H. k. Chagger, The Formation of Sio2 From Hexamethyldisiloxane Combustion in Counterflow Methane-Air Flames

7/25/2019 H. k. Chagger, The Formation of Sio2 From Hexamethyldisiloxane Combustion in Counterflow Methane-Air Flames

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1859

Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 18591865

THE FORMATION OF SiO2

FROM HEXAMETHYLDISILOXANECOMBUSTION IN COUNTERFLOW METHANE-AIR FLAMES

H. K. CHAGGER, D. HAINSWORTH, P. M. PATTERSON, M. POURKASHANIANand A. WILLIAMS

Department of Fuel and EnergyUniversity of Leeds

Leeds, LS2 9JT, UK

Silica formation from hexamethyldisiloxane (HMDS) oxidation was studied by means of a CH4N2/airopposed diffusion flame technique to which vaporized HMDS was added to the fuel flow. The CH4N2/air flame changed color from a pale-blue flame to whitish pink color when small amounts of HMDS wereintroduced in the flame. Increasing concentrations (1.3 mol %) made the flame more luminous, and asecond thin-flame zone, orange in color, appeared on the fuel side. Emission spectroscopy revealed theexistence of SiH and SiO species. SiO2particles were observed only in the postcombustion gases, andthe analysis of solid materials suggested the formation of fused silica particles, which were initially about10 nm in size, forming outside the flame zone in these experiments. The overall mechanism in additionto that for methane oxidation is suggested as follows:

C H Si O OH 2C H SiO H6 18 2 3 9

C H Si O O 2C H SiO O6 18 2 2 3 9

C H Si O HO 2C H SiO OH6 18 2 2 3 9

C H SiO M 3CH SiO M3 9 3

SiO HO HSiO O2 2

SiO OH SiO H2

SiO O M SiO M2

SiO O SiO O2 2

together with the formation of SiOH and SiO(OH) species. The agreement betweenthe model predictionsusing Sandia code OPPDIF and the experimental data was found to be satisfactory. The model appearsto be a useful tool in elaboration of chemistry of formation of SiO2in flames used to synthesize pure silicain this way.

Introduction

Flame synthesis is a useful technology for the syn-thesis of oxide powders like TiO2, Al2O3, and SiO2.Fumed silica obtained from this process has numer-ous commercial applications and is used as a pre-cursor material in the fiber-optic, ceramic, semicon-ductor, pigment, and dye industries. For theproduction of high-purity nanometer-sized particles

(nanoparticles in the 2100 nm size range), gas-phase flame synthesis is particularly attractive to thematerial industry as this process can be scaled upand is economically advantageous [1]. However, therole of flame species, such as OH, and carbon com-pounds may determine the purity of the final prod-uct.

The synthesis of silica by the oxidation of silaneand silicon tetrachloride in hydrogen or hydrocarbonflames has been studied both experimentally and

theoretically [24]. The study showed that both thefinal silica aggregate and primary particle size werestrongly affected by flame temperature and resi-dence time, whereas the nucleation and surface re-action did not affect particle dynamics. Thesestudies

were extended to show that coalescence was a rate-controlling step in the growth of silica [24]. Thebasic flame chemistry, such as kinetics, and ther-modynamic properties of simple silicon compounds

like SiH4and SiCl4 are fairly well documented [59] as a consequence of their use in chemical vapordeposition (CVD) and semiconductor manufactur-ing. Currently, industrial interest has been directedto the use of organosiloxanes as a precursor for thesesilica synthesis processes because of their ease ofavailability and economic advantage. These com-pounds include the highly methylated siloxanes anddisiloxanes and the cyclic siloxanes([(CH3)2SiO]n).


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1860 MATERIALS SYNTHESIS

Fig. 1. Schematic representation of an opposed diffu-sion flame. The fuel outlet corresponds to zero on the z-axis distance scale, and the air outlet corresponds to 10mm.

In this paper, the formation of SiO2from the or-ganodisiloxane, hexamethyldisiloxane (HMDS),(CH3)3SiOSi(CH3)3, is reported. The oxidation ofHMDS has been studied by the addition of smallamounts to a counterflow methane-air diffusionflame; a process that is similar to that used in indus-try where the production of high-purity fused silicaformation occurs through the diffusion flame oxi-dation of a vaporized silicon-containing compound.A number of previous studies have been undertakenin opposed diffusion flames of silane-air or of silanedecomposition in opposed H2O2 diffusion flames,and some of these cases have been numericallymod-eled [59]. In addition, the growth processes leading

to formation of silica particles have been investigated[24]. Although the general features of these flameshave been described, there has been little validationof the detailed chemical models, and there are un-certainties about both the reaction kinetics and thethermochemistry. To our knowledge, there havebeen no studies of HMDS combustion in opposeddiffusion flames to date, and in this paper, we reportthe results of such a study.

