Study On Preparation And Performance Of Multiple Absorbing Materials Based On Silicon Carbide Nanowires

Silicon carbide nanowires(SiC nanowires,SiC_NWs)are of particular interest due to their high specific surface area,abundant stacking layer dislocations,twinned interfaces and tunable conductivity,and the formation of conducting networks in the matrix with superior electromagnetic absorption properties to SiC bulk and particle forms.However,the dielectric constant of a single SiC phase is lower than that of carbon and ferromagnetic absorbing materials,and the usual way of introducing defects to increase the dielectric constant of SiC is difficult to approach a suitable impedance match,resulting in only a narrow effective absorption bandwidth(EAB),and the absorption strength and effective bandwidth of absorbers are difficult to increase significantly.The absorption strength and effective bandwidth of the absorber can not be increased significantly,which can not meet the requirements of high-performance absorbers.To solve the above problems,we can design and optimize the nanowire structure and introduce multiphase such as dielectric material and magnetic material.In this thesis,SiC_NWsare used as the substrate to build a composite material with various structures and multiple phases to prepare a high-performance electromagnetic wave absorber with"thin thickness,light mass,effective absorption bandwidth and strong electromagnetic absorption".The effect of the structure of each component on the absorbers'electromagnetic wave absorption performance was systematically investigated,the electromagnetic wave absorption mechanism of different absorbers was analyzed,and the application of absorbers in the field of electromagnetic shielding was investigated.The method of preparation and design of SiC_NWs-based multi-faceted composite absorbers were obtained.Polycarbosilane(PCS)is used as the raw material of SiC_NWs precursor,and the flexible SiC_NWs membrane is prepared by electrospinning combined with high-temperature pyrolysis.The 3D network structure constructed by SiC_NWs membrane and the conductive network of SiC_NWs can cause multiple reflections of electromagnetic waves,rich interfacial polarization,and dielectric losses such as dipole polarization can simultaneously promote the attenuation of electromagnetic waves,thus exhibiting excellent wave absorption performance.The minimum reflection loss(RLmin)is-41d B at 11.2 GHz and EAB is 5 GHz when the SiC_NWsmembrane is filled with only 10 wt%and the thickness is 2.0 mm.The flexible SiC_NWs membrane was prepared by electrospinning and carbon thermal reduction with two types of growth mechanisms:SiC_NWs were formed by direct in situ carbon thermal reduction of the precursor nanowires,and single-crystal SiC_NWs were grown by the vapor phase of the V-S mechanism.SiC_NWs membrane has excellent flexibility with a maximum bending angle of 160°.To address the problem of poor dielectric loss of single SiC_NWs,porous SiC/Si₃N₄nanowires(SiC/Si₃N₄ NWs)were prepared by electrospinning and nitrogen heat treatment.The 3D mesh structure composed of SiC/Si₃N₄ NWs with large specific surface area and abundant mesopores extends the attenuation path of electromagnetic waves and effectively improves the electromagnetic wave absorption performance through the interfacial synergy between carbon-rich SiC and Si₃N₄ nanograins.When the filling amount is 15 wt%,the EAB is as high as 7.12 GHz(10.88-18 GHz)covering the whole Ku-band with an RLmin of-48 d B at a coating thickness of 2.1 mm only.The strong absorption and wide EAB are achieved,which has important applications in the field of electromagnetic wave absorption in C,X and Ku-bands.To meet the demand of wave absorber lightweight,reduced graphene oxide(RGO)was introduced to prepare high specific surface area,laminar porous SiC_NWs@RGO composites by freeze-drying technique and heat treatment.The excellent structure of ultra-light 3D layered porous SiC_NWs@RGO broadens the attenuation path of electromagnetic waves through multiple reflections and scattering,and the synergistic effect of dielectric and conductivity losses gives the material excellent wave absorption performance.With SiC_NWs@RGO fillings as low as 5 wt%,RLmin reached-47 d B at12.7 GHz when the coating thickness is 2.2 mm and EAB up to 5.78 GHz(12.02-17.8GHz)with a coating thickness of only 2.0 mm,covering almost the entire Ku band.To address the problem of impedance mismatch of SiC_NWs,zeolite imidazole ester skeleton(ZIF-67)with Co-metal organic framework(MOF)structure was introduced to increase the loss mechanism and improve the impedance mismatch at the same time.B By constructing a beaded structure of ZIF-67@SiC_NWs,SiC_NWs and ZIF-67 are compounded to reduce the complex permittivity of SiC_NWs to enhance impedance matching,so that more electromagnetic waves enter the material,and the 3D conductive network composed of nanowires causes micro electric current produces conductivity loss,the defect dipole polarization inside SiC_NWsand the rich heterogeneous interface polarization between SiC_NWs/ZIF-67 produce dielectric loss together to improve its electromagnetic wave absorption performance.The RLmin is-56 d B at 11.7 GHz for a coating thickness of 2.2 mm when the ZIF-67@SiC_NWs is filled with 15 wt%,and the EAB is as high as 5.53 GHz when the coating thickness is only 1.8 mm.Using MOF as a template for preparing porous carbon,ZIF-67/PR/PF/silica precursor nanowires were prepared by electrospinning,followed by high temperature heat treatment under N2 atmosphere for the one-step synthesis of multifaceted composite Co/C nanochains(Co/C_NCs)@SiC/Si₃N_4NWs.A mesh-like structure is formed by interweaving Co/C_NCs and SiC/Si₃N_4NWs,with graphene-like carbon nanochains containing Co particles at the top and 3C-SiC,Si₃N₄ and carbon nanocrystals and a large number of mesopores randomly distributed in the nanowires.Dielectric losses caused by the rich heterogeneous interfacial polarization and dipole polarization between Co,C,SiC and Si₃N₄multiphase,conductive losses and multiple scattering caused by the conductive network of nanowires and nanochains,Co particles provide part of the magnetic loss capability thus improving the impedance matching.With the synergy of multiple loss mechanisms,the composite has excellent electromagnetic wave absorption performance.The coating thickness of Co/C_NCs@SiC/Si₃N_4NWs is only 1.9mm and the RLmin reaches-48 d B at 15 GHz.When the coating thickness is only 2.0mm,the EAB is as high as 5.92 GHz.