High-frequency Inductance Z-type Hexagonal Ferrite Materials

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 effects of preparation process on the microstructures and electromagnetic properties of hexaferrites and the relative mechanisms were studied in detail. The presintering temperature, firing 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.2. The modification mechanisms of different dopants were investigated. The variation of microstructures and electromagnetic properties of hexaferrites with different Sr²⁺, Ti4+-Zn²⁺, MnCO3, P2O5 and Y₂O₃ contents were investigated. The Sr²⁺ and Ti4+-Zn²⁺ addition can effectively increase the permeability. The Y3Fe5O12 phase was found in all samples with Y₂O₃, resulting in the improvement of electromagnetic properties of hexaferrites. The influence of the increasing grain size of the samples with P2O5 on the high permeability was discussed. The additive MnCO3 improved the complex permeability and the complex permittivity.3. The mechanisms of low-temperature sintering of the hexaferrites with Bi₂O₃, Bi₂O₃-SiO2 and Bi₂O₃-MgO were investigated. The influence of Bi₂O₃ additive on the phase composition, microstructures and electromagnetic properties of hexaferrties was revealed. The significant improvement of high-frequency characteristics was obtained for the samples with Bi₂O₃-SiO2 and Bi₂O₃-MgO additive. The high-frequency electromagnetic properties of Co2Z ferrite was also improved by sintered in oxygen atmosphere.4. Z-type hexaferrites prepared by sol-gel method were investigated. A novel route has been developed to prepare low-temperature sintered Z-type hexaferrites. 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 Z-type magnetic material with higher initial permeability and higher quality factor were obtained for the sample with NZHP.