如何切割纳米管

A pipefitter knows how to make an exact cut on a metal rod. But it's far harder to imagine getting a precise cut on a carbon nanotube, with a diameter 1/50,000th the thickness of a human hair. In a paper published this month in the British journal Proceedings¹ of the Royal Society A, researchers at Brown University and in Korea document for the first time how single-walled carbon nanotubes are cut, a finding that could lead to producing more precise, higher-quality nanotubes. Such manufacturing improvements likely would make the nanotubes more attractive for use in automotive, biomedicine, electronics, energy, optics（光学） and many other fields.

"We can now design the cutting rate and the diameters we want to cut," said Kyung-Suk Kim, professor of engineering in the School of Engineering at Brown and the corresponding author on the paper.

The basics of carbon nanotube manufacturing are known. Single-atom thin graphene sheets are immersed in（陷于，沉浸于） solution (usually water), causing them to look like a plate of tangled²（纠缠的，复杂的） spaghetti（意大利面条） . The jumbled³（乱七八糟的） bundle of nanotubes is then blasted by high-intensity sound waves that create cavities (or partial vacuums) in the solution. The bubbles that arise from these cavities expand and collapse⁴ so violently that the heat in each bubble's core can reach more than 5,000 degrees Kelvin, close to the temperature on the surface of the sun. Meanwhile, each bubble compresses at an acceleration⁵ 100 billion times greater than gravity. Considering the terrific energy involved, it's hardly surprising that the tubes come out at random⁶ lengths. Technicians use sieves⁷（筛子，滤网） to get tubes of the desired length. The technique is inexact partly because no one was sure what caused the tubes to fracture.

Materials scientists initially⁸ thought the super-hot temperatures caused the nanotubes to tear. A group of German researchers proposed that it was the sonic boomlets（小景气，短暂的繁荣） caused by collapsing⁹ bubbles that pulled the tubes apart, like a rope tugged¹⁰ so violently at each end that it eventually rips.

Kim, Brown postdoctoral researcher Huck Beng Chew, and engineers at the Korea Institute of Science and Technology decided¹¹ to investigate further. They crafted complex molecular¹² dynamics¹³ simulations using an array of supercomputers to tease out（梳理） what caused the carbon nanotubes to break. They found that rather than being pulled apart, as the German researchers had thought, the tubes were being compressed mightily¹⁴ from both ends. This caused a buckling¹⁵ in a roughly five-nanometer section along the tubes called the compression-concentration zone. In that zone, the tube is twisted into alternating 90-degree-angle folds, so that it fairly resembles a helix（螺旋，耳轮） .

That discovery still did not explain fully¹⁶ how the tubes are cut. Through more computerized simulations, the group learned the mighty¹⁷ force exerted by the bubbles' sonic booms caused atoms to be shot off the tube's lattice-like foundation like bullets from a machine gun.

"It's almost as if an orange is being squeezed, and the liquid is shooting out sideways," Kim said. "This kind of fracture by compressive atom ejection has never been observed before in any kind of materials."

The team confirmed the computerized simulations through laboratory tests involving sonication and electron microscopy of single-walled carbon nanotubes.

The group also learned that cutting single-walled carbon nanotubes using sound waves in water creates multiple kinks, or bent¹⁸ areas, along the tubes' length. The kinks are "highly attractive intramolecular（分子内的） junctions¹⁹（连接，接头） for building molecular-scale electronics," the researchers wrote.