Zhao Ming-Lei

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.

First Principles Study on Electronic Structure of PbFe0.5Nb0.5O3

The full potential linearized augmented plane wave (FLAPW) method is used to study the crystal structure and electronic structure properties of PbFe0.5Nb0.5O3 (PFN). The optimized crystal structure, density of states, band structure and electron density distribution have been obtained to understand the ferroelectric behaviour of PFN. The analysis result of the density of states shows there is an obvious change of Nb d states in the paraelectric-to-ferroelectric phase transition. The polarization result shows that the contribution to ferroelectricity of Nb atoms is larger than that of Fe atoms. In ferroelectric phase there is a hybridization of Fe d–O p and Nb d–O p in ferroelectric PFN. This is consistent with the result of the electronic band structure. This hybridization is responsible for the tendency to its ferroelectricity.The full potential linearized augmented plane wave (FLAPW) method was used to study the crystal structure and electronic structure properties of PbFe0.5Nb0.5O3 (PFN). The optimized crystal structure, density of states, band structure and electron density distribution have been obtained to understand the ferroelectric behavior of PFN. From the density of states analysis, it is shown that there is a hybridization of Fe d - O p and Nb d - O p in ferroelectric PFN. This is consistent with the calculation of electronic band structure. This hybridization is responsible for the tendency to its ferroelectricity.

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.

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.

Dielectric properties and structural defects in BaTi1?xSnxO3 ceramics

As a function of Sn doping concentration x, dielectric properties and X-ray diffraction measurements were carried out on BaTi1-xSnxO3 (BTSx) ceramics fabricated by the solid-state reaction route. Positron annihilation lifetime spectroscopy and coincident Doppler-broadening spectroscopy were also measured for the evaluation of defects in the BTSx ceramics. Dielectric properties measurement reveals that the permittivity of BTSx ceramic gradually increases with increasing Sn dopant content for x≤3%, and then decreases. This change of permittivity is found to agree well with the relative defect concentration estimated using two positron annihilation techniques. The S-W plot indicates that the defect species do not change with Sn doping. The variation correlations between defects and dielectric properties further proves that BTSx ceramics with the higher relative defect concentration present a lower permittivity.

Diffuse dielectric anomaly of (Na0.5K0.5Bi)0.5TiO3 crystal at high temperature

This paper investigates the dielectric properties of (Na0.5K0.5Bi)0.5TiO3 crystal at intermediate frequencies (1kHz≤f≤1MHz) in the temperature range of 30–560 °C. A pronounced high-temperature diffuse dielectric anomaly has been observed. This dielectric anomaly is shown to arise from a Debye-like dielectric dispersion that slows down following an Arrhenius law. The activation energy Eτ obtained in the fitting process is about 0.69eV. It proposes that the dielectric peak measured at low frequency above 400 °C is not related to the phase transition but to a space-charge relaxation.