Structure And Microwave Dielectric Characteristics Of Modified SrNdAlO₄and CaYAlO₄-Based Ceramics

MRAl04(M=Sr, Ca; R=Nd, Y) ceramics with K2NiF4structure generally have excellent microwave dielectric characteristics, while several methods should be used to modify the existing ceramics system in order to keep pace with the development of modern microwave technology. In the thesis, the modification of SrNdAl04and CaYAl04microwave dielectric ceramics has been investigated, and variation tendency of microwave dielectric characteristics in the modified ceramics has been determined together with the structure and microstructures.SrNdAl04ceramics were modified by Sr/Ti co-substitution, the single phase K2NiF4-type solid solutions with general formula Sr1+xNd1-xAl1-xTixO4(x=0.05.0.1.0.15,0.2,0.25,0.3,0.45,0.6,0.75,0.9) were obtained in the entire composition range. With increasing x, the dielectric constant εr increased linearly while the temperature coefficient of resonant frequency τf changes from negative to possitive The significantly improved Qf value was achieved in the present ceramics with increasing x, and it reached the maximum at x=0.6. The reduced interlayer polarization with Sr/Ti co-substitution in SrNdAlO4contributed to the improved Qf, while the decreased tolerance factor and increased inner stress brought the negative effects. The best combination of microwave dielectric characteristics was achieved at x=0.6(εr=23.6. Qf=86300GHz, τr=10.9ppm/℃).SrNdAlO4ceramics were modified by Ca-substitution, the single phase K2NiF4-type solid solutions with general formula (Sr1-xCax)NdAlO4(x=0,0.2,0.4,0.6,0.8,1) were obtained in the entire composition range. With increasing Ca-content, the dielectric constant εr increased slightly and the temperature coefficient of resonant frequency τf changed from-16.5ppm/℃to-11.5ppm/℃. Of value with different temperatures generally increased first and then decreased due to the oversintering, while it reached the maximum at x=0.6. Better microstructure and microwave dielectric characteristics were achieved at the middle of the composition points, because mixing enthalpy of these composition points were relatively smaller. The best combination of microwave dielectric characteristics was achieved at x=0.6(εr=18.9. Qf=91.300GHz, τf=-12.9ppm/℃).CaYAlO4ceramics were modified by create the composite ceramics with Mg2TiO4which had the opposite temperature coefficient of resonant frequency. The composite ceramics with general formula (1-x)CaYAlO4-xMg2TiO4(x=0.05.0.1,0.15,0.2.0.25,0.3) indicated the stable presence of CaYAlO4and Mg2TiO4as the major phases, and minor amount of Y2O3and MgTiO3secondary phases was detected. With increasing Mg2TiO4-content, the dielectric constant εr inceased slightly from19.1to20.6and the Of value slightly decreased. The temperature coefficient of resonant frequency τf decreased towards negative and reached zero at x=0.05. When the content of Mg2TiO4phase was relatively lower, the Qf value decreased not so much while the εr and τf were improved. The good combination of microwave dielectric characteristics of (1-x)CaYAl04-xMg2TiO4ceramics was achived at x=0.05:εr=19.1.Qf=55.000GHz,τf=-1.1ppm/℃.