Wang Chun-Lei

Green's function theory of phase transition in PbHPO4 and PbDPO4 ferroelectrics

Abstract In this paper, we investigated the newly found order-disorder ferroelectrics PbHPO4 along with its deuterated analogue PbDPO4. Because the major contribution of the spontaneous polarization is the dipoles forming by the proton (or deuteron) ordering, not the displacements of heavy atoms as in the case of KDP-type crystals. Therefore in the Hamiltonian, the influence of the lattice vibration may not be considered, but the 4th order interaction of pseudo-spin is included. By introducing the undetermined-constant into the Greens function method, we carried out the spontaneous polarization of PbHPO4 and PbDPO4 crystals, the results are good agreement with the experimental data.

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.

Thermoelectric properties of Sr0.61Ba0.39Nb2O6−δ ceramics in different oxygen-reduction conditions*

The thermoelectric properties of Sr0.61Ba0.39Nb2O6−δ ceramics, reduced in different conditions, are investigated in the temperature range from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping in the low temperature range, the Fermi glass behavior in the middle temperature range, and the Anderson localized behavior in the high temperature range. The thermal conductivity presents a plateau at high-temperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increase of the degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value is obtained to be ~0.19 at 1073 K in the heaviest oxygen reduced sample.The thermoelectric properties of Sr0.61Ba0.39Nb2O6−δ ceramics, reduced in different conditions, are investigated in the temperature range from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping in the low temperature range, the Fermi glass behavior in the middle temperature range, and the Anderson localized behavior in the high temperature range. The thermal conductivity presents a plateau at high-temperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increase of the degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value is obtained to be ~0.19 at 1073 K in the heaviest oxygen reduced sample.

Cu Doping Effect on Electrical Resistivity and Seebeck Coefficient of Perovskite-Type LaFeO3 Ceramics

Perovskite-type LaFe1?xCuxO3 (x = 0.10, 0.14, 0.18) solid solution is prepared with the conventional solid-state reaction technique. The electrical resistivity and the Seebeck coefficient are measured in the temperature range 473?1073K to elucidate the Cu doping effect on the thermoelectric properties of the LaFeO3. The electrical resisitivity of LaFe1?x Cux O3 shows semiconducting behavior. The temperature dependence of the electrical resistivity indicates that the adiabatic small-polaron hopping mechanism is dominant for their electric transportations. The activation energy decreases with the increasing Cu content as well as the increasing temperature. The Seebeck coefficient changes from a negative value to a positive value around 510 K, and increases with rising temperature up to 710K, then becomes saturated around 200?V/K. The Seebeck coefficient decreases with the substitution of Cu atoms in the temperature range of 573?1073 K, while the electrical resistivity decreases with the substitution of Cu atoms in the whole measured temperature. Overall the power factor increases with rising temperature, and the highest value of power factor is 54 ?W/K2 m for x = 0.10 of Cu doping.

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.

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.

Ferroelectric Properties of Polycrystalline Ceramics with Dipolar Defect Simulated from the Potts--Ising Model

Physical properties of polycrystalline ferroelectrics including the contributions of the fixed dipolar defects and the average grain size in the Potts-Ising model are simulated by using the Monte Carlo method. Domain pattern, hysteresis loop and switching current of the polarization reversal process are obtained. Two processes are considered in our simulation. In the first one, the grain texture of ferroelectric ceramics are produced from the Potts model, and then the Ising model is implemented in the obtained polycrystalline texture to produce the domain pattern, hysteresis loop and switching current. It is concluded that the defect has the ability to decrease the remnant polarization Pr as well as the coercive field Ec. The back switching is obviously observed after the electric field is off, and it shows some variation after introducing the fixed dipolar defect. Meanwhile, the spike of the switching current is found to lower with the increasing defect concentration and the decreasing average grain size.

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.