Study Of Z-type Hexaferrite Materials For UHF Application

With the development of communications technology to high-frequency and the rapid development of high-speed digital circuits,the electromagnetic interference(EMI) is becoming an increasingly serious problem and the excellent soft magnetic materials able to work in the high frequency band are required urgently.Especially,Z-type hexaferrties are among the most widely used soft magnetic materials,instead of spinel ferrite for high-frequency application due to their high magnetic anisotropy.It can be used to the chip inductors for high-frequency and anti-EMI countermeasure devices for ultra-high frequency.In this thesis,the synthesis mechanism and the basic mechanisms of modification and low-temperature sintering of Z-type hexagonal ferrite have been analyzed.Then a series of novel materials and its preparation are explored and investigated.The main results are as follows:1.The phase transition and reaction rules of hexaferrites prepared by a solid-state reaction method were investigated.It was found that,flake Co₃O₄ can be used to achieve single-phase Z-type hexaferrites at 1250-1300℃.With increasing Sr content, the calcination temperature range for Z-type phase will be narrowed.2.The effects of preparation process on the microstructures and electromagnetic properties of hexaferrites and the relative mechanisms were studied in detail.The presintering temperature,firing and cooling rate,sintering temperature,and holding time were changed regularly to investigate the phase formation and the microstructure evolvement of hexaferrites by means of X-ray diffraction(XRD) and scanning electron microscopy(SEM).The variation of complex permeability,dielectric constant and quality factor with preparation parameters was studied.The effect of premagnetization on the broadband was first proposed.It was found that the special microstructure is the important reason for high resonance frequency.3.The modification mechanisms of different dopants were investigated.The variation of microstructures and electromagnetic properties of hexaferrites with different Y₂O₃,Nb₂O₅ and Pb_0.95Sr_0.05(Zr_0.52Ti_0.48)O₃(PZTS) contents were investigated. The Y₃Fe₅O₁₂ phase was found in all samples with Y₂O₃,resulting in the improvement of electromagnetic properties of hexaferrites.The influence of the reducing K₁ of the samples with Nb₂O₅ on the high permeability was discussed.The additive PZTS improved the complex permeability and the complex permittivity.4.The mechanisms of low-temperature sintering of the hexaferrites with Bi₂O₃,Bi₂O₃-SiO₂ and Bi₂O₃-CuO were investigated.It was found that flake CO₃O₄ is conducive to the synthesis of single Z phase and the preparation of high-performance Z-type hexaferrite sintered at low-temperature.The influence of Bi₂O₃ additive on the phase composition,microstructures and electromagnetic properties of hexaferrties was revealed.The relationship between zinc content and magnetic properties of(Ba_0.7Sr_0.3)₃Cu_0.5Co_1.5-xZn_x Fe₂₄O₄₁(0≤x≤1.5) was discussed.The significant improvement of high-frequency characteristics was obtained for the samples with Bi₂O₃-SiO₂ additive.5.The phase transition and reaction rules of hexaferrites prepared by a improved sol-gel method were investigated.A novel route has been developed to prepare low-temperature sintered Z-type hexaferrites without any additive.The nanoparticles derived an improved sol-gel method were utilized to synthesize the low-temperature fired Z-type hexaferrite by a solid-state reaction method.The higher initial permeability and higher quality factor than that of the sample with 1.0 wt%Bi₂O₃ were obtained for the sample with proper NZHP.