Numerical Model

Calculations were carried out to simulate thecounterflow diffusion flame, as shown diagrammat-ically in Fig. 1, by means of the Sandia OPPDIF

code. The distance between the two burners in thetheoretical model can be varied. The distance cho-sen between the two opposing burners in this model

was 1 cm to reproduce the actual experimental con-ditions; this made the solution more difficult to con-

verge than the conventional distance of several cen-timeters. The model was applied to a fuel streamcontaining methane and nitrogen/HMDS on oneside and an oxidizer stream consisting of air on theother. The methane combustion mechanism was

modified to incorporate an oxidation mechanism forHMDS. The computations were conducted suchthat the gradient of axial velocitydV/dxwas constant

on both sides of the flame, and Eq. (i) held [10]1/2dV q dVair (i) dx q dx airfuel

whereqis the density, and this allows the use of Eq.(ii):

U V (ii)

r 2(x X )stag

where U is the radial velocity,ris the radius to anypoint in the flow defined by (x,r), and Xstag is theabscissa of the computed stagnation point. Thestretch rate selected to characterize the flame wasdefined by the expression (J/qfuel)1/2, whereJ is theconstant eigenvalue of the experimental flow config-uration, which is equal to

1 dP r dr

whereP is the pressure.The chemical mechanism used for the methane

oxidation is the natural gas oxidation scheme givenby GRI version 2.11 [11]. The Miller and Bowmanreaction scheme [12] was also tried, and it gave simi-lar results. Table 1 lists the additional reactionsadded to the mechanism to describe HMDS decom-position and the subsequent production of SiO andSiO2. The area of greatest uncertainty relates to thedecomposition of the HMDS. In previous modeling

studies [5,13], silane was taken to react in an anal-ogous manner to methane with allowance made forthe difference in bond energies of SiH and CH.Similarly, in this study, HMDS was assumed to be-have as a typical higher molecular weight hydrocar-bon in the initial stages of the flame. The SiO andSiO2reactions have been taken from the Britten etal. mechanism [13]. The reactions of the productCH3 are incorporated into the methane oxidationreaction scheme, which consists of 190 reactions intotal.

The rate constants are expressed in modified Ar-rhenius form

nk AT exp(E/RT) (iii)a

Reverse rate constants are calculated from the for-ward rates, and the appropriate equilibrium con-stants are computed from available or estimatedthermochemical data. Thermochemical data for Siintermediates such as HSiO were assumed to beidentical to that of C-analogs. SiO2was assumed tobe formed initially in the gaseous state.

The model was used to compute temperature andspecies concentration profiles for a counterflowCH4N2/air diffusion flame doped with HMDS. The


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SILICA FORMATION IN METHANE DIFFUSION FLAMES 1861

TABLE 1HMDS oxidation mechanism, the units are cm3, mol, s, and cal

Reaction

Rate Constant

A n Ea

1 C6H18Si2O OH 2C3H9SiO H 9.6 E 12 2 1002 C6H18Si2O O2 2C3H9SiO O 6.0 E 13 0 2613 C6H18Si2O HO2 2C3H9SiO OH 1.3 E 13 0 1034 C3H9SiO M 3CH3 SiO M 8.7 E 12 0 05 SiO HO2 HSiO O2 5.27 E 12 0 34,3006 SiO H2O HSiO OH 2.84 E 15 0 105,0007 SiO OH HSiO O 2.88 E 14 0 87,9008 SiO H2 HSiO H 1.31 E 15 0 90,0009 SiO H M HSiO M 1.74 E 11 0 11,600

10 SiO OH SiO2 H 4.0 E 12 0 5,70011 SiO O2 SiO2 O 1.0 E 13 0 6,50012 SiO O M SiO

2 M 2.5 E 15 0 4,370

kf ATn exp(Ea/RT).

model studied here is far from a complete HMDScombustion mechanism, as it does not fully considernucleation of SiO2to form solid particulates or thefull reactions of silicon-containing intermediate spe-cies.

