particle synthesis in flames

Proceedings

Proceedings of the Combustion Institute 31 (2007) 1773–1788

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of the

CombustionInstitute

Particle synthesis in flames

Paul Roth *

Institut fur Verbrennung und Gasdynamik, Universitat Duisburg-Essen, Lotharstraße, 47048 Duisburg, Germany

Abstract

From the view point of combustion science, the fundamentals of particle formation in a flame environ-ment are discussed. The principles of converting a gas phase precursor dopant into particles and also theirgrowth are addressed. Various experimental methods and examples for the synthesis of particles of bothsingle and mixed oxides are reviewed. First attempts to tune the stoichiometry of oxide particles by varyingthe combustion parameters of premixed flames are demonstrated. Some aspects of modelling and the prob-lems which still need to be solved are illustrated by means of the particles’ population balance equation.� 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keyword: Nanoparticles

1. Introduction

The present topic can easily be used as a leit-motiv to review the whole history of combustionscience and technology in the light of aspects ofparticle formation. Nearly every flame producesparticles, which are sometimes quite visible andobvious in a sooting flame, but sometimes nearlyinvisible. Flames often appear to be particle free,because our eyes or the respective diagnosticsare not sensitive or specific enough to detect them.We have learned to consider flames not only as areactive flow with internal energy transfer, butalso as a reactor for synthesizing mostly unwantedgaseous or particulate pollutants.

In the present case, particles are considered tobe desirable products of combustion and so rele-gate any energy aspects into the background.The formation and transport space for the parti-cles is the gas phase. This means that solid–solidreactions supported by transport and chemicalreactions in the gas phase, as well as self-propa-

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gating high temperature synthesis (SHS), are allexcluded. A further restriction is made withrespect to the material of the particles. The syn-thesis of carbonaceous particles is not considered,although they are probably on the one hand themost important unwanted combustion productsand on the other hand a desirable material, indus-trially produced in big quantities. Our under-standing of particle dynamics and particlediagnostics and their interaction with the fluidflow mostly comes from studying soot formation.

Gas phase combustion synthesis of inorganicparticles is used routinely today to make a varietyof commodities like SiO2, TiO2, Al2O3, etc.,amounting to millions of tons annually. Theyare used industrially as pigments, opacities, cata-lysts, flowing aids, for optical fibres and telecom-munication. The flame reactor is the workhorseof this technology developed mostly in the secondhalf of the 20th century. It has in some casessuperceded production routes by wet-phase chem-istry. Degussa has e.g., developed a H2/O2 flameprocess for synthesising of titania (Degussa P25-see Fig. 1), which is used in the expanding areaof photocatalysis, as well as cosmetics applica-tions. They have demonstrated the ability to

ute. Published by Elsevier Inc. All rights reserved.

mailto:[email protected]

Fig. 1. Degussa large scale industrial flame reactor.

1774 P. Roth / Proceedings of the Combustion Institute 31 (2007) 1773–1788

control the particle morphology, whilst achievinghigh production yields and large production rates.Flame synthesis is a cost-effective and versatileindustrial process for inexpensive materials; alsoit offers advantages over alternative material syn-thesis processes. The high temperature flame reac-tor can be designed for a wide range of operatingconditions. The process is self-purifying withrespect to the final powder product. The charac-teristics of flame-made particles are controlledby the following: the mixing of the reactants andprecursor, the overall composition, and the time-temperature behaviour, including rapid quenchingof the gas/particle flow. The required powdersshould be of high purity with a well controlled sizedistribution and morphology, which depend onthe particular application.

Beside the large scale industrial flame synthesisreactors, combustion scientists mostly from aca-demia have studied particle synthesis in nearlyall types of flames, including burner-stabilizedpremixed flat flames, stagnation point premixedflames, co-flow flames, counter flow flames, andmultidiffusion flames. Also well-stirred reactorsand non-stationary flames in closed vessels havebeen used to synthesize particles. Self-sustainingflames of e.g., hypergolic type, as well as normalflames doped with various particle precursors,have been used. The early studies were focusedon the development of new technologies and hadto demonstrate control over the process. Thecharacterization of the particulate product withrespect to size, structure, and morphology was

quite limited. It was gradually further developedin parallel with the gas-phase diagnostics of flamespecies and has profited much from ideas comingfrom aerosol science. Also new devices for charac-terizing material properties (TEM, XRD, AFM,and others) have contributed to the understandingand fine-tuning of particle synthesis.

With the start of the nano-age, interest inflame-synthesized particles—as also those pro-duced by other routes—was more and morefocused on size effects of nanostructured materi-als. Particles with a very narrow size distributionand well controlled phase composition and mor-phology called ‘‘functional nanoparticles’’ becamedesirable products. The reason for the size effectsexhibited by nanoparticles compared to the bulk,is their large surface to volume ratio. For a parti-cle of about 4 nm, half of the molecules formingthe nanostructure are actually at the surface withconsequences for the lattice structure. This causesdramatic changes in the physical and chemicalproperties compared to the bulk material andchanges, e.g., the melting temperature, themechanical properties, the band gap for semicon-ducting particles, the magnetic or the opticalproperties, as well as the catalytic behaviour. Asan example Fig. 2 shows that the magnetizationof c-Fe2O3 crystallites included in a SiO2 matrix,rapidly decreases with decreasing crystallite size.

