Physicists show unlimited heat conduction in graphene

May 13, 2014
Scanning tunnelling microscopy (STM) image of graphene on Ir(111). The image size is 15 nm × 15 nm. Credit: ESRF

Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz and the National University of Singapore have attested that the thermal conductivity of graphene diverges with the size of the samples. This discovery challenges the fundamental laws of heat conduction for extended materials.

Davide Donadio, head of a Max Planck Research Group at the MPI-P, and his partner from Singapore were able to predict this phenomenon with computer simulations and to verify it in experiments. Their research and their results have now been presented in the scientific journal Nature Communications.

"We recognized mechanisms of heat transfer that actually contradict Fourier's law in the micrometer scale. Now all the previous experimental measurements of the thermal conductivity of graphene need to be reinterpreted. The very concept of thermal conductivity as an intrinsic property does not hold for graphene, at least for patches as large as several micrometers", says Davide Donadio.

Are material constants alterable after all?

The French physicist Joseph Fourier had postulated the laws of heat propagation in solids. Accordingly, thermal conductivity is an intrinsic material property that is normally independent of size or shape. In graphene, a two-dimensional layer of carbon atoms, it is not the case, as our scientists now found out. With experiments and computer simulations, they found that the thermal conductivity logarithmically increases as a function of the size of the graphene samples: i.e., the longer the graphene patches, the more heat can be transferred per length unit.

This is another unique property of this highly praised wonder material that is graphene: it is chemically very stable, flexible, a hundred times more tear-resistant than steel and at the same time very light. Graphene was already known to be an excellent heat conductor: The novelty here is that its thermal conductivity, which was so far regarded as a material constant, varies as the length of graphene increases. After analyzing the simulations, Davide Donadio found that this feature stems from the combination of reduced dimensionality and stiff chemical bonding, which make thermal vibration propagate with minimal dissipation at non-equilibrium conditions.

Optimum cooling for nanoelectronics

In the micro- and nano-electronics, is the limiting factor for smaller and more efficient components. Therefore, materials with virtually unlimited hold an enormous potential for this kind of applications. Materials with outstanding electronic properties that are self-cooling too, as graphene might be, are the dream of every electronics engineer.

Davide Donadio, an Italian-born researcher, already dealt with nanostructures of carbon, crystallization processes and thermoelectric materials during his studies in Milan, his research stays at the ETH Zurich (Switzerland) and at the University of California, Davis (USA). Since 2010, he has been investigating, among others, thermal transport in nanostructures using theoretical physics and simulating the atomic behavior of substances with his Max Planck Research Group at the MPI-P.

Explore further: Researchers combine graphene and copper in hopes of shrinking electronics

More information: "Length-dependent thermal conductivity in suspended single-layer graphene." Xiangfan Xu, et al. Nature Communications 5, Article number: 3689 DOI: 10.1038/ncomms4689. Received 09 October 2013 Accepted 19 March 2014 Published 16 April 2014

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User comments : 18

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shavera
3.7 / 5 (3) May 13, 2014
A simplified explanation of what mechanisms are proposed to explain this behaviour would be appreciated.
El_Nose
5 / 5 (2) May 13, 2014
@shavera

the info you seek is behind the pay wall of Nature Communications as the article explains.

----

Thermal batteries anyone??
chaps
5 / 5 (1) May 13, 2014
This is incredible. I now wonder if a mechanism/method could be made/discovered to develop materials essentially reverse/slow the thermal conductivity properties on a logarithmic scale and make them less prone to the time-dependent loss of heat? I can only imagine the implications of such discovery to any kind of engine out there.
Writela
4 / 5 (2) May 13, 2014
a simplified explanation of what mechanisms are proposed to explain this behaviour would be appreciated
The electrons at the surface of graphene are compressed mutually in similar way, like the vagoons inside of squeezed train. Therefore they don't mediate the heat via mutual collisions, but in waves. And these waves spread the better, the larger the graphene flake are.
donadio_davide
5 / 5 (1) May 13, 2014
Thermal conductivity will not really be unlimited.
Xiangfan Xu, Luiz Felipe Pereira, Baowen Li, and all the team should be acknowledged.
Pejico
May 13, 2014
This comment has been removed by a moderator.
PhotonX
not rated yet May 13, 2014
a simplified explanation of what mechanisms are proposed to explain this behaviour would be appreciated
The electrons at the surface of graphene are compressed mutually in similar way, like the vagoons inside of squeezed train. Therefore they don't mediate the heat via mutual collisions, but in waves. And these waves spread the better, the larger the graphene flake are.
Thanks. Now, can you explain to a culturally-challenged American what a vagoon is?
Kedas
5 / 5 (1) May 14, 2014
Does ANY material have the exact same characteristics when you take only one layer of atoms?
Writela
5 / 5 (2) May 14, 2014
can you explain to a culturally-challenged American what a vagoon is?
Probably the railway car or its carriage. I do apologize for impertinent challenge.
Does ANY material have the exact same characteristics when you take only one layer of atoms?
Every material is indeed unique, but most of materials which consist of single layer (like the molybdenum disulphide) do share a similar characteristics. The superconductors and topological insulators work similarly too. IMO the trick here is, when the electrons are constrained to low-dimensional structure, they tend to behave like the atemporal Dirac fermions. It's rather difficult to describe such a behavior without math, but you can try to imagine the behavior of floaters at the stormy water surface, which aren't floating freely, but they're hanging on string parallel with water surface like the beads on abacus. The floaters will move jerkily, because the low-dimensional projection of harmonic motion is not harmonic anymore.
Pejico
May 14, 2014
This comment has been removed by a moderator.
russell_russell
not rated yet May 15, 2014
"Thermal conductivity will not really be unlimited. - DD"
Yes.
Fourier is safe not only from extrapolation. Practicalities are obstacles too. Solids of finite scales.
Birger
4 / 5 (1) May 15, 2014
Yes the conductivity will not be unlimited. Still, consider the applications of a material that "only" conducts heat twice as well as current materials...
Max_Velocity
5 / 5 (1) May 16, 2014
An intriguing discovery to be sure, but I do wish they'd stop calling graphene a '2D' material... it's not. It's just that one of the 3 dimensions is very, very small.
Writela
5 / 5 (1) May 16, 2014
There is important aspect of new physics, that the truth is not selfevident there (a multiverse?). So when you're insisting on some truth, you should always tell the conditions of your stance, i.e. with respect to what such a truth remains valid. The graphene is indeed "very thin" from practical perspective and indeed 3D material from purely formal mathematical perspective. But with respect to electron orbitals its thickness is already not so great - and this is what actually matters here.
When the electrons will get constrained in their free motion with some structure (geometrically frustrated), then the structure should be considered low-dimensional and the electrons will tend to undulate in another dimensions instead. When we squeeze some particle into small volume, its internal energy content cannot disappear, so that the character of oscillations will change into quantum jitterbugging and the particle will change into Dirac fermion.
FastEddy
not rated yet May 20, 2014
This is incredible. I now wonder if a mechanism/method could be made/discovered to develop materials essentially reverse/slow the thermal conductivity properties on a logarithmic scale and make them less prone to the time-dependent loss of heat? I can only imagine the implications of such discovery to any kind of engine out there.


