Ultrafine Refractory Particle Formation in Counterflow ...

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Co m b u st.Sci.a n d T e ch .,1 9 9 2Vo , l . 82, pp 169-183 Photocopying permitted by license only

UltrafineRefractoryParticleFormationin Counterflow D i f f u s i o nF l a m e s JOS E P HL. K A TZ*and CHE NG -HUNGHUNG De p a rt me not f Ch e mic a l Engineering,The Johns Hopkins University,Baltimore,Maryland 2121I, U.S.A. Abstract-The effect of process variables (percursor concentration, residence time, temperature, and electric field) on ceramic powder formation was studied using a counterflow diffusion flame burner. GeClo and SiClnwere usedas sourcematerialsfor the formation o[ GeO, and SiO, in hydrogen-oxygen flames. In-situ particle size and number density were determined using dynamic light scattering and 90' light scattering.A thermophoretic sampling method also was used to collect particles directly onto carbon coated grids, and their sizeand morphology examinedusing transmissionelectronmicroscopy.Increasing the precursor concentration caused larger particles to form and enhancedsurface growth effects;decreasing residencetime favored homogeneousnucleationas the particle formation route; using higher temperatures resultedin a larger sizefor the fundamentalparticlesbut lessaggregation.Using GeCloin the presenceof a 1250V/cm electric field, a three fold increasein particle size was obtained. Processvariables also determinedthe degreeof crystallinity; crystallineparticleswere usually found when mostly small particles were produced; amorphous particleswere found when high precursor concentration,long residencetime, and low temperatureswere used. These results show how desired particle characteristics(particle size, particle morphology, crystaliine or amorphous) can be generatedthrough control of flame conditions. Key words' ceramic powders,GeO, , SiO, , crystalline,amorphous,counter-flowdiffusion, flame, residence time, electric field

INTRODUCTION A number of refractory materials are produced as fine particles by combustion of appropriate precursors. In the pigment and reinforcing agent industries, tens of thousandsof tons of SiO2,TiO, and FerOr,areproducedannuallyin this way (Mezey, 1966; Wiseman, 1976; Herman, 1976). Flame generated particles are used in the production of optical fibers as SiOr-GeO, fumes which are deposited as a preform, treated to remove water and other undesirablesubstances,melted, and drawn (Cheo, 1990;Ulrich,1984; Kawachi et al., 1980).They are also used as the starting materials for the manufacture of high-tech ceramics.These ceramics are needed in extreme environments such as internal combustion, gas turbine, jet and rocket engines (Bowen, 1980;Sanders,1984).They are lighter and able to withstand much higher operating temperaturesthan the best superalloyscurrently available. Ceramics are also the support medium of most catalysts, and sometimes are also the active ingredient. The catalytic reactivity of mixed oxide powders, e.9.,TiO2-SiO2, is larger than that of the separated components (Rieck and Bell, 1986) and appear to be correlated to changesin their acid-baseproperties(Bielafiski and Haber, l99l). Flame generation may well be a way of obtaining catalyst materials with interesting new properties. In ceramic processing,the particle size, the particle size distribution, the presence of lack of agglomerates,and the structure are very important to the rate of sintering and microstructure development (Halloran, 1984). For example, large ordered rCorrespondence should be sent to: Prof. Joseph L. Katz, Department of Chemical Engineering, The Johns Hopkins University Baltimore, Maryland 21218,U.S.A.

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regions of particles in large gapsand mismatchesat their boundaries.To prevent their deleteriouseffectson strength, one needssize distributions with standard deviations greater than l0o/o(Dirksen et al., 1989).However, extremely broad sizedistributions have the disadvantagethat the largestparticles produce large grains, and thus defect filled grain boundaries in the sintered ceramic. This paper describes and discussesthe effects of temperature, residence time, precursorsconcentration, and d.c. electricfield strength on the formation and growth of ceramicparticles.GeCloand SiCl4wereusedas precursorsin a HrlAr-OrlAr flame to produce the glassyoxide powders,GeO, and SiOr. Oxides,whoseparticle formation processes are very differentfrom thosein glassymaterials(e.g.,TiO, and AlrOr), will be discussedin our next paper.

