SILICA NANOPARTICLES PRODUCED BY THERMAL ARC PLASMA. MODELLING

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1 Joural of Optoelectroics ad Advaced Materials Vol. 5, No. 3, September 003, p SILICA NANOPARTICLES PRODUCED BY THERMAL ARC PLASMA. MODELLING E. Balabaova * Istitute of Electroics, Bulgaria Academy of Scieces, Tzarigradsko Chaussee 7, Sofia 784, Bulgaria The preseted work summarises our results cocerig the modellig of silica vapour-tosilica aoparticles geeratio i both forms - o-aggregated ad aggregated, realised i a thermal arc plasma set up. It is show that some operatig parameters, such as iitial temperature of the vapour ad the vapour supersaturtio, play importat role i the mechaism of particle formatio ad growth, ad i the structure of the produced particles, respectively. (Received July 0, 003; accepted July 3, 003) Keywords: Silica aoparticles, Thermal arc plasma modellig. Itroductio I the recet years the powders cotaiig silica aoparticles have bee used i a lot of applicatios as fillers (reiforcig agets for producig "gree tires"), lubricats (tixopropic additios for liquids), additios ("free-flow" agets for toers), silica based catalysts (molecular sieves), etc. [- 3]. The various applicatios eed powders with differet structure, for example: for reiforcig agets powders cotaiig aggregates are suitable, for catalytic activity, o the cotrary, uaggregated powders are preferable. The structure depeds o the method of powder productio. The flame methods that are covetioally used for silica aopowders productio usually produce large aoparticle structures- aggregates [4-6]. I [7, 8] we had show that i a thermal arc plasma set up for maufacturig of silica sad to silica aopowders, there is a opportuity for geeratio of silica aoparticles i two forms - o-aggregated ad aggregated. The study of the uique coditios (thermal, hydrodyamic, kietic etc.) i the plasma set up leads to a better uderstadig of the processes for the aoparticle sythesis. By modellig these processes, it is possible to get iformatio about the relatio betwee the operatig parameters ad the particle structure. The preseted work summarises our results cocerig the modellig of silica vapour-to-silica aoparticles geeratio i o-aggregated ad aggregated form.. Modellig the processes carried out i thermal arc plasma set up.. Vaporisatio of silica sad Argo plasma jets or direct arc are used i our statioary plasma furace for the sad vaporisatio. The electric power of the arc is i the rage 8-30 kw. The sad vaporisatio is combied with SiO destructio to SiO ad oxyge. Because of that it is suitable to use reductors, * Correspodig author: emilia@ie.bas.bg

2 680 E. Balabaova which boud the oxyge ad shift the SiO destructio process to the right side, i.e. icreasig vaporisatio. Experimets were made without ad with use of reductor carbo durig the sad vaporisatio. The experimetal characteristics (specific surface area, mea surface diameter ad media mea diameter) of two samples of silica powders, obtaied i two regimes were discussed [8]. The samples were produced at the followig iitial mole ratios: SiO :Ar: =:.4 ad SiO :Ar:C =:4:. It was show that the sample, produced with use of carbo cotaied aggregate structures. To uderstad the coditios for the sad vaporisatio i both experimets, i the preset work thermodyamic equilibrium calculatios are made for the systems SiO -Ar ad SiO -Ar-C. The calculatios cocer temperature rage K ad pressure atmosphere. The stadard program is used for calculatio of chemical equilibrium cotets... Silica vapour- to-silica aoparticle coversio Silica vapour-to-silica aoparticle coversio is carried out i plasma flow reactor (see the diagram give i Fig. ). SiO (particles) Ar, SiO vapour SiO (particles) to filter O, T=300K Fig.. Diagram of the plasma flow reactor for silica aoparticle productio. I the reactor the hot flow (silica vapour ad plasma gas) comig from the plasma furace (vaporiser) is ed with a coolig flow (O ). The coolig flow, wth temperature of 300 K, eters the reactor trasversely to the hot flow. The coolig gas is take i a big excess to the silica vapour. The iitial ratios are SiO :Ar:O =:.4:0 ad SiO :Ar:C:O =:4::5, respectively. Durig the ig temperature variatio of the flow is observed, chemical reactios betwee the compoets of the hot flow ad the coolig gas as well as, aoparticle geeratio are carried out. The model of the processes i our aerosol flow reactor is developed previously [9]. It icludes the presetatio of: the fluid dyamics; the thermal- ad the mass- balaces of the flow; the chemical processes for aoparticle precursor sythesis ad the aoparticle growth by free-molecular coagulatio. Below are show the equatios presetig the fluid dyamics, the eergy- ad the mass balaces d(ρu) dx = m g i i ( x) ; () ρ = m c i i () dci gi ci du gi ( x) = + ;,... (3) dx u u dx u

