Compressive Behavior of Crosslinked Mesoporous Silica Aerogels at High Strain Rates
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Abstract
Aerogels are low-density mesoporous materials with exceptionally low dielectric constants, low thermal conductivities (up to 40 times better thermal insulators than the best fiberglass) and high acoustic impedance. Their practical applications, however, have been slow due to their hydrophilicity and brittleness. The fragility problem was resolved by nanocasting a ~2-4 nm thick polymer layer on the skeletal silica nanoparticles that strengthens the weak inter-nanopartical necks. The thin polymer coats conformally the skeletal framework without clogging the mesopores and reinforces the structure by chemically crosslinking the nanoparticles. With a density increase by only three times, crosslinking aerogels have the flexural strength increased by 300 times. The method has been applied for crosslinking aerogels consisting of oxides of more than 30 different elements from the periodic table. The mechanical properties of crosslinked silica aerogels with different polymers were improved to a similar level since the polymer bond energy was similar among polymers.Therefore, to improve the mechanical properties further, we turn to the network morphology. For this study we turned to certain micelle-templated aerogels known to have a worm-like microstructure with a macro/mesoporous skeletal framework. For comparison, macroporous monolithic silica aerogels consisting of both random and ordered mesoporous walls have been synthesized via an acid-catalyzed sol-gel process from tetramethoxysilane (TMOS) using a tri-block copolymer (Pluronic P123) as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as a micelle-swelling reagent. Although those monoliths are more robust than base-catalyzed silica aerogels of similar density, the mechanical properties can be improved dramatically by letting di-isocyanate react with the silanols on the mesoporous surfaces. The compressive behavior of both crosslinked templated silica aerogels with/without ordered mesostructure and non-templated silica aerogels was characterized under high strain rates using a long split Hopkinson pressure bar (SHPB). Their mechanical properties at different strain rates are compared with those of engineering plastics polymethyl methacrylate (PMMA) and polycarbonate (PC). The effect of water absorption and of low temperatures on the compressive behavior was also investigated.
