- Email: [email protected]
Impact of chlorine dissociation for modified chemical vapor deposition
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 355 (2009) 817–820
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Impact of chlorine dissociation for modified chemical vapor deposition Catherine K.W. Cheung, David F. Fletcher *, Geoffrey W. Barton School of Chemical and Biomolecular Engineering, University of Sydney, Building J01, Sydney, NSW 2006, Australia
a r t i c l e
i n f o
Article history: Received 13 December 2008 Received in revised form 4 March 2009 Available online 9 April 2009
PACS: 81.15.Gh 42.81.Bm 47.54.Jk
a b s t r a c t Modified chemical vapor deposition (MCVD) is the platform technology used to create a wide range of silica-based optical fibers. This paper reports on the extension of the reaction scheme embedded within a computational fluid dynamics model of the MCVD process to include chlorine dissociation and recombination. Simulations employing this modified kinetic scheme indicate that chlorine dissociation acts primarily as a ‘heat sink’ in cases where the operating conditions promote a high peak temperature in the narrow reaction zone where most of the SiCl4 oxidation occurs. The extended model allows a wider range of operating parameters to be examined in terms of the deposition profile of silica ‘soot’ particles on the substrate tube wall. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Chemical vapor deposition Silica optical fiber Chlorine dissociation CFD modeling
1. Introduction This note describes the extension of a computational fluid dynamics (CFD) model of the MCVD process used in optical fiber fabrication. The rationale for developing such a model has been to elucidate transport and wall deposition mechanisms for silica particles created in a narrow reaction zone. Chlorine dissociation was included within the reaction scheme to permit the model to simulate cases where high temperatures were experienced in the reaction zone. 2. Modification of reaction scheme to include chlorine dissociation/recombination In previous modeling studies [1,2], the sole reaction considered was the irreversible oxidation of silicon tetrachloride (SiCl4) [3].
SiCl4 ðgÞ þ O2 ðgÞ ! SiO2 ðsÞ þ 2Cl2 ðgÞ:
ð1Þ
In reality, chlorine dissociation also occurs but only to any significant extent at temperatures above 1500 K. This reaction is reversible and involves a bath gas/third body, M, with recombination occurring at lower temperatures.
Cl2 ðgÞ þ M $ 2Cl ðgÞ þ M:
ð2Þ
In the present modeling scenario, the major role of chlorine dissociation is as a high temperature ‘heat sink’ with the heat required for the dissociation process being released downstream from the reaction zone by subsequent chlorine recombination. However, these two reactions play a more important role within an overall scheme for cases where oxidation of both SiCl4 and GeCl4 takes place [4]. Such scenarios are common in practice, as co-deposition of SiO2 and GeO2 is frequently used to adjust the refractive index profile in optical fibers. This important case will be considered in the next phase of our MCVD model development. It should be noted that a variety of other chemical species are known to be present [5–7]. However, the associated reactions need not be considered in a modeling context as the concentrations of these species are expected to be very low under MCVD operating conditions [8]. The rate constants for chlorine dissociation and recombination were derived from literature data given in Baulch et al. [9] and Snider and Leaist [10], respectively, with argon as the bath gas (third body). Eq. (3) gives the rate constant for chlorine dissociation over the temperature range 1550–2800 K, while Eq. (4) gives the chlorine recombination rate constant for the temperature range 195– 2582 K. Note that the order of these two reactions are different and thus the rate constants have different units (kdiss has units of cm3 mol1 s1 while krecomb has units of cm6 mol2 s1). 1
* Corresponding author. E-mail address: d.fl[email protected] (D.F. Fletcher). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.03.003
kdiss ¼ 2:32 1013 ½cm3 mol
s1 expð46 960½cal=mol=RTÞ ð3Þ
LETTER TO THE EDITOR
818
C.K.W. Cheung et al. / Journal of Non-Crystalline Solids 355 (2009) 817–820 2
krecomb ¼ 9 1013 ½cm6 mol
s1 expð2400½cal=mol=RTÞ:
All other aspects of the CFD modeling of the MCVD system remained the same as in our earlier studies where SiCl4 oxidation was the only reaction considered [1,2].
in the reaction zone. However, the (bracketed) results in Table 1 clearly show that this additional effect has had a negligible impact on the silica deposition profile along the substrate tube, validating the original assumption to neglect chlorine dissociation/recombination in the previous simulations.
