Chien-Ming Lei

Synthesis and Characterization the Dielectric Properties of Cobalt Doping Hexagonal BaTiO3

BaTi1−xCoxO3 ceramics (x = 0.05∼0.25) have been synthesized with the solid state method and the Co-doping samples were formed hexagonal structure with x < 0.25. The XPS spectra show the two Co2+2p3/2 and Co2+2p1/2 peaks in full doping range, which indicates hexagonal BaTi1−xCoxO3 are stable by Co2+, not Co3+-ion. Although the radius of Co2+-ion is larger than the radius of Ti4+-ion, the volume of unit cell is still decreasing with Co concentration increasing due to oxygen vacancy increasing, which can induce the hydrostatic pressure in this system. The Qxf value and K value are both decreasing with Co-doping level increasing due to hydrostatic pressure increasing.

The Higher Energy Components in Ti2p Xps Spectrum of Ga Doped Barium Titanate

The X-ray photoelectron spectroscopy was applied to study a series of room temperature stabilised hexagonal barium titanate (h-BaTiO3), which have been synthesized with solid-state reaction method by adding Ga^(3+) ions into the process of producing BaTiO3. The doping levels vary from 5% to 25% (0.05 ≤ x ≤ 0.25). The photoelectron spectra collected at different spots of the samples showed inhomogeneous occurrences of the atomic replacement. The inhomogeneity increases significantly at and beyond 20% of doping levels (x ≥ 0.2). The expected appearance of Ti^(3+) ions, as a result of replacing Ti^(4+) with Ga^(3+), was observed only when the Ga concentration is 20% or higher. In all of the samples we observed unexpected components at higher binding energies than the expected Ti^(4+)2p3/2 and Ti^(4+)2p1/2 peaks in BaTiO3. These components stayed weak until the doping level reaches 20% and higher, and became stronger together with Ti^(3+) components (at lower binding energies than the corresponding Ti^(4+) peaks). These unexpected components may be attributed to the internal compressive stress due to the coexistence of Ba2TiO4 and BaTiO3 as described in [J. Appl. Phys. 95, 219 (2004)].

Investigating the Dielectric Property of Abnormal Grain Growth Hexagonal BaTiO3

In this work, the structural and dielectric properties of hexagonal BaTiO 3 have been investigated. The samples are partially doped with Ga 3+ at Ti site to stabilize the hexagonal structure at room temperature. The abnormal grain growth (AGG) is observed on h-BaTi 1 − x Ga x O 3 samples at a higher sintering temperature. The Qxf values can be improved by Ga-doping in BaTiO 3 . By the evanescent microwave microscope scanning probe technology (EMP), it is found that the regions of the abnormal growth grains have the higher K-value than that of the normal growth grains. The K-value and Qxf value can be concurrently enhanced of the h-BaTi 1 − x Ga x O 3 samples with AGG have been reported.

Effect of Sintering Temperature on the Microstructural, Magnetic and Microwave Properties of Magnesium-Manganese Ferrites

High quality spinel magnesium-manganese (Mg-Mn) microwave ferrites were fabricated by conventional mixed oxide process. In this study, we report the influence of sintering temperature on the microstructures, and on the magnetic and microwave properties of Mg-Mn ferrites. Green compacts of Mg-Mn ferrites were sintered at 1250 to 1350°C for 4 h. XRD patterns revealed that all sintered Mg-Mn ferrites had a cubic spinel structure. According to fracture surface SEM images, the 1350°C sintered samples exhibited the best densification microstructure. Experimental results also showed that the 1300 and 1350°C sintered samples exhibited a close saturation magnetization and the real part of alternative-current magnetic susceptibility from 50 to 2500 Hz. Among all as-prepared Mg-Mn ferrites, the 1350°C sintered samples had the lowest magnetic energy loss at the resonance frequency of 15 kHz, the narrowest magnetic resonance linewidth of 330 G at a frequency of about 9.86 GHz, and the highest Q×f value of 940 GHz at a frequency of about 6.5 GHz.

