedependent computed absorption signal with the time-dependent experimental absorption signal for two experimental conditions at 2137 and 1634 K. Figure 5 also shows that the raw and fitted signals are comprised in between ±20% of the ri in terms of computed absorption. Sources of uncertainty on the absorption cross-section include the quality of the optical set-up, i.e. the signal to noise ratio, the thermodynamic state, the mixture composition (including the presence of impurity), and the calculated concentrations, i.e. kinetic and thermodynamic parameters of the reaction model. Combining all these uncertainties, we estimated that the overall uncertainty on the cross-section is ±20%. An experimental relative uncertainty of ±20% for an indirect determination at such high temperatures seems reasonable as Vatsa and Volpp [38] reported an uncertainty of ±7% for direct determination at room temperature. All the individual ri values determined are reported in Table 1 and Figure 6. As indicated above, ri cannot be determined for all species at all temperatures. When the concentration is too low the determination of ri is difficult and can lead to large errors. rSiCl4 can be determined at all temperatures as it is the reactant. However, rSiCl4 values at high temperatures for which the decomposition is very fast can only be determined with a large uncertainty. In fact, at high temperatures, the experimental and computed absorption signals are consistent whatever the value of rSiCl4 comprised between 0 and 1016 cm2 molecule1, except at time zero and up to few tens of microseconds, see Figure 7. Nevertheless, all rSiCl4 are reported in Table 1 and Figure 6 as well as rSiCl2 , although rSiCl2 cannot be determined with sufficient accuracy in all the test cases. The same applies to rSiCl3 in some cases. The present study provides absolute absorption cross-sections for SiCl3, SiCl2 and Cl which are not yet available in the literature. Figure 6 reports the temperature dependency of absorption cross sections studied in this work. Absorption cross sections can be expressed as:
Figure 6. SiCl4, SiCl3, SiCl2 and Cl absorption cross sections at k = 121.6 nm. Reflected shock pressure vary between 120 and 155 kPa.
Figure 7. Comparison between the experimental and the calculated absorption profiles at k = 121.6 nm. Conditions: XSiCl4 ¼ 100 ppm; T5 = 2369 K; P5 = 137 kPa. Black line: raw signal. Red line: filtered signal. Solid blue line: calculated signal with rSiCl4 = 0 cm2. Dashed blue line: calculated signal with rSiCl4 = 1016 cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Figure 5. Comparison between the experimental and the calculated absorption profiles at k = 121.6 nm. Conditions: Top: XSiCl4 ¼ 100 ppm; T5 = 2137; P5 = 121 kPa. Bottom: XSiCl4 ¼ 100 ppm; T5 = 1634 K; P5 = 155 kPa. Blue lines: raw signal. Red lines: filtered signal. Black lines: calculated signal. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
rSiCl4 ¼ 4:47 1016 expð8:6 104 TÞ cm2
ð2Þ
rSiCl3 ¼ 4:80 1016 expð7:2 104 TÞ cm2
ð3Þ
rSiCl2 ¼ 6:36 1011 expð7:1 103 TÞ cm2
ð4Þ
rCl ¼ 1:06 1019 expð2:4 103 TÞ cm2
ð5Þ
Causley and Russell [39] report a value of 1.349 10 cm2 molecule1 at 123 nm for SiCl4 at room temperature. Recently, Ali [40] studied the VUV spectrum of SiCl4 and reported absorption 16
R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35
cross-section against photon energy at room temperature and low pressure (gas pressure in the cell in the range 0.5–6 Pa). At Lymana, a value of about 1016 cm2 molecule1 is reported. It is worthy to note that the variations of the cross-section rSiCl4 are very sharp around 121.6 nm (10.2 eV). Ibuki et al. [41] report a photoabsorption spectrum for SiCl4 with an absorption cross section of about 100 Mb (1016 cm2 molecule1) at 10.2 eV. Kameta et al. [43] reported a higher absorption cross section for SiCl4, about 1.8 1016, but at a much shorter wavelength, 85 nm. Table 1 shows that the absorption cross section of SiCl4 is found to be around 1016 cm2 molecule1 between 1444 and 1546 K for conditions at which only SiCl4 absorbs because it does not decompose. Up to 2100 K, the absorption cross section of SiCl4 remains in the vicinity of 1016 cm2 molecule1. Above 2100 K, rSiCl4 decreases with increasing temperature but, as discussed above, caution has to be exercised for the highest temperatures. rSiCl3 and rSiCl2 also exhibit the same trend whereas rCl exhibit a different trend, i.e. the cross section increases with increasing temperature. To our knowledge, no data are reported for Si-compound radicals except for SiH3 but in the 205–255 nm wavelength range [42].
