Changzheng Hu

Dielectric and nonlinear current–voltage characteristics of rare–earth doped CaCu3Ti4O12 ceramics

CaCu3Ti4O12 (CCTO) ceramicsdoped with rare earth (RE) oxides, including Y2O3, La2O3, Eu2O3, and Gd2O3, were prepared by the traditional solid–state reaction method in order to investigate the effect of RE oxide dopants on the electrical properties as a varistor. The phase identification and morphology of the ceramics were investigated by x–ray diffraction(XRD) and scanning electron microscope (SEM), respectively. A high voltage measuring unit and precision impedance analyzer were used to determine the nonohmic (J–E) behaviors and measure the dielectric properties and impedance spectroscopy of the ceramics, respectively. The results showed that RE oxides enhanced greatly the breakdown electric flied but reduced the nonlinear coefficient and the mean grain size of CCTO ceramics. There was a good linear relationship between ln J and E 1/2, which demonstrated that the Schottky barrier should exist at the grain boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. The depletion layer width determined by diffusion distance of RE ions and the effective surface states played important roles on the electrical properties of the ceramics.

Quantitative Description of the Diffuse Phase Transition of BNT-NKN Ceramics

A perovskite solid solution of (1-x)(Bi0.5Na0.5)TiO3-x(Na0.5K0.5)NbO3 (BNT-xNKN) (x = 0–0.08) synthesized by solid state reaction technique exhibits diffuse phase transition (DPT) behavior. Power law, Gauss-type quadratic law and Lorenz-type quadratic law was employed to characterize the dielectric permittivity peaks near Tm . The diffuseness parameters γ, δG and δA increase with the increase in the value of x, which implies a composition-induced diffuse transition. Among these laws simulating dielectric response in diffusion ferroelectric phase transition, the Lorenz-type quadratic law is the best to describe the characterization of DPT behavior in such a wide temperature range as observed in our experiment.

Ba4Ln2Fe2Nb8O30 (Ln = Eu, Gd) Ferroelectric Ceramics

Ba4Ln2Fe2Nb8O30 (Ln = Eu and Gd) ceramics were synthesized by high temperature solid state reaction. The phase composition, microstructure, dielectric and ferroelectric properties of them were characterized. Both ceramics adopt filled tetragonal tungsten bronze structure at room temperature. The diffusive phase transitions were observed for Ba4Eu2Fe2Nb8O30 and Ba4Gd2Fe2Nb8O30 around 163°C and 150°C, respectively. Ba4Eu2Fe2Nb8O30 and Ba4Gd2Fe2Nb8O30 ceramics are classical ferroelectrics at room temperature and show high dielectric constant of 118 and 160 and low dielectric loss 0.022 and 0.054 at 1 MHz, respectively. The remnant polarizations (2Pr) are 3.9μC/cm2 for Ba4Eu2Fe2Nb8O30 and 1.71μC/cm2 for Ba4Gd2Fe2Nb8O30.

Dielectric properties of unfilled tetragonal tungsten bronze Ba4PrFe0.5Nb9.5O30 ceramics

In order to found new dielectrics ceramics in tungsten bronze structure, unfilled tungsten bronze (TB) ceramics with nominal formula Ba4PrFe0.5Nb9.5O30 were prepared by the solid state reaction method. The microstructure and dielectric properties were studied using powder X-ray diffraction, field emission scanning electron microscope, and variable temperature dielectric test system. The results show that the ceramics have a single phase and belong to the space group of P4bm with the cell of a = b = 12.4839(3) Å, c = 3.9409(5) Å, V = 614.192(2) Å3. The frequency dependent dielectrics properties show that the ceramics have a Debye-like relaxation at room temperature, while the temperature dependent dielectrics properties indicate that the ceramics are a relaxor and the relaxation is due to the nanopolars and oxygen vacancies. The ceramics have a potential application in electronic ceramics as temperature-stable multilayer ceramic capacitors.

Structure and Electrical Properties of LiF Doped 0.996(0.95K0.5Na0.5NbO3-0.05LiSbO3)-0.004BiFeO3 Piezoelectric Ceramics

LiF was selected as a sintering additive to improve the sinterability of the 0.996(0.95K0.5Na0.5NbO3-0.05LiSbO3)-0.004BiFeO3 lead-free piezoelectric ceramics. Results suggested that at room temperature the phase structure of the studied ceramics shifted from the coexistence of two phases to a phase in which orthorhombic was dominant when the LiF value was higher than 1.5 mol%, meanwhile, a pronounced shift of the Curie point to higher temperature was observed. The sample with 2 mol% LiF exhibited a higher mechanical quality factor Qm value of ∼75.18. With increasing temperature from 30 to 120°C, the remnant polarization of the sample increased monotonically.