Studies On Preparation And Electromagnetic Characteristics Of Ferromagnetic Microwave Absorbing Materials

Now, for the sigle morphology and limited preparation method of FeSiAl alloy powders, the improved centrifugal atomization method was used in this paper to prepare spherical FeSi and spindly FeSiAl alloy powders. Meanwhile, electromagnetic interference and radiation damage on the human body have become more serious with the rapid development of electronics and communications equipment. To solve the above problem, an absorbing sheet employing ferromagnetic alloy powders as absorbent and chlorinated polyethylene as matrix was prepared. Besides, to broaden the absorption frequency band of the absorbing coatings filled with sigle absorbent, a single-layer coating employing composite absorbents and epoxy resin as matrix was introduced in this paper. We also prepared a double-laye absorbing coating introducing the SiO2 powders as wave-transmitting materials. The absorbent, absorbing patch and coatings were measured by SEM, XRD, vibrating specimen magnetometer, electromagnetic parameters and absorption properties testing device, respectively.The results showed spherical FeSi and spindly FeSiAl alloy powders were prepared at lower rotate-speed (<8000r/min) by the improved centrifugal atomization method. The forming process included the first droplet rupture, the first spheroidizing, the second droplet rupture and the second spheroidizing. The higher rotate-speed and degree of superheat were beneficial to obtain the finer powder, but the working life of equipment was reduced. The difference in morphology between FeSi and FeSiAl alloy powders resulted from the addition of Al element. For the FeSi alloy powders, when the content of Si was lower, the phase structure wasα-Fe(Si), and the alloy powders showed excellent electromagnetic properties, especially permeability; when the content of Si was higher, the phase changed into Fe1.34Si0.66 or FeSi compound, microwave absorption properties decreased dramatically. In addition, the decrease of particle size could improve microwave absorption properties of alloy powders. Spindly FeSiAl alloy powders with the near composition of Sendust alloy (Fe85Si9.6Al5.4) showed excellent electromagnetic properties, especially permeability. The higher Al or Si content would decrease obviously the electromagnetic properties. Spindly shape FeSiAl alloy powders showed more excellent electromagnetic properties than the spherical powders which was confirmed by theoretical analysis and contrast test. Carbonyl-iron powder and FeSiAl alloy powders as ferromagnetic absorbent both exhibited excellent electromagnetic properties especially permeability. The absorbing sheet with the smaller thickness showed good microwave absorption properties in lower frequency range. When the thickness was small, the absorption peaks moved towards the lower frequency range with the increase of absorbent content, but the rule was unsuitable to the thicker samples due to the broken impedance matching characteristics. The absorption peaks shifted to the lower frequency range with the increase of thickness, and the microwave absorption properties could be improved as the increasing thick while the impedance matching characteristics was good. For the sheets employing carbonyl-iron powder, when the ratio of absorbent to matrix was 16:1, the reflection loss of absorbing sheet with 1.5mm thickness was below -8dB in 2-3.7GHz, and the reflection loss was below -4dB in 650MHz-1.8GHz for the samples with 2mm thickness. For the sheets based on FeSiAl powders, the reflection loss was below -4dB in 590MHz-1.8GHz for the samples with 1.1mm thickness and the ratio of absorbent to matrix was 16:1.The microwave absorption properties was bad for the epoxy resin matrix absorbing coating employing single absorbent, such as ferromagnetic alloy powders and carbon black, but the properties could be improved by the application of composite absorbents consisting of ferromagnetic alloy powders/carbon black, and the density and cost of the coating could be reduced. For the FeSi powders/carbon black coating, when the ratio of FeSi powders:carbon black:epoxy resin is 1.8:0.2:1, the absorption bandwidth (reflection loss below -4dB) achieved 15.1 GHz for the samples with 1.2mm thickness. For the carbonyl-iron powder /carbon black coating, when the ratio of carbonyl-iron powder: carbon black: epoxy resin is 0.7:0.6:1, the reflection loss of the samples with 1.5mm thickness was below -4dB in 7.9-18GHz. For the absorbing coatings employing composite absorbents, the different types of absorbent could play their own role respectively in attenuating electromagnetic wave. Moreover, when the content of composite absorbents was higher, the conductive network with larger resistivity was formed in the coating, which was beneficial to suppress the unfavorable eddy-current effect. Besides, according to impedance matching principle, a double-laye absorbing coating with the introduction of the SiO2 powders firstly as wave-transmitting materials was designed. The addition of SiO2 powders improved the impedance matching characteristics of the coating, and the absorption properties increased, especially in low frequency range.