Experimental Methods

The oxidation of HMDS is studied in a counter-flow CH4N2/air diffusion flame to obtain some ex-

perimental measurements with which the results ofthe model may be compared quantitatively. The ad-

vantage of using a counterflow geometry is that, toa first approximation, flow along the stagnationstreamline can be described as one-dimensional.The basic characteristics of the flow field of such aflame are illustrated in Fig. 1. The experimental ar-rangement consists of two circular burners (d 2.5cm), containing wire mesh screens as flow straight-eners and mounted in square steel plates (10 10cm) to reduce external air entrainment, aligned ver-tically opposite to each other. The burners are sep-arated by a distance of 10 mm. Air (43.3 cm3 s1)flowed downward from the top burner, while thefuel mixture, methane diluted with nitrogen (11.7

cm3

s1

CH4 10 cm3

s1

N2) flowed upward fromthe bottom burner. The flow rates of all componentsof the oxidizer and fuel stream were controlled usingmass flowmeters. A flame was generated in the re-gion where the two opposing gas streams impinged.The fuel and air are mixed with each other within anarrow zone near the stagnation point mainly due todiffusion and can hold a stable diffusion flame, pro-

vided the air and fuel velocities do not exceed certaincritical values. The flame, approximately 2.5 cm in

diameter, was reasonably flat, stable, and uniform inthe horizontal plane. A known vapor pressure of theHMDS was added to the fuel stream by bubblingthe nitrogen through a bottle containing HMDS atroom temperature. This gave a typical maximumconcentration of 1.3 mol % of HMDS in the flamegases. All experimental measurements were madealong the stagnation streamline (zaxis), as shown inFig. 1, where 0 mm corresponds to the fuel outletand 10 mm is the air outlet. Temperature profiles

were obtained using a platinumplatinum 13% rho-

dium thermocouple and corrected for radiationlosses using Kaskans method [14]. The compositionof flame gases was obtained by microprobe samplingand gas chromatography (GC) analysis using flameionization detection (FID) or thermal conductivitydetection. The HMDS disappearance in the flame

was monitored by Fourier transform infrared(FTIR). Visible, UV, and FTIR emission spectros-copy were used to detect flame species. The visibleemission from the flame was focused using a mon-ochromator with a resolution of1 nm. Surface-area measurements were obtained using N2adsorp-tion at liquid nitrogen temperature (Quantasorbinstrument).

Results and Discussion

Study of the Flame with Added HMDS

The CH4N2/air diffusion flame with no HMDSis mainly blue with a pale-yellow zone on the richside. As small amounts of HMDS were added to thefuel stream, the flame changed color to whitish pinkand became more luminous. On increasing the


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Fig. 2. Temperature and composition profiles along thez axis of the CH4N2/air opposed diffusion flame with

added HMDS. Calculated profiles are representedby linesand experimental profiles as data points; CH4, CO,and T.

Fig. 3. Emission profiles obtained along thezaxis of theCH4N2/air opposed diffusion flame with added HMDS.Line a, 441.4 nm emission; line b, carbon emission, andline c, visible light emission.

concentration of the HMDS further, a secondary flatflame, orange in color, appeared on the fuel side, and

a stronger blue emission was observed on the leanside. These effects are similar to those previouslyobserved for silane combustion in H2O2counter-flow flames [7,8]. A white smoke is seen clearly torise from the diffusion flame, and significant particledeposition was observed on the plates surroundingthe burners. Laser light was found to be scatteredonly in the postcombustion gases, indicating thepresence of silica particles. No particles were ob-served in the flame zone using this technique. The

SiO2particles collected in the postcombustion gaseswere white in color, but those collected nearer to theflame were slightly brown, indicating the presence

of SiOx[8]. The particles collected in the postcom-bustion gases and deposited on the surroundingplates had a surface area of 130 10 m2 g1, com-pared with a value of 200250 m2 g1 for the par-ticles produced by an analogous H2N2/air diffusionflame also doped with HMDS [15].

