Structure And Microwave Dielectric Characteristics Of Ruddlesden-popper Ceramics

Dielectric materials with Ruddlesden-Popper structure have received extensive attention due to their significant application prospect in microwave communication systems.In the present thesis,the optimization of the sintering behavior of MLnAlO4(M = Ca,Sr;Ln = La,Sm)based ceramics have been investigated together with the microwave dielectric characteristics,and the composition,structure and microwave dielectric characteristics of SrLanAlnO3n+1(n = 1,2)and Srn+1TinO3n+1(n = 1,2)ceramics have been systematically investigated.The following primary conclusions have been obtained.SrLaAl1-x(Zn0.5Ti0.5)xO4 ceramics are developed by Zn2+/Ti4+ partial substitution for Al3+ in SrLaAlO4,and better microwave dielectric characteristics have been obtained.In the whole composition range,?r increases with the increasing x.?f can be adjusted from-30 ppm/? to 33.6 ppm/? and near-zero value is obtained at around x=0.5.The Qf values could be improved at the composition of x = 0.5 and then turn to decrease rapidly with further increasing x.The drop of Qf values could ascribe to the more unstable structure owing to the abnormal variations of the axial bonds(Sr/La-O2b and Al/Zn,Ti-O2),where lattice distortions and inhomogeneous regions are also confirmed from the high resolution electron microscopy(HRTEM)images.A new n = 2 type R-P compound with chemical formula of Sr2LaAlTiO7 is successfully synthesized and the excellent microwave dielectric properties are obtained.X-ray diffraction(XRD)Rietveld analysis confirms that the compound belongs to the I4/mmm space group and the interlayer polarization is considerably lower than that of SrLa2AlO7 ceramics.Moreover,a well ordered distribution of ions along c axis is confirmed from the HRTEM images of[001],[010],[110]zone axis.The optimized structure and microstructure lead to the outstanding microwave dielectric characteristics obtained in the present ceramics.The outstanding microwave dielectric characteristics with a dielectric constant range of 40?70 and ultra-low dielectric loss have been achevied in Srn+1TinO3n+1(n =1,2)ceramics.XRD patterns exhibit tetragonal R-P structures with I4/mmm space group in Srn+1TinO3n+1.Qf values increase with increasing sintering temperature until they reach the maximum and decrease slightly thereafter.The initial increase should benefit from the improvement of densification and grain size,while the following deterioration could ascribe to the increasing content of oxygen vacancy.?r and ?f are mainly determined by the composition and less sensitive with the evolution of microstructure.The addition of B2O3 could significantly lower the densification temperature of Ca1.15Sm0.85Al0.85Ti0.15O4 and Sr0.6Ca0.4LaAlO4 ceramics(?200?).For Ca1.15Sm0.85Al0.85Ti0.15O4 ceramics,a certain amount of B2O3(0?0.15 wt%)addition will not cause the change of phase composition and the high Qf value could be remained.While,with higher amount of B2O3 addition,the residual B2O3 will react with the matrix grains and deleterious secondary phase appeares.For Sr0.6Ca0.4LaAlO4 ceramics,a tiny amount of B2O3 addition could lead to the appearance of deleterious secondary phase together with a significant reduction of Qf value.Thus,the component and the amount of glass frit should be carefully designed when conducting experiments to lower the sintering temperature of microwave dielectric ceramics.The following ceramics with good combination of microwave dielectric characteristics have been developed:SrLaAl1-x(Zn0.5Ti0.5)xO4:?r = 23.5,Qf= 102,000 GHz,?f= 3.4 ppm/?(x = 0.5);Sr2LaAlTiO7:?r = 26.5,Qf= 110,850 GHz,?f= 2.95 ppm/?.