Disrupting Density-Dependent Property Scaling in Hierarchically Architected Foams

Creating lightweight architected foams as strong and stiff as their bulk constituent material has been a long-standing effort. Typically, the strength, stiffness, and energy dissipation capabilities of materials severely degrade with increasing porosity. We report nearly constant stiffness-to-density and energy dissipation-to-density ratios a linear scaling with density in hierarchical vertically aligned carbon nanotube (VACNT) foams with a mesoscale architecture of hexagonally close-packed thin concentric cylinders. We observe a transformation from an inefficient higher-order density-dependent scaling of the average modulus and energy dissipated to a desirable linear scaling as a function of the increasing internal gap between the concentric cylinders. From the scanning electron microscopy of the compressed samples, we observe an alteration in the deformation modality from local shell buckling at a smaller gap to column buckling at a larger gap, governed by an enhancement in the number density of CNTs with the increasing internal gap, leading to better structural stiffness at low densities. This transformation simultaneously improves the foams' damping capacity and energy absorption efficiency as well and allows us to access the ultra-lightweight regime in the property space. Such synergistic scaling of material properties is desirable for protective applications in extreme environments.