SrLaGaO₄-Based Microwave Dielectric Ceramics And Theoretical Investigation On Dielectric Constant Of K₂NiF₄-Type Materials

SrLaGaO₄ single crystal with K₂NiF₄-type structure has excellent microwave dielectric properties,and the Qf value is as high as 170,000GHz.However,little work on SrLaGaO₄ ceramics has been carried out.This thesis focuses on SrLaGaO₄ and SrLaGaO₄-based ceramics,in which the preparation,structure,microstructure and microwave dielectric properties of?1-x?SrLaGaO₄-xSr₂TiO₄ and?1-x?SrLaGaO₄-xSrLa?R0.5Ti0.5?O₄?R=Mg,Zn?solid solutions were investigated systematically.Moreover,based on the Clausius-Mosotti equation and Phillips-Van Vechten?P-V?theory,the dielectric constant of K₂NiF₄-type dielectric materials was discussed preliminarily.Dense?1-x?SrLaGaO₄-xSr₂TiO₄?r=0,0.2,0.4,0.5,0.6,0.8?ceramics were prepared by a standard solid-state reaction method.The structural evolution was investigated by Rietveld refinement and Raman spectroscopy.With x increasing,both the dielectric constant??r?and the temperature coefficient of resonant frequency??f?increase monotonously.Meanwhile,the Qf value is on the rise as a whole due to the decreasing layer mismatch and interlayer polarization.Moreover,the decreased secondary phase and increased grain size are also responsible for the improvement of the Qf value.The following optimal combination of microwave dielectric properties was achieved for x=0.5 sintered at 1450? in air for 3h:?r= 23.3,Qf= 129,700GHz,and ?f =-1.7 ppm/?.Dense?1-x?SrLaGa04-xSrLa?R0.5Ti0.5?O₄?R=Mg,Zn??x= 0.2,0.4,0,6,0.8?ceramics were prepared by a standard solid-state reaction method.With x increasing,?r and ?f increase simultaneously.The Qf value for R=Mg firstly increases and then declines while the Qf value for R=Zn tends to decrease as a whole.The Qf value is influenced by the tolerance factor?t?,porosity,grain size and the secondary phase.The following ceramics with the optimal combination of microwave dielectric properties had been developed:0.6SrLaGaO₄-0.4SrLa?Mg0.5Ti0.5?O₄??r-=23.3,Qf=89,400GHz,and ?f=-0.8ppm/??;0.8SrLaGaO₄-0.2SrLa?Zn0.5Ti0.5?O₄(?r=22.4,?Qf=72,900GHz,and ?f =-5.4ppm/??.Based on the Clausius-Mosotti equation and P-V theory,theoretical investigation on dielectic constant of SrLaBO₄?B=Al,Ga,?Mg0.5Ti0.5?,?Zn0.5Ti0.5??,Sr₂TiO₄ and corresponding solid solutions with K₂NiF₄-type structure was carried out.Considering that the coordination environment has impact on the polarizability of the ions in crystal,the molecular polarizability calculated by the sum of individual ion polarizabilities is inaccurate.Therefore,the deviation of predicted and measured values is relatively large.In the P-V theory,complex crystal is split up into bonding subformulas,and the dieletric constant can be predicted by the crystal bond parameters.The P-V theory overcomes the inaccuracy of the sum of individual ion polarizabilities to some extent,so the accuracy is higher for the dieletric constant obtained by P-V theory.Higher prediction accuracy is expected for P-V theory if the correction factors are taken into account.