Effect of composition and annealing on the dielectric properties of ZnO/mullite composite coatings

ZnO/mullite composite coatings were fabricated by atmosphere plasma spraying technology (APS). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were employed to investigate the microstructures and compositions of the feedstock and coatings. The permittivity–frequency properties were determined in the microwave frequency range of 8.2–12.4 GHz by network analysis. Our results show that both the real and imaginary part of permittivity increase greatly with increasing ZnO content over the frequency range of 8.2–12.4 GHz, which can be ascribed to interfacial polarization and orientation polarization. However, the composite coatings after 900 °C annealing for 5 h show much lower permittivity, due to the decrease of ZnO content resulting from the further reactions between the ZnO and mullite. Results indicate that the composite coating has excellent permittivity in room temperature, while for high temperature application, more investigation should be done.