直流电动机可控硅控制方案评述

近年来,可控硅控制的直流传动已经代替了惯用的 Ward-Leonard 电动机发电机组。固体直流传动有许多突出的优点,如维修简单、体积较小、重量较轻、效率较高、时间响应较快等等。     

Absence of volatile N-nitrosamines in cooked nitrite-free cured muscle foods

Nitrite-free cured pork systems were prepared using the preformed cooked cured-meat pigment (CCMP) and sodium ascorbate. Absence of volatile N-nitrosamines in cooked nitrite-free systems was confirmed using a gas chromatography-thermal energy analyzer (GC-TEA) methodology. Similar results were obtained when cod, cod surimi or mixtures containing pork with 15 or 50% cod or cod surimi were used. However, counterpart samples cured with sodium nitrite (156 ppm) and sodium ascorbate (550 ppm) produced N-nitrosodimethylamine at 1·0 ppb levels or less. Results demonstrate that nitrite-free curing of meat and meat/fish systems containing the preformed CCMP is successful in yielding products devoid of volatile N-nitrosamines.     

Temporal coding of time-varying stimuli

Temporal structure is an inherent property of various sensory inputs and motor outputs of the brain. For example, auditory stimuli are defined by the sound waveform. Temporal structure is also an important feature of certain visual stimuli, for example, the image on the retina of a fly during flight. In many cases, this temporal structure of the stimulus is being represented by a time-dependent neuronal activity that is locked to certain features of the stimulus. Here, we study the information capacity of the temporal code. In particular we are interested in the following questions. First, how does the information content of the code depend on the observation time of the cell's response, and what is the effect of temporal noise correlations on this information capacity? Second, what is the effect on the information content of reading the code with a finite temporal resolution for the neural response? We address these questions in the framework of a statistical model for the neuronal temporal response to a time-varying stimulus in a two-alternative forced-choice paradigm. We show that information content of the temporal response scales linearly with the overall time of the response, even in the presence of temporal noise correlations. More precisely, we find that positive temporal noise correlations have a scaling effect that decreases the information content. Nevertheless, the information content of the response continues to scale linearly with the observation time. We further show that finite temporal resolution is sufficient for obtaining most of the information from the cell's response. This finite timescale is related to the response properties of the cell.     

Chemistry of single-walled carbon nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.     

Role of cerium as a promoter and process optimization studies for dehydration of glycerol to acetol over copper chromite catalyst

Cerium-promoted silica supported copper chromite catalyst was synthesized from acid hydrolysis of sodium silicate by sol-gel method.The catalyst was characterized by Brunauer-Emmett-Teller(BET)method,field-emission scanning electron microscopy(FESEM),X-ray diffraction(XRD),H₂-temperature programmed reduction(H₂-TPR),NH3-temperature programmed desorption(NH3-TPD)and pyridine adsorbed Fourier transform infrared spectroscopy(Py-FTIR).Among cerium doped catalysts,5 wt%of Ce promoted copper chromite supported by 40 wt%of silica(SiCuCr40-Ce5)shows the largest BET surface area.XRD analysis of the reduced form of the catalyst shows both CeO₂/Ce₂O₃ redox system and CuO/Cu₂O/Cu redox system.Py-FTIR shows the maximum number of Lewis acid sites for SiCuCr40-Ce5 than others.The highest acetol selectivity with analytical reagent(AR)grade glycerol conversion is observed for SiCuCr40-Ce5 at 200℃for 3 h in a batch reactor at atmospheric pressure.Cerium promotion lowers the reaction te mperature with enhanced glycerol conve rsion and increased acetol selectivity.Though the above catalyst shows higher conversion for laboratory reagent(LR)grade glycerol but it reduces acetol selectivity.The addition of glucose into the LR grade glycerol further reduces glycerol conversion and decreases the acetol selectivity to zero.This may be due to the presence of iron as impurity in LR grade glycerol.XRD analysis of spent catalyst shows the absence of redox catalytic system and the pore volume reduces identified by BET analysis.Raman analysis of the spent catalyst shows graphite-like carbon deposition in the spent catalvst.     

