Structure Design And Performance Optimization Of Broadband Wave-absorbing Composites

With the development of radar detection technology,it is urgent for the omni-directional stealth materials,including lateral,forward and tail direction of aircrafts.The polymer matrix composites（PMC）electromagnetic（EM）wave absorbing materials designed for lateral and forward cryogenic components such as wings etc,can achieve relatively good EM wave absorption,which still needs to be improved.Ceramic matrix composites（CMC）designed for high-temperature components such as the engine tail nozzle etc,require efficient EM absorbing and good mechanical properties at high temperatures,and the SiC_f/Si₃N₄ composites fabricated by the chemical vapor infiltration（CVI）method has become the most important choice for high temperature absorbing materials.Though the SiC_f/Si₃N₄ composite have excellent mechanical properties,it can only achieve reflection less than-6 dB in 8～18 GHz,which needs to improved.Therefore,with the aim to realize reflection less than-10 dB in 8～18 GHz for SiC_f/Si₃N₄composites,the meso-structure design for absorbing materials is creatively implemented with the microstructure design,which would provide new thoughts for designing broadband and high temperature absorbing materials.Three types of broadband structures,including the double layer impedance matching type,the period stepped type and the sandwich structure,are designed to optimize the absorbing performances,respectively,and the correlations between performances and the EM as well as structural parameters are specified.The correlations between the absorbing properties and permittivities as well as structure parameters are specified.During each structural design,the absorbing properties of PMC would be first optimized and the design principles would be specified for designing absorbing materials with broadband absorbing performances.The SiC_f/Si₃N₄ with the same structures would be designed and fabricated,and the absorbing performances would be investigated to specify the mechanism of absorbing materials.（1）According to the transmission line theory and metal backplate model,the target values of dielectric parameters for the single layer and double layer materials to achieve full frequency effective absorption from 8 to 18 GHz are specified,and the results indicate that the single layer absorbing materials are difficult to meet the design requirements of permittivities and double layer impedance matching structure can realize the broadband design goal at a smaller thickness.Based on the existing four different SiC fibers:the type A（high resistivity of 10⁵Ω·cm）,the type B（moderate resistivity of 86.2Ω·cm）,the type C（medium low resistivity of 7.3Ω·cm）,the type D（low resistivity of 0.46Ω·cm）,the A and C SiC fibers are chosen as the braided performs and the epoxy（EP）is chosen as the matrix,and the double layer impedance matching C/A-SiC_f/EP was fabricated,which realized RL less than-10 dB in 8～18 GHz.（2）Based on the design principles of double layer absorbing composite,the double layer impedance matching C/A-SiC_f/Si₃N₄ was prepared.The design of SiC_f/Si₃N₄ from single-layer to multi-layer greatly improves its broadband absorption performance.The measured reflection loss curves are decreased from less than-2.5 dB to less than–8 dB from the single layer to double layer design.With temperature increasing to 600℃,the average change rate of the RL is less than 3.4%,and the insensitive high temperature absorption performance should be attributed to the temperature insensitive permittivity,which is induced by the compensating effect of the decreased interfacial polarization loss to the increased conductivity loss with the increasing temperature.（3）Based on the relations between the geometric structure parameters and material dielectric parameters and the broadband absorption performance of the period stepped composite,the principles for designing the period stepped absorbing materials are specified:the values of the two structure factors approximately obey the rule of the Golden Section;the intrinsic materials used in structural design should have medium dielectric and medium loss（typical absorbing material characteristics）.The prepared period stepped Fe/EP shows extremely broadband absorbing property with reflection loss less than-10 dB in the range of2.64～40 GHz.The effective permittivity would be minished by the period stepped structure,which would improve the impedance matching in ultra-broad band.（4）Based on the design principles of period stepped absorbing composite,the B-SiC_f/Si₃N₄with medium dielectric constant and loss could meet the requirement.The prepared period steppedB-SiC_f/Si₃N₄ achieves reflection loss-15.3 dB and-14.8 dB at 8 GHz and 18GHz frequencies,respectively,which realizes the synergistic strong absorption of EM waves at relatively low and high frequencies.The EM wave loss is the combing result of the coupling effect of unit cells in mesoscopic scale,the multiple reflection effect between pores in microstructures,and the polarization loss as well as the conductivity loss of SiC fibers in nanoscale.With the temperature increasing from room temperature to 500℃,the reflection loss at 8 GHz and 18 GHz frequencies is always less than-10 dB.The period stepped structure could effectively restrain the effective permittivity increasing with temperature,which would result in the insensitive high temperature absorption performance.（5）The design principles of sandwich metamaterial absorbing composite is proposed:the loss layer should be low conductive phase and the dielectric layer is a phase with low dielectric constant and low loss.With the aim to verify the effect of the sandwich structure on the absorbing performances,the sandwich metamaterial ITO/PVC was designed and prepared by using the indium tin oxide（ITO）thin film and polyvinyl chloride（PVC）as the conductive loss layer and dielectric layer,respectively.The designed sandwich structure can significantly reduce the dependence of material parameters and geometric structure parameters on the absorbing performances.The sandwich metamaterial absorbing composite shows excellent impedance matching characteristics,which allow large number of EM wave enter into the material,and the period loss layer enhances the coupling effect of EM wave at their edges,resulting in the RL less than-10 dB in 8～18 GHz.（6）Based on the design principles for sandwich absorbing structures,the sandwich SiC_f/Si₃N₄ was prepared by using the D-SiC_f/Si₃N₄ with low conductivity as the lossy layer and A-SiC_f/Si₃N₄ with high conductivity as the dielectric layer,respectively.The results show that the discontinuous mesoscopic design on the loss layer can improve the impedance matching and enlarge the surface current density,and the current conducting in the D-SiC fibers can lead to large ohmic loss,and the nanocrystalline in D-SiC fibers can attenuate the EM wave via polarization loss.These multi-scale loss effect would result in the reflection loss less that of-11.3 dB in 8～18 GHz at room temperature.With the temperature increasing to 600℃,the average of the reflection loss of the SiC_f/Si₃N₄ composite remains unchanged,which also shows insensitive high temperature properties.