| (M=Ba, Ca, Sr; Ln=La, Nd, Sm, Y) layered perovskites are of Ruddlesden-Popper structure with n=1, and consist of an intergrowth of one perovskite layer alternating with two rocksalt layers. The MLnAlO4(M=Ba, Ca, Sr; Ln=La, Nd, Sm, Y) ceramics are of good microwave dielectric properties, and have been investigated as promising candidates for microwave applications. On the other hand, the octahedral center is occupied by heterovalent ions for complex provskites A(B1, B2)O3, which are important electroceramic materials. In this thesis, heterovalent ions of R2+(R=Zn, Mg) and Ti4+were used to replace Al3+ in SrLaAl04, and the phase composition, microstructures and microwave dielectric properties of SrLa(R0.5Ti0.5) O4(R=Zn, Mg) ceramics were investigated. Single-phase SrLa(Ro.5Tio.5)04(R=Zn, Mg) ceramics with I4/mmm space group were prepared by a standaed solid-state reaction method. The refinement of XRD data indicated that the substitution by (R0.5Ti0.5)3+ ions in SrLaAlO4 ceramics have both effects on the interlayer polarization and tolerance factor. With introducing (R0.5Ti0.5)3+ ions, the average ionic radius increasing, make the structure stability of K2NiF4-type change, lead to the tolerance factor decreases, indicate increased interlayer mismatch, which arouse extra internal stress and subsequently the degraded Qf value. The dielectric constant is proportional to the polarization of the ion. Because of the higher polarizability for (R0.5Ti0.5)3+, the dielectric constants εr have made great progress. At the same time, Temperature coefficient of resonant frequency τf has changed from negative to positive because of the enhanced long-range coupling of Ti4+. The best combination of microwave dielectric characteristics are achieved:εr= 29.4, Qf=34,000 GHz, τf=+38 ppm/℃ for SrLa(Zn0.5Ti0.5)O4 sinted at 1400℃/3h,εr=25.5, Qf=72,000 GHz, τf=+29 ppm/℃ for SrLa(Mg0.5Ti0.5)O4 sinted at 1575℃/3hDielectric constant (εr), Qf value and temperature coefficient of resonant frequency (τf) are the three key parameters for characterizing low-loss microwave dielectric materials. Considering the mearuement accuracy, εr and τf are usually measured by parallel plate method, while Qf value by resonant cavity method. Benefiting from the rapid and continuous improvement of computer performance in the past decades, high measurement accuracy of εr has been realized for resonant cavity method with the aid of numerical calculation. Therefore, many research groups only use resonant cavity method to measure εr and τf as well as Qf value. Strictly to say, the temperature coefficient of resonant frequency of low-loss microwave dielectric material (τf,s) is defined as the τf value of an ideal resonant system that is homogeneously filled with one material, while it is usually measured as the τf value of a non-ideal resonant system (τf,r) such as cavity resonator. In the present work, finite element analysis is used to investigate the measurement error of τf,s by resonant cavity method. Several error sources contribute to the total measurement error, which is usually several ppm/℃ and decreases with the dielectric constant of the sample (εr,s). In comparison, the measurement error is much lower and usually within 1 ppm/℃ for paralleling plate method. It is strongly suggested to measure τf,s by paralleling plate method but not by resonant cavity method if τf,s is treated as τf,r, especially when εr,s is low or high measurement accuracy is needed. If resonant cavity method is used to measure τf,s the result should be corrected with the aid of numerical calculation to improve the measurement accuracy. |
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