Volume 92, Issue 12, 15 December 2002
Index of content:
- DIELECTRICS AND FERROELECTRICITY (PACS 77)
High-temperature phase transitions, dielectric relaxation, and ionic mobility of proustite, and pyrargyrite,92(2002); http://dx.doi.org/10.1063/1.1520720View Description Hide Description
The nature of phase transitions in natural and synthetic proustite, has been studied in darkness above 300 K and compared with its natural counterpart pyrargyrite, The behavior of proustite is characterized by silver ion mobility within the structure. Proustite and (to a lesser extent) pyrargyrite were investigated as a function of temperature by x-ray and neutronpowderdiffraction, dielectric spectroscopy, and dynamic mechanical analysis. At 305 K (280 K for pyrargyrite) proustite undergoes a second-order phase transition, exhibiting a positive nonsymmetry breaking spontaneous strain of the unit cell, with thermal expansion along  changing from negative to positive. This strain results from the onset of thermally induced hopping of silver ions, as revealed by impedance spectroscopy. It may be described as an almost undamped Debye oscillator, which is not present below (305 K) with an activation energy of 0.42 eV (0.40 eV for pyrargarite). Around 420 K the high-frequency conductivity of proustite begins to increase, accompanied by elastic softening. At this temperature increasing random disorder of silver within possible unoccupied sites in the structure leads to increased electrical conductivity and destabilizes the material. When almost all silver ions are disordered into a so-called “molten sublattice,” a transition to fast ion conduction (at 540 K in proustite and 490 K in pyrargyrite) is reached. The additional component to the thermal expansion disappears and a linear negative spontaneous strain suggests a second-order phase transition. Above the transition silver seems to be maximally disordered, the structure itself is weakened and the sample starts to decompose.
Analysis of electron transfer between electron irradiated metallic ball and insulators in vacuum: A specific alternative to the mirror method92(2002); http://dx.doi.org/10.1063/1.1522477View Description Hide Description
In order to improve the knowledge of dielectric properties of insulators, we have imagined an original method of characterization of the charge buildup. Electrons of an electron beam are implanted through a metallic ball directly in contact with the insulator in a scanning electron microscope. By calculating and modeling the capacitance and the electrostatic force between the ball and the insulator plane, it has been possible to determine the relationship between the injected charges in the metallic ball and its surface potential. The major role of the dielectric thickness has been evidenced when the insulator is placed on a grounded metallic plane. At high potential values, a dielectric breakdown of the medium surrounding the sphere occurs and electrical charges are transferred from the ball to the dielectric sample. This transfer has been evidenced and quantified in the case of sapphire and quartz. Analytical calculations and numerical simulations using the finite-element method have been performed for interpreting these experimental results.
Fabrication and investigation of ultrathin, and smooth films for miniaturization of microelectronic devices92(2002); http://dx.doi.org/10.1063/1.1524307View Description Hide Description
(PZT)thin films were fabricated on substrates at 375 °C by radio frequency magnetron sputtering. A mixture of (110) and (100) orientations was found in all the PZTthin films. However, the in-plane grain size increased with an increase in film thickness, all films had smooth surfaces, and the root mean square roughness of the PZT films was in the range of 1–1.5 nm. As the film thickness increased, a decrease in residual stress and volume density of the PZT films was observed. PZT films become poorly crystallized with a decrease in film thickness. The magnitude of the maximum displacement from atomic force microscopy in local piezoresponse hysteresis mode increased from in 40 nm to in 152 nm. We suggest that the degradation in piezoelectric properties with a decrease in film thickness resulted from degradation of the crystallinity observed using transmission electron microscopy analysis, size effects derived from the grain size, and the residual stress evaluated using a laser reflectance method.
Ferroelectric polarization in stretched piezo- and pyroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer films92(2002); http://dx.doi.org/10.1063/1.1524313View Description Hide Description
After suitable preparation, films of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] show strong piezo- and pyroelectric effects. We investigated the polarization buildup in stretched P(VDF-HFP) copolymerfilms, cast from a dimethylsulfoxide/acetone solution. The films were poled under varying or constant electric fields. We observed a hysteresis of the polarization as a function of the electric field during poling of the stretched P(VDF-HFP) copolymerfilms with varying electric fields. From the hysteresis curve, a polarization of 24 and a coercive field of 100 MV/m were determined. The polarization values are lower than those of PVDF films cast from solution and stretched under the same conditions. It seems that HFP comonomer leads to a decrease in the amount of switchable dipoles. Nevertheless, the P(VDF-HFP) copolymer is very attractive because of its high piezo- and pyroelectric coefficients as well as its better chemical and thermal stability and its predicted longer lifetime. A rather uniform polarization across the whole sample thickness was found in spatially resolved measurements of the piezo- and pyroelectric properties. For comparison with PVDF and for structural information, we also report and discuss infrared transmission as well as dielectric response.
Microwave loss-factor of as a function of temperature, biasing field, barium concentration, and frequency92(2002); http://dx.doi.org/10.1063/1.1524314View Description Hide Description
A correct and verifiable model of the dielectric loss factor of as a function of temperature, biasing field, barium concentration, and frequency has been developed in frequency range 0.1–100 GHz. Contributions to the loss factor of fundamental, quasi-Debye, acoustic, and low frequency relaxation mechanisms have been modeled. The specified frequency dependence of the loss factor has been taken into account. The fitting parameters of the model provide a good agreement between experimental and simulated data.