Regioselective cross-linking of silica aerogels with magnesium silicate ceramics

Abstract

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 m² g⁻¹. 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 m² g⁻¹. The cross-linking crust was composed of forsterite (Mg₂SiO₄), enstatite (MgSiO₃), 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 cross-linking 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 Mg₂SiO₄. 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.