放电等离子烧结制备纳米Ni块体材料

 采用自悬浮定向流法制备纳米Ni粉体,利用放电等离子烧结技术制备出了直径10 mm、厚2 mm,致密度为96.8 %,显微硬度为4.17 GPa的纳米块体材料。用X射线衍射仪、扫描电子显微镜和显微硬度计分析了烧结块体样品的相组成、晶粒尺寸、微观形貌和显微硬度。研究表明：随烧结温度的升高,块体样品的致密度和晶粒尺寸增大,当烧结温度为650 ℃时,致密度最高,晶粒尺寸为44.8 nm;显微硬度随烧结温度的增高先增大后减小,当烧结温度为550 ℃时,显微硬度最大为4.33 GPa;较高烧结温度下,断口微观形貌的纳米级韧窝出现,显示了韧性断裂的特征。     

Coherent structures of the turbulent round jet in different focal lengths

Graphical abstract  

     

Dynamics of a new Lorenz-like chaotic system

The present work is devoted to giving new insights into a new Lorenz-like chaotic system. The local dynamical entities, such as the number of equilibria, the stability of the hyperbolic equilibria and the stability of the non-hyperbolic equilibrium obtained by using the center manifold theorem, the pitchfork bifurcation and the degenerate pitchfork bifurcation, Hopf bifurcations and the local manifold character, are all analyzed when the parameters are varied in the space of parameters. The existence of homoclinic and heteroclinic orbits of the system is also rigorously studied. More exactly, for $b\ge 2a>0$ and $c>0$, we prove that the system has no homoclinic orbit but has two and only two heteroclinic orbits.     

The number of nonseparable maps on the projective plane

In this paper, we study the rooted nonseparable maps on the sphere and the projective plane with the valency of root-face and the number of edges as parameters. Explicit expression of enumerating functions are obtained for such maps on the sphere and the projective plane. A parametric expression of the generating function is obtained for such maps on the projective plane, from which asymptotic evaluations are derived. Moreover, if the number of edges is sufficiently large, then almost all nonseparable maps on the projective plane are not triangulation.     

基于比例积分控制原理预测钻井全过程原始地层温度的新方法研究

应用比例积分控制原理将瞬态传热模型预测结果与出口 温度实测数据逐步进行反馈可准确预测原始静态地层温度. 为此, 本文基于井下各控制组件质量、动量及能量守恒原理, 建立了实际井身结构与钻具组合条件下循环和停止循环期间井筒-地层温度分布全瞬态传热模型, 应用全隐式有限差分法进行求解, 并引入比例积分控制原理对比分析实测温度与预测温度的误差范围进而精确、 快速获取原始地层温度. 结合一口深井基础数据计算表明, 套管下入深度改变了井筒-地层间热交换效率, 进而影响了近井壁地层温度分布状况; 同时, 钻井过程中循环和停止循环作业过程改变了井下各控制组件的初始条件与边界条件, 致使近井壁原始地层温度分布距离产生变化. 建立的数学模型和研究方法可为石油钻井、地热井开采及地球深部原始地层温度信息准确、 经济、快速获取提供理论基础. 关键词： 原始地层温度 循环与停止循环 瞬态传热模型 比例积分控制原理     

超长定向含铁多壁碳纳米管阵列的制备

采用无模板化学气相沉积法,以二茂铁为催化剂,二甲苯为碳源,利用单温炉加热装置制备了定向碳纳米管阵列。运用扫描电子显微镜、透射电子显微镜、拉曼光谱和X射线衍射仪等对定向碳纳米管阵列的形貌、成分和物相进行细致的分析和表征。结果表明:制得的碳纳米管阵列具有良好的定向性和多壁管状结构,并且石墨化程度高;碳纳米管中除碳元素外,管中包含有少量以纳米颗粒和纳米线形式存在的铁及其化合物,主要成分是铁和碳化铁。结合碳纳米管的制备和透射电子显微镜分析表征结果,认为超长碳纳米管阵列的生长模式为底部生长方式,即经历催化剂分解、催化、成核、长大、中毒、凝聚成粒和连接成线的循环过程,正是由于碳源和催化剂的连续供应促成了碳纳米管阵列的快速定向生长。     

Doping dependence of charge order in electron-doped cuprate superconductors

Abstract

In the recent studies of the unconventional physics in cuprate superconductors, one of the central issues is the interplay between charge order and superconductivity. Here the mechanism of the charge-order formation in the electron-doped cuprate superconductors is investigated based on the t-J model. The experimentally observed momentum dependence of the electron quasiparticle scattering rate is qualitatively reproduced, where the scattering rate is highly anisotropic in momentum space, and is intriguingly related to the charge-order gap. Although the scattering strength appears to be weakest at the hot spots, the scattering in the antinodal region is stronger than that in the nodal region, which leads to the original electron Fermi surface is broken up into the Fermi pockets and their coexistence with the Fermi arcs located around the nodal region. In particular, this electron Fermi surface instability drives the charge-order correlation, with the charge-order wave vector that matches well with the wave vector connecting the hot spots, as the charge-order correlation in the hole-doped counterparts. However, in a striking contrast to the hole-doped case, the charge-order wave vector in the electron-doped side increases in magnitude with the electron doping. The theory also shows the existence of a quantitative link between the single-electron fermiology and the collective response of the electron density.     

Spin-dependent negative differential conductance in transport through single-molecule magnets

Transport properties are theoretically studied through an anisotropy single-molecule magnet symmetrically connected to two identical ferromagnetic leads. It is found that even though in parallel configuration of leads’ magnetizations, the total current still greatly depends on the spin polarization of leads at certain particular bias region, and thus for large polarization a prominent negative differential conductance (NDC) emerges. This originates from the joint effect of single-direction transitions and spin polarization, which removes the symmetry between spin-up and spin-down transitions. The present mechanism of NDC is remarkably different from the previously reported mechanisms. To clarify the physics of the NDC, we further monitored the shot noise spectroscopy and found that the appearance of the NDC is accompanied by the rapid decrease of Fano factor.     

Carbene‐Catalyzed Desymmetrization and Direct Construction of Arenes with All‐Carbon Quaternary Chiral Center

Multisubstituted arenes such as indanes with attached all‐carbon quaternary centers are unique scaffolds in synthetic functional molecules and sophisticated natural products. A key challenge in preparing such molecules lies in the enantioselective installation of the quaternary carbon centers. Conventional methods in this direction include asymmetric substitution reactions and substrate‐controlled cyclization reactions. These reactions lead to poor stereoselectivities and/or require long and tedious synthetic steps. Disclosed here is a one‐step organic catalytic strategy for enantioselective access to this class of molecules. The reaction involves an N‐heterocyclic carbene catalyzed process for direct benzene construction, indane formation, remote‐carbon desymmetrization, and excellent chirality control. This approach will enable the concise synthesis of arene‐containing molecules, including those with complex structures and challenging chiral centers.