Preparation And Properties Of Electromagnetic Wave Absorbing Materials With Gradient Distribution Of Absorber

With the application of electromagnetic wave absorbing materials in daily life and the military field,the traditional absorbing materials are difficult to adapt to the harsh environment in military operations due to its narrow absorption band,high density and high cost.Electromagnetic wave absorbing ceramic materials with gradient distribution of absorbers have the advantages of high temperature resistance,corrosion resistance,oxidation resistance,light in weight,and have good absorbing properties in low frequency,which show unique advantages in electromagnetic wave absorbing for its good absorbing performance and bearing capacity.Then.In this paper,three electromagnetic wave absorbing ceramics with gradient distribution of absorbers were prepared,The effects of the preparation process on the microstructure,physics,mechanics and absorbing properties of the three ceramics were studied in detail,and the absorption mechanism of gradient absorbent distributed electromagnetic wave absorbing materials was further studied.（1）The silicon nitride ceramics with gradient distribution of Fe₃O₄（G-Si₃N₄-Fe₃O₄）was prepared by chemical co-precipitation and directional infiltration.By observing the microstructure and phase analysis of G-Si₃N₄-Fe₃O₄ ceramics,it is found that there is a gradient distribution of Fe₃O₄ in G-Si₃N₄-Fe₃O₄ ceramics.The infiltration pressure has a great influence on the distribution of Fe₃O₄ in G-Si₃N₄-Fe₃O₄ ceramics,and then affects the electromagnetic absorption properties of G-Si₃N₄-Fe₃O₄ ceramics.When the infiltration pressure is 0.93 times of atmospheric pressure,the electromagnetic wave absorptivity and reflectivity of G-Si₃N₄-Fe₃O₄-0.93 ceramics to electromagnetic wave is calculated to be 92%and 8%,and the electromagnetic wave reflectivity is calculated to be-10.8 dB,which means the G-Si₃N₄-Fe₃O₄-0.93 ceramics is an electromagnetic absorbing material with excellent performance.（2）Porous Si₃N₄ ceramics with gradient distributions of SiC and pores are fabricated by directional nitridation sintering diatomite preforms with phenolic resin as a carbon source and pore forming agent.The nitrogen flow rate during sintering and the phenolic resin content in the preforms greatly impact the distribution of SiC and pores on the ceramics.Due to the small amount of SiC and high porosity on the surface of ceramics,electromagnetic wave can enter the interior of ceramics almost without reflection.With the increase of SiC content and the increase of interface polarization,the ceramic has good electromagnetic absorption performance.In the frequency range of 8-18 GHz,the average reflectivity is far lower than-10 dB.（3）Quartz ceramics with a uniform/gradient distribution of BaTiO₃（U/G-SO-BTO）are fabricated by cold pressing a powder blend with BTO followed by sintering.And using a combined technique of spreading the powder blends with gradually increased BTO layer-by-layer and sintering.The electromagnetic wave absorption properties of these two ceramics are studied in detail.For U-SO-BTO samples,the primary electromagnetic reflection is strong due to the aggravated impedance mismatch at their surfaces.The electromagnetic wave reflectivity of U-SO-BTO could only reach-7.0 dB when the sample thickness is 6 mm and the BTO content is 8.0 wt%,and it decreases slightly to-8.1dB when the sample thickness is increased to 10.0 mm and the BTO content is decreased to 5.0 wt%simultaneously.For G-SO-BTO samples,electromagnetic waves could enter with little reflection due to the weak surface impedance mismatch,and the electromagnetic waves entering these samples could propagate forward while being absorbed gradually with little reflection because of the weak impedance mismatch at the interfaces.The G-SO-BTO samples are promising excellent electromagnetic absorbing materials because their electromagnetic wave reflectivity could reach a level lower than-12.0 dB and could decrease further from-12.2 to-13.1 dB as the layer thickness increases from 1.0 to 2.0 mm.