Heat in Nanofilms: Why Thinner Doesn't Mean Faster ⚡ экспресс
In bulk aluminum nitride, heat is transferred very efficiently thanks to the long mean free path of phonons (quanta of thermal vibrations). However, in thin films, scattering on defects and boundaries changes the picture. An experimental transition from a ballistic regime (where thermal resistance does not increase with thickness) to a diffusive one was discovered in films from 1.6 nm to 2440 nm. In the ballistic regime, phonons have low speed and lifetime, and upon transition to diffusion, the lifetime of optical phonons increases by more than an order of magnitude and then stabilizes. Understanding such processes is important for the design of nanoelectronics.
Heat in a solid is a relay of atomic jolts: a line of people passing a hot object. In an ultrathin aluminum nitride film, the line is too short, and the object quickly hits the edge or a defect. The thermal resistance stalls—making it thinner doesn’t help.
When the film grows past a few hundred nanometers, the jolts start colliding with one another. The resistance rises, but the heat pulses themselves live ten times longer. However, a longer life doesn’t speed up transfer: in the market-like hustle of changing directions, the advantage vanishes.
Heat always flows toward the cold, increasing entropy—the measure of chaos. With light scattering analysis and brightness measurements, scientists captured this transition. The groundwork was laid by Max Born and Felix Bloch. Now, precise cooling of nanochips is a reality.
🎯 Diamond is one of the best heat conductors, but in nanofilms, the same trouble arises: boundary scattering drops its thermal conductivity by hundreds of times.
🎬 Stephen Baxter in 'The Time Ships' describes materials that arbitrarily change their thermal conductivity—almost like in these experiments.