| Ruddlesden-Popper compounds with layered perovskite structure possess the general chemical formula of A’2[An-1BnO3n+1], and they have attracted continuous scientific interests because of their excellent physical and chemical properties. In recent years, the research interests on these compounds have extended to the field of microwave dielectric materials. In the present thesis, the effects of structures and micro structures upon the microwave dielectric characteristics in SrLn2Al2O7(Ln=La, Nd, Sm) ceramics which belong to Ruddlesden-Popper compounds with n=2were investigated systematically. The improved sintering behavior and microwave dielectric characteristics were investigated by the formation of solid solutions with partical Ca substituting for Sr. Moreover, microwave dielectric characteristics of other ceramics in MO-Ln2O3-Al2O3(M=Sr, Ca; Ln=La, Nd) ternaty oxide system including (Sr, Ca)Ln2Al2O7ceramics were also investigated.SrLn2Al2O7(Ln=La, Nd, Sm) ceramics with I4/mmm space group were prepared by a standard solid-state reaction process. They are expected as the promising new candidates for microwave applications because of their excellent microwave dielectric characteristics. The refinement of XRD data indicated that there were two sites of Sr2+/Ln3-in SrLn2Al2O7(Ln=La, Nd, Sm) crystal structure and they had different occupation distributions. With decreasing the radius of rare-earth ion, Ln3+was inclined to occupy the A(2) site in rock-salt layer (RS), and the quantity of Sr2+in the A(1) site in double perovskite layer(Pi) incresed. Because of this occupation distribution, the interlayer polarization charge in rock-salt layer (RS) increased as the radius of rare-earth ion decreased; it casued the increase of innerstress in the structure. Therefore, the distortion of most bond lenths and bond angles increased, and the dielectric loss of SrLn2Al2O7(Ln=La, Nd, Sm) increased as the radius of rare-earth ion increased. The infrared reflectivity spectra research indicated that the translation vibration of Sr/La ions against the AlO6octahedron primarily contributed to dielectric constant and intrinsic dielectric loss. With rare-earth element varing from La to Sm, the τf value (the temperature coefficient of resonance frequency) of SrLn2Al2O7(Ln=La, Nd, Sm) materials shifted towards to the positive direction, and this variation was associated with the increased εr(dielectric constant).By Ca partical substituting for Sr,(Sr1-xCax)La2Al2O7and (Sr1-xCaA)Nd2Al2O7solid solutions were parepared, and their sintering behavior and microwave dielectric characteristics were significantly improved. With introducing Ca2+, the occupation distributions of cations in the crystal structure tended towards to the ideal state, and the free energy of formation of the solid solution was decresed. It can be the fundamental reason for the improvement of the sintering behavior. Because the Ca2+prefered different sites at different solid solutions, the least stacking fault in (Sr1-xCax)La2Al2O7and (Sr1-xCax)Nd2Al2O7solid solutions appeared at different components. The fomer one was at x=0.1and the latter one was at x=0.5. The distribution of stacking fault can be observed clearly by transmission electron microscope (TEM) analysis. The main reasons for significantly improved Qf value (the product of quality factor and resonance frequency) were the increasing densification and decreasing stacking fault of the solid solution. The τf values of the solid solutions also moved towards to the positive direction as x increased, and it can be associated with the increased εr at full densification.Other ceramics with good microwave dielectric characteristics, such as CaAl2O4, were found in MO-Ln2O3-Al2O3(M=Sr, Ca; Ln=La, Nd) ternaty system including (Sr, Ca)Ln2Al2O7ceramics. Important experimental basis was provided for expanding the range of composition tailoring for MLnnAlnO3n+1(n=1,2&∞) microwave dielectric materials with Ruddlesden-Popper structure and designing the composite materials with excellent microwave dielectric characteristics.A number of microwave dielectric ceramics with good combinations of microwave dielectric characteristics were obtained as following:◢SrLn2Al2O7(Ln=La, Nd, Sm): εr=18.2～21.6, Qf=64,680～71,680GHz, τf=-22.1～+4ppm/℃◢(St1-xCax)La2Al2O7(x=0.1～0.3): εr=19.7～20.1, Qf=135,400～110,700GHz, τf=-18.5ppm/℃◢(Sr1-xCax)Nd2Al2O7(x=0.5): εr=21.2, Qf=68,200GHz, τf=-0.5ppm/℃◢CaAl2O4: εr=8.5, Qf=90,000GHz, τf=-55ppm/℃... |
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