Preparative separation of flavonoids from the medicinal plant Davilla elliptica St. Hill. by high-speed counter-current chromatography

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Depairing critical current density of a mesoscopic square superconductor

In the present paper we consider the vortex lattice properties of a square superconductor such as the vortex patterns, the Gibbs free energy, the magnetization, and the depairing critical current density. We show that this last quantity shows a matching effect, that is, it shows a discontinuous behavior as a function of the applied magnetic field.

CURRENT-ALGEBRA OF SUPER WZNW MODELS

We derive the current algebra of supersymmetric principal chiral models with a Wess-Zumino term. At the critical point one obtains two commuting super-affine Lie algebras as expected, but, in general, them are intertwining fields connecting both right and left sectors, analogously to the bosonic case. Moreover, in the present supersymmetric extension we have a quadratic algebra, rather than an affine Lie algebra, due to the mixing between bosonic and fermionic fields; the purely fermionic sector displays an affine Lie algebra as well.

BARRIER VOLTAGE DEFORMATION OF ZNO VARISTORS BY CURRENT PULSE

The phenomenon of electrical degradation in ZnO varistors was studied by application of high-intensity current pulses. A wave shape of 8 X 20-mu-s and rectangular waves of 1 and 2 ms were used. The degradation was estimated by reference electric-field variation and by Schottky voltage barrier deformation. The results showed that current pulses reduce both the height and the width of the barrier voltage. It was also observed that the donor density N(d) did not change but the surface states density N(s) decreased with degradation.

Droplet counter-current chromatography of indole alkaloids from Tabernaemontana hilariana

This paper reports the separation of the indole alkaloids from the benzene extract of the root barks of Tabernaemontana hilariana (Apocynaceae). The crude alkaloid fraction was fractionated by droplet counter-current chromatography using a low polarity mixture (hexane:ethyl acetate:ethanol:water). Nine indole alkaloids (3-hydroxycoronaridine, coronaridine, voacangine, 3-(2-oxopropyl) coronaridine, voacangine hydroxyindolenine, ibogamine, voacangine pseudoindoxyl, coronaridine pseudoindoxyl and tabernanthine) were identified using thin laver chromatography gas chromatography coupled with mass spectrometry and nuclear magnetic resonance spectroscopy. Copyright (C) 1999 John Wiley & Sons, Ltd.

Settling analysis of MOS regulated current mirrors applied to micropower D/A converters

This paper provides an insight to the trade-off between settling time and power consumption in regulated current mirrors as building parts in micropower current-switching D/A converters. The regulation-loop frequency characteristic is obtained and difficulties to impose a dominant-pole condition to the resulting 2nd-order system are evaluated. Raising pole frequencies in micropower circuits, while meeting consumption requirements, is basically limited by parasitic capacitances. For such cases, an alternative is to impose a twin-pole condition in which design constraints are somewhat relieved and settling slightly improved. Relationships between pole frequencies, transistor geometry and bias are established and design guidelines for regulated current mirrors founded. By placing loop-transistors in either weak or strong inversion, small (W/L) ratios are allowed and stray capacitances reduced. Simulated waveforms suggest a good agreement with theory. The proposed approach applied to the design of a micropower current-mode D/A converter improves both simulated and experimental settling performance.

Electric measurements with constant current: A practical method for characterizing dielectric films

This article assesses the use of the constant current (CC) method for characterizing dielectric films. The method is based on charging the sample with a constant current (current stress) and measuring the corresponding voltage rise under the closed circuit condition. Our article shows that the CC method is an alternative to the constant voltage stressing method to study the electric properties of nonpolar, ferroelectric, and polar polymers. The method was tested by determining the dielectric constant of polytetrafluoroethylene, and investigating the electric conduction in poly(ethylene terephthalate). For the ferroelectric polymer poly(vinylidene fluoride), it is shown that hysteresis loops and the dependence of the ferroelectric polarization on the electric field can be obtained. (C) 2001 American Institute of Physics.

Effect of the bound nucleon form factors on charged-current neutrino-nucleus scattering

We study the effect of bound nucleon form factors on charged-current neutrino-nucleus scattering. The bound nucleon form factors of the vector and axial-vector currents are calculated in the quark-meson coupling model. We compute the inclusive C-12(nu(mu),mu(-))X cross sections using a relativistic Fermi gas model with the calculated bound nucleon form factors. The effect of the bound nucleon form factors for this reaction is a reduction of similar to8% for the total cross section, relative to that calculated with the free nucleon form factors.

CURRENT ASPECTS OF FUEL ETHANOL-PRODUCTION IN BRAZIL

Brazil has become a great producer of bioethanol using sugarcane as the basic raw material. Fed-batch process and continuous process are used. Biogas generation from vinasse, production of dry yeast, and autolyzed bagasse for animal feed are making the ethanol production less polluting and more profitable. Bagasse surplus has also been converted into electrical energy. Another alternative use for bioethanol is its conversion to petrochemical derivatives. Up to the present, however, this conversion has been carried out on only a small scale by the industry.

