Yuguo Wang

Size effects on the Curie temperature of ferroelectric particles

Abstract Landau-Devonshire theory for ferroelectric phase transition is applied to ferroelectric spherical particles. The extrapolation length is shown to be size dependent. The Curie temperature as a function of size is calculated. Theoretical results are compared with experimental data in the literature and good agreement is obtained.

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.

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.

Elastic properties of sodium lithium niobate piezoelectric ceramics at low temperature

Abstract The elastic stiffness coefficient of Li0.02Na0.98NbO3 was measured at low temperature. Around the ferroelectric-ferroelectric phase transition temperature, the elastic stiffness behaves anomalously with a large thermal hysteresis. The result is in agreement with previous measurement on dielectric, ferroelectric and pyroelectric properties.

Ion implantation in PbTiO3 films

Lead titanate films prepared by the Sol‐Gel process were ion implanted at various temperatures. The structural changes were examined by x‐ray diffraction, and the composition distribution was studied by secondary ion mass spectra and Rutherford backscattering. If the implantation is carried out at room temperature, Pb segregates in crystalline form. For high‐temperature implantation, both crystallized Pb and PbO were observed and the temperature‐induced crystallization of PbTiO3 was hindered. However, the implantation‐induced Pb and PbO are metastable. Annealing at high enough temperatures can convert the implanted films into a purely perovskite structure.

Structure changes in PbTiO3 films induced by ion implantation with Ar+ and N+

Abstract Structure changes of PbTiO 3 films induced by ion implantation with Ar + and N + were measured by X-ray diffraction and MeV 4 He + Rutherford backscattering. The lead segregation which occurred in the volume of the collision cascade and the amorphization of the surface layer of the films are discussed.

Spontaneous strain in finite size ferroelectrics

Abstract Both the surface and strain contributions are included in the free energy expression for ferroelectric films and particles. Using the relationship between the strain and polarization under stress-free conditions, the spontaneous strain is obtained via the polarization. In ferroelectric films, the average spontaneous strain decreases linearly with the reciprocal of the film thickness if the extrapolation length δ is positive and it increases linearly if δ is negative. Independently of δ, the inhomogeneity of spontaneous strain increases with decreasing film thickness whrn the thickness is large. In ferroelectric spherical particles, the average value and inhomogeneity of the spontaneous strain are also calculated. Theoretical results are compared with the experimental ones and good agreement is obtained.