| Electromagnetic microwave (denoted as EM) absorber is a kind of functional materials that can absorb EM (usually refer to the electromagnetic wave in the frequency range of 0.3-300 GHz) efficiently and convert EM energy into thermal energy, thus realized the attenuation of the microwave. It has important application in the field of electromagnetic interference and shielding as well as camouflage coating on the military aircraft. Modern microwave absorber has the properties of wide absorption band, small density and thickness, strong in absorption, high compaticity, high weather resistance and high temperature stable. Recently, great attention has been paid to the study of using multifunctional composite materials as microwave absorber considering the combination of different composites may take respective advantages, since one-component material cannot achieve all those properties.High temperature stable microwave absorber is a kind of material of great importance, which can preserve high microwave absorption performance under high temperature. Previous researches on high temperature stable microwave absorber were scarce due to the difficulties in finding merits of both strong absorption capability and heat resistance from one individual material. Most of the traditional high-temperature stable microwave absorbers are the dielectric ceramic materials that have heat resistance property. The ferrites though have the advantages of strong absorption and wide frequency band over those dielectric ceramics, still have difficulties in the application of high temperature stable absorbers, because of the characteristic of easily being oxidized under high temperature and low Curie temperature. The development of sol-gel method enables us to prepare ferrite@ceramic composite structures to solve these problems and improve many weaknesses of the traditional high temperature stable microwave absorbers.(1) In this research, we designed yolk/shell structural Fe3O4@ZrO2 material to study the microwave absorption property of the ferrite@ceramic composite structures. FeaO4 microspheres with tailored morphology were prepared via hydrothermal method. SiO2 layer was coated outside the Fe3O4 microspheres as template by using Stober method. Then ZrO2 layer was deposited on the SiO2 layer by using a modified polymer surfactant assisted sol-gel method. SiO2 layer was removed by treating the particles with base solution afterwards, and the obtained product was calcinated under 500℃ to obtain ZrO2 shells with good crystallinity.(2) Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), elemental mapping and line scanning, scanning transmission electron microscopy (STEM), and selected area electron diffraction (SAED) were used to study the structure, crystallinity and morphology of the product. It was revealed that the particle has a ZrO2 shell of 500 nm in diameter, 25-30 nm in thickness and a Fe3O4 core of 300 nm in diameter. XRD and SAED proved the purity and good crystallinity of the product.(3) This material shows great potential in acting as a high temperature stable microwave absorber. Even the measured temperature reach 500℃, the material still preserved over 98% of the reflection loss (RL) value compared with the room temperature property. In contrast, the microwave absorption property of Fe3O4@SiO2 decreases as the temperature rising, and the absorption frequency move towards the high frequency. These results proved that the ZrO2 layer played the role in protecting the microwave absorption property of the internal cores by restricting the oxidation of Fe3O4. |
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