Heat “waves” detected moving through pencil lead

At relatively balmy temperatures, heat behaves like sound when moving through graphite, study reports

From MIT:

Exotic ‘second sound’ phenomenon observed in pencil lead

The next time you set a kettle to boil, consider this scenario: After turning the burner off, instead of staying hot and slowly warming the surrounding kitchen and stove, the kettle quickly cools to room temperature and its heat hurtles away in the form of a boiling-hot wave.

We know heat doesn’t behave this way in our day-to-day surroundings. But now MIT researchers have observed this seemingly implausible mode of heat transport, known as “second sound,” in a rather commonplace material: graphite — the stuff of pencil lead.

At temperatures of 120 kelvin, or -240 degrees Fahrenheit, they saw clear signs that heat can travel through graphite in a wavelike motion. Points that were originally warm are left instantly cold, as the heat moves across the material at close to the speed of sound. The behavior resembles the wavelike way in which sound travels through air, so scientists have dubbed this exotic mode of heat transport “second sound.”

Researchers find evidence that heat moves through graphite similar to the way sound moves through air. Image: Christine Daniloff

The new results represent the highest temperature at which scientists have observed second sound. What’s more, graphite is a commercially available material, in contrast to more pure, hard-to-control materials that have exhibited second sound at 20 K, (-420 F) — temperatures that would be far too cold to run any practical applications.

The discovery, published in Science, suggests that graphite, and perhaps its high-performance relative, graphene, may efficiently remove heat in microelectronic devices in a way that was previously unrecognized.

“There’s a huge push to make things smaller and denser for devices like our computers and electronics, and thermal management becomes more difficult at these scales,” says Keith Nelson, the Haslam and Dewey Professor of Chemistry at MIT. “There’s good reason to believe that second sound might be more pronounced in graphene, even at room temperature. If it turns out graphene can efficiently remove heat as waves, that would certainly be wonderful.”

The result came out of a long-running interdisciplinary collaboration between Nelson’s research group and that of Gang Chen, the Carl Richard Soderberg Professor of Mechanical Engineering and Power Engineering. MIT co-authors on the paper are lead authors Sam Huberman and Ryan Duncan, Ke Chen, Bai Song, Vazrik Chiloyan, Zhiwei Ding, and Alexei Maznev.

“In the express lane”

Normally, heat travels through crystals in a diffusive manner, carried by “phonons,” or packets of acoustic vibrational energy. The microscopic structure of any crystalline solid is a lattice of atoms that vibrate as heat moves through the material. These lattice vibrations, the phonons, ultimately carry heat away, diffusing it from its source, though that source remains the warmest region, much like a kettle gradually cooling on a stove.

The kettle remains the warmest spot because as heat is carried away by molecules in the air, these molecules are constantly scattered in every direction, including back toward the kettle. This “back-scattering” occurs for phonons as well, keeping the original heated region of a solid the warmest spot even as heat diffuses away.

However, in materials that exhibit second sound, this back-scattering is heavily suppressed. Phonons instead conserve momentum and hurtle away en masse, and the heat stored in the phonons is carried as a wave. Thus, the point that was originally heated is almost instantly cooled, at close to the speed of sound.

Previous theoretical work in Chen’s group had suggested that, within a range of temperatures, phonons in graphene may interact predominately in a momentum-conserving fashion, indicating that graphene may exhibit second sound. Last year, Huberman, a member of Chen’s lab, was curious whether this might be true for more commonplace materials like graphite.

Building upon tools previously developed in Chen’s group for graphene, he developed an intricate model to numerically simulate the transport of phonons in a sample of graphite. For each phonon, he kept track of every possible scattering event that could take place with every other phonon, based upon their direction and energy. He ran the simulations over a range of temperatures, from 50 K to room temperature, and found that heat might flow in a manner similar to second sound at temperatures between 80 and 120 K.

Huberman had been collaborating with Duncan, in Nelson’s group, on another project. When he shared his predictions with Duncan, the experimentalist decided to put Huberman’s calculations to the test.

“This was an amazing collaboration,” Chen says. “Ryan basically dropped everything to do this experiment, in a very short time.”

“We were really in the express lane with this,” Duncan adds.

Upending the norm

Duncan’s experiment centered around a small, 10-square-millimeter sample of commercially available graphite.

Using a technique called transient thermal grating, he crossed two laser beams so that the interference of their light generated a “ripple” pattern on the surface of a small sample of graphite. The regions of the sample underlying the ripple’s crests were heated, while those that corresponded to the ripple’s troughs remained unheated. The distance between crests was about 10 microns.

