COLLOIDS AND A Colloids and Surfaces SURFACES ELSEVIER A: Physicochemical and Engineering Aspects 97 (1995) 217-225 Evolution of the fractal structure during sintering of SnO 2 compacted sol-gel powder Giancarlo Esp6sito de Souza Brito, Celso Valentim Santilli, Sandra Helena Pulcinelli * lnstituto de Quimica de Araraquara-UNESP, P.O. Box 355, 14800-900, Araraquara, SP, Brazil Received 18 April 1994; accepted 14 December 1994 Abstract The structural evolution during sintering of compacted SnO2 sol-gel powder was investigated using nitrogen adsorption isotherm analysis. Results show that for sintering temperatures up to 400°C the samples have a fractal pore size distribution. As the sintering temperature increases, a structural rearrangement occurs, allowing an increase of the efficiency of particle packing and the reduction of fractality. Above 400 ° C, the pore size growth associated with grain coalescence is the main structural change observed as the sintering temperature increases. Keywords: Fractal structure; Micropore evaluation; Sintering; SnO2 sol-gel powder 1. Introduction Inorganic solids prepared by the sol-gel tech- nique are of widespread interest for a broad range of practical applications as well as being model systems for the study of colloidal aggregation and sintering. Typically, materials prepared by this technique consist of aggregates that are formed of a random highly branched chain-like structure of primary particles [ 1 ]. The interest in these struc- tures lies in the fact that they exhibit mass fractal behavior, i.e. the density of the aggregates decreases as the distance from the cluster center increases. In particular, the fractal dimension (D) pro- vides quantitative information about the packing efficiency and the connectivity of the primary particles [2]. In general, the experiments required to show conclusively that a material is fractal and over which spatial scale it is fractal are both time- * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03084-0 consuming and expensive. This kind of analysis uses a combination of scattering techniques (light, small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS)) to study all length scales of interest [3]. From SAXS data, for exam- ple, we showed that SnO2 hydrosols and hydrogels exhibit fractal structure dimensions of 1.42 and 1.76, respectively. Meanwhile, the average size of the aggregates was not determined because it is greater than the limit of detection for this technique [4]. Compared to scattering measurements, a wide range of sizes may be probed by porosimetry (mercury intrusion or nitrogen adsorption iso- therms) with a single experiment. In this case, structural information about aggregates can be inferred by recognizing that the pore texture is a template of the solid phase. However, mercury porosimetry has been inadequate for this kind of analysis due to the compression that occurs in samples, compromising a reliable determination of the pore size distribution [5]. In this paper we explore the use of nitrogen