when the samples were annealed in vacuum, the concentration of O2 surrounding the annealed ZnO/Al2O3 coatings is extremely low. According to defect Eq. (2), the thermodynamic balance will move toward the right side, which implies that the oxygen atoms are preferential to be liberated from oxygen vacancies, resulting in higher concentration of oxygen vacancies thus higher electrical conductivity of ZnO particles. Consequently, the dielectric properties of annealed ZnO/Al2O3 coatings will be remarkably affected by the increased number of oxygen vacancies in the ZnO phase. On the one hand, the real part (ε ) which represents the degree of polarization occurring under the electromagnetic field could be affected with higher amount of oxygen vacancy defects. For example, the decreased value of ε of B4 coating is possibly attributed to the loss of relaxation polarization effect under the high frequency electromagnetic field. On the other hand, the imaginary part (ε ) of the coating which is closely related to the electrical conductivity according to Eq. (1), was found to be significantly increased after annealing treatment in vacuum