Hysteresis in Transport Critical-Current Measurements of Oxide Superconductors

We have investigated magnetic hysteresis in transport critical-current (I_c) measurements of Ag-matrix (Bi,Pb)₂Sr₂Ca₂Cu₃O_10–x (Bi-2223) and AgMg-matrix Bi₂Sr₂CaCu₂O_8+x (Bi-2212) tapes. The effect of magnetic hysteresis on the measured critical current of high temperature superconductors is a very important consideration for every measurement procedure that involves more than one sweep of magnetic field, changes in field angle, or changes in temperature at a given field. The existence of this hysteresis is well known; however, the implications for a measurement standard or interlaboratory comparisons are often ignored and the measurements are often made in the most expedient way. A key finding is that I_c at a given angle, determined by sweeping the angles in a given magnetic field, can be 17 % different from the I_c determined after the angle was fixed in zero field and the magnet then ramped to the given field. Which value is correct is addressed in the context that the proper sequence of measurement conditions reflects the application conditions. The hysteresis in angle-sweep and temperature-sweep data is related to the hysteresis observed when the field is swept up and down at constant angle and temperature. The necessity of heating a specimen to near its transition temperature to reset it to an initial state between measurements at different angles and temperatures is discussed.     

颗粒被凝固前沿排斥的临界速度模型

通过对颗粒增强金属基复合材料凝固过程中温度场，、质量分数场变化以及凝固前沿形状改变的数值模拟，建立了颗粒被凝固前沿排斥或捕获的临界速度模型，当凝固前沿的推进速度小于颗粒被凝固前沿所排斥／捕获的临界速度时，颗粒被凝固前沿所捕获的现象不会发生，并且在临界速度模型中保持其它参数为旧，而只考虑合金质量分数的影响时，发现随着合金质量分数的逐渐增加，颗粒被凝固前所排斥／捕获的临界速度逐渐减小。     

回转窑内物料流动模型研究

着重研究L/D≥ 10的回转窑内颗粒的运动 ,采用颗粒受力矢量分析确定单颗粒运动轨迹 ,从而推出单历程颗粒的位移和时间。然后引进差补方法计算物料平均轴向位移和平均停留时间 ,并分析了物料充满角对轴向位移速度的影响。同时对几种固体废弃物进行了流动试验 ,实验结果与理论分析得出的公式吻合较好。     

Sandwich shell finite element for dynamic explicit analysis

The contribution of this paper consists of new development of transverse shear stresses through the thickness and finding an expression for the critical time step for explicit time integration of layered shells. This work presents the finite element (FE) formulation and implementation of a higher‐order shear deformable shell element for dynamic explicit analysis of composite and sandwich shells. The formulation is developed using a displacement‐based third‐order shear deformation shell theory. Using the differential equilibrium equations and the interlayer requirements, special treatment is developed for the transverse shear, resulting in a continuous, piecewise quartic distribution of the transverse shear stresses through the shell thickness. Expressions are developed for the critical time step of the explicit time integration for orthotropic homogeneous and layered shells based on the developed third‐order formulation. To assess the performance of the present shell element, it is implemented in the general non‐linear explicit dynamic FE code DYNA3D. Several problems are solved and results are presented and compared to other theoretical and numerical results. Copyright © 2002 John Wiley & Sons, Ltd.     

STN-LCD低温发泡临界温度估算

导出了STN产品低温下出现黑点的临界温度估算公式，讨论了影响临界温度的各种因素，估算了在特定条件下出现低温黑点的临界温度。     

Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number

Critical heat flux (CHF) in nucleate pool boiling of binary mixtures was newly measured with a horizontal platinum wire, 0.5 mm in diameter, and heated by DC, over the full range of concentrations. Seven mixtures were selected with the intent to cover various types of mixtures: methanol/water, ethanol/water, methanol/ethanol, ethanol/n-butanol, methanol/benzene, benzene/n-heptane and water/ethylene glycol, each in the saturated state at atmospheric pressure. Total 311 raw CHF data were obtained at 75 concentrations including pure components.Aqueous mixtures of methanol and ethanol revealed significant increase of CHF compared to either CHF linearly interpolated between pure components or CHF predicted from a single component correlation with use of the mixture properties. Three organic mixtures showed more or less the same level as an interpolated CHF, while the remaining two mixtures of methanol/benzene and water/ethylene glycol gave the reduced CHF by 20% and 50% at most, respectively. Marangoni number was introduced as a controlling variable to explain the observed increased, invariable, or reduced CHF, and an empirical correlation was developed.     

