Characterization And Analysis Of Microstructure For Perovskite Microwave Dielectric Ceramics

Microwave dielectric ceramics refer to ceramic materials used in the microwave band circuit as dielectric materials to complete one or more functions, with the following excellent dielectric properties: high values of dielectric constant(?r), low dielectric loss(tan ?), near zero temperature coefficient of the resonant frequency(?f) and so on. In modern communication, they are widely used as the new key materials of satellite communications, mobile communications and GPS technology in filter, resonator, dielectric antenna and dielectric waveguide circuit. Therefore, microwave dielectric ceramics have become a new research hotspot in recent years.Ceramics with the general formula ABO3(A=Ba, Ca, Sr; B=Zr) and Ba(Mg1/3Ta2/3)O3(BMT) are all perovskite microwave dielectric ceramics with high quality factor(Q) and can potentially be used for high frequency or low loss communication application.The Ba Zr O3(BZ), Ca Zr O3(CZ), Sr Zr O3(SZ) and Ba(Mg1/3Ta2/3)O3(BMT) ceramics were synthesized by the conventional solid-state sintering technique. X-ray diffraction(XRD) was employed to analyze the crystal structures of these ceramics. Vibration modes were identified via vibration spectra(Raman spectroscopy and Fourier transform far-infrared reflection spectroscopy short for FTIR) and the relationship between the vibration modes were analyzed. Kramers-Kr?nig analysis(K-K) was adopted to obtain both the imaginary and real parts of the dielectric constant. Dispersion curve and high frequency dielectric constant were obtained via four-parameter model fittings and the contribution of each vibrational mode to the microwave properties was calculated. As well as the effect of different sintering temperature on Ba(Mg1/3Ta2/3)O3 ceramics was discussed. The main results are as follows:1. For the BZ ceramics, cubic perovskite structure is studied by X-ray diffraction. After stripping K?2, the XRD patterns prove that oxygen octahedron occurs in BZ ceramics and FTIR testifies this point. Seven Raman modes are identified and the A1g(O) mode, which has the highest intensity, is the breath vibration of the Zr O6 octahedron. At the same time, three IR-active modes are identified, F1u(2) regarded as the O-Zr-O bending mode has the most contribution to the dielectric properties.2. For the CZ ceramics, orthorhombic structure is confirmed by XRD data. Compared with the Joint Committee on Powder Diffraction Standard(JCPDS) cards, there exists a small number of Ca CO3 phase in CZ sample. According to the group theory, optic modes of CZ ceramics are as follow:?V =7Ag+5B1g+7B2g+5B3g+8Au+10B1u+8B2u+10B3uRaman shifts and full width at half-maximum(FWHM) values are discussed in detail. Far-infrared spectrum with six IR-active modes is fitted using the four-parameter semiquantum models. The F5 mode, with the most contribution to the dielectric properties, can be described as the inverted vibrations of Zr4+ atoms in Zr O6 octahedron, and the fitting curve shows mode F4 and F5 split because of the tiny difference between the partial structure and the average structure of the sample.3. For the SZ ceramics, orthorhombic structure is provided, according to XRD data. Raman spectra with ten active modes are assigned and B1 g mode, the stretching vibrations of Zr-O near 550cm-1, has the highest intensity. Far-infrared spectrum is fitted by the four-parameter models, F1u(4) mode splits into two parts. By comparing the contributions of each IR-active mode to dielectric properties, the dielectric properties are mainly affected by the modes involving more movements of heavier metal atoms.4. For the BMT ceramics, rhombohedral structure is confirmed by XRD and Rietveld refinement with 1:2 ordering structure. Raman spectra are assigned, F2g(Ba) mode has the highest intensity, the Raman shift of the F2g(Ba) mode has a negative correlation with the dielectric constants and a positive correlation with the temperature coefficients of constant, as well as a positive correlation with the dielectric losses. According to XRD, Raman and FTIR spectra, 1510? is the best temperature to sinter BMT ceramics.