C. L. Wang

Microstructure and electrical properties of CaCu3Ti4O12 ceramics

CaCu3Ti4O12 (CCTO) ceramics are prepared by the conventional solid-state reaction method under various sintering temperatures from 1000to1120°C at an interval of 10°C. Microstructures and crystalline structures are examined by scanning electronic microscopy and x-ray diffraction, respectively. Dielectric properties and complex impedances are investigated within the frequency range of 40Hz–110MHz over the temperature region from room temperature to 350°C. It has been disclosed that the microstructures can be categorized into three different types: type A (with the small but uniform grain sizes), type B (with the bimodal distribution of grain sizes) and type C (with the large and uniform grain sizes), respectively. The largeness of low-frequency dielectric permittivity at room temperature is closely related to the microstructure. Ceramics with different types of microstructures show the diverse temperature-dependent behaviors of electrical properties. However, the existence of some common characteristics is...

Dielectric dispersion of CaCu3Ti4O12 ceramics at high temperatures

CaCu3Ti4O12 ceramics has been prepared by the conventional solid-state reaction method under different sintering conditions. Dielectric properties have been investigated in the temperature range of 25–350°C within the frequency range of 40Hz–10MHz. It has been found that both the dielectric constants and the grain sizes increase with the sintering time. A relaxation has been observed in the frequency range of 100Hz–100kHz, apart from the one already known in the frequency range higher than 100kHz. High-temperature dielectric dispersion shows one large low-frequency response and two Debye-type relaxations. Activation energy values of these two relaxations are 0.084 and 0.678eV, respectively. An equivalent circuit model is suggested, which well explains the dielectric dispersion.

Dielectric relaxor properties of K0.5Bi0.5TiO3 ferroelectrics prepared by sol–gel method

Lead-free potassium–bismuth titanate, K0.5Bi0.5TiO3 (KBT), ferroelectric ceramics were fabricated from the natural sintering of powders prepared by the thermal decomposition of adequate precursor solutions. Their crystal structure was determined by x-ray diffraction, and the temperature dependence of dielectric constants were measured. The results show that KBT may be a kind of order–disorder relaxor ferroelectric with a first-order phase transition below the temperature of dielectric constant maximum.

High permittivity and low dielectric loss in ceramics with the nominal compositions of CaCu3−xLa2x∕3Ti4O12

Ceramics with the nominal compositions of CaCu3−xLa2x∕3Ti4O12 were prepared by the conventional solid-state reaction technique. It was found that these ceramics have the properties of rather high dielectric permittivity and sufficiently low dielectric loss. In particular, CaCu2.9La0.2∕3Ti4O12 ceramics sintered at 1050°C for 20h showed the high dielectric permittivity of 7500 with weak frequency dependence below 1MHz, the low dielectric loss less than 0.05 in the wide frequency range of 120Hz–200kHz (with the minimum 0.012 at 20kHz), and the small temperature coefficient about ±15ppm∕°C between −80 and 125°C. From the analysis, it was suggested that CaTiO3 secondary phase due to Cu deficiency and La doping plays the important roles in the observed excellent dielectric properties in CaCu3−xLa2x∕3Ti4O12 ceramics. The results indicated that they are a very promising class of high-permittivity materials for practical applications.

Phase transition properties of ferroelectric superlattices with three alternative layers

Abstract The Curie temperature, polarization and susceptiblity of ferroelectric superlattices with three alternative layers have been obtained using the transverse Ising model in the mean field approximation. The interface layers show crucial influence on the properties of superlattices when the period is small enough.

First-principles study on the optical properties of SrHfO3 and SrTiO3

Abstract The optical properties of cubic SrTiO3 and SrHfO3 has been calculated. The full potential linearized augmented plane wave method is used and exchange-correlation effects are treated by the generalized gradient approximation. In order to understand the optical properties of SrTiO3 and SrHfO3, the imaginary part of dielectric function ϵ2(ω), the optical absorption coefficient I(ω), the reflectivity R(ω) and the optical conductivity σ(ω) were calculated. The calculated ϵ2(ω) is in agreement with experimental results.

Pyroelectric properties of ferroelectric superlattice on transverse Ising model

The pyroelectric properties and the Curie temperature of a ferroelectric/ferroelectric superlattice are studied for the transverse Ising model. Two pyroelectric peaks are found for large-period superlattices. The pyroelectric peak at low temperatures disappears when the period is small enough. Certain layers in small-period superlattices remain ferroelectric above the Curie temperature of their bulk material.

First-principles study of the electronic structure of NaTaO3

Abstract The electronic structure of cubic NaTaO3 is reported. The full potential linearized augmented plane wave method is used and exchange-correlation effects are treated by the generalized gradient approximation. In order to understand the interaction between atoms, the density of states, band structure, and charge density distribution were calculated. From the density of states analysis, we conclude that there is significant hybridization between Ta d states and O p states. This is consistent with the analysis of the band structure and the charge density distribution. There are two kinds of electronic interactions in the crystal. The interaction between Ta and O is covalent, and the interaction between the Na and TaO3 is ionic.

Dielectric relaxation in barium strontium titanate

Abstract BaxSr1−xTiO3 ceramics with different grain size were prepared by solgel method. The dielectric relaxation of the samples was observed at room temperature. The experimental results showed that the relaxation frequency increases with decreasing grain size. An explanation of this phenomenon was given by an electric potential model that qualitatively agrees with the experimental results. Furthermore, the model showed that the height of the potential barrier had a simple relation with the grain size.

Dielectric and pyroelectric properties of SrBi4Ti4O15-based ceramics for high-temperature applications

Abstract Sr (Sr, Ca, Ba)0.1Bi3.9Ti3.9(Ta, Nb)0.1O15 ceramics were prepared by a conventional sintering technique and the dielectric, piezoelectric and pyroelectric properties were investigated. Among these materials, Sr1.1Bi3.9Ti3.9Ta0.1O15 was found to have the highest Curie temperature, largest piezoelectric constant d33 and pyroelectric coefficient p. In addition, MnCO3 and La2O3 additives can further improve the piezoelectric and pyroelectric properties. The Sr1.1Bi3.9Ti3.9Ta0.1O15+0.5 wt.% MnCO3+0.2 wt.% La2O3 sample, in particular, has excellent pyroelectric properties in the temperature range of 100–200xa0°C, making this material very suitable for pyroelectric-derived applications.