The formation of SiO2 from hexamethyldisiloxane combustion in counterflow methane-air flames

Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 1859–1865

THE FORMATION OF SiO2 FROM HEXAMETHYLDISILOXANE COMBUSTION IN COUNTERFLOW METHANE-AIR FLAMES H. K. CHAGGER, D. HAINSWORTH, P. M. PATTERSON, M. POURKASHANIAN and A. WILLIAMS Department of Fuel and Energy University of Leeds Leeds, LS2 9JT, UK

Silica formation from hexamethyldisiloxane (HMDS) oxidation was studied by means of a CH4–N2/air opposed diffusion flame technique to which vaporized HMDS was added to the fuel flow. The CH4–N2/ air flame changed color from a pale-blue flame to whitish pink color when small amounts of HMDS were introduced in the flame. Increasing concentrations (1.3 mol %) made the flame more luminous, and a second thin-flame zone, orange in color, appeared on the fuel side. Emission spectroscopy revealed the existence of Si–H and Si–O species. SiO2 particles were observed only in the postcombustion gases, and the analysis of solid materials suggested the formation of fused silica particles, which were initially about 10 nm in size, forming outside the flame zone in these experiments. The overall mechanism in addition to that for methane oxidation is suggested as follows: C6H18Si2O ` OH 4 2C3H9SiO ` H C6H18Si2O ` O2 4 2C3H9SiO ` O C6H18Si2O ` HO2 4 2C3H9SiO ` OH C3H9SiO ` M 4 3CH3 ` SiO ` M SiO ` HO2 4 HSiO ` O2 SiO ` OH 4 SiO2 ` H SiO ` O ` M 4 SiO2 ` M SiO ` O2 4 SiO2 ` O together with the formation of SiOH and SiO(OH) species. The agreement between the model predictions using Sandia code OPPDIF and the experimental data was found to be satisfactory. The model appears to be a useful tool in elaboration of chemistry of formation of SiO2 in flames used to synthesize pure silica in this way.

Introduction Flame synthesis is a useful technology for the synthesis of oxide powders like TiO2, Al2O3, and SiO2. Fumed silica obtained from this process has numerous commercial applications and is used as a precursor material in the fiber-optic, ceramic, semiconductor, pigment, and dye industries. For the production of high-purity nanometer-sized particles (nanoparticles in the 2–100 nm size range), gasphase flame synthesis is particularly attractive to the material industry as this process can be scaled up and is economically advantageous [1]. However, the role of flame species, such as OH, and carbon compounds may determine the purity of the final product. The synthesis of silica by the oxidation of silane and silicon tetrachloride in hydrogen or hydrocarbon flames has been studied both experimentally and

theoretically [2–4]. The study showed that both the final silica aggregate and primary particle size were strongly affected by flame temperature and residence time, whereas the nucleation and surface reaction did not affect particle dynamics. These studies were extended to show that coalescence was a ratecontrolling step in the growth of silica [2–4]. The basic flame chemistry, such as kinetics, and thermodynamic properties of simple silicon compounds like SiH4 and SiCl4 are fairly well documented [5– 9] as a consequence of their use in chemical vapor deposition (CVD) and semiconductor manufacturing. Currently, industrial interest has been directed to the use of organosiloxanes as a precursor for these silica synthesis processes because of their ease of availability and economic advantage. These compounds include the highly methylated siloxanes and disiloxanes and the cyclic siloxanes ([1(CH3)2SiO1]n).

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modified to incorporate an oxidation mechanism for HMDS. The computations were conducted such that the gradient of axial velocity dV/dx was constant on both sides of the flame, and Eq. (i) held [10] 1/2

q dV 41 1dV dx 2 q 2 1 dx 2 air

fuel

(i)

air

where q is the density, and this allows the use of Eq. (ii): U V 4 r 2(x 1 Xstag) Fig. 1. Schematic representation of an opposed diffusion flame. The fuel outlet corresponds to zero on the zaxis distance scale, and the air outlet corresponds to 10 mm.

