Demonstration of soot particle resizing in an

JOURNAL OF APPLIED PHYSICS 100, 124918 共2006兲

Demonstration of soot particle resizing in an ethylene flame by small angle x-ray scattering J. B. A. Mitchella兲 and J. Courbe PALMS, UMR 6627 du CNRS, Université de Rennes I, 35042 Rennes Cedex, France

A. I. Florescu-Mitchell Institute for Space Science, P.O. Box MG-36, 76900 Bucharest, Romania

S. di Stasio Aerosol and Nanostructures Laboratory, Istituto Motori CNR National Research Council of Italy, Via Marconi 8, 80125 Naples, Italy

T. Weiss European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France

共Received 13 June 2006; accepted 26 September 2006; published online 29 December 2006兲 The size distribution of soot nanoparticles in an ethylene flame has been mapped in an in situ small angle x-ray scattering measurement at the European Synchrotron Radiation Facility. It has been seen that an abrupt size distribution change occurs at about a third of the flame’s visible height and this is believed to be due to the oxidation and dehydrogenation of the particles. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2400105兴 INTRODUCTION

A recent contribution to emerging topics in nanoparticle aerosol science1 focused on the necessity to gain a full understanding of the formation and growth of soot precursors, ultrafine particles, and aggregates. In situ studies in the past have been conducted mainly using light absorption and scattering methods.2–6 In particular, static laser light scattering has been used to detect the restructuring of fractal soot aggregates in an ethylene-air diffusion flame.7 Nevertheless, scattering techniques at visible wavelengths are not able to probe very small particles since the differential Rayleigh scattering cross section depends on the ratio of the sixth power of the particle size to the fourth power of the wavelength. A number of ex situ methods have been implemented including scanning and transmission electron microscopies8–10 of flame soot particles sampled as surface coatings on grids and the direct aspiration of soot aerosol from flames prior to sizing on line by differential mobility analyzers 共DMAs兲.11 Extracted particles have also been studied using x-ray scattering techniques.12,13 In situ methods include sizing by analyzing the cooling rate of particles in laser induced incandescence measurements,14 and x-ray and neutron scattering techniques have been applied to this problem.13,15–24 In recent measurements, we have used small angle x-ray scattering 共SAXS兲 共Ref. 20兲 and small angle neutron scattering 共SANS兲 共Ref. 24兲 methods to characterize in situ nanoparticle size distributions in an ethylene diffusion flame. The SAXS experiments were performed at the ID09b beamline at the European Synchrotron Radiation Facility 共ESRF兲. Three different size modes in the range of 1 – 20 nm were observed to grow in an atmospheric pressure ethyleneair diffusion flame at different residence times. These were subprimary particles 共4 – 6 nm兲, primary particles a兲

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共10– 12 nm兲, and small nuclei 共⬃1.5– 2 nm兲. These results were in accord with a previous ex situ transmission electron microscopy 共TEM兲 study.9 SANS measurements were also performed by our group24 at the Laboratoire Léon Brillouin 共LLB兲, and soot particles in the size ranging from 6 to 50 nm were studied. In this paper, we report detailed measurements of size distributions for soot nanoparticles in the range from 6 to 50 nm radius, performed at the ID02/ ESRF beamline where the availability of the long sample-todetector distance allowed us to observe relatively larger particles 共⬎20 nm兲 compared to the previous SAXS study.21 EXPERIMENTAL METHOD

