The next time you boil a kettle, consider this: After turning it off, instead of staying hot and slowly warming the surrounding kitchen, it 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 behaviour resembles the wavelike way in which sound travels through air.
The new results represent the highest temperature at which scientists have observed second sound.
This suggests that graphite, and perhaps its high-performance relative, graphene, may efficiently remove heat in microelectronic devices in a way that was previously unrecognised.
“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,” said 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.”
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 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.