真空环境下小孔节流式静压气体轴承流场特性与轴承性能分析

以真空环境下具有排气系统的圆盘形简单孔式节流静压气体轴承为研究对象,忽略过渡流,将流场分成黏性流和分子流区域,利用基本流动方程,推导出黏性流转变为分子流的临界半径,计算转变位置处的临界压力,纠正了前人在相关研究中部分计算式推导和分析结果错误。在此基础上,研究了临界半径和临界压力之间的相互制约关系,气流通道、气腔压力、气腔大小等对临界半径和临界压力的影响,润滑气体分子直径、质量和温度对特征压力影响和对气腔压力的约束,给出了提高轴承性能需着重考虑的因素,为真空下静压气体轴承的性能计算和设计参数优化提供了参考。     

Extraordinary catalytic effect of Laves phase Cr and Mn alloys on hydrogen dissociation and absorption

In activation of hydrogen storage alloys, the nickel-group metals, especially Pd, act as the catalysts to dissociate hydrogen and turn the alloys into successful hydrogen absorbers. The Laves phase Sc and Zr alloys with Cr–Mn as common components exhibit extraordinary hydrogen activation properties matching Pd. Through a cracking mechanism, the bulk samples of these alloys rapidly absorb hydrogen at sub-atmospheric pressures and room-temperature, achieve absorption performance of those Pd surface-modified alloys, meanwhile retaining good reversibility. Among them, the ScCrMn exhibits significantly higher absorption rate than Pd, whereas, the ZrCrMn shows similar absorption kinetics and reversibility to Pd. The shortest initial-activation-time, highest initial-activation-rate and lowest allowed-activation-pressure achieved by ScCrMn are 15 s, −16.6 kPa/s and 0.46 kPa, respectively, in comparison with those of 18 s, 3.2 kPa/s and 0.13 kPa for Pd powder measured under equivalent conditions. The findings and associated magnetization measurements indicate that Cr and Mn upon alloying with certain lower valence metals possess surface electronic structures highly beneficial to hydrogen dissociation.     

Thermal stability,magnetic and mechanical properties of Fe–Dy–B–Nb bulk metallic glasses with high glass-forming ability

The effects of Dy addition on the thermal stability, glass-forming ability (GFA), magnetic and mechanical properties of quaternary (Fe_0.76−xDy_xB_0.24)₉₆Nb₄ (x = 0–0.07) bulk metallic glasses (BMGs) were investigated. Increasing Dy content from x = 0 to 0.05 extended the supercooled liquid region up to 112 K, allowing the fabrication by copper mold casting of BMGs rods with 5.5 mm in diameter. The high GFA was found to be related to the structure of primary crystalline phase. For the x = 0.05 alloy, the competitive formation process of the complex Fe₂₃B₆ and Dy₂Fe₁₄B phases enabled to obtain the largest GFA value. Moreover, the Fe–Dy–B–Nb BMGs exhibited good soft-magnetic properties, i.e., high saturation magnetization of 1.18–0.56 T and low coercive force of 1.9–21.6 A/m. In addition, the glassy alloy rods also showed high compressive fracture strengths of 4400–4150 MPa and high Vickers hardness of 1110–1090 kg/mm².     

Smooth feedrate planning for continuous short line tool path with contour error constraint

In the machining program for free form surfaces, the tool path is usually represented as continuous short lines. For the computer numerical control, the feedrate profile for short line tool path should be smooth and optimized in order to achieve high machining quality and high speed. In high speed machining, the feedrate profile also has a strong influence on contour accuracy. This paper presents a new real-time smooth feedrate planning algorithm for short line tool path, in which the contour error constraint is included. To realize contour error control, the feedrate is adaptively adjusted based on the curvature radius of the tool path, which is directly estimated from the short lines. The 7-phase jerk-limited look-ahead planning is employed to generate a smooth feedrate profile. The target feedrate filter (TFF) and planning units merging techniques are developed to improve the smoothness of the feedrate profile and reduce the overhead on look-ahead. The advantage of the proposed algorithm is that it is not only convenient to achieve the balance among accuracy, smoothness and productivity by adjusting parameters, but also efficient in design, which makes it possible to be implemented on low cost hardware platforms. Experiment results demonstrate the feasibility of the proposed algorithm on smooth feedrate planning and contour error control for continuous short line tool path.     

