Carbon nanotubes: The weird world of 'remote Joule heating'

Apr 10, 2012
Representation of Nanotubes. Credit - UMD

(Phys.org) -- A team of University of Maryland scientists have discovered that when electric current is run through carbon nanotubes, objects nearby heat up while the nanotubes themselves stay cool, like a toaster that burns bread without getting hot. Understanding this completely unexpected new phenomenon could lead to new ways of building computer processors that can run at higher speeds without overheating.

"This is a new phenomenon we're observing, exclusively at the , and it is completely contrary to our and knowledge of Joule heating at larger scales-for example, in things like your toaster," says first author Kamal Baloch, who conducted the research while a graduate student at the University of Maryland. "The nanotube's are bouncing off of something, but not its atoms. Somehow, the atoms of the neighboring materials-the silicon nitride substrate-are vibrating and getting hot instead."

"The effect is a little bit weird," admits John Cumings, an assistant professor in the Department of who oversaw the research project. He and Baloch have dubbed the phenomenon "remote Joule heating."

An Unreal Discovery

For the UMD researchers, the experience of the discovery was like what you or I might have felt, if, on a seemingly ordinary morning, we began to make breakfast, only to find certain things happening that seem to violate normal reality. The toast is burned, but the toaster is cold. The switch on the stove is set to "HI" and the teapot is whistling, but the burner isn't hot.

Of course, Baloch, Cumings and their colleagus weren't making breakfast in a kitchen, but running experiments in an electron microscopy facility at the A. James Clark School of Engineering at the University of Maryland.  They ran their experiments over and over, and the result was always the same: when they passed an electrical current through a carbon nanotube, the substrate below it grew hot enough to melt metal nanoparticles on its surface, but the nanotube itself seemed to stay cool, and so did the metal contacts attached to it.

For us non-scientists, their experience might not seem so strange at first glance-after all, food cooked in a microwave oven gets hot while the oven itself stays close to room temperature. The problem is that Baloch and Cumings weren't intentionally generating a microwave field. They were only passing a direct electrical current through the nanotube, which should have caused it to heat up. The data were telling them a story that didn't seem to make any sense-one about a plugged-in toaster that could burn bread without getting hot.

A phenomenon known as "Joule heating" dictates that an electrical current will cause travelling electrons to bounce off the atoms of a metal wire, making them vibrate in place. These vibrations create heat, and any conducting wire should show the effect, including the heating elements of toasters, hair dryers, and electric stovetops. Carbon nanotubes are known to conduct electricity like nanoscale metallic wires, so Baloch and Cumings expected to see the same effect when they passed current through a .

They used a technique developed in Cumings's lab called electron thermal microscopy, which maps where heat is generated in nanoscale electrical devices, to observe the effect of the current on a nanotube. They expected to see heat traveling along the length of the nanotube to metal contacts attached to it. Instead, the heat seemed to jump directly to the silicon nitride substrate beneath, heating it up while leaving the nanotube relatively cold.

But how is it even possible for the nanotube's electrons to vibrate the substrate's atoms if they're separated by distance, even one measuring in nanometers? Baloch and Cumings speculate a "third party" is involved: electrical fields.

"We believe that the nanotube's electrons are creating electrical fields due to the current, and the substrate's atoms are directly responding to those fields," Cumings explains. "The transfer of energy is taking place through these intermediaries, and not because the nanotube's electrons are bouncing off of the substrate's atoms. While there is some analogy to a microwave oven, the physics behind the two phenomena is actually very different."

Baloch adds that the remote Joule heating effect could have far reaching implications for computing technology. "What currently limits the performance of a computer's processor is the speed at which it can run, and what limits the speed is the fact that it gets too hot," he explains. "If you could find some way of getting rid of the waste heat more effectively, then it could run faster. A transistor that doesn't dissipate energy within itself as heat, like the in our experiment, could be a game-changer. This new mechanism of thermal transport would allow you to engineer your thermal conductor and electrical conductor separately, choosing the best properties for each without requiring the two to be the same material occupying the same region of space."

For the moment, an air of mystery still surrounds the phenomenon, which has been observed only at the nanoscale, and only in carbon materials. The next steps are to determine if other materials can produce the effect, and if so, what properties they must have. "We now know that can absorb energy from a current-carrying nanotube in this way, but we would like to test other materials, such as semiconductors and other insulators," Cumings explains.  "If we can really understand how this phenomenon works, we could start engineering a new generation of nanoelectronics with integrated thermal management."

This discovery was published in the April 8th advance online issue of Nature Nanotechnology. The research was supported by a grant from the U.S. Department of Energy Office of Basic Energy Sciences.