In this paper, the formation of SiO2 from the organodisiloxane, hexamethyldisiloxane (HMDS), (CH3)3Si–O–Si(CH3)3, is reported. The oxidation of HMDS has been studied by the addition of small amounts to a counterflow methane-air diffusion flame; a process that is similar to that used in industry where the production of high-purity fused silica formation occurs through the diffusion flame oxidation of a vaporized silicon-containing compound. A number of previous studies have been undertaken in opposed diffusion flames of silane-air or of silane decomposition in opposed H2–O2 diffusion flames, and some of these cases have been numerically modeled [5–9]. In addition, the growth processes leading to formation of silica particles have been investigated [2–4]. Although the general features of these flames have been described, there has been little validation of the detailed chemical models, and there are uncertainties about both the reaction kinetics and the thermochemistry. To our knowledge, there have been no studies of HMDS combustion in opposed diffusion flames to date, and in this paper, we report the results of such a study. Numerical Model Calculations were carried out to simulate the counterflow diffusion flame, as shown diagrammatically in Fig. 1, by means of the Sandia OPPDIF code. The distance between the two burners in the theoretical model can be varied. The distance chosen between the two opposing burners in this model was 1 cm to reproduce the actual experimental conditions; this made the solution more difficult to converge than the conventional distance of several centimeters. The model was applied to a fuel stream containing methane and nitrogen/HMDS on one side and an oxidizer stream consisting of air on the other. The methane combustion mechanism was

(ii)

where U is the radial velocity, r is the radius to any point in the flow defined by (x,r), and Xstag is the abscissa of the computed stagnation point. The stretch rate selected to characterize the flame was defined by the expression (1J/qfuel)1/2, where J is the constant eigenvalue of the experimental flow configuration, which is equal to

31r2 • 1 dr 24 1

dP

where P is the pressure. The chemical mechanism used for the methane oxidation is the natural gas oxidation scheme given by GRI version 2.11 [11]. The Miller and Bowman reaction scheme [12] was also tried, and it gave similar results. Table 1 lists the additional reactions added to the mechanism to describe HMDS decomposition and the subsequent production of SiO and SiO2. The area of greatest uncertainty relates to the decomposition of the HMDS. In previous modeling studies [5,13], silane was taken to react in an analogous manner to methane with allowance made for the difference in bond energies of Si–H and C–H. Similarly, in this study, HMDS was assumed to behave as a typical higher molecular weight hydrocarbon in the initial stages of the flame. The SiO and SiO2 reactions have been taken from the Britten et al. mechanism [13]. The reactions of the product CH3 are incorporated into the methane oxidation reaction scheme, which consists of 190 reactions in total. The rate constants are expressed in modified Arrhenius form k 4 ATn exp(1Ea/RT)

(iii)

Reverse rate constants are calculated from the forward rates, and the appropriate equilibrium constants are computed from available or estimated thermochemical data. Thermochemical data for Si intermediates such as HSiO were assumed to be identical to that of C-analogs. SiO2 was assumed to be formed initially in the gaseous state. The model was used to compute temperature and species concentration profiles for a counterflow CH4–N2/air diffusion flame doped with HMDS. The

SILICA FORMATION IN METHANE DIFFUSION FLAMES

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TABLE 1 HMDS oxidation mechanism, the units are cm3, mol, s, and cal Rate Constant Reaction 1 2 3 4 5 6 7 8 9 10 11 12

C6H18Si2O ` OH 4 2C3H9SiO ` H C6H18Si2O ` O2 4 2C3H9SiO ` O C6H18Si2O ` HO2 4 2C3H9SiO ` OH C3H9SiO ` M 4 3CH3 ` SiO ` M SiO ` HO2 4 HSiO ` O2 SiO ` H2O 4 HSiO ` OH SiO ` OH 4 HSiO ` O SiO ` H2 4 HSiO ` H SiO ` H ` M 4 HSiO ` M SiO ` OH 4 SiO2 ` H SiO ` O2 4 SiO2 ` O SiO ` O ` M 4 SiO2 ` M

A

n

Ea

9.6 E 12 6.0 E 13 1.3 E 13 8.7 E 12 5.27 E 12 2.84 E 15 2.88 E 14 1.31 E 15 1.74 E 11 4.0 E 12 1.0 E 13 2.5 E 15

2 0 0 0 0 0 0 0 0 0 0 0

100 261 103 0 34,300 105,000 87,900 90,000 11,600 5,700 6,500 4,370

kf 4 ATn exp(1Ea/RT).

model studied here is far from a complete HMDS combustion mechanism, as it does not fully consider nucleation of SiO2 to form solid particulates or the full reactions of silicon-containing intermediate species.