The last review at a Combustion Symposiumon gas-phase combustion synthesis of materialwas given by Brezinsky [1]. Consequently thisreview is restricted to the developments in that

Fig. 2. Example of nano-effect: magnetization ofc-Fe2O3 particles in SiO2 matrix as a function ofcrystallite size.

P. Roth / Proceedings of the Combustion Institute 31 (2007) 1773–1788 1775

field in the last 10 years. Further useful studies onflame or combustion synthesized particles are con-tained in comprehensive review papers of Pratsinis[2], Wooldridge [3], Pratsinis and co-workers [4,5],and Rosner [6]. No claim for completeness ismade here, and for many interesting examplesnot explicitly discussed here, the literature shouldbe consulted. I also apologize for choosing manyexamples originating from my own laboratoryalthough others are probably better.

Fig. 3. Principles of reactive particle formation sequencein a high temperature fluid flow.

2. Principles of particle formation and growth fromthe gas phase

Nanoparticles are mostly synthesized in labo-ratory flames by adding a precursor dopant, in agaseous or liquid state, to the unburnt gas. Suchprecursors are often compounds of metal, likehalides or organometals. They can also be dis-solved in water or in liquid hydrocarbons. Thekinetics of the combustion reactions are onlyloosely coupled to the precursor’s decompositionand the reactions forming particles. The energyof the exothermic oxidation reactions is used toincrease the temperature of the fluid flow, thusdriving the chemical reactions of the precursorgas. This results in vaporization of droplets of aliquid precursor, thereby initiating its decomposi-tion. Nuclei and clusters are formed, which fur-ther grow to nanoparticles by surface growthand/or coagulation and coalescence. This soundsquite similar to what is known about soot forma-tion in flames.

Such synthesis of particles in a fluid flow canalso be established by using energy sources otherthan combustion to start or sustain the partlyendothermic reactions of the precursor. Particu-larly advantageous for the synthesis of non-oxideor metallic particles are hot wall reactors, plasmareactors, or laser reactors. All are simple devices,in which the energy for increasing the temperatureis transferred from a hot wall or is directly cou-pled in the form of microwave or laser energy intothe fluid flow. They are quite useful and can

extend the synthesis of flame-made oxide particlesto the huge class of oxygen-free particles. Thesereactors also open the possibility of staged reac-tion processes for coated or mixed particles, seee.g., [7–12].

A typical sequence of basic steps illustratingparticle formation in a flowing gas is shown inFig. 3. It is based on an early representation byUlrich et al. [12–15] who studied very carefullythe SiO2 particle formation in flames. The precur-sor is injected as a gas or a liquid spray into theflow, which is rapidly heated up by either externalor internal energy transfer, e.g., by heat of com-bustion. A sprayed precursor rapidly evaporatesand starts to decompose like a primary gas phasecompound. Consumption of the gaseous precur-sors can proceed either by gas phase or by surfacereactions or by both. A complete description ofthe decomposition kinetics and the subsequentoxidation/hydrolysis reactions is rarely obtained.The decomposition kinetics can also interact withthe kinetics of combustion, e.g., via radical reac-tions. Radicals, intermediates and product mole-cules are formed, which polymerise or nucleateto the first clusters, whose thermal stability deter-mines in many cases the further evolution of theparticle-forming process. According to classicalnucleation theory based on bulk material proper-ties, the critical cluster size is often smaller thanthe dimension of the monomers. The clusterscan grow either at the gas kinetic collision ratewith sticking coefficients often assumed to beequal to one, or by the addition of monomers to


the cluster’s surface. The coalescence of cluster–cluster ensembles is normally very fast resultingin compact nearly spherical structures, whichcould be called particles. The typical time scaleof the gas-phase chemical reactions, includingthe cluster processes, is usually short comparedto that for the subsequent evolution of a particle.

When typical particle diameters become largerthan several nanometers, the further developmentof a particle is determined by surface growth andby the interdependence of coagulation and coales-cence. The importance of sintering was conclu-sively demonstrated, e.g., by Helble and Sarofin[16] and by Matsoukas and Friedlander [17].Brownian coagulation starts to form fractal struc-tures, which merge again into spheres by rapidcoalescence and surface growth. As coalescencerates show a strong dependence on both size andtemperature, Brownian coagulation finally winsthe race in the cooler parts of the flow, where frac-tal aggregates are formed. They are called ‘‘soft’’if the primary particles are interconnected byvan-der-Waals forces and ‘‘hard’’, if sinter necksexist. Material properties, residence time, andthe temperature of the flowing gas are the keyproperties which determine the morphology andcrystallinity of the agglomerates.

Beside the well-known fluid mechanical forcesdetermining the convective and diffusive mobilityof the particles, thermophoretic forces (due tolarge temperature gradients) also influence a par-ticle’s residence time. The addition of ionic speciescan increase the charging of primary particles and

Fig. 4. Self-organized Fe-particle cha

affect both the size of the primaries and the struc-ture of aggregates, especially in external electricfields, see e.g., [18–20]. As recently demonstrated[21] magnetic properties of primary particles alsocan affect the final structure of an agglomerate,so that long chain-like aggregates, consisting ofmore than 50 primaries, can be produced, asshown in Fig. 4. This is an example of the self-organization of magnetic nanoparticles.

