Zhong Wei-Lie

Nonlinear Electrical Behaviour of the TiO2?Sb2O3 System

The Sb2O3-doped TiO2 varistors were obtained by conventional ceramic processing. The 0.75 mol% Sb2O399.25 mol% TiO2 varistor has a nonlinear coefficient α = 7 and a minimum breakdown electrical field of 6 V/mm. The nonlinear electrical behaviour of TiO2Sb2O3 ceramics was explained by the introduction of defects in the crystal lattice that are responsible for the formation of Schottky potential barriers at grain boundaries.

Grain-Size Effect on the Dielectric Properties of Bi0.5Na0.5TiO3

We prepared bismuth sodium titanate (Bi0.5Na0.5TiO3) ultrafine powders by the sol-gel method. The dielectric properties of the pressed pellets and fired ceramics with different grain sizes as a function of temperature at various frequencies were studied. With decreasing grain size, the dielectric anomaly around 200°C increases, while the dielectric thermal hysteresis decreases. All the samples with grain sizes larger than 100 nm show dielectric peaks at temperature of about 350°C. The very little change in Tm observed down to the critical size indicates that Bi0.5Na0.5TiO3 is an order-disorder system above 200°C. In addition, the dielectric peak becomes lower with decreasing grain size and the ferroelectric critical size of Bi0.5Na0.5TiO3 was eventually determined to be about 100 nm according to the disappearance of dielectric peak.

Barium Titanate Prepared by Stearic Acid-Gel Method

Stearic acid-gel method was used to prepare barium titanate. All the samples show a cubic perovskite structure and thus no ferroelectricity is detected at room temperature. X-ray diffraction reveals that the grains can hardly grow even firing at high temperature so that the grain size in most of the samples is below the ferroelectric critical size. A tentative explanation is proposed of the peculiarities based on the nonstoichiometry at the surface layer.

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN), we calculated the electronic structure of CaTiO3, BaTiO3 and KTN by first principles calculation. From total energy analysis, it is shown that, with increasing cell volume, the crystals (CaTiO3, SrTiO3) will have a ferroelectric instability. For BaTiO3, the ferroelectricity will disappear as the cell volume is decreased. From the density of states analysis, it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics. This is consistent with the analysis of band structure.

Effect of trifluoroethylene monomers on molecular conformation of poly (vinylidene fluoride-trifluoroethylene) copolymer

Hartree–Fock and density functional theory (DFT) methods were employed to study poly (vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] molecular chains with different VDF contents. The dependence of dipole moment of P(VDF-TrFE) chains on VDF content obtained from our calculation is in good agreement with the experiment. The TrFE monomer plays an important role in introducing the gauche bond into copolymer chains. A possible mechanism was interpreted.

Piezoelectric ceramics with high coupling and high temperature stability

Abstract A new method for making piezoelectric ceramics with high stability and high coupling is described. Two ceramics in the PZT system with compositions on opposite sides of the morphotropic phase boundary are prepared first. By means of mixed sintering, a mixture or composite ceramic is obtained. If the compositions of the two components and their weight ratio are carefully adjusted and the sintering temperatures are appropriate, the composite ceramic will possess very high temperature stability and a high coupling coefficient.

Size and Temperature Dependence of Dielectric Constant of Ultrafine PbTiO3 Particles

Using sol-gel method, we prepared ferroelectric PbTiO3 fine particles with particle size covering the range 20-200 nm; the dielectric properties of powder compact were studied in the present work. The room temperature dielectric constant increases with the decrease in particle size and reaches a maximum value at the size of about 40 nm. The temperature dependence of dielectric constant involving the evolution of particle size during the measurement is discussed.

Size Effect in Ferroelectric Long Cylinders

The Curie temperature and polarization in a ferroelectric cylinder with infinite length have been examined using Landau free energy expansion. The Curie temperature and polarization decrease with decreasing cylinder diameter for the positive extrapolation length, and reach zero at the critical size. For negative extrapolation length, both Curie temperature and polarization increase with decreasing cylinder diameter.

Vibrational Modes Analysis of Poly (Vinylidene Fluoride-Trifluoroethylene)

The ground state geometry and vibrational modes of poly (vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] molecular chains have been investigated by ab initio molecular orbital calculation. The gauche bonds in P(VDF-TrFE) are introduced by TrFE monomers. The vibrational modes are localized within all-trans segments. These results cause the infrared absorption weaker and diffused, and may be responsible for dielectric relaxation of the copolymers.

An Electronic Structure of Ferroelectric PbZr0.5Ti0.5O3

An electronic structure of ferroelectric PbZr1/2Ti1/2O3 is presented. The full potential linearized augmented plane wave method is used and the exchange correlation effects are treated by the generalized gradient approximation. In order to understand the ferroelectricity of PbZr1/2Ti1/2O3, the density of states, charge density distribution and band structure are calculated. The results show that the atoms Pb and Ti act similarly as in pure PbTiO3. Like other perovskite ABO3 ferroelectrics, there is a hybridization between the Ti d and O p states, which is responsible for the tendency to ferroelectricity.