风沙稳定输运中起跳沙粒运动状态分布函数

本文用气固二相流模型模拟了风沙稳定输运过程。若起跳沙粒运动状态分布函数采用由实验得到的正态分布函数形式时,计算出的水平质量流量通量随高度的变化规律与实验结果不同。经深入分析,本文提出一新的起跳沙粒运动状态分布函数形式。其中初始起跳角的分布仍采用正态分布函数形式,而对初始起跳速度则采用指数函数和正态分布函数的分段函数形式。计算结果表明本文提出的起跳沙粒运动状态分布函数与实际情况吻合较好。     

单螺线管型高温超导磁体的设计与优化

高温超导带材的各向异性严重制约着超导磁体可运行临界电流的提高,从而影响着磁体的性能。因此,电磁优化设计在高温超导磁体设计中占据着至关重要的位置。主要采用有限元软件对磁体建模仿真,根据磁体磁场的分布,采用多种带材绕制磁体,进行设计优化。结果表明,在相同体积的有效带材基础上,优化之后的临界电流明显提高,储能量提升了约22%;同时,在计算磁体的可运行临界电流时,采用了多次仿真、迭代计算的方式求解磁体的可运行临界电流。这种方法在仿真阶段可直接求解出磁体的临界电流,计算较为方便,而且获取的是实时运行的临界电流,更符合实际情况。     

YBCO高温超导磁体临界电流密度快速确定方法

基于绘图法基本原理,以及YBCO高温超导带材的临界电流特性,采用有限元电磁场分析软件AnsoftMaxwell建立了空心圆柱磁体模型.根据磁场分布、并考虑到带材的临界电流密度随磁场大小和方向的变化关系,给出了影响磁体工作电流特性的磁场的分布.基于以上工作提出了一种利用Ansoft电磁场分析软件快速确定YB-CO高温超导带材临界电流密度的方法.     

银包套Bi2223多芯带材局部临界电流的变化对整个长样带材性能的影响

本文研究了Bi2223高温超导带材在自场和77K温度下，局部临界电流的统计特性对整根长样带材临界电流特性的影响，一般地，超导带材的Ⅰ-Ⅴ特性由标准的幂指数模型参数Ic和n值来描述，并以1μV/cm作为临界电流的判据；通过测量局部带材的临界电流Ⅰ-Ⅴ特性，得到了整根带材的临界电流特性；并通过统计方法，得到了局部带材临界电流分布的高斯几率分布函数，结果表明，带材局部临界电流分布不均匀，局部临界电流的变化对整个带材的临界电流影响很大；并用统计学的方法对长样带材的临界电流性能进行了分析和讨论。     

多超导线圈并联下的电流分布研究

与多超导线圈串联相比,多超导线圈并联通流的电流分布特性比较复杂。本文提出一种基于H方程和均质化方法的有限元模型,研究了4 YBCO单饼线圈并联下的电流分布,并与场路耦合模型对比。仿真结果表明,多超导线圈并联下的电流分布与各线圈的接头电阻、等效电感和等效超导电阻有关。线圈失超之前,各线圈的等效超导电阻几乎可以忽略,电流分布主要受线圈的接头电阻和等效电感控制;线圈开始失超后,电流分布主要受等效超导电阻影响,电流会重新分配,最终各线圈稳流值与各线圈的临界电流有关。     

两种描述钙基脱硫剂孔隙分布特性模型的分析

本文采用两种方法对钙基脱硫剂孔隙分布特性进行了模拟,结果证明孔长度分布满足对数正态分布的假设和满足同一分布的假设相比与实验结果更加吻合,反映出了孔隙分布的变化特性。     

高温超导电机电枢线圈临界电流密度研究

高温超导线材的临界电流密度受磁场的影响,在设计计算时,通常假设在线圈截面上电流密度是均匀分布的。但是,实际上在线圈截面上,电压是一致的。由于线圈中的磁场不均匀分布,各处的临界电流密度是不同的。采用数值模拟的方法,分析了高温超导线圈截面上的临界电流分布。结果显示,临界电流密度的分布是不均匀的,总的临界电流可比按均匀分布时的计算结果提高25%以上。     

