Microwave Absorption Properties Of The Metal-oxide Composite Materials

With the rapid development of wireless communications and high-frequency circuit devices in the gigahertz range, electromagnetic interference (EMI) problems have attracted increasing attention recently. Many kinds of microwave absorption materials (MAM) have been studied with the objective of solving the EMI problems. Among all MAMs, metallic magnetic materials are especially focused on due to high saturation magnetization, high permeability, high Curie temperature and low coercivity. However, the high electrical conductivity and high dielectric constant of metallic magnetic material limit the application. The composite materials cantaining metallic magnetic material and certain insulation material may be a way to solve these problems.In the work, three series of metal-oxide composite materials were prepared using ball milling method assisted by some chemical methods, and the effect of preparation conditions on the morphology, structure, static magnetic properties and microwave absorption characteristics was explored. The main results are as follows:(1) FeCo/Al2O3 composite material was prepared using mechanochemical method. The reaction is quick, and the XRD pattern shows there are only the diffraction peaks of FeCo and Al2O3. In one particle, the FeCo and Al2O3 disperse into each other. The effect of the ball milling time on the coercivity and saturation magnetization is visible:when the ball milling time is less than 10 h, the coercivity and saturation magnetization of samples increase with the increase of ball milling time; when the ball milling time is more than 10 h, the coercivity and saturation magnetization of samples almost keep in a certain range with the increase of ball milling time. The dielectric constant of samples increase with the increase of ball milling time, while permeability is almost no law. The microwave absorption can reach -20 dB in 3-18 GHz by changing the sample in samples milled 15 min and 30 min and adjusting the absorber thickness between 1 mm and 5 mm. But in the low frequency band, the absorber thickness is too large. (2) Samples were obtained by ball milling carbonyl iron, when alcohol, acetone and hexane were used as process control agent (PCA) respectively, and the ball milling time was 10 h. The carbonyl iron particles have different shape under different PCAs:when alcohol and acetone are used as PCA, the shape of particles is flake, and the aspect ratio of former is larger than the latter; when hexane is used as PCA, the particles have irregular shape. For the three kinds of PCA, the introduction volume of impurity varies widely. When acetone is used as PCA, sample has the most introduction volume of impurities, followed by alcohol. When hexane is used as PCA, the loss of sample is serious due to excessive cold-welding. Among the three kinds of PCA, when alcohol is used as PCA, sample has the highest permeability, followed by acetone, and when hexane is used as PCA the permeability of sample is much lower than the first two. But, when hexane is used as PCA, better impedance matching can reach due to the low dielectric constant, and the microwave absorption properties is much better than alcohol and acetone. In summary, alcohol can be used as the optimal PCA in fabrication of flake-shaped carbonyl iron particles due to the high permeability and low pollution.When alcohol is used as PCA, with the ball milling time increasing, spherical carbonyl iron particles are flatted into flake shape gradually, and the longer the ball milling time is, the smaller the thickness of flake-shaped particles is. Such morphology transition may be the reason why the magnetic spectra show more and more obvious domain wall resonance peak at about 500 MHz with the increasing the ball milling time.(3) The surface of flake-shaped carbonyl iron particles was coated with a thin layer of amorphous SiO2 by Stober method to obtain flake-shaped Fe/SiO2 core-shell particles. After coating SiO2, the dielectric constant of flake-shaped carbonyl iron particles decreased significantly, which is close to the permeability, so ideal impedance matching can be achieved, and microwave absorption performance improved greatly. By changing the coating time, Fe/SiO2 core-shell particles with the thickness of SiO2 layer 20 nm,60 nm and 70 nm were obtained. The thickness of SiO2 layer almost have no effect on the high-frequency characteristics of samples, and the microwave absorption properties of the three samples with different thickness of SiO2 layer are almost same.(4) Fe/Ni0.5Zn0.5Fe204 composite materials were prepared by mixing and milling Ni0.5Zn0.5Fe204 fabricated using traditional co-precipitation method and carbonyl iron. For the sample with weight ratio of Ni0.5Zn0.5Fe204 and carbonyl iron 1:9, the particle shape changes from flake to irregular shape gradually, which results in the dielectric constant and permeability decreased gradually. As dielectric constant of samples decreases with increasing the ball milling time, impedance matching becomes better and the performances of microwave absorption are promoted. For the samples with different weight ratio of Ni0.5Zn0.5Fe2O4 and carbonyl iron, the dielectric constant and permeability decrease gradually with the content of Ni0.5Zn0.5Fe2O4 increasing in the sample, and impedance matching becomes better gradually, and the microwave absorption performances are enhanced.