4. Conclusion We have measured indirectly the temperature dependency of the SiCl4, SiCl3, SiCl2 and Cl photoabsorption cross sections at the Lyman-a line over a wide temperature range and at a pressure of about 150 kPa. For silicon tetrachloride, the cross section is temperature independent in the 300–2100 K temperature range whereas the cross sections for SiCl3 and SiCl2 decrease exponentially and increase exponentially for chlorine atoms. These coefficients are needed for the reappraisal of elementary rate constants, for instance for the reaction SiCl4 + H = Products, previously determined without such refinements which were not possible at that time or for the determination of elementary rate constants either never studied or studied in other temperature ranges.
Acknowledgements The present work was carried out in the Institut für Verbrennung und Gasdynamik (IVG), Universität Duisburg-Essen. Laurent Catoire acknowledge the French Ministry ’’Ministère de la Défense’’ for a postdoctoral fellowship. The authors acknowledge S. Coronel (Caltech) for useful discussions.
35
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2013.01. 035. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
K. Bhaskaran, P. Roth, Prog. Energy Combust. Sci. 28 (2002) 151. S. Javoy, V. Naudet, S. Abid, C. Paillard, Int. J. Chem. Kinet. 32 (2000) 686. H. Mick, H. Matsui, P. Roth, J. Phys. Chem. 97 (1993) 6839. H.-J. Mick, M. Markus, P. Roth, V. Smirnov, Ber. Bunsen-Ges. Phys. Chem 99 (1995) 880. S. Javoy, R. Mével, G. Dupré, Chem. Phys. Lett. 500 (2010) 223. K. Takahashi, A. Giesen, P. Roth, PCCP 3 (2001) 4296. P.M. Patterson, J.W. Sutherland, R.B. Klemm, Int. Symp. Shock Waves Shock Tubes 17 (1990) 444. A. Kunz, P. Roth, J. Phys. Chem. A 103 (1999) 841. R. Sivaramakrishnan, J.V. Michael, Proc. Combust. Inst. 33 (2011) 225. S. Javoy, R. Mével, C.-E. Paillard, Int. J. Chem. Kinet. 41 (2009) 357. L. Catoire, R. Mével, A. Kunz, P. Roth, Proc. Combust. Inst. (2011) 477. R. Mével, S. Javoy, G. Dupré, Proc. Combust. Inst. 33 (2011) 485. M. Binnewies, K. Jug, Eur. J. Inorg. Chem. 2000 (2000) 1127. J. Chapple-Sokol, C. Giunta, R. Gordon, J. Electrochem. Soc. 136 (1989) 2993. C. Giunta, J. Chapple-Sokol, R. Gordon, J. Electrochem. Soc. 137 (1990) 3237. S.-L. Chung, M.-S. Tsai, H. Lin, Combust. Flame 85 (1991) 134. B. Hannebauer, F. Menzel, Z. Anorg. Allg. Chem. 629 (2003) 1485. J. Surugue, Techniques générales du laboratoire de physique, Éditions du Centre National de la Recherche Scientifique, 1966. H. Strobel, Chemical Instrumentation, Addison-Wesley Publishing Company, Inc., 1960. J. Monnier, Science et Défense 91, Nouvelles Avancées Scientifiques et Technique’s, Dunod, 1991. A.I. Boukai et al., Nature 451 (2008) 168. S. Javoy et al., Exp. Therm Fluid Sci. 27 (2003) 371. U. Itoh et al., J. Chem. Phys. 85 (1986) 4867. G. Cooper et al., Chem. Phys. 196 (1995) 293. M. Suto, L. Lee, J. Chem. Phys. 84 (1986) 1160. T. Just, Int. Symp. Shock Tubes Waves 13 (1981) 54. H.E. Hunziker et al., Can. J. Chem. 61 (1983) 993. C.D. Pibel et al., J. Chem. Phys. 110 (1999) 1841. H. Ismail et al., J. Phys. Chem. A 113 (2009) 1278. L. Catoire, D. Woiki, P. Roth, Int. J. Chem. Kinet. 29 (1997) 469. L. Catoire, D. Woiki, P. Roth, Int. J. Chem. Kinet. 29 (1997) 415. A. Kunz, P. Roth, Int. J. Chem. Kinet. 33 (11) (2001) 741. A. Kunz, P. Roth, Ber. Bunsen-Ges. Phys. Chem. 102 (10) (1998) 1492. D. Husain, P.E. Norris, J. Chem. Soc., Faraday Trans. 2 (74) (1978) 106. T. Moore, B. Brady, L. Robbin Martin, Combust. Flame 146 (3) (2006) 407. A. Kunz, P. Roth, Proc. Combust. Inst. 27 (1998) 261. L. Catoire, D. Woiki, P. Roth, Rev. Sci. Tech. Déf. 4 (1997) 73. R.K. Vatsa, H.-R. Volpp, Chem. Phys. Lett. 340 (2001) 289. G.C. Causley, B.R. Russell, J. Electron. Spectrosc. Relat. Phenom. 11 (1987) 383. S. Ali, Gas Phase Vacuum-Ultraviolet (VUV) Spectroscopy of small halogenated polyatomic molecules, Ph.D. thesis of University of Birmingham, 2007. T. Ibuki, M. Kono, Y. Asari, A. Hiraya, K. Shobatake, J. Chem. Phys. 106 (1997) 4853. A.V. Baklanov, L.N. Krasnoperov, J. Phys. Chem. A 105 (2001) 4917. K. Kameta et al., J. Chem. Phys. 99 (1993) 2487.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Measurement of the absorption cross sections of SiCl4, SiCl3, SiCl2 and Cl at H Lyman-a wavelength R. Mével a,⇑, L. Catoire b, M. Fikri c, P. Roth c a
