Volume 96, Issue 7, 01 October 2004
Index of content:
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
96(2004); http://dx.doi.org/10.1063/1.1785843View Description Hide Description
By means of transmission electron microscopy(TEM) the structure of chemically ordered regions in -doped has been studied. Large coherent ordered domains were obtained by the addition of . The results obtained by conventional, analytical, and high-resolution TEM are consistent with the random-site description of the -site order, the so-called random-layer model. Image simulations showed that a careful selection of the imaging conditions for high resolution transmission electron microscopy is necessary in order to determine the different -site occupancies. By comparison of simulated with experimental images it was possible to directly distinguish between the different ordering models.
96(2004); http://dx.doi.org/10.1063/1.1784612View Description Hide Description
Experimental data on fatigue in the Metal∕Ferroelectric∕Metal thin film structures are reported. A model is proposed based on the trapping and the releasing of the free carriers in the band-gap states located at the interfaces between the electrodes and the ferroelectricfilm. Fits of the experimental data with the plots calculated from the model show very good agreement. In particular, the fatigue dependence on both the frequency and the magnitude of the applied voltage is well reproduced by the model. Saturation of fatigue for a large number of cycles is also predicted.
Low frequency dielectric relaxation phenomena in conducting polypyrrole and conducting polypyrrole-zeolite composites96(2004); http://dx.doi.org/10.1063/1.1788846View Description Hide Description
The dielectric properties of polypyrrole-zeolite composites up to zeolite are studied in the frequency range from from room temperature to liquid nitrogen temperature. The complex permittivity formalism reveals a temperature dependent relaxation in all samples except for the zeolite composite. The frequency where a maximum of a loss peak is located varies with temperature by the Williams-Lander-Ferry law. The values of the activation energy of the relaxation process (which are of the order of polaronic dc conductivity) have the tendency to reach a minimum in the composition, which is a loss-free composite. The zeolite behaves as a dielectric where ionic relaxation dominates. The temperature variation of the strength of the dielectric mechanism follows a Curie law, apart from zeolite where the dielectric strength is practically constant. The frequencies, where loss peaks are maximum, as well as dc conductivity follow qualitatively the same temperature law, but the parameters are quite different. Moreover, the locations of the relaxation peaks diverge from the predictions of Barton-Nakajima-Namikawa model. Long-range electric charge transport (dc conductivity) and the relaxation that corresponds to short-range localized motion probably involve different processes.
96(2004); http://dx.doi.org/10.1063/1.1786675View Description Hide Description
Electric conduction in the solid oxide fuel cellselectrolyte,, has been elucidated by dc and ac measurements as a function of temperature from . The sum of resistances in the bulk and boundary, and , estimated in impedance analyses are nearly in agreement with dc resistance. Though at , their relative relation is reversed at . Since reciprocal resistance is proportional to conductivity, temperature dependencies of and yield activation energies for conductions in the bulk and boundary. Two relaxation processes show up in dielectric properties, i.e., low-frequency relaxation peak due to conduction in the boundary and high-frequency peak resulting from bulk conduction. Temperature dependencies of resonance frequencies and maxima of dielectric parameters in these relaxation peaks are used to estimate migration energies of ion, , and energies required for creation of a free oxygen vacancy assisting migration, . The sums of and in high- and low-frequency peaks are mostly in agreement with activation energies estimated from temperature dependencies of and .
96(2004); http://dx.doi.org/10.1063/1.1787586View Description Hide Description
0-3 composite ranging between 0 and 3, of ferroelectric ceramiclead zirconate titanate(PZT) and thermoplastic elastomer polyurethane (PU) were fabricated. The pyroelectric and dielectric properties of the hot-pressed thin film samples of various PZT volume fractions were measured. The experimental dielectricpermittivities and losses agreed reasonably well with the Bruggeman model. The room temperature pyroelectric coefficients of the composites were found to increase linearly with PZT volume fraction and substantially larger than expected. For example, for a composite with 30% PZT, its pyroelectric coefficient is about at room temperature, which is more than tenfold of a PZT∕PVDF composite of the same ceramic volume fraction. We propose a model in which the electrical conductivity of the composite system is taken into consideration to explain the linear relationship and the extraordinarily large pyroelectric coefficients obtained.