3. Diagnostics for particles

Diagnostic techniques for sizing particles areconfronted with the problem of coping with struc-tures ranging from nearly molecular dimensionsto say �100 nm. Although the very small clustersbehave like big molecules, characteristic finger-prints from molecular spectroscopy are generallynot available. The smallest particles have a veryhigh mobility and are quite sensitive to changesin the dispersion fluid. Particle probing and ex situparticle analysis must therefore be performed verycarefully, as discussed below.

There is still a need for diagnostics providingin situ measurements of the distribution of parti-cle sizes. The interaction of light with particlesin the form of static or dynamic light scatteringhas been used, see e.g., [22–28]. Good spatial res-olution can be obtained, but according to Mie-theory, the scattering decreases rapidly withdecreasing particle size and can reach valuescomparable to those of gas phase species. From

ins formed by magnetic forces.

Fig. 5. Principles of particle mass spectrometer (PMS)for calibration-free particle sizing.


sooting flames, light extinction, especially laserlight extinction, is known to be a useful detectiontechnique for small particles, see e.g., [29,30]. Forhigh particle concentrations, good signal qualityhas been obtained in systems with nano-particles.However, this line-of-sight technique needs signaldeconvolution. A quantitative interpretation alsovery much depends on the size- and material-de-pendent value of the refractive index. A spectrallyresolved light extinction technique has been devel-oped by D’Alessio et al. [31,32] and successfullyapplied to sooting flames. Data reduction to yieldparticle sizes is also somewhat critical.

In the last 10–15 years, a further laser baseddiagnostic for nano-particles has been developedcalled laser-induced-incandescence (LII), see e.g.,[33–35]. Whilst light scattering takes advantagemainly of the real part of the refractive index,the LII technique is based on the absorptive imag-inary part. In its simplest form, the particles areheated up by a laser and their laser-induced radi-ation is a measure of the number density of theparticles. The time-resolved version (TR-LII) isin principle able to deliver particle size informa-tion, see e.g., [36–46]. In this case, the particlesare heated up by a very brief laser pulse. The emis-sion of light from the particles during their subse-quent cooling-down to the temperature of thesurrounding gas is recorded. The time behaviourof this signal contains information on the particlesizes, as the cooling rate of small particles is fasterthan that of big ones. The experimental set-up isquite simple and TR-LII signals are easy toobtain. Data reduction to provide informationon particle size parameters is somewhat complexand needs much theory, including assumptions,e.g., for heat and mass transfer properties. Thisparticle sizing technique has been developed moreand more in recent years and its application is ofcourse not restricted to only carbonaceous parti-cles. Some variants of this technique, like two-col-our LII, are in use. The first international LIIworkshop, recently organized at University ofDuisburg-Essen has demonstrated the problemsand perspectives of this diagnostic technique forparticles [47].

Although in situ measurement of size parame-ters is important for controlling particles and thequality of particulate products, additional struc-tural, and morphological information is usuallyrequired, which can best be obtained from collect-ed particles. However, this requires careful andrepresentative particle sampling, which does notfalsify or change the particles’ properties duringthe collection procedure itself. The technicallymost fastidious and my favourite method ismolecular beam sampling combined with a parti-cle classification and particle deposition device;this is called a particle mass spectrometer(PMS). It is a variant of the classical massspectrometer for gaseous flame species, including

radicals, see e.g., [48,49]. The actual realizationof the PMS is schematically shown in Fig. 5, seealso [50]. It is assumed that nano-particles in aflame are partly charged either by the particle syn-thesis process itself or by an appropriate source. Asample of the aerosol is supersonically expandedthrough a platinum-plated and electricallygrounded quartz nozzle into a first vacuum cham-ber. The supersonic free jet formed by the expand-ing flow contains both particles and gaseousspecies. The flow conditions are such that thegas temperature decreases extremely rapidly, thusfreezing any physical or chemical rate processesinside the sample almost completely. The centreof the free jet is extracted by a sharp-edged skim-mer and moves as a particle-loaded molecularbeam into a second vacuum chamber. It can beassumed that the particles’ properties in the beamare equal to the respective properties at the sam-pling position in the flame. This means thatmolecular beam sampling can be regarded as anin situ diagnostic with only very weak repercus-sions on the reacting flow.

The molecular beam is directed through acapacitor, where the charged particles are deflect-ed from the beam according to their mass andcharge, thus forming a fan-shaped beam. The clas-sification of the particles due to their mass is per-formed by introducing a grounded plate into thefan-shaped beam carrying two symmetrical and


one central slit. By varying the voltage of thecapacitor, the fan of charged particles is scannedover the outer slits so that particles of differentmass can pass through. The PMS offers two pos-sibilities for further particle processing:

1. Particle collection on TEM grids either behindthe central slit (neutral particles) or behind thetwo other slits (charged particles) with furtherex situ size and structural imaging by e.g., highresolution electron microscopy (HR-TEM).

2. Collection of charged particles by two Faradaycups, to which they deliver their electrical char-ge. This enables on-line determination of theprobability density function (PDF) of the parti-cles’ mass without any further calibration.

Both possibilities have been successfully appliedto space-resolved nano-particle analysis in lowpressure flames and in other devices, see [51–53].

