Microwave Absorbing Properties Of Planar Rare Earth Intermetallic Compounds Powders/paraffin Composites

'Thin thickness, light weight, wide band and strong absorption" is required for microwave absorption materials in the electromagnetic compatibility (EMC), anti-electromagnetic interference and military stealth fields. The improvement of the microwave complex permeability is very important for the magnetic microwave absorption materials, according to the quarter-wavelength condition. Based on the double anisotropy model, the rare earth intermetallic compounds with planar magnetocrystalline anisotropy could exceed Snoek's limit. High permeability and high magnetic resonance frequency can be both obtained. The rare earth intermetallic compounds with planar magnetocrystalline anisotropy show the very significant potential as microwave magnetic materials and microwave absorption materials in the future.The magnetic moment distributing of the rare earth intermetallic compounds is significantly affected by the composition and temperature. Gererally, there are three kinds magnetocrystalline anisotropy for the rare earth intermetallic compounds: unaxial, cone and planar anisotropy.Based on the above theory, the Ce2(Co1-xFex)17(0≤x≤0.8) rare earth intermetallic compounds micropowders were prepared and the magnetocrystalline anisotropy was studied. The intermetallic compounds micropowders/paraffin composites were prepared. The complex permeability and microwave absorption properties were studied. Especially for the planar and cone anisotropy intermetallic compounds micropowders/paraffin composites, the microwave absorption mechanism is deeply analyzed. The affect of the oxide doping on microwave absorption properties for intermetallic compounds micropowders/paraffin composites is also studied. The main results are shown as follows:(1) In the range of0≤x≤0.8, Ce2(Co1-xFex)17intermetallic compounds possess uniaxial magnetocrystalline anisotropy with x≤0.45, possess cone magnetocrystalline anisotropy with x=0.5and0.55, and possess planar magnetocrystalline anisotropy with x≥0.6.(2) The microwave magnetic properties of uniaxial Ce2(Co1-xFex)17micropowders/paraffin composites with0≤x≤0.45obey the Snoek's limit. The initial permeability increases and the natural resonance frequency decreases with the x value increasing from0to0.45for the uniaxial Ce2(Co1-xFex)17micropowders /paraffin composites with the same volume concentration.(3) The microwave magnetic properties of planar Ce2(Co1-xFex)17micropowders/paraffin composites with0.6≤x≤0.8overcome the Snoek's limit. The initial permeability decreases and the natural resonance frequency increases with the x value increasing from0.6to0.8for the planar Ce2(Co1-xFex)17micropowders/paraffin composites with the same volume concentration.(4) The microwave magnetic properties of cone Ce2(Co1-xFex)17micro powders/paraffin composites with x=0.5and0.55overcome the Snoek's limit.(5) The complex permeability of the planar and cone anisotropy intermetallic compounds micropowders/paraffin composites is higher than that of the uniaxial Ce2(Co1-xFex)17micropowders/paraffin composites with the same volume concentration.(6) Simulated microwave reflection result shows that the reflection loss peak frequency, the absorber thickness and the electromagnetic parameters is coincident with the quarter wavelength condition for the Ce2(Co1-xFex)17micropowders/paraffin composites. In the frequency range of0.1-18GHz, with the absorber thickness increasing, the reflection loss peak moves to lower frequency, and two or more reflection loss peaks can be observed.(7) The reflection loss peak value increases first and then decreases with the absorber thickness increasing. The minimum reflection loss peak value is obtained and the perfect loss is achieved as Z=|Zin/Z0|=1. At the matching frequency of the perfect loss conditions, ε/μ>1.(8) The microwave reflection loss characteristics of the Ce2(Co1-xFex)17micropowders/paraffin composites can be modulated by changing the x value. The matching frequencies of the perfect loss conditions for the planar and cone anisotropy intermetallic compounds micropowders/paraffin composites are lower than that for the uniaxial Ce2(Co1-xFex)17micropowders/paraffin composites with the same volume concentration.(9) The microwave reflection loss characteristics of the Ce2(Co1-xFex)17micropowders/paraffin composites can be modulated by changing the volume concentration. With the increase of the volume concentration, both the complex permittivity and permeability increase, and the reflection loss peak of the perfect loss conditions moves to lower frequency.(10) The S11-open, S21-open and S11-short parameters of the Ce2(Co1-xFex)17(x=0.5,0.6) micropowder/paraffin composites of different thickness was measured directly, according to which, the microwave energy loss was calculated. The microwave energy loss includes the electromagnetic loss in the inner of the absorber and the interference loss between the reflection waves from the back interface and the front interface. And the interference loss is the most important contribution.(11) The SiO2@Ce2(Co0.4Fe0.6)17micropowders were prepared by directly ball milling the mixture of the Ce2(Co0.4Fe0.6)17quenched samples and SiO2powders. The XRD patterns are totally same with Ce2(Co0.4Fe0.6)17micropowders, and any diffraction peaks can not be seen in relation to the crystal structure of SiO2. The saturation magnetization of SiO2@Ce2(Co0.4Fe0.6)17micropowders is lower than that of Ce2(Co0.4Fe0.6)17micropowders. The SiO2@Ce2(Co0.4Fe0.6)17micropowders paraffin composites also show lower complex permittivity and lower complex permeability. Simulated microwave reflection result shows that the matching thickness and peak frequency is coincident with the quarter wavelength condition. The matching thickness is thinner and the matching frequency is higher for the perfect matching conditions.(12) The Ce2O3@Co0.4Fe0.6nanocomposites were fabricated by arc melting and followed ball milling. The CoFe nanocrystals are dispersed in the cerium oxide. The complex permittivity, permeability and microwave absorbing properties of Ce2O3@Co0.4Fe0.6powders/paraffin composites with orientation have been studied. The lower permittivity and the higher permeability for the oriented composites have been shown obviously than those for unoriented one. Simulated microwave reflection result shows that the matching thickness and peak frequency is coincident with the quarter wavelength condition. For the oriented one, the minimizing RLm has been reached with the matching thickness of about2.49mm at the corresponding frequency of3.7GHz. At this point, not only are the reflection waves out of phase180°, but also the input impendence of absorber Zin is equal to the impendence of air Z0.