Dielectric Properties Of Ferroelectric-Ferromagnetic Composite Ceramics

The permittivity of Barium strontium titanate （BST） can change with the DC bias,and BST can be used in tunable microwave devices, such as phase shifter, phased arrayantennas, resonator, frequency agility devices, et al. The decrease of permittivity with DCbias field will result in poor impedance match at the tunable device. It is likely thatferrite-ferroelectric composites can adjust the characteristic impedance of the device bychanging the permittivity of ferroelectric and permeability of ferrite so that thecharacteristic impedance remains unchanged.Ba_0.5Sr_0.5TiO₃/BaFe₁₂O₁₉and Ba_0.5Sr_0.5TiO₃/YIG composite ceramics were preparedby a solid-state reaction method in this thesis and their phase compositions and dielectricproperties were investigated. In order to decrease dielectric loss of BST/YIG, the effect ofMgO doping on dielectric properties of BST/YIG is also studied. The experimental resultswere as follows:（1） Ba_0.5Sr_0.5TiO₃/BaFe₁₂O₁₉composite ceramics consists of ferroelectric BST phaseand BaFe₁₂O₁₉phase. The addition of BaFe₁₂O₁₉decreases the permittivity ofBa_0.5Sr_0.5TiO₃. With the increase of BaFe₁₂O₁₉content, permittivity and dielectric lossobviously increase in Ba_0.5Sr_0.5TiO₃/BaFe₁₂O₁₉composite. The permittivity and dielectricloss is in the range of880-3300and0.015-0.32, respectively.（2） Ba_0.5Sr_0.5TiO₃/YIG composite ceramics consists of ferroelectric BST phase andYIG phase. YIG decreases the permittivity of Ba_0.5Sr_0.5TiO₃obviously. With the increaseof YIG content, the permittivity of composite ceramics decrease and dielectric loss has nodistinct change.0.9Ba_0.5Sr_0.5TiO₃/0.1YIG ceramics sintered at1260oC has the smallestpermittivity （314） and dielectric loss （0.005）.（3） Ba_0.5Sr_0.5TiO₃/YIG/MgO composite ceramics contains four phases, i.e.Ba_0.5Sr_0.5TiO₃, MgO, MgFe2O4and YFeO₃phase. YIG phase disappeared because YIGreacted with MgO to form MgFe2O4and YFeO₃. With the increase of MgO content, permittivity and dielectric loss of composite ceramics obviously decrease. With theincrease of YIG content, permittivity reduces obvious, but dielectric loss graduallyincreases.The ferrite-ferroelectrics (Ba_0.5Sr_0.5TiO₃/BaFe₁₂O₁₉and Ba_0.5Sr_0.5TiO₃/YIG)prepared in the thesis have high dielectric loss due to the high resistivity of ferrite. It ispossible to decrease the dielectric loss by doping low loss dielectrics, but the reactionbetween dielectrics and ferroelectrics or ferrites should be avoided.