No abstract

A method to vary locally the physical properties of a porous material is presented. A wet gel is first prepared following conventional sol-gel techniques. The pore walls are derivatized by adding to the gelling solution a silane carrying a polymerizable moiety such as trimethoxysilylpropyl. The solvent of the wet gel monolith is then exchanged with a solution of a monomer such as styrene and a photoinitiator such as 2,2′-azobis-isobutyronitrile. Illumination with ultraviolet light initiates polymerization which in turn engages the moiety dangling from the pore surfaces. Supercritically dried monoliths were characterized with techniques such as field-emission scanning electron microscopy (SEM), methylmethacrylate atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, and Brunauer-Emmett-Teller surface area measurements. These structural characterization techniques showed that the silica nanoparticles making up the backbone of the monoliths were cross-linked by a polymer conformal coating. Mechanical characterization was carried out with nanoindentation and the three-point flexural method and showed that the properties of uniformly photo-cross-linked monoliths could be varied by varying exposure time. So, for example, the monolith density could be varied between about 0.21 and 0.97 g • cm -3 , the porosity between 6 and 87%, and Young's modulus between 9 and about 1800 MPa. Overall, the characterization techniques show that photo-cross-linked monoliths have physical and mechanical properties comparable and often superior to those of monoliths obtained by thermally initiated cross-linking (see, for example,

Nanoporous monoliths with a typical aerogel core and a mechanically robust exterior ceramic layer were synthesized from silica aerogels cross-linked with polyacrylonitrile (X-PAN aerogels). Following supercritical drying, X-PAN aerogels were wrapped in Mg and heated to 850 C in inert atmosphere to generate Mg-PAN monoliths. Mg penetrated 2-3 mm inside the monolith, reacted with the silica framework and formed a mechanically strong crust with a hardness of 28 MPa, a reduced Young modulus of 374 MPa and a surface area of 170 m 2 g À1 . The core of the monoliths maintained chemical and physical characteristics close to those of native silica aerogels with a hardness of 0.81 MPa, a modulus of 7.42 MPa and a surface area of 370 m 2 g À1 . The cross-linking crust was composed of forsterite (Mg 2 SiO 4 ), enstatite (MgSiO 3 ), cristobalite and magnesium oxide (MgO). SEM and TEM showed that the Mg silicates bridged between silica aggregates, thus realizing the first known example of ceramic crosslinking of aerogels. Control experiments showed that carbon from PAN was responsible for the crust formation. A thin, mechanically weak crust formed in all other samples and was composed of MgO and Mg 2 SiO 4 . EELS showed the presence of disordered, graphite-type carbon in Mg-PAN aerogels and traces of amorphous carbon in all other control samples. The role of PAN was confirmed by using masking techniques, whereas acrylonitrile was photopolymerized in selected regions of aerogel monoliths. The crust formed only in these exposed regions. The role of carbon in crust formation is discussed, as well as potential applications of our technique for the fabrication of anisotropic ceramics.