Graduate Aerospace Laboratories, California Institute of Technology 1200 E. California Blvd, Pasadena, CA 91125, USA Ecole Nationale Supérieure de Techniques Avancées ENSTA-ParisTech 828, boulevard des Maréchaux, 91762 Palaiseau Cedex, France c Institut für Verbrennung und Gasdynamik (IVG), Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47048, Germany b
a r t i c l e
i n f o
Article history: Received 3 December 2012 In final form 23 January 2013 Available online 31 January 2013
a b s t r a c t Atomic resonance absorption spectroscopy coupled with a shock tube is a powerful technique for studying high temperature dynamics of reactive systems. Presently, high temperature pyrolysis of SiCl4–Ar mixtures has been studied behind reflected shock waves. Using time-resolved absorption profiles at 121.6 nm and a detailed reaction model, the absorption cross sections of SiCl4 , SiCl3 , SiCl2 and Cl have been measured. Results agree well with available data for SiCl4 and constitute, to our knowledge, the first measurements for SiCl3 , SiCl2 and Cl at the Lyman-a wavelength. These data are relevant to silica particle production from SiCl4 -oxidant mixtures combustion synthesis. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Atomic resonance absorption spectroscopy (ARAS) coupled with a shock tube is a powerful technique for studying high temperature dynamics of reactive systems [1]. It has been widely used to determine elementary reaction rates for a variety of chemical systems including H2–O2 [2], silane-based mixtures [3–5] or tin [6], among others [7–9]. Because of its very high sensitivity, the ARAS technique allows working with highly diluted mixtures. However, experimental measurements of rate constants with a reasonable accuracy is complicated by two main limitations of the method, (i) the role of secondary reactions, and (ii) the lack of spectral selectivity in the UV region as compared to the IR region. Even by working with very diluted mixtures (up to a few ppm of reactants), chemical isolation still remains difficult to reach and the profiles obtained are due to the reaction of interest and other interfering secondary reactions. Furthermore, reactants, intermediates and products can potentially absorb at the working wavelength. Javoy et al. [10], Catoire et al. [11], and Mével et al. [12] previously underlined the importance of modeling these additional contributions for N2O–Ar, SiCl4–N2O and SiH4–N2O mixtures, respectively. Indeed, if these absorptions are not considered, rate constant values would be affected by quite large uncertainties. Especially for intermediates, these absorptions are generally neglected because the absorption coefficients needed are not known. In addition, the absorption coefficients may be temperature-dependent and even for reactants and products these dependencies are generally not established for wide temperature ranges. ⇑ Corresponding author. E-mail address: [email protected] (R. Mével). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.01.035
