Preparation And Studies Of Structure,Morphology, Magnetic Properties Of Co₂Z-type Hexaferrite

Co2Z-type hexaferrite with high initial permeability, high resonance frequency, high quality factor, high thermal stablity and low loss property is a promising material for high frequency (300MHz-IGHz) application. In this thesis, Co2Z-type planar hexaferrite powders were synthesized using the molten salt method. The influence of different ions substitution and iron content on the microstructures and magnetic properties of the ferrite powders and sintered rings was investigated. Co2Z hexaferrite rings were fabricated by a two-step process. The effects of different materials and ions substitution on the high-frequency magnetic properties of Co2Z hexaferrite sintered rings were investigated. The primary works are as follows:A series of Ba3Co(2-x)Fe(24+x)O41(0≤x≤0.4) ferrite powders were prepared by the molten salt method. The phase composition, morphology, and magnetic properties of ferrite powders were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), and vibrating sample magnetometer (VSM) techniques. All the samples showed Z-phase with smooth surface and hexagonal plate shape. It was found that the coercivity (Hc) firstly increased and then decreased with the increasing of x, and the Hc had a minimum value around x=0.3. The saturation magnetization (Ms) was insensitive to the increase of x.Ba3Co1.6Fe(2n+0.4)O41(10.8≤n≤12.0) ferrite powders were synthesized by the molten salt method. The influence of iron content on the properties of Co2Z hexaferrite was investigated. The results of SEM showed that the gains size increased gradually and the gains thickness decreased with the decreasing of n. The Hc decreased with the decreasing of n at the same calcination temperature, while the Ms is insensitive to the calcination temperature and iron content. In order to prepare sintered rings for dynamic magnetic property measurements, the calcined powders were mixed with binder and pressed to rings with a steel mold. Then the rings sintered in air environment or oxygen-rich environment. The initial permeability increased with the increasing of sintering temperature for the samples with the same amount of iron. The initial permeability of sintered rings firstly increased and then decreased with the decreasing of n. The sintered ring had a larger permeability when the powder calcined at a lower temperature.Hexagonal ferrite samples Ba3Co1.7Fe(23.5-2x)(ZnTi)xO41(0.80≤x≤0.95) which Fe3+ions were partially substituted for Ti4+and Zn2+had been prepared by a molten salt method. The ferrite powders showed hexagonal plate shape when calcined at1250℃and1280℃. The gains size firstly increased and then decreased with the increasing of x. The gains thickness increased significantly and the hexagonal plate structure was destroyed when the powder calcined at1300℃. The samples had smaller coercivity at x=0.80,0.85and calcined at1280℃, which pressed to rings for dynamic magnetic property measurements. The sintered rings showed Z-phase with trace of W-phase. The real part of permeability had a maximum value21around x=0.8. The curve of the imaginary part μ" vs.f had a bround peak located around400-900MHz due to the resonance of domain wall displacement, and a significant shoulder around1.5-3GHz corresponding to the natural resonance due to spin rotation in Z-phase.The above studies showed that the coercivity of powder was lowest when the amount of Co2+ions substituted for Fe2+x=0.3(e.g. Ba3Co1.7Fe24.3O41). The permeability of sintered rings was highest when the iron concent n=10.8(e.g. Ba3Co1.6Fe22O41). The Ba3Co1.7Fe21.9O41and Ba1.5Sr1.5Co2Fe21.6O41were fabricated by a two-step calcination and sintering process. The precursors calcined at1080℃were well indexed to M-and Y-phases, and the hysteresis loops present "wasp waist" shape. The calcined power mixed with binder and pressed to rings for sintering, and the initial permeability decreased with the increasing of sintering temperature. The initial permeability of sintered rings is larger through using a-Fe2O3as raw material for Ba3Co1.7Fe21.9O41. The permeability of Ba1.5Sr1.5Co2Fe21.6O41is larger when the rings sintered at air atmosphere.