SnO2, ZnO and related polycrystalline compound semiconductors: An overview and review on the voltage-dependent resistance (non-ohmic) feature

The present review describes mainly the history of SnO2-based voltage-dependent resistors, discusses the main characteristics of these polycrystalline semiconductor systems and includes a direct comparison with traditional ZnO-based voltage-dependent resistor systems to establish the differences and similarities, giving details of the basic physical principles involved with the non-ohmic properties in both polycrystalline systems. As an overview, the text also undertakes the main difficulties involved in processing SnO2- and ZnO-based non-ohmic systems, with an evaluation of the contribution of the dopants to the electronic properties and to the final microstructure and consequently to the system's non-ohmic behavior. However, since there are at least two review texts regarding ZnO-based systems [Levinson, L. M., and Philipp, H. R. Ceramic Bulletin 1985;64:639; Clarke, D. R. Journal of American Ceramic Society 1999;82:485], the main focus of the present text is dedicated to the SnO2-based varistor systems, although the basic physical principles described in the text are universally useful in the context of dense polycrystalline devices. However, the readers must be careful of how the microstructure heterogeneity and grain-boundary chemistry are capable to interfere in the global electrical response for particular systems. New perspectives for applications, commercialization and degradation studies involving SnO2-based polycrystalline non-ohmic systems are also outlined, including recent technological developments. Finally, at the end of this review a brief section is particularly dedicated to the presentation and discussions about others emerging non-ohmic polycrystalline ceramic devices (particularly based on perovskite ceramics) which must be deeply studied in the years to come, specially because some of these systems present combined high dielectric and non-ohmic properties. From both scientific and technological point of view these perovskite systems are quite interesting. (c) 2007 Elsevier Ltd. All rights reserved.

Stepped light-induced transient measurements of photocurrent and voltage in dye-sensitized solar cells based on ZnO and ZnO:Ga

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Modelling of the influence of the layer pollution thickness in high voltage polluted insulators

An experimental model and a mathematical model with the introduction of a ramp in the channel of Obenaus model are presented. The aim is to present a better reproduction of the real layer pollution deposited on the HV insulators. This better reproduction is obtained from two types of thickness variation: the introduction of a ramp (soft variation) and the introduction of a step (sudden variation). The computational simulations and the experimental data suggest that the introduction of the ramp is the better reproduction of the layer pollution. The ramp approximates to the real layer pollution more than the step.

New 12kW three-phase 18-pulse high power factor AC-DC converter with regulated output voltage for rectifier units

This work presents a new high power factor three-phase rectifier based on a Y-connected differential autotransformer with reduced kVA and 18-pulse input current followed by three DC-DC boost converters. The topology provides a regulated output voltage and natural three-phase input power factor correction. The lowest input current harmonic components are the 17th and the 19th. Three boost converters, with constant input currents and regulated parallel connected output voltages are used to process 4kW each one. Analytical results from Fourier analyses of winding currents and the vector diagram of winding voltages are presented. Simulation results to verify the proposed concept and experimental results are shown in the paper.

On designing linearly-tunable ultra-low voltage CMOS gm-C filters

A linearly-tunable ULV transconductor featuring excellent stability of the processed signal common-mode voltage upon tuning, critical for very-low voltage applications, is presented. Its employment to the synthesis of CMOS gm-C high-frequency and voiceband filters is discussed. SPICE data describe the filter characteristics. For a 1.3 V-supply, their nominal passband frequencies are 1.0 MHz and 3.78 KHz, respectively, with tuning rates of 12.52 KHz/mV and 0.16 KHz/m V, input-referred noise spectral density of 1.3 μV/Hz1/2 and 5.0μV/Hz1/2 and standby consumption of 0.87 mW and 11.8 μW. Large-signal distortion given by THD = 1% corresponds to a differential output-swing of 360 mVpp and 480 mVpp, respectively. Common-mode voltage deviation is less than 4 mV over tuning interval.