EXPERIMENT

DESCRIPTION

A rectangular counterflow diffusion flame burner was used in this study (Chung and Katz, 1985).This burner (seeFigure 1) differs from traditional counterflow burners (Pandya and Weinberg, 1964;Tsuji, 1982)by its rectangularinstead of cylindrical geometry,its separateend sections,and its fusedsilica end windows. With this burner, the chemical and physical processescontrolling particle formation, growth, and

PARTICLE FORMATION IN FLAMES

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agglomerationcan be studiedin a uniform and highly controllable flame environment. The burner consistsof two vertically opposedtubes of rectangularcrosssection.Each tube consistsof three channels,a central main channel, and two end channels.Fused silica plates connect the outsides of the two end channels of the two opposed tubes, thus forcing the combustion gasesto flow out through the front and back. Flangesfitted to both the top and bottom of the burner keep the gas outflow parallel to the burner surfaces, and minimize entrainment of surrounding air. Oxidizer (oxygen, 99.6oA,diluted by argon, 99.996%) flows downward from the top tube, while fuel (hydrogen, 99.99o/o,diluted by argon, 99.998o/o,and also containing a low concentration of the appropriate precursor) flows upward from the bottom tube. Temperatures were controlled by changing the amounts of argon added to the fuel and oxidizer streams. Residencetimes were controlled by changing the flow rates of the fuel and oxidizer gas streams.No precursor vapor is added to the mixtures which are fed to the end channels.By precursor we mean the sourcematerial (e.9.,SiHo, TiCl4, AI(CH3)3) which after combustion resultsin the formation of a ceramic oxide. A flame is generatedin the region where the two opossedgas streams impinge. This flame is very flat and uniform in the horizontal plane. Temperature measurementsmade using thermocouples and speciesconcentration profiles measuredby light absorption confirm this uniformity. The basic geometricalcharacteristics of the flow field are also illustrated in Figure l. All measurements,temperature,particle size, and light scatteringintensity, were made on the stagnation point streamline(i.e., f : 0). We have also made studiesas a function of d.c. electricfield. The electrodesare the top wire screens(12.7mm x 63.5mm) and their surrounding flanges (each is 9.3mm x 63.5mm). D.c. electricfields of any desiredstrength(up to 4100V) were generatedby connecting severalbatteriesin series.Polarity of the electrodescould be invertedusing a double pole-doublethrow switch.The data presentedin Figures l2 and l3 are plotted as V/cm, which was calculatedby dividing the measuredvoltage by the 15mm distancebetweenthe burner flanges. The flow rates of all the componentsof the oxidizer stream and the fuel streamwere measuredusing calibrated flowmeters.Known vapor concentrationsof precursors which are liquid at room temperature,i.e., GeClo(Alfa, 99o/opure) and SiClo (Alfa, 99oh pure), were produced by bubbling a small part of the fuel stream through a gas washingbottle containing the appropriateliquid (and measuringthis flow with a gas flowmeter). The vapor feed rates were calculatedusing the known vapor pressuresof the precursors and the measuredfuel stream flow rates. Temperature was measured by two techniques with overlapping ranges: below 2000K, by using silica coated Pt-PtlO%Rh thermocouples,and correcting for radiation losses(Chung andKatz,l985); above 1500K, by measuringthe rotational fine structure in the UV absorption spectraof OH, thus determining the distribution of the ground state population of hydroxyl radicals, and from it, their rotational temperature (Dieke and Crosswhite, 1962; Kostkowski and Broida, 1956). The uncertaintiesin our rotational temperature measurementare greatly improved over thosewe previouslypublished(Chung andKatz,l985) becauseof (l) a 6 fold increase in light intensity entering the monochromator, obtained by rotating the light beam 90" using a right angle fused silica prism held at a 45o angle, and (2) a 3 fold decreasein noiseusing a combination of analogfiltering and digital averaging.Theseimprovements were especiallyuseful when the OH concentrationswere low and thus the absorbances were weak. Rotational temperaturesdown to - 1500K could be measured.Figure 2 shows the temperature profiles for the actual flame conditions used in this study, measuredby both the optical method (solid symbols) and using thermocouples(open

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Z (mm) FIGURE ? Temperature profiles as a function of height in the burner, i.e., the Z direction.Symbols (r, l) are for flame l, (r, a) are for flame 2, and (f ,b) ur. to, na-e g.

