Nanotubes spring eternal

Forests of carbon nanotubes have acted as brushes, electron emitters, light-absorbing antennae, and even as glue mimicking the sticky fibers under geckos’ feet. Now, researchers say such arrays of tiny carbon tubes are something else again—an amazing new kind of foam.

RESILIENT RIPPLES. An electron micrograph shows how aligned carbon nanotubes buckle at their bases after 1,000 compression cycles. Above this region (not shown) the tubes remain straight. A. Cao et al./Science

After growing thumbnail-size forests of nanotubes up to 2 millimeters tall, Pulickel M. Ajayan of Rensselaer Polytechnic Institute in Troy, N.Y., and his colleagues used a standard testing instrument to repeatedly apply and release vertical pressure.

For the first few hundred compressions, nanotube arrays that were flattened to 15 percent of their original heights rebounded fully. Subsequently, the recovery was always more than 80 percent of full height, even after 10,000 cycles.

Because the springy arrays are 87 percent air, the researchers liken them to foam. Each 40-nanometer-wide, multiwalled tube is so strong that flattening the foam requires hundreds of times more pressure than needed to squeeze ordinary foams, the team reports in the Nov. 25 Science.

The arrays may ultimately absorb impacts and vibrations in micromachinery or, perhaps, even in human joints, Ajayan says. If the nanotube forests can be grown a lot larger, they might someday compete with conventional foams, such as packing and acoustic foams, he adds.

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