An accurate low-voltage analog memory-cell with built-in multiplication

A CMOS memory-cell for dynamic storage of analog data and suitable for LVLP applications is proposed. Information is memorized as the gate-voltage of input-transistor of a gain-boosting triode-transconductor. The enhanced output-resistance improves accuracy on reading out the sampled currents. Additionally, a four-quadrant multiplication between the input to regulation-amplifier of the transconductor and the stored voltage is provided. Designing complies with a low-voltage 1.2μm N-well CMOS fabrication process. For a 1.3V-supply, CCELL=3.6pF and sampling interval is 0.25μA≤ ISAMPLE ≤ 0.75μA. The specified retention time is 1.28ms and corresponds to a charge-variation of 1% due to junction leakage @75°C. A range of MR simulations confirm circuit performance. Absolute read-out error is below O.40% while the four-quadrant multiplier nonlinearity, at full-scale is 8.2%. Maximum stand-by consumption is 3.6μW/cell.

A low-voltage programmable-gain CMOS amplifier with very-low temperature-drift

A new topology for a LVLP variable-gain CMOS amplifier is presented. Input- and load-stage are built around triode-transconductors so that voltage-gain is fully defined by a linear relationship involving only device-geometries and biases. Excellent gain-accuracy, temperature-insensitivity; and wide range of programmability, are thus achieved. Moreover, adaptative biasing improves the common-mode voltage stability upon gain-adjusting. As an example, a 0-40dB programmablegain audio-amplifier is designed. Its performance is supported by a range of simulations. For VDD=1.8V and 20dB-nominal gain, one has Av=19.97dB, f3db=770KHz and quiescent dissipation of 378μW. Over temperatures from -25°C to 125°C, the 0. ldB-bandwidth is 52KHz. Dynamic-range is optimized to 57.2dB and 42.6dB for gains of 20dB and 40dB, respectively. THD figures correspond to -60.6dB@Vout= 1Vpp and -79.7dB@Vout= 0.5 Vpp. A nearly constant bandwidth for different gains is also attained.

A low-voltage triode-MOSFET four-quadrant multiplier with optimized current-efficiency

A low-voltage, low-power four-quadrant analog multiplier with optimized current-efficiency is presented. Its core corresponds to a pseudodifferential cascode, gain-boosting triode-transconductor. According to a low-voltage 1.2μm CMOS n-well process, operand differential-amplitudes are 1.0Vpp and 0.32Vpp for a 1.3V-supply. Common-mode voltages are properly chosen to maximize current-efficiency to 58%. Total quiescent dissipation is 260μW. A range of PSPICE simulation supports theoretical analysis. Excellent linearity is observed on dc characteristic. Assuming a ±0.5% mismatch on (W/L) and VTH THD at full-scale is 0.93% and 1.42%, for output frequencies of 1MHz and 10MHz, respectively.

Effect of powder milling and dopant addition on the current-voltage characteristics of SnO2-based ceramics

Non-linear electrical properties of SnO2-based ceramics were investigated as a function of powder agglomeration condition and as a function of dopant addition. All doped powders presented a single phase, cassiterite, as evidenced by X-ray diffraction analysis. The effect of milling was quite evident, with non-milled powder showing higher agglomerated particle size than milled powder. Cr addition seemed to increase the non-linear coefficient. Cu and Mn rendered dense ceramics, but α values for systems with Mn were higher than for systems with Cu.

Electrode poling of cellular polypropylene films with short high-voltage pulses

Recently, piezoelectric cellular polypropylene (PP) was proposed as a new type of quasi-ferroelectric. The observed hysteresis of the charge density as a function of the electric field could be explained as field-dependent charging inside the gas-filled voids. Interestingly enough, the measurable poling behavior of the macroscopic dipoles formed by charges that are trapped at the internal void surfaces is phenomenologically completely identical to the cooperative poling behavior of microscopic molecular dipoles in ferroelectric polymers. Therefore, it can be assumed that charge separation (or charge redistribution) and subsequent trapping in cellular PP is a rather fast switching process. In order to examine the poling dynamics, we developed an experimental setup for pulsed poling. High-voltage pulses with a duration of 45 μs (FWHM) were applied in direct contact to two-side metallized cellular PP films. The pulsed poling yields piezoelectricity in the cellular PP. We study and discuss the dependence of the resulting piezoelectricity on the poling field. We also characterize the charge separation during application of higher electric poling fields of up to -10 kV in direct contact to the two-side metallized films for longer times.