The measurements are made using a 12.46 keV, ␭ = 0.099 nm, x-ray beam with a spot size of 100 ␮m width ⫻ 100 ␮m height and the scattered rays were detected using a pinhole camera25 arrangement where the source-to-detector distance could be varied from 1.5 to 10 m. This fast readout, low noise, position sensitive detector with a size of 220 ⫻ 220 mm2 with 2048⫻ 2048 pixels2 and a spatial resolution of 200 ␮m is mounted inside an evacuated tank so that the interference from air scattering is eliminated over most of the path between the sample and the detector surface. The photon flux arriving on the sample is typically 4 ⫻ 1012 photons/ s. The experimental setup is illustrated in Fig. 1. The diffusion flame is formed on an 11 mm diameter cylindrical tube supplied with ethylene fuel with purity better than 99.9%. This tube is surrounded by a 100 mm diameter tube through which air from a compressor is made to pass in order to act as a screen against flame perturbation caused by ambient air movements. A fuel flow rate of 62 ml/ min of ethylene and 39 l / min of air yielded a flame with a visible height of about 40 mm. This is the flame height that we have used in all our studies using both neutrons and x rays and

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FIG. 1. 共Color online兲 Experimental layout.

was chosen to meet the exigencies of the initial available setup for varying the sample height at the ESRF. The flame is scanned vertically and horizontally by the x-ray beam in order to sample the soot particles that formed during the combustion process, as a function of height. Data are taken with and without the flame present, and the resulting images subtracted, in order to yield scattering data, from the soot particles with background air scattering were removed. This flame has been characterized by us previously and details are available in Ref. 22. The soot volume fraction in this flame was measured using a helium-neon laser beam absorption method, and the results are shown in Fig. 2. It can be seen that it reaches a maximum of about 4.0⫻ 10−6 in the center at a height of 25 mm above the burner. DATA ANALYSIS

There are a number of ways in which small angle x-ray scattering can be analyzed depending upon whether one supposes that the particles are spherical or not and, further, that their sizes are distributed according to a single or multimode distribution. In a first analysis, a typical assumption is to represent the soot particles by spheres having the same radius r, being randomly distributed in the gas, and with no interaction between them 共i.e., in a dilute suspension兲. This is a reasonable approximation in that scanning electron micros-

FIG. 2. 共Color online兲 Plots of soot volume fraction measured by He–Ne laser light absorption as a function of radial distance for various heights above the burner 共HABs兲.

copy 共SEM兲 studies of soot fractal aggregates8,10 have shown that for a given height above the burner, the size range of larger subunit building blocks is quite narrow: usually between 15 and 50 nm. For a single particle, the scattering power I共q兲 is then given by26 2 I共q兲 = ␳soot 具兩⌺共q兲兩2典sphere

共1兲

q=兩q兩

where ␳ is the scattering density in the scattering volume, in the case of SAXS the density of electrons in the scatterer. The arrowlike brackets indicate that an average is taken over all orientations of the particles. The momentum transfer vector is q = 共4␲ / ␭兲 sin共␪ / 2兲 where ␭ and ␪ are the wavelength of the incident radiation and the scattering angle, respectively. ⌺共qជ 兲 = 兰Vreiqជ rជdrជ is evaluated over the volume V = 共4 / 3兲␲r3 of the sphere modeling the soot elementary particle where r is the particle radius. This leads to the following expression for the scattering intensity per particle:

冋

2 Vr2 3 I共q兲 = ␳soot

sin共qr兲 − qr cos共qr兲 共qr兲3

册

2 2 = ␳soot Vr2F共q兲,

共2兲 where F共q兲 is known as the form factor. The total scattered intensity from all the particles in the sampling volume is then 2 Vr2F共q兲, Itotal共q兲 = n p␳soot

共3兲

where n p is the number of particles per unit volume. Figure 3 shows a simulation of this expression for two different radii: 10 and 20 nm. It is clear from this figure that as soon as one is dealing with a particle ensemble that is polydisperse in size, the deep wells found in this graph will become washed out unless one particular narrow size is dominant. Another way to represent these data is the so-called Kratky plots, namely, the plots of Iq2 vs q. This representation, used often in SAXS studies of protein folding and unfolding,27,28 is known to render a sharp peak in the graph if the particles are globular. Such a simulation is shown for spherical particles in Fig. 4 for particles with radii of 10, 15, 20, 30, and 50 nm. Figure 5 shows Kratky simulations for other forms 共rods of several radii to length ratios and a thin disk兲. The case of spheres with R = 30 yields the first maximum of the Kratky plot at q = 0.07 nm−1. In the low q limit, the form factor for rods is given by