temperature profiles are not precisely bell ler temperature flames, becausethe upper exhaustgasesbut the temperatureof the the addition of a water cooling loop (to prevent premature decomposition of precursor vapor). Particle sizeswere determined_usingDynamic r-ighf Scattering(DLS) (chang and Penner,l98l; ohsawa et a\.,1983;pecoia, 19g5;d'ernard,lggg').'The experim"enial apparatusfor the DLS measurements consistedof a 5 W argon ion laseroperatingat 514.5nm at a single line power of l.5w (seeFigure 3). Tie burner can be mo"ved reproduciblyusinga precisionslidefrom the opticalline usedfor absorptionmeasurements to the laser line, the line used for scattlring measurements. On the laser line, spatialresolutionand signal-to-noise ratio weresiriultaneouslyimproved by focusing theiaser beam using a 300mm focal length lens, thus resultingin a beam waist of 0'l6mm' Forward scatteredlight was collectedaf a shallowangle(7.6") and focused usign a 150mm focal length lens (usedin 2f optics) onto u pn6to--uriipri..ipl,ir, EMI9785B) after pa_ssing through a (l nm half bandwidth)laJerline filter. The output signal from the PMT was fed to a Fast Fourier Transform Analyzer(Rapid Systels .R?qg0) Usually the frequency range was set to about five timei of the fiequency at half height. Frequenciesup to 2001H2 were used,dependingon particles iir.. in. frequency at half height (A/) was determined uy htting a rirentiian profile to tn" power spectrum obtained averaging 64 scans. Particle diameters (d) were then .by calcllated using relationshipsbetweenihe f.equency at half height una' tir. diffusion coefficient (D) of the particles (Hinds and Reist, lri721,1.e.,

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in this calculation. the particlesare assumedto be both sphericaland mono-dispersed particle We estimatean uncertaintyin the diameterof about 4oh dueto the uncertainties in temperature and frequency at half height. A thermophoretic sampling method (Dobbins and Megaridis,1987)was also usedto collectparticlesdirectly onto carbon coated grids, and their sizeand morphology were examinedusing a high resolution transmissionelectronmicroscope(TEM, Philips EM420ST). A 0.1 secondexposure time of the grids in the flame was usedto obtain optimal concentrationsof particles on the grids. Scatteringintensitieswere measuredat 90o to the Argon ion laserlight beam (see Figure 3). The laser light beam was chopped at 1035Hz by a mechanicalchopper (Wang, 1989).Cross polarization scatteringwas eliminatedusing a polarizer. Two thin rectangular slits (0.25mm x l0mm), separatedby a distance of l30mm, restrictedthe viewed region to a length of 12mm and resultedin a 0.4mm spatial resolution in the vertical direction. Signalsfrom the PMT (Hamamatsu R928) were fed to a lock-in amplifier (Princeton Applied Researchl864). The combination of a 514.5nm laser line filter in front of the PMT and the lock-in amplifier completely eliminated any effectsdue to room lights or to emission from the flame. The light scattering intensity, measuredby a PMT located at an angle 0 from the probe laser beam, is related to the particle number densityN by (Flower, 1983) Io:

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J. L. KATZ AND CHENG.HUNGHUNG

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TABLE I Flameconditions Flame No

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2'780 2620 2290 2670 2700 2770

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2263 2062 1239 1322 2989 3712

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O, Flow Rate (cc/min)

Ar Flow Rate (cc/min)

l 168 1440 2561 923 2088 2s92

l 8l 8 l 46l 87'l 937 2n9 2630

167 '757 25'77 485 1098 I JOJ

The momentum ratio is 0 8 for all flames The equivalenceratio is 0.62 for flame I and is 0.7 for all other flames