3. Results
4.2. Impact of feed gas temperature and wall temperature profile
A wide range of simulations (outlined in Table 1) were conducted to quantify the impact of including the chlorine dissociation/recombination reactions. Note that details of the different wall temperature profiles used (i.e. single- and multi-pass) are given in [2]. Initial simulations covered cases examined previously for which high gas temperatures were not experienced, and thus the chlorine dissociation/recombination reactions would have only a modest impact. Subsequent runs examined cases that had previously been impossible to simulate due to model convergence issues associated with physically unrealistic (high) temperatures. The incorporation of chlorine dissociation/recombination allowed all such cases to be successfully simulated, with the chlorine dissociation reaction acting as an important ‘heat sink’ in the high temperature region. The particle deposition statistics for all cases are summarized in Table 1, while Fig. 1 shows the temperature and mass fraction profiles along the substrate tube. The sharp boundaries present in Fig. 1(a) show the location of the narrow reaction zone where the majority of the SiCl4 is reacted and heat is released. Note that in reporting results, position along the tube is given as a normalized parameter (ND) defined as the distance along the tube divided by the tube radius (0.019 m).
The feed gas temperature in the base case was 298 K. Increasing this to 473 K made little difference to the position of the reaction front (see Fig. 1(a)) or to the deposition statistics (see Table 1) for a given wall temperature profile with the additional thermal energy in the feed stream having little impact on system behavior. Changes to the wall temperature profile had a far greater impact. Comparing simulation results for the single-pass and multi-pass profile cases showed that SiCl4 oxidation occurred further upstream in the latter case due to the higher wall temperatures encountered in the early part of the substrate tube. These observations are clearly important from an operational perspective.
ð4Þ
4. Discussion 4.1. Impact of including chlorine dissociation/recombination Initially, both the base case and a high wall temperature case (i.e. a multi-pass profile with a peak of 2300 K) that had been successfully modeled previously [2] were repeated but now with chlorine dissociation/recombination included in the reaction scheme. The thermal conditions for some chlorine dissociation are reached in both cases (see Fig. 1) at the highest temperatures encountered
4.3. Impact of high SiCl4 inlet composition Simulations were also performed with high SiCl4 feed compositions. The SiCl4:O2:He mass ratios used were 60:30:10 and 75:15:10, the latter being equivalent to an approximately equimolar ratio of SiCl4 to O2. These cases had previously led to unrealistically high temperatures. However, the inclusion of chlorine dissociation/recombination into the reaction scheme reduced the maximum temperature in both cases to physically sensible values. The deposition statistics (see Table 1) and deposition patterns (see Fig. 2) for cases with a higher SiCl4 composition and a single-pass wall temperature profile were found to be very similar to those for the base case. The only notable difference is that the reaction front, best seen in Fig. 1(a), was found to be somewhat narrower on increasing the SiCl4:O2 mass ratio, although this was expected as the greater amount of reactant leads to higher temperatures and hence faster reaction. Simulation results for the multipass wall temperature profile case were also as expected with the reaction zone moving upstream relative to the single-pass temperature profile case.