In a recent experiment [54] we have demon-strated the sizing of Fe2O3 nano-particles synthe-sized in a low pressure H2/O2/Ar flame byapplying simultaneously both TR-LII and PMStechniques. Figure 6 shows the working principlesof the set-up used and examples of the raw signals

Fig. 6. Set-up for simultaneous sizing of Fe2O3 nano-particles in a low pressure flame and raw signals fromPMS and TR-LII.

obtained using the two techniques. From theI/U—spectrum of the PMS (I = flux of chargedparticles arriving at the Faraday cup, U = deflec-tion voltage) a mean particle size of dp = 7.1 nmcan easily be determined. The TR-LII raw signalshows a specific feature: as the particles are verysmall, the whole process of particle heat-up bythe pulsed laser and the subsequent particle cool-ing takes only a few ns. A certain time overlap ofboth process steps is also very obvious, whichmakes the evaluation of such signals more compli-cated. A first attempt to determine the mean par-ticle size from the TR-LII signal based on amodified theory results in a value which is veryclose to the PMS particle size. The simultaneousapplication of both these particle sizing techniquesis expected to be quite helpful in the further devel-opment and improvement of the TR-LIIdiagnostic.

A further particle probing and data analysistechnique is known from aerosol science, seee.g., [55]. A sample is taken from a particle-ladenflame through a small orifice in a tube and rap-idly quenched by an inert gas flow. The suddenhigh dilution immobilizes the particles, reducesparticle collisions, and enables transport by flowto subsequent devices without significant changesin particle size and structure. The flowing parti-cles passes an electrical charger, where the parti-cles acquire their assumed equilibrium chargedistribution, and is subsequently classified by adifferential mobility analyser (DMA). Thisinstrument is basically a cylindrical capacitor,through which the flow of particles, togetherwith a clean gas flow is directed. The particlesare deflected from the laminar streamlines andmove according to their electrical mobilitythrough the electric field. Particles with a certainrange (or class) of mobility can leave the mainflow through a slit in the inner electrode. Bychanging the voltage on the capacitor, the vari-ous mobility classes of the particles are scannedthrough. The final particle processing is similarto that described above for the PMS. The mobil-ity-selected particles can either be deposited onTEM grids for ex situ imaging or counted by acondensation nucleus counter (CNC) afterenlarging their size by heterogeneouscondensation.

By way of a comparison of the PMS- andDMA-based particle sizing routes, the last onemight have some disadvantages, especially fornano-particles. The dilution and probing of a vis-cous flow, diffusion losses in the DMA, and theinterpretation of a particles’ mobility in terms ofits size are some critical points.

The most common and easy to realize way ofcollecting particles out of a flowing fluid isthermophoretic sampling, introduced by Georgeet al. [56] and further improved by Dobbinsand co-workers [R.A. Dobbins, private


communication] [57] for soot particles. A coldsurface, e.g., a TEM grid, placed along a flame’sstreamlines causes a strong temperature gradient,along which the particles move towards the sur-face, driven by thermophoretic forces. Theinserted probing surface inevitably influencesthe flowing flame. However, the good thing isthat the thermophoretic drift velocity is in theKnudsen regime independent of particle size.The deposition of particles is therefore not size-selective. Further analysis depends on the avail-able instrumentation and can be made e.g., byatomic force microscopy (AFM), by TEM oreven by optical diagnostics. It is advantageousto ‘‘shoot’’ the cold surface through the flame,thereby avoiding any restructuring of the collect-ed particles by the influence of the hot flame gas-es. A recently performed comparison [58]between PMS-collected particles and thermopho-retically sampled particles from the same flameshow surprisingly good agreement.

There are some further diagnostic techniquesfor particles in use; they cannot be described here.Also the various physical methods used to charac-terize a particle’s material properties with respectto crystallinity, phase composition, etc., arebeyond the scope of this article.

Fig. 7. Various laboratory flame types used for nano-particles synthesis.

4. Examples of flame synthesized particles

Many of the early experimental studies onthe gas phase combustion synthesis of particleswere initiated and accelerated by industrialneeds. As reviewed by Pratsinis [59], the historicevolution was motivated by the industrialimportance of fumed silica, titania, and aluminiaproduced in large scale flame reactors. Since theearly 1990s combustion scientists became moreinterested in this subject, which was also dis-cussed at Combustion Symposia, and researchwas intensified towards the manufacture ofadvanced tailored materials. New routes forthe synthesis of non-oxide ceramic particles inself-sustaining chemical systems were exploredby Calcote et al. [60,61] and realized by, e.g.,Axelbaum and co-workers [62–65], by Glassmanet al. [66,67], and by Gerhold and Inkrott [68].This is all described in the review article byBrezinsky [1].

Because of the limitations set by this arti-cle, I must restrict myself to the synthesis ofoxide particles produced in doped laboratoryflames. Four basic types of burners have beenused: two burning premixed and the othertwo diffusion flames, as shown in Fig. 7.The discussion of the experimental work isgrouped by the kind of material produced.The examples are selected such that the vari-ous types of flame and the specific diagnosticsused are addressed.