sizemeasuredby DLS, Eq. (3) allows one to determineN, the number densityof the scatteringparticles.The real parts of the refractiveindicesusedin the number density calculationswere 1.65for GeO, and 1.46for SiO, (Weastand Astle. 1982). RESULTS AND DISCUSSION The flames used in this study were fuel lean flames (equivalenceratios of 0.7) with momentum ratios of 0.8 (i.e., the ratio of the momenta of the fuel and oxidizer streams).This combination placed the temperaturemaximum I mm above the center of the burner while placingthe stagnationplane2 mm below the centerof the burner. Three sets of conditions, resulting in flames with different peak temperatureswere used in this study. For convenience,we refer to them as flames l, 2, or 3 (see Table I and Figure 2). Their gas velocities(and thus their residencetimes)are about the same (after correcting the velocity for their temperature differences). Their temperature gradients are about the same in the region in which the particles were formed (-7 to - I mm). We also examined whether our results are affectedby residencetime. Flames 4, 5, and 6 have different residencetimes but have almost the sametemperaturedistribution as flame 2 in the region where the important processesare occurring, i.e., their temperature are essentiallyidentical between - 5 mm and - 3 mm and differ very little over the entire lower regior, -i mm to - I mm. Sincethe growth mechanism and the structure of flame generatedmaterials have been extensively discussedby Megaridis and Dobbins (1990)and Schaeferand Hurd (1990),this paper mostly will discussthe effect of processvariables on particle size,the particle growth processes, and on the particle's degree of crystallinity. Effect of Precursor Concentrationon the Particle Size Figure 4 shows the effect of increasingthe GeCl4 concentration on particle size. Relative to the resultsobtainedusing a concentrationof 0. loh in flame 2, increasing the concentration by a factor of 2 resulted in an approximate doubling of the maximum particle diameter. Increasingit by a factor of 3.3 resultedin a 2.5 fold increasein maximum particle size. Figure 5 shows the effect of increasingGeClo concentrationon number density.At low GeCloconcentrations,particle formation

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Z (mm) FIGURE 4 Diameter of GeO, particles measuredusing DLS (as a function ol height in the burner) producedusing GeClnconcentrationsof,0.33% (E),0.2% (a), and 0.1% (O), using the flame 2 set of conditions.

appearsto proceedprimarily through a homogeneousnucleationroute, as evidenced by the rapid increaseand then rapid decreasein the number density. However at higher GeCloconcentrations,heterogeneous condensation(surfacegrowth) is probably the dominant growth mechanismsincethe number densitiesremainsalmost constant in the region, - 6 to - 4.8 mm, but their diameterincreases. Figure 6, which consists of 3 TEM micrographs of the particles collectedat the location where the peak scatteringintensity occurs, confirms this conclusion.One seesthat the size of the fundamentalparticles(i.e., the isolatedparticlesand also the individual particlesin the aggregates)is much larger at the highest GeCloconcentrations( - 30 nm) than at the lowest GeCloconcentration( - 9 nm). A circle has beendrawn on eachmicrograph. The outsidediametersof thesecirclesrepresentther DLS measureof the sizeof the particles(84, 68, and 38 nm respectively).One seesthat theseDLS sizes,which are to s c o1

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effective hydrodynamic sizes,are reasonablerepresentationsof the sizes of the aggregates. Effect of Temperature on Particle Size Figure 7 showsscatteringintensity profiles for threedifferent peak temperatureflames on adding 0.2Vo GeClo to the fuel stream. For the higher temperature flames, the nucleationof particlesstarts early and closeto the entranceregion. For the lowest temperatureflame, the location where particlesstart nucleatingis pushed upward, into the flamecenterregion.If one comparesthe temperatureat the locationsof peak scatteringintensities,one finds almost exactly the sametemperaturesat thesecorrespondinglocationsfor all threeflames.If one further comparesthe temperatureat the location where scatteringbegins,one again finds almost exactly the iame temperatures at the correspondinglocations.This impliesthat the onsetof nucleationand the growth to scattering size dependsprimarily on temperature. Figure 8 shows the effect of temperature on particle size as measuredusing DLS. The peak particle sizesare about the samefor flames I and 2, probably becausetheir

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Z (mm) FIGURE 7 Scatteringintensity for GeO, particlesas a function of height in the burner for flameswith peak temperaturesof (tr) 2780K (flame 1), (t)2620K (flame 2), and (O) 2290K (flame 3).

differencein temperature is small. However, the peak particle sizein flame 3 is larger. A TEM examinationof the particlestaken at the location of the peaksof scattering intensity shows that the fundamental particles are larger in the highest temperature flame (flame l) than in the lowest temperatureflame (flame 3), suggestingthat higher temperaturefavors surfacegrowtl as the growth mechanism.TheseTEM micrographs also show that the particlesare more chain-likeat lower temperatures. Effect of ResidenceTime on Particle Size Three additional setsof flame conditions,with gas flow rates 0.7, 1.4,and 1.8 times that of flame 2 were analyzed. Their gas phase temperature proflles were about the sameas in flame 2, and their GeCloconcentrationswere exactlythe same,i.e.,0.2o/o.