Study the Raman Spectra and Dielectric Properties of Hexagonal BaTi1-xGaxO3 Ceramics

A series of polycrystalline Ga-doped Barium titanate (BaTi1-xGaxO3) were synthesized for the studies on the dopant-concentration dependence of Raman spectra and dielectric properties. The Qx f value of Ga-doping samples increases up to about 20% of Ga concentration, but the dielectric constant (K-value) decreases while the A1g phonon mode (636 cm−1) experiences an increasing red-shift. The phonon mode becomes soft with Ga-ion increasing, and the FWHM of Raman peak increases with Ga-ion increasing until tetragonal structure appeared. The effective ionic weight, ionic radii and Coulomb interaction are important influences, which induce the chemical pressure and reduce the excess charge in Ti-site in the Ga-doping h-BaTiO3 system. The relation between Raman mode shifter and K-value, and the correlation of the Qx f value and the FWHM of related phonon vibrations have been reported and examined.

Electrical transport properties of CoMn0.2−xGaxFe1.8O4 ferrites using complex impedance spectroscopy

In this study, we report the influence of Ga content on the microstructural, magnetic, and AC impedance properties of Co-based ferrites with compositions of CoMn0.2−xGaxFe1.8O4 (x=0, 0.1, and 0.2) prepared by the solid-state reaction method. Experimental results showed that the as-prepared Co-based ferrites had a single-phase spinel structure; the Curie temperature of Co-based ferrites decreased with increasing Ga content. All ferrite samples exhibited a typical hysteresis behavior with good values of saturation magnetization at room temperature. The electrical properties of Co-based ferrites were investigated using complex impedance spectroscopy analysis in the frequency range of 100 kHz-50 MHz at temperatures of 150 to 250 oC. The impedance analysis revealed that the magnitudes of the real part (Z’) and the imaginary part (Z”) of complex impedance decreased with increasing temperature. Only one semicircle was observed in each complex impedance plane plot, which revealed that the contribution to conductivity was from the grain boundaries. It was found that the relaxation time for the grain boundary (τgb) also decreased with increasing temperature. The values of resistance for the grain boundary (Rgb) significantly increased with increasing Ga content, which indicated that the incorporation of Ga into Co-based ferrites enhanced the electrical resistivity.

Study the sintering temperature effect and frequency dependence of magnetic property of (LaSr)MnO3 ceramic

ABSTRACT The (LaSr)MnO3 ceramics have been synthesized with the solid state method to study the sintering temperature effect and temperature dependent of AC magnetic susceptibility measurement, which is showed that all samples obey Neel magnetic relaxation behavior. Low temperatures AC magnetic susceptibility has been measured vary temperatures, which were fitted the Debye relaxations model to calculate the resonance frequency, and the magnetic anisotropy energy, which is decreasing with sintering temperature increasing.

Study the Phase Transition and Dielectric Property of Sr-Doping Hexagonal Ba(Ti,Ga)O3 Ceramic

In this work, we study the phase transition and structure properties of hexagonal Ba1-xSrxTi0.8Ga0.2O3 (x = 0.01, 0.03, 0.05, 0.07, and 0.09) systems, and also investigate the microwave dielectric property of this system. The all samples are prepared by the mixed oxide method, the structure of samples are transferred from hexagonal phase to mixed phases then tetragonal phase through the Sr-doping process, which is consistent with the thermodynamic prediction on literature. The samples are included the tetragonal phase, whose dielectric property is poorer than the hexagonal phase samples, so it also points out that the hexagonal BaTiO3 material has very good dielectric property and a large potential for application.

Electrical Properties of Lanthanum Strontium Titanates Modified by Microwave Sintering

The pure perovskite La0.3Sr0.7Ti0.9Nb0.1O3 (LSTN) with high densification and conductivity was achieved by 1500°C conventional sintering (CS) in activated carbon reduction atmosphere. The low conductivity of porous LSTN was obtained for low sintering temperature 1300°C in same process. Porous and well-crystallized LSTN could be obtained by microwave reduction sintering (MS). The activated carbon post-treatment with long dwell time improved the conductivities of microwave sintered LSTN samples. The conductivity of microwave sintered LSTN increased fast in short post-reduction time and was higher than conventional sintered LSTN for same sintering temperature. The microwave sintering essentially modified the LSTN electrical properties.