Thickness dependence of leakage current in BaBi2Ta2O9 thin films

BaBi2Ta2O9 thin films having a layered structure were fabricated by metalorganic solution deposition technique. The films exhibited good structural, dielectric, and insulating properties. The room temperature resistivity was found to be in the range of 10(12)-10(14) Omega cm up to 4 V corresponding to a field of 200 kV/cm across the capacitor for films annealed in the temperature range of 500-700 degrees C. The current-voltage (I-V) characteristics as a function of thickness for films annealed at 700 degrees C for 1 h, indicated bulk limited conduction and the log(I) vs V-1/2 characteristics suggested a space-charge-limited conduction mechanism. The capacitance-voltage measurements on films in a metal-insulator-semiconductor configuration indicated good Si/BaBi2Ta2O9 interface characteristics and a SiO2 thickness of similar to 5 nm was measured and calculated. (C) 1999 American Institute of Physics. [S0003-6951(99)00830-X].

A low-voltage wide-swing programmable-gain current amplifier

A CMOS low-voltage, wide-swing continuous-time current amplifier is presented. Exhibiting an open-loop architecture, the circuit is composed of transresistance and transconductance stages built upon triode-operating transistors. In addition to an extended dynamic range, the current gain can be programmed within good accuracy by a rapport involving only transistor geometries and tuning biases. Low temperature-drift on gain setting is then expected.In accordance with a 0.35 mum n-well CMOS fabrication process and a single 1.1 V-supply, a balanced current-amplifier is designed for a programmable gain-range of 6 - 34 dB and optimized with respect to dynamic range. Simulated results from PSPICE and Bsim3v3 models indicate, for a 100 muA(pp)-output current, a THD of 0.96 and 1.87% at 1 KHz and 100 KHz, respectively. Input noise is 120 pArootHz @ 10 Hz, with S/N = 63.2 dB @ 1%-THD. At maximum gain, total quiescent consumption is 334 muW. Measurements from a prototyped amplifier reveal a gain-interval of 4.8-33.1 dB and a maximum current swing of 120 muA(pp). The current-amplifier bandwidth is above 1 MHz.

Effect of combined bending strain and thermal cycling on the voltage-current characteristic curves of Bi-2223 tapes

High critical temperature superconductors are evolving from a scientific research subject into large-scale application devices. In order to meet this development demand they must withstand high current capacity under mechanical loads arising from thermal contraction during cooling from room temperature down to operating temperature (usually 77 K) and due to the electromagnetic forces generated by the current and the induced magnetic field. Among the HTS materials, the Bi2Sr2Ca2Cu3Ox, compound imbedded in an Ag/AgMg sheath has shown the best results in terms of critical current at 77 K and tolerance against mechanical strain. Aiming to evaluate the influence of thermal stress induced by a number of thermal shock cycles we have evaluated the V-I characteristic curves of samples mounted onto semicircular holders with different curvature radius (9.75 to 44.5 mm). The most deformed sample (epsilon = 1.08%) showed the largest reduction of critical current (40%) compared to the undeformed sample and the highest sensitivity to thermal stress (I-c/I-c0 = 0.5). The V-I characteristic curves were also fitted by a potential curve displaying n-exponents varying from 20 down to 10 between the initial and last thermal shock cycle.

High current-density anodic electro-dissolution in flow-injection systems for the determination of aluminium, copper and zinc in non-ferroalloys by flame atomic absorption spectrometry

An automatic Procedure with a high current-density anodic electrodissolution unit (HDAE) is proposed for the determination of aluminium, copper and zinc in non-ferroalloys by flame atonic absorption spectrometry, based on the direct solid analysis. It consists of solenoid valve-based commutation in a flow-injection system for on-line sample electro-dissolution and calibration with one multi-element standard, an electrolytic cell equipped with two electrodes (a silver needle acts as cathode, and sample as anode), and an intelligent unit. The latter is assembled in a PC-compatible microcomputer for instrument control, and far data acquisition and processing. General management of the process is achieved by use of software written in Pascal. Electrolyte compositions, flow rates, commutation times, applied current and electrolysis time mere investigated. A 0.5 mol l(-1) HNO3 solution was elected as electrolyte and 300 A/cm(2) as the continuous current pulse. The performance of the proposed system was evaluated by analysing aluminium in Al-allay samples, and copper/zinc in brass and bronze samples, respectively. The system handles about 50 samples per hour. Results are precise (R.S.D < 2%) and in agreement with those obtained by ICP-AES and spectrophotometry at a 95% confidence level.