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Z (mm) FIGURE 9 Diameter of GeO, particlesmeasuredusing DLS as a function of height in the burner for different gas flow rates: (tr) 5cm/sec(flame 4), (a) Tcmisec (flame 2), (Q) llcmisec (flame 5), and (o) l3cm/sec (flame6).

Figure 9 shows that the maximum particle diametersare approximatelyinversely proportional to the gas flow rates. The number densityprofiles,shown in Figure 10, suggesta significantinfluenceof gas flow rate on the mechanismof condensation.At lower gas flow rates, no new particlesare formed after the initial burst of nucleationof particlesbut their diameter increases,suggestingthat surface growth is the favored mechanism.However, at higher gas flow rates, homogeneousnucleation is the primary particle formation route, as evidencedby the rapid increaseand then the rapid decreasein number density.Figure I l, which consistsof 2 TEM micrographsof the particlescollectedat the location where the peak scatteringintensityoccurs,confirmsthis conclusion,i.e., -5.8nm for flame 4 and Z: -4.4mm for flame 6. One seesthat the at Z: fundamentalparticlesizeis much largerin flame4, - 3l nm, than in flame6, - l6 nm. 10s ^

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Z (mm) FIGURE l0 Number densityof GeO, particlesas a function of height in the burner for differentgasflow rates: (D) 5 cm/sec(flame 4), (a) 7 cm/sec(flame 2), ( O) I I cm/sec(flame 5), and (o) l3 cmi sec (flame 6).

PARTICLE FORMATION IN FLAMES

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FIGURE ll Transmissionof electronmicrograph of GeO, particlesobtained by adding 0.2oh of GeClo to (A) flame 4 and sampledat Z : -5.8mm, (B) flame 6 and sampledat Z : -4.4mm.

The circlesdrawn on eachmicrograph,representthe DLS measureof the sizeof the particles(77 and 38 nm respectively).As can be seen,the DLS sizesare reasonable representationsof the sizesof the aggregates. The influenceof gas flow rate on the location in the burner where particles are first observed can be seenfrom Figure 10. The higher is the flow rate, the higher is the location where particles are first observed and thus, the higher is the temperature, since the temperature profile and the precursor concentration are the same all four cases.This onset of particle formation at higher temperaturessuggeststhat the nucleationprocessis diffusion limited. Effect of An Electric Field on Flame Characteristics A flame with a temperature profile similar to flame I was used in this study. In the absenceof any precursors, the flame remains flat, even at electric fields as high as 4000V, i.e.,2667V/cm. At higher voltages,sparksoccur. On adding low concentrations of SiClo (at an electric field of 200V/cm) the flame shifted up or down slightly, dependingon polarity. Usually the changewas larger when the bottom half of the burner was negative relative to the top half. This shift was most pronounced at the

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flame edges,causing the flame to appear bent. At electricfields higher than 200V/cm, a wave in the flame appearedand travelled repeatedlyfrom one end of the burner to the other (along the burner's long axis). On adding SiClothe flame remained wavy at all field strengths (up to the break down voltage of -2667Ylcm). But, on adding GeCloinstead of SiClo, the wave disappearedat fields higher than 1000V/cm, and the flame front (in the center part of the burner) changed from blue-white to clear blue. Particles could be seenmoving from the burner mouth towards the top half of the burner when the bottom was positive relative to the top. But they moved out of the burner at a vertical position close to the bottom, i.e., between Z : - 5.6 and -4.2mm, when the polarity of the plates was reversed,showing that the charge on the particles is positive. An increasein particle velocity as a result of the electric field was also observed,causing their residencetime in the burner to decrease. Figure 12 shows how the current changedas a function of electric field strength. It increasedrapidly at low fieldsand then flattenedfor fields between200 and 1200V/cm. A somewhat lower current was obtained when the bottom plate was negative (open symbols) than when the bottom plate was positive (solid symbols).This differencein current may be due to the fact that all particles are formed in the lower half of the burner and also exit there. Their mobility is much lessthan that of electrons(and any small negativeions).A negativelower plate pushestheseelectrons(and ions) through the particles, causing somerecombination. However, a positive lower plate pulls then down to itself, resulting in less recombination and thus a larger current in the battery-burner circuit. More details on the experimental setup and on the effectsof electric fields are available in our recent paper (Katz and Hung, 1990). Effect of Electric Field on Particle Size Particle diameters,obtained by DLS measurements,are presentedin Figure 13.They were measured both in the absenceand presenceof an electricfield, using GeClo and SiCl, as precursors,SiCf at 0 and l20V/cm and GeCloat0,120 and 1250V/cm. Since 120V/cm had little effecton particle size,resultsare shown only for GeClo. For GeO, at 1250V/cm, the particle size increasedby a factor of 2 to 4. The net charge of the particles is positive but a particle could be positive at one end and negativeat the other end. In regions where the number density decreasedrapidly, i.e., - 5 to -4nm (no electric field) and -6.5 to -5.5mm (with electric field), the particle diameter