Silicon tetrachloride is widely employed to produce solid SiO2 particles [13] which are used to form protective and/or insulating layers on semi-conductors [14,15]. One of the synthesis method used consists of doping a hydrogen–oxygen flame with SiCl4 [16,17]. Whether a thermal or plasma chemical vapor deposition process or a flame synthesis process is used to perform the synthesis of solid SiO2, the chemical reaction mechanism involved is very complex. The large number of applications of semi-conductors and electronic components [18–21] demonstrate the continuous relevance of improving the accuracy of elementary reaction rate parameters through chemical kinetics studies. In the present study, a validated SiCl4–Ar chemical kinetics model is used to interpret ARAS experiments at 121.6 nm performed with mixtures containing up to 200 ppm SiCl4. Absorption coefficients and their temperature-dependency were extracted for molecular SiCl4, SiCl3 and SiCl2 radicals, and Cl atoms. 2. Experimental The experiments were carried out in a stainless steel pressure driven shock tube with an internal diameter of 79 mm. It is divided by a thin aluminium diaphragm, into two sections, a driver section of 3.5 m and a driven section of 5.7 m in length. The internal surface has been specially prepared for ultra high vacuum purposes. The driven section can be heated and pumped down to pressures below 5 106 Pa by a turbomolecular pump. Gas mixtures were prepared manometrically in a stainless steel UHV storage cylinder which could be baked out and evacuated using a separate turbomolecular pumping unit. The residual gases in all UHV devices were analyzed by quadrupole mass-spectrometers and were found to be practically free of hydrocarbons. Silicon tetrachloride and
32
R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35
Argon used in the present study are highly pure (P99.999% for SiCl4 and P99.9999% for Ar). Silicon tetrachloride, which is a liquid at STP conditions, was injected and evaporated in a stainless steel vessel. The shock tube is equipped with diagnostics for atomic resonance absorption spectroscopy (ARAS) consisting of a microwave excited discharge lamp (microwave EMS Mikrotron 200 Mark III), the optical absorption path in the shock tube (equal to the diameter of the tube), a 1 m vacuum UV monochromator (McPherson 225 GCA) and a solar blind photomultiplier (EMR 542 G-08-12). The lamp was operated with a flowing gas mixture of 1 mol% H2 in He maintained at a pressure of about 600 Pa and a microwave power of about 50 W. It should be noted that the emission line shape, which depends on the optical depth of the plasma and unexcited layers, and the spectral response of the monochromator are not precisely known. Moreover, unknown parameters, such as the broadening coefficients, have to be estimated in order to calculate the absorption line shape. For all the aforementioned reasons, comparison of the measured absorption cross section with theoretical prediction is very uncertain and was not attempted in the present study. In most ARAS studies, a calibration procedure is employed rather than theoretical calculations in order to link the light absorption and the species concentration [3,6,22]. Nevertheless, the determination of cross sections in different experimental conditions with different apparatus are generally consistent. Many examples can be cited: (i) for silane, the value measured by Mével et al. [12] at 130.5 nm agrees with the values reported by Itoh et al. [23], Cooper et al. [24] and Suto and Lee [25], (ii) for nitrous oxide, the value obtained by Javoy et al. [10] at 130.5 nm is in good agreement with the values measured by Just [26], and (iii) for the vinyl radical, data measured by Hunzinker et al. [27] at 310 K by using photochemical modulation spectroscopy at 423.2 nm are consistent with data reported by Pibel et al. [28] at 298 K by using cavity ring-down spectroscopy at 423 nm and by Ismail et al. [29] at 293 K by using initial absorption of iodine radical from C2H5I at 423.2 nm.
