A Comparison of Tio 2 -Sio 2 Aerogels and Xerogels
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A COMPARISON OF T102-SIO2 AEROGELS AND XEROGELS
G. COGLIATI*, M. GUGLIELMI*', T.M. CHE*** AND T.J. CLARK..
*TEMAV, Laboratori di Medicina, Bologna, Italy. Universita di Padova, Dipartimento di Ingegneria Meccanica, Padova, Italy. Enimont America Inc., Advanced Materials Department, Monmouth Junction, N.J. 08852
ABSTRACT Titania-silica gels containing up to 30% titania have been obtained from silicon and titanium alkoxides and from silicon alkoxides and TiCI4. These gels were then processed by normal drying and by hypercritical methods. The thermal evolution of these gel/glass systems, specifically with respect to crystallization behavior has been followed using x-ray diffraction and Raman spectroscopy. From these studies, it was found that not only the different chemistries, but also the different processing methods significantly affect the microstructure of these titania-silica systems. INTRODUCTION Silica-titania glasses have a very low thermal expansion (1,2) and high refractive indices (3,4). Unfortunately, they are difficult to fabricate by conventional techniques owing to their very high melting temperatures. As a result, many investigators (2,4-9) have employed sintering methods, such as sol-gel, which require much lower processing temperatures to prepare TiO2-SiO2 glasses. A major problem in producing bulk glass by the sol-gel route is formation of cracks during the drying stage. In practice, ambient xerogel methods require considerable drying times, whereas with the use of hypercritical drying techniques, large aerogel monoliths can be obtained in a few hours with 100% certainty (10). To date, few comparisons of multicomponent aerogels and xerogels have been made. In this paper, based on x-ray diffraction and Raman data , we compare various TiO 2 -SiO 2 gels made using TiCI4 or Ti(OC 3 H7)4 with silicon tetramethoxide, and dried by the two different methods. EXPERIMENTAL PROCEDURE Preoaration of aels The procedure followed in this study for the preparation of xerogels and aerogels is summarized schematically in Figure 1. Colloidal solutions of hydrolyzed silicon and titanium alkoxides or silicon alkoxide and titanium tetrachloride gelled in several hours at room temperature. In all cases, gelation occurred more rapidly at higher Ti/Si ratios. The resulting transparent and homogeneous bulk gels were then dried by both normal and hypercritical methods. The xerogel and aerogel monoliths studied received heat treatments in which the samples, when heated to a given temperature and cooled, were either extracted for characterization or heated to the next higher heat treatment temperature. The heat treatments of 300, 700, and 900 oC were conducted in air for 24 hours soaking time, whereas the 1200 and 1500 oC heat treatments received 2 hour soak times in a helium atmosphere.
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
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Si(OCH3 )4 [TMOS ] + CH 3 OH + H2 0 TMOS: CH 3 OH:H 2 0 = 1:2.5:1
I
Metal Alkoxide Route
T.H..O 4R)
097.0
TMOS:Ti(OR) 4 = 0.97:0.03 TMOS:Ti(OR)4 = 0.93-0.07
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