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z (mm, FIGURE 13 Diameterof GeO, particlesmeasured usingDLS (produced using0.02%GeClo)as a functionof heightin the burnerat differentelectricfield strengths.Solid symbolsare for a positively chargedbottomplate;opensymbolsarefor a negatively chargedbottomplate,andhalf-filledsymbolsare for no field.Solidlinesarefor 1250V/cm(andalsofor no field):dashedlinesare for l20V/cm. increased from -80nm to 150-350nm, i.e., a three fold increase. For GeO, at l20Y lcm, the particle size increases slightly and the number density decreasesslightly. However, these small changes may not be significant because of measurement uncertalntles. The lack of relationship between the flame current and particle size can be seen by comparing Figure 12and 13. Figure 12 shows that the flame current increased by a

factor of about 100from 0 to l20Vicm, but increasedby a factor of only 5 from 120 to 1250V/cm. However, Figure l3 shows that the particle size increasednegligibly between0 to l20V/cm but increasedby a factor of about 3 from l20to 1250V/cm. Effect of Process Variables on Particle Crystallinity The particlescan be singlecrystal,poly-crystalline,or amorphous.Thesephasescan be detectedusing electron microscopy by their different diffraction patterns, e.9., ordered and separatedspots, co-axial rings, or if they are amorphous, uniform scattering.Sixflames(seeTableI)andfourGeCloconcentration(0.33o/o,0.2oA,0.1o, and 0.05%) were used in this study. On adding 0.2%oof GeCloto flames 1,2, and 3 (note that these flames have markedly different peak temperatures),one obtains crystalline particlesin flame l, but amorphous particlesin flames2 and 3. All samples were collected at the location where the scatteringintensity sharply decreased.This occurs when the temperaturesare close to GeOr's melting temperature (- 1300K). On adding 0.2oh of GeClo to flames 2, 4, 5, and 6 (markedly different flow rates but similar temperature profiles), one finds that crystalline particles are formed in flames 5 and 6 and amorphous ones in flames 2 and 4. On adding different concentration of GeClo(0.33o/o,0.2yo,0.|yo, and 0.05%) to flame2, the particlesformed are amorphous for the higher concentrations of GeClo Q.33% and0.2"/r) and are crystalline for the lower concentrations(0.1% and 0.05%). Most of the crystalsare polycrystallinebut single crystals are also found when adding 0.05% GeClo in flame 2. Which factors determinethe particle's crystallinity is not yet clear,however,crystallineparticles were usually found when mostly small particles were produced.

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CONCLUSIONS This study presentsour presentunderstandingof the effectsof processvariableson the particleformation processes and resultantparticlesizeand morphology.We have shown how one obtains desiredproperties.For example,if one wants the smallest sizedparticlespossibleone should use low precursorconcentration,short residence times, and high flame temperatures.Particle sizeson the order of l0nm can be obtained. If one wants amorphous particles, high precursor concentration, long residencetime, and low temperatureshould be used.Other setsof conditionspromote chain formation, and by using mixtures,one can obtain very interestingnanophase structures.This will be discussedin our next paper. ACKNOWLEDGEMENT This paper is basedon researchsupported by the Division of Materials Sciences,Office of Basic Energy Sciences, U.S. Departmentof Energy via Grant DE-FG02-88ER45356

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