A linearly-tunable OTA-C sinusoidal oscillator for low-voltage applications

A low-voltage, low-power OTA-C sinusoidal oscillator based on a triode-MOSFET transconductor is here discussed. The classical quadrature model is employed and the transconductor inherent nonlinear characteristic with input voltage is used as the amplitude-stabilization element. An external bias VTUNE linearly adjusts the oscillation frequency. According to a standard 0.8μm CMOS n-well process, a prototype was integrated, with an effective area of 0.28mm2. Experimental data validate the theoretical analysis. For a single 1.8V-supply and 100mV≤VTUNE≤250mV, the oscillation frequency fo ranges from 0.50MHz to 1.125MHz, with a nearly constant gain KVCO=4.16KHz/mV. Maximum output amplitude is 374mVpp @1.12MHz. THD is -41dB @321mVpp. Maximum average consumption is 355μW.

Effect of growth regulators in citral content in lemongrass in different seasons (in Portuguese)

The purpose of the present research was to evaluate the effect of plant growth regulators in yield citral content in lemongrass (Cymbopogon citratus (D.C.) Stapf), in different seasons. The field experiment was conducted on Sao Manuel Experimental Farm, Agronomical Science College, Paulista Estatal University, Botucatu, Brazil, which soil was classified in alic yellon red latosol (Haplorthox). Plants were randommly assigned blocks to treatments with three repetitions. The experimental groups consisted of GA3 (50 and 100 mg.l-1);Ethrel (100 and 200 mgl-1); CCC (500 and 1000 mg.l-1); Alar 85 (1000 and 2000 mg.l-1); Accel (20 and 40 mg.l-1) and control group. Five applications of plant growth regulators were realized every other three months. After 40 days of each foliar spray, the plants were cut and determined the citral yield. The essential oils were extracted from leaf tissue by hydrodistillation in a Clevenger apparatus. Samples of 250g of leaf and put in Clevenger apparatus with deionized water for 3h to determine the yield of essential oil. The citral content was analyzed by GC. In the present study the concentrations of plant growth regulators used did not increase citral content.

A low-voltage programmable-gain current-mode amplifier

A CMOS low-voltage, wide-band continuous-time current amplifier is presented. Based on an open-loop topology, the circuit is composed by transresistance and transconductance stages built around triode-operating transistors. In addition to an extended dynamic range, the amplifier gain can be programmed within good accuracy by the rapport between the aspect-ratio of such transistors and tuning biases Vxand Vy. A balanced current-amplifier according to a single I. IV-supply and a 0.35μm fabrication process is designed. Simulated results from PSPiCE and Bsm3v3 models indicate a programmable gain within the range 20-34dB and a minimum break-frequency of IMHz @CL=IpF. For a 200 μApp-level, THD is 0.8% and 0.9% at IKHz and 100KHz, respectively. Input noise is 405pA√Hz @20dB-gain, which gives a SNR of 66dB @1MHz-bandwidth. Maximum quiescent power consumption is 56μ W. © 2002 IEEE.

Stable and metastable ferroelectric polarization analyzed by the ramp voltage technique

The study of the stable and the metastable ferroelectric polarization of poly(vinylidene fluoride), PVDF, was performed using two successive equal sign ramp voltages, mediated by a short-circuit period. Rates from 10 V/s up to 0.7 MV/s were used. Results showed that they follow different formation kinetics; that the stable part decreases for higher ramp voltage rates and its apparent coercive field increases.

Using artificial intelligence for the control of medium voltage substations

This paper introduces a method for the supervision and control of devices in electric substations using fuzzy logic and artificial neural networks. An automatic knowledge acquisition process is included which allows the on-line processing of operator actions and the extraction of control rules to replace gradually the human operator. Some experimental results obtained by the application of the implemented software in a simulated environment with random signal generators are presented.