Figure 1. Comparison between experimental and calculated (solid lines) Cl atom profiles during the thermal decomposition of SiCl4 in argon. Experimental data were obtained at k = 134.7 nm [36].
3. Computational procedure and results Previous shock-tube experiments using SiCl4–Ar mixtures [30,31] revealed strong absorption at the Lyman-a wavelength, k=121.6 nm. At first glance this represents a drawback for the study of the elementary kinetics of SiCl4 by using the ARAS technique. However, this allows for the study of the thermal decomposition of SiCl4 in a direct way, as well as the direct determination of rSiCl4 at Lyman-a wavelength for temperatures at which no decomposition is observed. Nevertheless, it is tenable that radicals (SiCl3, SiCl2, SiCl) and atoms (Si, Cl) formed during SiCl4 thermal decomposition also absorb, and the fact that no apparent absorption change is visibly observable does not mean that SiCl4 does not decompose. For instance, if SiCl3 has the same ri as SiCl4, the absorption will remain constant although a significant amount of SiCl4 has disappeared. It is therefore almost impossible to deduce directly from the present experiments the absorption coefficients ri , except as specified just above for SiCl4 for the lowest temperatures studied (roughly 1400 K). Nevertheless, an indirect approach is possible and these ri can be deduced from the absorption signals interpreted in terms of a validated detailed chemical kinetics model. A SiCl4–Ar chemical kinetics model has been built using available data from the literature [32–35]. It is given as a Supplemental material. It consists of 10 reactions and 8 species, including Ar. The model validation is demonstrated in Figures 1 and 2 against the formation of Cl and Si atoms during the thermal decomposition of SiCl4 highly diluted in Ar for a wide temperature range and pressures around 150 kPa [36]. The mixtures studied are reported in
Figure 2. Comparison between experimental and calculated (solid lines) Si atom profiles during the thermal decomposition of SiCl4 in argon. Experimental data were obtained at k = 251.6 nm [36].
Table 1. A typical absorption signal at a wavelength of 121.6 nm, behind a reflected shock, is reported in Figure 3 together with the SiCl4, SiCl3, SiCl2 and Cl profiles computed using the detailed chemical kinetic model. From Figure 3 it is clear that SiCl4 decomposes thermally at about 1800 K although the absorption signal remains constant. This computational observation is experimentally
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R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35 Table 1 Experimental conditions and SiCl4, SiCl3, SiCl2 and Cl absorption cross section values. XSiCl4 are ppm. Balance is Ar. N 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
XSiCl4 100 100 100 100 100 100 100 100 100 100 200 100 200 200 100 100
P5 (kPa)
T5 (K)
rSiCl4
rSiCl3
rSiCl2
rCl
(cm2)
(cm2)
(cm2)
(cm2)
144 153 155 154 142 139 120 121 145 136 128 136 135 143 137 124
1444 1546 1634 1826 2069 2082 2094 2137 2158 2158 2184 2253 2275 2310 2369 2415
9.96 1017 1.03 1016 9.40 1017 9.81 1017 9.98 1017 1.14 1016 1.17 1016 9.30 1017 6.89 1017 1.09 1016 6.22 1017 8.04 1017 5.61 1017 3.44 1017 2.52 1017 6.01 1017
– – 1.95 1016 1.54 1016 6.90 1017 7.75 1017 6.80 1017 1.08 1016 1.26 1016 1.05 1016 1.02 1016 – 8.70 1017 1.06 1016 1.49 1016 7.82 1017
– – – – – 1.55 1017 – – – 2.25 1017 – – 1.34 1017 4.40 1018 2.20 1018 2.35 1018
– – 2.87 1018 9.52 x1018 2.60 1017 2.13 1017 2.90 1017 2.62 1017 2.67 1017 1.91 1017 2.58 1017 2.62 1017 1.90 1017 2.18 1017 2.60 1017 2.53 1017
1600 K, absorption by Cl atoms is significant. Experiments were performed at temperature up to 2415 K from which the absorption cross sections for SiCl4, SiCl3, SiCl2 and Cl are extractable. Although the spectral resolution of the monochromator is 0.02 nm, Cl atoms seem able to absorb at the Lyman-a wavelength probably because of the pressure broadening and shift of the spectral lines. The temperature remains too low to extract the Lyman-a absorption crosssection of SiCl radicals. Because the signals were noisy, a Savitzky– Golay filter with a third-order polynomial and a span of 101 data points was used. Figure 4 shows an absorption signal that is filtered. The absorption coefficients ri are extracted from the filtered signals by using multi-linear regression according to the Lambert– Beer law:
