Wigner ensemble Monte Carlo simulation without splitting error of a GaAs resonant tunneling diode

Journal of Computational Electronics - A Monte Carlo technique for the solution of the Wigner transport equation has been developed, based on the generation and annihilation of signed particles...     

Catalytic activity of a mechanically mixed Ga2Oe/H-MFI catalyst under distance preservative conditions

The distance between active centers is the key factor in studying the activity of a gallium promoted zeolite catalyst. Special conditions, preserving the long distance between gallium and zeolite acid center (i.e. distance preservative conditions), were found in mechanically mixed catalysts. Catalyst samples were tested with the n-butane aromatization reaction under these conditions. The observed catalytic cooperation between active centers was ascribed to a bifunctional catalyst and/or a remote control mechanism.     

Room temperature migration of ion beam injected point defects in crystalline silicon

We have investigated the room temperature diffusion and trapping phenomena of ion beam generated point defects in crystalline Si by monitoring their interaction with dopants, native contaminants such as C and O, and other defects. Spreading resistance measurements show that a small fraction ( 10⁻⁷–10⁻⁶) of the defects generated at the surface by a 40 keV Si implant is injected into the bulk. These defects undergo trap-limited diffusion and produce dopant deactivation and/or partial annihilation of preexisting deep (several micron) defect markers, produced by MeV He implants. It is found that in highly pure, epitaxial Si layers, these effects extend to several microns from the surface, demonstrating a long range migration of point defects at room temperature. A detailed analysis of the experimental evidences allows us to identify the Si self-interstitials injected into the bulk as the major responsible of both dopant deactivation and partial annealing of vacancy-type defects (divacancies, phosphorus-vacancy and oxygen-vacancy) generated by the implants. Finally, a lower limit of 6 × 10⁻¹¹ cm²/s is obtained for the room temperature diffusivity of Si self-interstitials.     

Coupling Impedance in a Circular Particle Accelerator, a Particular Case: Circular Beam, Elliptic Chamber

As a first step the concept of coupling impedance (c.i.) is extended to characterize the relationship between cause and effect relevant to distinct points. Then we can exploit a new law that relates the transverse c.i. to the lonitudinal one by means of a second derivative. When cause and effect coincide we get the standard definition of c.i. At a first sight the computation of the coupling impedance for the present model seems a trivial problem. In fact the c.i. is proportional to the Green function of the relevant model, which can be found as an open series, for instance in the treatise of Morse and Feshbach. But the authors themselves warn that this series is poorly convergent and in addition should not be differentiated. We succeeded in closing the series. The problem formally seems to be solved. But this procedure could however turn out to be sterile for numerical purposes, where we obviously need series expansions, unless one finds better alternative expansions. This is the final and possibly the most important step, where a different extremely rapidly convergent expansion is found. The use of this expansion has led to the recognition of a vast number of numerical results that are synthesized in the graphic representations of this work. The longitudinal c.i. is scarsely affected by the ellipticity and excentricity of the beam, so that, unless the beam is far off the central beam position, it behaves as in a circular cross section pipe.     

Automatic ultrasonic inspection for internal defect detection in composite materials

The detection of internal defects in composite materials with non-destructive techniques is an important requirement both for quality checks during the production phase and in-service inspection during maintenance operations. Visual inspection allows only the analysis of surface characteristics of materials and, then, if internal faults occur inside composite structures, a deeper analysis is required. A comparison between the reactions of different materials to ultrasonic signals can be used to highlight the difference in the internal structures and also to detect the depth position of these anomalies. However, ultrasonic data are difficult to interpret since they require the analysis of a continuous signal for each point of the material under consideration. An automatic procedure is necessary to manage large data sets and to extract significant differences between them.In this paper, we address the problem of automatic inspection of composite materials using an ultrasonic technique. We consider two main steps for interpreting ultrasonic data: the pre-processing technique necessary to normalize the signals of composite structures with different thicknesses and the classification techniques used to compare the ultrasonic signals and detect classes of similar points.     

Dynamic Photoswitching of Electron Energy Levels at Hybrid ZnO/Organic Photochromic Molecule Junctions

The functionality of interfaces in hybrid inorganic/organic (opto)electronic devices is determined by the alignment of the respective frontier energy levels at both sides of the heterojunctions. Controlling the interface electronic landscape is a key element for achieving favourable level alignment for energy and charge transfer processes. Here, it is shown that the electronic properties of polar ZnO surfaces can be reversibly modified using organic photochromic switches. By employing a range of surface characterization techniques combined with density functional theory calculations, it is demonstrated that self‐assembled monolayers (SAMs) of photochromic phosphonic acid diarylethenes (PA‐DAEs) can be employed to reversibly change the electronic properties of polar ZnO/SAM structures by light stimuli. The highest occupied molecular orbital level of PA‐DAE is raised by 0.7 eV and the lowest unoccupied one lowered by 0.9 eV, respectively, upon illumination by ultraviolet light and the levels shift back to their original position upon illumination by green light. The results thus provide a pathway to tailor hybrid interface electronic properties in a dynamic manner upon simple light illumination, which can be exploited to reversibly tune the electrical properties of photoswitchable (opto)electronic devices.     

Dielectric behavior of some polar high polymers at ultra-high frequencies (microwaves)

The dielectric behavior of different polar high polymers at ultra-high frequencies has been investigated by means of a dielectrometer, suitably modified to permit measurements at different temperatures. Experimental measurements were made at about 9 × 10⁹ cps over the temperature range of ?150 to 200°C. for polyoxymethylene, polythiomethylene, poly(3,3′-chloromethyl)oxetane (Penton), polycarbonate of 4,4′-dioxydiphenyl-2,2′-propane (Makrolon), poly(vinyl alcohol), poly(vinyl acetate), poly(vinyl chloride), vinyl chloride–vinyl acetate copolymer, and two ABS plastics, type B (blend) and type G (graft). On comparing the dielectric behavior of the examined materials at ultra-high frequencies with the corresponding ones determined at low or at radiofrequencies, it is observed that, in the microwave region, all relaxation peaks, either connected with cooperative motions in main chain (primary processes) or with local motions in the backbone or in side chains (secondary processes), usually observed at lower frequencies, tend to disappear; the corresponding relaxation effects, however, manifest themselves through a progressive increase of losses with increasing temperature, which is particularly marked above the glass transition temperature T_g. The latter transition, in spite of the very high frequency, is easily distinguished, in most cases, by the sudden change of slope in the tan δ versus temperature curve which accompanies its onset. This is explained on the basis of the very wide distribution times of molecular relaxation processes in polymers and the increase in strength of the secondary relaxation effects, which is verified at T_g, as a consequence of the increased kinetic energy of macromolecules and of the larger free volume for orientation of side chains. Each case is discussed separately and the experimental results interpreted on the basis of the molecular structure and chain mobility of the examined polymers.     

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².