Formation, Electric And Magnetic Properties Of0-3Barium Titanate/Nickel Zinc Ferrite Cosintered Composite Ceramics

The0-3perovskite ferroelectric-ferrite ferrimagnetic composite ceramics are important multiferroic materials. They can be used to fabricate multifunctional devices, such as capacitance/inductance integrated filters which can promote the miniaturization of integrated circuits, as well as tunable devices and those devices for modulating electromagnetic waves which require impedance matching. However, these materials usually possess high dielectric loss (-1) at low frequencies due to lots of space charges induced by the ionic inter-diffusion between the constituent phases. Also impurity phases were easily formed in these cosintered materials, resulting in the weakening of the respective ferroic properties of the two phases. In this thesis, it was reported the dielectric loss of0-3ferroelectric/ferrimagnetic BTO/NZFO (BaTiO3/Ni0.5Zn0.5Fe2O4) ceramic composites was significantly reduced and the formation of impurity was depressed by decreasing the direct ionic inter-diffusion between the constituent crystalline phases due to the inertia noncrystalline phase among them. The effect of phase composition, chemical composition, different NZFO precursors and PBO (2PbO-B2O3) glass doping on the formation and electromagnetic properties of BTO/NZFO (BaTi03/Nio.55Zno.45Fe2.o2904) composite ceramics, as well as the mechanism of the structure-properties relationship in these ceramic composites were investigated in detail. The main results are listed below.Firstly, it was found a large amount of hexagonal BTO were formed if the highly active NZFO precursor was synthesized by combustion method from ferric nitrate, which was ascribed to the severe inter-diffusion and inter-substitution of the cations of the two phases. It rendered the dielectric loss much increased and the effective permeability significantly reduced. In order to suppress such interaction between BTO and NZFO, noncystalline content in the NZFO precursor was increased by replacing ferric nitrate with ferric citrate. In the composite ceramics prepared from such precursor, the interaction was successfully reduced and the impurity was not observed, which can be attributed to the easy accommodation of the diffusing cations by the noncystalline phase. As a result, the tangent loss of the composite with NZFO below its percolation threshold was reduced to be lower than0.1even at frequencies as low as100Hz, which was advantageous to the actual application of the perovskite/ferrite composite ceramics in fields like integrated filers. Secondly, PBO glass doping can block the conductive avenue within the NZFO clusters, such that the conductivity of BTO/NZFO (BaTiO3/Ni0.55Zn0.45Fe2.029O4) composite with NZFO above the percolation threshold can be reduced by1～2orders of magnitude. Also, the doping of PBO can significantly reduce the sintering temperature and promote the particle growth of NZFO by the liquid-phase sintering mechanism. The effective permeability of the composites can be increased by200%at most due to the growth of NZFO particle and washing of the impurity ions at grain boundaries.Thirdly, the concentration of the hopping electrons and thus the colossal permittivity and conductivity in both NZFO and BTO/NZFO (BaTiO3/Ni0.5Zn0.5Fe2O4) composites with high NZFO content can be reduced much by decreasing the Fe content to be lower than its stoichiometric composition. The frequency range in which the dielectric loss was lower than0.1can be widened from1MHz and higher to1KHz by annealing the as-sintered composites with low NZFO content in oxygen for24h. Moreover, excessive Ti can improve the compatibility of the two phases due to the depressing of the formation of hexagonal BTO.Fourthly, the effect of percolation transition on the densification and electromagnetic properties were also analyzed. It was found the density, permittivity and permeability were controlled by the percolation transition of NZFO. Both the concentration dependence of the effective permittivity and permeability were consistent with the Kirkpatrick’s effective medium equation. The polarization and the lack of magnetism of BTO controlled the electromagnetic response below the threshold, while the colossal polarization and ferrimagnetism of NZFO dominated above the threshold.Fifthly, for the colossal dielectric response of NZFO at high concentrations, a hybrid model concerning the hopping charges and conductivity inhomogeneity was proposed, which was further demonstrated to originate physically from the variable-range-hopping of the localized polarons in NZFO. The experimental dielectric spectra of the composites were demonstrated to be consistent with the theoretical hybrid dispersion equation obtained, which promoted the understanding of the dielectric and conductive mechanisms in ferrites and multiferric composite ceramics.Sixthly, the effect of internal demagnetizing field, which was not accounted sufficiently before, was analyzed theoretically by considering the effective permeability of a randomly oriented ferromagnetic.sphere with180°-domain structure. The results suggested the demagnetizing field arising from the nonmagnetic BTO can shield the response of spin rotation to some extent depending on the fraction of NZFO, thus weakening the permeability of NZFO and BTO/NZFO composites. A modified effective medium equation and a new dispersion equation of permeability of ferrite were proposed and then confirmed by the experimental data. By analyzing the experimental magnetic spectra, three factors reducing the permeability of NZFO in the composites were found:the increase in the magnetic anisotropy, the coefficient of the restoring force of domain-wall motion and the demagnetizing factor. This result was contributable to the understanding of the magnetic properties of0-3ferroelectric/ferromagnetic composites.