Nominal Negative Contact Thermal Resistance between Boron Nitride Nanotubes with a Gold Interlayer

Thermally conductive polymer composites are competitive candidates for thermal interface materials, which find extensive applications in the thermal management of modern electronic devices. To date, numerous filler materials have been explored, including carbon nanotubes, ceramic particles, and metal nanowires. In particular, the requirement of electrical insulation in electronic devices warrants the rising attention paid to boron nitride nanotube- (BNNT) and nanosheet- (BNNS) based composites. One key issue in understanding the performance of polymer composites is the contact thermal resistance between nanofillers, with and without a polymer interlayer between the fillers. Previous studies have revealed interesting transport phenomena such as non-intrinsic ballistic contact resistance due to phonon reflection and bi-directional modulation of the contact thermal resistance from a polymer interlayer between BNNTs, which provide insights into enhancing the thermal performance for BNNT-based polymer composites.<br/>Here we report on efforts towards further reducing the contact thermal resistance through introducing a gold interlayer at the contact region, which could help eliminate phonon reflections with minimal added diffusive thermal resistance. Systematic measurements have been done to compare the contact thermal resistance between bare BNNTs (<i>R</i>_C) with those including a gold interlayer (<i>R</i>_C,Au). As the low resistance gold interlayer removes phonon reflection resistance, <i>R</i>_C,Au is smaller than <i>R</i>_C in the entire measurement temperature range. For example, the contact thermal conductance is enhanced by ~200% for a 50 nm BNNT at room temperature and the enhancement decreases with increasing tube diameter as the phonon reflection effect is smaller for larger tubes. Surprisingly, opposite to the trend for <i>R</i>_C, <i>R</i>_C,Au starts to decrease quickly as the temperature drops below 150 K and becomes negative below 80 K for the 50 nm BNNT sample. This negative <i>R</i>_C,Au indicates that some interesting transport mechanism has been introduced by the inclusion of the gold interlayer. Since the gold interlayer is not included in the thermal pathway, the observed nominally negative contact thermal resistance must occur through altering the thermal transport along the tube. One possible reason is that the gold interlayer boosts thermal transport along the tube through introducing surface phonon polaritons (SPhPs) into the tube. As a quasi-particle comprised of a photon strongly coupled with polar optical phonons, this hybrid mode benefits from the high heat capacity associated with optical phonons and the non-negligible group velocity of the optical phonons. A nominally negative <i>R</i>_C,Au, extracted by subtracting the thermal resistance of the corresponding bare BNNT from the total measured thermal resistance of the contact sample, could occur if the inclusion of the gold interlayer provides an additional thermal transport pathway, here we propose that the inclusion of the gold interlayer serves as a launcher for stimulating SPhPs which in turn can carry heat along the BNNT. This hypothesis is further supported by the fact that the propagation length of SPhPs increases as temperature drops. Therefore, the relative contribution of SPhPs enhances at lower temperatures and indeed, <i>R</i>_C,Aubecomes smaller as temperature drops and after <i>R</i>_C,Au turns negative, its magnitude keeps increasing (<i>R</i>_C,Au becomes more negative) as the temperature drops further. The above findings disclose rich physics in thermal transport through BNNTs and their contacts with a gold interlayer, which could be utilized to improve thermal performance of BNNT-based polymer composites.