| With the need of reducing the electromagnetic pollution of electronic components and the developing national defense stealth weapons,electromagnetic absorbing materials have been paid more and more attention,and people have been working on developing absorbing materials with good performance.Certainly,the key to improve the microwave absorbing properties of materials is to promote their permittivity and permeability,meanwhile ensure the impedance matching.In this thesis,the RE-Fe(Nd-Fe-B and La-Fe-Si)rare earth alloy powders were flattened by ball milling to improve their permeability.Subsequently,in order to increase the motion path of the electromagnetic wave and enhance the absorption efficiency,the textured structure was used obtained by dealloying method.Finally,as prepared powders were annealed at 500 °C for 1 h to improve the impedance matching.The phase constitution,morphology,magnetic properties and electromagnetic parameters before and after the treatment were studied by X ray diffraction,scanning electron microscope,vibrating sample magnetometer and vector network analyzer,respectively.Moreover,the influence of textured structure on the absorbing properties of flaky magnetic absorbing materials was mainly studied and the internal mechanism was discussed.Through above research,the method of reusing RE-Fe rare earth alloy powders was also explored.The main findings of this thesis are shown as follows:(1)The optimal process for preparing flaky Nd-Fe-B alloy powder was proposed:the time of dry milling and wet milling was 2 h and 4 h,respectively.Furthermore,the adding ratio of process control agent was 1:1,and the corresponding addition amount was 0.3g for both anhydrous ethanol and oleic acid.The particle size of the flaky powder was in the range of 4-8μm,and the thickness was 200 nm.(2)With the process of dealloying and subsequent annealing,textured structure successfully formed on the surface of the flaky Nd-Fe-B alloy powder,and the powder was mainly composed of α-Fe and amorphous phase.The change of powder surface structure and phase transformations significantly increased the permittivity and permeability of the powder,and improved its impedance matching,so as to greatly enhance its absorption performance.The RL of raw powder was only-4.52 dB with the thickness d=5.0 mm of the matching coating,and the RL of the 20 min sample reached-45.76 dB,and the thickness of the matching coating was only 2.4 mm.(3)Textured structure was also obtained on the surface of flaky La-Fe-Si alloy by using the above optimization process and dealloying.The phase constitution changed from single phase La(Fe,Si)13 to the combination of a-Fe and La(Fe,Si)13,which caused remarkable increase of the permittivity and permeability.The impedance matching was enhanced and the powder obtained good absorbing properties.The reflection loss RL of sample with 20 min dealloying treatment reached-32.30 dB when d=7.0 mm.But for the sample without dealloying,the minimum value of RL was only-9.44 dB.(4)The mechanism analysis exhibited that the formation of textured structure gradually formed with the increase of dealloying time.Then the greater interface polarization and higher permittivity were achieved,and the impedance matching performance of the powders were enhanced.The magnetic loss mechanism also changed from the natural resonance to the exchange resonance.At the same time,the textured structure effectively improved the interference cancellation in electromagnetic wave scattering,and reduced the coating thickness to form interference cancellation,which was beneficial to the light-weight of the absorbing coating. |
