| Summary: | 碩士 === 國立臺灣師範大學 === 物理學系 === 93 === In this thesis, we adopted the Extended X-ray Absorption Fine Structure (EXAFS), Raman scattering and x-ray diffraction techniques to detect the micro-structure of a series of perovskite microwave materials and the correlation with microwave dielectric properties were also investigated. The size of oxygen-octahedrons in perovskite ceramics can be revealed by these measurements, and can be directly correlated with the microwave dielectric properties.
The EXAFS measurements of xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3 (hereafter xBMT +(1-x)BMN) with Ta and Nb as core ions revealed the structural properties of the oxygen-octahedrons. The rigid oxygen-octahedrons give the tight oxygen bonds which imply the reluctant motion of the cations and anions due to external electromagnetic waves. We found the Ta-O bond length decrease with the Ta concentrations in xBMT +(1-x)BMN, therefore, the decreasing of dielectric constant as Ta concentration increases in xBMT +(1-x)BMN is expected. The distortion of the oxygen-octahedrons in xBMT +(1-x)BMN were also revealed by the EXAFS measurement, and the small distortion gives high Qxf values as expected, such as Ba(Mg1/3Ta2/3)O3 and Ba(Mg1/3Nb2/3)O3 ceramics. The largest distortion in xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3 were found for x=0.75, and this causes its Qxf value is lower than the compound with x=0.25. The smallest Qxf value were expectedly found for the compound with x=0.5, due to the distortion of the oxygen-octahedrons and the B”-site ordering effect. However, the 1:2 ordering factors in these compounds can not be easily detected by EXAFS.
The substitution of Ti ions for both Ta and Mg ions in xBaTiO3-(1-x) Ba(Mg1/3Ta2/3)O3 are the cause of small lattice constants, low quality values, and the high dielectric constants. The oxygen octahedrons were deteriorated due to Ti substitution, and this causes dielectric constant increase with Ti concentration. The result of Qf value decrease with Ti concentration is mainly due to the degrading of 1:2 ordered structure, which were determined by the increasing of A1g(O) phonon width.
The result of complex perovskites of xSr(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Ta2/3)O3 ( hereafter xSMT+(1-x)BMT) clearly reveal a three-stage variation in the crystal structure due to the Sr substitution. For x≦0.5, the 1:2 ordered structure is still a dominate structure, and the dielectric constant increases and the Qf value declines as Sr substitution increases. This is mainly due to the smallness of the Sr2+ ion and the slightly twist of the TaO6 oxygen octahedrons caused by the Sr substitution. For 0.5≦x≦0.75, the sample structure is no longer dominated by the 1:2 ordered structure. At x≧0.75, the disappearance of the 1:2 ordered phonons reveals that the new crystal symmetry has appeared. Still, the characteristics of the oxygen octahedrons are strongly related to the microwave properties.
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