4.1. Single oxide particles

The classical and most extensively studied sin-gle oxide, synthesized in nearly every type offlames is SiO2. Typical precursors used are silane(SiH4) or tetramethylsilane (Si(CH3)4); typicalflame gases are either H2/O2 or CH4/O2. Earlywork by Ulrich [12] and by Hardesty andWeinberg [18] goes back to the 1970s. Furtherstudies on the flame synthesis of SiO2 werefocused on characterizing the environment whereparticles form by measuring temperature and theconcentrations of gas phase species with LIF,Raman, REMPI, and others, see e.g., [59,69–72].Beside laser light scattering, the particles’ struc-tures were mostly obtained from thermophoreticprobing with subsequent TEM analysis, see e.g.,[73–75]. Experiments illustrating the time behav-iour of particle growth are rare. Roth et al. [76]measured in their doped, low pressure premixedH2/O2/Ar flame the formation of primary SiO2

particles. In the enlarged flame zone, the growthof particles along the flame coordinate was fol-lowed by molecular beam sampling and PMSanalysis. A clear increase in size, ranging from 3

prim

ary

part

icle

siz

e dp

p/ n

m

residence time / ms


to 8 nm, was obtained, as discussed below. Anexperiment performed at Degussa [A. Gutsch, pri-vate communication] shows the time evolution ofagglomerates synthesized in a diffusion typeindustrial flame. In this case, the average size ofthe primary particles is nearly the same, whereasthe number of primaries m forming an agglomer-ate increases; this is shown in Fig. 8. These twoexamples illustrate the evolution of particles whencoalescence and coagulation dominate.

Besides silica, many other single oxides havebeen synthesized in laboratory flames includingTiO2, Al2O3, GeO2, PbO, V2O5, Fe2O3, SnO2,ZrO2, ZnO, and WO2, see reviews [2–6]. The pic-ture which can be drawn from all these findings isrelatively consistent. The most important parame-ters determining particle morphology are the con-centration of precursor and the combinedinfluence of flame temperature (particle tempera-ture) and the residence time of particles. Burnerparameters seem to have only a minor influence.Knowing the limitations of every summarizingstatement, the relative variations of the keyparameters and their combined effects on the syn-thesized particles can be standardized in a simpli-fied form as follows:

8

Fig. 8. Growth of particle agglomerates at nearlyconstant primary particle size, obtained from an indus-trial flame reactor, after Degussa.

increase of

precursor

concentration

fi

>>>>>>>>>>>>>>>>><>>>>>>>>>>>>>>>>>:

increase of

primary particle

size

increase of

temperature

fi
increase of
precursor

decomposition,

with increase in

monomer

formation rate,

high coalescence

rate, finally

smaller primary

particles.

high temperature,

short residence time

fi
compact particles
low temperature,

long residence time

fi
highly structured
agglomerates

Fig. 9. Example of DMA sized TiO2 particles obtainedfrom a atmospheric premixed stagnation point flame,after Zhao et al. [77].

Recently Zhao et al. [77] described a new typeof flame (atmospheric premixed stagnation pointflame), combined with a classical aerosol diagnos-tic technique (dilution sampling and DMA), tostudy the formation of TiO2 particles. It was dem-onstrated that this type of flame is suited to pro-duce ultrapure, ultrafine, single crystal particleswith a narrow size distribution. A sequence ofDMA measurements, which agree with evaluatedTEM pictures of collected particles, is shown inFig. 9. The TTIP (titanium tetraisopropoxide)

loading is lowest in sample with smallest meandiameter. With increase in TTIP loadings, themean particle diameter increases. The insert shows


the PSDF (particle size distribution function)properties, Ædpæ (mean particle diameter), r(width), and FWHH (full-width at half height).The observed trends of the particle size parame-ters with precursor concentration are as men-tioned above.

A further example of particle synthesis in apremixed flame comes from my own laboratory.Figure 10 illustrates the formation and growthof Fe2O3 in a low pressure H2/O2/Ar flame dopedwith Fe(CO)5 [78]. The evolution of particle sizewas in this case measured by the particle massspectrometer described above. The inset inFig. 10 shows a particle mass PDF measured ata distance along the flame coordinate of 80 mm.The main figure illustrates the growth of particlesfrom about 3.4 to 6.0 nm as a function of axialdistance from the burner head. The temperatureeffect addressed earlier is also visible.

New synthesis routes for single and mixed(see later) oxides have been extensively exploredby Pratsinis and co-workers [4,5] using variousmodifications of their single and multi-diffusionco-flow flames. Their work is more stronglyrelated to practical chemical engineering aspectsand always has applications of their newlydesigned or conventional powders in view. Theclassical spray pyrolysis, which is more or lessa drying process, has been further developed[4,79–82] towards flame spray pyrolysis andemulsion combustion. Precursors dissolved inwater or in an organic solvent have been sprayedinto their flames. Classical single oxides, likeSiO2, TiO2, and SnO2, have been synthesizedwith high production rates in scalable reactors.Also Y2O3, Eu2O3 and other rare earth metaloxides, partly doped with Eu, Tb, or Tm, havebeen synthesized in spray flames, see e.g.,[83–85]. Such nano-particles are attractivebecause of their high fluorescent intensity andfluorescence lifetime, and show promise in sensorapplications.

Fig. 10. Example of time-resolved PMS-sized Fe2O3

particles obtained from a premixed doped low pressureflame, after Janzen et al. [78].

