Two-piece model of hot point generation in reverse biased p−n diodes

This paper discusses the process of the generation of hot points in reverse biased p?n diodes. In this work we use a simple lumped thermal model of the diode. The stability of the nearly uniform distribution of the current through the junction of the diode is analysed. This analysis determines, for every diode, the maximum value of the current under which the junction will be free of current constriction, or hot points. It is concluded that, for diodes which are small enough, a hot point cannot appear. Nevertheless, if a nearly ideal voltage generator is used to drive the diode, a thermal instability of the whole junction can appear.     

A liquid-liquid extractor model based on UNIFAC

A steady state liquid-liquid extractor model using the UNIFAC group contribution method for phase equilibria has been developed. A heuristic method, which limits the number of stage calculations per iteration to a low number is used, according to the residuals calculated in the previous iteration, the method either selects or rejects a stage for the calculations. With this approach, the same accuracy in results have been obtained at about half the computing time with respect to without use of heuristic method. The model has been applied to simulate staged extractors as well as continuous extractors (plate columns). The model is based on a very robust flash calculation routine and on a versatile package (UNIFAC) for the prediction of thermodynamic properties.     

Star couplers using fused biconically tapered multimode fibres

We report the fabrication and testing of 4-, 7-and 19-fibre fused biconical taper transmissive star couplers for use with single multimode fibres. Overall insertion losses on six couplers range from ?1.3 dB to ?2.6 dB. Losses are nearly independent of N, the number of fibres, but dispersion of the powers in the output fibres increases significantly with N.     

Hydrogenation of CO and CO2 on carbon black-supported Ru catalysts

The activity and selectivity of both unsupported Ru and carbon black-supported Ru catalysts toward the hydrogenation of CO and CO₂ have been investigated in order to learn about the effect of metal particle size on both reactions. The specific activity for both reactions decreases with metal particle size and the product distribution obtained in the hydrogenation of CO (the hydrogenation of CO₂ only yields methane) is also a function of metal dispersion; thus, the proportion of methane produced increases and the olefin/paraffin decreases with decreasing Ru particle size. This behaviour is attributed to changes in the electronic properties of the Ru crystallites in close contact with the graphitised carbon black used as support.     

Supersensitivity induced by the toxin of Brazilian scorpion venom (Tityus serrulatus Lutz e Mello)

The purified toxin of the Brazilian scorpion venom (TsTX) enhances the pharmacological actions of directly and indirectly acting substances on several preparations. The enhancing action of TsTX is more pronounced for the indirectly acting drugs, which is highly suggestive of a prejunctional site of action. Secondarily, TsTX does also produce an unspecific increase of sensitivity at the level of the pharmacological receptor itself. The mechanism by which these changes are brought about remains to be determined.     

Vapor-liquid equilibrium in mixtures of 1,3 dioxolane-water

Vapor-liquid equilibrium measurements were carried out with a Stage-Müller apparatus on mixtures of 1,3 dioxolane-water at isobaric conditions. Thermodynamic consistency of data was checked, and liquid-phase activity coefficients were correlated using Wilson's equation, which was applied to various groups of data, each with a restricted range of temperature variation. The parameters δ_kj of Wilson's equation proved to be independent of the temperature.     

Analysis of Frequency Selective Surfaces at Millimeter Wave Band Using the Hertz Vector Potential Method

In this work, the Hertz vector potential method was used to obtain the dyadic Green’s function components for frequency selective surfaces (FSS) on an anisotropic layer. The considered structures are composed by periodic arrays of conducting crossed dipole and rectangular patch elements. Results of reflected power were obtained by using the moment method. Curves for the reflected power versus frequency are presented as function of the structural parameters, assuming x, y, and z orientation for the optical axis in the anisotropic substrate.     

Remote atmospheric breakdown for standoff detection by using an intense short laser pulse

A remote atmospheric breakdown is a very rich source of UV and broadband visible light that could provide an early warning of the presence of chemical-biological warfare agents at extended standoff distances. A negatively chirped laser pulse propagating in air compresses in time and focuses transversely, which results in a rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof-of-principle laboratory experiments are performed on the generation of remote atmospheric breakdown and the spectroscopic detection of mock biological warfare agents. We have generated third harmonics at 267 nm and UV broadband radiation in air from the compression and focusing of femtosecond laser pulses. Fluorescence emission from albumin aerosols as they were illuminated by the femtosecond laser pulse has been observed.     

Efficiency optimization for a diode-pumped Nd:YAG ceramic slab laser

With the help of photometric calculations based on ray-tracing algorithms, we have optimized the efficiency of the optical pumping of a Nd:YAG ceramic slab laser. The slab pumping is performed by means of two horizontal diode laser array stacks. The use of two small reflecting walls allows the sort of duct coupling that is capable of significantly improving the performance of the system. Our first experiments with a simple direct coupling provided a maximum extraction of slightly more than 160 W at a 20% slope efficiency level. The use of the optimized short duct coupling leads us to the extraction of 350 W with a slope efficiency of 51%, making use of the same diode arrays. The laser design is suitable for the construction of cw sources with a power output above 1 kW.