Papers by Author: Fei Zhao

Abstract: In this work hexagonal perovskite powders of CaLa4Ti4O15 were prepared by citrate sol-gel method. The calcined temperature was 1150oC, which is lower than that of the conventional solid method. Dense CLT ceramics were produced at sintering temperature of 1600oC. The crystal structures and microstructures were investigated by XRD, TEM and SEM, respectively. Compared with samples produced by solid method, the sol-gel derived CLT ceramics exhibited a near zero temperature coefficient of resonant frequency (τf ) of -6 ppm/oC, and other microwave properties , εr ~ 43, Q×f ~ 42000 GHz were also obtained. Moreover, the sintering temperature of the CLT ceramics could be reduced to 1400 oC by addition of B2O3-ZnO.

Abstract: Microwave dielectric ceramics of CaO-2ZnO-xTiO2 (x=6~9) were prepared by citrate sol-gel method. The phase formation and structural properties of the ceramics were investigated as a function of titania content (x). Monophase of Ca2Zn4Ti16O38 polytitanates were formed at x = 8. With the increase of x, residual rutile phase were observed, whereas with the decrease of x, the CaTiO3 and Zn2TiO4 phases were detected. The microwave dielectric properties of the sintered ceramics were investigated using resonance method. The variation of dielectric properties of the ceramics as a function of titania content (x) was investigated. For the monophase of Ca2Zn4Ti16O38 polytitanate at x = 8, the dielectric properties showed minimum dielectric constant of 48.1, minimum temperature coefficient of resonant frequencies +44.1 ppm/oC, and maximum Q×f value of 31000 (at 6.921GHz). Deviating from x = 8, the dielectric constant and temperature coefficient of resonant frequencies slightly increased, and the Q×f value decreased. The variation was concerned with the presence of secondary phase.

Abstract: Zinc titanate ceramics having excellent dielectric properties and low sintering temperature are promising materials for RF/microwave multilayer devices. In this paper, the ZnO-V2O5 addition was added to promote the sintering process in order to obtain low-temperature sintered ceramics with high quality factors. The sintering addition could also restrain ZnTiO3 phase from decomposition. Using this method, low-temperature sintered zinc titanate ceramics with excellent microwave dielectric properties of εr~ 25.3, Q×f~15200GHz, and τf ~ -16 ppm/oC were obtained at sintering temperature of 800oC. The low-fired mechanism, microstructure, phase formation and microwave dielectric properties of ceramics were investigated by XRD, SEM, EDS and network analysis techniques, respectively.

Abstract: The crystal structure, phase relation, microstructure and dielectric properties of microwave dielectric ceramics comprised of Zn(Nb1-xTax)2O6 were investigated systematically using XRD, SEM and a network analyzer in this study. XRD results show that the Zn(Nb1-xTax)2O6 ceramics have the orthorhombic for all x values. However, two kinds of solid solutions phase regions based on columbite structure and ZnTa2O6 structure were observed. SEM results show that the growth of grains is restrained with increase of Ta content in Zn(Nb1-xTax)2O6 ceramics. The dielectric properties exhibited a significant dependence on the crystal structure and compositions of the ceramics. The dielectric constant (εr) of Zn(Nb1-xTax)2O6 ceramics increased with increasing Ta content. However, the quality factors (Q×f) of this system show a different trend, and decrease with increasing Ta content. At x=0.65, a zero temperature coefficient of resonant frequency ( τ f) can be realized in Zn(Nb1-xTax)2O6 ceramics system.

Abstract: The nano-sized Ba(Zr0.2Ti0.8)O3 powder was prepared by citrate sol-gel method. The crystal structure and the particle size of the powder were characterized by XRD. The morphology of calcined powder and the sintered ceramics was observed by TEM and SEM, respectively. The particle size of the powders calcined at 600oC is about 10 nm. The dense ceramics can be obtained at sintering temperature of 1220 oC, which is much lower than that of the powder prepared by the conventional solid method. The effects of Al2O3 additive on the sintering behavior, microstructure and dielectric properties of Ba(Zr0.2Ti0.8)O3 ceramics were also investigated.

Abstract: The Ba5(Nb1-xTax)4O15 (x=0~0.5) solid-solution ceramics with hexagonal perovskite structure were prepared by solid-state oxide route. The phases and structure of the sintered ceramics were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of composition and sintering temperature on the bulk density, microstructure and microwave dielectric properties were also investigated. As the value of x increased from 0 to 0.5, the Q×f value increases from 36,049 to 49,493 GHz (at ~9GHz), whereas the dielectric constant (εr) decreases from 39.21 to 33.03 and the temperature coefficient of resonant frequency (τf ) decreases from 78.6 to 19.4 ppm/oC. The results indicate that the microwave dielectric properties of Ba5(Nb1-xTax)4O15 ceramics can be easily modified in a large range by changing the relative content of Ta to Nb.