Duncan then shone onto the sample a third laser beam, whose light was diffracted by the ripple, and its signal was measured by a photodetector. This signal was proportional to the height of the ripple pattern, which depended on how much hotter the crests were than the troughs. In this way, Duncan could track how heat flowed across the sample over time.

If heat were to flow normally in the sample, Duncan would have seen the surface ripples slowly diminish as heat moved from crests to troughs, washing the ripple pattern away. Instead, he observed “a totally different behavior” at 120 K.

Rather than seeing the crests gradually decay to the same level as the troughs as they cooled, the crests actually became cooler than the troughs, so that the ripple pattern was inverted — meaning that for some of the time, heat actually flowed from cooler regions into warmer regions.

“That’s completely contrary to our everyday experience, and to thermal transport in almost every material at any temperature,” Duncan says. “This really looked like second sound. When I saw this I had to sit down for five minutes, and I said to myself, ‘This cannot be real.’ But I ran the experiment overnight to see if it happened again, and it proved to be very reproducible.”

According to Huberman’s predictions, graphite’s two-dimensional relative, graphene, may also exhibit properties of second sound at even higher temperatures approaching or exceeding room temperature. If this is the case, which they plan to test, then graphene may be a practical option for cooling ever-denser microelectronic devices.

“This is one of a small number of career highlights that I would look to, where results really upend the way you normally think about something,” Nelson says. “It’s made more exciting by the fact that, depending on where it goes from here, there could be interesting applications in the future. There’s no question from a fundamental point of view, it’s really unusual and exciting.”


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March 15, 2019 4:06 pm

It seems there’s a new definition of putting lead in your pencil.

A glass of fine Scottish malt no longer competes.

Gunga Din
March 15, 2019 4:08 pm

I only read the headline for to the post, “Heat “waves” detected moving through pencil lead”, but I’m sure that if the study does’t support “CAGW”, it will include an erasure.

Chuck in Houston
March 15, 2019 4:10 pm

Wow. Experiments following computer simulation. This could be a very big deal if it can be scaled.

David Dibbell
March 15, 2019 4:18 pm

Interesting! Perhaps this a thermal version of electrical superconductors.

March 15, 2019 4:21 pm

Pencil lead contains graphite, but is mostly clay.

Chuck in Houston
March 15, 2019 4:22 pm

Repost. I guess I used a different email address when I posted a moment ago. Anyway –

My points were a) good to see a good experiment following computer simulations AND b) this could be a very big deal if verified, and can be scaled.

March 15, 2019 4:26 pm

That’s real science. It was also an effective use of a model. Make a prediction, test the prediction.

March 15, 2019 4:27 pm

Nice article, but 120 Kelvin = -153 degree Celcius

M Courtney
Reply to  F. Udo
March 16, 2019 12:27 am

Anything above -199°C is practical as that’s the temperature of liquid nitrogen.
Besides, this is proof of concept at temperatures that are far higher than before and using materials that are commercially available.
It’s good work is this.

Crispin in Waterloo
Reply to  F. Udo
March 16, 2019 5:15 am


That is a working temperature at the cold end of a natural gas fired thermoacoustic refrigerator so even with conventional industrial equipment is capable of maintaining that temperature. It is also easily reached with a Stirling Engine run backwards. Three stage desktop ones can reach 3 Kelvins.

Consider the implications of superconductors and heat flow at the speed of sound. We may be on the way to a new form of energy generation. Chips are already small enough. We need big power applications.

John Bell
March 15, 2019 4:37 pm

It seems to defy some law of thermodynamics somehow, heat has no momentum, in fast they claim heat flows from cold to hot? I doubt it a lot!

Chuck in Houston
Reply to  John Bell
March 15, 2019 4:52 pm

The second law is where we get heat goes to cold, although it’s more about a system’s entropy tending to increase. Then we have to consider boundary conditions etc…

But that part caught my attention as well. Not sure how to rationalize it in my head but they are measuring the relative differences of a wave’s peaks and troughs along a length of carbon.

Clear enough? 😉

Robert W. Turner
Reply to  John Bell
March 15, 2019 5:38 pm

Phonons in harmony, good band name.

Reply to  John Bell
March 15, 2019 8:55 pm

The 2nd Law is limited to heat engine models and closed systems. It is constantly being cited as applicable to systems beyond its proper and limited frame.

March 15, 2019 4:43 pm

Slightly off-topic, another highly unusual property of acoustic phonons was recently discovered: phonons have their own gravitational mass and can generate a tiny gravitational field. But it the coefficient is negative, so these phonons are repelled, not attracted, by the gravity of other mass.

