三维问题的局部块分解预条件

利用块三对角矩阵的嵌套局部块分解构造了一个不完全分解预条件子,并考虑了其修正型变种,分析了两者的存在性及若干性质.针对标准七点差分矩阵,给出了预条件后的实际条件数.结果表明,采用局部块分解预条件时条件数与矩阵阶数的2/3次幂成正比,而采用修正型预条件时条件数与矩阵阶数的立方根成正比.最后考虑了预条件的高效实现并在主频为550MHz、内存为256M的微机上作了若干数值实验,并与其它较有效的预条件方法进行了比较.     

铁磁性锰氧化物掺杂的ZnO压敏电阻性能研究

利用通常的电子陶瓷制备工艺制备了铁磁性锰氧化物La_07Sr_03MnO₃掺杂的ZnO陶瓷. 晶界处存在La_07Sr_03MnO₃(LSMO)和LaMnO₃(LMO)两种杂相. 样品中绝缘相LMO的含量显著影响着样品的电学性能. 掺杂后的样品仍具有一定的铁磁性. 在样品上施加磁场后,样品电阻值增加,表现为正磁电阻性质. 正磁电阻的出现,是由于磁场的存在 关键词： ZnO 压敏电阻 锰氧化物 正磁电阻     

一种基于哈特曼传感器的角锥棱镜检测方法

在使用角锥棱镜前对其进行检测,选取误差较小的棱镜是一项关系到整个测量工作结果的重要任务。因此,提出一种用哈特曼波前传感器全面检测角锥棱镜角度误差和面形误差的技术方案,从理论上分析了哈特曼法的测量原理,深入研究了光经过角锥棱镜后的波面重建,给出了波面特征与角锥棱镜误差参量之间的关系。在试验中验证了这种方法的正确性和可行性。     

Abnormal positive thermal expansion in Mo substituted ZrW2O8

ZrW₂O₈ displays the unusual property of an isotropic bulk contraction in volume as a function of temperature. We report here on the positive thermal expansion (PTE) property caused by substituting Mo for W sites in ZrW₂O₈ at room temperature. The room temperature crystal structure of ZrW_2−xMo_xO₈ compounds, which were synthesized using a low temperature route, could be divided into ordered phase with α-ZrW₂O₈ structure (0≤x≤0.5) and disordered phase with β-ZrW₂O₈ (0.5<x≤1.5) and c-ZrMo₂O₈ structure (1.5<x≤2). ZrW_2−xMo_xO₈ adopting β-ZrW₂O₈ structure shows abnormal PTE property at room temperature due to the existence of water molecules, while ZrW_2−xMo_xO₈ adopting the other two structures (α-ZrW₂O₈ and c-ZrMo₂O₈) shows negative thermal expansion (NTE) property from room temperature until decomposition for α-ZrW₂O₈ structure or trigonal phase transition for c-ZrMo₂O₈ structure. The reason lies in the fact that the oxygen migration caused by the Mo substitution leads to the re-arrangement of W(Mo)O₄ tetrahedra lying along the 3-fold axis, only particular W/Mo ratio (0.5<x≤1.5) produces a special crystal structure, which allows the entrance of water molecules.     

基于COMSOL软件下截锥体电阻的计算

本文应用Comsol软件讨论了截锥体内电势和电流的分布,并计算了截锥体的电阻.     

On an optimal robust digital image watermarking based on SVD using differential evolution algorithm

It is shown that the optimal robust digital image watermarking based on singular value decomposition using differential evolution algorithm proposed by Veysel Aslantas [Opt. Commun. 282(5):769-777 (2009)] has a serious fundamental flaw and should not be used for proof of ownership application.     

计算全息图产生的锥面波在激光准直中的应用

本文提出了用计算全息图产生锥面波的方法.将全息图置于激光光路中,它的衍射图样中心在空间形成了一条准直线.文中对锥面波的传播、准直精度及准直距离作了理论分析,给出了实验结果.     

Alkali-metal-induced topological nodal line semimetalin layered XN2 (X= Cr,Mo, W)

Based on first principles calculations and the K·p effective model, we propose that alkali metal deposition on the surface of hexagonal XN₂ (X= Cr, Mo, W) nanosheets induces topologically nontrivial phases in these systems. When spin orbit coupling (SOC) is disregarded, the electron-like conduction band from N-p_z orbitals can be considered to cross the hole-like valence band from X-d²_z orbitals, thereby giving rise to a topological nodal line state in lithium-functionalized XN₂ sheets (Li₂MoN₂ and Li₂WN₂). Such band crossing is protected by the existence of mirror reflection and time reversal symmetry. More interestingly, the bands cross exactly at the Fermi level, and the linear dispersion regions of such band crossings extend to as high as 0.9 eV above the crossing. For Li₂CrN₂, the results reveal the emergence of a Dirac cone at the Fermi level. Our calculations show that lattice compression decreases the thickness of a Li₂CrN₂ nanosheet, leading to phase transition to a nodal line semimetal. The evolution of the band gap of Li₂XN₂ at the Γ point indicates that the nontrivial topological character of Li₂XN₂ nanolayers is stable over a large strain range. When SOC is included, the band crossing point is gapped out giving rise to quantum spin Hall states in Li₂CrN₂ nanosheets, while for Li₂MoN₂, the SOC-induced gap at the crossing points is negligible.     

强耦合尘埃等离子体中正电扰动下的尘埃声激波

研究了强耦合尘埃等离子体的尘埃声波的线性色散关系和尘埃声孤波的非线性传播。考虑一个包含电子、离子、正电扰动尘埃颗粒的完全电离的三成分模型等离子体。假定其电子、离子数密度服从玻尔兹曼分布,而大质量的尘埃成分用一组经典流体方程描述,对系统方程进行线性化,得到了尘埃声波的线性色散关系,发现离子的集中参数对色散关系的影响很大。用约化摄动法对系统方程进行展开,得到了描述小振幅孤波的伯格斯方程。基于伯格斯方程研究了尘埃声孤波的基本特性,发现尘埃颗粒的强耦合效应对尘埃声孤波有很大的修正作用。该研究结果有助于理解尘埃空间等离子体中局域波的一些特性。