“Mechanically Strong Lightweight Polymer Nanoencapsulated Aerogels”
Hongbing Lu, University of North Texas
Monolithic, low-density (down to 1.0 mg/cm3, less than dry air density) 3-D assemblies of nanoparticles (e.g, silica), known as aerogels, are characterized by large specific surface areas and high porosity; they demonstrate low thermal conductivity, low dielectric constants and high acoustic impedance. Traditional aerogels, however, are extremely hygroscopic and fragile materials, 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 and 2004).
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 by Leventis (see, for example, Leventis, Acc. Chem. Res. 2007, 40, 874-884) 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 may combine a high specific compressive strength with the thermal conductivity of Styrofoam.
In collaboration with Leventis we have carried out experiments to characterize the thermo-mechanical behavior of this new class of aerogels using experimental facilities such as a long split Hopkinson pressure bar and ultra-high-speed photography. Digital image correlation was used to measure surface deformations. X-ray nano-computed tomography was used to determine the structures for simulations. Material point method (MPM) was used to simulate the deformations to determine the structure-property relationship. Results indicate that polymer nanoencapsulated aerogels have superior mechanical properties, with the specific energy absorption reaching 192 J/g (J. Mater. Chem. 2008, 18, 2475-2482). The simulation results demonstrate the capability of the MPM in simulations of porous nanostructured materials under compression, experiencing elastic, compaction and densification stages. The work indicates a paradigm in the design of porous nanostructured materials, comprising three degrees of freedom, namely the chemical identity of the nanoparticles, the crosslinking polymer and the nanostruture morphology. Currently technology is evaluated for applications as lightweight multifunctional materials with high-specific strength combined with acoustic attenuation, artificial heart valve leaflets, energetic materials (J. Am. Chem. Soc. 2009, 131, 4576-4577) and energy-absorption materials for ballistic impact.
Hongbing Lu received his doctorate in aeronautics from Caltech in 1997, his master’s in engineering mechanics from Tsinghua University in 1988 and his bachelor’s in solid mechanics from Huazhong University of Science and Technology in 1986. He was on the faculty of the School of Mechanical and Aerospace Engineering at Oklahoma State University for 13 years before joining UNT this fall. His research areas include nano-indentation, visco-elasticity, experimental mechanics and the mechanics of nanostructured materials. His work has been funded by NSF, AFOSR, NASA, NIH and industry. He is on the editorial board of Mechanics of Time-Dependent Materials, and he chaired the Time-Dependent Materials Division in the Society for Experimental Mechanics in from 2000 to 2003. He received an NSF Career Award in 2000 and was elected an associate fellow of the American Institute of Aeronautics and Astronautics in 2008.