Despite their isostructural similarity to graphene, large-scale preparation of hexagonal boron nitride nanosheets (hBNNs) has been challenging, mainly due to their strong interlayer interactions initiated by the electronegativity difference in their elemental components. In this study, a large-scale synthesis of hBNNs via a ball milling (BM) technique is reported. The hBN exfoliation was optimized by analyzing the effect of the ball milling process parameters (ball to powder ratio, speed, and time) on the quality and yield of hBNNs. hBNNs prepared at the optimum milling conditions (10:1 ball to powder ratio, 250 rpm rotation speed, and 1.5 hours) were examined in terms of yield, stability, structure, and surface chemistry. The yield of hBNNs was significantly high and reached 38% and 60% at 30 minutes and 4.5 hours milling times, respectively. hBNNs prepared at the optimum milling conditions were utilized to fabricate thermally conductive linear low-density polyethylene (LLDPE) nanocomposites. The ultimate application of these nanocomposites is replacing metal evaporators in multi-effect distillation desalination. Therefore, it is a prerequisite to have high thermal conductivity and is essential to have enhanced mechanical durability, thermal stability, and wettability. Thus, the performance of these nanocomposites and conventional composites containing bulk hBN were analyzed in terms of processability, thermal conductivity (κ), and mechanical properties. Finally, a mitigation method to alleviate BMinduced defects and amorphizations was proposed and investigated. This treatment was essential in alleviating hBNNs defects and enhancing the thermomechanical properties of LLDPE nanocomposites, making them excellent candidates for heat exchanger applications.

No abstract

Emergent heat transfer and thermal management applications require polymer composites with enhanced thermal conductivity (κ). Composites filled with non-spherical fillers, such as hexagonal boron nitride (hBN) and Graphite (Gr), suffer from processing-induced filler orientations, resulting in anisotropic κ, commonly low in the through-plane direction. Here, the effects of extrusion and compression molding-induced orientations on κ of hBN- and Gr-filled polyethylene composites were investigated. The effect of extrusion on the hBN orientation was studied using dies of various shapes. The shaped extrudates exhibited hBN orientations parallel to the extrusion flow direction, which prompted additional hBN orientation during compression molding. κ of the composites produced with shaped extrudates varied from 0.95 to 1.67 W m−1 K−1. Pelletizing and crushing the extrudates improved κ, by exploiting and eliminating the effect of extrusion-induced hBN orientations. Gr-filled composites showed better κ than hBN composites due to the higher intrinsic conductivity and bigger particle sizes. A maximum κ of 5.1 and 11.8 W m−1 K−1 was achieved in composites with oriented hBN and Gr through a thin rectangular die and stacking the sheets to fabricate composites with highly oriented fillers.

No abstract