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.