Quasi-Isotropic Thermal Conduction in Percolation Networks: Using the Pore-Filling Effect to Enhance Thermal Conductivity in Polymer Nanocomposites

A quasi-isotropic percolation network with enhanced thermal conductivity was prepared with a hexagonal boron nitride/poly(methyl methacrylate) (h-BN/PMMA) composite using a double filler-to-polymer structure (D-structure) approach. Using a three-dimensional (3-D) polygonal network of PMMA beads and the additional application of a PMMA resin with a different solubility, a secondary polymer-assisted filler percolation, the D-structure, was generated. The 3-D thermal percolation routes based on the D-structure generated in-plane (20 vol % of h-BN) and out-of-plane (30 vol % of h-BN) percolations of the polymer nanocomposites with quasi-isotropic thermal properties. Moreover, compared to a bare PMMA sheet, the composites showed 44 times enhancement of out-of-plane thermal conductivity (6.34 W m–1 K–1) and 51 times enhancement of in-plane thermal conductivity (7.34 W m–1 K–1) with 50 vol % h-BN filler loading. The dual 3-D thermal percolation routes, with in-plane and out-of-plane percolations, were generated in the polymer nanocomposites even after incorporation of a two-dimensional h-BN filler. A COMSOL thermal conducting simulation was designed to elucidate the creation of high thermal conductivity pathways through the composites. Thermal percolation thresholds over the generation of D-structure networks were revealed by correlating the infrared (IR) camera, COMSOL thermal conducting simulation, and infrared microscopy analyses.