Enhanced permittivity and multi-region microwave absorption of nanoneedle-like ZnO in the X-band at elevated temperature

We fabricated nanoneedle-like ZnO (ZnO_n) by a facile combustion synthesis route, demonstrating the original observations on the complex permittivity of ZnO_n ranging from 298 to 573 K and in the X band. Both real and imaginary permittivities (ε′ and ε′′) of ZnO_n depend on temperature and frequency. The ε′ increases while the ε′′ possesses a maximum value with increasing temperature. The maximum value of ε′′ appears at 473 K, and the corresponding loss tangent increases by 100% than that at 298 K. The relaxations in ε′′ mainly arise from dipole polarization and interfacial polarization in ZnO_n. Multi-region microwave absorption of ZnO_n appears at the investigated temperature and frequency as well as thickness. The minimum reflection loss value reaches -44 dB, which is superior to the previous reports. These findings provide a novel and feasible strategy to tune microwave absorption towards a structural absorber at elevated temperature.