lnð1 AðtÞÞ ¼ LðrSiCl4 ½SiCl4 ðtÞ þ rSiCl3 ½SiCl3 ðtÞ þ rSiCl2 ½SiCl2 ðtÞ þ rCl ½ClðtÞÞ
ð1Þ
where AðtÞ is the computed time-dependent absorption, ½SiClx ðtÞ represents the computed time-dependent concentration in SiClx species, ½ClðtÞ is the computed time-dependent concentration in
Figure 3. Absorption signal at k = 121.6 nm and SiCly and Cl computed profiles during SiCl4 pyrolysis. y = 2, 3 and 4.
confirmed by considering Figure 1 in which Cl atom formation is observed by using the ARAS technique at 134.7 nm during the thermal decomposition of SiCl4 at a temperature of about 1800 K. The same observation is reported by Catoire et al. [37] at around 1700 K by using the ARAS technique for chlorine atom at 134.7 nm for SiCl4–Ar mixtures which were 1000 times more concentrated than the mixture shown in Figure 1. The experiment reported in Figure 3 was performed with highly concentrated SiCl4 mixtures, therefore, depending on the temperature, it is likely that species other than SiCl4 and SiCl3 are responsible for the absorption at 121.6 nm: basically, for the lowest temperatures, up to about 1550 K, the absorption is mostly due to SiCl4, then in the 1600– 1800 K temperature range it is mostly due to SiCl4 and SiCl3. SiCl2 plays a more important role at temperatures above 2000 K. Above
Figure 4. Raw and filtered absorption signals at k = 121.6 nm. Conditions: XSiCl4 ¼ 100 ppm; T5 = 2369 K; P5 = 137 kPa.
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R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35
Cl-atoms, and L is the optical path length, in the current experiments, L is the diameter of the tube. Figure 5 compares the timedependent computed absorption signal with the time-dependent experimental absorption signal for two experimental conditions at 2137 and 1634 K. Figure 5 also shows that the raw and fitted signals are comprised in between ±20% of the ri in terms of computed absorption. Sources of uncertainty on the absorption cross-section include the quality of the optical set-up, i.e. the signal to noise ratio, the thermodynamic state, the mixture composition (including the presence of impurity), and the calculated concentrations, i.e. kinetic and thermodynamic parameters of the reaction model. Combining all these uncertainties, we estimated that the overall uncertainty on the cross-section is ±20%. An experimental relative uncertainty of ±20% for an indirect determination at such high temperatures seems reasonable as Vatsa and Volpp [38] reported an uncertainty of ±7% for direct determination at room temperature. All the individual ri values determined are reported in Table 1 and Figure 6. As indicated above, ri cannot be determined for all species at all temperatures. When the concentration is too low the determination of ri is difficult and can lead to large errors. rSiCl4 can be determined at all temperatures as it is the reactant. However, rSiCl4 values at high temperatures for which the decomposition is very fast can only be determined with a large uncertainty. In fact, at high temperatures, the experimental and computed absorption signals are consistent whatever the value of rSiCl4 comprised between 0 and 1016 cm2 molecule1, except at time zero and up to few tens of microseconds, see Figure 7. Nevertheless, all rSiCl4 are reported in Table 1 and Figure 6 as well as rSiCl2 , although rSiCl2 cannot be determined with sufficient accuracy in all the test cases. The same applies to rSiCl3 in some cases. The present study provides absolute absorption cross-sections for SiCl3, SiCl2 and Cl which are not yet available in the literature. Figure 6 reports the temperature dependency of absorption cross sections studied in this work. Absorption cross sections can be expressed as:
Figure 6. SiCl4, SiCl3, SiCl2 and Cl absorption cross sections at k = 121.6 nm. Reflected shock pressure vary between 120 and 155 kPa.
