Synergistic enhancement of thermal conductivity in composites filled with expanded graphite and multi-walled carbon nanotube fillers via melt-compounding based on polymerizable low-viscosity oligomer matrix

• We proposed a novel melt-compounding based on a polymerizable low-viscosity oligomer.
• Dispersion of the second multi-walled carbon nanotubes was improved by the method.
• Synergistic enhancement in thermal conductivity of composite was found by the method.
• Construction of thermally conductive pathways was the most important physical factor.
• X-ray computed tomography was suitable for evaluation of the conductive pathways.

We found that the thermal conductivity of the polycarbonate (PC) composite filled with both 9.9 wt% expanded graphite (EG) and 0.1 wt% multi-walled carbon nanotube (MWCNT) fillers was synergistically improved by 49% compared to that of the PC composite filled with 10 wt% EG alone. In order to induce the optimal internal structure favorable for thermal conduction by enhancing the dispersion of the second MWCNT fillers, we applied a two-step melt-compounding to fabricate composites using a MWCNT masterbatch based on a polymerizable oligomer resin, cyclic butylene terephthalate (CBT), which is characterized by ultra-low viscosity and excellent impregnability during initial melting. Three-dimensional (3D) non-destructive characterization using X-ray micro computed tomography (micro-CT) was utilized to demonstrate the synergistic enhancement and to verify dispersion and 3D thermal network of the fillers in the composites accurately. The synergistic enhancement was significantly affected by the formation of the efficient thermally conductive pathways and dispersion of the second MWCNT fillers.