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FIG. 3. 共Color online兲 Graphic representation of Eq. 共2兲 for particles with radii of 10 and 20 nm. 2 2

e−q r /4 F共q兲 = 2qH where 2H is the length and r the radius, while that for a thin disk is 2 2

F共q兲 =

e−q H /3 q 2r 2

with 2H the disk thickness and r the radius.29 The data for each case have been normalized for ease of comparison. It can be seen that a sharp peak can also occur for wide rods but even so such a plot can provide some information concerning the particle shape and, in particular, can distinguish between spheres and very thin rods or thin disks. A second method has also been employed in the present work and here we assume that the particle sizes are polydisperse and have used a data analysis package called GNOM 共Ref. 30兲 to determine the particle size distribution. In this procedure, the Fourier transform of the I共q兲 function is taken to yield the size distribution function P共r兲, and it is assumed in our analysis that P共r兲 is evaluated in the range rmin ⬍ r ⬍ rmax where rmin and rmax are determined by the upper and lower bounds to the measured q range. P共r兲 is actually the

FIG. 4. 共Color online兲 Simulated Kratky plots for spherical particles with radii of 10, 15, 20, 30, and 50 nm obtained using Eq. 共2兲 for the intensity. Also shown is the sum of the five graphs.

J. Appl. Phys. 100, 124918 共2006兲

FIG. 5. 共Color online兲 Simulated Kratky plots for spherical particles with a radius of 25 nm, disklike particles of 2 nm thick by 20 nm radius, and rodlike particles with diameters of 2, 20, and 50 nm by 200 nm long. The plots have been normalized in intensity for ease of comparison.

volume distribution function, i.e., for each r it represents the product of the volume of the particles times the relative number concentration of identical particles with radius r lying in the scattering volume. RESULTS AND DISCUSSION

Plots of scattered intensity versus q for heights above the burner made along the centerline of the flame between 10 and 34 mm are shown in Fig. 6. Data were taken with 0.5 mm height resolution but only a few curves are shown for reasons of clarity. It is clear that no apparent structure is present, leading to the conclusion that the system of particles is polydisperse. Figure 7 shows corresponding Kratky plots for 10, 15, 20, and 25 mm heights above the burner 共HABs兲. It can be seen that a peak at about 0.06– 0.08 nm−1 appears for each measurement indicating that the particles are spherical in form. The fact that the peak is seen, by comparison with the simulation in Fig. 4, to be asymmetric indicates that the distribution of particles is polydisperse in size. Particle volume distributions, determined using the

FIG. 6. 共Color online兲 Plots of scattered x-ray intensity vs q for heights above the burner 共HABs兲 of 10.5, 15.5, 20, 26, 31, and 34 mm.

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FIG. 7. 共Color online兲 Measured Kratky plots of Iq2 vs q for heights above the burner 共HABs兲 of 10, 15, 20, and 25 mm.

J. Appl. Phys. 100, 124918 共2006兲

FIG. 9. 共Color online兲 Particle size distributions measured by x-ray scattering as a function of the height above the burner 共HAB兲 between 10 and 15 mm as measured with a detector-sample distance of 5 m.

GNOM

inversion package from the measured data between 10 and 31 mm HABs, are shown in Fig. 8. In Fig. 9, only data between 10 and 15 mm HABs are shown for clarity. It is seen from these figures that at 10 mm HAB, the primary particles have a distribution centered around 22 nm radius but between 12 and 12.5 mm HABs, and the form of the primary particle distribution changes to bimodal with a new peak appearing, centered at 10 nm radius and this peak continues to grow, shifting to about 12 nm at z = 14 mm HAB and about 15 nm at z = 15 mm HAB. As the HAB is increased, the peak at 22 nm continues to grow in intensity but is dominated by the lower radius peak from z = 13 mm HAB on. The highest soot concentration is found at z = 18 mm HAB with a peak in the distribution at 16 nm. Higher in the flame, the soot concentration diminishes and from z = 23 mm HAB on, the peak shifts again to lower radius values attaining a value of 10 nm at z = 27 mm HAB and diminishing rapidly in intensity above that. This effect is most probably due to the reshaping of soot primaries as a coupled effect of the oxidation and graphitization processes, which cause a reduction of the external size of the particle. In particular, it is known9,31 that oxidation of soot particles can