4.2. Mixed oxide and coated particles

Mixed particles are nanocomposites contain-ing two different crystal phases, which are mixedon a nm length scale. Coated particles are a var-iant; here one particle phase is covered orencapsulated by another. Such nanocompositesare of huge technical interest, especially for cat-alytic applications, but also e.g., as high temper-ature superconductors or super-paramagneticmaterials. Flame synthesis has been successfullyapplied to produce mixed oxide powders bothin laboratory see e.g., [86–89], and industrialquantities [90].

Katz and co-workers [91,92] are some of thepioneers who synthesized TiO2/SiO2 in theircounterflow diffusion flame. They used TiCl4and SiCl4 as gaseous precursors and controlledthe structure of the final particles by alteringvarious process parameters. They convincinglyshowed, how variations in the precursor ratioaffect the particles’ structure. Friedlander andco-workers [93,94] later extended the under-standing of how mixed particles form; they dis-cussed the influence of precursor chemistry,temperature, and thermodynamics on the intra-particle and innerparticle homogeneity. The mis-cibility/immiscibility of the two compounds,together with intraparticle transport, can limitthe approach of the equilibrium phase distribu-tion. Figure 11 illustrates the various possiblecombined effects. The upper part shows the pos-sible intraparticle morphologies, the lower onethe interparticle homogeneity. For radical drivenprecursor decomposition, uniform morphologiesfor miscible compounds must be expected. Incase of immiscible phases, both segregated anduniform particle morphologies are possible.For thermally driven precursor chemistry, thevariety of possible particle morphologies areeven higher.

The motivation for the synthesis of mixed oxi-des mostly comes from materials research. Conse-quently, other nanocomposites like V2O5/TiO2,V2O5/Al2O3, SiO2/GeO2, TiO2/Al2O3, TiO2/SnO2, Ta2O5/SiO2 have been synthesized in coflowand counterflow diffusion flames as well as in sprayflames, see e.g., [24,88,91,92,95]. Sometimes theformation of particle chains and particulate nee-dles have been observed, see [96]. The detailedstructure of the nanocomposites is partly deter-mined by the material properties. Also carbon-coated oxide particles have been synthesized inflames, see e.g., [96].

An interesting further example illustrating theimportance of gas phase kinetics is the formationof Fe2O3/SiO2 mixed oxides in a premixed CH4/O2 flame or a H2/O2/Ar premixed low pressureflame [89,97]. The thermal decomposition of theiron precursor Fe(CO)5 is much faster than theradical driven decomposition of the Si-precursor

Fig. 11. Schematics of mixed oxide and coated particle formation in flames, after Ehrman et al. [94].


(Si2(CH3)6 or SiH4), see e.g., [73]. Consequently,the iron oxide particles are formed first in theflame and are later encapsulated by SiO2. Suchpowders exhibit super-paramagnetic propertiesand are meanwhile commercially available.

A last example which attracted my attention,is the controlled deposition of noble metal clus-ters on oxide particles by Pratsinis’s group [98].Their classical spray pyrolysis burner is shownin Fig. 12. The metal-containing liquid mixtureof precursors is sprayed through the innermostcapillary into the outer premixed CH4/O2 flame,which is surrounded by flowing oxygen. The finespray droplets are ignited and particle synthesisoccurs. The two metal precursors used in thisstudy were titanium iso-propoxide and platinumII acetyl-acetonate, both dissolved in hydrocar-bon solvents. Quite interesting is the use of amoveable quench device. It consists of a water-cooled ring, through which quench gas can beinjected radially into the spray flame. TiO2 par-ticles of homogeneous morphology and sphericalshape were synthesized showing very small plat-inum clusters on their surface. The detailed mor-phology depends very much on the quenchingprocedure. This has direct applications in heter-ogeneous catalysis and gas sensing.

4.3. Tuning of the stoichiometry of oxide particles

The detailed stoichiometry of oxide particlessynthesized in flames affect their crystalline mor-phology and lattice structure. Many metals existin various oxidation states, and can form a varietyof sub-oxides. The oxygen vacancies in the parti-cles lead, e.g., to an increased charged carriermobility in case of semiconducting oxides. Thisis of interest in the development of gas sensingstructures and optoelectronic devices.

The formation of stable and fully oxidizednanoparticles is normally obtained in flamesburning under lean conditions. The propertiesof the combustion flowfield, in which the parti-cles are synthesized, can have a profound effecton the properties of the product particles. Anearly example is the formation of superconduc-ting oxides by spraying respective solutions intovarious H2/O2/Ar flames [99,100]. The oxygenstoichiometry in the particles depends on thedistribution of oxygen concentration in theflames and determines the success of the synthe-sis process. A further good example is the for-mation of the tin oxides SnO and SnO2 byHall et al. [101,102]. It has been demonstratedthat by varying the combustion properties of

Fig. 12. Flame spray pyrolysis burner with quench-cooling device, inset: TiO2 particles with Pt-clusters, after Schulzet al. [98].


the multielement diffusion flame burner, bothstable oxides were obtained.

