| Summary: | 博士 === 國立臺灣師範大學 === 物理研究所 === 91 === In this study, full-band dielectric spectroscopy and microwave evanescent microscopy were used to study the intrinsic and extrinsic dielectric mechanism of microwave dielectric materials. Bi1.5Zn1.0Nb1.5O7 and Ba(Mg1/3Ta2/3)O3 ceramics are investigated to reveal the mechanisms for high dielectric constant and high quality factor, respectively. The studied spectrum regimes include: low-frequency band (1 kHz－3 MHz), microwave frequencies (~GHz), submillimeter wave (THz) band, infrared frequency band, and optical (visible light) band. The dominant polarization mechanisms in each band are studied, and the correlation between these mechanisms is discussed.
Displasive disorders are found in both the A and O’-sites of Bi1.5Zn1.0Nb1.5O7 pyrochlore structure, A2B2O6O’1, which have significant influences on the dielectric properties of the materials at microwave (~GHz) and lower frequency (1 kHz－3 MHz) bands. The highly polarizable A-site ions, Bi3+, and the asymmetries at A- and O’-sites cause prominant dielectric contributions from A-O and A-O’ related modes, resulting in high dielectric constants at microwave frequencies. Large damping coefficients of these modes also lead to high dielectric losses. In contrast, the displacement disorders mechanism is not operating for high-Q Ba(Mg1/3Ta2/3)O3 ceramics and the dominant dielectric mechanism for theses materials is the polarization of rigid B”O6 octahedra in the complex perovskite structure A(B’1/3B”2/3)O3. Microwave dielectric constants are influenced by the rigidity of the three-dimensional oxygen-octahedron network, while the microwave quality factors, Q×f values, are related to the distortion of B”O6 oxygen octahedron.
In the study of extrinsic mechanism, EMP images reveal that samples with inferior dielectric properties usually contain large proportion of low-K and low-Q secondary phase. Moreover, scanning Raman spectroscopy reveals the presence of grains with distorted crystal structure. These defects degrade the macroscopic dielectric properties, and are the main extrinsic mechanisms modifying ceramic properties.
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