Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.     

Critical size and shape of CdO ex-Cd(OH)2 crystallites

The present paper reports the extent to which the size and shape of oxide crystallites resulting from a decomposition reaction depend on the crystallite size of the initial material. If those sizes are sufficiently large i.e. higher than critical sizes, the resulting oxide crystallites assume exactly these critical sizes. They are the maximum size that can be reached by oxide crystallites in the absence of any evolution process such as ageing or sintering and are specific to a particular shape of these crystals, which is independent of initial crystallite size. Alternatively, if the initial material crystals are below these critical sizes, at least in one direction, then the dimensions and shape of the oxide crystallites depend on the size of the initial crystallites. These results were interpreted within the framework of the structural conversion mechanism related to the decomposition reaction and showing great similarities with a shear transformation.     

RDX和HMX机械刺激临界反应阈值试验研究

为了研究RDX和HMX在机械刺激下的临界反应阈值以及丙酮重结晶工艺对它们的影响，采用BAM撞击感度仪和摩擦感度仪测试了不同温度条件下的临界撞击能量和临界摩擦力。结果显示，80、60、40 ℃和25 ℃下，RDX的临界撞击能量分别为5.0、7.5、7.5、7.5 J，HMX的分别为4.0、4.0、5.0、5.0 J；RDX的临界摩擦力分别为120、120、128、144 N，HMX的分别为108、108、108、120 N。丙酮重结晶后，RDX在25 ℃的临界撞击能量和临界摩擦力分别为5.0 J、128 N；HMX在80、60、40 ℃和25 ℃下的临界撞击能量均为7.5 J，临界摩擦力分别为108、108、120、128 N。撞击感度和摩擦感度的结果表明:在25~80 ℃范围内，RDX和HMX的机械感度随着温度的提高呈下降趋势，重结晶工艺对RDX和HMX的机械感度存在着一定的影响。     

Critical and Flash Points for Metastable Systems in a Mean-Field Approach

By using, as a model, an analytical equation of state which describes a system that can exist in a liquid or vapor phase, scaling properties regarding the critical temperature and the flash temperature were found. The flash temperature is defined as the highest temperature at which a self-bound system can exist in hydrostatic equilibrium. The flash temperature can provide an alternate natural dimension-dependent scale of temperature, other than the critical temperature.     

Determination of the Critical Point of Gold

Wire-shaped gold specimens are placed in a new, improved high-pressure vessel, which is part of a fast capacitor-discharge circuit and in which static pressures above 600 MPa can be reached with distilled water as the pressure-transmitting medium. The specimens are self-heated resistively by a current pulse. The current through the specimen, the voltage drop across it, and its temperature are recorded as a function of time with submicrosecond resolution. The radial expansion of the specimen is determined with a CCD camera, Experiments are performed at different pressures. When the critical pressure is exceeded, there is no liquid–gas phase transition; hence, no sudden change in the thermal expansion rate is observed. The results for temperature, pressure, and specific volume at the critical point of gold are as follows: T _c =7400±1100 K, p _c=530±20 MPa, and v _c=0.13±0.03 × 10^–3m³·kg^–1.     

复合材料层合板临界屈曲载荷分散性

基于随机场理论, 将纤维和基体性能以及纤维体积分数作为随机场变量, 利用局部平均法对随机场进行离散。结合MATLAB与ANSYS的PDS模块对复合材料层合板临界屈曲载荷进行Monte-Carlo模拟, 分析各类随机场变量、随机场的相关长度、对称性和边界条件对临界屈曲载荷分散性的影响。结果表明: 不同随机场变量对层合板屈曲载荷分散系数影响的程度不同, 纤维体积分数的影响最大, 其次为纤维性能与基体性能; 屈曲载荷的分散系数存在尺寸效应, 随着板尺寸的增加, 屈曲载荷分散系数逐渐减小; 减小相关长度可有效地减小屈曲载荷的分散系数; 纤维正对称铺设所引起的屈曲载荷分散系数稍大于反对称铺设情况, 而两对边固支板的屈曲载荷分散系数一般大于四边简支板的结果。     

1/f Fluctuations in Critical Modes of Flow of Superheated Liquid

Experimental studies are performed into the variation of the shape of a jet of boiling liquid flowing out of a high-pressure vessel under conditions of high superheating. The mechanisms of bulk heterogeneous and homogeneous nucleation are identified, and the correlation between these mechanisms and critical modes of outflow is determined. The photometry of transmitted laser radiation is used to investigate the dynamics of photocurrent fluctuations. It is found that, in the case of intensive bulk boiling in a jet of superheated liquid, the fluctuation power spectrum has a low-frequency component (flicker noise) diverging as 1/f. This effect is associated with nonequilibrium phase transitions proceeding in the system.     

