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The remarkable electrical and mechanical properties of metallic
carbon nanotubes make them promising candidates for interconnects in
future nanoscale electronic devices. But, like tiny metal wires,
nanotubes grow hotter as electrical current is increased. At some
point, a nanotube will burn apart like an element in a blown fuse.
"Heat dissipation is a fundamental problem of electronic transport
at the nanoscale," said Jean-Pierre Leburton, the Gregory Stillman
Professor of Electrical and
Computer Engineering at Illinois and co-author of a paper
published in the Dec. 21 issue of the journal Physical Review
Letters. "To fully utilize nanotubes as interconnects, we must
characterize them and understand their behavior and operating
limits."
Up to now, no coherent interpretation had been proposed that
reconciled heat dissipation and electronic transport and described
thermal effects in metallic carbon nanotubes under electronic
stress, said Leburton, who is also a researcher at the
Beckman Institute for
Advanced Science and Technology, at the
Micro and Nanotechnology
Laboratory, and at the
Frederick Seitz Materials Research Laboratory. "Our theoretical
results not only reproduce experimental data for electronic
transport, they also explain the odd behavior of thermal breakdown
in these nanotubes."
For example, in both theory and experiment, the shorter the
nanotube, the larger the current that can be carried before thermal
breakdown occurs. Also, the longer the nanotube, the faster the rise
in temperature as the threshold current for thermal heating is
reduced.
In nanotubes, heat generated by electrical resistance creates
atomic vibrations in the nanostructure, which cause more collisions
with the charge carriers. The additional collisions generate more
heat and more vibrations, followed by even more collisions, in a
vicious cycle that ends when the nanotube burns apart, breaking the
circuit.
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 "Short nanotubes can carry more current before burning apart
because they dissipate heat better than longer nanotubes," Leburton
said. "Although the entire nanotube experiences resistance heating,
the electrical contacts at each end act as heat sinks, which in
short nanotubes are relatively close to one another, leading to
efficient heat removal."
This phenomenon also explains why the highest temperature always
occurs in the middle of the nanostructure, Leburton said, "which is
the furthest point away from the two ends, and where burning occurs
in longer nanotubes under electrical stress."
In another important finding, Leburton and his colleagues have
revised the common belief that charge carriers go ballistic in short
metallic nanotubes having high currents. Researchers had previously
thought that charge carriers traveled from one terminal to the other
like a rocket: that is, without experiencing collisions.
"We have shown that the high current level in short metallic
nanotubes is not due to ballistic transport but to reduced heating
effects," Leburton said. "Owing to their large concentration, the
charge carriers collide efficiently among themselves, which prevent
them from going ballistic. Even in short nanostructures, the current
level is determined by a balance between the attractive force of the
external electric field and the frictional force caused by the
nanotube thermal vibrations. The collisions among charge carriers
help the energy transfer to the nanotubes, which results in heat
dissipation."
Co-authors of the paper are Leburton, electrical and computer
engineering professor Andreas Cangellaris, and graduate student
Marcelo Kuroda.
The work was funded by the National Science Foundation and the
Beckman Institute.
[News release from
University of Illinois at Urbana-Champaign] |