The temperature profile and concentration pro-files of some stable species obtained along the zaxisfor the CH4N2/air diffusion flame with addedHMDS are shown in Fig. 2. The data points repre-sent the experimental measurements, and the linesare the calculated results from the model, which willbe discussed later. Figure 3 shows the overall visiblelight emission and the continuum emission (at 441.4nm) profiles [8]. The flame emission at 441.4 nm wasscanned along the z axis, with the methane flamealone and with added HMDS. The latter case gavean enhanced signal (20%) compared with methanealone, which was associated with visible enhance-ment of the yellow zone to become a reddish yellowzone. The methane-air flame exhibited the charac-teristic emissions of OH* (310 nm), CH* (431 nm),and (516.5 nm), with visible yellow and blueC*2zones. When HMDS was added, an orange colorzone appeared on the fuel side, and the intensityincreased with further addition of HMDS, and theblue emission on the lean side also intensified in thesame way. These emissions have previously been ob-served [8,9]; the red emission is apparently a contin-uum, while the blue emission is unidentified specif-ically in this flame but thought to be due to SiO blue

band (420 nm) and SiO2 bands (421430 nm), al-though the most distinctive SiO lines appeared at284 and 254 nm [16]. The FTIR emission studies ofthe flame indicated that the gas in the flame zoneexhibited the expected emissions for CH (4.55lm),OH (2.732.79lm), SiH (4.354.76lm), and SiO (9.159.77 lm) and that the HMDS rapidly de-composed in the initial stages of the flame [17].

Modeling of HMDS Decomposition

Decomposition of HMDS is proposed to proceedvia the following reactions:

C H Si O OH 2C H SiO H (1)6 18 2 3 9

C H Si O O 2C H SiO O (2)6 18 2 2 3 9

C H Si O HO 2C H SiO OH (3)6 18 2 2 3 9

C H SiO M 3CH SiO M (3)3 9 3

Table 1 lists the kinetic values used for the decom-position of HMDS, which involves the attack of O 2,HO2, and OH, on the HMDS molecule resulting inthe formation of a short-lived organosilicon species.


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TABLE 2Bond dissociation energies of carbon and silicon

BondBond Energy(kcal mol1)

CH 99.1SiC 68.3SiO 108.1

Fig. 4. Some computed species profiles along thezaxisof the CH4N2/air opposed diffusion flame with addedHMDS.

Fig. 5. Calculated profiles for some silicon species alongthez axis of the CH4N2/air opposed diffusion flame withadded HMDS. Solid lines represent fast kinetics, anddashed lines represent slower kinetics for HMDS decom-position. Lines 1 and a, HMDS; lines 2 and b, SiO; andlines 3 and c, SiO2.

The values were chosen, by analogy with similarreactions of hydrocarbons, to account for the dis-appearance of HMDS by approximately 3 mm in theflame. The destruction of the intermediate species(CH3)3SiO depends upon the bond strengths, whichare given in Table 2. The SiC bond is weaker andbreaks first, leaving the SiO species [18]. The use ofreactions (1)(4) for HMDS seem to be valid andshows that the HMDS decomposes rapidly, which isconsistent with experimental observation. Somecomputed profiles obtained for the methane flamespecies are shown in Fig. 2 together with some ex-perimental data for CH4and CO, and theagreement

between experimental and calculation is quite sat-isfactory. The calculated profiles of O2are shown forillustrative purposes. Figure 4 shows computed pro-files of representative radical species calculated us-ing OPPDIF. This model is further validated by theposition of the yellow and blue reaction zones (Fig.3); The yellow zone coincides with the calculatedacetylene concentration, and theblue zone coincides

with the product of [CO] and [O] (Fig. 4). Com-puted profiles for some silicon species are shown in

Fig. 5. The computed HMDS disappearance shownin these figures demonstrates how sensitive it is tothe decomposition rate. The species SiO is formedrapidly and undergoes further reaction to form SiO2.The model at this stage is not complete in that thecalculated SiO2profiles represent SiO2in the gase-ous state and does not take nucleation or particle

formation into consideration. One interesting aspectof the variation of the rate of HMDS decompositionis that the ultimate SiO2 production appears rela-tively insensitive to its initial value.

The reactions that are postulated involving SiO aregiven below, where the data for the heats of forma-tion at 298.15 K have been taken from the literature[1921].

On the Lean Side of the Reaction Zone

SiO OH SiO H2

1DH 0.1 kcal mol (10)

SiO O SiO O2 2


SiO O (M) SiO (M)2

1DH 102.5 kcal mol (12)

SiO is electronically analogous with CO. On this ba-sis, it is possible to expect the reaction

SiO O SiO* (13)2


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to occur in a similar way to CO O andCO*2may possibly explain the enhanced blue emissionsand the blue SiO2bands.

