Boosting the in-plane thermal conductivity of nanofibrillated cellulose films: alignment engineering of cross-linked AlN whiskers

Abstract

With the rapid advancement of foldable electronic devices, ultra-light, flexible and anisotropic composite films with high in-plane thermal conductivity have garnered significant attention. Aluminum nitride (AlN), as a ceramic material with extremely high intrinsic thermal conductivity, has been considered as one of the ideal thermally conductive fillers. However, conventional angular AlN particles (AlNp) need to maximize the filling fraction to achieve high thermal conductivity, yet resulting in increased costs and decreased mechanical strength of composites. In this work, innovative one-dimensional AlN whiskers (AlNw) with high aspect ratio were synthesized using a modified direct nitridation method, which were further combined with nanofibrillated cellulose (NFC) to prepare anisotropic thermal conductive films using a facile vacuum filtration method. The composite films exhibited an exceptionally high in-plane thermal conductivity of 22.78 W m⁻¹ K⁻¹ at 70 wt% AlNw loading, which could be attributed to the alignment of cross-linked AlNw along the horizontal direction and the strong hydrogen bonding interactions between AlNw and NFC. The higher thermal conductivity enhancement effect of AlNw over AlNp was also demonstrated by an actual thermal transfer application and finite element simulations. Furthermore, the NFC/AlNw composite films also demonstrated superior thermal stability, mechanical strength, flexibility, and electrical insulation performance, indicating their broad potential applications in the field of thermal management materials.