[High Energy Physics – Theory]
“The gravitational mass carried by sound waves”
Angelo Esposito, Rafael Krichevsky, Alberto Nicolis
(Submitted on 23 Jul 2018 (v1), last revised 2 Mar 2019]

Reply to  Johanus
March 15, 2019 6:15 pm

Wow, just wow.

Joel O’Bryan
Reply to  Johanus
March 15, 2019 6:47 pm

Just because it appears in Arxiv? it Is not peer reviewed.

Reply to  Joel O’Bryan
March 15, 2019 8:14 pm

But Physical Review Letters is peer reviewed, where the paper was published in the 1 March 2019 issue (Vol. 122, Iss. 8). It is the flagship “letter journal” in Physics, where results of seminal research by distinguished physicists is published weekly.

Alastair Brickell
Reply to  Johanus
March 15, 2019 9:46 pm

March 15, 2019 at 4:43 pm

Very, very interesting. It’s great that unlike climate science ideas in the rest of the scientific world are not settled.

So what would happen to sound in and around a black hole one wonders…Obviously there’s no air but could some other medium conduct phonons and which way would they go?

Many thanks Anthony for letting us know about interesting things other than AGW from time to time.

Reply to  Alastair Brickell
March 15, 2019 10:20 pm

Sound still cannot travel in a vacuum. This notion of “phonon mass” must be viewed as part of the medium, but the innovation revealed here is that this mass is carried by the phonon from one place to another. Experiments are being set up to measure the amount of mass being transported. For a large event like an earthquake, it is estimated that billions of tons of this phonon mass will be transported, and should generate detectable gravitational fields.

Having said that, it is possible that phonons could be transported across the Universe, in the sense that true vacuum (i.e. large regions with “nothing” in them) does not really exist. The Universe is filled with all kinds of inter-stellar gases and particles. So given a large enough scale and large enough “bang”, sound waves might indeed travel from one star to another, transporting phonon mass with them.

Reply to  Johanus
March 16, 2019 12:43 am

“The Universe is filled with all kinds of inter-stellar gases and particles.”
You can’t have gases or particles without the electron cloud, look at the periodic table. It’s my belief , that space isn’t empty, it’s filled with electric potential (electrons) . Without electrons creating separation (space) all mass would collapse in on it self. How big is the electron cloud holding Earth together ?????

Crispin in Waterloo
Reply to  Johanus
March 16, 2019 5:20 am

He is referring to the fact that what was the cold zone was having the heat pulled out of it to the formerly hot zone, which had been emptied of heat by rapid flow.

He did not say heat in the cold some moved into a hotter zone. The pattern was “inverted” by the rapid cooling of the hot zone.

Crispin in Waterloo
Reply to  Johanus
March 16, 2019 5:32 am


There are large scale “sound waves” moving through the large structures of the universe which take eons to travel through space. Consider that masses repel and attract so there are “pressure waves” rippling through the strings of galaxies. That was reported maybe three years ago (?).

As there is no such thing as an utter vacuum we can assume there are other forms of matter filling all the visible space. This is a strong deviation from the Standard Model which we can assume is in need of large corrections including its Big Bang. If the whole universe has always existed and is filled with a rippling superfluid, it puts a new perspective on the CMB and the meaning of red shift. We already know that the red shift of quasars and the galaxies that contain them are completely, wildly different. Obviously red shift doesn’t equal distance.

As the waves (pressure waves?) ripple through vast stretches of the universe, the idea that sound doesn’t move through a vacuum requires a new definition of “sound”. Let him hear it, who has ears to hear.

Alastair Brickell
Reply to  Crispin in Waterloo
March 16, 2019 2:15 pm

Crispin in Waterloo
March 16, 2019 at 5:32 am

I quite agree…there’s still a lot we don’t know or understand. That’s exactly why this discovery is just so interesting.

Maybe we need yet another new kind of ears (to add to our newly discovered gravitational wave ones)!

Alan Tomalty
Reply to  Crispin in Waterloo
March 16, 2019 10:39 pm

That is why the Thunderbolts project is exciting. Their theory of the electric universe was just recently partially confirmed by NASA ‘s discovery of huge electric currents around earth. The Thunderbolt physicists have offered to debate main stream physicists about their theory claiming that Einstein’s theory of gravity is all wrong but of course the main stream physicists refuse. Sound familiar?

Reply to  Johanus
March 16, 2019 1:39 am

…”the coefficient is negative, so these phonons are repelled, not attracted, by the gravity of other mass.”..

