| Continuous SiC fibre reinforced ceramic matrix composites have the potential to be the structural and electromagnetic?EM?wave absorbing composite materials used at high temperature.However,the previous studies shown that the dielectric constant of SiC fibres was too high or excessive low,leading to the poor EM wave absorbing performance;the flexural strength of as-prepared ceramic matrix composites was less than 300 MPa;the focus of the EM wave absorbing properties and the optimization of as-prepared composites was the theoretical calculation.Aiming to optimize the EM wave absorbing properties,strength and toughness of the ceramic matrix composites,the SiC fibre reinforced Si3N4 ceramic matrix composites?SiCf/Si3N4?were developed innovatively.The influences of the microstructure and surface modification on the EM wave absorbing SiC fibre used in SiCf/Si3N4 were studied in detail.The mechanical properties,thermal properties,EM wave absorbing properties and the formation mechanism of SiCf/Si3N4 were also studies.The main research contents and results are as follows:?1?The relationship between the microstructure near stoichiometric SiC fibres and SiC fibres with low oxygen and high carbon content and their EM properties were studied.Meanwhile,the influence of fibres surface modification on their performance was studies.Results show that the near stoichiometric SiC fibre is composed of surface carbon layer with the thickness of 5 nm,SiC nanocrystals,a small amount of free carbon and amorphous phase.The unique microstructure makes the near stoichiometric SiC fibres have the outstanding EM wave absorbing properties and the near stoichiometric SiC fibres can be used as EM absorbing reinforced fibre of SiCf/Si3N4.The surface glue of near stoichiometric SiC fibres carbonized by vacuum heat treatment can effectively improve their EM wave absorbing properties and the effective absorbing bandwidth of near stoichiometric SiC fibres increase from 2.6 GHz to 3.8 GHz;SiC fibres with low oxygen and high carbon content have high C/Si ratio?1.41?,leading to the characteristics of high dielectric constant and loss because of the existence of a large number of free carbon.Then,SiC fibres with low oxygen and high carbon content could be used as the dielectric loss layer of a multilayer EM wave absorbing material.?2?The influences of Si3N4 content on the mechanical properties,thermal properties and EM wave absorbing properties of near stoichiometric SiC fibres reinforced Si3N4 ceramic matrix composites?2D SiCf/Si3N4?were studied in detail.The results show that the flexural strength and fracture toughness of 2D SiCf/Si3N4 increase up to 380±34 MPa and 12.9±1.2MPa·m1/2 with the increase of Si3N4 matrix content because of the enhanced synergies between the fibre bundle/beam.The thermal expansion coefficient of amorphous Si3N4 matrix was about 3.55×10-6 K-1,lowering than that of SiC matrix(4.6×10-6 K-1).Therefore,the engineering line thermal expansion coefficient of 2D SiCf/Si3N4 was lower than 2D SiCf/SiC?about 13%?;2D SiCf/Si3N4 has good EM wave absorption properties,whose electromagnetic reflection coefficient at the X and Ku band are less than 5 dB and 4 dB?>68%absorption and>60%absorption?,respectively.And the near stoichiometric SiC fibres are the main absorbing components of 2D SiCf/Si3N4.?3?The optimization design methods of the EM wave absorbing properties of 2D SiCf/Si3N4 were studied.The three layers of Jaumann absorption type 2D SiCf/Si3N4?2D H-SiCf/Si3N4?were developed according to the optimization.The change rule and evolution mechanism of the mechanical and EM wave absorbing properties were studied before and after the optimization.The results show that the two type absorbers of impedance matching layer and three layer Jaumann absorber have the characteristics of strong absorption and multiple reflection in X and Ku band,whose reflection coefficient could be lower than to-8dB and-10 dB,respectively;in X band,the reflection coefficient of 2D H-SiCf/Si3N4 decrease to-7.7 dB?>83%absorption?from-5 dB of 2D SiCf/Si3N4;in Ku band,the reflection coefficient of 2D H-SiCf/Si3N4 decrease to-5.7 dB?>73%absorption?from-4 dB of 2D SiCf/Si3N4;although the flexural strength of 2D H-SiCf/Si3N4 is lower than 2D SiCf/Si3N4about 6%because of the introduction of the dielectric loss type SiC fibre with lower strength and modulus,the mechanical properties of 2D H-SiCf/Si3N4 are excellent at room temperature.?4?The evolution law and mechanism of the mechanical and EM wave absorbing properties of SiCf/Si3N4 at high temperature were investigated.Results show that at 800?,the tensile/flexural strength and fracture toughness of 2D SiCf/Si3N4 and H-SiCf/Si3N4 have fallen slightly than that at room temperature,and the descend range of tensile/flexural strength are about 80%because the length and amount of pull-out fibres decreased due to enhanced interfacial shear strength at high temperature.The real part of permittivity of 2D SiCf/Si3N4and H-SiCf/Si3N4 increase by 20%and 11%with temperature rise to 800oC at 10 GHz,respectively.The imaginary part of permittivity of 2D SiCf/Si3N4 and H-SiCf/Si3N4 increase by 109%and 53%with temperature rise to 800oC at 10 GHz,respectively.The main reason why the permittivity increase is that the polarization/loss capacity and conductivity are enhanced at higher temperature.The reflection coefficient of 2D H-SiCf/Si3N4 increases about7.3%from room temperature to 800 oC,which was lower than that of the 2 D SiCf/Si3N4?30%?.Therefore,the design of SiCf/Si3N4 with multilayered absorbing material is expected to achieve the combination of EM wave absorbing properties at room and high temperature. |
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