Controllable synthesis of diamond/graphite composite films with tunable dielectric properties for microwave attenuation

Abstract

Diamond/graphite composite films with tunable dielectric properties were prepared by microwave plasma chemical vapor deposition. The effect of methane flow rate on the morphology, phase composition, electric conductivity and dielectric properties of the films were investigated. The results showed that the grain structure of the film transformed from spherical shape to needle-like structures with the flow rate of methane increased from 1.5 sccm to 3.0 sccm. Meanwhile, the sp3/sp2 ratio decreased from 1.77 to 1.04 and the graphitization degree of sp2-bonded carbon was enhanced. Furthermore, the electric conductivity was improved from 0.01 S/m to 63.04 S/m and the average dielectric loss tangent in the range of 75–110 GHz was significantly increased from 0.09 to 1.31 with the increasing of methane flow rate. The results demonstrated that methane flow rate play an important role in governing dielectric properties of diamond/graphite composite films. Moreover, the possible microwave attenuation mechanism was explored. The dominating attenuation mechanism of the film changed from polarization loss to conductive loss with the increase of methane flow rate. The microwave attenuation ability of the film was enhanced with the synergistic increase of both conductive loss and polarization loss.