The pore morphology of fluoride catalyzed xerogels
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I. INTRODUCTION The sol-gel process has attracted widespread attention because it allows the preparation of films and monoliths of glasses with very high homogeneity, the fabrication of glasses with special chemical composition, and the processing can be performed at relatively low temperatures.1 With this new glassmaking technique it is also possible to produce porous glasses which have a high internal surface area and pores with narrow pore size distribution. By changing the experimental conditions, a great variety of different average pore sizes can be obtained. This makes these glasses an ideal host for studies of the effect of fluid confinement in small pores on the properties of the fluids.2"5 In the sol-gel process, hydrolyzed silicon alkoxides Si(OR)4-, e.g., R = CH3, C2H5, polymerize via condensation reactions with the elimination of water or alcohols1'6 = Si—OH + HO — Si== ^ = S i — O — S i = + H2O = Si—OH + RO — S i = -> = S i — O — S i = + ROH
(1) (2)
This polycondensation leads to the gelation of the sol, which after drying is densified to a glass by heat treatment. The properties of the final glass are determined by the chemical and physical conditions during the process of preparation. They depend on the initial ratio of alkoxide: alcohol:H2O,7 the solvent,89 pH-value,1112 temperature,12 pressure,13 the presence of catalysts,14 and the time and conditions of drying and firing.6 The drying and firing process is the critical stage in the production of monolithic glasses. Too rapid drying causes the cracking of the glass. So-called "drying control chemical additives (DCCA's)", like, e.g., formamide, have been introduced recently.15 They modify the pore structure of the gel, thus permitting relatively rapid drying of the gel "'Current address: Philipps-Universitaet, Physikalische Chemie, Hans Meerweinstrasse, D-3550 Marburg/Lahn, West Germany. J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
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without cracking. However, the removal of DCCA during the heating process can be a source of problems. Recently, we performed a detailed study of the effect of fluoride anion catalysis on the sol-gel process.16 It has been shown that the F~-anion is one of the most effective catalysts in accelerating the gelation process in both acid and basic cases.16'17 For example, the concentration of 10~3 M NaF in a tetramethoxysilicate solution at a pH = 6.4 reduces the gelation time from 33 h to several min.16 We employed 29Si-NMR and Raman spectroscopy in order to investigate the effect of the F~-anion on the various stages of the gelation process.16 It was shown that fluoride not only drastically accelerates the gelation process but also leads to a different polymerization process in which a loose and relatively open gel network with a high percentage of — OCH3 groups is formed at the gelation point. Fluorine has recently also become a popular dopant for lowering the refractive index of silica glasses in optical fiber fabrication.18 In their study Pope and Mackenzie14 have
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