In situ growth and structure of fractal silica aggregates in a flame

OH 4 5 2 2 2 1 -0 0 7 1 ABSTRACT. A premixed flame aerosol reactor was used to produce titania particles by oxidation of titanium isopropoxide vapor. The growth, aggregation of particles, and the agglomerate structure were determined as a function of height in the flame using in situ light scattering and transmission electron microscopy (TEM) measurements. A methodology to determine the sintering characteristic time using light scattering data was established. The light scattering data provided the evolution of the fractal dimension which was then related to the normalized surface area change using a computer simulation. The sintering equation was redeveloped in terms of the normalized surface area, thus not having to account for coagulation effects. Experimental results indicate that isolated titania particles were observed at the high temperatures due to fast sintering. An agglomerate was obtained at downstream locations with an associated change in fractal dimension due to sintering.

Section: Aerosol Science and Technologymentioning

confidence: 99%

Section: Light Scattering From Fractal Aggregatesmentioning

confidence: 99%

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Study of the Sintering of Nanosized Titania Agglomerates in Flames UsingIn SituLight Scattering Measurements

Yang

Biswas

1997

“…The resulting equation for the structure factor does not appear to be analytically solvable, but Mountain and Mulholland calculated it numerically. Hurd and Flower (1988) proposed that the fractal aggregates had sharp spherical perimeters, hence the cutoff is described by the autocorrelation function of a sphere. Because we have shown that the density autocorrelation function is equivalent to a self convolution, this cutoff may be called the "overlapping spheres" cutoff because the convolution integral essentially overlaps the real space density function.…”

Section: Explicit Forms For the Structure Factormentioning

confidence: 99%

Light Scattering by Fractal Aggregates: A Review

Sorensen

2001

1,033

756

This paper presents a review of scattering and absorption of light by fractal aggregates. The aggregates are typically diffusion limited cluster aggregates (DLCA) with fractal dimensions of D ' 1:75, which occur frequently in aerosols and colloids, but other types of aggregates are discussed as well. The results of the review are quite general. A scaling description of the scattering of waves forms the basis of our understanding. This description yields the optical structure factor S(q ) of the aggregate, where q is the scattering wavevector, a key quantity. The rigorous approach to the structure factor, the Fourier transform of the density correlation function, is also given and various forms provided in the literature are compared and the best selected. Light scattering is directly related to the structure factor under the assumption of no internal multiple scattering. This so-called Rayleigh -Debye -Gans (RDG) approximation is compared to more rigorous electromagnetic approaches and found to be quite accurate for fractal aggregates. This results despite the presence of internal multiple scattering, which leads to depolarization, and which is also extensively described. For ensembles of aggregates, the effects of aggregate polydispersity on scattering experiments are described and methods for proper analysis are given. A description of optical particle sizing and morphology analysis is given for a complete in situ characterization of the aggregate system. The review relies strongly on straightforward physical reasoning and experimental examples, largely from aerosols, especially those of carbonaceous soot. This paper not only reviews and consolidates, but also presents new results including a scaling analysis of multiple scattering, which leads to depolarization, with a successful comparison to available data and an understanding of structural subtleties; demonstration of the signi cance of a phase shift parameter for evaluating the range of validity of the RDG theory; a Maxwell -Garnet analysis of fractal aggregate scattering; calculations of the albedo and successful comparison to available simulation data; and speculations on multiple scattering and the interior eld. INTRODUCTIONThe purpose of this review is to unify our knowledge of how fractal aggregates scatter (and absorb) light. This is of value for light scattering diagnostics of aerosols and colloids and for understanding the optics of these systems in the environment. The subject has seen able and useful reviews before by Teixeira (1986), Martin and Hurd (1987), Charalampopoulos (1992), and a brief overview by myself (Sorensen 1997). Very recently Fuller and Mackowski (2000) have given an excellent review of the superposition theory of scattering from aggregates. My desire here is to provide an up-to-date and comprehensive review useful for the experimentalist. Any theory that does appear will be based on simple physical arguments rather than detailed mathematical analysis. Many of the general results to be described here apply to nonfractal aggreg...

“…Ulrich and coworkers (Ulrich 1971;Ulrich et al 1976;Ulrich and Subramanian 1977;Ulrich and Riehl 1982) proposed a theoretical model for particle growth in a premixed at ame by treating coagulation and coalescence separately. Hurd and Flower (1988) measured the fractal dimension of silica aggregates in a premixed at ame using a light scattering technique and observed the decrease of primary particle size along the ame height. Chang and Biswas (1992) determined the size distribution of silica particles in a methane/air at ame using the method of angular dissymmetry light scattering.…”

Section: Introductionmentioning

confidence: 99%

Simulation of Growth of Nonspherical Silica Nanoparticles in a Premixed Flat Flame

Lee¹,

Oh²,

Choi³

2001

The growth of nonspherical silica nanoparticles in a premixed at ame has been simulated, including the effects of convection, diffusion, thermophoresis, chemical reactions, coagulation, and coalescence. Considering both radiation effect and multistep chemical reactions of methane/air including both oxidation and hydrolysis of SiCl 4 , combustion analysis in a premixed at ame was done rst to obtain temperature, concentration of gas species, and ow elds. The predicted ame temperatures were in good agreement with the previous experimental data. Two-dimensional aerosol dynamics in which both particle volume and surface area are independent variables was then analyzed to investigate the growth of nonspherical silica particles. Several different models of coalescence of silica particles were studied: viscous ow sintering, atomistic diffusion sintering, fast sintering, and hybrid sintering models. Since the residence time was short and temperatures were not high enough for perfect coalescence of silica particles in the present study, the resulting particles were partially sintered or open-structured aggregates. The variations of total volume/number concentration and diameter of average volume along the ame height were obtained and compared with experimental data. Bi-modal size distributions were obtained at some ame heights.