Significantly reduced intrinsic dielectric constant and loss of nano-silica by direct fluorination

The surface modification of ceramic nanofillers is a critical process for preparation of high-performance polymeric microwave dielectric composite. Up to now, the commonly adopted modification methods based on silane coupling agent can improve the dispersibility and compatibility of ceramic nanofillers, while they can't significantly affect the intrinsic dielectric properties of ceramic nanofillers in the microwave band. In this work, we presented a surface modification approach of direct fluorination utilizing F2/N2 to modify nano-silica (nano-SiO2). F2 molecules can directly react with hydroxyl groups as well as uncondensed SiO3/2 to generate a large amount of Si–F and -SiF2- bond with low polarizability on nano-SiO2. As a result, the dielectric constant and loss of nano-SiO2 in the microwave band are greatly reduced. In particular, the dielectric constant of nano-SiO2 at 10 GHz is reduced by 49% from 5.72 to 2.92 and the dielectric loss is reduced by 85% from 0.1490 to 0.0223. In addition, the grafting of fluorine can improve the dispersibility of nano-SiO2 in polyimide (PI) matrix. These superiorities are thus conducive to the preparation of the PI/F–SiO2 composite (PI compounding with fluorinated nano-SiO2) with low dielectric constant and low loss in microwave band. In addition, the PI/F–SiO2 composite film also possesses enhanced thermal-dimensional stability and mechanical properties because of better compatibility of F–SiO2 in PI matrix.