A team of researchers from the University of Texas at Dallas, in collaboration with Brazilian scientists, has made a groundbreaking discovery regarding carbon nanotube films. These materials exhibit unusual mechanical behaviors when stretched or uniformly compressed, opening up new possibilities for advanced applications such as composites, artificial muscles, gaskets, and sensors. The study was recently published in the April 25 issue of *Science*.
Most materials tend to become thinner when stretched, similar to how a rubber band stretches and thins out. However, this special type of carbon nanotube film, known as "buckypaper," behaves differently. When stretched, it actually expands in width, and under uniform compression, it grows both in length and width. This unique behavior challenges traditional expectations.
This phenomenon is related to Poisson's ratio, which measures how a material deforms when stretched or compressed. For most materials, stretching causes lateral contraction, resulting in a positive Poisson’s ratio. But in this case, the researchers found that by adjusting the composition of the buckypaper, they could achieve a negative Poisson’s ratio — meaning the material expands laterally when stretched.
The team used an ancient technique similar to making paper from fiber slurries to create their nanotube films. Their slurry contained a mix of single-walled and multi-walled carbon nanotubes. By increasing the proportion of multi-walled nanotubes, they were able to shift the Poisson’s ratio from approximately +0.06 to -0.20, a significant change in mechanical behavior.
To explain this, the researchers compared the structure of the nanotubes to collapsible wine racks. When the structure is flexible, stretching causes narrowing (positive ratio), but when the structure becomes rigid and the pillars can extend, stretching leads to widening (negative ratio).
In addition to their unique mechanical properties, these hybrid nanotube films showed improved strength-to-weight ratios and stiffness. Compared to pure single- or multi-walled nanotubes, the composite version was 1.6 times stronger, 1.4 times stiffer, and had 2.4 times better elastic modulus.
This research suggests that combining different types of carbon nanotubes could enhance performance in various nanomaterials, including membranes and other nanostructures. The ability to control Poisson’s ratio also opens up new design opportunities for nanotube-based sensors, actuators, and smart materials. This discovery could have wide-ranging implications in engineering, materials science, and even biomedical applications.
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