| Wave-absorbing materials have been widely used in electronic communication,data transmission,military stealth,electromagnetic protection and other fields.However,how to design and develop wave absorbing materials that can work in high temperature and strong corrosion is still an urgent problem to be solved.The absorbing material consists of transmission wave matrix and electromagnetic wave absorbent.Silicon nitride and alumina ceramics are considered ideal for high temperature wave absorption matrix materials due to low ditivity and good high temperature resistance,but with high density.At the same time,commonly used wave absorbent materials,such as carbon materials and magnetic materials,are easy to oxidation,decompose and fail in the process of composite with ceramics and sintering at high temperature.Polymer precursor ceramics（PDCs）have become one of the important candidate materials for high temperature wave absorption materials due to their high temperature resistance,antioxidant resistance,low ceramic conversion temperature,coating and protection effect on wave absorber materials,and their unique composition and dielectric properties of regulation.However,PDCs ceramics in the transformation of organic precursor to ceramic,difficult material compaction,poor mechanical performance,difficult to be used as structural parts.In this paper,a composite material of porous ceramic load SiCN precursor ceramics is designed,and the liquid polysilane precursor with magnetic wave absorption phase is immersed in porous Si3N4 and Al2O3 ceramics,which can be analyzed in situ from SiC and free carbon,and the magnetic particles introduced in collaboration can effectively lose and absorb the electromagnetic waves.The influence of material composition and microstructure on wave absorption properties,the mechanism and the electromagnetic properties were studied.The main research conclusions are as follows:（1）The porous Si3N4 ceramics were prepared by pressureless sintering method at 1700℃by the pore-forming agents of bezoic acid,stearic acid and polymethacrylic acid.Effects of type and content of pore-forming agent on phase composition,pore shape and distribution,apparent porosity and high frequency dielectric properties of porous Si3N4 ceramics were studied.When 30～50 wt.%benzoic acid was added as pore-forming agent,the porosity of the material can be 30.3%～48.7%.the bending strength was 43.3～96.4 MPa,and the average of the dielectric constant in the band range of 2～18 GHz is about 4.1～5.5.The size of the pores is appropriate and the distribution is uniform.At this point the dielectric properties of porous Si3N4 ceramics are low ang the porosity is high,so it is suitable as the matrix of the absorbing materials.SiCN/Si3N4 composite ceramics were prepared using polymer as precursor to be fully impregnated in porous ceramics.The effects of impregnation times and porosity rate on composite phase composition,microstructure and electromagnetic wave absorption properties were studied.It turned out that the new phases of β-SiC nanotubes and free carbon appeared inside the impregnated sample,which can promote the absorbing properties by dielectric loss.The minimum reflectivity value of the sample with 30 wt.%benzoic acid at about 13 GHz is close to-15 dB.The absorption rate of the electromagnetic wave of the sample is close to 99%,indicating that the impedance matching degree between the sample and the air is higher,and the absorption and consumption of the electromagnetic wave inside the sample are more,and that the wave absorption performance is better.（2）Based on porous Si3N4 ceramics,using SiCN as the precursor and the different iron-containing compounds as iron sources,SiCN（Fe）/Si3N4 composite ceramic absorbing materials were prepared by impregnation-pyrolysis method at 1000℃.When iron（Ⅲ）acetylacetone was used as iron source,adding 5 wt.%iron（Ⅲ）acetylacetone has the best absorbing property.The ε’ of the sample containing 5 wt.%of iron（Ⅲ）acetylacetone at around 15 GHz was about 0.5.The highest peak tan δE the sample was 3.49.It shows that the impedance between the material and the air matches well.It is advantageous for the incident electromagnetic wave to enter the material.The reflectivity（R）of the sample is lower than-20 dB.It shows that the electromagnetic wave absorptivity of the sample is more than 99%at this point.A obvious circular arc appears on the Cole-Cole curve of the sample,indicating dielectric relaxation in the environment of high frequency electromagnetic waves.When nano-iron oxide is used as iron source,after three impregnation cycles,the lowest ε’ was 2.6.The maximum ε"value was 1.72,and the tan δE was around 0.49.The constant C0 in 2～18 GHz band is constant,indicating that the magnetic loss mechanism is the eddy current loss.（3）Porous Al2O3 ceramics were prepared by using carbon powder as pore-forming agent.SiCN（Fe）/Al2O3 composite porous absorbing ceramic materials were prepared by using iron（Ⅲ）acetylacetone and nano-iron oxide as iron sources,respectively.When iron（Ⅲ）acetylacetone was used as iron source,the pyrolysis temperature is 1000℃ and the thickness of 3 wt.%sample is 3.5 mm,has a minimum reflectivity with-18 dB of about 12 GHz.The reflectivity between 9～13 GHz is below-10 dB,an effective absorption bandwidth of more than 90%of electromagnetic waves has reached 4 GHz.As the content of α-Fe gradually increases,the loss mechanism of electromagnetic waves gradually changes from dielectric loss to be dominated by magnetic loss.When nano-iron oxide is used as iron source,the diffraction peak height ofα-Fe2O3 and α-Fe increases gradually with the increase of impregnation pyrolysis times.When electromagnetic wave is incident into the material,α-Fe2O3 and α-Fe will cause eddy current loss inside the sample.Thus,the loss and absorption of electromagnetic wave are enhanced,which is beneficial to the improvement of absorbing properties of composites.The reflectivity of the sample after impregnation and pyrolysis is the smallest about 15.5 GHz,which is about-10 dB.At this time,the absorptivity of electromagnetic wave reaches 90%.The impedance matching value of the sample is also high,and the impedance matching degree between the sample surface and air is better.The polarization relaxation occurs inside the sample,while the electron motion produces a secondary alternating electromagnetic field,preventing the external electromagnetic field from penetrating into the interior of the material and having a small skin depth value.（4）CST software is used to design the structure of porous ceramic matrix composites and simulate the absorbing properties.On the basis of previous experiments,the structure and electromagnetic parameters of the material are constructed and set firstly,the absorptivity of electromagnetic wave is 90%～99%in the wide band of 6～18 GHz.The three-layer cyclic periodic metamaterial structure in the horizontal direction of infinite plane is constructed based on the basic parameter values of the prepared materials.The S parameters and the reflectivity are calculated by TE polarization method and frequency domain simulation.The electromagnetic wave absorptivity is above 90%in the range of 4.7～18 GHz,and the electromagnetic absorptivity is close to 100%at 7.152 GHz.The current density of the outer and upper surfaces of the periodic structure of the sample is high,and the electric field distribution and the electric field response between the structural layers are also strong.The near field coupling is produced.Based on the electromagnetic parameters of the prepared material,the simulation calculation of the patch antenna device is carried out.The radiation effect of the antenna at 8.55 GHz is better,and the gain is the highest.From the port direction,the closer it is to the patch,the more energy it radiates,the greater the gain,and the better the radiation performance of the patch antenna.CST simulation can provide an effective reference and guidance for the performance optimization and application of porous ceramic based absorbing materials.A composite of porous ceramic loaded SiCN with good absorbing properties is prepared in this paper,and its loss and absorption mechanism of electromagnetic wave at different frequencies are analyzed.It provides some experimental basis and theoretical basis for the practical application of porous ceramic based composite absorbing materials.It has a broad application prospect in the field of high-temperature wave absorption materials. |
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