Table 1 Simulations conducted incorporating chlorine dissociation/recombination. Tube rotation = 34 rpm; gas flow = 2 105 kg s1 Inlet gas temperature = 298 K (unless otherwise specified) SiCl4:O2 mass ratio = 40:50 (unless otherwise specified)a Simulation Conditions
Peak temp. (K)
Particle size (nm)
Total deposition (%)
Mean deposition location (ND)
Single-pass Base case
2000
High feed temperature (473 K)
2000
High SiCl4:O2 mass ratioa (60:30)
2000
10 10 000 10 10 000 10 10 000 10 10 000
56 (55)b 72 (72) 56 71 54 73 54 76
14.9 (15.0) 15.2 (15.3) 14.9 15.2 14.8 15.2 14.7 15.2
10 10 000 10 10 000 10 10 000
42 (41) 53 (51) 44 55 44 56
13.9 (13.9) 14.2 (14.2) 13.8 14.1 15.7 15.7
a
Highest SiCl4:O2 mass ratio (75:15)
2000
Multi-pass High peak wall temperature
2300
High feed temperature (473 K)
2000
a
High SiCl4:O2 mass ratio (60:30) a
2000
Remaining mass is helium which is a constant (10%) in all cases. Same operating conditions as previously [2] but with the addition of chlorine dissociation/recombination kinetics. Values shown in brackets are for the corresponding case without chlorine dissociation/recombination modeled. b
LETTER TO THE EDITOR
C.K.W. Cheung et al. / Journal of Non-Crystalline Solids 355 (2009) 817–820
819
(a) SiCl4 mass fraction (MF) Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
(b) Temperature (K) Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
(c) Cl2 MF Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
(d) Cl MF Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
Gas flow direction Fig. 1. Profiles along the substrate tube: (a) SiCl4 mass fraction, (b) temperature, (c) Cl2 mass fraction, and (d) Cl mass fraction.
8 SiCl44 MF MF == 0.4 0.4 (base) (base) SiCl4 SiCl4 SiCl SiCl4 MF=== 0.6 0.4 MF 0.6 4 MF SiCl4 SiCl4 MF=== 0.75 0.4 SiCl4 MF MF 0.75
Deposition (%)
7 6 5 4 3 2 1 0 10
12
14
16
18
20
ND Fig. 2. Deposition distribution along the substrate tube as a function of inlet SiCl4 mass fraction (the lines drawn in the figure are a guide to the eye only.)
5. Conclusions Previous modeling studies of the MCVD process have all neglected the possibility of chlorine dissociation at high temperatures. We have shown here that inclusion of this reaction is essential to accurately model scenarios with high operating temperatures. In such cases, this reaction acts as a local ‘heat sink’, capping the maximum temperature. As might be expected, significant chlorine dissociation is limited to the narrow, high temperature, reaction zone where most of the SiCl4 oxidation occurs. Chlorine recombination takes place downstream of the reaction zone as the gas temperature falls, and acts as a mechanism for transporting heat downstream away from the reaction zone. This extended model allowed a wider range of operational scenarios to be studied
LETTER TO THE EDITOR
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C.K.W. Cheung et al. / Journal of Non-Crystalline Solids 355 (2009) 817–820
and is an important intermediary step in the development of a comprehensive model that includes the simultaneous reaction of SiCl4 and GeCl4 and the subsequent co-deposition of silica and germania on the substrate tube wall. Acknowledgment This research was supported under the Australian Research Council’s Discovery Project funding scheme (DP0452166). References [1] C.K.W. Cheung, D. Haley, D.F. Fletcher, G.W. Barton, P. McNamara, J. Non-Cryst. Solids 353 (44–46) (2007) 4066.
[2] C.K.W. Cheung, D.F. Fletcher, G.W. Barton, P. McNamara, J. Non-Cryst. Solids 355 (4&5) (2009) 327. [3] P. Kleinert, D. Schmidt, J. Kirchhof, A. Funke, Krist. Tech. 15 (9) (1980) K85. [4] D.L. Wood, K.L. Walker, J.B. MacChesney, J.R. Simpson, R. Csencsits, J. Lightwave Tech. LT-5 (2) (1987) 277. [5] R.M. Atkins, V.A. Hoban, J. Mater. Res. 4 (3) (1989) 641. [6] D.L. Wood, J.B. MacChesney, J.P. Luongo, J. Mater. Sci. 13 (8) (1978) 1761. [7] D.R. Powers, J. Am. Ceram. Soc. 61 (7&8) (1978) 295. [8] K.B. McAfee Jr., R.A. Laudise, R.S. Hozack, J. Lightwave Tech. LT-1 (4) (1983) 555. [9] D.L. Baulch, J. Duxbury, S.J. Grant, D.C. Montague, J. Phys. Chem. Ref. Data 10 (Suppl. 1) (1981) 1. [10] N.S. Snider, D.G. Leaist, J. Phys. Chem. 81 (11) (1977) 1033.
