Low-density dry objects consisting of 3D assemblies of nanoparticles are referred to as aerogels and are pursued for their low thermal conductivity, low dielectric constants, high acoustic impedance, and for their mesoporosity that can hold functional guests for numerous electrical, electronic, optical, magnetic and catalytic applications. That versatility, however, comes at a high cost: fragility. We took a shot at that issue with X-aerogels in which skeletal inorganic nanoparticles (e.g., silica and over 30 other oxides) are coated conformally and bridged covalently with a nano-thin layer of polymer. The process left the porosity almost intact, but the new 3D core-shell superstructures displayed an unprecedented mechanical strength that allowed applications unthinkable for aerogels before, e.g., in ballistic protection. Since the exceptional mechanical properties of X-aerogels came with the polymer coating, the question became whether the inorganic framework was even needed at all: purely polymeric aerogels with the nanostructure of the inorganic backbone and the interparticle bonding of X-aerogels should be as strong materials. The critical factor for making all-polymer aerogels was identified with an early phase separation of colloidal nanoparticles capable for interparticle covalent bonding. That hypothesis was confirmed by synthesis of polyurea, polyurethane, polyimide, polyamide, polyacrylic, phenolic including polybenzoxazine, and ROMP-based aerogels. All were extremely strong materials. Most of those all-polymer aerogels turned out to be carbonizable. Thus, synthesis of interpenetrating hybrid organic-inorganic networks followed by carbothermal reduction led to a wide array of carbide, and purely metallic aerogels (Fe, Co, Ni, Sn, Cu). Coming back full circle, currently we explore sol-gel inorganic powders whose elementary building blocks are coated conformally with carbonizable polymers. Those powders allow preparation of large monolithic artifacts in any shape at will. The method has been demonstrated with nanoporous SiC and Si3N4 ceramics as high-temperature catalyst supports, and metallic Fe(0) aerogels as thermites and explosives.

Nanomaterials in 3D: A 20-year adventure from mechanically strong polymer-crosslinked aerogels to porous metals and ceramics

255th National Meeting (2018)

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