The mesoporous surfaces of TMOS-derived silica aerogels have been modified with amines by copolymerization of TMOS with APTES. The amine sites have become anchors for cross-linking the nanoparticles of the skeletal backbone of the aerogel by attachment of di-, tri-, and tetra-functional epoxies. The resulting conformal coatings increase the density of the native aerogels by a factor of 2−3 but the strength of the resulting materials may increase by more than 2 orders of magnitude. Processing variables such as the amount of APTES used to make the gels, the epoxy type and concentration used for cross-linking, and the cross-linking temperature and time were varied according to a multivariable design-of-experiments (DOE) model. It was found that while elastic modulus follows a similar trend with density, maximum strength is attained neither at the maximum density nor at the highest concentration of −NH2 groups, suggesting surface saturation effects. Aerogels cross-linked with the trifunctional epoxide always show improved strength compared with aerogels cross-linked with the other two epoxides under identical conditions. Solid 13C NMR studies show residual unreacted epoxides, which condense with one another by heating cross-linked aerogels at 150 °C.

We describe a new mechanically strong lightweight porous composite material obtained by encapsulating the skeletal framework of amine-modified silica aerogels with polyurea. The conformal polymer coating preserves the mesoporous structure of the underlying silica framework and the thermal conductivity remains low at 0.041 ( 0.001 W m -1 K -1 . The potential of the new cross-linked silica aerogels for load-carrying applications was determined through characterization of their mechanical behavior under compression, three-point bending, and dynamic mechanical analysis (DMA). A primary glass transition temperature of 130 °C was identified through DMA. At room temperature, results indicate a hyperfoam behavior where in compression cross-linked aerogels are linearly elastic under small strains (<4%) and then exhibit yield behavior (until 40% strain), followed by densification and inelastic hardening. At room temperature the compressive Young's modulus and the Poisson's ratio were determined to be 129 ( 8 MPa and 0.18, respectively, while the strain at ultimate failure is 77% and the average specific compressive stress at ultimate failure is 3.89 × 10 5 N m kg -1 . The specific flexural strength is 2.16 × 10 4 N m kg -1 . Effects on the compressive behavior of strain rate and low temperature were also evaluated.