JOURNAL OF MATERIALS SCIENCE 19 (1984) 1656-1665 Synthesis of monolithic silica gels by hypercritical solvent evacuation M. PRASSAS*,J. PHALIPPOU, J. ZARZYCKI Laboratory of Materials Science and CNRS Glass Laboratory, University of Montpellier ll, 34060 Montpellier Cedex, France The method of obtaining monolithic dry gels by hypercritical solvent evacuation is presented. The influence of various parameters on the possibility of obtaining crack-free pieces is studied in some detail. The influence of these parameters on the final properties of the gel is also discussed. 1. Introduction Gel powders of various chemical compositions have been successfully converted into "optically clear" glasses by melting or hot pressing [ 1-4]. Apart from the great advantage of a low pro- cessing temperature, it was recognized that hom- ogeneous glasses could be prepared with high purity in this way [5, 6]. Cost considerations, however, show clearly that neither of the above processes could be advantageous as far as the conventional melting and forming technologies are concerned [7]. The most promising route to obtain glasses from gels is to prepare monolithic (without cracks) dried gels. An additional requirement to obtain monolithic glass is that this material has to be sintered without destroying its integrality. Thus, glasses prepared from monolithic gels without hot pressing or melting conserve their purity, because of the absence of contamination from dies or containers. The synthesis of monolithic silica gels has been reported by several investigators. So far only the use of colloidal silica solution led to monoliths which had at least one dimension higher than 10 cm [8, 9]. Some difficulties were encountered when metal alkoxides were used. In fact, drying times from two weeks for silica to a few months for more complicated systems are needed to avoid cracking [ 10]. Moreover, not much is known about the pro- cedure to prepare gels of a size comparable to that previously mentioned. *Present address: Corning Europe, 77210 Avon, France. Even in this case, some cracks appear at high tempeatures which are necessary to ensure the sintering of the gel. Our purpose is to present a very simple, fast and reproducible process to obtain dry monolithic silica gels which have at least one dimension higher than 15 cm. The main characteristic of our process is that the evacuation of the solvent within the pores of the gel is carried out under hypercritical conditions. The parameters affecting the monolithicity of the gels will be presented. The textural and struc- tural features of this kind of gel have also been studied. 2. Gel synthesis and the problem of monolithicity Two methods are generally used for the prepar- ation of silica gels. 1. Destabilization of a colloidal silica sol. 2. Hydrolysis and polycondensation (dehy- dration) of silicon alkoxides. The different stages of each of these processes and the characteristic properties of the resulting material have recently been investigated [11]. Whatever the preparation technique, the freshly prepared silica gels contain an appreciable amount of solvent (usually 70 to 90 wt %) which must be eliminated. A fast drying process either (i)under reduced pressure at room temperature; or (ii) by heating at atmospheric pressure, promotes the crumbling of the gel into grains of few mm 3 volume. The 1656 0022-2461/84 $03.00 + .12 9 1984 Chapman and Hall Ltd.