Preparation And Dielectric Properties Of Barium Strontium Titanate-based Composite Ceramics

A series of (Ba,Sr)TiO₃ ceramics, STO-BTO composite ceramics and BSTO-BTO composites were prepared by conventional solid-state reaction method. Temperature dependence of dielectric constant was measured. Crystal structure and microstructure were analyzed through X-ray diffraction patterns and scanning electron microscope photographs of the samples, respectively.(1) Ba_1-xSr_xTiO₃ ceramics were prepared by the conventional solid-state reaction method at sintering temperatures of 1350℃, 1400℃and 1450℃for 2 h, respectively. XRD analysis confirms that all of the BST ceramics are the good solid solution with the monophase of perovskite crystal structure. At room temperature, BST ceramics show a change of crystal structures from the cubic phase to the tetragonal one at x=0.3. Microstructures change significantly with both Sr content and sintering temperature. For those compositions with x=0⁰.8, there are three phase transitions, which temperatures decrease linearly with Sr content and are independent of sintering temperature. Dependence of dielectric peak values on Sr content and sintering temperature is discussed.(2) xSTO-(1-x)BTOcomposite ceramics were prepared by a two-step synthesis method at sintering temperatures of 1275℃1300℃and 1325℃for 2 h, respectively. XRD analysis confirms that all of the xSTO-(1-x)BTO samples have main phases of Ba_1-xSr_xTiO₃ solid solution crystal structure , a small amount of other components of barium strontium titanate phases are also exist in the composite ceramics. Microstructures change significantly with both Sr content and sintering temperature. For the samples at same sintering temperature, the Curie temperature gradually shifts to low temperature with increasing STO content. All samples show relaxation properties, the Curie peak values are significantly less than BSTO solutions.(3) xBSTO70-(1-x)BTO and yBSTO80-(1-y)BTO composites were prepared by a two-step synthesis method. Study results show all the samples have the grain size of 0.5 -1 μm; with the sintering temperature increases, the density of the samples has improved. The diffusion between BSTO80 and BTO is more notable than that between BSTO70 and BTO. This is probably because the ion concentration difference of BSTO80 and BTO is larger than that of BSTO70 and BTO. The ε'-T spectrum of the composites shows bimodal structure. In the sintering condition of 1300℃/10min, the 0.7BSTO70-0.3BTO, 0.8 BSTO70-0.2BTO and 0.7BSTO80-0.3BTO composites show a good dielectric temperature stability in the temperature range of -70℃--10℃, -80℃-0℃ and -120℃--20℃, respectively.