Journal of Non-Crystalline Solids 355 (2009) 817–820
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Impact of chlorine dissociation for modified chemical vapor deposition Catherine K.W. Cheung, David F. Fletcher *, Geoffrey W. Barton School of Chemical and Biomolecular Engineering, University of Sydney, Building J01, Sydney, NSW 2006, Australia
a r t i c l e
i n f o
Article history: Received 13 December 2008 Received in revised form 4 March 2009 Available online 9 April 2009
PACS: 81.15.Gh 42.81.Bm 47.54.Jk
a b s t r a c t Modified chemical vapor deposition (MCVD) is the platform technology used to create a wide range of silica-based optical fibers. This paper reports on the extension of the reaction scheme embedded within a computational fluid dynamics model of the MCVD process to include chlorine dissociation and recombination. Simulations employing this modified kinetic scheme indicate that chlorine dissociation acts primarily as a ‘heat sink’ in cases where the operating conditions promote a high peak temperature in the narrow reaction zone where most of the SiCl4 oxidation occurs. The extended model allows a wider range of operating parameters to be examined in terms of the deposition profile of silica ‘soot’ particles on the substrate tube wall. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Chemical vapor deposition Silica optical fiber Chlorine dissociation CFD modeling
1. Introduction This note describes the extension of a computational fluid dynamics (CFD) model of the MCVD process used in optical fiber fabrication. The rationale for developing such a model has been to elucidate transport and wall deposition mechanisms for silica particles created in a narrow reaction zone. Chlorine dissociation was included within the reaction scheme to permit the model to simulate cases where high temperatures were experienced in the reaction zone. 2. Modification of reaction scheme to include chlorine dissociation/recombination In previous modeling studies [1,2], the sole reaction considered was the irreversible oxidation of silicon tetrachloride (SiCl4) [3].
SiCl4 ðgÞ þ O2 ðgÞ ! SiO2 ðsÞ þ 2Cl2 ðgÞ:
ð1Þ
In reality, chlorine dissociation also occurs but only to any significant extent at temperatures above 1500 K. This reaction is reversible and involves a bath gas/third body, M, with recombination occurring at lower temperatures.
Cl2 ðgÞ þ M $ 2Cl ðgÞ þ M:
ð2Þ
In the present modeling scenario, the major role of chlorine dissociation is as a high temperature ‘heat sink’ with the heat required for the dissociation process being released downstream from the reaction zone by subsequent chlorine recombination. However, these two reactions play a more important role within an overall scheme for cases where oxidation of both SiCl4 and GeCl4 takes place [4]. Such scenarios are common in practice, as co-deposition of SiO2 and GeO2 is frequently used to adjust the refractive index profile in optical fibers. This important case will be considered in the next phase of our MCVD model development. It should be noted that a variety of other chemical species are known to be present [5–7]. However, the associated reactions need not be considered in a modeling context as the concentrations of these species are expected to be very low under MCVD operating conditions [8]. The rate constants for chlorine dissociation and recombination were derived from literature data given in Baulch et al. [9] and Snider and Leaist [10], respectively, with argon as the bath gas (third body). Eq. (3) gives the rate constant for chlorine dissociation over the temperature range 1550–2800 K, while Eq. (4) gives the chlorine recombination rate constant for the temperature range 195– 2582 K. Note that the order of these two reactions are different and thus the rate constants have different units (kdiss has units of cm3 mol1 s1 while krecomb has units of cm6 mol2 s1). 1
* Corresponding author. E-mail address: d.fl[email protected] (D.F. Fletcher). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.03.003
kdiss ¼ 2:32 1013 ½cm3 mol
s1 expð46 960½cal=mol=RTÞ ð3Þ
LETTER TO THE EDITOR
818
C.K.W. Cheung et al. / Journal of Non-Crystalline Solids 355 (2009) 817–820 2
krecomb ¼ 9 1013 ½cm6 mol
s1 expð2400½cal=mol=RTÞ:
All other aspects of the CFD modeling of the MCVD system remained the same as in our earlier studies where SiCl4 oxidation was the only reaction considered [1,2].
in the reaction zone. However, the (bracketed) results in Table 1 clearly show that this additional effect has had a negligible impact on the silica deposition profile along the substrate tube, validating the original assumption to neglect chlorine dissociation/recombination in the previous simulations.
