Nanostructure designed powders of optical active materials Me x SiO y obtained by ultrasonic spray pyrolysis V. Jokanovic ´ a, * , M.D. Dramic ´anin a ,Z ˇ . Andric ´ a , T. Dramic ´anin a , M. Plavs ˇic ´ b , S. Pas ˇalic ´ c , M. Miljkovic ´ d a Institute of Nuclear Sciences ‘‘Vinc ˇa’’, P.O. Box 522, 11001 Belgrade, Serbia b Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia c MSEP Serbia, Nemanjina 22-26, 11000 Belgrade, Serbia d University of Nis ˇ, Center for Electron Microscopy, 18000 Nis ˇ, Serbia Available online 10 July 2007 Abstract A new approach to the two-level design and self-assembling of Me x SiO y (Me = Gd, Y and Ca or their mixtures) nano-powders from SiO 2 sols is described. The nanostructure design of various optical active materials based on different compounds of Me x SiO y is dis- cussed. These particles were formed by the combination of hydrothermal synthesis and ultrasound spray pyrolysis. The morphology and structural design of the particles and sub-particles are defined by their synthesis procedure and processing parameters. The mean particle size and size distribution of all the Me x SiO y based compounds were determined by SEM analysis. These results were compared with the particle diameter evaluated from the theoretical model of the breaking-up of capillary waves. For all the synthesized powders, the obtained results indicate the possibility of a rigorous design of the structure of the particles throughout their entire hierarchy. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Nano-powder systems of different compounds based on Me x SiO y (Me = Gd, Y and Ca or their mixtures) as a matrix cover a wide range of potential applications, e.g., integrated opto-couplers, light color converters, fluores- cence tubes, X-ray detectors, xerograph and various dis- play devices, as photo-luminescent, cathode-luminescent and electro-luminescent materials, and laser and electronic devices [1–4]. Theoretical and experimental expertise available for the structural and substructural design of various ceramic sys- tems (alumina, mulite, cordierite, calcium hydroxyapatite, titanium dioxide, wolfram bronzes dopped with phospho- rus and argentum iodate) have been applied on systems of different compounds of Me x SiO y (Me = Gd, Y and Ca or their mixtures) [5–10]. The mechanism of droplets formation during ultrasonic spray pyrolysis and droplet transformation into hollow or fully dense spheres of Me 2 SiO 5 (Me = Gd, Y and Ca) is considered in this paper. Based on the estimated differences between the calculated values for the particle diameters and the experimentally determined values, the thickness of the walls of hollow particle spheres were found. It is shown that the developed theoretical method [5–10] successfully describes the nano-structural design of all the investigated compounds. 2. Experimental A silica sol made by a hydrothermally method which is described elsewhere [11,12] was the common precursor for the synthesis of all the silicate systems studied in this arti- cle. After the hydrothermal treatment, the pH of the silica sol was adjusted to nine and then the sol was concentrated until a silica concentration of 0.1 mol/l was attained. Depending on the synthesized silicate system, the required 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.05.043 * Corresponding author. E-mail address: vukoman@vin.bg.ac.yc (V. Jokanovic ´). www.elsevier.com/locate/optmat Available online at www.sciencedirect.com Optical Materials 30 (2008) 1168–1172