Method of measuring loaded Q-factor of single-ended cavity resonators using reflection bridge

A bridge method of determining the loaded Q-factor and coupling coefficient of single-ended cavity resonators is described. The measuring system consists of a hybrid-T or 'magic T' bridge with one of the collinear arms terminated by the cavity and the other by a precision attenuator followed by a variable precision short. The experimental procedure essentially measures the reflection coefficient of a cavity resonator as a function of frequency by balancing the bridge, using an oscillator with variable frequency amplitude modulation as the source.     

Theoretical Analysis of Solidly Mounted Film Bulk Acoustic Resonator with Nanocrystalline Diamond as High Impedance Material of Bragg Reflector

纳米金刚石（NCD）作为固贴式薄膜体声谐振器（SMR）布拉格反射栅高声阻抗材料的理论分析

鄂羽佳,张森,代兵,朱嘉琦,韩杰才

（哈尔滨工业大学 复合材料与结构研究所,哈尔滨 150001）

中文说明：

本文设计了一种采用纳米金刚石（NCD）作为布拉格反射栅高声阻抗材料的固贴式薄膜体声波谐振器（SMR）,可以显著提高布拉格反射栅的性能。利用Mathcad软件研究了布拉格反射栅对SMR性能的影响,以及由不同高声阻抗材料、不同层数组成的布拉格反射栅对SMR品质因子Q的影响。结果表明,选用纳米金刚石作为高声阻抗材料的布拉格反射栅能有效地降低能量损失,且可以有效地提高SMR的性能。高声阻抗材料的声阻抗越高,SMR性能越好。建立MASON模型,利用先进设计系统（ADS）模拟了金属钨（W）和NCD两种高声阻抗材料组成的布拉格反射栅对SMR器件所产生的寄生因素的影响。结果表明,金属材料所引起的寄生效应会显著降低SMR的品质因子Q。在6 GHz以下的频率范围内,在SMR正常工作的情况下,选用NCD作为高声阻抗材料的器件性能优于W,因此,NCD是SMR设计中高声阻抗材料的较好选择,在高频和低损耗下,器件的性能得到了改善。同时,布拉格反射栅的最佳层数为6层。

关键词：固贴式薄膜体声波谐振器（SMR）,纳米金刚石（NCD）,布拉格反射栅,品质因子（Q）,寄生效应

     

Atomic layer deposition of Al2O3 on porous polypropylene hollow fibers for enhanced membrane performances

Porous polypropylene hollow fiber (PPHF) membranes are widely used in liquid purification.However,the hydrophobicity of polypropylene (PP) has limited its applications in water treatment.Herein,we demonstrate that,for the first time,atomic layer deposition (ALD) is an effective strategy to conveniently upgrade the filtration performances of PPHF membranes.The chemical and morphological changes of the deposited PPHF membranes are characterized by spectral,compositional,microscopic characterizations and protein adsorption measurements.Al2O3 is distributed along the cross section of the PP hollow fibers,with decreasing concentration from the outer surface to the inner surface.The pore size of the outer surface can be easily turned by altering the ALD cycles.Interestingly,the hollow fibers become much more ductile after deposition as their elongation at break is increased more than six times after deposition with 100 cycles.The deposited membranes show simultaneously enhanced water permeance and retention after deposition with moderate ALD cycle numbers.For instance,after 50 ALD cycles a 17％ increase in water permeance and one-fold increase in BSA rejection are observed.Moreover,the PP membranes exhibit improved fouling-resistance after ALD deposition.     

Coma, hyperammonemia, metabolic acidosis, and mutation: lessons learned in the acute management of late onset urea cycle disorders

Urea cycle disorders are an important and treatable cause of hyperammonemia in the newborn and pediatric age group. Presentation in adolescence or adult life is rare and can manifest as frequent vomiting and behavioral changes. An inherited metabolic disorder should be considered in adults with obvious or occult encephalopathy. Failure to diagnose and treat rapidly may lead to irreversible neuronal damage. An improved understanding of the diagnosis and management of late-onset urea cycle disorders is needed to assist nephrologists in providing optimal care. This report describes the clinical characteristics of a young man with first presentation of hyperammonemia in adult life.