Resumo em Inglês:

During his scientific career, spanning the time from the early 1930's to the present date, Bernhard Gross has contributed to many disciplines of physics and engineering. Some of these fields, such as radiation effects in solids and charge storage and transport in dielectrics, are integral parts of electret research, an area which Gross has promoted, and whose development he has guided, over the past six decades. The present article focusses on the outstanding contributions of Bernhard Gross to electret research and discusses the impact of his work on the evolution of this field.

Resumo em Inglês:

Electron irradiation of solids produces a backemission of secondary electrons (energies between 0 and 50 eV) and reflected primaries (energies between 50 eV and that of the incident beam). For insulators, it is shown that an externally applied positive electric field penetrating into the solid material, energizes electrons generated by the primary irradiation and enables them to travel back to the surface of incidence and be emitted (stimulated secondary emission).

Resumo em Inglês:

Recent studies of charge trapping and charge transport in spolymer films irradiated and charged with monoenergetic electrons of range smaller than the sample thickness and thereafter stored or annealed under various conditions are discussed. An analytical model used to describe the phenomena takes the following parameters into consideration: Charge and energy deposition profiles, charge drift due to a shallow-trap-modulated mobility, deep trapping without release, trap filling due to a finite trap density, and ohmic relaxation due to a radiation-induced conductivity during irradiation and its delayed component after irradiation. The model calculations show the effect of various parameters on the shape of the initial charge distribution and on its evolution with annealing time. - Experiments with the laser-induced pressure-pulse (LIPP) method on 12 and 25<FONT FACE="Symbol">m</font>m thick fluorocarbon and polymide films, charged with 10 or keV electron beams, respectively, yield the charge distributions after irradiation and the changes of the distributions due to annealing of the samples at 120oC for various times. A comparison of experimental and analytical results reveals the trapping kinetics and permits to estimate the deep-trap density and the <FONT FACE="Symbol">mt</font>-product, where <FONT FACE="Symbol">m</font> is the trap-modulated mobilit and <FONT FACE="Symbol">t</font> is the trapping time.

Resumo em Inglês:

The effects of high-energy irradiation on the charging of insulators is reviewed with a viewpoint similar to the seminal work of Professor Bernhard Gross. Recent results relating to insulation breakdown and to insulators in space radiation are more easily explained because of Gross' fundamental studies. A number of important issues remain unsolved, yet the first steps to solutions have been provided by Gross.

Resumo em Inglês:

Photostimulable x-ray storage phosphors in form of image plates are a promising alternative to conventional two-dimensional x-ray detectors. By absorption of ionizing radiation, electrons and holes are generated and captured locally to form a dose proportional latent image. The stored information is read out by scanning with a focussed HeNe laser. This leads to a local excitation of the trapped electrons which subsequently recombine with the trapped holes causing the emission of light. The information is then recorded by means of a photomultiplier, digitized by an A/D converter and displayed with the aid of a computer. The present state of the field of storage phosphors is reviewed in the following areas: the image principle, today's understanding of the nature of the storage centers, their physical generation, and the optically stimulated charge transfer paths. In addition, improvements and applications of these phosphors will be discussed for existing and future commercial devices.

Resumo em Inglês:

This paper contains a brief overview on the recent developments of corona charging of polymers, with emphasis on the current corona triode. This latter method, which has been successfully applied to several types of polymer, is a legacy from Prof. Bernhard Gross' work in São Carlos, Brazil. Following a short introduction to corona charging, the experimental setups are described, especially with regard to the advantages in the constant current method. A few examples are given of the use of the constant current corona triode in the investigation of electrical properties of nonpolar and ferroeleectric polymers. The application of corona charging to pole nonlinear optic (NLO) polymers is discussed, including the perspectives for the constant current charging method for the NLO field.

Resumo em Inglês:

Electronic charge transport in insulating polymers is briefly discussed, particularly the measurement of charge carrier mobility. The more frequently used techniques of space charge profile measurent in dielectrics are then described, and data for polyethylene published during the last 5-6 years are reviewed.

Resumo em Inglês:

The properties of space charge (homocharge) and dipoles (heterocharge) in various electrets have been in the focus of interest since the investigations of B. Gross on carnauba wax. In this article we present a review on the role and the interaction of charges and dipoles in the polymeric electret polyvinylidenefluoride (PVDF). It covers the importance of charge injection for the orientation of the dipoles at low fields as well as the role of charge trapping for the stabilization of the polarization. Investigations of the thermal stability of the polarization and the charge balance are leading to a detailed analysis of the depth and localization of the charge traps. Furthermore, it was possible to identify the space charge as ions created electrochemically during the poling process.

Resumo em Inglês:

Mono-, bi-, and multimorphs are poled electrets with +/0, +/-, and more complicated alternating-polarity dipole-orientation patterns, respectively. Because of their unusual macroscopic piezo- and pyroelectrical as well as nonlinear optical properties, such structures are of particular interest for special transducer and optical waveguide applications. In this contribution, the preparation of such dipole-polarization patterns in ferroelectric and amorphous polymer electrets and their nondestructive probing by means of thermal and acoustical techniques are reviewed.

Resumo em Inglês:

The influence of polar species on the transport of charge carriers in molecular solids is reviewed. Guest molecules with permanent dipole moments introduced into molecular crystals can act as traps localizing carriers provided certain energetic conditions are fulfilled. In this case their polar character may manifest itself in a field-dependent decrease of the trap depth. Moreover, irrespective of the positions of their energy levels, polar impurities may locally modify the polarization energy thus creating traps on neighbouring molecules. The effect of polar dopants in disordered molecular solids can be described within the model due to Bässler: the presence of polar species broadens the density-of-states function thus decreasing the effective mobility of charge carriers.

