Preparation And Microwave Electromagnetic Properties Of Fex And FeX@SiO₂ Composite Particles

In order to obtain good performance electromagnetic wave absorbing materials,the flake FeSi and FeSiAl alloy powders were prepared by ball milling.The SiO₂ was coated on the flake powder by sol-gel method.The microstructure of the composite powder was preliminarily realized by hydrogen heat treatment.Regulation.The microstructure of the powder was analyzed and characterized by XRD,SEM,EDS and TEM.The oxidation resistance of the powder was evaluated by TG-DSC.The electromagnetic parameters of the powder were tested by VSM and VNA.The electromagnetic wave absorption properties of different powders were evaluated by the transmission line theorem based on electromagnetic parameters.The preparation process research shows that the ball milling time has a certain influence on the degree of flaking of the powder.The longer the milling time,the higher the flaky degree and the finer the particles.In the SiO₂ coating experiment,the TEOS dosage,reaction time and reaction temperature all have an effect on the coating effect.For the FeSi powder,when the amount of TEOS is 5 mL,the reaction temperature is 20°C,and the reaction time is 4 h,a good coating effect is obtained;the coating parameter of FeSiAl powder is 7.5mLTEOS,temperature 40°C,reaction time 4 h.After coating two ferromagnetic@SiO₂ composite powders,the SiO₂ dielectric layer islayer is amorphous,and the cladding layer has a thickness of about 50 nm.The two composite powders were heat treated at different temperatures,and the phase of FeSi@SiO₂ did not change.The phase of the alloy powder in FeSiAl@SiO₂ was transformed from Al_0.5Fe₃Si_0.5 phase to Al_0.3Fe₃Si_0.7.7 phase.The oxidation resistance of the coated composite powder is improved.For the FeSi powder,after coating,the rapid oxidation temperature is increased from 700°C to 900°C,and the mass increase at 1200°C is decreased from 4.6%to 3.9%;FeSiAl alloy powder,the rapid oxidation temperature increased from 900°C to 1000°C,the mass increase to 1200°C decreased from 7.0%to 4.4%.The longer the powder milling time,the greater the defect density,the higher the dielectric constant and magnetic permeability,and the better the absorption at low frequencies.The complex dielectric constant of FeSi alloy decreases with the decrease of size,and the complex permeability increases with the decrease of size.The small size FeSi powder has better impedance matching,higher absorption peak and better absorption performance.The dielectric layer of the FeSi@SiO₂ composite powder effectively insulates the alloy particles,the dielectric constant is lowered,and the magnetic permeability is lowered.The absorption peak after coating of FeSi powder is increased because the decrease in dielectric constant after coating increases the impedance matching of the coating.After the heat treatment,the defects of the FeSi alloy powder inside the composite powder are reduced,the electrical conductivity is increased,the dielectric constant is sharply increased,and the increase of the eddy current effect leads to a decrease in the magnetic permeability.The absorption performance after heat treatment is deteriorated,and the impedance matching of the coating is deteriorated.For the FeSiAl alloy,the large-sized particles have high electrical conductivity,an increase in dielectric constant,and an increase in eddy current loss and an increase in magnetic permeability.When the size is reduced,the impedance matching is better,and the electromagnetic wave absorption performance is more excellent.The FeSiAl@SiO₂composite powder has a small dielectric constant,a greater decrease in magnetic permeability after coating,and a decrease in magnetic loss,resulting in deterioration of absorbing properties.The dielectric constant and magnetic permeability of FeSiAl@SiO₂after heat treatment are improved,the magnetic loss and dielectric loss are simultaneously increased,and the electromagnetic wave absorption performance is improved,which is related to the phase transition of the alloy particles.As a result of the calculation of the transmission line theorem,it is known that by adjusting the ball milling time,the size,and performing dielectric coating and hydrogen heat treatment,an absorbing coating having different electromagnetic wave absorbing properties can be obtained.Overall,the flake FeSi alloy powder and the treated composite powder have better absorption at low frequencies.The typical coating has a reflection loss of-72.9 dB,a matching thickness of 2-3 mm,and an effective bandwidth of 6 GHz.The flake FeSiAl alloy powder has better reflection loss at 4-18 GHz.The typical coating has a reflection loss of-49 dB,a matching thickness of only 1.7 mm,and an effective bandwidth of 6 GHz.It can be applied to different working environments by selecting different powders and treatment processes,which has high application value.