3 Silica aoparticles produced by thermal arc plasma. Modellig 68 dp d( ρu + dx dx p = RT c i ) = 0 ; (4) (5) d dx ρu u + Hi( T) ci = [ g ( x) Hi( T )] i (6) where: x is the axial co-ordiate; u is the velocity; ρ is the desity; T is the temperature of the gas ture; T is the temperature of the ig gas; p is the pressure; R is the gas costat; is the umber of compoets; m i, c i, H i, are the molecular mass, the volume cocetratio ad the ethalpy of the i-th compoet, respectively; g i, g i are the source terms presetig the variatio of the i-th compoet quatity i uit volume i uit time due to chemical reactios ad ig, respectively. The term g i, presetig the coolig gas (O ) variatio, is approximated by the liear fuctio g O ( x) = g x g x x (7) where x is axial co-ordiate; g - g = is the full quatity of the ig gas; x - x is equal to the ig legth. It is kow that the ig legth depeds o the reactor diameter ad o the diameter ad the place of the coolig gas orifice. This legth for the ig of trasverse flows (as it is i our case) is cosidered to be 3d [9]. The model cosiders the chemical reactios carried out betwee silica vapour (SiO) ad coolig gas (O ). I the cases whe carbo is used for the sad vaporisatio, reactios betwee CO ad O are icluded too. The list of reactios is: k SiO + O SiO + O (8) k SiO + O + O SiO + O (9) O O O k O + O (0) k CO + O CO + O () k CO + O + O CO + O () The reactios (8) ad (9) lead to productio of the precursor of silica aoparticles, SiO gas molecule. The coversio rate of SiO to SiO (κ, %) is a importat parameter presetig the SiO oxidatio processes. [ SiO] κ = 00, % [ SiO] + [ SiO ] (3) where [SiO] ad [SiO ] are the mole fractios of the correspodig species.

4 68 E. Balabaova The further coversio of the precursors to aoparticles is the homogeeous ucleatio ad free-molecular coagulatio. Such coversio ca be realised whe the silica vapour reaches high supersaturatio. The supersaturatio is determied by the saturatio ratio (S) which is the ratio betwee the SiO mole fractio i the gas ture (vapour partial pressure) ad the saturated vapour pressure at the correspodig temperature. I our model the particle growth via free-molecular coagulatio is preseted by a discretesectioal method, described i [0]. I this method the particles are icluded i discrete groups (sectios), depedig o their masses - m, m, m, where m is the mass of the precursor. The set of equatios used i the method is: M dz l = 3 Cl, zzl + C z z + C z C lzzl z 4,,, λ (4) dt l< l= dz dt 0 M = z C, z λ z (5) 0 o l l 0 l= l l C, l = ( + ) F ( m,, T) ρ (6) F ( m, ρ, T) = 3 m π ρ ( kt) (7) Here z, z l are the umber cocetratios of particles i groups ad l (cm -3 ); z 0 is the precursor cocetratio. M is the umber of groups; C l is the coagulatio coefficiet for the particle groups (cm 3 /s); ρ is the mass desity of the particles, (for silica particles it is take to be. g/cm 3 ), k is Boltzma's costat, T is absolute temperature. The term λz presets the variatio of the particle cocetratio with the variatio of the volume durig the coolig of the gas ture i the reactor. After solvig the set of equatios 4-7, oe gets the particle size distributio fuctio (PSDF). 3. Results ad discussio To determie the silica vaporisatio temperature i both cosidered systems (SiO -Ar ad SiO -Ar-C), thermodyamic calculatios are made. I them the iitial mole ratios of the compoets correspod to that of the experimets, amely SiO :Ar=:.4 ad SiO :Ar:C=:4:. The equilibrium cotets, obtaied after thermodyamic calculatios, are preseted i the Figs. ad 3..0E+0.0E-.0E-.0E-3 SiO(s) SiO O Ar O Mole fractios.0e-4.0e-5.0e-6.0e-7 Si.0E-8.0E Temperature, K

