Phonon Polariton-Mediated Thermal Transport in Low-Loss Boron Nitride Nanotubes

Phonon polaritons (PhPs), quasi-particles originating from interactions between infrared photons and optically active phonons, have attracted recent attention as effective energy carriers for heat conduction in solids. Benefiting from their long propagation lengths and high group velocities, both of which could be orders of magnitude higher than the corresponding values for phonons, PhP-mediated thermal transport along thin films and nanowires have been recently demonstrated. However, suffering from the low number density, the PhP contribution to thermal conductivity is often limited with measured values below 0.5 W/m-K.
Boron nitride has attracted extensive research interest in nanophotonics because of its fascinating optical properties including the hyperbolic polariton dispersions in the Reststrahlen bands where multiple PhP branches are accessible. In addition, boron nitride has been widely studied as nanofillers for thermally conductive but electrically insulting nanocomposites that can find extensive application in electronic device thermal management. Here we report on our recent discovery of strong thermal effects produced by the non-equilibrium PhPs in boron nitride nanotubes (BNNTs).
In an effort to reduce the contact thermal resistance (R_C) between BNNTs, a thin Au interlayer was introduced to eliminate the resistance due to phonon back reflection. Indeed, the Au interlayer yields a ~200% enhancement in the contact thermal conductance between two 50 nm diameter BNNT segments around room temperature. Surprisingly, opposite to the temperature dependence for R_C between bare BNNTs, the extracted contact thermal resistance with the Au interlayer starts to decrease as the temperature drops below 150 K and eventually becomes negative below 80 K for the 50 nm BNNT sample. This unexpected observation implies unrevealed transport mechanism(s) in BNNTs as compared to similar studies for multi-walled carbon nanotubes (MWCNTs), where R_C always remains positive even with a thin Au interlayer. The sharp contrast between non-polar CNTs and polar BNNTs, and the fact that Au pads can serve as efficient PhP launchers, suggest that it could be that the Au interlayer introduced PhPs in the BNNTs, which alters the thermal circuits and leads to a nominal negative R_C. To verify this hypothesis, individual BNNTs with and without Au-coating were measured to estimate the effects of PhPs on heat conduction along the tube. A remarkable polariton-mediated thermal conductivity of 9.5 W/m-K was observed at room temperature. Importantly, the PhP thermal conductivity increases with the sample length, suggesting ballistic PhP transport, which is supported by the long propagation lengths from the near-field optical characterizations. Interestingly, the measured polariton thermal conductivity shows a clear upturn in the temperature range (395-470 K) corresponding to the Reststrahlen band of BNNTs, which marks the first experimental verification of the strong thermal effects of the hyperbolic phonon polaritons from multiple branches. The rich physics disclosed in PhP-mediated heat conduction in BNNTs can be used to devise strategies for PhP-based heat dissipation.