Fractal Analysis of Flame-Synthesized Nanostructured Silica and Titania Powders Using Small-Angle X-ray Scattering Jingyu Hyeon-Lee, † Gregory Beaucage,* ,† Sotiris E. Pratsinis, ‡ and Srinivas Vemury § Department of Materials Science and Engineering, ML 0012, and Department of Chemical Engineering, ML 0171, University of Cincinnati, Cincinnati, Ohio 45221, and Lucent Technologies, G010, 2000 Northeast Expressway, Norcross, Georgia 30071 Received March 17, 1998. In Final Form: July 21, 1998 The morphology of flame-generated silica and titania aggregates is characterized by small-angle X-ray scattering (SAXS). Nearly all these powders display mass-fractal morphologies, which are composed of ramified aggregates of nanoscale primary particles. Primary particle size, aggregate size, fractal dimension, and specific surface area are obtained from this analysis. The mass-fractal dimension varies from 2.5 to 1.6 for flame generated silica and titania aggregates in single and double diffusion flame reactors. However, titania powders made in a single diffusion flame reactor appear as nonaggregates and nonfractals. Silica powders synthesized with an imposed electric field in a laminar, premixed flame reactor are mass fractals with narrowly confined fractal dimensions from 1.5 to 1.9 regardless of aggregate size. Introduction Aggregates of small primary particles can be produced by various aerosol processes, including flame processes for the production of materials such as titania, fumed silica, and carbon black. During these processes, structural features such as particle and aggregate size, structure, and surface area vary, affecting the properties of the final product. 1 Although aggregation behavior on molecular or colloidal scales is important, relatively little attention has been paid to it, partly because of the highly disordered nature of these materials and difficulties in characteriza- tion. However, with fractal concepts, the description of morphological features of aggregates has been facilitated. The primary tools for description of the morphology of aggregates using fractals are electron microscopy 2-5 and X-ray 6-9 or light scattering. 10,11 Megaridis and Dobbins obtained a fractal dimension of about 1.7 for flame- generated carbonaceous soot and fumed silica based on transmission electron microscopy (TEM), suggesting that cluster-cluster aggregation is an important growth mechanism. More recently, Koylu et al. 3 analyzed flame synthesized carbonaceous soot and found that aggregates are mass-fractal with fractal dimension (d f ) of about 1.7. Although microscopy is quite useful in measurements of primary particle size, it is generally difficult to obtain primary particle size distribution from microscopy and the measurement thus relies on a tedious process of micrograph measurements. Microscopic characterization of the morphology of secondary, mass-fractal structures is even a more difficult and subjective process. Moreover, the occlusion of 3-dimensional structure by overlap of a number of aggregates or even superimposed structure from the same aggregate, in a typical micrograph, makes the process quite difficult. Small-angle X-ray scattering (SAXS) offers an alternate to this process since it is by nature a 3-dimensional averaging technique with a direct measure of the radius of gyration, R g . Characterization of 3-dimensional mass- fractal morphologies is simple in a typical scattering experiment. Combustion aerosol aggregates have been characterized using SAXS in terms of a statistical analysis. 6-9 Schaefer et al. 6 obtained fractal dimensions between 1.7 and 1.9 for fumed silica aggregates from SAXS and found that the surface of fumed silica is self-affine with surface fractal dimensions between 2.5 and 3. For these flame-generated silica powders, the features of kinetic growth processes were also suggested. In addition to SAXS, ultra small-angle X-ray scattering (USAXS) scattering patterns can be extended to close to the millimeter size scale by careful combination of SAXS data with light scattering data. In the past, such combined scattering curves and global scattering functions 12-14 have been used to describe the three levels of structure for fine powders, primary, secondary (aggregate) and tertiary (agglomerate). 12-14 Fumed silica and titania powders made in a diffusion flame reactor and a laminar, premixed flame reactor are analyzed in this study. When particles are synthesized in these reactors, the characteristics of the flame such as mixing configuration or oxidant type can greatly affect † Department of Materials Science and Engineering, University of Cincinnati. ‡ Department of Chemical Engineering, University of Cincinnati. § Lucent Technologies. (1) Pratsinis, S. E. Prog. Energy Combust. 1998, 24 (3), 197. (2) Megaridis, C. M.; Dobbins, R. A. Combust. Sci. Technol. 1995, 71, 95. (3) Koylu, U. O.; Xing, Y.; Rosner, D. E. Langmuir 1995, 11, 4848. (4) Neimark, A. V.; Koylu, L. O.; Rosner, D. E. J. Colloid Interface Sci. 1996,180, 590. (5) Samson, R. J.; Mulholland, G. 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