Analogue switch for very low-voltage applications

A new analogue switch suitable for operation at very low-voltage supply in a standard CMOS technology is presented. The proposed switch is based on 'quasi-floating-gate' transistors and has a simple and compact structure. For illustrative purposes, two sample-and-hold circuits operating from a single supply voltage close to the threshold voltage of a transistor, and using the proposed technique, are presented. Experimental results obtained from prototypes in a 1.5 /spl mu/m CMOS technology are provided.     

Room-temperature laser emission of GaInNAs-GaAs quantum wells grown on GaAs (111)B

In this work, we present room-temperature laser emission at 1.206 /spl mu/m from GaInNAs-GaAs quantum-well (QW) laser diodes (LDs) grown on misoriented GaAs (111)B substrates for the first time. Details of the structure and the molecular beam epitaxial growth of the lasers are discussed. We found that the postgrowth rapid thermal annealing increased the optimum emission, while the in situ self annealing effect in these QWs is almost negligible. The optimum annealing cycle (30 s at 850/spl deg/C) is comparable to that found for the cladding-free GaInNAs single QW samples grown on GaAs (111)B. Finally, the optical and electrical characterization of these LD devices is presented. The LDs show a room-temperature threshold current density of 2.15 kA/cm/sup 2/, with a differential quantum efficiency of 37%, under pulsed conditions.     

How Focusing Phenomena in the Instructional Environment Support Individual Students' Generalizations

This article sets forth a way of connecting the classroom instructional environment with individual students' generalizations. To do so, we advance the notion of focusing phenomena, that is, regularities in the ways in which teachers, students, artifacts, and curricular materials act together to direct attention toward certain mathematical properties over others. The construct of focusing phenomena emerged from an empirical study conducted during a 5-week unit on slope and linear functions in a high school classroom using a reform curriculum. Qualitative evidence from interviews with 7 students revealed that students interpreted the m value in y = b + mx as a difference rather than a ratio as a result of counterproductive generalization afforded by focusing phenomena. Classroom analysis revealed 4 focusing phenomena, which regularly directed students' attention to various sets of differences rather than to the coordination of quantities.     

A multiple-switch high-voltage DC-DC converter

Series connection of power devices has evolved into a mature technique and is widely applied in HV DC power systems. Static and dynamic voltage balance is ensured by shunting individual devices with dissipative snubbers. The snubber losses become pronounced for increased operating frequencies and adversely affect power density. Capacitive snubbers do not exhibit these disadvantages, but they require a zero-voltage switching mode. Super-resonant power converters facilitate the principle of zero-voltage switching. A high-voltage DC-DC power converter with multiple series-connected devices is proposed. It allows the application of nondissipating snubbers to assist the voltage sharing between the multiple series-connected devices and lowers turnoff losses. Simulation results obtained with a circuit simulator are validated in an experimental power converter operating with two series-connected devices. The behavior of the series connection is examined for MOSFETs and IGBTs by both experimental work with a 2 kW prototype and computer simulation. Applications can be found in traction and heavy industry, where the soft-switching power converter is directly powered from a high-voltage source     

A study on some trace elements in Chilean seafood

Levels of essential and toxic trace elements in six marine species greatly in demand in the international market (canned pink clams, razor clams, clams, king crab, sardines, and frozen albacore tuna fish) were studied. A special laboratory adequately equipped for handling and preparing biological materials, is described. The elements As, Br, Na, Se, Hg, Cr, Fe, Zn and Rb were determined by INAA, while copper and cadmium content were determined using RNAA. Significant differences were found on comparing the contents of some toxic elements in mollusk and other samples of marine food. Various certified reference materials (CRMs) of the IAEA, NIST and NIES were analyzed for quality assurance purposes.     

Simultaneous determination of copper and lead in ethanol fuel by anodic stripping voltammetry

The presence of trace metals in car fuels plays an important role in the engine maintenance. In addition, these metals contribute for the environmental contamination in big cities and their control is necessary. Square Wave Stripping Voltammetry (SWSV) is a very sensitive technique for elemental trace determination and was applied for ethanol fuel analysis. The first studies were done searching for the best conditions for copper determination in alcoholic medium, utilizing gold electrodes. During these studies, the possibility of the simultaneous determination of copper and lead in the same experiment was observed. Two procedures for the analysis of these metals were adopted: The direct quantification of metals in alcohol–water mixtures and a second way that involves the evaporation of the organic solvent and re-suspension of the ions with water+electrolyte. Good recovery values were obtained for synthetic samples spiked with known amounts of metals. The results obtained for the two methods were in good agreement. The detection limits for copper and lead in 75% ethanol–water ratio solution were calculated as 120 and 235 ng l⁻¹, respectively, for 15-min deposition time.     

Design of ordered bimetallic complexes,Part 2:Trans-1,2-cyclohexanediaminetetraacetate bimetallates

Summary Knowledge of the kinetic and thermodynamic behaviour of aqueous solutions containing two divalent cations andtrans-1,2-cyclohexanediaminetetraacetate has been used to design synthetic pathways to ordered bimetallic complexesPart 1: E. Escriva, A. Fuertes and D. Beltran,Transition Met. Chem.,9, 184 (1984).     

Rapid spectrophotometric determination of chlorate with dimedone bisthiosemicarbazone monohydrochloride

Summary Dimedone bisthiosemicarbazone monohydrochloride has been examined to evaluate its usefulness as a reagent for rapid and sensitive Spectrophotometric determination of chlorate (0.5–5.0 ppm) by oxidation of the reagent in 70% perchloric acid medium and measurement of the yellow product. The molar absorptivity at 417 nm is 1.86×10⁴ l·mole^–1·cm^–1. The relative standard deviation is 1.6% for 2.5 ppm of chlorate.

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Spektropbotometrische Bestimmung von Chlorat mit Dimedonbisthiosemicarbazon-monocblorhydrat (I)

Zusammenfassung Die Verwendbarkeit von I als Reagens für die rasche und empfindliche spektrophotometrische Bestimmung von 0,5–5,0 ppm Chlorat durch Oxydation von I in 70%iger Perchlorsäure und Messung des gelben Reaktionsproduktes wurde untersucht. Die molare Absorptivität bei 417 nm beträgt 1,86×10⁴ l·mol^–1·cm^–1. Die relative Standardabweichung beträgt 1,6% für 2,5 ppm Chlorat.

     

Stereoselective synthesis of 4 alpha-hydroxy-8,12-guaianolides from santonin

Hydroxyester 2, easily obtained from santonin (1), has been transformed into 10 alpha-hydroxyguai-3-en-8,12-olide 6, a good intermediate for the synthesis of natural 8,12-guaianolides. Compound 6 was obtained from 2 by photochemical rearrangement of its acetyl derivative 7, stereoselective hydrogenation on Pd/C, reduction, regioselective elimination, hydrolysis, and lactonization. The synthesis of the natural guaianolides 3-5 was carried out in two sequences in which the regioselective elimination of a hydroxyl group at C10 with triflic anhydride or SOCl2 to afford, respectively, the endo or exo double bond on C10 and the regioselective opening of the C3-C4 alpha-epoxide were the key steps.