Microstructure And Electromagnetic Wave Absorption Properties Of Si₃N₄/SiC Composites Based On SLS/PIP Process

With the rapid development of omnidirectional radar detection system,the exposure symptoms of high-temperature ceramic components such as the engine tail nozzle of high-speed aircraft is becoming obvious.Designing and preparing complex structural ceramic matrix absorbing materials that can serve under harsh conditions such as high temperature and strong corrosion is an urgent problem to be solved.However,the ceramic matrix absorbing materials reported in the studies are mainly prepared by traditional processes,which cannot achieve the preparation of complex structure,and the absorbing performance of which cannot meet the requirements of broadband effective absorption in 8～18 GHz.The electromagnetic absorbing performance still needs to be improved.Based on it,Polymer derived Si C（PDCs-Si C）/Si₃N₄ ceramic matrix microwave absorbing materials were prepared by combining selective laser sintering（SLS）3D molding and precursor infiltration pyrolysis（PIP）process in this work,and the dielectric properties and absorbing properties were analyzed following three aspects:process optimization,micro-material design and macro-structure design,which aims to achieve the goal that the electromagnetic wave reflection loss of PDCs-Si C/Si3N4 composites less than–10 d B,further provide a new idea for the design and preparation of high temperature absorbing materials.The main work and conclusions are as follows:PDCs-Si C/Si₃N₄ composites were prepared by SLS/PIP process.The effects and mechanisms of annealing treatment,PIP cycle and sol modification on the microstructure,dielectric and wave absorption properties of the composites were investigated.The results showed that dielectric properties of composites are mainly controlled by the conductivity loss,dipole polarization loss and interfacial polarization loss caused by free carbon,β-Si C and Si C_w.The dielectric constant of the prepared materials tunable between 10 and 40 through adjusting annealing temperature and PIP cycle numbers.On the basic of that,The free carbon content and the continuity of the conductive phase were changed via sol infiltration,and good impedance matching was obtained.The effects of the three sol-modified treatments on the dielectric constants followed below order:Al（H₂PO₄）₃>Ti O₂>Al₂O₃.At a annealing temperature of1600°C and 4 times PIP cycles,the composites modified by Al（H₂PO₄）₃ shows the minimum reflection loss of-47.61 d B,and the effective absorption bandwidth up to3.52 GHz.The heterogeneous element-doped PDCs-Si C/Si₃N₄ composites were prepared by adding Al,Fe,Ti and Zr elements,and the effects of four elements doping on the microstructure,dielectric and wave absorption properties of the composites were investigated.It was found that Al and Fe doping caused the increase of hole traps,residual free carbon content（or graphitization of free carbon）and a large number of heterogeneous interfaces,which leading to the increase of dielectric loss;Ti and Zr doping caused the increase of residual free carbon and heterogeneous interfaces,which was benefit for the improvement of dielectric loss.However,in all cases,the strong resonance peaks in dielectric constant caused by multiple dielectric resonances lead to the large dielectric losses,which reduce the impedance matching characteristics of the material with free space and decrease the wave absorption performance.C_sf,MWCNTs and Si C_w were doped into the PDCs-Si C/Si₃N₄ composites,and their microstructure,dielectric and absorbing properties were analyzed.It is found that C_sf,MWCNTs and Si C_w have good conductivity loss or polarization loss ability,which is beneficial to improve the dielectric loss ability of doped composites.However,the higher content of C_sf and Si C_w with larger radial size will increase the pores and defects in the doped composite body,inducing to the high PCS content,and reduce the absorbing properties of the material.The effective absorption bandwidth of the composites with a content of 2 vol.%MWCNTs increased from 2.72 GHz to 3.2 GHz,and that with 4 vol.%Si C_w reaches 3.04 GHz.The PDCs-Si C/Si₃N₄ composites with different types of triple period minimal surface（TPMS）structure were designed to optimize the geometric parameters of the composites.It is found that the electromagnetic absorption properties of G-type,P-type,F-RD-type and I-WP-type TPMS structural composites are significantly improved compared with bulk materials,and the G-type structure has better absorbing properties.The backplane and basic unit combination and double-layer base unit combination can effectively improve the absorbing performance of the composite material,and double-layer base unit combination is better.When the side lengths of the upper and lower base units are 5πmm and 6πmm,respectively,and the wall thickness is 2 mm,the reflection loss of the composite material below-10 d B in the frequency range of 6.9～18 GHz,and the effective absorption bandwidth reaches 11.1GHz.The simulation results are confirmed to be consistent with tested through experimental verification.In this study,the target of reflection loss less than-10 d B and electromagnetic wave absorption efficiency more than 90%in the full frequency range of 8-18 GHz was finally achieved through process optimization,material composition and macroscopic structure design of 3D printed PDCs-Si C/Si₃N₄ composites,which met the electromagnetic stealth requirements of high temperature resistant absorbing materials in X and Ku bands.