Characterization And Analysis Of Microstructure For Complex Perovskite Microwave Dielectric Ceramics

Complex perovskite microwave dielectric ceramics general exhibit excellent microwave dielectric properties: high values of dielectric constant (ε_r), high quality factor ( Q·f) and near zero temperature coefficient of the resonant frequency (Ï„_f), therefore, it was widely used as resonators, filters, dielectric capacitors, dielectric substrates, dielectric antenna and dielectric waveguide circult in telecommunication devices, and the key new materials of mobile communications, satellite communications and GPS technology, largely determine the size and performance limits of microwave communication devices and system.Ceramics with the general formula A(B'_1/3B"(2_/3))O₃(A=Ba, Sr; B'=Mg, Zn, Ni or Co; B"=Ta and Nb) possess the highest quality factor Q fin microwave frequency regime among the microwave dielectric materials and can potentially be used for high frequency communication application.The Ba[(Zn_1-xMg_x)_1/3Nb_2/3]O₃ (BZMN, x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0), (Ba_0.3Sr_0.7)[(Zn_xMg_1-x)_1/3Nb_2/3]O₃ (BSZMN) (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) and (Ba_1-xSr_x)(Zn_1/3Nb_2/3)O₃ (BSZN) (x = 0, 0.5, 0.65, 0.7, 1.0) solid solution ceramics were synthesized by the conventional solid-state sintering technique. Vibration spectra (Raman spectroscopy and Fourier transform far-infrared reflection spectroscopy, short for FTIR) and X-ray diffraction (XRD) were employed to evaluate the correlation between crystal structures and vibration modes of these solid solutions as a function of Ba²⁺ ions replaced by Sr²⁺ ions in the A-site and Zn²⁺ ions replaced by Mg²⁺ ions in the B-site.1. For the BZMN ceramics, 1:2 ordered structure appears for Mg-rich samples where x≥0.6. The Ba[(Zn_0.4Mg_0.6)_1/3Nb_2/3]O₃ has a 1:2 ordered monoclinic unit cell, which is distorted by the antiphase tilting of the oxygen octahedra. The variation in the Raman spectra (the intensity of band changing new mode appearing and the Ag(Nb) modes near 236.5 cm^-1 which correlate to the 1:2 ordered structure present a suddenly drop behavior in the Mg²⁺ content of x=0.6.) indicates that the crystal structures of the BZMN solid solutions have changed with the increase in the Mg²⁺ content, i.e. phase transition occurrence, and the degree of order increases. It is suggested that the FTIR modes near 300 cm^-1 and 500 cm^-1 are correlated to the 1:2 ordered phase.2. For the BSZMN ceramics, it is verified that these ceramics present a phase transition for Zn-rich samples ( x≥0.6). From the XRD analysis, the ceramics with x≥0.8 are the transformed phases which have the pseudocubic structures. Raman result shows that the samples with Zn²⁺ content of x≤0.4 present monoclinic phases. The phase transition is a gradual process, that is, the phase transition occurs from monoclinic phase to pseudocubic phase with increasing Zn²⁺ content. The sample where x=0.6 is the transition state, i.e., at x=0.6, pahse transition begins to appear but is not complete. In the Raman spectra, the appearance of the additional phonon modes indicates that the BSZMN systems exist monoclinic phase in the range of Zn²⁺ content, 0≤x <0.6. The disappearance of the the Eg(O) modes is caused by the phase transition from monoclinic phase to pseudocubic phase. Tilting of oxygen octahedra is the main reason for the phase transition. The phase transition is also verified by the FTIR spectra.3. For the BSZN ceramics, with the increase in the Sr²⁺ content, the lattice structure of ceramics turns gradually from disordered cubic structure to ordered structure and antiphase tilting of the oxygen octahedra occurs, which is the main reason for phase transition and variation of crystal structure. The appearance of phase transition is associated with variation of the symmetry structure,from cubic ( Pm3 m, where x=0) to pseudocubic (I4/mcm, where 0 .65≤x <1) and next to hexagonal ( P 3m1, where x =1). New phonon modes appear at around 250 cm^-1 where x≥0.65 in Raman spectra, and there is also a different phonon mode appears at 156 cm^-1 in FTIR spectra at the same range stated above. The appearance of the new phonon modes are the characteristic of the ceramics whose oxygen octahedra have tilted with the Sr²⁺ concentration. The reverse trend of Raman shift and the dielectric constant is found.It is sugest that the phonon modes are sensitive to structure, rather than to the mass of vibrating ions.