Simultaneously Enhanced In-Plane and Out-of-Plane Thermal Conductivity of a PI Composite Film by Tetraneedle-like ZnO Whiskers and BN Nanosheets

In the preparation of high-performance polyimide (PI) thermally conductive composite films, it is generally a challenge to improve both in-plane thermal conductivity (λin-plane) and out-of-plane thermal conductivity (λout-of-plane). Herein, boron nitride nanosheet-coated tetraneedle-like zinc oxide whisker composite particles (ZnO@BN) were obtained by electrostatic self-assembly, and then, a ZnO@BN/PI thermally conductive composite film was prepared by the "two-step method". The ZnO@BN composite particles have the advantages of three-dimensional morphology structure, high thermal conductivity, and low interfacial thermal resistance. A continuous and three-dimensional enhanced thermal conductivity network structure of ZnO@BN in the PI matrix was obtained to simultaneously enhance the λin-plane and λout-of-plane of the composite film. At a 30 wt % ZnO@BN content, the λin-plane of the ZnO@BN/PI composite film reached 2.235 W m–1 K–1, which was 1075% higher than that of pure PI, and the λout-of-plane reached 0.853 W m–1 K–1, which was 469% higher than that of pure PI. The thermal conductivity anisotropy (λin-plane/λout-of-plane) of ZnO@BN/PI is significantly improved compared to that of BN/PI, decreasing from 4.44 to 2.62. This work provides a strategy for the preparation of high-performance thermally conductive composites.