Resumo em Inglês:

Relations and distinctions which are relavant to small-signal electrical-relaxation behavior are reviewed and applied to the important problem of identifying the physical processes leading to dispersed relaxation response. Complex-nonlinear-least-squares fitting of a response model to frequency-response data is found not to allow one to distinguish unambiguously in most cases between conductive-system response of Wagner-Voigt type, which may be characterized by a distribution of conductive system relaxation times [DCRT], and dielectric- system response of Maxwell type, characterized by a distribution of dielectric-system relaxation times [DDRT]. In general, one must include a parallel conductivity element <FONT FACE="Symbol">s</font>CP, as well as a high-frequency-limiting dielectric constant, in a conductive-system fitting model used to represent dielectric-system data with non-zero dc conductivity. Contrary to earlier predictions of Gross and Meixner, accurate numerical inversion of a set of exact frequency- response data to estimate the distribution with which it is associated shows that no discrete line necessarily appears in a DCRT associated with a truncated continuous DDRT. A discrete line can appear in general, however, when <FONT FACE="Symbol">s</font>CP <FONT FACE="Symbol">¹</font> 0 and is unaccounted for in an inversion process. The novel result is established that a data set mathematically described in terms of a dielectric system with dc leakage and involving a Maxwell-circuit exponential distribution of relaxation times may be well represented within usual experimental error by a Wagner-Voigt conductive system involving a form of the important Kohlrausch-Williams-Watts response model.

Resumo em Inglês:

The time dependent current in disordered systems under a step applied voltage for a planar synmetry is deduced according to the continuous time random walk approximation. Known dielectric response functions like Cole-Cole, Davidson-Cole, Havriliak-Negami and a few others are used as hopping time distribution functions in order to generate conductive responses. A theoretical relation exists between the dielectric and the conductive response which is the same one prevailing between the time derivative of the creep and the relaxation function, as found long ago by Gross (J.Appl.Phys.,18, 212, (1947)). A truncated version of the Widder method, in connection with MapleTM software facilities, was employed to obtain graphical primitives of Laplace transforms.

Resumo em Inglês:

A brief review is provided of the ways electrical properties of semiconducting polymers can be exploited. Emphasis is given to the analysis of electrical conductivity data, in particular to the importance of interchain processes that must be considered along with intrachain processes for bulk conductivity. Among the large variety of possible applications of thin films of semiconducting polymers, the focus is placed on recent breakthroughs in producing diodes, field effect transistors and electroluminescent devices out of these materials. The fabrication of ultrathin films from semiconducting polymers, using the Langmuir-Blodgett and the self-assembly techniques, is discussed to illustrate the importance of processibility issues for any real application including these polymers.

Resumo em Inglês:

We compute the Casimir pressure between an unusual pair of parallel plates, namely, a perfectly conducting plate (<FONT FACE="Symbol">e®¥)</font> and an infinitely permeable one (<FONT FACE="Symbol">m®¥</font>) with the generalized <FONT FACE="Symbol">z</font>-function method. The result for this problem, which has been rarely discussed in the literature, is a repulsive Casimir force. The <FONT FACE="Symbol">z</font>-function method provides a very compact and economic way of obtaining the final result.

Resumo em Inglês:

Oscillations in the vertical magneto-transport of a GaInAs-AlInAs superlattice were observed and explained as being the modulation of miniband properties. We explain that such a modulation is due to the Landau levels of the highly (Si) n+ doped regions. It is shown that phase and amplitude of these oscillations are dependent on the angle between electric and magnetic fields. This phenomenon is thus similar to the Shubnikov - de Haas effect in bulk materials, even though the active region of the superlattice is not intentionally doped. Very highly doped three-dimensional regions were grown on each side of the superlattice to allow for the process of Ohmic contact formation.

Resumo em Inglês:

Polarized first-order Raman scattering studies at 300 K were carried out in L-asparagine monohydrate crystal in order to obtain the general assignment of its phonon spectra. A careful analysis of the vibrational spectra shows that the assignment of the fundamental vibrational modes can be done on the basis of amino, carboxilic and water groups vibrations, and correlations with previous data reported for other amino acids. However, some vibrations were correctly assigned from the study of its temperature - dependence behavior performed in the range 10 - 300 K. The assignment proposed cofirm the zwitterionic structure of L-asparagine monohydrate crystal.

Resumo em Inglês:

Calcium Fluoride buffer lavers were grown on Si < 100 > substrates using the low energy cluster beam deposition technique. The films were annealed at various temperatures ranging between 500oC and 700oC. The SEM studies showed that as-deposited films were well oriented along the c-axis and had very smooth surface morphology. The annealed films on the contarary, showed lowering of peak intensities and roughening of the surface. The dielectric constant derived from the C-V measurements at 1 MHz were 2.01 and 18 for as-deposited and annealed films respectively.

Resumo em Inglês:

We obtain an analytic representation for the profile of the surge wave in the sea, due to a tropical storm at a low latitude. The results reproduce the known experimental data adequately. In the calculations we take into account the influence of Rossby's second parameter and we analyse its contribution to the surge wave, which is proved to be sufficiently small to be neglected in higher latitudes. The obtained analytical solution allows us to study in detail the surge wave formation in the presence of such a storm. To solve the problem, Laplace method and Green functions are used. This allows us to improve previous results obtained by Evsa in 1989.