快猫短视频

TINY tubes of carbon may conduct electricity without any resistance, at
temperatures stretching up past the boiling point of water. The tubes would be
the first superconductors to work at room temperature.

Guo-meng Zhao and Yong Sheng Wang of the University of Houston in Texas found
subtle signs of superconductivity. It wasn鈥檛 zero resistance, but it鈥檚 the
closest anyone鈥檚 got so far. 鈥淚 think all the experimental results are
consistent with superconductivity,鈥� Zhao says. 鈥淏ut I cannot rule out other
别虫辫濒补苍补迟颈辞苍蝉.鈥�

At the moment no superconductor will work above about 130 kelvin (鈥�143
掳C). But if a material could carry current with no resistance at room
temperature, no energy would be lost as heat, meaning faster, lower-power
electronics. And electricity could be carried long distances with 100 per cent
efficiency.

Zhao and Wang studied the effects of magnetic fields on hollow fibres of
carbon known as 鈥渕ultiwall carbon nanotubes鈥�. Each nanotube is typically a
millionth of a metre long, several billionths of a metre in diameter and with
walls a few atoms thick. The nanotubes cling together in oblong bundles about a
millimetre in length.

The researchers did not see zero resistance in their bundles. They think this
is because the connections between the tiny tubes never become superconducting.
But they did see more subtle signs of superconductivity within the tubes
themselves.

For example, when the researchers put a magnetic field across a bundle at
temperatures up to 400 kelvin (127 掳C), the bundle generated its own weak,
opposing magnetic field. Such a reaction can be a sign of superconductivity. And
when the team cooled the bundles from even higher temperatures then turned the
external field off, they stayed magnetised. A current running around within the
tubes could generate this lingering field if there wasn鈥檛 any resistance to make
it fade away.

While each effect could have a more prosaic explanation, they varied in
similar ways as the temperature of the bundles changed. The correlation suggests
superconductivity was responsible, Zhao and Wang argue in a paper to be
published in Philosophical Magazine B.

However, their argument doesn鈥檛 convince Paul Grant, a physicist with the
Electric Power Research Institute in Palo Alto, California. 鈥淕enerally,
superconductivity is such a dominating effect that when it occurs it just shouts
out at you,鈥� Grant says. 鈥淚t doesn鈥檛 appear in these indirect ways.鈥�

Superconductivity theories do not forbid the phenomenon at very high
temperatures, says Sasha Alexandrov, a theoretical physicist at Britain鈥檚
Loughborough University. A material becomes superconducting when its electrons
pair up. Normally such negatively charged particles would repel each other, but
in a positively charged crystal structure, vibrations called phonons help them
get together. In carbon nanotubes, the frequency of these vibrations is very
high, which, in theory at least, means superconductivity at higher temperatures.
鈥淭he results on the magnetic response are very intriguing, and favour the
explanation they present,鈥� Alexandrov says. 鈥淚t鈥檚 certainly possible,鈥� agrees
David Caplin, head of the Centre for High Temperature Superconductivity at
Imperial College, London.

To decide whether or not the nanotubes really are superconductors, you need
to measure the resistance through a single tube, Alexandrov says. 鈥淭o be
convinced, I鈥檇 like to see zero resistance.鈥