高温超导磁体临界电流的磁矢量分析法及其验证

临界电流是高温超导磁体的关键参数,文中提出了一种名为"磁矢量分析法"的高温超导临界电流计算方法。这种方法以高温超导带材性能测试结果为依据,以有限元仿真软件为手段,通过对计算高温超导磁体内部电磁感应强度矢量的分布,采用插值运算来确定高温超导磁体的临界电流。通过在77K温度下进行1:1的磁体实验,磁矢量分析法的可行性得到了验证。     

铁磁材料作用于Bi-2223带材临界电流的研究

主要研究了铁磁材料对Bi-2223带材的临界电流的影响。通过计算铁磁材料在不同布局下对带材磁场分布的影响,进行了相应的临界电流测试实验。分析表明,将铁磁材料垂直放在带材边缘能有效地提高带材的临界电流。     

低阻抗二极管产生的强流电子束能谱分布的数值模拟

 给出了低阻抗二极管产生的电子束能谱分布及外加磁场对二极管阻抗影响的数值模拟研究结果。结果表明，即使在外加电压恒定的条件下，二极管产生的电子束也具有一定的能谱分布，这说明用二极管电压、电流波形计算脉冲电子束能谱分布是不正确的。另外,外加磁场对低阻抗二极管的阻抗特性具有较大影响，其阻抗随外加磁场的增大而减小。分析认为这是由于外加磁场强度的变化改变了二极管中束电子的运动轨迹。当没有外加磁场或外加磁场较小时，低阻抗二极管产生的电子束发生自箍缩，此时二极管电流是自箍缩饱和顺位流；当外加磁场足够强时，电子束的自箍缩被抑制，二极管电流是没有箍缩时的空间电荷限制电流。束电流小于自箍缩临界电流的二极管其阻抗将不随外加磁场的变化而变化。     

The statistical distribution of magnetic critical currents determined by HTSC film morphology

An analysis is made of the geometric and morphological properties of YBCO superconducting films and of their effect on the magnetic and transport phenomena. A study is presented of the statistical characteristics of critical currents derived from the variation of trapped magnetic flux induced by passing a pulsed transport current. It is shown that the critical currents in the materials under study have a specific statistical distribution, whose main properties are due to the morphology of their structure and can be determined by geometric-probability analysis. The superconducting film is considered as a percolation system. An empirical normal-phase cluster-area distribution function was used to derive the distribution function of magnetic critical currents, which describes adequately the experimental data on how transport current affects trapped magnetic flux. The critical current for transition of a film to the resistive state has been calculated. Fiz. Tverd. Tela (St. Petersburg) 41, 1739–1742 (October 1999)     

Resistive state of superconducting structures with fractal clusters of a normal phase

The effect of morphologic factors on magnetic flux trapping and critical currents in a superconducting structure, which presents a type II percolation superconductor with pinning centers, is considered. The role of pinning centers is played by fractal clusters of the normal phase. The properties of these clusters are analyzed in detail: their statistics is studied, the distribution of critical currents of depinning is found, and the depen-dences of the main statistical parameters on the fractal dimension are obtained. The effect of fractal clusters of the normal phase on the electric field caused by the motion of the magnetic flux after the vortices have been broken away from pinning centers is considered. The current-voltage characteristics of superconducting structures in a resistive state are obtained for an arbitrary fractal dimension. It is found that the fractality of the boundaries of normal-phase clusters forces magnetic flux trapping, thereby increasing the critical current.     

