Zhiliang Pan
Vanderbilt University – Nashville, Tennessee, USA
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Polymer composites of low-cost and good processability are widely employed in electronic packaging. However, the continuously increasing power consumption of modern electronics demands polymer composites of higher and higher thermal conductivity for more efficient heat dissipation. Towards this goal, researchers have explored various high thermal conductivity fillers to boost the composite thermal performance. In recent years, boron nitride nanotubes (BNNTs) have caught significant attention because of their high thermal conductivity and stability and insulting electrical property for thermally conductive and electrically insulating composites. So far, the success of boosting the composite thermal conductivity remains moderate and fall below the prediction of classical particle mixing theory, which has been attributed to the contact thermal resistance between fillers and across filler-polymer interface. Efforts of understanding the thermal transport mechanisms at these contacts and interfaces based on measurements at bulk level suffer from large uncertainties and various assumptions that have to be made. In contrast, measurements at individual filler level, while challenging, could provide more solid data to disclose the underlying transport mechanisms.
Here we report on systematic studies of contact thermal resistance between individual BNNTs with and without a thin polymer interlayer, which reveal interesting transport phenomena and rich physics. First, the results indicate ballistic phonon transport across direct contacts between BNNTs, originating from the long phonon mean free path (~100 nm at 300 K) along the cross-plane direction. This ballistic transport leads to significant contact resistance from the back-reflection of phonons, which could contribute up to > 80% of the contact thermal resistance for a 47 nm BNNT at low temperature. More interestingly, an unexpected bidirectional modulation of the contact thermal resistance by a thin PVP interlayer was discovered. Analysis indicates that this bidirectional modulation is due to the competing effects induced by the PVP interlayer. On one hand, the polymer interlayer poses additional thermal resistance because of the added interfaces between BNNTs and PVP, and the thermal resistance of the thin PVP layer. On the other hand, the PVP interlayer effectively eliminates the resistance associated with phonon back-reflection. For thinner BNNTs with more significant phonon back-reflection resistance, the polymer interlayer reduces the contact thermal resistance while the opposite results are observed for larger tubes where phonon back-reflection is less significant. These findings disclose interesting thermal transport phenomena at contacts between BNNTs, which could provide important insights into designing BNNT-based polymer composites with enhanced thermal conductivity.
Email: zhiliang.pan@vanderbilt.edu