Materials Science and Engineering A334 (2002) 53 – 58 Influence of process parameters on the morphological evolution and fractal dimension of sol – gel colloidal silica particles Daniela C.L. Vasconcelos a , Wagner R. Campos b , Vanderley Vasconcelos b , Wander L. Vasconcelos a, * a Laboratory of Ceramic Materials, Department of Metallurgical and Materials Engineering, Federal Uniersity of Minas Gerais, Rua Espı ´rito Santo, 35 -2 ° andar sala 206 -Centro, CEP 30160 -030, Belo Horizonte MG, Brazil b CDTN-Nuclear Energy Research Center -Belo Horizonte MG, Brazil Received 19 March 2001; received in revised form 23 July 2001 Abstract Colloidal silica particles were synthesized from tetraethoxysilane (TEOS), ammonium hydroxide (NH 4 OH), ethanol (C 2 H 5 OH) and deionized water. Obtaining different structures was possible due to the variation of molar ratios of NH 4 OH, C 2 H 5 OH and H 2 O with respect to the precursor. The nature of the particles was evaluated using X-ray diffraction, energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). The morphology of the particles was described using electron microscopy (SEM). Pore structure was characterized by nitrogen gas sorption-desorption, showing colloidal silica particles with solid-pores surface areas varying from 0.48 m 2 g -1 up to 567 m 2 g -1 ; pore volume varying from 0.0013 up to 0.88 cm 3 g -1 and average pore radii varying between 1.4 and 36 nm. The fractal dimension of the silica particles and agglomerates varied from 2.0 to 2.12. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol – gel; Colloidal silica; Fractal dimension; Structology www.elsevier.com/locate/msea 1. Introduction Colloidal silica particles and agglomerates are syn- thesized by sol–gel method from hydrolysis of a metal alkoxide, followed by condensation and polymerization reactions [1 – 6]. Colloidal silica powders are extensively used commercially as catalytic supports, for modifying surface properties of synthetic films, increasing the hardness, durability and integrity of coatings, and oth- ers [1 – 4]. Depending on the synthesis process parame- ters, the structure of colloidal particles may vary from isolated spheroidal particles to agglomerates of complex structures [4]. Therefore, a major concern associated with obtaining colloidal silica particles is the control of structural features, such as average diameter and mor- phology of particles. The aim of this work is 2-fold, the description of important structural features of sol – gel colloidal silica particles obtained by using tetraethoxysilane (TEOS) and ammonium hydroxide (NH 4 OH) and the applica- tion of the concept of fractals for describing the mi- crostructure of these systems. 2. Experimental procedure Colloidal silica particles were produced, at room temperature, from hydrolyzed TEOS, ethyl alcohol (C 2 H 5 OH) using ammonium hydroxide (NH 4 OH) as a catalyst. The precursor solutions were obtained using molar ratios of TEOS:NH 4 OH:H 2 O:C 2 H 5 OH equal to 1:1–10:4–16:6–60, as described in Table 1. The nature of the particles was distinguished by X-ray diffraction (XRD) (powder method; PW1710, Philips), energy dispersive spectroscopy (EDS) (JSM 5410, Jeol) and Fourier transform infrared spec- troscopy (FTIR) with diffuse reflectance accessory * Corresponding author. Tel.: +55-31-3238-1821; fax: +55-31- 3238-1815. E-mail address: wlv@demet.ufmg.br (W.L. Vasconcelos). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5093(01)01762-2