Structural hierarchy enhances conflicting mechanical properties

In many materials, there is often a tradeoff between strength and toughness, resulting in strong materials that are often very brittle, and vice versa. This is a problem when designing materials for protective applications, because both of these mechanical properties are essential. To address this, researchers at the University of Wisconsin–Madison have designed a carbon nanotube (CNT) foam that is both ultrastrong and tough, by architecting its structure across multiple length scales. They published their findings in a recent issue of Extreme Mechanics Letters (https://doi.org/10.1016/j.eml.2022.101899).

(a) Chemical vapor deposition-synthesized pristine vertically aligned carbon nanotube (CNT) sample on a 5 mm × 5 mm substrate. (b, c) Scanning electron microscope (SEM) images showing the mesoscale, hexagonally packed hollow cylinders made of vertically aligned CNTs; inner diameter (D_in) and thickness (t) of a cylinder and the gap (g) between adjacent cylinders are indicated. (d) An SEM image showing the entangled forest-like morphology of vertically aligned individual CNTs in microscale. (e) A transmission electron microscope image showing the multiwalled structure of an individual CNT at the nanoscale. Credit: Extreme Mechanics Letters.

The key enabling discovery involved the hierarchical structure of the foam, which contains micron-scale cylinders made from bundles of CNTs. The researchers found that by changing the size of the mesoscale cylinders, they could induce nanoscale changes in the self-assembly of the constituent CNTs. They then used those nanoscale changes to enhance the bulk mechanical properties of the foam.

“What we want to understand are fundamental process–structure–property relations so that we can exploit them to design new materials,” says Ramathasan Thevamaran, whose group performed the study. CNT foams are already known to have mechanical properties that outperform many commercially available materials. What the research team wanted to know, says Thevamaran, was whether “we do better using our structural mechanics understanding.”

To investigate this, the researchers grew 180 foam samples using vertically aligned bundles of CNTs arranged into a hexagonal pattern of hollow, mesoscale cylinders. In order to test how different structural parameters change the mechanical properties of the foam, the researchers varied the diameter and thickness of the cylinders as well as the gap between them. They then measured the stress–strain response of each foam sample by repeatedly compressing it by up to 50% strain in order to measure three mechanical properties—the specific energy absorption, specific peak stress, and the specific modulus.

The researchers discovered that the mechanical properties synergistically improved as they decreased the thickness of the cylinders. To determine what was causing this change, they used scanning electron microscopy to compare the morphology at different thicknesses. “When we looked into the microstructure, what we noticed is these carbon nanotubes tend to be aligned more and packed more when we do size-confined synthesis,” Thevamaran says. “We call it a size effect.” As a result, thinner cylinders contained a higher density of nanotubes, which the research team termed their intrinsic density, resulting in stiffer cylinders that can absorb more energy.

The researchers also found that the mechanical properties improved when decreasing the gap between cylinders due to their increased lateral interactions. Using this collective information, the team created design rules that model the combined effect of these lateral interactions and the intrinsic density on the bulk mechanical properties. They obtained the best results when they minimized both the thickness and the gap between cylinders, resulting in enhanced interactions at both the nano- and mesoscale. The mechanical properties of the resultant foam outperformed both nonarchitected CNT foams as well as other commercially available materials, like polyurethane foams.

“The vast parameter space presented in this work shows the exciting potential for unexpected properties owing to interactions at different scales,” says Carlos Portela, an assistant professor in mechanical engineering at the Massachusetts Institute of Technology who was not involved with the research. Portela says, “This study presents the possibility of leveraging truly nanoscale effects in materials whose overall dimensions approach a centimeter—this is exciting and provides hope for other emerging nanoarchitected materials.”

Cullen Walsh