Yes, incredible! And the thermal conduction properties are just the beginning ...

Electrical resistance approaching zero = significantly better than Silver/Gold/Copper ...

Energy storage an order of magnitude greater in batteries, capacitors ... (http://phys.org/n...les.html )

Tensile strength greater than other Carbon structures, far greater than steel ...

And lots of experimental work is being done of the kitchen table.
FastEddy
not rated yet May 20, 2014
Does ANY material have the exact same characteristics when you take only one layer of atoms?


A profound question. Probably applies to metals, mostly, but ...

There may have been a whole lot of science missed during practical applications in the semiconductor industry ... Vacuum deposition of ultra-thin and single atom layers = there may be unusual properties hiding in there ...
FastEddy
not rated yet May 20, 2014
... Does ANY material have the exact same characteristics when you take only one layer of atoms?

writela: "... IMO the trick here is, when the electrons are constrained to low-dimensional structure, they tend to behave like the atemporal Dirac fermions. ... imagine the behavior of floaters at the stormy water surface, which aren't floating freely, but they're hanging on string parallel with water surface like the beads on abacus. The floaters will move jerkily, because the low-dimensional projection of harmonic motion is not harmonic anymore ..."

Like whale oil on water calms the stormy waves?

(I believe it was Ben Franklin who "discovered" the single atom thickness characteristic of oil on water ... but failed to recognize it as such.)
antialias_physorg
5 / 5 (1) May 20, 2014
but I do wish they'd stop calling graphene a '2D' material... it's not. It's just that one of the 3 dimensions is very, very small.

Speaking of dimensions with something at that level is iffy at best. We're not talking 'solid balls', here.

The "2D" in 2D-materials does refer to the crystalline structure (Miller-Bravais Index if you so will). Which is 2D, not 3D.

Does ANY material have the exact same characteristics when you take only one layer of atoms?

Probably not - though you can't make a 2D sheet from just any material. Some effects (edge/plane effects) average out in bulk materials. So we should expect some weird stuff happening when go to geometries where these effects play a significant role.
FastEddy
not rated yet May 20, 2014
Writela: "There is important aspect of new physics, that the truth is not selfevident there (a multiverse?). So when you're insisting on some truth, you should always tell the conditions of your stance, i.e. with respect to what such a truth remains valid. ..."

Its all relative, don't you know ...

"... The graphene is indeed "very thin" from practical perspective and indeed 3D material from purely formal mathematical perspective. But with respect to electron orbitals its thickness is already not so great - and this is what actually matters here. ..."

Insightful. Writela, you may have hit on the "cause" of this high thermal conductivity. Do most atomic material's electron orbits shrink when cold, expand when warmer ? ... Or both: atomic motion activity reduced and electron orbits shrink when cooler ? ... Or is it the single layer topography that brings this increase in thermal conductivity ? (Which would imply that there are other materials besides Carbon that would display this.)
antialias_physorg
5 / 5 (1) May 20, 2014
Do most atomic material's electron orbits shrink when cold, expand when warmer ?

No. The orbitals are quantized. You do get a broadening of spectral lines with higher temperatures as the atoms, as a whole, move (doppler shifts), but you do not get 'squeezing' of orbitals.
With higher temperatures you also have higer probabilities of electrons being in higher oribitals - so you get additional lines. (And you get fine structure and hyperfine structure splitting depending on quantized spin and nuclear dipole moment)

The quantized nature is part of the wavelike properties of the electrons (as stabel orbits need to be integer multiples of the wavelength)