Study On Grain Orientation And Low Frequency Electromagnetic Properties Of FeSiAl Alloy Absorbents

High performance low-frequency absorbers are important for combating low-frequency radar detection.However,the low-frequency absorbents with high permittivity and low permeability used for frequencies below 4 GHz,so it can't produce effective absorption at low frequencies.At the same time,broadband absorbers are generally incapable of taking into account low frequencies.The research work in this paper is aimed at reducing permittivity,improving low-frequency permeability and the design of low-frequency broadband absorbers.The main research contents and conclusions are as follows:?1?A pre-heat treatment-milling two-step method was proposed to prepare grain-oriented flaky FeSiAl alloy absorbents with low permittivity and high permeability.The results show that the pre-heat treatment increases grain size,reduces internal strain,and produces slight oxidation on the surface of sample.Milling increases the aspect ratio of the FeSiAl alloy particles and obtains the grain orientation of the?400?crystal plane.The slight oxidation of the surface reduces permittivity of flaky FeSiAl alloy powder obtained by milling.When pre-heating temperature is 1000°C and milling time is 20 h,permittivity is the smallest,permeability is the highest at 1 GHz,and grain orientation is 47.3%of the sample,The minimum reflection loss is-5.3 dB at a thickness of 1 mm.?2?A method for preparing flaky FeSiAl alloy absorbent rich in DO₃ phase and higher grain orientation by annealed was proposed,which improved low-frequency permeability and absorption performance of absorbers.The results show that the energy of the crystal is the lowest when the?400?plane of the FeSiAl is parallel,a preferred orientation of the?400?crystal plane is formed during grain growth.In the meantime,as the heat treatment temperature increases,the DO₃ phase content increases and the internal stress decreases,thereby increasing the magnetic permeability of the sample.When annealing temperature was 700°C,the sample had a saturation magnetization of 126 emu/g and grain orientation of 58.4%.When the volume fraction of FeSiAl/paraffin composite is 35 vol%,the minimum reflection loss value is-7.1 dB,and peak frequency is 1.06 GHz at 1 mm thickness.?3?Using our FeSiAl alloy as a low frequency absorbent.We present a simple design strategy for broadband absorbers by gradient stacking three magnetic absorbers with different matching frequencies at high,medium and low frequencies,respectively.In this strategy,the bottom layer was made of high dielectric material with magnetic resonance and matching frequency at low frequency;the surface layer was made of low dielectric material with magnetic resonance and matching frequency at high frequency,while the medium layer was made of medium dielectric material with magnetic resonance and matching frequency at middle frequency.Based on this structure,the broadband absorption characteristics was achieved by optimizing thickness of each layer under the limitation of the total thickness.We designed and fabricated two wideband absorbers:one exhibited reflection loss of lower than-8 dB within frequency range of 2¹8 GHz at total thickness of 2.4 mm;the other showed reflection loss of lower than-4 dB within frequency range of 2.2¹8 GHz at total thickness of 1 mm.