Dielectric Properties Of BST Ferroelectric-Ferromagnetic Composite Ceramics

Barium strontium titanate(BST) is a very exciting class of microwave materials because their dielectric constant depends nonlinearly on applied electric field.This bias-field dependence offers the possibility of making a wide variety of electrically tunable microwave devices. Ferrite materials have been used for magnetically tunable microwave devices whose electrical response can be tuned by applying a directcurrent(dc) magnetic field. Ferrite and ferroelectric materials individually provide the magnetic and the electrical tunability for microwave devices, respectively. However, tuning by permeability or permittivity only leads to changes in the impedance. With dual tuning one can maintain a constant microwave impedance while tuning the frequency or propagation velocity.Dual tuning microwave devices could potentially be accomplished by ferroelectric-ferrite composites. Therefore, it will be meaningful to study the dielectric and magnetic properties of ferroelectric-ferrite composites.Ba0.5Sr0.5Ti O3/Mg Fe2O4,Ba0.5Sr0.5Ti O3/Mg Fe2O4/Mg O and Ba0.5Sr0.5Ti O3/Li Fe5O8 ferroelectric- ferromagnetic composite ceramic were synthesized by using the traditional solid state reaction method and the results were as follow:Ba0.5Sr0.5Ti O3/Mg Fe2O4 composite ceramicsconsistof only BST phase and Mg Fe2O4 phase. The permittivity of Ba0.5Sr0.5Ti O3/Mg Fe2O4 decreases with the increase of Mg Fe2O4 content when the content of Mg Fe2O4 is smalland the sintering temperature is low. For Ba0.5Sr0.5Ti O3/Mg Fe2O4 composite ceramics with high Mg Fe2O4 content(>40%), higher sintering temperatures result in dramatic increased dielectric constant and dielectric loss due to percolation effects. The tunabilityof Ba0.5Sr0.5Ti O3/Mg Fe2O4 sintered at 1300 oC decreases with the increase of Mg Fe2O4 content. The tunability of 0.5BST/0.5Mg Fe2O4 decreases with the increase of the sintering temperature. The tunability of 0.5BST/0.5Mg Fe2O4 sintered at 1200 oC is even higher than that of 0.9BST/0.1Mg Fe2O4 sintered at 1300 oC. This may open a new route to control the tunability of composites. The permittivity of the Ba0.5Sr0.5Ti O3/Mg Fe2O4 composite ceramic changes in the range of 60–1675, and the Lichtenecker model and the corrected Lichtenecker model predict the relation between Mg Fe2O4 content and the dielectric constant very well. Saturation magnetization of BST/Mg Fe2O4 composite ceramics increase with the increase of Mg Fe2O4 content. When the content of Mg Fe2O4 is 60%, the saturation magnetization of the composite ceramic is 16.6emu/g.BST/Mg Fe2O4/Mg O composite ceramic is only composed of BST phase, Mg Fe2O4 phase and Mg O phase. The tunability and the dielectric loss of composite ceramics decrease with the adding of Mg O. The permittivity of Ba0.5Sr0.5Ti O3/Mg Fe2O4 composite ceramic changes in the range of200- 560. Saturation magnetization of the BST/Mg Fe2O4 composite ceramics decreases with the increase of Mg O content. Saturation magnetization of BST/Mg Fe2O4/Mg O composite ceramics is in the range of 5.1emu/g- 9.9emu/g.The electric field dependence of dielectric constant of BST/Mg Fe2O4/Mg O was well described byusing the ??multi-polarization mechanism?? model.BST forms diphasic ferroelectric-ferromagnetic composites with Li Fe5O8. The dielectric constant of BST/Li Fe5O8 compositeceramic decreases with the increase of Li Fe5O8 content and varies in the range of 70 – 580. With increasing the sintering temperature, thedielectric constant of BST/Li Fe5O8 increases and the dielectric loss decreases. 0.6BST/04 Li Fe5O8sintered at 1200 oC has the smallest dielectric loss: 0.07 at 10k Hz.