Explore further: NIST offers electronics industry two ways to snoop on self-organizing molecules

More information: Kamal H. Baloch, Norvik Voskanian, Merijntje Bronsgeest, and John Cumings. Remote Joule heating by a carbon nanotube. Nature Nanotechnology. Published online 8 April 2012. dx.doi.org/10.1038/NNANO.2012.39

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User comments : 14

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axemaster
4.5 / 5 (2) Apr 10, 2012
I assume the lattice vibrational energies of a carbon nanotube are quantized, and there are probably only a few of them (as compared to more complex structures). Is it possible that the mean interaction length of the electrons is too short to reach the minimum vibrational energies? If that were the case, one might expect them to dump energy into the substrate.

Either way this is a very interesting development and I will be following it with great interest.

EDIT: I just read the source. Apparently the currents in the nanotube are coupling to vibrational modes in the substrate. I'm not sure how that would work, but still very cool. It brings up the question though - we should be able to do this with other materials as well.
Cave_Man
not rated yet Apr 10, 2012
I think what they are observing is simply an electro-dynamic field generation due to the electrical properties of the nanotube. Similar to how we make an electromagnet except in this case it is an electrical field and the nanotube is not becoming magnetized only acting as a conduit for electricity to form a field of energized particles around the nanotube.

Pretty much the reverse of how carbon nano tubes make great antenna.
Au-Pu
2.3 / 5 (3) Apr 10, 2012
Paragraph 9 It should read bouncing off the substrate's atoms.
Not bouncing off of the substrate's atoms.
Why cannot intelligent people get such a simple thing right.
"Off of" developed in the old frontier days when education was poor and ignorance was bliss.
Cave_Man
not rated yet Apr 10, 2012
Lol I didnt need to read the article, I already had it. Then I went back and read this:

But how is it even possible for the nanotube's electrons to vibrate the substrate's atoms if they're separated by distance, even one measuring in nanometers? Baloch and Cumings speculate a "third party" is involved: electrical fields.

Re: Au-Pu (gold plutonium?? or are you and Indian convenience store clerk?)
A new form of English has been established thanks to the modern computer age of twits and i fags. Its called simple English which is right on the nose if you ask me. Go to wikipedia and view an article in standard English and simple English, it should make you retch.
Tennex
1 / 5 (3) Apr 10, 2012
Carbon nanotubes are known to conduct electricity like nanoscale metallic wires
well, this is not quite true. As the above picture illustrates, the nanotube come in three geometric conformations, only one of which is conductor, the remaining ones are semiconductive. The electrons are moving in ballistic transport in the later forms, i.e. they're doing tiny jumps, which could induce the heating of the substrate at distance. This theory could be verified with observation of remote heating with nanotubes of different geometry.
bg1
not rated yet Apr 10, 2012
Perhaps the charge carriers in the current are driven by repulsion to the outside of the tube and the magnetic field associated with this current interacts with surrounding charges that are in motion thereby creating inductive currents outside the tube, thereby creating heating outside the tube. How does this effect vary with temperature?
Graeme
not rated yet Apr 11, 2012
Sounds to me like the electric current is flowing in the substrate instead and not in the nanotube.
retrosurf
not rated yet Apr 11, 2012
Selective radiation.
Narrowband, efficient infrared emission.
Husky
Apr 11, 2012
This comment has been removed by a moderator.
coldwelder
not rated yet Apr 11, 2012
Temperature is usually defined in terms of a Boltzmann Distribution of particulate velocities which do not exist in a single tube as there are only about 4 phonon modes active. Radiation however does and radiation can cause heating in nearby electrical conductors or semiconductors.
Raphs
not rated yet Apr 11, 2012
An amazing phenomenon, i as a mechanical engineer has not much to do with modern physics, but does having a current in the substrate while no heat loss in carbon nano-tubes coupling this with the assumption that the magnetic field induced within the tube induces current within the substrate imply that nano-tubes are acting as superconductors ?
yoatmon
not rated yet Apr 11, 2012
This may be a novel innovation for a heating system in EVs.
axemaster
not rated yet Apr 11, 2012
An amazing phenomenon, i as a mechanical engineer has not much to do with modern physics, but does having a current in the substrate while no heat loss in carbon nano-tubes coupling this with the assumption that the magnetic field induced within the tube induces current within the substrate imply that nano-tubes are acting as superconductors ?

Unfortunately no. The nanotubes are still undergoing some heating, they are offloading something like 85% of the heating to the substrate, but not 100%.

Hey, here's a thought. What if the substrate was a thin film superconductor? Would the nanotubes still excite resonance modes in it? If so, the superconductor probably wouldn't heat at all, allowing the efficiency to skyrocket, i.e. removing 85% of the effective resistance.
henkel024
not rated yet Apr 14, 2012
jdaviqwert
not rated yet Apr 17, 2012
Selective radiation.
Narrowband, efficient infrared emission.

The conduction electrons are obviously causing some EM emission, there seem to be no other options. However the wavelength is undetermined.