Two calculations concerning the critical aluminium content in Fe20Cr5Al alloys

Alloys for high temperature applications rely on a protective oxide layer formed by selective oxidation. In Fe20Cr5Al type alloys the useful lifetime is governed by the depletion of aluminium by oxidation down to a critical aluminium content. Following spallation of the oxide scale the bare metal is exposed to air and catastrophic failure due to breakaway oxidation occurs if the aluminium content is too low. In this paper a model for the calculation of the critical aluminium content is presented. Based on Wagner's ideas on selective oxidation in ternary alloys, the critical aluminium content is calculated as that corresponding to the transition between external and internal oxidation. The resulting aluminium contents are temperature dependent and cover the range of values 0 to 3.5 wt.% reported in the literature. In the second part of the paper aluminium depletion profiles in Fe20Cr5Al plates have been calculated assuming a parabolic growth law for the oxidation process. The solution of the differential equation is an extension of the one dimensional solution presented by Whittle and coworkers for chromium depletion in FeCr sheet material, to three dimensions. Because of the increased consumption of aluminium at the corners of the sample, the critical aluminium content is first reached there. Based on the calculation, the reduction in lifetime, due to premature breakaway oxidation at the corners can be estimated.     

Comparison of the critical liquid volume fraction to rectilinear-diameter methods for prediction of the critical density of ethylene and oxygen

A comparison of the prediction of the critical density for ethylene and oxygen from available coexistence density values using three methods is presented. The conventional rectilinear diameter, the rectilinear diameter with an additional term to represent the postulated curvature near the critical point, and the critical liquid volume fraction were each utilized to predict critical density values for ethylene and oxygen. The correlating functions and fitted constants for the three techniques are given for different ranges of saturation density values. The accepted saturation lines for ethylene and oxygen calculated from new correlations of thermodynamic properties for these fluids were used in this study. The critical density of ethylene was predicted using coexistence density values from 220 to 230 K and from 220 to 282.34 K. Similarly, the critical density for oxygen was predicted using coexistence densities from 100 to 116 K, from 100 to 154.004 K, and from 100 to 154.571 K. Values of the critical density predicted using the three methods are compared with the accepted critical density values for these fluids. The ability of the three methods to predict accurate critical density values using saturation densities at temperatures removed from the critical point is assessed.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

甲醇-大豆油二组分物系临界参数的计算及其应用

对甲醇-大豆油二组分物系的临界参数采用 Lyderson 基团贡献法和 Prausnitz 混合规则进行计算,计算临界温度的平均误差为2%结合甲醇-大豆油二组分物系临界参数的计算结果,对由甲醇和大豆油制生物柴油的管式反应器中的流体相态进行了分析。管式反应器沿轴向依次分为扩散区、互溶区和分相区;反应温度影响甲醇和大豆油混合状况和相态的轴向分布,进而影响生物柴油收率;反应压力影响物系的密度和油脂在甲醇中的溶解度,从而影响相态的轴向分布和生物柴油收率;利用管式反应器制生物柴油较适宜的操作条件在物系的临界点附近。     

Learning about being critical

Abstract. This short paper is a response to the article by McGrath in this issue which argues that information systems (IS) researchers need to be more explicit about 'being critical'. I accept her point, and I use this paper to offer a sketch of my personal journey in learning about criticality, and some thoughts from where I am now on various aspects of carrying out critical IS research.