Electron Diffraction Analysis of the Structure of SiO 2 Gel-Film
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ELECTRON DIFFRACTION ANALYSIS OF THE STRUCTURE OF SiOi
GEL-FILM
HISASHI OHSAKI4, MICHEL A. AEGERTER• AND TAKAKI SHICHIRI" *Universidade de Sgo Paulo, Instituto de Fisica e Quimica de S~o Carlos, Sgo Carlos 13560, Brazil. **Osaka City University, Department of Physics, Faculty of Science, Sumiyoshi-ku, Osaka 558, Japan.
ABSTRACT The structure of self-supported SiOz gel-films prepared from acid and basic TEOS solutions is analysed by high energy transmission electron diffraction method. The reduced radial distribution function (RDF) curves show that all the films are already well dense despite the low drying temperature (•50* C) and short drying time (•30 s). The Si-O bond length of the gelfilms prepared from highly acid and basic solutions is about 1.58 A; it is smaller than that of bulk vitreous silica (1.61 A) but similar to that of 60 A thick evaporated a-SiO2 film.
INTRODUCTION Sol-gel process has a huge potential for producing functional materials using simple procedures. The gel materials are generally porous and a thermal treatment is necessary to transform them into dense glasses and/or ceramics. The microscopic and atomic structures of gel materials are therefore important parameters for the densification process. In this study, the atomic network of thin SiO2 gel-films is analysed for the first time by using electron diffraction method. EXPERIMENTAL The sols were prepared by mixing 15 ml TEOS and 28 ml ethanol. Sols labelled A-i, A-2 and I have been acidified by addition of aqueous HC1 in order to get apparent pH (pH*) of 0.6, 1.6 and 2.6 (isoelectric point of SiOz), respectively, while the pH^ of sample B has been adjusted to 10.5 by adding aqueous NH+OH. The ratio TEOS:HZO:ethanol is kept almost constant for all the sols (1:5.8:7.1 in mole ratio). The sols were aged for 5 days without any evaporation. The gel-films were prepared by the following procedures. A #100 mesh Cu grid with 0.24 mm diameter hole size was immersed into a sol and withdrawn vertically. Thin films of sol retained in the holes of the mesh grid were self-supported by the surface 0 tension and they were then dried in warm air at 40 - 50 C for 10 - 30 s. An electron diffraction apparatus was operated at an electron energy of 55.8 kev; the electron-beam size was about 70 um and the beam current was of the order of 10"q A. The range of 3 the scattering angle corresponds to Smin=0. Ai and snm =34 k., where s=(41E/A)sin(0/2), 8 is the angle between the incident and diffracted beam, and A is the electron wavelength. The intensity of transmitted electron-beam was measured by a Faraday cage with an aparture of 0.1 mm in diameter; the Faraday cage captured the electron-beam with diverging angle until 0.016O(s=O.03 W' ). The transmittance for sample A-I, A-2, I and B were 9.4, 9.2, 16 and 5.4 % and the film thicknesses estimated from these values Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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are about 500, 500, 400 and 600 A, respectively. The small values of transmittance indicate the occurence multiple scatter
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