PNNL

Abstract

Planarly aligning two-dimensional (2D) platelets is a challengeable task due to their additional degrees of orientational freedom compared to those of one-dimensional building blocks. This study reports a sequential dual-alignment approach, employing an extrusion printing-induced force and rotating magnetic field (RMF)-induced force couple for platelet alignment in a yield-stress matrix bath. It is hypothesized that the partial alignment of platelets by a directional shear force facilitates the axial rotation of the platelets for planar alignment under an external force couple-induced torque, and the sequential dual alignments result in a synergistic alignment effect. The realization of planar alignment of 2D modified hexagonal boron nitride (mhBN) is reported when using the sequential dual-alignment approach. The dual-alignment approach induces a synergistic effect on thermal conductivity improvement compared to the cumulative influence of individual extrusion and magnetic field-assisted approaches. Particularly, the thermal conductivity of the 40% mhBN/epoxy composites, aligned with the sequential dual-alignment approach, is 692% higher than that of unaligned composites, and this outperforms the sum of using two individual methods collectively (133% improvement only) by more than five times, meaning a five-time more synergistic effect. A three-dimensional mhBN/epoxy heat sink is further printed and evaluated to demonstrate the feasibility for device fabrication.