Is everyone thinking what I am thinking : the mysterious “dark energy ,” the repulsive force that accelerates cosmic expansion against gravitational attraction?

Dr. Strangelove
Reply to  Johanus
March 16, 2019 4:28 am

The so-called gravitational mass by sound waves can be explained more easily by Newtonian and statistical mechanics. A phonon with kinetic energy E and moving at velocity v has a mass m.
E = 1/2 m v^2
m = 2/v^2 E
In the paper, the factor 2/v^2 is replaced by a formula of the speed of sound in the medium and mass density of the medium.

The so-called negative gravitational mass is simply an outward pressure P (repulsive force) exerted by phonons. Negative pertains to direction of force which is opposite of gravity or inward pressure (attractive force)
∑ E = P dV
The summation of kinetic energies of photons equals the product of outward pressure P and change in volume dV. The phonons are contained in volume V and outward pressure increases the volume by dV. Phonons are quantized heat energy. dV is commonly called thermal expansion.

Dr. Strangelove
Reply to  Dr. Strangelove
March 16, 2019 4:40 am

spelling mistake: “kinetic energies of phonons” not photons

Jean Parisot
March 15, 2019 4:43 pm


March 15, 2019 4:56 pm

This may well be connected to the concept of “Discrete Breathers” which is the case where individual doping atoms in a heterogeneous matrix may oscillate at frequencies higher than the normal cutoff frequency. Excitation by crossed lasers was the predicted means of creating the oscillations. I was involved in discussions regarding how to demonstrate the effect in carbon nano tubes. Terrific to see that it works in graphite as well.

Joel O’Bryan
March 15, 2019 5:17 pm

Ha! Heat sound in graphite… That’s nothing!
Yes, The evil Carbon atom works its magic in many mysterious ways.
According to climate scientists like Mann and Schmidt it can even alter meso-scale storm intensity without changing the temperature! Amazing, I know.

March 15, 2019 5:46 pm

The measurement technique was very clever … maybe too clever. I will believe this when they clamp a chunk of graphite to a CPU and remove heat faster than can be done by a heat pipe.

1 – Maybe they aren’t measuring what they think they’re measuring.
2 – Most published research findings are wrong.

michael hart
Reply to  commieBob
March 16, 2019 1:11 am


“This was an amazing collaboration,” Chen says. “Ryan basically dropped everything to do this experiment, in a very short time.”
“We were really in the express lane with this,” Duncan adds.

Something about this makes me feel slightly uncomfortable. Is this guy really focusing on the science?

March 15, 2019 6:22 pm

Very cool!

March 15, 2019 6:30 pm

“so scientists have dubbed this exotic mode of heat transport “second sound.”

let’s for a moment stop calling this heat (the raised energy state effect of absorbing energy) and call it infrared light, which as we know is absorbed by some things and like with other forms of light, passes through things which are transparent to it.

now let’s also remember different media have different refractive indices which basically means it slows light at a different rate to other things.

Putting those things together they’ve observed infrared passing through graphite which appears according to their observations to be at least semi-transparent to the wavelength they subjected it to, and it also slowed as it passed through it too – and they’ve dubbed this .. ‘second sound’!?

wow ..

Peter Pearson
Reply to  Karlos51
March 17, 2019 10:42 am

Thank you, Karlos51. Yes, we can save the second law of thermodynamics by noting that this might not be the conventional meaning of the word “heat”. The teakettle analogy is definitely misleading showmanship.

Gary Pearse
March 15, 2019 6:33 pm

Interesting for sure. But I was underwhelmed by the pedestrian use for it they reckoned. Yeah, it could work for cooling computer components buy hey we are moving heat fast here. Maybe carbon fibers that take solar heat from your outside wall, dash into your house where it creates a hot interior wall, etc.

The other thing was the conservation of momentum of the phonons. Whether they wiggle off in one direction or bag and forth with collisions with other molecules, it seems to me momentum is conserved. Its merely shared with suroundings until attenuated (diluted ) by sharing with neighbors.

Reply to  Gary Pearse
March 16, 2019 6:54 am

Perhaps they have learned something from the lack of commercial success of many other ‘significant breakthroughs.’

I remember some thirty years ago reading about Buckyballs (fullerene), and how it was going to impact everything from electronics to medicine. Today? A few commercial applications, like lubricants, but not much else that I am aware of. And how many ‘game-changing breakthroughs’ in battery technology have we seen bite the dust for various reasons?

March 15, 2019 7:16 pm

What if the heat waves could be made coherent? — i.e. lase the heat through some material as a non-diffusing “beam.”

