The effect of chemical composition of a crystallization medium on the cBN stoichiometry

Abstract

The effect of chemical composition of the crystallization medium in synthesis on and some properties of cubic boron nitride (cBN) has been studied. Cubic BN crystals with the lowest free boron content and a boron-nitrogen ratio close to the stoichiometric one are grown in a multicomponent crystallization medium in a Li₃N-BN system. These crystals are characterized by a higher strength and thermal stability.     

“绿色”液体激光介质的热流场特性研究

选取掺钕四氟化钆钠(Nd3+:NaGdF4)纳米晶分散到二甲基亚砜作为"绿色"液体介质,构建单侧抽运和液体横流系统,使用ANSYS软件对液体介质稳定热流场分布进行模拟分析。结果表明:当流道形状、流动状态、壁面相同时,液体流场分布基本相同,泵浦光的频率对流场分布影响很小;液体介质入口的流速会影响流线分布,流速越快,流线越密;泵浦区域中,介质沿着液体流动方向温度逐渐升高;在液体流向变化小的区域,可获得较为均匀的温度分布。     

两层耦合可激发介质中螺旋波转变为平面波

采用Br-Eiswirth模型研究了两层耦合可激发介质中螺旋波的动力学,两层介质通过网络连接,即在每一层介质上,每一列选一个可激发单元作为中心点,在一层介质上同一列的可激发单元只与另一层介质上对应的中心点及其8个邻居有耦合.数值模拟结果表明:通过这种局部耦合,在适当小的耦合强度下两耦合螺旋波可实现同步,增大耦合强度会导致螺旋波漫游和漂移,造成螺旋波不同步,观察到螺旋波与静息态、低频平面波和不规则斑图共存现象.在适当强的耦合强度下,还观察到两螺旋波转变成同步的平面波消失现象.对产生这些现象的物理机理做了讨论.     

Polarization of plane wave propagating inside elastic hexagonal system solids

Based on the reported physical parameters for hexagonal system solids,we have calculated the effects of anisotropy on polarization of plane P-wave propagation.Herein we report the results of calculations and the newly observed physical phenomena.It is found that,for a given propagation,if the polarization is parallel to the wave vector,so also to the Poynting vector.In such a case,the phase velocity is identical to the energy velocity;the quasi P-wave degenerates to a pure P-wave along the propagation.It is also noted that if the polarization is parallel to the Poynting vector but not to the wave vector,the propagating wave cannot be a pure P-wave.Furthermore,the polarization in a quasi P-wave may deviate from the wave vector for more than 45°,but the deviation from the Poynting vector is always less than 45°.The energy velocity of a quasi SV-wave can be larger than that of the quasi P-wave in some propagation directions,even though the phase velocity of a quasi SV-wave may never be larger than either the phase velocity or energy velocity of the quasi P-wave.Finally,in case of parameters ε=0 and δ*≠0,the polarization of a quasi P-wave has an observed symmetry at a 45°phase angle.The anisotropy of a hexagonal system solid determines if a pure P-wave can be created and what the propagation direction is for a plane wave propagating inside such a hexagonal system solid.     

Nd:LMA自锁模飞秒激光器腔结构最佳化设计

Nd:LMA晶体相对低的非线性折射率n₂,使得针对Ti:Sapphire自锁模激光器设计的腔,对Nd:LMA介质而言并没有提供足够的自幅度调制(SAM).为了在Nd:LMA激光器上实现自锁模,在腔内引入一个具有大的非线性系数n₂、快的响应时间的SF57介质,这样就使得激光腔的结构比较特殊.在这篇文章里利用ABCD传输矩阵对该激光器的结构从理论上进行了分析和计算,使腔结构达到了最佳化设计.     

Chemical bonding in representative astrophysically relevant neutral,cation, and anion HCnH chains

Most existing studies assign a polyynic and cumulenic character of chemical bonding in carbon-based chains relying on values of the bond lengths. Building on our recent work, in this paper we add further evidence on the limitations of such an analysis and demonstrate the significant insight gained via natural bond analysis. Presently reported results include atomic charges, natural bond order and valence indices obtained from ab initio computations for representative members of the astrophysically relevant neutral and charged HC_2k/2k+1H chain family. They unravel a series of counter-intuitive aspects and/or help naive intuition in properly understanding microscopic processes, e.g., electron removal from or electron attachment to a neutral chain. Demonstrating that the Wiberg indices adequately quantify the chemical bonding structure of the HC_2k/2k+1H chains—while the often heavily advertised Mayer indices do not—represents an important message conveyed by the present study.     

表面等离子共振现象的新应用——微弱磁场的测量

提出了一种新的表面等离子体共振传感器, 它包含三层结构: 棱镜、金属薄膜及二能级介质. 通过理论分析发现, 与通常表面等离子体共振系统不同, 这一物理系统中同时存在两种共振效应 (表面等离子体共振和能级间量子跃迁的共振效应), 它们共同作用的结果导致一系列新的物理现象, 其中一个令人感兴趣的现象是入射光的反射率对外场导致的微小能级移动十分敏感 (这一现象是通常的表面等离子体共振系统所不具有的). 由于能级移动依赖于外场, 所以最终入射光的反射率对外场具有灵敏的响应. 本文以外磁场导致能级移动的情况进行了理论计算, 结果表明, 这种表面等离子体共振系统的入射光的反射率对外加磁场极其敏感. 这一特性可以用来测量物质表面附近的微弱磁场, 有可能发展成为一种新型检测技术.     

Simultaneous effects of magnetic field and space porosity on compressible Maxwell fluid transport induced by a surface acoustic wave in a microchannel

Peristaltic motion induced by a surface acoustic wave of a viscous, compressible and electrically conducting Maxwell fluid in a confined parallel-plane microchannel through a porous medium is investigated in the presence of a constant magnetic field. The slip velocity is considered and the problem is discussed only for the free pumping case. A perturbation technique is employed to analyze the problem in terms of a small amplitude ratio. The phenomenon of a “backward flow” is found to exist in the center and at the boundaries of the channel. In the second order approximation, the net axial velocity is calculated for various values of the fluid parameters. Finally, the effects of the parameters of interest on the mean axial velocity, the reversal flow, and the perturbation function are discussed and shown graphically. We find that in the non-Newtonian regime, there is a possibility of a fluid flow in the direction opposite to the propagation of the traveling wave. This work is the most general model of peristalsis created to date with wide-ranging applications in biological, geophysical and industrial fluid dynamics.     

Development of boundary element method to dynamic problems for porous media

Biot's consolidation theory is extended to a general class of viscoelastic bodies defined by Riemann-Stieltjes integral convolutions. From a new reciprocity theorem, proved for the governing equations including the inertia terms, the basic integral representations of the displacement fields and pore pressure are obtained. It is shown that, in the absence of internal inputs, a formulation of the dynamic problem in terms of the boundary unknown fields only is possible.