Hot highway in graphite bands

April 21, 2023

(Nanowerk News) Scientists from the Institute of Industrial Science at The University of Tokyo studied the flow of heat energy in bands of purified graphite and showed that heat can move more like a liquid, rather than scatter randomly, under certain conditions (Nature Communications, “Observations of Poiseuille phonon flow in isotope-purified graphite bands”).

This work could lead to more efficient heat dissipation from electronic devices, including smartphones, computers and LEDs.

Prior to modern understanding of thermodynamics, scientists sometimes thought of heat as a liquid called a “calorie”. However, we now know that heat is actually the random kinetic energy possessed by the vibrating atoms or molecules that make up a material. Sometimes, vibrations can be thought of as physical particles called phonons, which are the main contributors to heat conduction in semiconductors. Surprisingly, in certain materials such as graphite, the phonons may indeed behave very much like liquids. However, this theory remains relatively obscure.

Now, a research team led by the Institute of Industrial Science at The University of Tokyo has used the theoretical and experimental results to better understand the fluid-like properties of phonons. They showed that when graphite samples were prepared from isotopic pure carbon, meaning only carbon-12 atoms, heat could be conducted much more quickly, almost like water flowing through a pipe. This is called “Poiseuille phonon flow,” based on the theory of a viscous fluid flowing in a closed tube. The effect is strongest on graphite at about 90 Kelvin. However, natural graphite contains about 1% of other carbon isotopes, particularly carbon-13, which limits this effect in natural samples. Illustration of Poiseuille phonon flow in graphite bands. In the hydrodynamic regime, the heat flux (represented by red arrows) manifests a parabolic profile maintained by the collective motion of phonons through a band structure of limited width. (Image: Huang et al.)

“Our study clarifies the theoretical criteria for the formation of Poiseuille phonon streams in graphite, a material that exhibits strong anisotropy, which has never been clear before,” said lead author Dr. Xin Huang.

Graphite, also known as pencil, is very cheap and easy to produce. As a result, it is already used for heat dissipation in some electronic devices which generates a lot of energy waste during operation. Using pure graphite which has at most 0.02% carbon-13, the team was able to observe a heat conductivity that was more than double the value of natural graphite. The fact that this increase occurs only over a certain temperature range is evidence that the fluid-like collective motion of phonons is the mechanism.

“In conventional Poiseuille flow, the highest velocity is near the center, which we propose to occur with the phonons in our experiments,” said senior author Professor Masahiro Nomura. 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 if they increase their density inside the device.

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