Experimental Methods The oxidation of HMDS is studied in a counterflow CH4–N2/air diffusion flame to obtain some experimental measurements with which the results of the model may be compared quantitatively. The advantage of using a counterflow geometry is that, to a first approximation, flow along the stagnation streamline can be described as one-dimensional. The basic characteristics of the flow field of such a flame are illustrated in Fig. 1. The experimental arrangement consists of two circular burners (d 4 2.5 cm), containing wire mesh screens as flow straighteners and mounted in square steel plates (10 2 10 cm) to reduce external air entrainment, aligned vertically opposite to each other. The burners are separated by a distance of 10 mm. Air (43.3 cm3 s11) flowed downward from the top burner, while the fuel mixture, methane diluted with nitrogen (11.7 cm3 s11 CH4 ` 10 cm3 s11 N2) flowed upward from the bottom burner. The flow rates of all components of the oxidizer and fuel stream were controlled using mass flowmeters. A flame was generated in the region where the two opposing gas streams impinged. The fuel and air are mixed with each other within a narrow zone near the stagnation point mainly due to diffusion and can hold a stable diffusion flame, provided the air and fuel velocities do not exceed certain critical values. The flame, approximately 2.5 cm in

diameter, was reasonably flat, stable, and uniform in the horizontal plane. A known vapor pressure of the HMDS was added to the fuel stream by bubbling the nitrogen through a bottle containing HMDS at room temperature. This gave a typical maximum concentration of 1.3 mol % of HMDS in the flame gases. All experimental measurements were made along the stagnation streamline (z axis), as shown in Fig. 1, where 0 mm corresponds to the fuel outlet and 10 mm is the air outlet. Temperature profiles were obtained using a platinum–platinum 13% rhodium thermocouple and corrected for radiation losses using Kaskan’s method [14]. The composition of flame gases was obtained by microprobe sampling and gas chromatography (GC) analysis using flame ionization detection (FID) or thermal conductivity detection. The HMDS disappearance in the flame was monitored by Fourier transform infrared (FTIR). Visible, UV, and FTIR emission spectroscopy were used to detect flame species. The visible emission from the flame was focused using a monochromator with a resolution of 51 nm. Surfacearea measurements were obtained using N2 adsorption at liquid nitrogen temperature (Quantasorb instrument). Results and Discussion Study of the Flame with Added HMDS The CH4–N2/air diffusion flame with no HMDS is mainly blue with a pale-yellow zone on the rich side. As small amounts of HMDS were added to the fuel stream, the flame changed color to whitish pink and became more luminous. On increasing the

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Fig. 2. Temperature and composition profiles along the z axis of the CH4–N2/air opposed diffusion flame with added HMDS. Calculated profiles are represented by lines and experimental profiles as data points; v CH4, ¶ CO, and m T.

Fig. 3. Emission profiles obtained along the z axis of the CH4–N2/air opposed diffusion flame with added HMDS. Line a, 441.4 nm emission; line b, carbon emission, and line c, visible light emission.

concentration of the HMDS further, a secondary flat flame, orange in color, appeared on the fuel side, and a stronger blue emission was observed on the lean side. These effects are similar to those previously observed for silane combustion in H2–O2 counterflow flames [7,8]. A white smoke is seen clearly to rise from the diffusion flame, and significant particle deposition was observed on the plates surrounding the burners. Laser light was found to be scattered only in the postcombustion gases, indicating the presence of silica particles. No particles were observed in the flame zone using this technique. The

