| More and more electronics and equipment widely used in military and civil applications, such as radar, local area network, computers, mobile phones, 3G and 4G of mobile telecommunications technology, work at gigahertz frequencies. Therefore, microwave absorbing materials and electromagnetic interference(EMI) materials are required to have a higher resonant frequency and higher permeability. Thinner thickness, lighter weight, wider frequency band of microwave loss and more intensity of the microwave absorption are needed for absorbers. High-frequency magnetic performances of Ferrites with the spinel structure are restricted by Snoek’s limit, so these materials usually can’t not be applied in gigahertz range of frequencies. According to the bianisotropy theory, materials with the easy-plane anisotropy can exceed the restriction of Snoek’s limit.The flake shell-structure Fe/Fe4 N powder with the shape-induce planar anisotropy and inoxidizability and the rare-earth 3d transition intermetallics with the intrinsic magnetocrystalline easy-plane anisotropy have been prepared based on the bianisotropy model, and their high frequency properties and microwave absorbing performance were measured and discussed. According to quarter-wavelength, microwave absorption properties dependence of the absorber thickness for flake carbonyl-iron powders nitrided in various time were calculated and discussed, perfect matching frequencies and perfect matching thickness dependences of the magnetic powder volume percent were given, and microwave absorbing performance of micronized powders, leading to smaller particles and less eddy-current loss, were discussed. The details and results of our work are listed as follows:1. Choice of an appropriate volume percent for compositesThe inner demagnetization factors of various volume percent for spherical carbonyl-iron particles were determined by the ferromagnetic resonance(RMR). The results indicated that high frequency properties of composites dependence of the volume percent is mainly owing to magnetic dilution characterized by the inner demagnetization factor. The best magnetic powders’ volume percent of composites is between 25 % and 40 % for the design and research work.2. Nitridation of flake carbonyl-iron powders(i) The nitriding temperature of flake carbonyl-iron can be divided four districts: non-react district, phase transition district and α-Fe reduction district. the best temperature 350 ℃ was selected for carbonyl-iron surface nitridation.(ii) The initial permeability of nitridation products is close to flake carbonyl-iron when nitriding time is within 1 h. The real part of permeability and natural resonant frequency decrease along with increase of nitriding time. When nitriding time is equal to 1 h, the weight concentration of N can reach 25 %. The nitriding time should be controlled less than 1 h.(iii) According to quarter-wavelength model, the relation between matching frequency and matching thickness of carbonyl-iron and its nitrides composites was discussed for 30 min, 45 min and 60 min of nitriding time. The Musal model and the Musal chart were used to explain the calculation error by quarter-wavelength model.3.(Nd1-x Smx)2Fe17N3-δ powders(x = 0.0 ~ 1.0)(i) The preparation of(Nd1-x Smx)2Fe17N3-δ powders was optimized and high-purity and irregular-shape(Nd1-x Smx)2Fe17 N3-δ particles were obtained.(ii) Lattice parameters, saturation magnetizations and coercivity forces of(Nd1-xSmx)2Fe17, ball-milling(Nd1-x Smx)2Fe17 and(Nd1-x Smx)2Fe17N3-δ for various x were measured and calculated. The easy magnetization type of(Nd1-x Smx)2Fe17N3-δ were determined by XRD of composite oriented in the rotating magnetic field. The result shows that(Nd1-x Smx)2Fe17N3-δ is easy-plane when x is between 0 and 0.4,(Nd1-xSmx)2Fe17N3-δ is easy-axis when x is between 0.5 and 1.0 and(Nd1-x Smx)2Fe17N3-δ may be easy-cone when x is between 0.4 and 0.5.(iii) Permeability of 35 vol. %(Nd1-x Smx)2Fe17N3-δ/paraffin composites for x between 0.0 and 0.4 is higher than the others. The oriented composites for x between 0.0 and 0.4 have higher real part of permeability than those with the same x value and have larger value of Snoek’s product larger than Snoek’s limit. On the contrary, orientation of the composites for x between 0.4 and 1.0 makes no obvious difference to permeability.(iv) Permeability and permittivity of oriented Nd2Fe17N3-δ/paraffin composites were measured for various volume concentrations. According to the theory of transmission line and quarter-wavelength model, microwave absorption RL was calculated for fixed matching thicknesses(1.0 mm and 1.5 mm), fixed matching frequencies(2 GHz and 13 GHz) and the perfect matching frequency(|Zin/Z0| = 1) of composites, respectively. The matching frequency and matching thickness dependences of the volume concentration were explained by the input impedance curve calculated by the quarter-wavelength model.4. Further refinement of Ce2Fe17N3-δ powders(i) The optimized conditions for preparing Ce2Fe17N3-δ powders were obtained according to the purity and repeatability of Ce2Fe17N3-δ: the Ce2Fe17 powders must be sieved by the sample sieve with 500 mesh and nitriding time is 4 h.(ii) Zr O beads which have greater density than the agate was selected as ballmilling beads. By changing the diameter of beads and ball-to-powder weight ratio, optimizing beads are mixed by 5.0 mm beads and 3.5 mm beads with 1:3 weight ratio, and the ball-to powder weight ratio is 150:2.(iii) Permeability and permittivity of 35 vol. % non-oriented and oriented Ce2Fe17N3-δ/paraffin composites were measured for various ball-milling solutions, respectively. According to the results, eddy current loss was reduced by decreasing the thickness or diameter of particles. These composites fabricated using various milling media have excellent microwave absorption performance.5. Eddy current and domain-wall vibration in rare earth-iron nitrides(i) It is confirmed that eddy curent and domain-wall vibration can be present in rare earth-iron nitrides in 0.1-18.0 GHz by the theoretical calculation.(ii) According to the Effective Medium Theory(EMT), it is demonstrated that there is a linear relation between the effective permeability of composites and the intrinsic permeability of magntic particles in the composites.(iii) Permeabilities of irregular Nd2Fe17N3-δ/paraffin composite, irregular Ce2Fe17N3-δ/paraffin composite and flaked Ce2Fe17N3-δ/paraffin composite were fitted quite well considering the eddy current, the domain-wall vibration and the natural resonance. |
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