Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure

The thermal conductivity along the out-of-plane direction in polyimide (PI) blend films containing ZnO nano-pyramidal particles (ZnO-NPs) was investigated. PI blend films composed of a sulfur- and a fluorine-containing PI were prepared via spin-coating and thermal curing of precursor solutions containing ZnO-NPs. Microphase-separated structures with "vertical double percolation (VDP)" morphology were spontaneously formed in the films, in which two phases were separately aligned along the out-of-plane direction, and ZnO-NPs were preferentially precipitated in the fluorine-containing PI phase. The blend film exhibits 410% enhancement of thermal conductivity at 27 vol% of ZnO-NPs, whereas only 90% enhancement was observed in the monophase PI film containing homogeneously dispersed ZnO-NPs. These results indicate that the VDP structure with selective incorporation of ZnO-NPs functions as an effective thermal conductive pathway.