SiO2 particles collected in the postcombustion gases were white in color, but those collected nearer to the flame were slightly brown, indicating the presence of SiOx [8]. The particles collected in the postcombustion gases and deposited on the surrounding plates had a surface area of 130 5 10 m2 g11, compared with a value of 200–250 m2 g11 for the particles produced by an analogous H2–N2/air diffusion flame also doped with HMDS [15]. The temperature profile and concentration profiles of some stable species obtained along the z axis for the CH4–N2/air diffusion flame with added HMDS are shown in Fig. 2. The data points represent the experimental measurements, and the lines are the calculated results from the model, which will be discussed later. Figure 3 shows the overall visible light emission and the continuum emission (at 441.4 nm) profiles [8]. The flame emission at 441.4 nm was scanned along the z axis, with the methane flame alone and with added HMDS. The latter case gave an enhanced signal (20%) compared with methane alone, which was associated with visible enhancement of the yellow zone to become a reddish yellow zone. The methane-air flame exhibited the characteristic emissions of OH* (310 nm), CH* (431 nm), and C*2 (516.5 nm), with visible yellow and blue zones. When HMDS was added, an orange color zone appeared on the fuel side, and the intensity increased with further addition of HMDS, and the blue emission on the lean side also intensified in the same way. These emissions have previously been observed [8,9]; the red emission is apparently a continuum, while the blue emission is unidentified specifically in this flame but thought to be due to SiO blue band (420 nm) and SiO2 bands (421–430 nm), although the most distinctive SiO lines appeared at 284 and 254 nm [16]. The FTIR emission studies of the flame indicated that the gas in the flame zone exhibited the expected emissions for C–H (4.55 lm), O–H (2.73–2.79 lm), Si–H (4.35–4.76 lm), and Si– O (9.15–9.77 lm) and that the HMDS rapidly decomposed in the initial stages of the flame [17]. Modeling of HMDS Decomposition Decomposition of HMDS is proposed to proceed via the following reactions: C6H18Si2O ` OH 4 2C3H9SiO ` H

(1)

C6H18Si2O ` O2 4 2C3H9SiO ` O

(2)

C6H18Si2O ` HO2 4 2C3H9SiO ` OH

(3)

C3H9SiO ` M 4 3CH3 ` SiO ` M

(3)

Table 1 lists the kinetic values used for the decomposition of HMDS, which involves the attack of O2, HO2, and OH, on the HMDS molecule resulting in the formation of a short-lived organosilicon species.

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TABLE 2 Bond dissociation energies of carbon and silicon

Bond

Bond Energy (kcal mol11)

C–H Si–C Si–O

99.1 68.3 108.1

Fig. 5. Calculated profiles for some silicon species along the z axis of the CH4–N2/air opposed diffusion flame with added HMDS. Solid lines represent fast kinetics, and dashed lines represent slower kinetics for HMDS decomposition. Lines 1 and a, HMDS; lines 2 and b, SiO; and lines 3 and c, SiO2.

Fig. 4. Some computed species profiles along the z axis of the CH4–N2/air opposed diffusion flame with added HMDS.

The values were chosen, by analogy with similar reactions of hydrocarbons, to account for the disappearance of HMDS by approximately 3 mm in the flame. The destruction of the intermediate species (CH3)3SiO depends upon the bond strengths, which are given in Table 2. The Si–C bond is weaker and breaks first, leaving the SiO species [18]. The use of reactions (1)–(4) for HMDS seem to be valid and shows that the HMDS decomposes rapidly, which is consistent with experimental observation. Some computed profiles obtained for the methane flame species are shown in Fig. 2 together with some experimental data for CH4 and CO, and the agreement between experimental and calculation is quite satisfactory. The calculated profiles of O2 are shown for illustrative purposes. Figure 4 shows computed profiles of representative radical species calculated using OPPDIF. This model is further validated by the position of the yellow and blue reaction zones (Fig. 3); The yellow zone coincides with the calculated acetylene concentration, and the blue zone coincides with the product of [CO] and [O] (Fig. 4). Computed profiles for some silicon species are shown in

Fig. 5. The computed HMDS disappearance shown in these figures demonstrates how sensitive it is to the decomposition rate. The species SiO is formed rapidly and undergoes further reaction to form SiO2. The model at this stage is not complete in that the calculated SiO2 profiles represent SiO2 in the gaseous state and does not take nucleation or particle formation into consideration. One interesting aspect of the variation of the rate of HMDS decomposition is that the ultimate SiO2 production appears relatively insensitive to its initial value. The reactions that are postulated involving SiO are given below, where the data for the heats of formation at 298.15 K have been taken from the literature [19–21]. On the Lean Side of the Reaction Zone SiO ` OH 4 SiO2 ` H DH 4 10.1 kcal mol11

(10)

