Structures And Microwave Dielectric Characteristics Of SrRAlO₄-Based (R=La,Nd,Sm) Ceramics

MRAlO4 (M= Sr. Ca:R= La. Nd. Sm) ceramics with K2NiF4 structure have excellent microwave dielectric characteristics together with the merits of easy preparation and low cost. and their promising applications are expected in the high-band microwave and millimeter-wave elements. In this thesis, the effects of the composition, crystal structure and microstructures in SrRAlO4 ceramics upon the microwave dielectric characteristics were investigated systematically, and the modification of the microwave dielectric characteristics were discussed. The following primary conclusions were obtained:Composition tailoring for SrSmA104 ceramics has significant effect on phase constitution, microstructure. and microwave dielectric characteristics. K2NiF4-type structure major phase with minor amount of secondary phases is observed in the ceramics with nominal composition Sr1+xSm1-xAlO4-x/2. and the type of secondary phase is dependent on x. Sr2.25Sm0.75AlO4.875 secondary phase is detected for x> 0. while the secondary phase for x< 0 is SrSm2AlO7. Different secondary phases and microstructures have different effects on the microwave dielectric characteristics. The stoichiometric composition is the key point to guarantee the optimum Qfvalue.Sr/Ti co-substitution in SrSmAlO4 ceramics has both effects on the interlayer polarization and tolerance factor. With increasing x, the interlayer polarization is reduced which is benificial to Of value, while the tolerance factor is decreased which is deteriorated to Of value. However, excessive A-site ionic radius difference (Sr2+/Sm3+:1.31 A/1.132 A) induces the extra increase of internal stresses in the rock-salt layer, which finally reduces the Qf value. HRTEM and SAED observation could confirm that the lattice distortion increases with increasing x.The fine structure evolution of Sr1+xSm1-xAl1-xTixO4 (x= 0,0.05,0.10,0.15) ceramics has been investigated through the analysis of Rietveld refinement of X-ray diffraction data. Raman spectra, and IR reflectivity spectra. The abnormal variation of Sr/Sm-O(2b) and Al/Ti-O(2) is observed for compositions from x= 0.10 to x= 0.15. which results in the extra internal stresses and subsequently makes a sharp decrease of Qf value. It indicates that the Of value is sensitive to the structure evolution induced by ions arrangement along c axis.Through the Ca2+-substitution for Sr2+, (Sr1-xCax)SmAlO4 (x=0,0.1,0.3,0.5,0.7, 0.9,1.0) ceramics with K2NiF4-type solid solution structure are synthesized with the significantly improved Qf value. According to the analysis of IR reflectivity spectra, it is well understood that the decreased A-site ionic radius difference reduces the intrinsic dielectric loss. Meanwhile, the inhomogeneous microstructure is suppressed by the formation of solid solution due to the structure adaptability of A-site centered dodecahedron, which reduces the extrinsic dielectric loss. Moreover, the substitution of Ca2+ weakens the Sr-O bonding and in turn makes theσf value more negative.(1-x)SrLaAlO4-xSr2Ti04 solid solution ceramics with K2NiF4-type structure can be synthesized in the entire range of 0≤x≤1. With increasing x, the dielectric constant s, increases from 17.6 to 31.0 due to the high polarizability of Ti4+. Meanwhile, the enhanced long-range coupling of Ti4+ makes theτf-value vary from-32 ppm/℃to 122 ppm/℃. The effects of competition balance of layer mismatch and interlayer polarization lead Qf value to increase first and then decrease. The degradation of Of value in the vicinity of x=0.2 can be attributed to the compositional inhomogeneity and fluctuation. The lattice distortion caused by the compositional inhomogeneity and fluctuation can be observed by TEM microanalysis, which should be originated from large positive mixing enthalpy during the formation of solid solution.Some good combinations of microwave dielectric characteristics are obtained:1) (Sr1-xCax)SmAlO4 (x=0.7):εr=18.7, Qf= 125,100 GHz,τf=-9.7 ppm/℃;2) (1-x)SrLaAlO4-xSr2TiO4 (x=0.4):εr= 18.5, Qf= 95,000 GHz,τf=-8.9 ppm/℃.