Construction And Electromagnetic Wave Absorption Performance Of SiCN Precursor Ceramic Aerogel Composites

With the rapid development of electronic equipment and communication technology,the problem of electromagnetic pollution and communication security becomes more and more serious.Excellent microwave absorption materials are one of the important means to reduce electromagnetic pollution and improve communication safety.In addition,the microwave absorption material is an important basic material to realize the stealth of weapons and improve their survivability.Therefore,it is of great scientific significance and application value to research and develop electromagnetic absorption materials.SiCN ceramic aerogel has three-dimensional network structure,large specific surface area,lightweight,excellent high-temperature stability,corrosion/oxidation resistance and excellent microwave absorption performance.However,the shortcomings of SiCN aerogel,such as loose,brittle and poor mechanical properties,limit its further application.In this thesis,mullite felt and porous Si₃N₄ with good mechanical properties were innovatively introduced to improve the mechanical properties of SiCN precursor ceramic aerogels.The liquid precursors of SiCN ceramic aerogels are impregnated into mullite felt or porous Si₃N₄ by high pressure impregnation technology.Through high-temperature pyrolysis,SiCN ceramic aerogel composites were formed to realize the structure-function integration.The main research results and contents of this thesis are as follows:?1?Preparation and microwave absorption mechanism of SiCN ceramic aerogels:According to the mechanism of SiCN aerogel formed by the Hydrosilylation between Si-H in the precursor of SiCN and C=C in divinyl benzene,we regulated the composition and structure by changing the molar ratio of Si-H/C=C.The results indicated that with the increase of Si-H/C=C molar ratio,the specific surface area of SiCN ceramic aerogel decreased significantly and the content of C increased,thus achieving the regulation of electromagnetic wave dissipation ability.When the mass ratio of SiCN ceramic aerogels to paraffin is 30:70 and the Si-H/C=C molar ratio is1:1.5,the reflection loss?RL?value of SiCN ceramic aerogels can reach-54.35 d B and the corresponding effective band width is 4.16 GHz,showing excellent microwave absorption performance.?2?Microstructure regulation and microwave absorption mechanism of SiCN_a/Mullite_m composites:High pressure impregnation technology was used to prepare mullite felt reinforced SiCN ceramic aerogel composites.By adjusting the SiCN aerogel content,Si-H/C=C molar ratio,and pyrolysis temperature,the composition,pore structure and graphitization degree of free carbon of the composites were regulated,and finally SiCN_a/Mullite_m composites with excellent microwave absorption properties were obtained.The results indicated that the content of SiCN aerogels could be regulated by changing the impregnation times?12.8-37.5 wt%?.When the pyrolysis temperature was 1000°C,SiCN_a content was 22 wt%,and Si-H/C=C molar ratio was 1:1,SiCN_a/Mullite_m composites had the minimum RL value of-39.38 d B.When the pyrolysis temperature was 1000°C,SiCN_a content was 37.5 wt%,and Si-H/C=C molar ratio was 1:1.25,the absorption bandwidth of SiCN_a/Mullite_mcomposites almost covered the entire X-band.The excellent microwave absorption performance of SiCN_a/Mullite_m composite materials can be attributed to the following reasons:First,the three-dimensional network structure composed of a large number of pores can cause multiple reflection and scattering of electromagnetic wave,and thus extend their propagation path and increase the probability of electromagnetic wave attenuation.Secondly,the introduction of mullite fiber constitutes conductor-semiconductor-insulator combination structure of typical microwave absorption materials,which leads to the enhancement of interfacial polarization abilities of composites.Thirdly,the introduction of mullite fiber improves its impedance matching ratio,which is beneficial to broaden the absorption band.?3?Microstructure regulation and electromagnetic wave absorption mechanism of SiCN_a/Si₃N₄composites:On the basis of?2?,porous Si₃N₄ with better mechanical properties was used as the skeleton material,and the SiCN ceramic aerogel was impregnated in it to form SiCN_a/Si₃N₄ composites.At the same time,the composition,structure and properties of composites can be regulated by changing the technological parameters such as impregnation times and pyrolysis temperature.The results showed that when the impregnation times were 4 times,the content of SiCN ceramic aerogel reached the maximum value,?5 wt%,and the specific surface area of the composites was about 2.5 times higher than that of porous Si₃N₄.No significant new phase formation was found at the selected pyrolysis temperature?800-1100°C?,which proves that the introduction of SiCN ceramic aerogels has no adverse effect on porous Si₃N₄.When the pyrolysis temperature was 1000°C,SiCN_a/Si₃N₄ composites had the best microwave absorption properties,and its effective absorption frequency could be effectively controlled within the range of 6.06-14.79 GHz,showing outstanding microwave absorption performance and realizing structure-function integration at the same time.