SiO ` O2 4 SiO2 ` O DH 4 16.7 kcal mol11

(11)

SiO ` O (`M) 4 SiO2 (`M) DH 4 1102.5 kcal mol11

(12)

SiO is electronically analogous with CO. On this basis, it is possible to expect the reaction SiO ` O 4 SiO*2

(13)

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MATERIALS SYNTHESIS

to occur in a similar way to CO ` O 4 CO*2 and may possibly explain the enhanced blue emissions and the blue SiO2 bands. On the Rich Side of the Reaction Zone In the reducing atmosphere on the fuel side of the flame, other reactions of SiO are possible. The brownish particles that were observed were assumed to be a mixture of SiO2 and SiO and/or Si [8]. These particles, called SiOx, could be responsible for the red continuum emission and could be produced via SiO ` SiO2 4 2 SiO1.5

(14)

In addition, SiO may react with hydrogen atoms that lead to the formation of HSiO. The overall reactions can be given as SiO ` H ` (M) 4 HSiO ` (M) DH 4 120 kcal mol11

(9)

followed by HSiO ` OH 4 SiO ` H2O DH 4 164 kcal mol11

(16)

SiO2 Particle Growth Silica particles are formed when HMDS is added to the CH4–N2/air diffusion flame; however, different views prevail regarding the process of nucleation and the actual formation of the first particles. Chong and Rogg [22] have stated that if the nuclei are formed with a radius smaller than a critical radius, then they re-evaporate, and only those having radii larger than the critical radius grow. This phenomenon results in the formation of nanosized particles because of differential limit growth rates; these then coagulate, forming a floc [23]. A difficulty is the calculation of the size of the critical nucleus, as the equilibrium vapor consists not only of SiO but also of some hydroxylated products such as SiO(OH) or HSiO(OH) [24]. The species SiO is a relatively stable entity that reacts with O, OH, or O2 to form SiO2 in the gas phase. The SiO2 then condenses to produce liquid drops, and it has been proposed that the molten oxide droplets grow by Brownian collisions according to [2] 16/5 N 4 (BcT1/2C1/6 o t)

(iv) cm3,

where N is the particle concentration per B is a constant dependent on the density and molecular weight of silica (equal to 6.8 2 10112), c is the sticking coefficient (ratio of successful to actual collisions), T is absolute temperature, C0 is the number of silica molecules per cm3 in the combustion gases,

and t is the growth time. Likewise, the surface area may be expressed as SA(m2 g11) 4 1.81 2 108(T1/2 cC0t)12/5

(v)

where SA 4 3/(qR), R is the radius of the particle, and q is the density of silica. This Brownian growth can be illustrated using some recent data for particle formation in premixed H2–O2–N2 flames with added HMDS [25]. For a lean flame with T 4 2414 K (v 4 18.5 m s11), C0 4 5 2 1016 molecules SiO2 per cm3, and employing a sticking coefficient of 0.3, the following particle dimensions were computed: At t 4 5.4 2 1014 s, there are 3.8 2 1012 particles cm13 with a surface area of 262 m2 g11, yielding a radius of 5.2 nm, and the number of SiO2 molecules per particle is 1.3 2 104. At t 4 4.9 2 1013 s, there are 2.7 2 1011 particles cm13 with a surface area of 108 m2 g11, yielding a radius of 12.6 nm, and the number of SiO2 molecules per particle is 1.85 2 105. Butler and Hayhurst [25] found that samples taken early in the flame had small diameters (5–10 nm), whereas those sampled farther downstream had diameters of the order of 20–30 nm, which agrees quite well with the Brownian growth model using a sticking coefficient of 0.3. There are also important reactions involving OH, as these reactions can result in the formation of silica-containing hydroxyl bonds. Among all the intermediate and final products, SiO2 is considered to be the major one, but the species OSiOH and OHSiOH are thermodynamically favored, especially in the flame zone by the reactions [20] SiO2 ` H 4 OSiOH DH 4 158.1 kcal mol11

(15)

SiO ` OH 4 OSiOH DH 4 158.2 kcal mol11

(16)

These reactions may explain the experimental observation that the particulate SiO2 is not present in the flame zone. However, once the concentration of OH and H decay, these species will also decline to yield SiO2 particles in an oxygen-containing burned gas zone. Conclusions Hexamethyldisiloxane (HMDS) was added to a CH4–N2/air counterflow diffusion flame. The Sandia OPPDIF code was used to formulate a mechanism for HMDS oxidation in this flame. Good agreement was obtained between experimental measurements and computed results. The model illustrates that HMDS decomposes rapidly to yield SiO, which is subsequently oxidized to SiO2. The model, however, does require further refinement in the future to include nucleation of the product SiO2 to form solid particles.

