Controlling thermal properties using 2D and 3D phononic crystals
We have shown experimentally and theoretically for the first time that it is possible to change the phonon thermal conductance strongly based on the wave properties of the phonons [Zen2014]. This was achieved by fabricating a 2D nanoscale mesh structure (or a so called phononic crystal), whose period is of the same order as the wavelength of the phonons that carry heat, about a micrometer in this case. Then the phonon waves interact strongly with the phononic crystal structure and change their speed by almost an order of magnitude. Because the waves move much more slowly, the thermal conductance is strongly reduced. We also showed theoretically that this reduction gets stronger as the period increases [Puurtinen2016a], and, indeed, we have experimentally observed a whopping 130 times reduction with a periodicity 4 µm [Maasilta2016].
More recently, we studied experimentally, whether the predicted thermal conductance reduction with increasing period has experimental limits [Tian2019]. Indeed, we found out that there was an optimal period ~10 µm with the highest reduction, after which the conductance started increasing again. This was successfully modeled as a crossover to the diffusive limit, where boundary scattering from the hole edges started to dominate.
In addition, and quite counter-intuitively, a period much smaller than the average phonon wavelength was predicted to lead to an increase of thermal conduction as compared to the uncut membrane [Puurtinen2016a]. In addition to thermal conduction, we have also shown theoretically that sub-Kelvin heat capacity can be modified strongly, as well [Puurtinen2016b]. In the future, these demonstrated concepts can possibly be used in many ways.
