用于三电平逆变器的IGBT提高DC／AC转换的效率

当我们谈论效率时，我们主要指的是电力效率。然而，这种用法并不是效率一次本身的完整含义。在电力电子应用巾，工程师在开发新产品过程中的追求的主要目标是以最大限度地发挥不同类型的效率，如电效率和热效率，以及优化谐波失真和产品整体的尺寸。     

Novel rotor position extraction based on carrier frequency component method (CFCM) using two reference frames for IPM drives

In this paper a novel interior permanent-magnet (IPM) motor's rotor position extraction method from the carrier-frequency component signal, derived from pulsewidth-modulated (PWM) inverter switching, using two reference frames is presented. This method has been utilized for IPM motor vector control without a mechanical rotor position detector, extracting rotor position angle from the switching carrier-frequency (10 kHz) component current. It is effective for IPM motors, which have magnetic saliency, sinusoidal distributed stator winding, and are supplied by a PWM voltage-source inverter. The performance of two IPM motors' vector control without mechanical rotor position detector utilizing this method has been investigated. Experimental results demonstrating good dynamic and steady-state performance achieved are presented and discussed.     

The impact of natural and artificial weathering on the visual,colour and structural changes of seven tropical woods

This study investigates the effects of wood weathering on changes in its macroscopic and colour characteristics in connection with changes in its molecular and anatomical structure. Seven hardwoods suitable for outdoor architecture—bangkirai, cumaru, cumaru rosa, ipé, jatobá, kusia, and massaranduba—were exposed to the exterior out of ground contact for 1–36 months according to EN 927-3, and for 1–12 weeks in Xenotest with water spraying according to partly modified EN 927-6. With prolonged weathering, the following changes occurred in the top surfaces of all tropical woods: (1) visual—creation of longitudinal macro-cracks, (2) spectrophotometry and CIE-L ^? a ^? b ^? —darkening in exterior exposure mainly due to pollutants, except for ipé, and vice versa lightening in Xenotest, as well as greening and blueing in both modes of exposure, (3) FTIR—faster decrease of guaiacyl than syringyl lignin, absolute decrease of conjugated and unconjugated carbonyl groups in the newly formed lignin-polysaccharide-extractive substrate in the photo-oxidized and washed-out cell walls, and decrease of cellulose crystallinity, (4) SEM—damaging of cell-walls by micro-cracks, and their degradation by thinning. Connections between changes of the individual characteristics of weathered woods, for example, between the colour (ΔE*, etc.) and the molecular structure (carbonyls, etc.), were also determined.     

Response of rainfed bread and durum wheat to source,level and timing of nitrogen fertilizer on two Ethiopian Vertisols: II. N uptake,recovery and efficiency

In trials conducted at 2 highland Vertisol sites in Ethiopia in 1990 and 1991, 2 locally popular wheat cultivars, 1 spring bread wheat (Triticum aestivum L.) and 1 durum wheat (T. durum Desf.), were supplied with nitrogen (N) fertilizer at 0, 60 and 120 kg N ha^–1 in the form of large granular urea (LGU), standard urea prills or ammonium sulfate. N was applied all at sowing, all at mid-tillering or split-applied between these two stages (1/3:2/3). While durum wheat exhibited the highest N concentration in grain and straw, bread wheat, because of its higher productivity, resulted in a greater grain and total N uptake. In general, split application of N and use of LGU as N source enhanced grain and total N uptake, apparent N recovery and agronomic efficiency of N, particularly under severe water-logging stress. Where significant, the interactions among the experimental factors substantiated the superior responsiveness of the bread wheat cultivar to fertilizer N, and the beneficial effects of split N application and LGU as an N source. Split application of N tended to nullify the positive effects of LGU, presumably by approximating the delayed release of N achieved with LGU. Considering the potential benefits to Ethiopian peasant farmers and consumers, split application of N should be advocated, particularly on water-logged Vertisols; LGU could be an advantageous N source assuming a cost comparable to the conventional N source urea.     

Exploring the Substrate Specificity and Enantioselectivity of a Baeyer–Villiger Monooxygenase from Dietzia sp. D5: Oxidation of Sulfides and Aldehydes

Baeyer–Villiger monooxygenases (BVMOs) are valuable enzymes for specific oxyfunctionalization chemistry. They catalyze the oxidation of ketones to esters, but are also capable of oxidizing other chemical functions, namely aldehydes and heteroatoms such as sulfur, nitrogen, selenium and boron. The oxidation specificity and enantioselectivity of a newly characterized BVMO (BVMO4) from a strain of Dietzia towards sulfide- and aldehyde substrates have been studied. BVMO4 could react with sulfides containing an aromatic group. The presence of a substituent on the aromatic group was tolerated when they were in the meta- and para position and the oxidations yielded predominantly the (R)-sulfoxides. Similarly, BVMO4 displayed a higher activity for aldehydes containing a phenyl group, but long aliphatic aldehydes, namely octanal and decanal, were also accepted as substrate by this enzyme. The major oxidation products of the aldehyde substrates were the respective carboxylic acids in contrast to formate ester that was obtained in most of the previous reports. The Baeyer–Villiger oxidation of the substrate 2-phenylpropionaldehyde was studied in further detail and the corresponding acid product was obtained with good regio- and enantioselectivity. This is a unique feature for BVMO4 and is of great interest for further exploration of an alternative biocatalytic process.     

