Weilie Zhong

Finite-size effects in ferroelectric solid solution

We studied the effect of grain size on the ferroelectric phase transitions in ceramics (x = 0.3, 0.5 and 0.7) prepared by a sol-gel method. The samples were observed and analysed by SEM, TEM and x-ray diffraction. The phase transitions were investigated by means of dielectric measurements. With the decrease of grain size, the Curie temperature decreases and the Curie peak becomes lower and broader and eventually disappears. The diffuseness of the dielectric response is understood on the basis of a diffuse phase transition model. The size dependence of can be described by , where is the for bulk , D is the grain size, and C and are two constants. This equation gives three ferroelectric critical sizes of 317 nm, 246 nm and 176 nm for x = 0.3, 0.5 and 0.7 respectively.

Oriented PbTiO3 films from metalorganic precursors

PbTiO3 films on various substrates were prepared by a metalorganic decomposition process using a multilayer spinning technique. Films on LaAlO3 and MgO substrates were preferentially c‐axis oriented, and those on SrTiO3 have a completely c‐axis orientation. The mechanism leading to the orientation was discussed. Contrary to the c‐axis orientation, a‐axis orientation can be induced on otherwise random films if they were annealed under pressure.

Size effect on the dielectric properties of BaTiO3

BaTiO3 ultrafine particles of different size were prepared by Sol-Gel process. Their microstructure was examined at room temperature. The temperature dependence of the dielectric properties of the pressed pellets and fired ceramics of these particles was studied. With the decrease of particle size d, the Curie temperature decreases while the tetragonal-orthorhombic and orthorhombic-trigonal phase transition temperature increase, and the three dielectric peak become lower and broader and eventually disappear in the following sequence: trigonal-orthorhombic, orthorhombic-tetragonal, tetragonal-cubic. The critical size at which ferroelectricity disappears was determined by dielectric measurement, which agrees with that determined by XRD technique.

Pyroelectric properties of ferroelectric‐polymer composite

General formulae of the pyroelectric coefficient and the figure of merit (p/e) of a ferroelectric‐polymer composite are theoretically derived in terms of the volume fraction q, the depolarization coefficient nx, the dielectric constant of the ferroelectric particles and the dielectric constant of the composite. The value of p/e can be expected to be larger than that of pure ferroelectric ceramics. Good agreement is obtained between theoretical and experimental results.

Polarization and Dielectric Susceptibility of Ferroelectric Superlattice

The temperature dependence of dielectric susceptibility as well as the spontaneous polarization of a ferroelectric superlattice were discussed based on the Ising model in a transverse field. When the slab thicknesses are larger, the temperature dependence of the spontaneous polarization shows a steplike structure. The dielectric susceptibility becomes infinite at the Curie temperature T c and shows finite peaks corresponding to the rapid decrease in the spontaneous polarization near the bulk Curie point of each slab. The height of the finite peaks decreases with the decrease in the slab thicknesses. When they become so thin that the polarization profiles are predominantly controlled by the interface, the finite peaks in susceptibility as well as the steps in polarization disappear.

Effect of Mn2+ on the electrical nonlinearity of (Ni, Nb)-doped SnO2 varistors

The reason that the (Ni, Nb)-doped SnO2 varistors exhibit poorer densification and electrical nonlinearity than the (Co, Nb)-doped SnO2 varistors is explained. The effect of Mn2+ on the electrical nonlinear properties of SnO2 based ceramics were investigated. The sample doped with 0.10 mol% MnCO3 exhibits the highest reference electrical field of 686.89 V/mm, the highest electrical nonlinear coefficient of 12.9, which is consistent with the highest grain-boundary defect barriers. It can be explained by the effect of the substitution of Sn4+ for Mn2+, which facilitate the formation of the defect barriers, and the maximum of the substitution. The shrinkage rates increase with the doping of MnCO3, although the sample doped with 0.5 mol% MnCO3 appears the highest density (ρ=6.87 g/cm3). In order to illustrate the grain boundary barriers formation in SnO2.Ni2O3.Nb2O5.MnCO3 varistors, a grain-boundary defect barrier model was introduced.

Ferroelectric, Piezoelectric and Pyroelectric Properties of Sr1+xBi4-xTi4-xTaxO15 Ceramics (x=0–1)

The dielectric, ferroelectric, piezoelectric and pyroelectric properties of bismuth layer-structured ferroelectric (BLSF) Sr1+xBi4-xTi4-xTaxO15 (x=0–1) ceramics were investigated. The Curie temperature of this system is from 540°C to 30°C, and all samples show ferroelectricity at room temperature. From piezoelectric measurements, Sr1.1Bi3.9Ti3.9Ta0.1O15 was found to have the largest piezoelectric coefficient and electromechanical coupling factor of compounds in this system. The thermally stimulated current measurements showed that Sr1.1Bi3.9Ti3.9Ta0.1O15 also has excellent pyroelectric responses, and the enhancement of pyroelectric properties may partially result from a relatively large temperature–dependent internal electric field which was observed from the polarization–electric field (P–E) hysteresis loops.

Strain analysis of PbTiO3 fine particles and films prepared by sol-gel method

Abstract The micro-strain as well as the grain size in PbTiO3 fine particles and films was determined by x-ray diffraction. For isolated particles, the micro-strain supposedly resulting from non-equilibrium defects decreases monotonically with the increase in sintering temperature. The micro-strain in films is higher than that in isolated particles because there are many defects accumulated at the grain boundary. As the sintering temperature increases the micro-strain in films first decreases and then increases. The decrease in micro-strain at lower sintering temperatures can be attributed to the thermally induced defect-elimination, while the increase in micro-strain at higher sintering temperatures is supposed due to the extension of the transition region at the surface and interface.

Surface analysis of PbTiO3 films prepared by the sol‐gel method

The surface of PbTiO3 films prepared by the sol‐gel method was analyzed by the use of x‐ray photoelectron spectra combined with secondary‐ion mass spectra and infrared absorption spectra. The results suggested that traces of organic materials still existed at least in the surface layer of the films. The outmost surface was partly covered by adsorbed oxygen. The surface layer of the film deviated from stoichiometry because of the shortage of Ti; the thickness of it was estimated about 400 A.

Ferroelectric superlattice with first-order phase transition

Abstract The Mean field expressions derived from the transverse Ising model are presented to describe the first-order ferroelectric phase transitions. The theory is used to discuss the static properties of ferroelectric superlatticcs formed by inserting periodically a monolayer of material B into material A; the bulk properties of material B are assumed paraelectric. Both the Curie temperature and the critical value of the spin average are dependent sensitively on the period of the superlattice when the period is small. For the case of large period, the Curie temperature is reduced slight compared with the bulk value of material A; while the spin average is reduced remarkably.