Hocus Locus
March 15, 2019 7:42 pm

Empirical observations can be sometimes misleading. There might be some time sensitive polarizing issue in the sensor technology thinks it ‘saw’ heat waves and is really seeing a beat frequency/zero crossing in the measurement system. Is a point along the wave front suddenly and really ‘becoming’ cold, or is the heat simply not visible to the sensor/measurement at the moment?

Graphite is weird you know, slippery and tricksy, yessss.

Dr. Strangelove
March 16, 2019 1:35 am

This can be explained by quantum mechanics. The wave-particle duality in QM means phonons have a wave property. Diffusion in heat conduction can be explained by the wave property of phonons. Normally phonons have varying wave frequencies. Wave cancellation occurs as phonons move and encounter more waves of different frequencies. Diffusion is the weakening of the wavefunction as wave packet moves farther from its point of origin.

The so-called second sound is a resonance of phonon wave and sound wave. That’s why the phonons move at the speed of sound. The frequency of photon waves matches the natural acoustic frequency of the material. In resonance, there is no wave cancellation. The wavefunction is preserved as it moves so no diffusion of heat.

Peta of Newark
March 16, 2019 3:01 am

I’m with ‘Strangelove’ and Karlos on this one – the guy here is, shall we say, ‘confused about what he’s observing.
A sound wave moving through something is a mechanical excitation – so is heat. He could get the same effect by tapping the pencil on his desk.

But hey, Its got ‘lasers’ -it’s cool! Hey Ma, look what I found!

Otherwise I really rather have fair admiration for these visitors from Planet Zog..
(Elon, got that rocket working yet? Not for me tho – am not ‘good with heights’)

Esp when he (not Elon) declares:

— meaning that for some of the time, heat actually flowed from cooler regions into warmer regions.
“That’s completely contrary to our everyday experience, and to thermal transport in almost every material at any temperature,”
So very succinctly and completely trashes the theory of the GHGE – the (cool) atmosphere is NOT going to push heat into the (warm) dirt, soil or water.

He’s obviously a Zoggian native and/or less than 30 years old and not Au Fait with modern science

And just take a look at what ‘Modern Science’ is doing to our kids – causing fear & despair in 6 years olds.
Six Year Old Children are now frightened and scared-to-death about going to school.
Thanks. someone.
Coming clear now about where/when science went down the pan?


Why doesn’t someone ‘Write a letter about that’………….

Peta of Newark
Reply to  Peta of Newark
March 16, 2019 3:03 am

(U kno Y)

Bloke down the pub
March 16, 2019 3:06 am

For those who need to brush up on their entropy.

old construction worker
March 16, 2019 3:33 am

” meaning that for some of the time, heat actually flowed from cooler regions into warmer region.” Interesting. Graphite acting like a sponge. Questions. Is “heat”from the cooler region trying to dilute the “heat” in the warmer region which would be a cooling effect. What happens to the “second sound” once the beam is shut off?

March 16, 2019 4:16 am

graphite and graphene are rather “big” in all sorts of research recently Im seeing
batteries were one
hmm so if they get hot whats going where if this data is correct/
could be more fun than watching lithium batteries explode.
EEEElon where is ya?

March 16, 2019 6:24 am

Heat amplification by stimulated emission of phonons? HASEP? No, I don’t think so.

You don’t get a pulse, or a wave, of energy in a laser unless you start with a pulse going through material in an excited state, or you carefully create a wave during the excitation of the material (mirrors reflecting photons, restricting them to a specific repeated path). Otherwise, when you put material in an excited state, you just get a ‘lightbulb’, incoherent photons streaming in all directions.; no pulses (waves).

Now, the following would be fascinating and, in my book, not too surprising: that you could find a specific material that when a rod of it it is cladded by a heat resistant material, and is heated to the point of radiating, then when a sudden burst of heat is applied to one end, you get a burst of heat, maybe measurably greater than the original burst, at the other end, and the rod is left in a cooler state. That would imply that the wave of heat stimulate existing energy in the rod to flow with the wave.

But the way they are doing it, no.

Tom Johnson
March 16, 2019 6:31 am

Do the Quantum Physicists now have a new ‘particle’ to deal with? Heaton? “Hey Martha, it’s too hot in here. Turn on the Heaton generator to get rid of some of it.”

Pa Wi
March 16, 2019 7:07 am

This sounds interesting! If it is replicable, its certain to be valuable in some applied engineering science.

March 20, 2019 7:17 am

April 1st is coming soon !
If we make CO2 from burning Graphite and release it , It will solve the GorBalloney warming problem !
/sarc off

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