Structure And Modification Of BaO-Ln₂O₃-TiO₂-Based Microwave Dielectric Ceramics

With the rapid development of the microwave ceramics technology,more and more research has been conducted on the novel microwave dielectric ceramics,which has high dielectric constant,low dielectricloss,andnearzerotemperaturecoefficientofresonantfrequency.The BaO-Ln₂O₃-TiO₂-based solid solutions have important applications in microwave circuits because of their high dielectric constant,high Q×f and small temperature coefficient of resonant frequency.The effects of preparing methods on the phase compositions and doping were investigated with X-ray diffraction?XRD?,scanning electron microscopy?SEM?,energy dispersive X-ray spectroscopy?EDS?,Raman spectroscopy and network analyzer.The author discussed the Raman shift and full width at half maximum?FWHM?of the Raman mode with reference to the group theory which influenced the microwave dielectric properties.Finally the high quality microwave dielectric ceramics were produced successfully.The effects of sintering process on the microstructures of BaO-Ln₂O₃-TiO₂ ceramics prepared by the conventional solid state reaction method had been studied.The XRD shows the different phases at the different calcining temperatures of BaO-Ln₂O₃-TiO₂ ceramics.Thermal analysis?DSC and DIL?and XRD indicate the physical and chemical changes.The optimal sintering process parameters were determined at calcining temperature of 1100?,sintering temperature of 1350?with reference to relative density and microstructure.The effects of ionic substitution on the structures and microwave dielectric properties of BaO-Ln₂O₃-TiO₂ceramics were studied.In the paper,improvement of quality factor?Q×f?by substitution small Sr ions for Ba ions in Ba_6-3xLn_8+2xTi₁₈O₅₄4 solid solutions,where x=0.5 is reported.The results show that Sr²⁺ionic substitution could improve the sintering characteristics of BaO-Ln₂O₃-TiO₂-based ceramics.In these solid solutions for Ca²⁺doping amounts are greater than1mol%,the quality factor?Q×f?appears extremely low due to the inner strain resulting from the occupation of large Ba ions at relatively small A1-sites.As Ca ion is much smaller than Ba ion which leads to the crystal lattice constants,cell volume has been greatly changed.BaO-Ln₂O₃-TiO₂ ceramics doped with 5 mol.%Sr²⁺exhibited the optimal microwave dielectric properties of?_r=76.3,Q?f=12450 GHz,?_f=0.1 ppm/°C.The effects of SmAlO₃,NdAlO₃ and BaTi₄O₉ on the phase composition,sintering characteristics,microstructures and microwave dielectric properties of BaO-Ln₂O₃-TiO₂ ceramics were investigated.The addition of LnAlO₃?Ln=Nd,Sm?resulted in the formation of a corresponding second phase,which affected the grain size distribution and density of the BaO-Nd₂O₃-TiO₂ based ceramics.The surface topography was changed from quasi square to columnar with the addition of LnAlO₃,especially when the content of LnAlO₃ was above 10 wt%.The dielectric constant and the resonant frequency temperature coefficient??_f?decreased with the increasing addition of LnAlO₃.The value of Q×f is not apparently related to the addition of LnAlO₃.The best effect was obtained in the ceramics doped with LnAlO₃,where the samples with 12.5 wt%LnAlO₃ had?_r=62.78,Q?f=11108 GHz,?_f=-1.55 ppm/?.The effects of BaTi₄O₉ added to BaO-Nd₂O₃-5TiO₂ ceramics had the similar changes.BaO-Sm₂O₃-TiO₂ ceramics added with 30 mol.%BaTi₄O₉ exhibited the optimal microwave dielectric properties of?_r=52.7,Q?f=12065 GHz,?_f=-1.25 ppm/?.The addition of NdAlO₃ resulted in the formation of a corresponding second phase,which affected the grain size distribution and density of the BaO-Sm₂O₃-TiO₂ based ceramics.The surface topography was changed from quasi square to columnar with the addition of NdAlO₃,especially when the content of NdAlO₃ was above 10 wt%.The cell volume and the grain size reduced with the increasing addition of NdAlO₃.The dielectric constant and the resonant frequency temperature coefficient??_f?decreased with the increasing addition of NdAlO₃.The value of Q×f is not obvious related to the addition of NdAlO₃.BaO-Sm₂O₃-5TiO₂ ceramics added with 7.5 mol.%NdAlO₃exhibited the optimal microwave dielectric properties of?_r=73.22,Q?f=10300 GHz,?_f=-1.05ppm/?.The relationships between lattice vibrations and microwave dielectric properties of BaO-Ln₂O₃-TiO₂ ceramics were studied with the group theory.The dielectric constant and Q×f value of the ceramic were related to the peak position and FWHM of A_g and B_1gg modes in Raman spectroscopy.When the peak of A_g and B_1gg mode shifted toward a higher wave number,it means the cell volume and the dielectric constants were reduced.When the FWHM of A_g and B_1gg modes in Raman spectroscopy becomes large,it reflects the Q×f value of the ceramic has decreased.When the doping ions are the same as the substituted ionic charges,and the radius difference is relatively large,the dielectric constant changes and the reason of the blue shift or red shift are determined by the relationship between the phonon frequency and?q/m?^1/2.