Origins of mechanical preconditioning in hierarchical nanofibrous materials

Disrupting Density-Dependent Property Scaling in Hierarchically Architected Foams

“…In this work, we report a transformation from a poor higher-order scaling of material properties with density (α, β ≈ 3) to a desirable linear scaling (α, β ≈ 1) as a function of architecture in hierarchical vertically aligned carbon nanotube (VACNT) foams with a mesoscale architecture of hexagonally close-packed thin concentric cylinders (Figure ). The macroscale VACNT foams (sometimes referred to as VACNT arrays or VACNT forests) exhibit a structural hierarchy that spans multiple length scales: from the graphitic structure at the atomic scale, tubular multiwalled CNTs (MWCNTs) at the nanoscale, and an entangled forest-like morphology of nearly vertically aligned MWCNTs at the microscale. , Morphology, resultant bulk density, and elastic stiffness of the VACNT foams can be tailored by varying different synthesis parameters during the floating catalyst chemical vapor deposition (FCCVD) process. − FCCVD-synthesized VACNT foams typically have a bulk density close to that of polymeric foams (∼200 kg/m 3 ), which can be further reduced by increasing the hydrogen concentration in the carrier gas during the synthesis . However, due to a strong dependence of the mechanical properties on density (α, β ≈ 3), both strength and stiffness severely degrade as the foam gets lighter. , Introducing mesoscale architecture is an another way to tailor the density of VACNT foams. ,, Various mesoscale architectures can be introduced by growing VACNT foams on a photolithographically prepatterned chromium-coated substrate (see Methods) to reduce the density while improving the structural stiffness.…”