| Summary: | 博士 === 國立成功大學 === 資源工程學系碩博士班 === 95 === The A(B'1/3B" 2/3)O3-type (A= Ba+2, Ca+2; B' = Mg+2 , Zn+2; B" = Nb+5, Ta+5) complex perovskite compound shows ordering structure in B-site cations and exhibits a very low dielectric loss at microwave frequency. The high cost of Ta-based complex perovskite ceramics makes it less cost-effective for many applications. Therefore, this study aims to investigate the structure-microwave dielectric property relations in the Nb2O5-based complex perovskite ceramics.
Barium zinc niobate, Ba(Zn1/3Nb2/3)O3, complex perovskite has been reported to have excellent microwave dielectric properties with close relation of its ordered structure and quality factor. This study investigates the effect of calcination condition on the evolution of ordered structure and on quality factor with Raman spectroscopy, x-ray diffractometry, and transmission electron microscopy. The present results reveal that the low temperature of synthesis causes the inhibition of ordered domain growth during sintering. In contrast, the 1:2 ordered domain in double-calcined Ba(Zn1/3Nb2/3)O3 powder grows significantly with an increase of sintering temperature and soaking time. It is attributed that the relatively higher degree of 1:2 ordering and better homogeneity are achieved by double calcinations. The quality factor is highly promoted by using double-calcined Ba(Zn1/3Nb2/3)O3 powder and sintered at 1350˚C / 48 h. Certainly, the quality factor closely relates to the size of ordered domain, degree of 1:2 ordering and relative density in Ba(Zn1/3Nb2/3)O3ceramic.
Furthermore, the dielectric properties and microstructural characteristics in solid solutions of Ba1-xLax[Zn(1+x)/3Nb(2-x)/3]O3 (BLZN) are investigated by measuring and observing these properties respectively. The 1:1 ordered structure of BLZN is explained by the random-site model for the distribution of B-site cations. The decrease in the tolerance factor (t) by lanthanum substitution causes the tilting of oxygen octahedra. It appears that the onset of antiphase and inphase tilting causes the variation in the temperature coefficient of resonant frequency. In the untilted region where t ≥ 1.01, the TCF shows a linear increase with decreasing tolerance factor. The region of antiphase tilting where 0.965 ≤ t < 1.01 causes the rapid decrease in TCF , including the reverse sign. The TCF slowly increases where t < 0.965, which is due to the presence of inphase tilting of oxygen octahedra. The is predicted by using the tolerance factor, and the near zero of TCF is obtained with lanthanum substitution in the solid solution of the BLZN system.
Moreover, this study investigates the effect of CaZrO3 (CZ) substitution on the evolution of ordered structure in a Ca(Mg1/3Nb2/3)O3 (CMN) system. It indicates that a (1-x)CMN-(x)CZ solid solution has the 1:2 and 1:1 ordered structure distorted by the antiphase, the inphase tilting of oxygen octahedra and the antiparallel shift of A-site cation. A distinct correlation is noted between the transition of the ordered structure and microwave dielectric properties. The decrease in the tolerance factor by zirconium substitution causes the large anharmonicity, which decreases the quality factor and increases the relative permittivity. Furthermore, the reduction in the tolerance factor causes the increase in because of an increase in tilting amplitude.
The A5B4O15 (A = Ba+2, Sr+2, Mg+2, Ca+2; B = Nb+5) cation-deficient perovskites ceramics also have good microwave dielectric properties. The effects of strontium substitution for barium on the structure and microwave dielectric properties of Ba5Nb4O15 compounds are investigated. The symmetric stretching vibration of Ba-rich compounds splits into two narrow Raman-active modes and indicates the existence of a strong anharmonic lattice in (Ba1-xSrx)5Nb4O15 system. Nevertheless, the symmetry changes from P-3m1 to P-3c1 for the Sr-rich compounds by the Rietveld analysis of XRD patterns. The relative permittivity and temperature coefficient of resonant frequency increases linearly with an increase of strontium content in Ba-rich compounds, but decreases in Sr-rich compounds. The transition of symmetry influences the variation of TCF and K. The behavior of TCF and K is correlated to the variation of symmetric stretching vibrations of NbO6 octahedra and c/a ratio of lattice parameter in the unit cell.
Furthermore, the microwave dielectric properties of (Ba1-xMgx)5Nb4O15 ceramics are also investigated. The formation of second phase MgNb2O6 causes the TCF decrease, including reverse sign and near zero of TCF is obtained.
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