Electromagnetic Wave Absorption Properties of Nanoscaled ZnO

Abstract

Microwave absorbing material (MAM) is a kind of functional material that can absorb electromagnetic wave effectively and convert electromagnetic energy into heat or make electromagnetic wave disappear by interference (Kimura et al., 2007). MAM is currently gaining much attention in the field of civil and military applications. For example, the materials have been widely applied to minimize the reflection of microwave darkrooms, airplanes, steamboats, tanks and so on (Zou et al., 2008). Generally, the electromagnetic absorbing performance of any MAM is linked to its intrinsic electromagnetic properties (i.e. conductivity, complex permittivity and permeability) as well as to extrinsic properties such as the thickness and working frequencies. It is clear that the microwave absorption properties can be improved by changing the above parameters. However, the traditional MAMs or novel nanomaterials still have some disadvantages such as high density, narrow band, and low absorptivity (Zou et al., 2006). Therefore, demands for developing more economical MAMs with “low density, wide band, thin thickness, and high absorptivity” are ever increasing. Wurtzite-structured ZnO is of great importance for its versatile applications in optoelectronics, piezoelectricity, electromagnetic wave absorption, laser, acous-optical divices, sensors, and so on (Wang et al., 2007). One-dimensional nanostructures of ZnO, such as nanowires, nanobelts, and nanotetrapods, have been a hot research topic in nanotechnology for their unique properties and potential applications. Moreover, several types of three-dimensional ZnO nanostructures have been synthesized. Because of the high surface/volume ratio and integrated platform, three-dimensional oxide networks have been demonstrated for building ultrasensitive and highly selective gas sensors and optoelectronics applications (Zhu et al., 2007). It is worth mentioning that the ZnO nanostructures have shown great attraction for microwave radiation absorption and shielding material in the high-frequency range due to their many unique chemical and physical properties (Zhuo et al., 2008). Some research works focused on nanoscaled ZnO as a vivid microwave absorption material due to their light weight, high surface/volume ratio, and semiconductive and piezoelectric properties (Wang & Song, 2008). On the other hand, carbon nanotubes (CNTs) as conductive filler have been widely studied in MAMs due to the unique spiral and tubular structure since the discovery of CNTs by Iijima in 1991 (Iijima, 1991). CNTs/polymer composites exhibit a strong microwave absorption in the frequency range of 2-18 GHz and have the potential application as broad