Fluorine incorporation in silica glass by MCVD process – a critical study

The chemistry of fluorine incorporation in silica glass preparatory to preform fabrication by MCVD process was critically analysed on the basis of the thermodynamic properties of the constituent reagents. The theoretical findings were compared with the experimental results, pointing out the anomalies observed by different workers. The addition of phosphorus is found to have an insignificant role in modifying the fluorine incorporation level. The accepted model of dependence of fluorine content in the glass on the partial pressure of SiF4 to the power of 0.25 finds poor agreement with the experimental data at low fluorine dopant concentration.     

COMPARISON OF TWO ANALYTICAL METHODS FOR THE DETERMINATION OF DIMETHYLNITROSAMINE IN BEER AND ALE, AND SOME RECENT RESULTS

The results obtained by a rapid Celite column extraction method for the determination of dimethylnitrosamine (DMN) in beer and ale were compared with those obtained by a more elaborate vacuum distillation method. The end determination in both cases was done by gas-liquid chromatography using a thermal energy analyzer (TEA) detector. The two sets of results for 28 samples of beers from various countries were highly comparable. Recent survey results for both Canadian and imported beer and ales suggest a significant decrease in the levels of DMN in these beverages from that observed 2–3 years earlier. This decrease is probably due to the improvements in the malt drying techniques instituted in Canada as well as in other countries.     

固体氧化物燃料电池阳极基底造孔剂的研究

采用甘氨酸-硝酸盐法制备La0.7Sr0.3Cr0.5Mn0.5O3-d(LSCM)钙钛矿型阳极材料,采用传统干压成型法制备LSCM阳极基底。在阳极基底中分别加入不同种类和不同含量的造孔剂,采用扫描电子显微镜(SEM)观察阳极基底微观结构,并用质量体积法测量阳极的孔隙率。研究结果表明,当选用质量分数为8%的淀粉作为造孔剂时,阳极基底的微观性能最佳并得到最大的孔隙率。     

Voltage-current characteristics and power relation in arc plasma in a transverse magnetic field †

The variation of current and the corresponding voltage across a mercury are plasma in a varying transverse magnetic field (0 to 3000 g) has been measured for various initial discharge currents. The current gradually decreases and the voltage across the arc increases with the increase of the magnetic field, the power consumed gradually increases and, attaining a maximum value at a certain value of the magnetic field which is different for different initial currents, gradually decreases with the further increase of the magnetic field. Assuming Beckman's expression (1948) for the axial field and the electron density distribution in a transverse magnetic field and Langmuir's expression (1925) for the current in the are plasma, the observed variation of current and voltage can be satisfactorily explained. A theoretical expression has been derived for the magnetic field at which the power consumed becomes a maximum, by assuming the above deductions and also taking into consideration the variation of electron temperature with magnetic field. The value of the magnetic field thus calculated agrees fairly well with the experimental results, especially for low values of initial currents, and the discrepancy for higher currents has been attributed to the limitations of the above deductions for higher values of H/P     

A NOVEL THREE-PHASE SINUSOIDAL PULSE-WIDTH MODULATION WAVEFORM GENERATOR

Abstract

Pulse Width Modulation is widely used in static power inverters (d.c. to a.c. converters) to control the output voltage and the harmonic content in it. Sinusoidal pulse width modulation is known to produce the least harmonic content in the inverter output. This paper describes a novel control circuitry that generates the three-phase sinusoidal pulse width modulation signals using a single phase sinusoidal reference signal and a triangular carrier signal. The advantages of this circuit are discussed and experimental results are presented     

A MICROPROCESSOR BASED CONTROL OF DC MOTOR DRIVES USING PROPORTIONAL-INTEGRAL (P-I) AND INTEGRAL-PROPORTIONAL (I-P) CONTROLLERS

ABSTRACT

This paper presents a Microprocessor based Digital control system for a DC Motor Drive using the commonly used P-I (Proportional-Integral) controller and a relatively new I-P (Integral-Proportional) controller. The relative merits and demerits of both P-I and I-P controllers are evaluated and compared. Important aspects, such as, the starting speed and current response; responses to step changes in the speed reference and the load torque; error signal processing; gain sensitivity, etc. are analyzed. It is shown that the I-P control scheme offers some distinctive advantages over the P-I control scheme. Some experimental and simulation results are also presented.     

新戊二醇改性PET的合成与表征

以对苯二甲酸为主要原料，通过加入2%～15%（摩尔）新戊二醇，合成了一系列不同二醇配比的共聚酯，用示差扫描量热方法研究了该体系的热性能。结果表明，共聚酯的熔点和结晶度均随新戊二醇的加入而降低。     

On the derivation of bilinear models for optimization of the sampling periods in discrete systems

This paper deals with the development of some bilinear models for non-periodic sampling in continuous plants by modelling the sampling interval as a component of the input vector. Using such models, simple optimization techniques, which are valid for on-line computation of the sampling intervals, are derived.     

基于两维图论聚类分析的烤烟外观质量特征区域归类

采用2003～2008年全回烤烟中部烟叶外观质量评价数据,运用两维图论聚类分析方法进行了烤烟外观质量特征区域的归类.结果表明:①烤烟外观质量特征区域差异主要体现为色度、油分和身份的差异;②依据烤烟外观质量特征的相似性、地理位置的相连性以及自然生态环境的相近性原则将我国烤烟外观质量特征归类区域分为5个区20类.将第1～4类归为I区,包括黑龙江、吉林、辽宁和甘肃;第5～8类归为Ⅱ区,包括山东、河南、陕西北部、安徽北部;第9～10类归为Ⅲ区,包括陕西南部、湖北、湖南西部、重庆、贵州西北部;第11～16类归为Ⅳ区,包括安徽南部、湖南东南部、广东、江西、广西东北部、福建;第17～20类归为V区,包括贵州中部和南部、云南、四川南部、广西西南部.     

NMR characterization of silicon nitride: slurry homogeneity by T2-weighted proton imaging

The Si3N4/water slurries were studied by proton nuclear magnetic resonance (1HNMR) imaging for homogeneity. 1H nuclear spin echo signals from Si3N4/water slurries were observed by a (π/2)-τ-π-τ-echo pulse sequence. Bloch’s equations were used to calculate the spin-spin relaxation times (T2) from these echo intensities. The T2 for the protons in these slurries was measured to be 53.7±0.1 ms. The T2-weighted imaging technique utilizing (π/2)-τ-π-τ multiple pulse sequence was mixed with a “shape pulse” for radio frequency (RF)-excitation to detect nuclear spin echo signals for image construction. Sinc shape pulses were used to mix with both the (π/2) and the π pulses as a frequency carrier because of the mobility of water molecules in the slurry. The nuclear spin echo intensities were transformed into three-dimensional pictures by magnetic field gradients generated by coils along x, y and z-co-ordinates. Axial-section slices were taken to map the water distribution of the slurry in an NMR tube. A stable and well-dispersed Si3N4/H2O slurry, with ammonium polymethacrylate as dispersant, was observed for several hours. Agglomerization of this slurry was detected after 15 h of standing and NMR imaging shown in contour plots depicted clearly the location and the degree of agglomerization. The water distribution can also be presented in three dimensions by stack plotting of the water intensity profiles. This revised version was published online in November 2006 with corrections to the Cover Date.