Volume 96, Issue 9, 01 November 2004
Index of content:
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
96(2004); http://dx.doi.org/10.1063/1.1796511View Description Hide Description
The solid solution between the normal ferroelectric(PZT) and relaxor ferroelectric (PNN) was synthesized by the columbite method. The phase structure and dielectric properties of where and the composition was fixed close to the morphotropic phase boundary (MPB) were investigated. With these data, the ferroelectric phase diagram between PZT and PNN has been established. The relaxor ferroelectric nature of PNN gradually transformed towards a normal ferroelectric state towards the composition 0.7PZT-0.3PNN, in which the permittivity was characterized by a sharp peak and the disappearance of dispersive behavior. X-ray diffraction analysis demonstrated the coexistence of both the rhombohedral and tetragonal phases at the composition 0.8PZT-0.2PNN, a new morphotropic phase boundary within this system. Examination of the dielectric spectra indicates that PZT-PNN exhibits an extremely high relative permittivity near the MPB composition. The permittivity shows a shoulder at the rhombohedral to tetragonal phase transition temperature , and then a maximum permittivity (36 000 at ) at the transition temperature at the MPB composition. The maximum transition temperature of this system was at the composition with the relative permittivity of 32 000 at .
96(2004); http://dx.doi.org/10.1063/1.1790573View Description Hide Description
Highly -oriented rhombohedral lead zirconate titanate thin films were prepared on substrates by suitably combining pulsed laser deposition with sol-gel coating. Two distinctive types of the backscattering geometry were employed for the polarized Raman scattering study: (i) normal backscattering in which the propagation direction of relevant phonons is parallel to the principal  polar axis of the rhombohedral point group for observing phonons, and (ii) side-view backscattering in which the phonon-propagation direction is normal to the  direction for isolating phonons. Room-temperature Raman spectra of the -oriented film (with ) having the high-temperature rhombohedral form were examined and compared with those of the -oriented low-temperature rhombohedral film which is characterized by the tilting of oxygen octahedra about the principal  direction. Similarities and differences in the observed Raman spectra between these two distinctive films were examined in the light of the Raman selection rules for and space groups.
96(2004); http://dx.doi.org/10.1063/1.1797549View Description Hide Description
The broadband complex permittivity is monitored continuously in hydrating cement paste over the frequency range of and from initial mixing to several weeks of cure. Measurements are made by time domain reflectometry (TDR) dielectric spectroscopy, using an adjustable capacitance sensor, which can be embedded in the material in situ. The results are fit to a relaxation model, which includes terms representing (1) a Cole-Davidson relaxation near , which grows initially and then decreases with an advancing cure; (2) a Debye relaxation near , which grows initially and then decreases with an advancing cure; (3) a free-water relaxation near , which decreases with an advancing cure; and (4) an ion conductivity and electrodepolarization, which decreases with an advancing cure. The model is fit continuously as function of cure time extracting parameters for the relaxation amplitudes, relaxation frequencies, and distribution parameters as a function of cure time. The results are contrasted with measurements in tricalcium silicate and tricalcium silicate with varying ion content, revealing differences that may indicate the nature of the processes involved. Alternative methods for extracting reaction-rate information directly from the TDR transient are presented, providing a robust monitoring procedure usable in the field. Such methods are demonstrated using a variation in temperature and comparison with measurements of reaction heat obtained by calorimetry.
96(2004); http://dx.doi.org/10.1063/1.1792391View Description Hide Description
Electrical breakdown in homogeneous liquid water for an voltage pulse is analyzed. It is shown that electron-impact ionization is not likely to be important and could only be operative for low-density situations or possibly under optical excitation. Simulation results also indicate that field ionization of liquid water can lead to a liquid breakdown provided the ionization energies were very low in the order of . Under such conditions, an electric-field collapse at the anode and plasma propagation toward the cathode, with minimal physical charge transport, is predicted. However, the low, unphysical ionization energies necessary for matching the observed current and experimental breakdown delays of precludes this mechanism. Also, an ionization within the liquid cannot explain the polarity dependence nor the stochastic-dendritic optical emission structures seen experimentally. It is argued here that electron-impact ionization within randomly located microbubbles is most likely to be responsible for the collective liquid breakdown behaviors.
Temperature stability of the piezoelectric and elastic response of dc biased  and  oriented single crystals96(2004); http://dx.doi.org/10.1063/1.1789271View Description Hide Description
Temperature and field dependent investigations of the longitudinal electromechanical properties of - and -oriented single crystals have been performed. Electromechanical equivalence was found between  and  orientations in the ferroelectric rhombohedral phase region, but not in the ferroelectric tetragonal one. A dc bias of 0.4 MV∕m did not affect this equivalence. Softening of the dielectric constant, piezoelectric coefficient , and Young’s modulus were observed along both the  and  orientations in the phase region on heating towards the one. Above the phase transition, a significant anisotropy was observed in the electromechanical and elastic coefficients. These results demonstrate that the enhanced length extensional (33-mode) electromechanical coupling of rhombohedral crystals far from the morphotropic phase boundary is not constrained to the orientation, but rather to the (110) plane.