Critical Currents of High-Temperature Superconductors in Homogeneous Magnetic Fields

In this paper we report on the measurements of the critical current of a tape-formed high-temperature superconductor in a magnetic field corresponding to the field at different positions in a (transformer, reactor or magnet) winding. An applied magnetic field reduced the perpendicular component of the self-field of the sample. The measurements were performed in liquid nitrogen on a silver-sheathed Bi-2223 conductor. The results show a significant increase of the critical current with reduction of the perpendicular component of the self-field.     

Reentrant Orientational Phase Transitions and Critical Points at Quantum Orientational Melting

An effect is studied of the crystal field on a system of rotors under the conditions of quantum orientational melting. It is shown that even very small crystal fields can substantially change the behavior of the system: at a certain positive value of the crystal field, a critical point appears at the classical side of the phase separation line; another critical point appears at the quantum side with further increasing of the field; then these critical points degenerate into a multicritical point and then the phase separation line disappears. For negative crystal fields stable states of the system of rotors can exist with a negative order parameter which can be treated as the orientational analog of the easy-plane-type ordering in magnets.     

Critical properties of metal-insulator transition in low dimensions

Anomalous metallic states near the Mott insulator are analyzed in terms of the critical scaling toward the metal-insulator transition. Continuous transitions from a metal to the Mott insulator are characterized either by the mass divergence as in the bi-component systems or the vanishing carrier number as in the valence bond systems. A set of critical exponents in these two cases is deduced from the scaling theory. The correlation exponent v and the dynamical exponent z satisfy v = 1/2d and z = 2d when the mass diverges in d dimensions, while v = 1/2and z = 2 if the carrier number vanishes. Crossovers between quantum critical regime and thermal critical regime inferred from this theory shed light on understanding experimental indications in high-T_c cuprates and other strongly correlated systems.     

Critical phenomena in solids

In general a solid-solid phase boundary does not terminate in a critical point due to symmetry requirements. However, there exists systems like pure cerium metal and rare earth compounds like samarium sulphide where an isostructural solid-solid phase boundary terminates at a critical point. The critical point for the pressure-induced semiconductor-metal phase transition in SmS has been determined for the first time using the thermoelectric power data collected upto 12 kbar pressure and 850° C. These results are discussed on the basis of some theoretical models developed for the mixed valent compounds.     

Critical parameters of channeling in nanotubes

The critical channeling parameters, which determine the process of ion dechanneling from carbon nanotubes, are considered. Analytical expressions for the threshold total energy and critical transverse energy of channeled particles, their minimum distance to the channel walls, and the critical angles of channeling in chiral and nonchiral nanotubes are obtained.     

Critical thrust force at delamination propagation during drilling of angle-ply laminates

A new formulation for the critical thrust force necessary to propagate the delamination generated during the drilling operation of an antisymmetric angle-ply laminate is proposed by modeling the delamination zone as an elliptical shape. The critical thrust force is analytically derived with the consideration of bending, twisting and mid-plane extension of the delamination zone. And then to maximize the critical thrust force, the optimal design of an angle-ply laminate is performed to find the optimal number of fiber per millimeter, optimum diameter of fiber and optimum lamination angle using ADS (Automated Design Synthesis).     

Critical heat flux for two-phase flow of helium I

This paper describes a study performed to obtain the critical heat flux data on liquid helium I in a vertical tube under forced convection. The pressure of the fluid ranges from 1.1 to 2.0 atm (from 0.11 to 0.20 MN m^?2), and the quality up to 1. The results show that the critical heat flux decreases with increasing pressure and quality.     

