Accurate large-signal GaAs MESFET and HEMT modeling for power MMIC amplifier design

This article describes, from an industrial user's point of view, how large-signal GaAs MESFET and HEMT modeling can be done accurately and efficiently for power MMIC amplifier design. The method is based on commercially available CAD tools enhanced by in-house software (e.g., small-signal parameter extraction, generation of load-pull contours). The Materka model is shown to predict accurately the large-signal characteristics of GaAs MESFETs, but not of pseudomorphic HEMTs. For these devices, a modified Angelov model is found to be adequate. A method for determining the numerous large-signal model parameters is presented. Model verification is achieved by comparing simulated and on-wafer measured data like static I(V)-characteristics, multiple bias S-parameters, gain compression characteristics, and load-pull contours. Results of device scaling and calculations of optimum load impedances are discussed. The close fit to the measured data proves that an excellent basis for large-signal power MMIC design has been established.     

Attraction personnelle et attraction sociale : Lorsque la saillance catégorielle annule la relation similitude-attraction.

Research on interpersonal attraction has repeatedly demonstrated that people are more attracted to similar others. In the present article, our goal is to show that this similar-attraction relationship is moderated by people's current level of self-categorization. Students were required to constitute work groups by selecting five classmates who shared to varying degrees similar attitudes. Under standard conditions (no group salience) participants' choice supported the similar-attraction relationship. However, when implicitly reminded of their gender identity, participants did not show anymore attraction to others who shared more similar attitudes. The results are discussed in terms of the self-categorization theory and suggestions are made to distinguish between two forms of attraction: interpersonal attraction and social attraction. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of δ phase precipitation on the stress corrosion cracking resistance of alloy 718 in PWR primary water

The negative influence of δ phase on the intergranular stress corrosion cracking (IGSCC) resistance of alloy 718 is commonly taken for granted. In addition, δ phase formed at low temperature (about 1023 K) do not present the same characteristics than the one formed at higher temperatures (from 1173 to 1273 K). The aim of the present study is then to understand how δ phase precipitation could enhance crack initiation in alloy 718, whatever the form of δ phase is. For that purpose, several heat treatments leading to δ phase precipitation were realized on two alloy 718 heats, one sensitive to IGSCC and the second not. Specific slow strain rate tensile tests carried out on thin tensile specimens in simulated PWR primary medium at 633 K conclusively prove that δ phase has no effect on the intrinsic sensitivity to intergranular crack initiation of tested heats.     

Surface nano-aggregation and photocatalytic activity of TiO2 on H-type activated carbons

The main objective of this work is to detect any associative or synergistic effects between TiO₂ and activated carbon in 4-chlorophenol photodegradation. Different activated carbons (AC) were prepared from Tabebuia pentaphyla wood by means of physical activation with CO₂ or by extensive carbonization under N₂ flow at temperatures from 450 °C up to 1000 °C during 1 h. Results indicate a clear correlation between photocatalytic activities of titania with texture and surface chemistry of AC. Kinetic results of 4-chlorophenol photodegradation indicate that for most of mixed TiO₂ and AC solids a synergistic effect between both solids is observed. Surface nano-aggregation of TiO₂ on AC was observed by scanning electronic microscopy and dispersion of TiO₂ nanoparticles was improved as a function of more basic surface pH of AC. In conclusion, an increase of electronic density in carbon support clearly introduces an enhancement in titania's photoactivity for 4CP photodegradation. This beneficial effect indicates that it is possible to obtain clean water in a much shorter period of time by employing some selected AC in conjunction with TiO₂.     

Réalisation de filtres multidiélectriques sur un substrat biréfringent,déformable et de faible épaisseur (mylar)

La répartition spectrale d'un filtre interférentiel à bande étroite est fixée par le type d'empilement et le choix des diélectriques.Nous étudions une technique qui permet de changer le pouvoir de transmission d'un filtre terminé, en faisant varier l'épaisseur optique du substrat. Ce substrat, qui fait partie intégrante du filtre, est élastique et biréfringent. Cette variation peut être obtenue soit par déformation élastique du substrat qui entraîne une modification de l'épaisseur mécanique, soit par utilisation d'un polariseur qui permet de changer la valeur de l'indice utilisé.     

Services de transfert de données sur machines parallèles

We study in this paper the feasibility of using general purpose distributed memory parallel computers to implement some functions of high speed networks. We concentrate on the switching problem of the xtp high performance protocol (dealing with the network and transport layers of the osi stack), and on smds (switched multimegabit data service). We use methods and tools developed within the Pampa project (parallelizing methods, formal specification tools) to parallelize and prototype these kinds of switching systems on parallel architectures, and to make performance evaluations.     

DIVA: Recognition of typical situations for turbine generator diagnosis

DIVA is an expert system for turbine generator diagnosis. Its aim is to help a plant operator interpret vibration evolution to diagnose developing faults. Interpretation of vibration evolution is primarily a recognition process which is achieved by a particular model of heuristic classification. DIVA tries to recognize the observed situation among a hierarchy of typical situations.The purpose of this paper is to present the model we used for heuristic classification, based on prototypes. Prototypes represent typical problem situations and may also be seen as experts with special knowledge of what to do in such situations. We argue that this model presents significant advantages to deal with complex problems such as vibration surveillance.     

Calcul d'energie de fautes d'empilement dans le dioxyde d'uranium

Energy computations of some (100), (110) and (111), planar defects were performed using an ionic bond model for stoichiometric uranium dioxyde. The repulsive contribution to the fault was estimated in two different ways, i.e. using the Born-Mayer classical treatment, or potentials derived from shell model calculations. The stability of the various defect configurations has been studied; on the basis of our numerical values, we may conclude that dislocation dissociation is unlikely in stoichiometric uranium dioxyde.     

Specific limited hydrolysis and phosphorylation of food proteins for improvement of functional and nutritional properties

Limited specific hydrolysis of casein byStaphylococcus aureus V8 protease was used to produce 2% and 6.7% hydrolysates (2 and 6.7% of the peptide bonds hydrolyzed), each containing five polypeptides (by gel filtration) ranging in size from ∼ 16,000 to ∼1,000 daltons. The mixtures of polypeptides had substantially increased solubilities at pH 4.0–4.5, near the isoelectric point of casein. In general, the emulsifying activity index was less for the hydrolysates than for casein; the emulsion stability was higher for the 2% hydrolysate than was the emulsion from casein. Phosphorylation of zein markedly increased the water solubility of zein above and below pH 4. When the free amino acids tryptophan and/or lysine were added to zein in the presence of POC1₃, some amino acids were covalently bound to zein, in addition to covalent attachment of phosphate groups. Threonine did not become incorporated into zein by this method. These derivatives were much more soluble than zein above and below pH 4, the minimum solubility point. A derivative containing 0.98 mol P/mol of zein, along with 1.05% tryptophan and 0.24% lysine, had a relative growth effect onTetrahymena thermophili of 49% that of casein, in comparison to 4.5% for unmodified zein. All the modified zeins had improved emulsifying activity indices.