5 Silica aoparticles produced by thermal arc plasma. Modellig 683 Fig.. Equilibrium cotet of the species i the vaporiser calculated for the iitial mole ratio SiO : Ar : = :.4..0E+0 Ar.0E- CO.0E- SiC(s) Mole fractios.0e-3.0e-4.0e-5.0e-6 SiO CO SiO(s) SiO.0E-7 Si O.0E-8 O.0E Temperature, K Fig. 3. Equilibrium cotet of the species i the vaporiser, calculated for the iitial mole ratio SiO : Ar : C = :4:. The results show that i the system without carbo, homogeeous equilibrium (without SiO - solid fractio i the system) is reached at the temperature of 3000 K. I the system SiO :Ar:C=:4: homogeeous equilibrium is reached at 000 K. The observed lower value of the SiO vaporisatio temperature is explaied by the chemical reactios betwee silica vapour ad carbo (bodig of the oxyge) ad also with the larger ratio SiO : Ar used i this case. The ifluece of differet iitial ratios SiO : Ar:C o the vaporisatio was discussed previously []. It was established that large ratios SiO : Ar ad also the use of C decrease the vaporisatio temperature. As it was metioed above, i the flow reactor the hot gas ture (silica vapour ad plasma gas) ad coolig gas (O ) are ed ad oxidatio of SiO to SiO is carried out. The rate of SiO to SiO coversio plays importat role i the complex processes realised i the reactor. I Fig.s 4 ad 5 the variatio of the coversio rate (κ) ad the variatio of the O -amout, eterig the reactor (g O) are show for the cases without ad with use of carbo, respectively. It ca be see that i both cases the SiO oxidatio is fiished very soo after the start of the O ig. The time of the complete SiO oxidatio (time whe κ=0) is called "oxidatio time". The time of the complete O ig is called "ig time". The compariso of these times for both cases show that they are shorter whe oe uses carbo.

6 684 E. Balabaova κ, % κ O, % 40 O E-3 4E-3 6E-3 8E E-.4E- time, s 0 Fig. 4. Evolutio of the SiO coversio rate (κ) ad of the O amout, itroduced i the flow reactor (g O), calculated for the mole ratio SiO : Ar : O = :.4: k, % κ O O-, % E E E E - 3 t i m e, s 0. 0 Fig. 5 Evolutio of the SiO coversio rate (κ) ad of the O amout, itroduced i the flow reactor (g O), calculated for the mole ratio SiO : Ar : C: O = :4::5. The flow temperature ad the vapour supersaturatio are other importat parameters. Their variatios are show i Fig. 6.

