Landau thermodynamic potential for BaTiO_3

In the paper, the description of the dielectric and ferroelectric properties of BaTiO_3 single crystals using Landau thermodynamic potential is addressed. Our results suggest that when using the sixth-power free energy expansion of the thermodynamic potential, remarkably different values of the fourth-power coefficient, \beta (the coefficient of P^4_i terms), are required to adequately reproduce the nonlinear dielectric behavior of the paraelectric phase and the electric field induced ferroelectric phase, respectively. In contrast, the eighth-power expansion with a common set of coefficients enables a good description for both phases at the same time. These features, together with the data available in literature, strongly attest to the necessity of the eighth-power terms in Landau thermodynamic potential of BaTiO_3. In addition, the fourth-power coefficients, \beta and \xi (the coefficient of P^2_i P^2_j terms), were evaluated from the nonlinear dielectric responses along [001], [011], and [111] orientations in the paraelectric phase. Appreciable temperature dependence was evidenced for both coefficients above T_C. Further analysis on the linear dielectric response of the single domain crystal in the tetragonal phase demonstrated that temperature dependent anharmonic coefficients are also necessary for an adequate description of the dielectric behavior in the ferroelectric phase. As a consequence, an eighth-power thermodynamic potential, with some of the anharmonic coefficients being temperature dependent, was proposed and compared with the existing potentials. In general, the potential proposed in this work exhibits a higher quality in reproducing the dielectric and ferroelectric properties of this prototypic ferroelectric substance.Comment: 7 figures, 5 table

Metal-Ferroelectric-Metal heterostructures with Schottky contacts I. Influence of the ferroelectric properties

A model for Metal-Ferroelectric-Metal structures with Schottky contacts is proposed. The model adapts the general theories of metal-semiconductor rectifying contacts for the particular case of metal-ferroelectric contact by introducing: the ferroelectric polarization as a sheet of surface charge located at a finite distance from the electrode interface; a deep trapping level of high concentration; the static and dynamic values of the dielectric constant. Consequences of the proposed model on relevant quantities of the Schottky contact such as built-in voltage, charge density and depletion width, as well as on the interpretation of the current-voltage and capacitance-voltage characteristics are discussed in detail.Comment: 14 pages with 4 figures, manuscript under revision at Journal of Applied Physics for more than 1 year (submitted May 2004, first revision September 2004, second revision May 2005

Anharmonicity of BaTiO_3 single crystals

By analyzing the dielectric non-linearity with the Landau thermodynamic expansion, we find a simple and direct way to assess the importance of the eighth order term. Following this approach, it is demonstrated that the eighth order term is essential for the adequate description of the para/ferroelectric phase transition of BaTiO_3. The temperature dependence of the quartic coefficient \beta is accordingly reconsidered and is strongly evidenced by the change of its sign above 165 C. All these findings attest to the strong polarization anharmonicity of this material, which is unexpected for classical displacive ferroelectrics.Comment: 4 figures, to be published in Phys. Rev.

Nowe materiały na polimerowe przepusty średnich napięć

Results of research and comparison of properties of four electrical materials for the manufacture of high-voltage transformer bushings «oil-air» are presented. Basing on investigations of tracking-erosion resistance, water and oil resistance, dielectric strength, resistance to fire and hydrophobicity, the material of type «FX» was selected. This material is based on unsaturated polyester resin that meets the anticipated operational impacts.W artykule przedstawiono wyniki badań nowych materiałów do produkcji polimerowych przepustów transformatorowych średnich napięć przeznaczonych do pracy w środowisku „olej-powietrze”. Testom poddano próbki gumy silikonowej SR oraz kompozytów na bazie żywic poliestrowych i epoksydowych. Badania obejmowały testy na odporność na prądy pełzające, odporność na działanie wody i oleju, wytrzymałości elektrycznej, palności oraz hydrofobowości. Po analizie wyników stwierdzono, że najbardziej odpowiednim materiałem będzie kompozyt żywicy poliestrowej zbrojony włóknem szklanym oznaczony jako próbki FX. Może być on również stosowany do wytwarzania izolatorów napowietrznych średniego napięcia. Stwierdzono, że guma silikonowa mimo, że może być stosowana do produkcji izolatorów napowietrznych, jednak ze względu na dużą chłonność oleju i pęcznienie nie powinna być używana jako materiał na przepusty „olej-powietrze”. Natomiast materiały IS oraz EN nie nadają się do produkcji zarówno przepustów jak i izolatorów napowietrznych średnich napięć

MÖSSBAUER STUDY OF THE STATE OF IMPURITY ATOMS IN TiO2 - AND BaTiO2 -BASED INSULATING AND SEMICONDUCTIVE CERAMICS

L'étude de l'état chimique des atomes d'impureté de fer, d'antimoine et de terbium (de dysprosium) dans des matériaux à base de bioxyde de titane de baryum témoigne de l'apparition dans ces matériaux d'une micro-hétérogénéité, même à des niveaux de dopage assez faibles (au-dessous de 1 à 2% at.). Nous avons trouvé que les propriétés des matériaux étudiés dépendent dans une large mesure de l'interaction chimique entre les impuretés de type donneur et de type accepteur ; cette interaction provoque une augmentation mutuelle de la solubilité de ces impuretés dans la matrice ; cet effet est analogue à l'influence mutuelle d'impuretés "donneurs" et "accepteurs'' dans des semiconducteurs classiques tels que le silicium et le germanium /9/. Ces régularités peuvent apparemment être étendues à un groupe assez large de matériaux de type oxyde.The study of the chemical state of iron, antimony and terbium (dysprosium) impurity atoms in materials based on titanium dioxide and barium titanate points to microheterogeneity appearing in these materials already at rather low impurity concentrations (below 1 to 2 at.%). We have found that of great importance ot the properties of the materials studied is chemical interaction between donor and acceptor impurities which results in a correlation increase of their solubilities in the matrix, this effect being similar to the mutual influence of donor and acceptor impurities in classical semiconductors such as silicon and germanium /9/. The regularities revealed can apparently be extended to a fairly large group of oxide materials