3. Results
4.2. Impact of feed gas temperature and wall temperature profile
A wide range of simulations (outlined in Table 1) were conducted to quantify the impact of including the chlorine dissociation/recombination reactions. Note that details of the different wall temperature profiles used (i.e. single- and multi-pass) are given in [2]. Initial simulations covered cases examined previously for which high gas temperatures were not experienced, and thus the chlorine dissociation/recombination reactions would have only a modest impact. Subsequent runs examined cases that had previously been impossible to simulate due to model convergence issues associated with physically unrealistic (high) temperatures. The incorporation of chlorine dissociation/recombination allowed all such cases to be successfully simulated, with the chlorine dissociation reaction acting as an important ‘heat sink’ in the high temperature region. The particle deposition statistics for all cases are summarized in Table 1, while Fig. 1 shows the temperature and mass fraction profiles along the substrate tube. The sharp boundaries present in Fig. 1(a) show the location of the narrow reaction zone where the majority of the SiCl4 is reacted and heat is released. Note that in reporting results, position along the tube is given as a normalized parameter (ND) defined as the distance along the tube divided by the tube radius (0.019 m).
The feed gas temperature in the base case was 298 K. Increasing this to 473 K made little difference to the position of the reaction front (see Fig. 1(a)) or to the deposition statistics (see Table 1) for a given wall temperature profile with the additional thermal energy in the feed stream having little impact on system behavior. Changes to the wall temperature profile had a far greater impact. Comparing simulation results for the single-pass and multi-pass profile cases showed that SiCl4 oxidation occurred further upstream in the latter case due to the higher wall temperatures encountered in the early part of the substrate tube. These observations are clearly important from an operational perspective.
ð4Þ
4. Discussion 4.1. Impact of including chlorine dissociation/recombination Initially, both the base case and a high wall temperature case (i.e. a multi-pass profile with a peak of 2300 K) that had been successfully modeled previously [2] were repeated but now with chlorine dissociation/recombination included in the reaction scheme. The thermal conditions for some chlorine dissociation are reached in both cases (see Fig. 1) at the highest temperatures encountered
4.3. Impact of high SiCl4 inlet composition Simulations were also performed with high SiCl4 feed compositions. The SiCl4:O2:He mass ratios used were 60:30:10 and 75:15:10, the latter being equivalent to an approximately equimolar ratio of SiCl4 to O2. These cases had previously led to unrealistically high temperatures. However, the inclusion of chlorine dissociation/recombination into the reaction scheme reduced the maximum temperature in both cases to physically sensible values. The deposition statistics (see Table 1) and deposition patterns (see Fig. 2) for cases with a higher SiCl4 composition and a single-pass wall temperature profile were found to be very similar to those for the base case. The only notable difference is that the reaction front, best seen in Fig. 1(a), was found to be somewhat narrower on increasing the SiCl4:O2 mass ratio, although this was expected as the greater amount of reactant leads to higher temperatures and hence faster reaction. Simulation results for the multipass wall temperature profile case were also as expected with the reaction zone moving upstream relative to the single-pass temperature profile case.