拉伸应变对Bi2223/Ag带材I_c不均匀性的影响

研究了Bi2223/Ag带材的Ic和n值沿带长方向的不均匀性在拉伸应变下的变化。结果表明,Ic和n值的大小并不是逐点一一对应的;当应变小于不可逆拉伸应变时,Ic随应变增加缓慢下降,n值随应变增加缓慢下降并有小幅震荡;Ic的均匀度随应变增加基本上没有变化,n值的均匀度随应变增加变好并有小幅振荡,而且变化范围很小。     

聚变装置中真空室上涡流的分析

本文研究了聚变装置真空室上的涡流问题,将真空室上的涡流看作面电流密度并定义一矢势的法向分量来描述它。详细地叙述了求解真空室上感应涡流的计算方法,对HTU托卡马克实验装置真空室上涡流的大小、分布形式及衰减情况进行了分析。     

Income distribution: Boltzmann analysis and its extension

The paper aims at describing income distribution in moderate income regions. Starting with dividing income behaviors into the two parts: random and deterministic, and by introducing “instantaneous model” for theoretical derivations and “cumulative model” for positive tests, this paper applies the equilibrium approach of statistical mechanics in the study of nonconserved individual income course. The random income follows a stationary distribution similar to the Maxwell–Boltzmann distribution in the instantaneous model. Combining this result with marginal analysis, the probability distribution of individual income process that is composed of the random and deterministic income courses approximately obeys a distribution law mixing exponential function with a logarithmic prefactor. Using the census or income survey data of USA, UK, Japan, and New Zealand, the distribution law has been tested. The results show that it agrees very well with most of the empirical data. The discussion suggests that there might be essentially different income processes to happen in moderate and high income regions.     

脆性断裂统计理论

本文试图用统计方法,将金属脆性断裂的微观过程与宏观过程结合起来,把断裂理论建立于微裂纹发展动力学的统计基础上。脆性断裂实质上是在小的范性变形过程中微裂纹成核长大的非平衡统计过程和单个主裂纹的传播过程。本文导出了描述这种非平衡统计过程的微分积分方程,并从位错机理出发研究了微裂纹动力学,从而解出了微裂纹的分布函数,求出了金属试样的断裂几率,进而导出了延伸率、断裂强度、范性功、裂纹扩展力、断裂韧性、临界裂纹长度、范性-脆性转变温度以及它们的统计偏差与其它有关物理量之间的函数关系。 关键词：     

多层PbBi-SiO膜的超导临界电流特性

用真空热蒸镀法制备了超导多层PbBi-SiO膜。测量了这些样品的超导临界参量。观测到同一样品上的临界电流在不同磁场和温度下有不同的特征。 关键词：     

Influence of charging current fluctuations on the grain velocity distribution in weakly-ionized plasmas

The distribution function of dust particles immersed in a weakly-ionized plasma is calculated with regard to electron and ion absorption by grains and fluctuations of grain-charging currents. The dust-particle energy is shown to undergo abrupt changes due to specific velocity dependence of the grain friction coefficient and stochastic character of grain charging. Their effect can crucially influence the state of the grain. This model can give some indication concerning the possibility of structure formation and melting of a dusty crystal structure. The hysteresis in the transition from the one-peak distribution function to twin-peak distribution function can be observed with changing of plasma parameters.     

Inhomogeneous current distribution in wide high-temperature superconducting small-angle grain boundaries

Using space resolved magneto-optical microscopy we have studied the development of the critical state in high temperature superconducting small-angle grain boundaries in magnetisation experiments. It has been found that with respect to the position in the grain boundary plane this critical state develops in a highly inhomogeneous manner. Towards the centre of the grain boundary there exists a distinct suppression of the inter-granular currents flowing across the grain boundary. A strong correlation between the static (critical current) and the dynamical properties (e.g. electric field or flux-line velocity) of the vortex system inside the grain boundary appears to be the principal mechanism for the observed inhomogeneous current distribution in wide bi-crystalline current bridges.Received: 6 August 2004, Published online: 14 December 2004PACS: 74.25.Sv Critical currents - 74.72.Bk Y-based cuprates - 74.78.Bz High-T _c films - 74.81.Bd Granular, melt-textured, amorphous and composite superconductors     

延时断裂统计理论

本文研究了广泛应力范围内(1》(ασ)/(Kt)》1)热激活延时断裂统计理论,文中从微裂纹按原子键机理随机演化导致断裂的思路出发,导出了微裂纹分布函数、断裂几率、可靠性、强度和寿命的统计分布函数及其统计平均值。 关键词：