Research On Micro-structure Adjustment And Electromagnetic Wave Absorption Properties Of SiC Porous Materials

The development of 5G communication technology,Ultra-High Frequency（UHF）radar detection and precision guidance technologies has been pushing electromagnetic（EM）wave absorbing materials to face urgent demand in the fields of national defense stealth and anti-EM interference.In order to meet the strict requirements of practical application conditions,such as extreme high temperature environment etc.,EM absorbing materials are demanded to be thin,lightweight and have broadband yet strong absorption capability.The high temperature resistance of EM absorbing materials is also a key index.Traditional magnetic absorbing materials are limited by the disadvantages of high weight density,poor thermal stability.It is difficult to serve for a long time in harsh conditions for magnetic EM absorber.In this thesis,the design and regulation of porous SiC with different morphology were studied using biomass template method,pore-making method,in-situ growth and freeze casting.The formation processes of porous SiC were studied.The effects of porous morphology and microstructure on EM parameters and absorbing properties of materials at room temperature and high temperature were systematically discussed.The main research contents are summarized as follows:Firstly,biomass-derived porous SiC with defect were synthesized through drying,carbonization and carbothermal reduction using plant cellulose and bacterial cellulose.The biomass template with special pore structure is easy to construct a large number of defect interface.The formation process of SiC with different morphologies in porous structure prepared by biomass template were analyzed in the perspective of thermodynamics and kinetics.The minimum reflection loss（RL）of the porous SiC derived from plant cellulose prepared at 1600°C is-22 d B,and its effective absorption band covers from 12.8 GHz to18 GHz at a thickness of 2.2 mm.The effective absorption band can cover from 9.7 GHz to 12.4 GHz with a thickness of3.1 mm in X-band.The minimum RL of the porous SiC derived from bacterial cellulose prepared at 1500°C is-23.5 d B,and its effective absorption band covers the frequency range of 11.0 GHz-18.0 GHz with 2.1 mm.The effective absorption band can cover from 8.2 GHz to 12.4 GHz at 3.2mm in X-band.The content,number and density of defects of the generated SiC can be regulate in different biomass templates by changing the reaction temperature,thereby inducing the differential dielectric constant and EM wave absorption properties.Secondly,in order to further improve the high temperature absorbing properties of porous SiC materials,the microstructure design of porous SiC with micro-nano hierarchical morphology was carried out inspired by the research idea that introducing SiC nanowires as the second component in the biomass template method can enhance the EM absorption performance.Benefiting from the abundant micro-nano hierarchical morphology,porous SiC possess excellent X-band absorbing properties at room temperature and high temperature.For carbon nanotube（CNT）-derived binary porous SiC,broadband effective absorption can be achieved in ranging from room temperature to 600°C with effective absorption band of 8.2 GHz-11 GHz at a fixed thickness.At 600°C,the optimal reflection loss at 9.8 GHz is–34 d B.The second part induced the in situ growth of the secondary SiC component inside the SiC porous skeleton by magnetic catalyst.Molten catalyst is a prerequisite for the growth of the secondary SiC phase.At a fixed thickness of 2.4 mm,broadband effective absorption can be achieved 10.2 GHz～12.4 GHz in ranging from room temperature to 600°C.At 600°C,the optimal reflection loss at 12.1 GHz is-51 d B.In addition,the compressive strength of binary SiC porous material were significantly improved to 15-fold by constructing of micro-nano hierarchical morphology.The method of constructing SiC porous structure with micro-nano hierarchical morphology provides reference for the preparation of lightweight multifunctional EM absorbing materials.The SiC aerogel study was carried out to realize the preparation of ultralight structures.Assembling low dimensional EM nanomaterial into porous structure is an effective way to optimize EM wave absorption properties at room temperature and high temperature.We proposed fabricating all-ceramic porous SiC with directional structure via freeze casting of graphene oxide（GO）doped SiC nanowires and carbothermal reduction reaction to convert GO into the secondary phase SiC for enhanced EM attenuation performance.The SiC aerogel has an ultralight density of0.2 g·cm^-3 and anisotropic EM properties.It is worth noting that the permittivity of the EM wave incident parallel to the layered structure of SiC aerogel are larger than that of the transverse incident.In the parallel direction,SiC aerogel demonstrates dramatically enhanced EM wave attenuation properties at 25℃,including an ultralow RL_min of-58 d B at 3.2 mm.SiC aerogel exhibits good absorbing property at400℃,where the RL peak is close to-64.4 d B at 2.8 mm.In addition,the SiC aerogel can reveal a whole X-band effective attenuation at a fixed thickness of 3.3 mm in the investigated temperature range from room temperature to 400℃.In addition,the thermal conductivity also presents anisotropy in different directions,and the lowest thermal conductivity is only 0.049 W·m^-1·K^-1.Based on the above microstructural design method for directional assembly of nanomaterials,the SiC aerogel can serve as a new design strategy for excellent lightweight all-ceramic microwave attenuation material for high temperature applications.