Preparation And Study Of A(Fe_0.5Nb_0.5)O₃（A=Sr,Ba） Nano-powder By Coprecipitation Method And It’s Dielectric Properties

Fe-based complex perovskites A(Fe0.5B0.5)O3(A=Ba, Sr, Ca; B=Nb, Ta, Sb)ceramics materials have attracted considerable attention due to their generally exhibit a giant dielectric constant step(of the order of 103-105) over a broad temperature and frequency interval. However, dielectric loss of these materials synthesize by solid-state reaction method is not small enough and the giant dielectric constant step is not spread to meet the requirement of the practical application. So, the key challenge in these materials is to extend its giant dielectric constant step and simultaneously suppress the dielectric loss. In the present thesis, Sr(Fe0.5Nb0.5)O3 and Ba(Fe0.5Nb0.5)O3 powders and ceramics were synthesized by co-precipitation methods, the optimum synthesis condition have been determined, the microstructure and dielectric properties of Sr(Fe0.5Nb0.5)O3 and Ba(Fe0.5Nb0.5)O3 ceramics have been systematically investigated. Ba(Fe0.5Nb0.5)O3 ceramics particles have been coated with Al2O3 and the dielectric properties of Al2O3@ Ba(Fe0.5Nb0.5)O3 ceramics have been investigated.The Sr(Fe0.5Nb0.5)O3 nano-powders are synthesized by co-precipitation methods. The optimum synthesis parameters of the Sr(Fe0.5Nb0.5)O3 powders as follows: the system temperatures is 30 ℃, the p H value is 10, the initial concentration is 0.25 mol/L and the calcinations temperature of the precursor powders is 950 ℃ for 2 h. The average grain size and dielectric constant of Sr(Fe0.5Nb0.5)O3 ceramics increase with increasing sintering temperature. The experimental results indicate that the dielectric properties of Sr(Fe0.5Nb0.5)O3 ceramics by two-steps sintering at 1450 ℃ for 3 h as follows: high dielectric constant(ε’=2296), low dielectric loss(tand=0.20), excellent frequency stability(?ε’=16%,) and temperature stability(?ε’=18.9%). A Debye-like relaxation is indicated in the present Sr(Fe0.5Nb0.5)O3 ceramics and the relaxation is a thermal activation process. The activation energy of dielectric relaxation decrease with increasing sintering temperature, it is related to potential barrier height of electron-hopping between the Fe2+ and the Fe3+.The Ba(Fe0.5Nb0.5)O3 nano-powders are synthesized by co-precipitation methods.The calcination temperature(950 ℃) of co-precipitation methods is much lower than calcination temperature(1200 ℃) of solid state reaction method. The dielectric loss of Ba(Fe0.5Nb0.5)O3 ceramics is lower by co-precipitation method(tand=0.11) compared with solid-state reaction method. In addtion, the frequency stability and temperature stability have been improved by the co-precipitation method. It is related to competition balance of the lowand high-temperature dielectric relaxations. The complex plane impedance plots further demonstrated that barrier layers create the heterogeneity in Ba(Fe0.5Nb0.5)O3 ceramics, consisting of semiconducting grains and insulating grain boundaries, which are responsible for the giant dielectric constant.The Ba(Fe0.5Nb0.5)O3 ceramics particles are coated with Al2O3 by co-precipitation methods. The amorphous Al2O3 is discovered the grain boundary of Ba(Fe0.5Nb0.5)O3 ceramics. After coated Al2O3 for Ba(Fe0.5Nb0.5)O3 ceramics, the dielectric loss is reduced, the frequency response range of high dielectric is extended and the temperature stability is enhanced. The insulativity of ceramics grain boundary is improved by coated Al2O3.The Al2O3@Ba(Fe0.5Nb0.5)O3 ceramics exist electric heterogeneity microstructure, which are responsible for the high dielectric constant. The uniform grain size distribution and excellent dielectric properties of Al2O3@Ba(Fe0.5Nb0.5)O3 ceramics with 2wt% have been obtained.