The Structure And Microwave Dielectric Properties Of CaRAlO₄ (R=Nd,Y) Based Ceramics

Ca/Ti co-substituted CaNdAlO₄ and (Sr_1-xCa_x)YAlO₄ ceramics were prepared, and the microwave dielectric characteristics were evaluated together with the structure and microstructures. The following conclutions were obtained:CaNdAlO₄ was modified by Ca/Ti co-substitution to attain the Ca_1+xNd_1-xAl_1-xTi_xO₄ (x=0, 0.025, 0.05, 0.1, 0.15, 0.2) microwave dielectric ceramics. K₂NiF₄-type single phase structure was formed for x <0.20, while small amount of CaTiO₃ second phase appeared for the composition of x = 0.20. Lattice parameters a and c linearly increased and decreased, respectively, suggesting the formation of ideal Ca_1+xNd_1-xAl_1-xTi_xO₄ solid solutions. The microwave dielectric properties were improved by such co-substitution. Lowering of the inter-layer polarization with increasing x favored the improvement of Qf Value, while the enhancement of inter-layer stress resulting from the decreasing tolerance factor, as well as the appearance of second phase brought the negative influence on the Qf value. The variation of Qf value with x was the result of the competition between these two aspects. The maximum value was found at x=0.15 with Qf= 93,400 GHz, 1.4 times of that at x=0. Moreover, the dielectric constant of Ca_1+xNd_1-xAl_1-xTi_xO₄ ceramics increased with x. whileÏ„_f evolved from negative to positive to get a near-zero value. The optimum microwave dielectric characteristics were abtained at x=0.15:ε_r = 19.5, Qf = 93,400 GHz.Ï„_f= -2 ppm/℃.The stability of K₂NiF₄ structure in the system of (Sr_1-xCa_x)YAlO₄ increased with increasing x. At x=0 and 0.2, no K₂NiF₄-type phase was formed, and K₂NiF₄-type phase emerged at x=0.4 where the Y₂O₃ second phase was also observed. The second phase faded with increasing x, and K₂NiF₄-type single phase structure was attained for CaYAlO₄ (x=1.0) ceramics. Qf value of (Sr_1-xCa_x)YAlO₄ ceramics soared with x, and reached the maximum at x=0.8. When x>0.8, Qf value remained around 50,000 GHz. With increasing x, both the enhanced stability of the perovskite layer and the release of inter-layer pressure in the K₂NiF₄-type structure due to the decreased A-site cation size diference led to the improvement of Qf value. Furthermore, the fading of second phase also favored Qf value. On the other hand, the enhanced size mis-match between the perovskite layer and rock salt layer tended to lower the Qf value. The two aspects resulted in the observed variation of Qf value in (Sr_1-xCa_x)YAlO₄ ceramics. The dielectric constant remained around 18.0, whileÏ„_f decreased with x. The best microwave dielectric characteristics were attained at x=1.0 : Qf =52,000 GHz,ε_r=17.9,Ï„_f=4.0 ppm/℃.