Gripping Tale: Metal oozes in nanotubes’ grasp

May 24, 2006 at 11:58 am

Chalk up another feat of astounding strength for the hollow threads called carbon nanotubes. When they squeeze in on enclosed crystals of hard metals, those substances collapse into thin shafts, an international team of scientists reports.

SQUEEZE PLAY. Time-lapse sequence shows a multiwalled carbon nanotube (seen in cross section as two bands of thin lines) compressing into a thin shaft the end of a rod-shaped nanocrystal of iron carbide (black bar labeled Fe3C). L. Sun et al./Science

The nanotubes exert pressures up to 400,000 atmospheres, or about a tenth of that at the Earth’s core, say Florian Banhart of the University of Mainz, Germany, and his colleagues. Their findings appear in the May 26 Science.

The nanotubes’ newfound capability “opens up a window to directly watch atomic-scale development of pressure-induced phenomena,” say Zhongwu Wang and Yusheng Zhao of Los Alamos (N.M.) National Laboratory in a commentary accompanying the report. The fields of materials science, chemistry, condensed-matter physics, geophysics, planetary science, and nanotechnology all stand to benefit, they add.

However, the nanotube-induced pressures don’t rival those produced in the diamond anvils used in many high-pressure studies (SN: 5/14/05, p. 309: Available to subscribers at Metal Rebel: Under extreme pressure, sodium breaks the rules for turning into liquid), Wang and Zhao note.

To carry out the experiments, Banhart and his coworkers in Mainz and in Finland, Mexico, and the United States first vaporized carbon containing various metals under conditions that would create multiwalled carbon nanotubes. The tubes simultaneously acquired crystalline cores of iron, cobalt, or iron carbide, a compound that hardens steel.

To make the tiny tubes even narrower, the team placed them in the vacuum chamber of an electron microscope at a temperature of 600°C and blasted them with the microscope’s high-energy electrons.

The beam knocked some carbon atoms out of the tubes. However, at the high temperature, the remaining carbon atoms were mobile enough to cinch together and establish new bonds. As that repair process narrowed the tubes and tightened their grips, the metal crystals inside collapsed into narrower, more-elongated shapes.

Banhart notes that at room temperature, the beam destroys nanotubes, but he predicts that even at 200°C, bombarded nanotubes would squeeze forcefully.

Because the cinching process occurs inside an electron microscope, “we can now study the deformation of nanocrystals,” Banhart says. “This has not been accessible to observation before.” He and other scientists find nanocrystals of increasing interest because of their extraordinary properties, including their hardness.

In the new experiments, the researchers were surprised to find that the nanocrystals, unlike larger-scale metal crystals, deform without developing any apparent structural flaws. So far, the team can’t fully explain that observation, Banhart says.

In 1996, Banhart used electron bombardment of carbon nanostructures to make diamonds. He and Pulickel M. Ajayan, now of Rensselaer Polytechnic Institute in Troy, N.Y., and a coauthor of the new work, forced carbon onions—concentric nanospheres of carbon—to compress their cores (SN: 8/31/96, p. 139).

Both sets of experiments have demonstrated a process that could be called “nano-shrink-wrapping,” comments David Tomanek of Michigan State University in East Lansing.

He notes that high pressures lower the melting points of metals and make elements mix that ordinarily wouldn’t. New materials built from such nano-shrink-wrapped components might have remarkable properties, says Tomanek.