Analyzing Thermal Energy Flow in Purified Graphite Ribbons

Scientists from the Institute of Industrial Sciences at the University of Tokyo studied the flow of heat energy in purified graphite ribbons and showed that heat can move like a liquid, rather than propagate randomly, under certain conditions. This work could remove heat more efficiently from electronic devices, including smartphones, computers, and LED lights.

Before the modern understanding of thermodynamics, scientists sometimes thought of heat as a fluid called a caloric. However, we now know that heat is actually the random kinetic energy possessed by the vibrating atoms or molecules that make up matter.

Sometimes vibrations can be thought of as particles of material called phonons, which are the main contributors to thermal conductivity in semiconductors. In a surprising twist, in some materials like graphite, phonons may actually behave in a very similar way to a liquid. However, this theory has remained relatively obscure.

Now, a team of researchers led by the University of Tokyo’s Institute of Industrial Sciences has used theoretical and experimental findings to better understand the nature of liquid-like phonons. They showed that when a sample of graphite is composed of isotope-pure carbon, meaning there are only carbon-12 atoms present, heat can be conducted more quickly, almost like water flows through a pipe.

This has been called the “phonon Poiseuille flow” based on the theory of the flow of viscous fluids in a closed tube. The effect was strongest in graphite at a temperature of about 90 K. However, natural graphite contains about 1% other isotopes of carbon, mainly carbon-13, which limits this effect in natural samples.

“Our study clarified the theoretical criteria for the formation of a Poiseuille phonon flux in graphite, a material that shows strong anisotropy, which was not apparent before,” says lead author Dr. Xin Huang. Graphite, also known as lead lead, is very inexpensive and easy to produce.

As a result, it is actually used to dissipate heat in some electronic devices which generate a lot of wasted energy during operation. Using purified graphite with a maximum of 0.02% carbon-13, the team was able to observe a thermal conductivity that was more than double the value of natural graphite. The fact that this enhancement only occurred within a certain temperature range was evidence that the liquid-like collective motion of the phonons was the mechanism.

“In a conventional Poiseuille flow, the velocity is higher near the center, which is what we propose to happen with the phonons in our experiments,” says Professor Masahiro Nomura, senior author. In addition to graphite, this phenomenon is also observed in solid helium and black phosphorus.

Theoretically, this phenomenon is also possible even at room temperature. This work can help keep sensitive computer processors cool, even as they become more dense inside the hardware.

The results have been published in the journal Nature Communications.