Seminars

Monolithic, low-density (down to 1.0 mg/cm3, less than dry air density) three dimensional assemblies of nanoparticles (e.g, silica, carbon nanotube), known as aerogels, are materials characterized by large specific surface areas and high porosity; they demonstrate low thermal conductivity, low dielectric constants and high acoustic attenuation. Traditional aerogels, however, are extremely hygroscopic and fragile, limiting their applications to a few specialized environments, such as the materials for capture of hypervelocity particles in space (NASA’s Stardust Program) and as integrated structural and thermal insulation materials for electronic boxes aboard planetary vehicles (the Mars Rovers in 1997, 2004, 2012).

The aerogel fragility problem is traced to the weak points in aerogels’ framework, the necks connecting neighboring spherical secondary nanoparticles. This problem has been resolved successfully using polymer nanoencapsulation of the skeletal network of inorganic nanoparticles to bridge the nanoparticles and stiffen all the necks. The resulting polymer cross-linked aerogels, or X-aerogels, may combine a high specific compressive strength with the thermal conductivity of Styrofoam.

Reasoning that the exceptional properties are derived from the hierarchical  nanostructures, in addition to X-aerogels, work was conducted to synthesize organic aerogels, including polyurea and polyurethane aerogels. With tuning of the nanostructures, these polymeric aerogels could be made to be hydrophobic. Some of the polymeric gels are strong enough to prevent collapse under capillary force in ambient drying process to allow fabrication of large pieces, thus removing limitation of size of the aerogels that can be made. The physical, chemical and mechanical properties of these ductile aerogels were characterized using techniques SEM, TEM, and SANS. Digital image correlation was used to measure large nonlinear surface deformations. X-ray computed tomography was used to determine the structures for simulations to determine the deformation mechanism. Results indicate that both X-aerogels and purely organic aerogels have superior mechanical properties, with the specific energy absorption reaching 192 J/g in compression. A highly sensitive acoustic impedance tube determined drastic airborne sound transmission loss in these materials. The work indicates a paradigm in the design of porous nanostructured materials, comprising three degrees of freedom, including the chemical identity of the nanoparticles, the crosslinking polymer, and the nanostruture morphology. Currently technology is evaluated for applications as lightweight materials for artificial heart valve leaflets, and energy-absorption materials for force protection under blunt impact, among others.