SILICA FORMATION IN METHANE DIFFUSION FLAMES Acknowledgments This work was supported by the EPSRC. In addition, we thank Drs. C. J. Butler, V. Dupont, A. N. Hayhurst, and I. G. Sayce and Prof. R. Walsh for their helpful discussions.

14. 15.

REFERENCES

16.

1. Stamatakis, P., Natalie, C. A., Palmer, B. R., and Yuil, W. A., Aerosol Sci. Technol. 14:316–321 (1991). 2. Ulrich, G. D., Combust. Sci. Technol. 4:47–57 (1971). 3. Ulrich, G. D., Milnes, B. A., and Subramanian, N. S., Combust. Sci. Technol. 14:243–249 (1976). 4. Ulrich, G. D. and Subramanian, N. S., Combust. Sci. Technol. 17:119–126 (1976). 5. Koda, S., Prog. Energy Combust. Sci. 18:513–528 (1992). 6. Zachariah, M. R., Chin, D., Semerjian, H. G., and Katz, J. L., Combust. Flame 78:287–298 (1989). 7. Koda, S. and Fujiwara, O., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1986, pp. 1861–1867. 8. Chung, S.-L., Tsai, M.-S., and Lin, H.-D., Combust. Flame 85:134–142 (1991). 9. Chung, S. L. and Katz, J. L., Combust. Flame 61:271– 284 (1985). 10. Dupont, V., Pourkashanian, M., Richardson, A., and Williams, A., Proc. of the Eighth International Conf. on Transport Phenomenon, 1995. 11. Gas Research Institute, Natural Gas Oxidation Mechanism Version 2.11, Taylor and Francis, in press. 12. Miller, J. A. and Bowman, C. T., Prog. Energy Combust. Sci. 15:287–338 (1989). 13. Britten, J. A., Tong, J., and Westbrook, C. K., Twenty-

17.

18.

19.

20. 21. 22.

23. 24. 25.

1865

Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 192–202. Kaskan, W. E., Combust. Flame 2:229–286 (1958). Chagger, H. K., Hainsworth, D., Pourkashanian, M., Patterson, P. M., and Williams, A., Joint Meeting of the Portuguese, British, Spanish and Swedish Sections of the Combustion Institute, April 1–4, Funchal, 1996. Pearse, R. W. B. and Gaydon, A. G., The Identification of Molecular Spectra, Chapman and Hall Ltd, London, 1963. Cross, A. D. and Jones, R. A., An Introduction to Practical Infra-Red Spectroscopy, Butterworths, London, 1969. Walsh, R., The Chemistry of Organic Silicon Compounds (S. Patai and Z. Rappoport, Eds.), Pt. 1, Wiley, Chichester, 1989, pp. 371–391. Chase, Jr., M. W., Davies, C. A., Downey, Jr., J. R., Frurip, D. J., McDonald, R. A., and Syverud, A. N., JANAF Thermochemical Tables, 3rd ed., J. Phys. Chem. Ref. Data, Vol. 14, Suppl. No. 1 (1985). Hildenbrand, D. L. and Lau, K. H., J. Chem. Phys. 101:6076–6079 (1994). Darling, C. L. and Schlegel, H. B., J. Phys. Chem. 97:8207–8211 (1993). Chong, K. H. and Rogg, B., Joint Meeting of the British and German Sections, The Combustion Institute, 29 March–2 April, Queen’s College, Cambridge, 1993. Wright, P. G., Proc. R. Soc. Edinburgh 64:65–80 (1962). Horton, J. H. and Goodings, J. M., Can. J. Chem. 70:1069–1081 (1992). Butler, C. J. and Hayhurst, A. N., Joint Meeting of the Portuguese, British, Spanish and Swedish Sections of the Combustion Institute, April 1–4, Funchal, 1996.