Preparation And Electromagnetic Performance Investigation Of Sc₂Si₂O₇-Based Ceramic Materials

Ceramic matrix composites are the candidate materials for the new generation of hot end components of aeroengines due to their light weight,high temperature resistance,high strength and excellent electromagnetic performance.However,ceramic matrix composites are susceptible to be to be corroded by high temperature water-vapor environment,resulting in the performance degradation.At present,rare earth silicate coatings are mainly used to protect composites from corrosion in high temperature water-vapor environment,but few reports on the electromagnetic properties.How to ensure the application of ceramic matrix composites with high temperature resistance,corrosion resistance and excellent electromagnetic properties,and realize multifunctional integration becomes a new challenge for the research of ceramic matrix composites.In this manuscript,the multi-functional ceramic matrix composites with a high-temperature-resistant and water-vapor-resistant performance were prepared.The electronic structure,state density and energy band structure of intrinsic Sc₂Si₂O₇ and Fe-doped Sc₂Si₂O₇ were calculated by first-principles,in order to provide the theoretical basis for adjusting the microstructure and dielectric properties of Sc₂Si₂O₇ ceramics.On this basis,CNTs,Si Cnp,and BSAS were used to modify the electromagnetic properties of Sc₂Si₂O₇materials,respectively.Finally,the prepared BSAS/Sc₂Si₂O₇ coatings were applied to C/Si C composites to study the rule of water-vapor environment on the electromagnetic properties of the composites.The main researches and results are as follows:（1）The Fe-doped Sc₂Si₂O₇ceramics were prepared by sol-gel method.The effect of Fe-doped content on electromagnetic properties was studied.The polarization loss and magnetic loss of Sc₂Si₂O₇ increase first and then decrease with the addition of Fe doping content.They reach the maximum when the Fe-doped content is 6%,indicating that the addition of Fe is beneficial to improving the electromagnetic wave absorption performance of Sc₂Si₂O₇.Meantime,the results show that the substitution of Fe for Sc atoms in Sc₂Si₂O₇generates impurity levels at band structure,resulting in difference in spin magnetic moment,which is beneficial to enhancing electromagnetic wave absorption.（2）The CNTs were prepared in porous Sc₂Si₂O₇ ceramics to form conductive CNTs/Sc₂Si₂O₇ ceramics by catalytic chemical vapor deposition（CCVD）.The CNTs with higher conductivity and specific surface area increase the relative complex permittivity of Sc₂Si₂O₇ ceramics.The defects in the multiphase ceramics enhance the polarization loss,and the nano-heterogeneous interface is conducive to carrier transport and electron transition.Furthermore,the 3D conductive network structure provides multistage channels for electromagnetic wave energy transmission,so as to improve the electromagnetic wave absorption performance of the multiphase ceramics.When the content of CNTs is 1.56 wt.%,the CNTs/Sc₂Si₂O₇ ceramics display excellent room temperature and high temperature electromagnetic wave absorption properties in the X-band.The RC_min is-33.5 d B（298 K）and-45.9 d B（573 K）,respectively,and EAB is 4.2 GHz.（3）The Si C/Sc₂Si₂O₇ and Si Cnws/Sc₂Si₂O₇ ceramics were prepared by introducing Si C/Si Cnws into porous Sc₂Si₂O₇ ceramics using precursor infiltration and pyrolysis（PIP）.The defects and hetero-interface in the ceramics increase their dielectric properties and absorption properties.The results show that the Si Cnws has low conductivity,large specific surface area,abundant defects and hetero-interfaces,and its electromagnetic wave absorption performance is significantly superior to that of nano-Si C,reflecting the electromagnetic performance of wide-band and strong absorption.The RC of Si Cnws/Sc₂Si₂O₇ ceramics is below-10 d B,covering the whole X-band at room/high temperature.The RC_min of the composite ceramics was-35.5 d B at room temperature,and the RC of the composite ceramics was below-30 d B when the temperature was 323 K-873 K,achieving over 99.9%electromagnetic wave absorption.（4）A wealth of hetero-interfaces and defects are constructed in the BSAS/Sc₂Si₂O₇ materials by controlling the BSAS content,which is beneficial to the increase of polarization loss.When the BSAS content is 30 wt.%,the composites possess the RC_min of-2.63 d B（achieving more than 20%of electromagnetic wave absorption）at 11.7 GHz with the thickness of 4 mm,which effectively improves the electromagnetic absorption capacity.（5）The water-vapor resistant of Sc₂Si₂O₇/BSAS ceramic coatings were prepared on the surface of highly conductive C/Si C composites to obtain multifunctional integrated composites.Their water-vapor resistance and shielding performance were studied,and the diffusion mechanism of the oxygen atoms of the composites in water-vapor environments was revealed.The results show that the coated C/Si C composites effectively prevent the substrate from being corroded.The absorption shielding effectiveness（SE_A）increases,the reflective shielding effectiveness（SE_R）decreases,and the total shielding performance（SE_T）is 30 d B when the exposure time reaches the 10 hours at 1250oC water-vapor environment,which can prevent more than 99.9%of electromagnetic waves.