On the Rich Side of the Reaction Zone

In the reducing atmosphere on the fuel side of theflame, other reactions of SiO are possible. Thebrownish particles that were observed were assumedto be a mixture of SiO2and SiO and/or Si [8]. Theseparticles, called SiOx, could be responsible for thered continuum emission and could be produced via

SiO SiO 2 SiO (14)2 1.5

In addition, SiO may react with hydrogen atoms thatlead to the formation of HSiO. The overall reactionscan be given as

SiO H (M) HSiO (M)1DH 20 kcal mol (9)

followed by

HSiO OH SiO H O2

1DH 64 kcal mol (6)

SiO2Particle Growth

Silica particles are formed when HMDS is addedto the CH4N2/air diffusion flame; however, differ-ent views prevail regarding the process of nucleationand the actual formation of the first particles. Chong

and Rogg [22] have stated that if the nuclei areformed with a radius smaller than a critical radius,then they re-evaporate, and only those having radiilarger than the critical radius grow. This phenome-non results in the formation of nanosized particlesbecause of differential limit growth rates; these thencoagulate, forming a floc [23]. A difficulty is the cal-culation of the size of the critical nucleus, as theequilibrium vapor consists not only of SiO but alsoof some hydroxylated products such as SiO(OH) orHSiO(OH) [24].

The species SiO is a relatively stable entity thatreacts with O, OH, or O2 to form SiO2 in the gasphase. The SiO2 then condenses to produce liquiddrops, and it has been proposed that the molten ox-

ide droplets grow by Brownian collisions accordingto [2]

1/2 1/6 6/5N (BcT C t) (iv)o

whereN is the particle concentration per cm3,B isa constant dependent on the density and molecular

weight of silica (equal to 6.8 1012),cis the stick-ing coefficient (ratio of successful to actual colli-sions),Tis absolute temperature, C0is the numberof silica molecules per cm3 in the combustion gases,

andt is the growth time. Likewise, the surface areamay be expressed as

2 1 8 1/2 2/5SA(m g ) 1.81 10 (T cC t) (v)0

where SA 3/(qR),R is the radius of the particle,and q is the density of silica. This Brownian growthcan be illustrated using some recent data for particleformation in premixed H2O2N2flames with addedHMDS [25]. For a lean flame with T 2414 K (v 18.5 m s1),C0 5 1016 molecules SiO2percm3, and employing a sticking coefficient of 0.3, thefollowing particle dimensions were computed: At t 5.4 104 s, there are 3.8 1012 particles cm3

with a surface area of 262 m2 g1, yielding a radiusof 5.2 nm, and the number of SiO2 molecules perparticle is 1.3 104. Att 4.9 103 s, there are2.7 1011 particles cm3with a surface area of 108m2 g1, yielding a radius of 12.6 nm, and the numberof SiO

2molecules per particle is 1.85 105. Butler

and Hayhurst [25] found that samples taken early inthe flame had small diameters (510 nm), whereasthose sampled farther downstream had diameters ofthe order of 2030 nm, which agrees quite well withthe Brownian growth model using a sticking coeffi-cient of 0.3.

There are also important reactions involving OH,as these reactions can result in the formation of sil-ica-containing hydroxyl bonds. Among all the inter-mediate and final products, SiO2is considered to bethe major one, but the species OSiOH and OHSiOHare thermodynamically favored, especially in theflame zone by the reactions [20]

SiO H OSiOH2


SiO OH OSiOH


These reactions may explain the experimental ob-servation that the particulate SiO2is not present inthe flame zone. However, once the concentration ofOH and H decay, these species will also decline to

yield SiO2particles in an oxygen-containing burnedgas zone.

Conclusions

Hexamethyldisiloxane (HMDS) was added to a

CH4N2/air counterflow diffusion flame. The SandiaOPPDIF code was used to formulate a mechanismfor HMDS oxidation in this flame. Good agreement

was obtained between experimental measurementsand computed results. The model illustrates thatHMDS decomposes rapidly to yield SiO, which issubsequently oxidized to SiO2. The model, however,does require further refinement in the future to in-clude nucleation of the product SiO2to form solidparticles.


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Acknowledgments

This work was supported by the EPSRC. In addition, we

thank Drs. C. J. Butler, V. Dupont, A. N. Hayhurst, andI. G. Sayce and Prof. R. Walsh for their helpful discussions.

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Documents

H. k. Chagger, The Formation of Sio2 From Hexamethyldisiloxane Combustion in Counterflow Methane-Air Flames