Physical-Chemistry Parameters of SiO 2 Monoliths Processed Via Sol - Gel
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PHYSMAL-C'HEPAISY PI? PR• •s
OF° MO MONOTI W& SOL 6i
A. KL ELU&% M. I&~ EUkS and~ ML nL NUNIES Depumeld OTiChemtsty Thdsn u~nfwyf Iuik Ea• sýio de V mZoLlc CL - 9, 170 Lhslom, Pongl ABSTRACT. High quality glasses may be prepared from liquid precursors, via sol - geL Starting materials and variable control determine the final characteristics of silica monoliths for optical fiber preforms, prepared via sol - gel, as we have found in previous studies. The rate of hydrolysis. and condensation reactions, as wcU as the time required for densification of a gel, is dependent on water/alkoxide ratio, pH and the presence of drying control chemical additives. The physical and optical properties of the preforms are also determined by the specific conditions of the sol - gel processing, In this study we present an additional analysis of pH effects on the kdnetics, before and during the material syneresis. The effect of CO as a contaminant, is also considered. Samples are prepared from hydrolysis 0 of tetrametoxysane in methanol - water mixtures, in the presence of a - picoline. At 2 r, the initial values of pH range between 8 and 6 and show an exponential decrease as the hydrolytic polymensation proceeds, tending to acidic or more acidic values. These steps also are followed by TEM and SEM and are correlated with the sort of aggregates formed in the material along the percolation process. Optical properties of the preforms are interpreted in terms of the clusters formed from those aggregates. Details on instrumental monitoring of [HI] along the alcogelation process are discussed. INTRODUCTION. The formation of glass by sol-gel processing allows the preparation of high quality glassy materials, such as optical fiber, films and in generale, second harmonic generation [1-3], in a direct manner without using chemical vapor deposition (CVD) or modified chemical vapor deposition (MCVD) techniques [4-6]. This method offers the possbility of producing new glass compositions not attainable by conventional methods. The properties of glass produced by this technique are determined by several factors, including type of alkoxide, type of solvent water and silicon alkoxide contents, catalyst and pH value of the solution and addition of drying control chemical agent (DCCA) [7,8]. Silica optical fibers preforms were prepared by hydmlitic policondensation of tetramethoxysilane (TMSO) in water-methanol media. In our proposed method, a-picoline is added to the initial alkooxide system to act as a DCCA [29]. The general scheme for the chemical reactions is:
Hydroysis
kH Si(OCH 3) 4 + nHOH
-
Si(OCH 3 ) 4 .n (OH)n + nCH 3 OH
(1)
Condensation
ka -Si-OCH 3 + OH-Si- -- -Si-O-Si-+CH 3 OH
(2)
Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Mgaterials Research Society
346
-Si-OH + OH-Si- -* -Si-O-Si-+HOH
(3)
The factors generating loss in optical fibers include intrinsic loss mechanisms are tail of infrared absorption by Si-O coupling, tail of ultraviolet absorption due to electron transition and Rayleigh scattering due to spatial fluctuations
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