Structure And Microwave Dielectric Characteristics Of Modified SrLaAlO₄-Based Ceramics

Sr1-xCaxLaAlO4 and (Sr0.6Cao.4)1+xLa1-x(Al1-xTix)04 ceramics were prepared, and the microwave dielectric characteristics were investigated together with the structure and microstructures. The following conclusions were obtained:SrLaAlO4 was modified by Ca substitution for Sr to attain the Sr1-xCaxLaAlO4 (x=0,0.2,0.4,0.6,0.8,0.9) microwave dielectric ceramics. K2NiF4-type single phase structure was formed for x<0.8, while only minor amount of LaAlO3 secondary phase appeared for the composition of x=0.8 and 0.9. Lattice parameters a and c both decreased linearly with increasing x, suggesting the formation of Sr1-xCaxLaAlO4 solid solutions. The microwave dielectric characteristics were improved through Ca substitution. The internal stress as a result of the distortion in rock-salt layer (caused by excessive ionic radius difference between A-site ions) with increasing x was beneficial to the improvement of Of value, while the increase of internal stress resulting from the decreasing tolerance factor, as well as the appearance of the secondary phase indicated the negative influence on the Of value. As a result of the competition of these aspects. Qf value inceased first to a maximum value at x=0.4 with Qf= 150,500 GHz, about 1.3 times of that at x=0, and then tured to decease with increasing x. Moreover, the relative permittivityεr of Sr1-xCaxLaAlO4 ceramics increased slightly with x, whileτf was adjusted towards zero. The best combination of microwave dielectric characteristics was achieved at x=0.4:εr=18.1, Qf=150,500 GHz,τf=-26.2 ppm/℃.As to the composition Sro.6Cao.4LaAlO4 with optimum microwave dielectric characteristics, itsτf still needed to be improved to achieveτf=±5 ppm/℃for microwave resonator application. Thus, the effects of introduction of Ti into B-site on the microwave dielectric characteristics of Sro.6Ca0.4LaAlO4 ceramics were further investigated. (Sr0.6Ca0.4)1+xLa1-x(Al1-xTix)O4 (x=0.025,0.05,0.1,0.15,0.2,0.25,0.3, 0.45,0.6,0.75) were prepared the standard solid state reaction process. K.2NiF4-type single phase structure was formed for 00, Qf value increased gradually first with increasing x and reached the maximum at x=0.25, then it turned to decrease for x>0.25. But Qf value of all these compositions of x>0 is smaller than that of x=0. As x increased,εr of (Sr0.6Ca0.4)1+xLa1-x(Al1-xTi)O4 ceramics was improved apparently andτf increased as well. The maximum of Qf value and near-zeroτf were attained at x=0.25 and 0.45 respectively. At x=0.25:εr=19.8,Qf=138,500 GHz,τf=-11.3 ppm/℃, while at x=0.45:εr=21.4, Qf=112,600 GHz,τf=-1.8 ppm/℃.