Laminar, premixed flames burning with vari-ous fuel/oxygen ratios seem to be ideal for synthe-sizing metal oxide particles with variousstoichiometries. The combination of basic com-bustion parameters with requirements of particleproperties is crucial for the synthesis of well-de-fined materials. Ifeacho et al. [103] have synthe-sized SnO2 � x nanoparticles in their lowpressure premixed H2/O2/Ar flame with x varyingbetween 0 6 x 6 1. The key parameter which con-trols the oxygen content in the particles is theH2/O2 ratio of the unburnt gas. The compositionof the particles was carefully determined by vari-ous physical methods including Auger electronspectroscopy. A further example of stoichiometrytuning by combustion is the synthesis of WO3 � x

in a premixed flame [104]. It has been demonstrat-ed by impedance spectroscopy how the particleresistance depends on x.

5. Modelling aspects

The formation of nanoparticles in a flame canbe formally described by combining the laws for aflowing and reacting fluid with the populationbalance equation for suspended particles. If theprecursor is sufficiently dilute, the flow field canbe decoupled from the particles’ rate processesand delivers the temperature and transport fieldfor the formation and growth of particles. Foraxisymmetric laminar flames of cylindrical geom-etry (as in Fig. 7a–c) a further simplification canbe made by reducing the spatial variables to oneaxial coordinate z.

Initially, the modelling effort to describe a par-ticle’s evolution from molecular dimensions tonanosized particles necessarily focuses on idealiza-tions. In early examples, only coagulation of par-ticles was considered with the assumption ofinstantaneous coalescence. Only one particle statevariable, i.e., the volume v of a particle, was taken

Fig. 13. Comparison of characteristic times for monomer formation, particle coalescence, and particleagglomeration, data for Fe precursor Fe(CO)5 andFe-particles.


into account. The problems associated with theevolution of particle size can be illustrated by dis-cussing the population balance equation (forspherical particles), as formulated by Friedlander[105]

onðvÞot¼ S � o

ovnðvÞdv

dt

� �� onðvÞ

ot

� �l

þ 1

2

Z v

0

bðv; v� xÞnðvÞnðv� xÞdx

� nðvÞZ 1

0

bðv; xÞnðxÞdx; ð1Þ

where n is the number density of the particles ofvolume v. The terms on the right hand side arethe source (or birth) term and those for surfacegrowth and the loss of particles, as well as thewin and loss terms by particle coagulation. Thelatter terms contain the collision frequency, b.Various solutions of the above equation takingonly coagulation into account are known, seee.g., [106–109], with different models for describ-ing the particle size distribution.

Largely unknown in the above equation arethe source term and that for surface growth. Bothcontain reaction kinetic parameters, which caninclude large reaction mechanisms with unknownrate coefficients. If every monomer is a stable par-ticle, the source term is equal to its chemical for-mation rate and can be related to thedecomposition rate of the precursor

S ¼ d½Monomere�dt

/ � d½Precursor�dt

: ð2Þ

For gaseous precursors global one-step reactionshave been proposed, see e.g., [110,111]. On theother hand, when the critical cluster size is compa-rably large, the source term S can be related to thenucleation rate. The reality should be somewherein between. Further work is needed to clear upthe chemistry of the birth of clusters and the kinet-ic steps involving clusters and the growth of earlyparticles. These data can hardly be obtained fromflame experiments. Like in the early days of com-bustion modelling, the input of such kinetic datamust come, e.g., from shock tube studies andlow pressure flow reactor experiments, see e.g.,[112,113].

As is obvious from many experimental find-ings and Fig. 8, particles are not always spheri-cal, as assumed above. They can have quitedifferent morphologies ranging from being quitecompact to very fluffy aggregates, which areformed if coalescence is slow compared to thecollision rate of particles with one another.Aggregates coagulate (in the free molecularregime) faster than fully fused particles of thesame total volume, because they have largerarea for collisions. This affects the collision fre-quency, b, in the coagulation kernel and can be

taken into account by introducing a fractaldimension, Df.

In the case of particle coalescence, the interde-pendence of sintering and agglomeration must befully taken into account. Agglomerates with part-ly fused primary particles must be expected. Aspointed out by Friedlander [105], Pratsinis [2],Rosner [6], another state variable for the particlenumber density n must be introduced,n = n(v,a), with ‘‘a’’ being the surface area ofthe particle, Eq. (3). A coalescence term must alsobe added to the above equation

onðv; aÞot

¼ onot

� �see above

� o

oanðv; aÞda

dt

� �: ð3Þ

Koch and Friedlander [114] have proposed a sim-ple relaxation equation for the decrease in surfacearea of an aggregate

dadt¼ 1

sf

ða� afÞ; ð4Þ

where a is the surface area of the aggregate, af isthe surface area of a completely fused sphere ofsame volume. The characteristic coalescence timedepends on the assumed fusion mechanism andalso on the particle’s environment. Figure 13 illus-trates a comparison of characteristic times formonomer formation, with those for particleagglomeration and particle coalescence. The datacalculated for Fe particles in the free molecularregime with surface diffusion being the main coa-lescence mechanism. Other models for particle fu-sion are also known, such as grain boundarydiffusion, viscous flow, and volume diffusion.Calculations of particle sintering using moleculardynamics have confirmed the relevance of theKoch–Friedlander Eq. (4), see e.g., [115].