Undoped, nitrogen-doped and boron-doped multiwalled carbon nanotube/poly(vinyl alcohol) composite as active layer in simple hydrostatic pressure sensors

We report on hydrostatic pressure sensors prepared using 10 % w/w multiwalled carbon nanotube (MWCNT)/poly(vinyl alcohol) composites. Three types of carbon nanotubes were used in the composites: undoped MWCNTs, nitrogen-doped MWCNTs and boron-doped MWCNTs (B-MWCNTs). The sensor response was tested using an alternating current input, measuring the capacitance and conductance outputs. The sensors based on the three composites show a linear capacitance and conductance sensitivity pressure dependence in the range 50–120 kPa, but a higher sensitivity to pressure dependence above this interval. The highest angular coefficient of the sensitivity, which reached 0.092 kPa^?1, was observed for the capacitance sensitivity of the B-MWCNT based composite.     

Dietary oxysterols are incorporated in plasma triglyceride-rich lipoproteins, increase their susceptibility to oxidation and increase aortic cholesterol concentration of rabbits

Early fatty streaks and advanced lesions are characterized by the deposition of cholesterol and cholesterol oxidation products (oxysterols). Oxysterols have been shown to be cytotoxic and pro-atherogenic compared to cholesterol and are found in cholesterol-rich processed foods. The consumption of dietary oxysterols may be significant in the onset and development of vascular disease. In order to study the short term effects of low levels of ingested dietary oxysterols on lipoprotein and aortic cholesterol and oxysterol levels, rabbits were fed either standard chow, chow supplemented with 1.0% oxidized cholesterol (containing 6% oxysterols), or 1.0% purified cholesterol (control). To determine the distribution and uptake of oxysterols after a 2-week dietary period, triglyceride-rich plasma lipoproteins, low density lipoproteins and aorta were analyzed by GC-MS. The concentration of 7beta-hydroxycholesterol was similar in all groups but the oxidized cholesterol-fed animals showed five times the concentration of 5alpha,6alpha-epoxycholesterol and double the level of 7-ketocholesterol in triglyceride-rich lipoproteins compared to the purified cholesterol-fed animals. The presence of 7-ketocholesterol in LDL was exclusive to animals fed the oxidized cholesterol diet. In addition, oxidation of triglyceride-rich lipoproteins was significantly greater in rabbits fed oxidized cholesterol compared to the pure cholesterol-fed animals. The oxidized cholesterol-fed animals also had a 64% increase in total aortic cholesterol, despite lower plasma cholesterol levels compared to the pure cholesterol control animals. Taken together these results suggest that dietary oxysterols may substantially increase the atherogenicity of lipoproteins.     

Accumulation of chylomicron remnants in homozygous subjects with familial hypercholesterolaemia

Despite the obvious differences between the USA and UK health care systems, they share the characteristics of being motivated and managed in relation to cost and process rather than quality (the improved health status of patients). Whilst governments and insurers across the world use the rhetoric of quality, they, as epitomized by the behaviours of UK and USA decision makers, fail to define, measure and implement quality outcome policies. These behaviours are examined and some of their causes are explored briefly. Competition, as designed and used in public (e.g. UK National Health Service) and private (e.g. USA managed care) markets is shown to fail both to identify quality outcome targets and to provide evidence-based and efficient mechanisms to motivate decision makers to be orientated towards continuous quality outcome improvement in health care. A central policy challenge is, consequently, not just the measurement of quality outcomes but also their management into practice.     

Luminescent Mononuclear and Dinuclear Iridium(III) Cyclometalated Complexes Immobilized in a Polymeric Matrix as Solid-State Oxygen Sensors

Oxygen quenching of the luminescence of mononuclear and dinuclear Ir(III) cyclometalated complexes immobilized in the pPEGMA matrixes has been studied. Linear Stern-Volmer plots, even when experiments at different emission wavelengths have been performed, were evidenced. Despite the different luminescence lifetimes of the chromophores in the absence of quencher, similar Stern-Volmer slopes have been calculated. This behavior was tentatively interpreted by taking into account the size and charge of the chromophores. Increased sizes and lower charges seem to enhance the sensitivity of the systems. Such findings could be of interest for the design of new solid-state luminescent oxygen sensors with improved performance.     

Influence of neodymium addition on microstructure,tensile properties and fracture behavior of cast Al-Mg2Si metal matrix composite

The influence of Nd on the microstructures,tensile properties and fracture behavior of cast Al-18 wt.%Mg2Si in situ metal matrix composite was investigated.Experimental results showed that,after introducing a proper amount of Nd,both primary and eutectic Mg2Si in the Al-18 wt.%Mg2Si composite were well modified.The morphology of primary Mg2Si was changed from irregular or dendritic to polyhedral shape,and its average particle size was significantly decreased from 47.5 to 13.0 μm.Moreover,the morphology of the eutectic Mg2Si phase was altered from flake-like to a thin laminar,short fibrous or dot-like structure.Tensile tests revealed that Nd addition improved the tensile strength and ductility of the material.Compared with those of unmodified composite,the ultimate tensile strength and percentage elongation with 0.5% Nd were increased by 32.4% and 200%,respectively.At the same time,Nd addition changed the fracture behavior from brittle to ductile.