Preparation And Wave-absorbing Properties Of Silicon Nitride Heat Resistant Composites

Silicon nitride material is a kind of ceramic material with special structure,which has excellent high temperature oxidation resistance,stable chemical properties and high degree of domestic raw material.In addition,it also has the characteristics of low dielectric constant and dielectric loss value,high temperature and high resistance,and generates a series of silicon nitride composite materials through surface modification in carbon materials,magnetic materials,polymers,etc.,not only by increasing the interface polarization,multiple reflection and other ways to improve electromagnetic wave absorption.At the same time,combined with the characteristics of silicon nitride resistance to high temperature to improve the heat resistance of the composite material,can work in different environments.In this paper,a series of silicon nitride composite materials were prepared.FTIR,XRD,XPS,SEM,TGA and other methods were used to characterize and analyze the elemental composition,microstructure,structure and thermal properties of the composite materials.Vector network analyzer（VNA）was used to test the electromagnetic parameters of a series of composite materials.The results are as follows:（1）MWCNT/Si₃N₄ composites were prepared by using carbon nanotubes and Si₃N₄ as raw materials,first by simple surface modification and then by solvothermal method.The results show that the carboxyl group on the surface of the carbon tube and the amino group on the surface of the silicon nitride form amide bonds,indicating that the binary composite is successfully prepared.The heat resistance of the composite is excellent,and the 5%heat loss temperature is 614℃.Comparing samples with different proportions,it can be seen that when the feeding ratio of carbon tube and silicon nitride is 3:At 4.24 GHz,the maximum reflection loss value can reach-41.73 dB,and the corresponding thickness is 4 mm.When the sample thickness is 1.5mm,the matched absorption bandwidth can reach 5.6 GHz,and the matching thickness of the sample can cover the S-Ku band.They are closely attached to each other through solvothermal reaction,and intertwine with each other to increase the contact area of multiple interfaces,which is more conducive to the generation of multiple reflections and interface polarization.The synergistic effect of carbon nanotubes and silicon nitride composites improves the impedance matching of the composites and the microwave absorption performance of the composites.（2）MWCNT/Fe₃O₄/Si₃N₄ composite was prepared based on the modified carbon nanotubes and Si₃N₄ in Chapter 2,and the magnetic loss mechanism of nano-sized ferric tetroxide was introduced.The results show that the ternary composite has excellent heat resistance and 5%heat loss temperature is 612℃.When the feed ratio is 3:15:20,the minimum reflection loss value of the sample is-23.3 dB at 14.4 GHz,the corresponding thickness is 2.5 mm,and the corresponding effective absorption bandwidth is 5.36 GHz.The addition of nano-sized ferric tetroxide can form a conductive network with the carbon tube,which is more conducive to the generation of conductance loss.A large amount of spherical ferric tetroxide is accumulated on the surface of silicon nitride,making the contact surface with the outside world larger,which is conducive to improving the interface polarization.The introduction of ferric tetroxide will make the loss path more diverse.（3）The conductive polymer was selected to improve the absorbing ability of the composite,and different proportions of MWCNT/Si₃N₄/PANI ternary composites were prepared by in situ polymerization of aniline monomers.The results show that the composite has excellent heat resistance.When the 5 % heat loss temperature is 623 ℃ and the sample feeding ratio is 1:4,the minimum reflection loss value of the composite is-42.57 dB,the corresponding frequency value and matching thickness are divided into 8.8 GHz and 3.5 mm,and the corresponding absorption bandwidth is 4.06 GHz.A large amount of polyaniline is coated on the surface of carbon tube and silicon nitride,and the stacking and winding of the three get larger contact area,which is conducive to the occurrence of multiple reflection and improve the interface polarization.With the increase of aniline content,the microwave absorption performance of the composites was improved.Figures [40] Tables [7] References [103]...