Disrupting Density-Dependent Property Scaling in Hierarchically Architected Foams

Creatinglightweight architected foams as strong andstiff as theirbulk constituent material has been a long-standing effort. Typically,the strength, stiffness, and energy dissipation capabilities of materialsseverely degrade with increasing porosity. We report nearly constantstiffness-to-density and energy dissipation-to-density ratios alinear scaling with density in hierarchical vertically alignedcarbon nanotube (VACNT) foams with a mesoscale architecture of hexagonallyclose-packed thin concentric cylinders. We observe a transformationfrom an inefficient higher-order density-dependent scaling of theaverage modulus and energy dissipated to a desirable linear scalingas a function of the increasing internal gap between the concentriccylinders. From the scanning electron microscopy of the compressedsamples, we observe an alteration in the deformation modality fromlocal shell buckling at a smaller gap to column buckling at a largergap, governed by an enhancement in the number density of CNTs withthe increasing internal gap, leading to better structural stiffnessat low densities. This transformation simultaneously improves thefoams' damping capacity and energy absorption efficiency aswell and allows us to access the ultra-lightweight regime in the propertyspace. Such synergistic scaling of material properties is desirablefor protective applications in extreme environments.

Automated summary

Creating lightweight architected foams as strong and stiff as their bulk constituent material has always been a challenge. However, through the use of hierarchical vertically aligned carbon nanotube foams, we have successfully achieved constant stiffness-to-density and energy dissipation-to-density ratios, enabling a linear scaling with density. This transformation not only improves the foams' structural stiffness at low densities, but also enhances their damping capacity and energy absorption efficiency, allowing access to the ultra-lightweight regime in the property space. Such synergistic scaling of material properties is highly desirable for protective applications in extreme environments.