On the current delivery limit of semiconducting carbon nanotubes

The current carrying capacity of single-walled semiconducting carbon nanotubes (CNTs) is studied by self-consistent quantum simulations using the non-equilibrium Green’s function formalism with the self-consistent Born approximation. The simulation shows that the current carrying capacity depends on the bias regime and is drastically different from that of metallic tubes. For long CNTs (with a length much longer than zone boundary and optical phonon scattering mean free path), the current saturates around 20 μA in the forward bias regime with unipolar transport due to phonon scattering. In ambipolar transport regime, the current delivery limit is still about 20 μA due to recombination of electron and hole currents. In contrast, for short semiconducting CNTs, the current delivery capacity can be above 25 μA in the unipolar transport regime and further double in the ambipolar transport regime. In reverse bias regime, the current of a long CNT can exceed 20 μA due to the second subband conduction and increased electron injection from the drain. The simulation provides a coherent explanation to the dependence of current delivery limit on bias regime and channel length, which is consistent with recent experiments.     

Efficiency increase of an induction motor by improving coolingperformance

Improving the efficiency of induction motors, which are the most energy consuming electric machines in the world, saves much energy. The efficiency can be increased by improving cooling performance as well as by using better materials or by improving electromagnetic performance with better design. This paper presents the relationship between the efficiency or the losses and the temperature of coils with experiments as well as simulations by changing parameters such as the load and the flow rate of cooling air. The losses and the efficiency are calculated from an equivalent circuit method as well as experiments. Coil temperatures variation affects much on the efficiency. The internal cooling method is better than that of external cooling for the coil temperature reduction. Several cooling methods are compared focusing on the fan efficiency and performance, from which the values of the efficiencies of the motors are expected. The fan efficiency as well as the fan performance should be considered for the optimum fan design to increase the total efficiency of a motor. The simulations are validated by the comparison with the experiments     

A simple and accurate method of probability of bit error analysisfor asynchronous band-limited DS-CDMA systems

This paper considers probability of bit error (P_e) analysis in asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. It presents a simple and accurate method of P_e analysis. The proposed method can serve as an attractive alternative to the only two techniques currently available for band-limited systems: the standard Gaussian approximation (SGA) and the characteristic function method. The former is prone to inaccuracy while the latter, large computational complexity. The method generalizes the simplified improved Gaussian approximation (SIGA) derived previously for rectangular pulses. This paper also outlines a generalization of another method referred to as the improved Gaussian approximation (IGA). Numerical examples demonstrate the far greater accuracy of the generalized SIGA with respect to the SGA. The examples consider the IS-95 and square-root raised cosine (Sqrt-RC) pulses as well as uniform and nonuniform received power conditions     

Particle characteristics of a stable fluidized bed aerosol generator

An aerosol generator consisting of a vibrating system for feeding dust into a fluidized bed was developed and tested to determine its dust output characteristics. The dust feed unit can produce 0–40 g min⁻¹ of coal dust and shows constant output up to 3 h operation durations. These correspond to mass concentrations of 0–101 g m⁻³ of coal particles for an air flowrate of 395 l min⁻¹ through the aerosol generator. The aerosolized coal particles show constant particle size distribution with time for up to h of testing under varied operation parameters. The normalized particle size distribution remains almost identical for a given feed material for a range of dust loadings. The time required to reach steady state aerosol generation is negligible for the sizes of coal particles used in this investigation.     

Wave-attenuation estimation in fluid-filled steel pipes: The first longitudinal guided wave mode

Attenuations of the first (or fundamental) longitudinal guided wave modes propagating in liquid-filled steel pipes are numerically investigated. Several filling liquids transported by the steel pipe are considered in the investigation. In the numerical modeling stage, a sink is considered for abandoning standing wave modes caused by the internal liquids; hence, the attenuation dispersion curves become simpler. From the attenuation dispersion curves, two specific attenuation values corresponding to 1 MHz and 2.68 MHz are selected; then, the concept of parametric density is introduced to predict attenuation for a certain filling material. With this concept, it is possible to approximately calculate attenuation values without a complex numerical attenuation calculation. This investigation may provide fundamental data to inspectors using ultrasonic guided-wave techniques in the petrochemical industry and in the field of water supply, two branches of the economy that are always under pressure owing to the demand of increasing productivity and that are challenged owing to stricter environmental rules, thus necessitating promising, low-cost inspection techniques. The text was submitted by the authors in English.     

宝山水电站坝区配电的选型及应用

本文通过ZWB组合式变电站在黑龙江省宝山水电站工程中的应用,论述了在坝区配电中选用组合式变电站的作用和必要性,并简要介绍了宝山电站组合式变电站的选型要点。     

Rapid determination of chlorophylls in vegetable oils by laser-based fluorometry

The presence of chlorophylls in vegetable oils is considered undesirable due to their adverse effects on oxidative deterioration, bleachability and hydrogenation. The usual methods for determination of chlorophylls in vegetable oils are absorption spectrophotometry, fluorometry and liquid chromatography using fluorescence detectors. This paper outlines a fluorometric method using a laser as the excitation source. The chlorophylls in crude palm oil, rapeseed oil, soya bean oil and olive oil were determined directly by laser-based fluorometry using (a) a helium-neon laser (632-8 nm, 5 mW) as the excitation source and an optical-multichannel analyser (OMA) with an intensified diode array as the detector, and (b) a helium-neon laser and a photomultiplier tube as the detector. The method is especially suitable for the detection of chlorophylls in crude palm oil since no pre-extraction step is required. The levels of chlorophylls measured in vegetable oils using the laser technique was also compared against values obtained by conventional spectrophotometric methods. The simplicity and rapid quantification of laser based fluorometry suggest that this technique can be readily applied to the detection or a wide variety of fluorescing species in vegetable oils.     

Mode I interlaminar fracture toughness of commingled carbon fibre/PEEK composites

Carbon fibre/poly (ether-ether-ketone) (PEEK) composites were fabricated from plain weave cloth using the commingled yarn of carbon fibres with PEEK filaments. The undirectional specimen was made from the warp of commingled yarn and the weft of PEEK yarn, while the two-dimensional specimen was made from commingled yarns both of the warp and the weft. During the hot-pressing process, PEEK filaments melt to form the matrix of the composite. The interlaminar fracture toughness of the commingled composite was measured and compared with that of the prepreg composite. The critical strain energy release rates,/'G _Ics, obtained for the commingled composites were higher than the prepreg composite. In particular, the two-dimensional composite exhibited higherG _Ic than the unidirectional commingled composite. Factors increasing the fracture toughness of commingled composites have also been investigated by SEM observation of the fractured surface.     

A wideband monolithic RMS-DC converter using micromachineddiaphragm structures

This paper reports a monolithic rms-dc converter for measuring wideband rms signals. Two identical thermoelements for thermal rms-dc conversion are formed using planar diaphragm structures and micromachined dielectric windows. Each thermoelement consists of two polysilicon heaters and a thin-film temperature sensor located on a window and operated at constant temperature using on-chip control feedback circuitry. The thermoelements achieve a thermal efficiency greater than 7°C/mW with a thermal time constant less than 5 ms. The on-chip control circuitry is realized using a standard 3 μm p-well CMOS process with minor modifications for process compatibility with the dielectric window formation. This converter measures rms signals over a full scale range of 1 Vrms, handles crest factors in excess of 5, exhibits a typical nonlinearity of less than 1%, and achieves a 3 dB bandwidth greater than 20 MHz