Nanocardboard as a nanoscale analog of hollow sandwich plates

Abstract

Corrugated paper cardboard provides an everyday example of a lightweight, yet rigid, sandwich structure. Here we present nanocardboard, a monolithic plate mechanical metamaterial composed of nanometer-thickness (25–400 nm) face sheets that are connected by micrometer-height tubular webbing. We fabricate nanocardboard plates of up to 1 centimeter-square size, which exhibit an enhanced bending stiffness at ultralow mass of ~1 g m−2. The nanoscale thickness allows the plates to completely recover their shape after sharp bending even when the radius of curvature is comparable to the plate height. Optimally chosen geometry enhances the bending stiffness and spring constant by more than four orders of magnitude in comparison to solid plates with the same mass, far exceeding the enhancement factors previously demonstrated at both the macroscale and nanoscale. Nanocardboard may find applications as a structural component for wings of microflyers or interstellar lightsails, scanning probe cantilevers, and other microscopic and macroscopic systems.

Photophoretic Levitation of Macroscopic Nanocardboard Plates

“…Figure 1 shows a version of our previously reported nanocardboard31—a microarchitected metamaterial—that is designed to maximize photophoretic lift force while minimizing the weight. The design features arrays of five channels, with in‐plane dimensions of 25 × 600 µm and 75 µm gaps (Figure 1D), and results in air flowing through the plate at an effective “flow‐though” velocity on the order of 1–10 mm s −1 .…”

“…To predict the lift force as a function of the levitation height, we developed a theoretical model based on the well‐known lubrication theory for thin‐film flows under a disk (for example, in air bearings). Figure 2D shows the lift force, the weight, and the theoretical equilibrium height, at which the two are equal, for both the single‐channel design that we used previously for mechanical tests31 and the optimized five‐channel nanocardboard plates. Plates of both designs were successfully levitated, although the observed gaps varied from sample to sample and over time, with the full ranges shown by colored bands in Figure 2D.…”

Hollow Atomic Force Microscopy Cantilevers with Nanoscale Wall Thicknesses

“…Similar recovery and mechanical robustness of hollow alumina structures and architected materials have been observed previously. [29,30,[47][48][49] Our hollow cantilevers also exhibited improved wear resistance. Such resistance can be attributed to the wear-resistant properties of ALD alumina [50,51] and the resilience of hollow ALD-alumina-made structures.…”

“…We fabricate our hollow cantilever probes by following a procedure similar to those previously reported for hollow plates with ultrathin walls (Figure 1). [29][30][31] The process involves steps of 1) conformally coating a commercially available solid silicon probe with a shell of alumina (Al 2 O 3 ) by ALD, 2) defining an etch hole using pulsed-laser machining, and 3) removing the internal silicon through the etch hole using XeF 2 isotropic etching.…”

Controlled levitation of nanostructured thin films for sun-powered near-space flight

“…At the same time, a substantial temperature difference must typically be generated across a small thickness of such an ultralightweight structure. Recently, we reported hollow nanocardboard plates that have submilligram masses, are thermally insulating enough to generate a few degrees of temperature difference between their top and bottom, and maximize the total photophoretic flow using thousands of microchannels, which was sufficient to levitate such highly engineered structures ( 10 , 27 ).…”