Figure 7. Comparison between the experimental and the calculated absorption profiles at k = 121.6 nm. Conditions: XSiCl4 ¼ 100 ppm; T5 = 2369 K; P5 = 137 kPa. Black line: raw signal. Red line: filtered signal. Solid blue line: calculated signal with rSiCl4 = 0 cm2. Dashed blue line: calculated signal with rSiCl4 = 1016 cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Figure 5. Comparison between the experimental and the calculated absorption profiles at k = 121.6 nm. Conditions: Top: XSiCl4 ¼ 100 ppm; T5 = 2137; P5 = 121 kPa. Bottom: XSiCl4 ¼ 100 ppm; T5 = 1634 K; P5 = 155 kPa. Blue lines: raw signal. Red lines: filtered signal. Black lines: calculated signal. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
rSiCl4 ¼ 4:47 1016 expð8:6 104 TÞ cm2
ð2Þ
rSiCl3 ¼ 4:80 1016 expð7:2 104 TÞ cm2
ð3Þ
rSiCl2 ¼ 6:36 1011 expð7:1 103 TÞ cm2
ð4Þ
rCl ¼ 1:06 1019 expð2:4 103 TÞ cm2
ð5Þ
Causley and Russell [39] report a value of 1.349 10 cm2 molecule1 at 123 nm for SiCl4 at room temperature. Recently, Ali [40] studied the VUV spectrum of SiCl4 and reported absorption 16
R. Mével et al. / Chemical Physics Letters 561–562 (2013) 31–35
cross-section against photon energy at room temperature and low pressure (gas pressure in the cell in the range 0.5–6 Pa). At Lymana, a value of about 1016 cm2 molecule1 is reported. It is worthy to note that the variations of the cross-section rSiCl4 are very sharp around 121.6 nm (10.2 eV). Ibuki et al. [41] report a photoabsorption spectrum for SiCl4 with an absorption cross section of about 100 Mb (1016 cm2 molecule1) at 10.2 eV. Kameta et al. [43] reported a higher absorption cross section for SiCl4, about 1.8 1016, but at a much shorter wavelength, 85 nm. Table 1 shows that the absorption cross section of SiCl4 is found to be around 1016 cm2 molecule1 between 1444 and 1546 K for conditions at which only SiCl4 absorbs because it does not decompose. Up to 2100 K, the absorption cross section of SiCl4 remains in the vicinity of 1016 cm2 molecule1. Above 2100 K, rSiCl4 decreases with increasing temperature but, as discussed above, caution has to be exercised for the highest temperatures. rSiCl3 and rSiCl2 also exhibit the same trend whereas rCl exhibit a different trend, i.e. the cross section increases with increasing temperature. To our knowledge, no data are reported for Si-compound radicals except for SiH3 but in the 205–255 nm wavelength range [42].
4. Conclusion We have measured indirectly the temperature dependency of the SiCl4, SiCl3, SiCl2 and Cl photoabsorption cross sections at the Lyman-a line over a wide temperature range and at a pressure of about 150 kPa. For silicon tetrachloride, the cross section is temperature independent in the 300–2100 K temperature range whereas the cross sections for SiCl3 and SiCl2 decrease exponentially and increase exponentially for chlorine atoms. These coefficients are needed for the reappraisal of elementary rate constants, for instance for the reaction SiCl4 + H = Products, previously determined without such refinements which were not possible at that time or for the determination of elementary rate constants either never studied or studied in other temperature ranges.
Acknowledgements The present work was carried out in the Institut für Verbrennung und Gasdynamik (IVG), Universität Duisburg-Essen. Laurent Catoire acknowledge the French Ministry ’’Ministère de la Défense’’ for a postdoctoral fellowship. The authors acknowledge S. Coronel (Caltech) for useful discussions.
35
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2013.01. 035. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
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