Progress on microwave absorption materials with core-shell structure

Abstract

Microwave absorber is a kind of functional material, which can absorb electromagnetic (EM) wave effectively and then convert EM energy into thermal energy or make EM wave dissipate by interference. The researches of microwave absorbers are important to satisfy the need of electromagnetic interference in daily life and industrial production, also meet the application requirements of stealth technology in military. Nowadays, the rapid increase in the use of telecommunications equipment, digital systems, and fast processors result in the increasing demand for the development of novel high-efficient microwave absorbers with light weight, thin thickness, wide absorption frequency, and strong absorption characteristics. The structures of microwave absorber are found closely related to the absorption properties. In recent years, many research institutions including our research group, found that the core-shell structure has good absorption performance. The core-shell structured nanocomposites, which combined functionalities of cores and shells, show great potential for significant effect on broadening the absorption spectrum and increasing the absorption intensity because of the complex impedance material matching, interface polarization and multi structure interface scattering. For example, the combination of magnetic materials (e.g., Fe3O4, Co, Ni) and dielectric materials (e.g., TiO2, SnO2, carbon nanotubes) in one entity may take advantages of both the magnetic and dielectric materials, which then can be used as high-performance microwave absorbers. It is well known that Fe3O4, which have been widely used as microwave absorbers because of low cost and strong absorption characteristics usually suffer from ease of oxidation at high-tempera- ture and relatively narrow absorption frequency bandwidth at applied frequency range. While, the magnetic nanoparticles encapsulated within carbon-nanotube composites and Fe3O4/TiO2 core/shell nanotubes were found with lower reflection loss and wider absorption frequency range than single-component absorbers. Meanwhile, remarkable progress has been made in the study of the absorbing mechanism of core-shell structure absorbing materials, which is a great contribution to the design of new structures to meet the new demands of absorbing materials. For example, the particle shapes and grain sizes of magnetic absorbents are of importance for high performance microwave absorbing materials, which are well known. However, research into the relationships among microstructure, electromagnetic property, and microwave absorption performance is rarely reported, especially from the influence of microcosmic magnetism distribution on microwave absorption performance is still lacking previously. While, based on the micro-magnetic investigation via electron holography and Lorentz microscopy, the micromagnetic analysis of the absorber provides a series of direct evidences regarding the effect of grain size on the microcosmic magnetic property, which was important for the further comprehension of the microwave absorption mechanism and new insight into the microstructure design strategy. Other researches, which focus on the influence of cores or shells to absorption performance, are also meaningful to absorption mechanism explanation. The development of science and technology has never stopped the demand for high performance microwave absorbing materials. It is still a difficult but meaningful task for absorbers practical applications to design microwave absorption materials with minimal coating thickness, even light weight, ultra-wide bandwidth, stronger absorption and higher heat-resistance characteristics