Table 1 Simulations conducted incorporating chlorine dissociation/recombination. Tube rotation = 34 rpm; gas flow = 2 105 kg s1 Inlet gas temperature = 298 K (unless otherwise specified) SiCl4:O2 mass ratio = 40:50 (unless otherwise specified)a Simulation Conditions
Peak temp. (K)
Particle size (nm)
Total deposition (%)
Mean deposition location (ND)
Single-pass Base case
2000
High feed temperature (473 K)
2000
High SiCl4:O2 mass ratioa (60:30)
2000
10 10 000 10 10 000 10 10 000 10 10 000
56 (55)b 72 (72) 56 71 54 73 54 76
14.9 (15.0) 15.2 (15.3) 14.9 15.2 14.8 15.2 14.7 15.2
10 10 000 10 10 000 10 10 000
42 (41) 53 (51) 44 55 44 56
13.9 (13.9) 14.2 (14.2) 13.8 14.1 15.7 15.7
a
Highest SiCl4:O2 mass ratio (75:15)
2000
Multi-pass High peak wall temperature
2300
High feed temperature (473 K)
2000
a
High SiCl4:O2 mass ratio (60:30) a
2000
Remaining mass is helium which is a constant (10%) in all cases. Same operating conditions as previously [2] but with the addition of chlorine dissociation/recombination kinetics. Values shown in brackets are for the corresponding case without chlorine dissociation/recombination modeled. b
LETTER TO THE EDITOR
C.K.W. Cheung et al. / Journal of Non-Crystalline Solids 355 (2009) 817–820
819
(a) SiCl4 mass fraction (MF) Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
(b) Temperature (K) Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
(c) Cl2 MF Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
(d) Cl MF Base case (single-pass) High wall temp, multi-pass High inlet temp, single-pass High inlet temp, multi-pass High SiCl4 inlet MF, single-pass High SiCl4 inlet MF, multi-pass Highest SiCl4 inlet MF, single-pass
Gas flow direction Fig. 1. Profiles along the substrate tube: (a) SiCl4 mass fraction, (b) temperature, (c) Cl2 mass fraction, and (d) Cl mass fraction.
8 SiCl44 MF MF == 0.4 0.4 (base) (base) SiCl4 SiCl4 SiCl SiCl4 MF=== 0.6 0.4 MF 0.6 4 MF SiCl4 SiCl4 MF=== 0.75 0.4 SiCl4 MF MF 0.75
Deposition (%)
7 6 5 4 3 2 1 0 10
12
14
16
18
20
ND Fig. 2. Deposition distribution along the substrate tube as a function of inlet SiCl4 mass fraction (the lines drawn in the figure are a guide to the eye only.)
5. Conclusions Previous modeling studies of the MCVD process have all neglected the possibility of chlorine dissociation at high temperatures. We have shown here that inclusion of this reaction is essential to accurately model scenarios with high operating temperatures. In such cases, this reaction acts as a local ‘heat sink’, capping the maximum temperature. As might be expected, significant chlorine dissociation is limited to the narrow, high temperature, reaction zone where most of the SiCl4 oxidation occurs. Chlorine recombination takes place downstream of the reaction zone as the gas temperature falls, and acts as a mechanism for transporting heat downstream away from the reaction zone. This extended model allowed a wider range of operational scenarios to be studied
LETTER TO THE EDITOR
820
C.K.W. Cheung et al. / Journal of Non-Crystalline Solids 355 (2009) 817–820
and is an important intermediary step in the development of a comprehensive model that includes the simultaneous reaction of SiCl4 and GeCl4 and the subsequent co-deposition of silica and germania on the substrate tube wall. Acknowledgment This research was supported under the Australian Research Council’s Discovery Project funding scheme (DP0452166). References [1] C.K.W. Cheung, D. Haley, D.F. Fletcher, G.W. Barton, P. McNamara, J. Non-Cryst. Solids 353 (44–46) (2007) 4066.
[2] C.K.W. Cheung, D.F. Fletcher, G.W. Barton, P. McNamara, J. Non-Cryst. Solids 355 (4&5) (2009) 327. [3] P. Kleinert, D. Schmidt, J. Kirchhof, A. Funke, Krist. Tech. 15 (9) (1980) K85. [4] D.L. Wood, K.L. Walker, J.B. MacChesney, J.R. Simpson, R. Csencsits, J. Lightwave Tech. LT-5 (2) (1987) 277. [5] R.M. Atkins, V.A. Hoban, J. Mater. Res. 4 (3) (1989) 641. [6] D.L. Wood, J.B. MacChesney, J.P. Luongo, J. Mater. Sci. 13 (8) (1978) 1761. [7] D.R. Powers, J. Am. Ceram. Soc. 61 (7&8) (1978) 295. [8] K.B. McAfee Jr., R.A. Laudise, R.S. Hozack, J. Lightwave Tech. LT-1 (4) (1983) 555. [9] D.L. Baulch, J. Duxbury, S.J. Grant, D.C. Montague, J. Phys. Chem. Ref. Data 10 (Suppl. 1) (1981) 1. [10] N.S. Snider, D.G. Leaist, J. Phys. Chem. 81 (11) (1977) 1033.