With the above two state variables for thevolume, v, and surface area, a, of an aggregate,

-


a quantitative description of particle morphologyby the two-dimensional particle population bal-ance is possible. This approach has successfullyverified experimental results, see e.g., [6,116,117].The trends are developing towards multivariantpopulation balances, which need both furtherinputs of physical rate parameters and numericalefforts to find reliable solutions.

6. Conclusions

Particle synthesis in flames is an overlappingfield of research; it can be explored from twodirections: material science and combustion sci-ence. It may be erroneously considered to be atthe fringes of both disciplines, but it can contrib-ute to and benefit from both. Material science isinterested in solid state products, includingnanomaterials with tailored new properties.Flame synthesis is from this scientific point ofview only one, and often not the most attractive,process among various other possibilities. Com-bustion science considers particle formation morefrom the viewpoint of pollutant formation.Often, interest is limited to characterizing particlesizes, so that strategies for using and applyingthe particles are not addressed properly enough.Researchers like Friedlander, Pratsinis, and oth-ers, who are working in the flower gardenbetween the disciplines addressed can bridge thedifferent interests from their viewpoint of chemi-cal engineering.

Understanding the dynamics of larger, highlystructured aggregates, which appear later in aflame, is not driven much by processes likechemical reactions. It is therefore obvious thatthe very early processes in a flame, like precursordecomposition, monomer formation, clusterkinetics, and the early formation of particlesshould be the natural playground of combustionresearchers. As all the processes addressed arestrongly controlled by temperature, this propertymust be carefully measured, together with theconcentrations of species in the region of particleinception. This seems to be even more importantfor those particles with mixed components, wherethe chemistry becomes critical. Also experimentalmethods for sizing and collecting particles in theregion of their birth are very necessary includingshock tube and flow reactor techniques.

From the point of view of specific applications,flame technology made uniquely possible themanufacture of carbon blacks, fumed silica andlightguide preforms. As such, it is quite possibleto contribute to the development of new materialsand products, which previously have not beenmade by other techniques. Recent results in theflame synthesis of catalysts, biomaterials (dentaland orthopaedics), and sensors support thisoptimism.

By way of a final remark, I would like to men-tion a similarity in the research involved in devel-oping gasdynamic and chemical lasers with thepresent topic of nanoparticles. To improve anddevelop lasers, various nonequilibrium flows werestudied, including the mixing of reactants inhypersonic flows, with the subsequent initiationof reaction by a shock wave. Why are we not try-ing this approach for nanoparticle synthesis?

Acknowledgments

The author thank Profs. S.E. Pratsinis, ETHZurich, A.N. Hayhurst, Cambridge University,and C. Schulz, University of Duisburg-Essen, forhelpful discussions. I am also grateful to A.Kowalik, P. Ifeacho, and B.F. Kock for their helpon the manuscript. The financial support of theGerman Science Foundation (DFG) and the sup-port of A. Gutsch, Degussa AG, are gratefullyacknowledged.

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Comments

Robert Hurt, Brown University, USA. Your presenta-tion emphasized short residence time as the main require-ment for obtaining unagglomerated nanoparticles bycombustion synthesis, and it gave less emphasis to lowreactant concentration or pressure. What is the possibilityof practical large-scale production of unagglomeratednanoparticles near atmospheric pressure in other-than-ul-tra-dilute systems—say through careful engineering con-trol of residence time and quench rate alone?

Reply. Large-scale production of unagglomeratednanoparticles near atmospheric pressure is hard to ob-tain. You are mentioning the two key elements: shortresidence time and high quench rate. A careful designof the time–temperature profile of the reacting flow lim-its the particle growth process and avoids strong particlesintering. The agglomeration of particles in the coolerpart of the flow in the form of weak agglomerates isnot very problematic. We have demonstrated for a larger

hot wall reactor (typical length: 3 m) that weeklyagglomerated silicon particles of small primary particlesize can be obtained by a careful design. In Germany,I could help to initiate a new research program on par-ticle synthesis in a hypersonic flow reactor. Such a devicecan also be useful to realize the above mentioned key de-sign elements.

d

Alessandro Gomez, Yale University, USA. Most ofyour excellent presentation was focused on gas-phaseparticle synthesis. Yet much of the industry interest isnow focused on multicomponent (multi-oxides) particlesfor which the gas-phase route may be limited by whatsome colleagues refer to as the ‘‘tyranny of thermody-namics’’. What is the possible role of spray pyrolysis,which does not suffer from this limitation, in the synthe-sis of these particles in combustion?


Reply. By the phrase ‘‘tyranny of thermodynamics’’you probably mean the limitation in gas phase particlesynthesis caused by the limited vapor pressure of the pre-cursor materials. As described in the paper, the classicalspray pyrolysis is more or less a drying process, whichhas been further developed by Pratsinis and coworkers([4,5] in paper) toward flame spray pyrolysis andemulsion combustion. This is indeed a possibility toovercome the problem of limited vapor pressure. A

metal-containing liquid mixture of precursors is sprayedinto a premixed or diffusion flame. This method is morestrongly related to chemical engineering aspects andgood for practical application in industry. A detailedunderstanding of what is happening during dropletevaporation and homogeneous and heterogeneous reac-tion is not available. Nevertheless, flame spray pyrolysisoffers new possibilities to synthesize interesting newmaterials more or less by trial and error.

particle synthesis in flames

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