Study On Design And Performance Of The Microwave Absoring Coatings Based On Water Varnish

With the rapid development of electromagnetic (EM) wave, the negative impacts of it emerge gradually. Therefore, the industry of EM absorbing coatings is developed quickly. With common absorbing coatings, it is hard to reach the research goal of thin thickness, light mass, wide range and strong absorption. Structure design is an effective way to achieve the technical requirements by providing a channel for EM waves. Therefore, traditional microwave absorbers and designed discrete-structure microwave absorbers are studied from the structure design angle.In this dissertation, the morphology and electromagnetic characteristics of carbonyl iron powder (CIP) and Fe-Si-Al (FSA) are studied with the scanning electron microscopy (SEM), the Vibrating Sample Magnetometer (VSM) and the vector network analyzer (VNA), and the microwave absorbing (MA) mechanism of these absorbents are analyzed as well. Water Varnish (WV) based microwave absorbing coatings are manufactured with aluminum (Al) sheet as the base-plate. Therefore, the effects of absorbent categories, absorbent contents and thickness of coatings are studied meticulously. Meanwhile, considering the structural and design issues, the conventional microwave absorber (CMA) is cut and the mechanism of discrete-structure is discussed. Besides, the influence factors of discrete-structure, including absorbent content, thickness of coatings and the size of discrete-structure unit are studied. The results show that:CIP and FSA are micrometer-scale magnetic loss absorbents belonging to soft magnetic materials. In the range of 2-18GHz, the microwave is attenuated by the two absorbents mainly via natural resonance and eddy-current loss. Al-based coatings with CIP and FSA are as the absorbent respectively, the reflection loss (RL) peaks of coatings shift towards low-frequency regions with the increasing of the absorbent content or coating thickness. With the increasing of the absorbent content, the absorption ability of the coatings increases, and when the absorbent content exceeds a certain threshold, the MA properties will become poorer because of the impedance mismatching. For thickness, there is a similar changing trend. The increment of the absorption ability will decrease continuously with the thickness increase.The MA properties of discrete-structure absorber (DMA) are much better than that of CMA. After the discrete-structure processing (5mmx5mm), the minimum RL value of CIP/WV CMA with a mass ratio of 0.5:1 and thickness of 1.5mm are improved from-12.5dB to-31.2dB and the effective RL (RL<-10dB) band is enlarged to 4GHz.The performance of discrete-structure is significantly influenced by the absorbent content and the size of the discrete-structure unit. The higher the absorbent, the more distinct the effect of discrete-structure is. And as the absorbent content increases, the size of discrete-structure unit is required to be smaller. After processing the discrete-structure with the unit size of 2mmx2mm, the minimum RL value of CIP/WV CMA with a mass ratio of 1:1 and thickness of 2.0mm are improved from-5.2dB to-17.6dB and the effective RL (RL<-10dB) band is enlarged to 7.7GHz.In this dissertation, the normalized impedance of the absorbing coatings is calculated to study the influence of impedance. For the MA coatings with higher absorbent content, the calculated wave impedances from electromagnetic (EM) parameters of absorbing materials and transmission line theory maintain consistency. For FSA/WV CMA with a mass ratio of 0.4:1 and thickness of 1.5mm, the normalized impedance calculated from electromagnetic (EM) parameters of absorbing materials and the transmission line theory is 0.17 and 0.27 respectively. The normalized impedance of DMA calculated by the transmission is 0.41, the impedance matching performance is improved significantly.