多模梯度折射率塑料光纤带宽的分析

结合WKB法对多模梯度折射率塑料光纤(GI-POF)色散和带宽进行了综合分析,研究了在GI-POF中材料色散对带宽影响的程度和规律,给出较佳折射率指数(g)的范围,并说明实测带宽高于理论带宽的原因.     

简单拓扑图及几乎交错链环补中的闭曲面*

研究了在曲面拓扑图的每个球泡中最多有一个鞍点的情况下,拓扑图的简单性与曲面亏格的关系. 并利用正方形素方割的性质讨论了素、非分离几乎交错链环补空间中的闭、不可压缩、分段不可压缩曲 面的性质.     

基于最大熵模型的微博传播网络中的链路预测

微博是基于用户关注关系建立的具有媒体特性的实时信息分享社交平台.微博上的信息扩散具有快速性、爆发性和时效性.理解信息的传播机理,预测信息转发行为,对研究微博上舆论的形成、产品的推广等具有重要意义.本文通过解析微博转发记录来研究影响信息转发的因素或特征,把微博信息转发预测问题抽象为链路预测问题,并提出基于最大熵模型的链路预测算法.实例验证的结果表明:1)基于最大熵模型的算法在运行时间上具有明显的优势;2)在预测结果方面,最大熵模型比同类其他算法表现优异;3)当训练集大小和特征数量变化时,基于最大熵模型的预测结果表现稳定.该方法在预测链路时避免了特征之间相互独立的约束,准确率优于其他同类方法,对解决复杂网络中其他类型的预测问题具有借鉴意义.     

Quantum thermodynamics

Quantum thermodynamics is an emerging research field aiming to extend standard thermodynamics and non-equilibrium statistical physics to ensembles of sizes well below the thermodynamic limit, in non-equilibrium situations and with the full inclusion of quantum effects. Fuelled by experimental advances and the potential of future nanoscale applications, this research effort is pursued by scientists with different backgrounds, including statistical physics, many-body theory, mesoscopic physics and quantum information theory, who bring various tools and methods to the field. A multitude of theoretical questions are being addressed ranging from issues of thermalisation of quantum systems and various definitions of ‘work’ to the efficiency and power of quantum engines. This overview provides a perspective on a selection of these current trends accessible to postgraduate students and researchers alike.     

多因素影响下的ADS-B地面站覆盖及三维显示

针对多因素影响下的ADS-B地面站覆盖范围进行了研究。在以往只考虑地形遮蔽条件的信号覆盖截止算法基础上,加入对电磁信号的传播有显著衰减影响的气象因素,如大气环境衰减,雨衰减和沙尘衰减,得到更贴近实际情况的覆盖效果。最后以我国新疆地区一组实际应用的ADS-B监视系统为例,计算在当地气象条件下的地面站覆盖效果并进行仿真,并在UGIS(三维地理信息系统)中进行三维可视化分析。仿真结果表明,多种影响因素结合得到的覆盖范围比单一的地形条件覆盖范围更加精确。     

基于四节点RSSI的三维空间定位算法

针对提高室内三维空间的定位精度和速度的问题,从减少节点数量、建立特定坐标系的角度提出了一种基于四节点和特定坐标系的三维空间定位算法。该算法引入位置相对固定的四个节点,提高解算速度;充分考虑有效反射区对衰减指数影响,根据伯鲁斯特角将天线辐射场区分为叠加加强区和叠加减弱区,建立三维坐标系进行定位,结合信号强度指示(Received Signal Strength Indicator, RSSI)衰减模型,在叠加减弱区定位,把三维空间问题转化到二维空间,降低定位算法的复杂度。文中分别阐述方法的原理与验证,结果表明,四节点定位算法的解算速度比降低复杂度(Complexity-Reduced Trilateration Approach , COLA)定位算法有较大提高,该算法平均定位误差小于2米。     

Calibrating the X‐ray attenuation of liquid water and correcting sample movement artefacts during in operando synchrotron X‐ray radiographic imaging of polymer electrolyte membrane fuel cells

Synchrotron X‐ray radiography, due to its high temporal and spatial resolutions, provides a valuable means for understanding the in operando water transport behaviour in polymer electrolyte membrane fuel cells. The purpose of this study is to address the specific artefact of imaging sample movement, which poses a significant challenge to synchrotron‐based imaging for fuel cell diagnostics. Specifically, the impact of the micrometer‐scale movement of the sample was determined, and a correction methodology was developed. At a photon energy level of 20 keV, a maximum movement of 7.5 µm resulted in a false water thickness of 0.93 cm (9% higher than the maximum amount of water that the experimental apparatus could physically contain). This artefact was corrected by image translations based on the relationship between the false water thickness value and the distance moved by the sample. The implementation of this correction method led to a significant reduction in false water thickness (to ～0.04 cm). Furthermore, to account for inaccuracies in pixel intensities due to the scattering effect and higher harmonics, a calibration technique was introduced for the liquid water X‐ray attenuation coefficient, which was found to be 0.657 ± 0.023 cm^?1 at 20 keV. The work presented in this paper provides valuable tools for artefact compensation and accuracy improvements for dynamic synchrotron X‐ray imaging of fuel cells.     

基于改进Mallat算法的倍频程分析方法研究

倍频程分析是噪声测量系统中的重要环节,针对传统倍频程分析方法中滤波器相对衰减值测量精度不高和声压级谱测量不准确的缺陷,本文提出了一种基于改进Mallat算法的倍频程分析方法。本文提出的改进Mallat算法:第一,对信号进行FFT运算,删除由于滤波器的非锐截止特性导致的频率混淆分量;第二,利用矩形窗函数与信号相乘,消除由于卷积运算时会导致的边界效应;最后,应用虚拟仪器软件开发平台Lab VIEW搭建了基于改进Mallat算法的倍频程分析系统。仿真对比实验结果验证了改进Mallat倍频程分析方法对噪声信号的相对衰减值测量精度更高,声压级谱显示更加准确。     

Cross‐linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells with dichloromethyl phosphinic acid as a cross‐linker

Phosphoric acid doped polybenzimidazole (PBI) membranes have been covalently cross‐linked with dichloromethyl phosphinic acid (DCMP). FT‐IR measurements showed new bands originating from bonds between the hydrogen bearing nitrogen in the imidazole group of PBI and the CH₂ group in DCMP. The produced cross‐linked membranes show increased mechanical strength, making it possible to achieve higher phosphoric acid doping levels and therefore higher proton conductivity. Oxidative stability is significantly improved and thermal stability is sufficient in a temperature range of up to 250°C, i.e. within the temperature range of operation of PBI‐based fuel cells. Copyright © 2008 John Wiley & Sons, Ltd.