Critical flow condition in open channel hydraulics

Summary Transition between sub- and supercritical flows in open channel occurs at the so-called critical point, for which critical flow conditions appear. This particular flow state has been originally introduced for flows with parallel streamlines. However, as streamlines are considerably sloped and inclined in the vicinity of the critical point, these effects have to be accounted for. The present investigation explores in detail these effects. In particular, present investigations include i) first order analysis expressing discharge in terms of upstream head and channel geometry (calibration of discharge measurement structures) and ii) determination of surface profiles for transitional flow states by accounting for the effective pressure and velocity distributions. Analysis is based on a first order model with restriction to typical channel bottom and sidewall geometry. Results are compared with observations, and a fair agreement between the two is noted.With 13 Figures     

Generalized Hydrodynamic Equations: Application to the Investigation of Gas Flows in Channels with a Step

Generalized hydrodynamic equations, which include explicitly the Kolmogorov fluctuations of hydrodynamic quantities, are used for mathematical simulation of vortex gas flows. The calculations are performed in the range of Reynolds number values from 64 to 10000 for two-dimensional unsteady nonisothermal flow of compressible gas in a channel with a step.     

Drop Tower Experiment to Study the Capillary Flow in Symmetrical and Asymmetrical Channels: Experimental Set-up and Preliminary Results

Owing to the development of the space exploration activities, the in-orbit management of fluids such as the transportation of propellant liquid in microgravity becomes the important direction of microgravity fluid research, and one of main problems is the stability behaviors of free surface flow in capillary channel of PMD. In the present study, an experiment set-up of the fluid transport with two different capillary channels has been developed on the Beijing Drop Tower platform. Both symmetrical and asymmetrical flow channels, with the same cross-sectional areas and lengths and different cross-sectional geometries were used and HFE-7500 is chosen as test liquid. 10 times of the drop-down experiments were performed for investigation of the capillary flow characters in different volumetric flow rates, and the three main patterns of capillary flows: subcritical flow, critical flow and supercritical flow were found in experiments, these patterns are distinguished by the movement of the point of lowest surface over time. Meanwhile, the critical flow rates at which free surface becomes instable observed in our experiments are (1) 2.7 ±0.2ml/s for the critical flow rate of asymmetrical channel; and (2) 2.2 ±0.2ml/s for symmetrical channel flow, respectively.     

Critical Success Factors for New Product Development in Biotechnology Companies

The development of new products has become increasingly critical for the competitiveness of companies, due to increased diversity and variety of products, the reduction of product life cycles, and, primarily, globalization of markets. Thus, the identification and prioritization of critical success factors for developing new products is important, particularly for the high degree of risk and uncertainty involved. The main objective of this article is the identification and prioritization of critical success factors in new product development projects in biotechnology companies in the state of Minas Gerais, Brazil. The research method used was a survey questionnaire, which was sent to a sample of 31 biotechnology companies in Minas Gerais state. The principal findings of this research are that interpersonal skills/relationships of the project leader and technical skills are the most critical factors for successful new product development in this industry. The implications for the biotechnology industry in general, as well as for those responsible for managing new product development, are discussed.     

Critical Assessment 22: bainitic forging steels

Recent developments of direct heat-treated steels for high strength forgings have led to some new steel grades with excellent service properties. Quenched and tempered steels are nowadays more and more replaced by precipitation hardened ferritic–pearlitic steels and by bainitic steels. Based on the experience on upper, lower and carbide-free bainite, a critical assessment is performed concerning the most important influencing parameters and processing routes. Steel designs and cooling concepts are discussed regarding their optimal chemical composition, achievement of a homogeneous microstructure and a low sensitivity to processing parameters. Some controversial aspects are illustrated and further need for research work is highlighted.     

Advances in the mechanism of cleavage fracture of low alloy steel at low temperature. Part I: Critical event

The critical event of cleavage is variable for different types of specimens made of the same steel. In notched specimens (Charpy V or 4 PB) over a wide temperature range as low as -196°C, the critical event is the propagation of a ferrite grain-sized crack (30-40μm). In precracked specimens at a moderately low temperature (around -110°C) it is the propagation of a second phase particle-sized crack (< 10μm). At ever lower temperatures (-150°C - -196°C) the cleavage fracture is nucleation-controlled. No matter whether a notched specimen or a precracked specimen is used, as long as a fibrous crack has been initiated and propagated in it, the critical event is the propagation of a ferrite grain-sized crack and the fracture behavior can be handled as in a specimen with an acute notch. The difference of ‘σ ’values measured in a notched specimen and a precracked specimen is caused by a change of the critical event in these two specimens. This revised version was published online in August 2006 with corrections to the Cover Date.