Scientists at the Institute of Industrial Science at the University of Tokyo have studied the flow of thermal energy in ribbons of pure graphite and shown that under certain conditions, heat can move more like a liquid, rather than diffusing randomly. The work could lead to more efficient cooling of electronic devices, including smartphones, computers, and LEDs.

Researchers have found that under certain conditions, heat can move like a fluid in pure graphite, improving the cooling efficiency of electronic devices. The phenomenon, known as "phonon flow," has been observed to more than double the thermal conductivity of natural graphite and has potential applications in smartphones, computers, and LEDs.Source: Institute of Industrial Science, The University of Tokyo

Before the modern understanding of thermodynamics, scientists sometimes thought of heat as a liquid called a "fluid." However, we now know that heat is random kinetic energy possessed by the vibrating atoms or molecules that make up a material. Sometimes these vibrations can be thought of as physical particles called phonons, which are the main contributors to heat conduction in semiconductors. In some materials, such as graphite, phonons may indeed behave very similarly to fluids. However, the theory remains relatively vague.

Now, a research team led by the University of Tokyo's Institute of Industrial Science has used theoretical and experimental results to better understand the fluid properties of phonons. They showed that when graphite samples are made of isotopically pure carbon, meaning only carbon-12 atoms are present, heat can be conducted more quickly, almost like water flowing in pipes. This is called "phonon flow" and is based on the theory that viscous fluids flow in closed pipes. This effect is strongest in graphite, which has a temperature of about 90 Kelvin. However, natural graphite contains about 1% other carbon isotopes, especially carbon 13, which limits this effect in natural samples.

"Our study elucidates a theoretical criterion for the formation of phonon flow in graphite, a material that exhibits strong anisotropy, which was previously unclear," said lead author Dr. Xin.

Graphite, the most common form of pencil lead, is very cheap and easy to produce. Therefore, it has been used to dissipate heat from some electronic devices, which generate a lot of exhaust gas during operation. Using purified graphite with up to 0.02 percent carbon-13, the team observed a thermal conductivity of more than twice that of natural graphite. The fact that this enhancement occurs only in a specific temperature range supports the fluid-like collective motion of phonons as a mechanism.

The senior author Professor said: "In regular flows, the velocity is highest near the center, which is what happens to phonons in our experiments. Besides graphite, this phenomenon has also been observed in solid helium and black phosphorus. In theory, this phenomenon is possible even at room temperature. The work could help sensitive computer processors stay cool even as they increase in density within the device.