Studies On Preparation And Electromagnetic Properties Of Z-type Hexaferrite In UHF

With the rapid development of electronic technologies, new type communications components and household appliances continually replaced by new ones quickly.Therefore, magnetic materials with cut-off frequencies of only several hundreds MHz can’t meet the demand of new microwave devices. Due to their large magnetic anisotropy, the hexaferrites magnetic materials have higher cut-off frequencies. They thus possess great potential applications in the microwave field. Z-type hexaferrites(Ba3Co2Fe24O41) is one of six kinds of hexaferrites, which exhibit low coercivity, high natural resonance frequency and high initial permeability in the GHz region.In this thesis, crystal structures, physical properties and synthesis mechanism about Z-type hexaferrites have been firstly investigated. Subsequently, Z-type hexaferrites have been studied from the aspects of ion substitutions, doping modification and preparation process in detail. The contents of the thesis are organized as follows.(1) The effects of different kinds of ion substitutions, especially rare earth ions,have been studied on the crystal structures, microstructures and electromagnetic properties with the measurement frequency beyond 1 GHz. In this thesis, all kinds of positive ion contained in Z-type hexaferrites have been separately substituted and the substitution ions include both magnetic metal ions and nonmagnetic metal ions. The results showed that a lot of factors, such as ionic radius, Bohr magneton effectively,valence state can affect the final electromagnetic properties by means of controlling the saturation magnetization and magneto-crystalline anisotropy. Besides, some low melting point oxides can influent Z-type phase synthesis temperature and magnetic loss as well.(2) The effects of impurity doping have also been studied on the electromagnetic properties. The results showed that samples sintered at 1000 oC with 3 wt.% Bi2O3 added show that high Q factor(more than 10) in the frequency of 920 MHz can be obtained.Thus, they can be used to increase the gain of antennas. Besides, spinel ferrites with large magnetocrystalline anisotropy have also been used to dope hexaferrites. The results show that the electromagnetic wave absorption properties were enhanced due to the increase of the imaginary part of dielectric constant. The reason that the imaginary parts of dielectric constant are obviously increased can be ascribed to the appearance of interfacial polarization, which is resulted from the increase of micro interface.(3) The effects of oxygen atmosphere during sintering have been investigated.The results showed that sintering under oxygen atmosphere can avoid the formation of Fe2+, which will effectively weaken the eddy current effect and decrease the eddy current loss. Accordingly, the improved Q factor has been observed.