Design Of The Microstructure Of SiC Nanowire Reinforced Ceramic Composites And Optimization Of The Electromagnetic Properties

Electromagnetic（EM）wave absorbing materials are widely used in military weapons and equipment,precision instruments and human protection fields.Especially with the development of aerospace equipment and advanced weapons,the high temperature stability of EM absorbing materials is required.SiC nanowire(SiC_nw)is an important dielectric material with excellent high temperature and chemical stability,low density,high specific surface area,rich stacking faults and twin interfaces.At present,there are few studies on the EM properties of SiC_nwreinforced ceramics,and the mechanical properties of the reported EM absorbing SiC_nw/ceramic materials have not been paid enough attention to.There are few studies on the synergistic system of SiC_nw as absorbent and reinforcement,which greatly limits its practical application.The aim of this paper is to prepare high temperature-resistant structural EM absorbing SiC_nw reinforced ceramics.Then,SiC_nw reinforced ceramics were prepared by three methods:SiC_nw/C/Si₃N₄ porous ceramics with self-assembled SiC_nw preform,SiC_f/SiC_nw/Si₃N₄composites with secondary enhancement of SiC_nw,and 3D printed SiC_nw/SiOC ceramics with in-situ synthesis of SiC_nw.According to the structural characteristics of the three materials,the influence of microstructure and phase composition on their EM absorbing and mechanical properties was studied,and the mechanisms of EM loss and mechanical reinforcement were revealed.The main research contents and results are as follows:1.Based on the intrinsic properties of SiC_nw and the modification of SiC_nw based materials,the EM absorbing properties of SiC_nw and SiC_nw/C aerogels were studied.There were abundant stacking faults and twin crystals inβ-SiC_nw,and the polarization loss was the main EM wave consumption mode.SiC_nw/C aerogel was prepared by the combination of SiC_nw（mainly provide polarization loss）and C（mainly provide conductance loss）.The carbon film improved the conductance loss ability of the aerogel;Abundant heterogeneous interfaces(between SiC_nw and carbon film,between microcrystalline carbon and amorphous carbon,and at stacking faults and twin crystals in SiC_nw)and defects（defects and residual functional groups in carbon film）enhanced the dipole polarization loss capacity of the aerogel;and the directional SiC_nw/C layers consumed the EM waves by multiple reflection and diffraction.SiC_nw/C aerogel had a density of only 9.2 mg/cm³,and when the sample thickness was 3.3 mm,the reflection loss（RL）could reach-31 d B,and the effective absorption bandwidth（EAB）covered the entire X band,which was very competitive in EM absorbing materials.2.SiC_nw/C/Si₃N₄ porous ceramics were prepared by combining self-assembled SiC_nw/C aerogel preform and Si₃N₄ ceramics coating,and their EM absorbing and mechanical properties were studied.In terms of macrostructure,the porous ceramic was composed of oriented SiC_nw/C/Si₃N₄ layers,and inside of the layer was sandwich-like structure.In terms of microstructure,there were a large number of stacking faults and twin crystals in SiC_nw,and microcrystalline carbon was dispersed in the amorphous phase.By adjusting the relative content of SiC_nw and C,the ratio of polarization loss and conductance loss was regulated,and the EM absorption performance of the porous ceramic was optimized:when the sample thickness was2.9 mm,the EAB covers the entire X band,the RL value reached-39.7 d B,and the EM absorption performance remained stable when the operating temperature was up to 873 K.The density of the porous ceramic was 0.92 g/cm³,the content of reinforcement（absorbent）was about 1 wt.%,and the longitudinal compressive strength reached 191.3±27.5 MPa.Therefore,the SiC_nw/C/Si₃N₄ porous ceramic met the requirements of modern EM wave absorbing material of"strong,wide,light,thin and stable",and as an excellent EM absorbing material had a certain mechanical bearing capacity.3.SiC_f/SiC_nw/Si₃N₄ composites were prepared by introducing SiC_nw as secondary reinforcement into high-strength-and-toughness continuous SiC fiber reinforced Si₃N₄ ceramic matrix（SiC_f/Si₃N₄）composites,and their electromagnetic and mechanical properties were studied.SiC_nw existed at the interface between the fiber and the matrix,and the content of SiC_nwincreased with the increase of PIP time.After the introduction of SiC_nw,the interface between the fiber and the matrix was weakened,which was conducive to the interface debonding and the fiber pulling out to release energy.As the content of SiC_nw continued to increase,the amount of Si₃N₄ matrix continued to decrease,the interfacial bonding was greatly weakened,and the load could not be effectively transferred from the matrix to the fiber,resulting in fiber bundle pulling out and even interlaminar stripping.When PIP one time,SiC_f/SiC_nw/Si₃N₄ composite had the best EM wave absorption performance and bending strength:when the sample thickness was 2.8 mm,the EAB was 3.5 GHz and the RL reached-31.1 d B.The bending strength reached333±29 MPa,which was 27.6%higher than that of SiC_f/Si₃N₄composite.Therefore,SiC_f/SiC_nw/Si₃N₄ composite was a structural EM absorbing material with excellent EM absorbing and mechanical properties and had great application potential.4.In-situ SiC_nw/SiOC ceramics were prepared by an intelligent manufacturing technology of UV curing 3D printing,and their EM absorbing properties were studied.With the increase of the fabrication temperature,the pores in the ceramics increased,the content of SiC nanowires/crystals increased,the proportion of microcrystalline carbon increased,and the real and imaginary parts of the dielectric constant increased.The SiOC ceramics prepared at 1100^oC had the best EM wave absorption performance:when the sample thickness was 2.7 mm,the EAB reached 3.17 GHz（covered more than 75%of the X band）,and the RL reached-22.9 d B.The SiC_nw/SiOC ceramics prepared at 1500 ^oC had the highest dielectric constant,and it was an EM shielding material with high dielectric loss characteristics:the EM shielding efficiency reached 29.81 d B,the reflection shielding efficiency and the absorption shielding efficiency were 6.64 d B and 23.16 d B respectively.Therefore,it was an EM absorption type-shielding material.3D printing of in-situ SiC_nw/SiOC ceramics provided a new idea for the preparation of complex structural EM absorbing materials.