title: Fabrication and Electromagnetic Wave Absorption Performance of SiC-based Composites
Creators: name: Wang, Jinyan
Publisher: name: University of Liverpool Repository
Year: 2024

description: As is known to all, electromagnetic waves (EMW) are everywhere. Different bands of EMW have different applications. Nowadays, the rapid development of wireless technology has put forward higher requirements for equipment. To meet the needs, the power density of the electronic equipment is increasing. As a result, EMW pollution is ever-increasing. EMW pollution has become the fourth source of pollution after air, water, and noise pollution. To decrease the radiation and minimise harmful effects, researchers have increasingly engaged in finding various EMW absorption materials. Because of the complexity and limitation of practical surroundings, the EMW absorption materials should possess the characteristics of being thinner, lighter, broader, and stronger and having harsh environmental resistance. Silicon carbide (SiC) is a dielectric absorber and can be applied in harsh working environments due to its high-temperature oxidation resistance and good chemical inertness. However, the EMW absorption performance of normal SiC is poor. Morphology design and compounding with other materials are effective ways to improve the EMW absorption performance of SiC. In this project, different morphological SiC were prepared through carbon or silica templates and combined with dielectric MnO2 or magnetic absorber Ni to form composites. This project systematically studied the synthesised mechanism, the influence of morphology, concentration, and size on the absorption performance and analysed the loss mechanism of each material, achieving the products with high absorption performance. Firstly, the monodisperse carbon spheres were obtained through a hydrothermal reaction of sucrose and then mixed with waste glass powder or commercial silicon in 2 different proportions to prepare different morphological SiC. When the weight ratio of the carbon source to the silicon source is 1:1, there is sufficient SiO vapour, and the diffusion rate is higher than the deposition rate, the porous fluffy spheres can be formed. When the weight ratio is 4:1, the deposition rate is significantly higher than the diffusion rate, and nanofibrous SiC can be obtained. Due to the high specific surface area, high conductivity and uniform monodisperse spheres of porous fluffy SiC, the products have premium performance and the effective absorption bandwidth (EAB) can reach 4.81 GHz at 1.6 mm. Compared with the sample prepared through commercial silicon, the performance of SiC obtained through waste glass is higher, proving the waste glass can fully replace commercial silicon to prepare SiC absorbers, which can help recycle the waste glass and protect the environment. Comparing the porous fluffy SiC with other morphological SiC reported before, we found that hollow SiC has better EMW absorption performance. Then, SiO2 spheres are used as sacrificial templates and covered with resorcinol formaldehyde resin to prepare hollow SiC (H-SiC). However, H-SiC is so light that the highest filling ratio is only 20 %, and the EAB is 2.54 GHz at 10 mm. Next, the abundant MnO2 is covered with H-SiC through a hydrothermal strategy to prepare uniform core–shell H-SiC@MnO2 composites to strengthen the absorption performance. The influence of shell thicknesses and core sizes on absorption performance was systematically studied. The results show that the EAB of H-SiC@MnO2 can reach 5.71 GHz at only 1.8 mm with a core size of 360 nm and a shell thickness of 90 nm, which improved by 225 % compared to pure H-SiC. Both SiC and MnO2 are dielectric absorbers. In order to take full advantage of the hollow spherical structure and synergism of dielectric loss and magnetic loss, we prepared H-SiC@Ni composites with boosted absorption performance. The influence 3 of the concentration of raw materials and reaction temperature on absorption performance was studied. The results indicate that when the volume of Na2S2O3 solution is 20 ml, the deposition temperature is 600 ℃, and the reduction temperature is 750 ℃, the EAB of H-SiC@Ni can reach 8.8 GHz at 2.6 mm, which is a promising candidate for high absorption absorber.