Thermal conductivity of boron nitride-filled thermoplastics: Effect of filler characteristics and composite processing conditions

The thermal conductivity of boron nitride (BN)-filled poly(butylene terephthalate) (PBT) was investigated as a function of particle size, aspect ratio, surface area, surface chemistry, and concentration of BN as well as composite processing methods and conditions. In the low filler concentration region, a larger BN surface area resulted in lower thermal conductivity of the composites as a result of phonon scattering at interfaces. In the high filler concentration region the ease in forming filler networks, as reflected by the aspect ratio of BN, played a more dominant role. A percolation-like behavior was observed when BN networks were formed while the thermal conductivity at close vicinity of the percolation threshold was not completely governed by the scaling law of classic percolation theory. High shear force employed in extrusion was effective in dispersing BN agglomerates into fine platelets while also inducing PBT degradation. When a low screw speed was used in extrusion followed by injection molding, the samples exhibited significantly lower thermal conductivity, which may be attributed to flow-induced orientation of BN platelets in the direction perpendicular to the heat flow, relatively low concentration of filler at sample surfaces (skin-core effect), and agglomeration of the BN platelets.