7 Silica aoparticles produced by thermal arc plasma. Modellig 685 Saturatio ratio E+3 E+ E+ E+0 E+9 E+8 E+7 E+6 E+5 E+4 E+3 E+ E+ E+0 T T S S E-4 E-3 E- E- time, s Temperature, K Fig. 6 Evolutio of the temperature ad the saturatio ratio durig the O ig i the flow reactor, i the cases of: SiO : Ar: C: O = :4::5 (T, S) ad SiO : Ar: O = :.4:0 (T, S). I both cases a iitial icreasig of the temperature ca be see. This is explaied by the exothermic effect of the oxidatio reactios. After the complete SiO oxidatio (at t= s ad t= s, respectively) the temperature drops with a costat rate util the ed of the coolig gas ig (i.e. to t m = s ad t m = s, respectively). I the case of pure silica vapour, the iitial temperature icreases from 3000 K to over 4700 K durig the complete SiO oxidatio. The saturated vapour pressure is very high i this temperature rage ad the SiO partial pressure ca ot reach it, so there is o opportuity for cluster formatio durig the oxidatio. The supersaturatio is reached after the complete oxidatio, log after the start of the ig. Three differet values of supersaturatio (7.6, 75 ad 95) have bee chose as iitial values for the homogeeous ucleatio ad the free-molecular coagulatio oset. Usig eqs. 4-7, the correspodig particle size distributio fuctios (PSDFs) are calculated (see Fig. 7). The choice of the supersaturatio is arbitrary, sice we do ot have experimetal proof of the superaturatio at which the process starts. I Fig. 7 the experimetal PSDF is also show. As it is see the best agreemet betwee the experimetal ad the calculated PSDFs is observed at the highest supersaturatio Iegral PSDF, % diameter, m Fig. 7. PSDFs calculated at the supersaturatio: 7.6 (), 75 (), 95 (3); experimetal PSDF (4).

8 686 E. Balabaova I the case whe carbo is used durig the vaporisatio, the iitial temperature of the gas ture, eterig i the reactor is 000 K. At this temperature, the SiO saturated vapour pressure is very low, ad it ca be easily reached ad overreached. Eve the small SiO mole fractio, obtaied at the begiig of SiO oxidatio is quite eough for reachig a large supersaturatio. After that durig the oxidatio, sice the temperature icreases very fast ad the correspodig saturatio vapour pressure becomes very high, the saturatio ratio decreases. Whe the oxidatio is completed ad the temperature starts to decrease, the saturatio ratio icreases costatly. This type of variatio of the saturatio ratio suggests a more complicated mechaism of aoparicle growth, compared to those of pure silica vapour. We could ot apply free-molecular coagulatio model (eqs.4-7) directly because the vapour codesatio at the very begiig of the ig is carried out simultaeously with chemical oxidatio reactios. A ew model is eeded i which the ifluece of the chemical source of precursors o coagulatio should be estimated. The coalescece (fluid fusio) of the particles must be cosidered, too. It is kow that ear to temperature 000 K the SiO particles ca ot completely fuse because of the high viscosity of SiO at this temperature, so the productio of aggregate structures becomes possible. We have some experimetal results, provig the cotet of aoparticle aggregates i a sample which is produced with use of carbo [ 8]. 4. Coclusio The coditios for productio of silica aoparticles i thermal arc plasma set up are discussed for two experimetal cases. It is show that some operatig parameters such as iitial vapour temperature ad vapour supersaturatio are importat for the mechaism of particle formatio ad growth, ad, respectively, for the structure of the produced particles. Refereces [] A. Blume, B. Freud, i Silica 98, Mulhouse, Frace, -4 Sept., 6, 998. [] H. Barthel, M. Dreyer, T. Gottschalk-Gaudig i Silica 00, Mulhouse, Frace, 3-6 Sept. 00, report umber 68. [3] G. Schulz, R. Fricke, G Lischke, B. Parlitz, ISPC-, Loughborough, Eglad, 993, editor J. Harry, p.-7. [4] D. W. Schaefer, A. Hurd, Aerosol Sciece ad Techology,, 876 (990). [5] W. Koch, S. Fridlader J. Colloid ad Iterface Sciece, 40, 49 (990)3 [6] M. Choi, J.Lee, H.W. Kim, Joural of Naoparticle Research,, 69 (999). [7] E. Balabaova, D. Oliver i "Silica 00", Mulhouse, Frace, 3-6 Sept. 00, report umber 4. [8] E. G. Balabaova, D. H. Oliver, i: "Naosciece & Naotechology '0", editors E. Balabaova, I. Dragieva, Hero press, Sofia, 9, 00 [9] E. Balabaova, Vacuum 69, 07 (003). [0] E.G. Balabaova, A. A. Levitsky, D. H. Oliver: Czech. J. Phys. 44, 39 (994). [] E. Balabaova, i "Silica 98" Mulhouse, Frace, -4 Sept., 67, 998.