Mobile Learning projects – a critical analysis of the state of the art

This paper provides a critical analysis of Mobile Learning projects published before the end of 2007. The review uses a Mobile Learning framework to evaluate and categorize 102 Mobile Learning projects, and to briefly introduce exemplary projects for each category. All projects were analysed with the criteria: context, tools, control, communication, subject and objective. Although a significant number of projects have ventured to incorporate the physical context into the learning experience, few projects include a socializing context. Tool support ranges from pure content delivery to content construction by the learners. Although few projects explicitly discuss the Mobile Learning control issues, one can find all approaches from pure teacher control to learner control. Despite the fact that mobile phones initially started as a communication device, communication and collaboration play a surprisingly small role in Mobile Learning projects. Most Mobile Learning projects support novices, although one might argue that the largest potential is supporting advanced learners. All results show the design space and reveal gaps in Mobile Learning research.     

盾构机吊装安全控制技术研究

盾构机吊装一般采用解体后单件吊装,但单件最大重量仍然在100 t左右,加上在深基坑边缘作业,周边空间又十分狭小,是高风险的作业活动.文章根据苏州近年来较丰富的吊装经验,总结了盾构机吊装的技术与安全控制方法,供生产实践中参考.     

A new parametric equation of state and quark stars

It is still a matter of debate to understand the equation of state of cold matter with supra-nuclear density in compact stars because of unknown non-perturbative strong interaction between quarks. Nevertheless, it is speculated from an astrophysical view     

压降法测定钻孔的抽放影响半径试验研究

钻孔的抽放影响半径是进行抽放方法选择、确定钻孔布置参数以及评价抽放效果的重要依据,对其准确测定对矿井瓦斯抽放具有重要意义,压降法测定钻孔的抽放影响半径是一种简单有效的方法。对压降法的测定原理及方法进行了探讨研究,并将该方法应用于岳城煤矿进行试验研究,结果表明:压降法测定钻孔的抽放影响半径是一种行之有效的方法,并确定了岳城煤矿在采用孔径94 mm的钻孔进行煤层瓦斯抽放14 d时,其抽放影响半径为3 m,这对该矿的瓦斯抽放及防治工作具有重要意义。     

Evaluation of the dynamic performance variation of a serial manipulator after eliminating the self-weight influence

Manipulators used in the industrial field usually have a very stiff structure but without an equivalent payload on their end-effectors. This is because the stiff structure is used to prevent excessive deformation which will negatively impact the positioning accuracy of the manipulator, especially when the manipulator is fully extended. However, the stiff structure increases the weight of the manipulator and consumes much of the output of the constituent joint actuators in order to overcome the gravitational force resulting from the heavy structure. To cope with this problem, the concept of gravity balance was proposed decades ago, and there have been several approaches suggested to eliminate the influence of the self-weight of the structure. With the help of gravity balance, the output of the constituent joint actuators can fully be used to drive the manipulator and save considerable energy when the manipulator is in static or low-speed applications.For decades, many papers have discussed how to make a manipulator in gravity balance or how to design and apply a gravity balance mechanism to satisfy a certain application. However, none of them discuss what the influence on the dynamic performance of a manipulator is after it is equipped with a gravity balance mechanism or how to evaluate that influence. To rectify this insufficiency, this article utilizes acceleration radius to be the index of measuring the dynamic performance before and after a manipulator is equipped with a gravity balance mechanism and proposes a new index, the maneuverability ratio, to provide quantitative information to measure whether the dynamic performance of the manipulator increases or not after the gravity balance mechanism is applied.     

周期环境中尺度结构线性种群模型的适定性

该文考虑周期环境中具有尺度结构线性种群动力系统，应用Volterra积分方程及算子谱半径知识证明了该线性系统解的存在唯一性，并且给出解关于出生率和死亡率的单调性结果。