proceed from within their interiors thus eventually causing the internal rearrangement of carbon layers and a reshaping of the reformed particles to slightly smaller units. In our previous measurement using neutron scattering,24 a size distribution centered around 20 nm radius was found for HABs of 10 and 15 mm, moving to around 23 nm for a HAB of 20 mm and falling again to 20 nm at a HAB of 30 mm. The reason why the neutron scattering measurements yield slightly larger values than the SAXS measurement can be understood in the following. Figure 10 shows radial distributions of the soot particle volumes for a height above the burner of 16 mm. It is seen in this figure that the particles at the center of the flame are smaller than those further out, with the largest particles being found at around 2 – 2.5 mm from the center. While a very small 共100 ⫻ 100 ␮m2兲 x-ray beam spot was used in the present work, the neutron beam used at LLB 共Ref. 24兲 had dimensions of 7 ⫻ 7 mm2 and thus detected particles found in the broader range of sizes shown in the radial distributions. This experimental property of the neutron beam would explain the discrepancy between the peak particle size between the two measurements. SANS serves as a good compliment to SAXS. Of course, the much higher intensity of the x-ray

FIG. 8. 共Color online兲 Particle size distributions measured by x-ray scattering as a function of height above the burner 共HAB兲 as measured with a detector-sample distance of 5 m.

FIG. 10. 共Color online兲 Particle size distributions measured by x-ray scattering for different radial positions in the flame and at a height above the burner 共HAB兲 of 16 mm.

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beam makes the SAXS measurements much faster and allows a much finer grid of measurements. The state of hydrogenation of the particles can in principle, however, be determined using neutrons by comparing the results for deuterated and nondeuterated compounds. Neutrons scatter off the nucleus of the target atom, and the scattering power for coherent scattering of neutrons from deuterium is greater than from hydrogen nuclei.32 共Coherent scattering is determined in the SANS method and being directional yields information on the target structure.兲 X rays scatter off the inner shell electrons of the target atom and do not detect hydrogen at all since the scattering cross section is negligible for this atom. A SANS measurement using a deuterated and nondeuterated fuel is planned for the near future. These measurements demonstrate that x rays can be used to interrogate particle size distributions at the nanometer scale in situ in environments of low particle density and high temperature. Future studies will involve using fuel additives that modify the particle size distribution. The goal of these studies will be to provide information on the mechanism of such additives. Particle size distributions will also be studied in other media including arcs and other air plasmas. ACKNOWLEDGMENT

One of us 共S.d.St.兲 is grateful to MIUR for the partial support to this work by Grant No. FIRB RBAU01CXNP. S. K. Friedlander and D. Y. H. Pui, J. Nanopart. Res. 6, 313 共2004兲. A. D’Alessio, A. Di Lorenzo, A. F. Sarofim, F. Beretta, S. Masi, and C. Venitozzi, Proceedings of the 15th International Symposium on Combustion 共The Combustion Institute, Pittsburgh, 1974兲, p. 1427. 3 R. J. Santoro, H. G. Semerjian, and R. A. Dobbins, Combust. Flame 51, 203 共1983兲. 4 R. Puri, T. F. Richardson, R. J. Santoro, and R. A. Dobbins, Combust. Flame 92, 320 共1993兲. 5 U. O. Koylu and G. M. Faeth, J. Heat Transfer 116, 971 共1994兲. 6 U. O. Koylu, C. S. McEnally, D. E. Rosner, and L. D. Pfefferle, Combust. Flame 110, 494 共1997兲. 1 2

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