碳化物价电子结构及其界面电子密度分析

运用固体与分子经验电子理论计算了六种过渡金属碳化物(MC)的价电子结构及其部分低指数晶面的电子密度.计算结果表明:MC最强键为最近邻M-C键,相邻(111)面间以最强键结合,滑移困难.通过对NiAl和MC的不同晶面的共价电子密度的分析发现:(110)NiAl和(100)MC在一级近似范围内基本上保持连续;采取适当的制备工艺,使复合材料中存在尽可能多的(110)NiAl∥(100)MC,有可能使这种复合材料具有更优异的力学性能.     

Electron microscopy of carbon nanotubes

High-resolution electron microscopy was used to study multiwall carbon nanotubes obtained by the arc-discharge technique and double-and single-wall nanotubes produced by the arc-discharge catalytic synthesis. The structure of conical layer nanotubes obtained by the CVD technique is characterized in detail. It is established that heat treatment of nanotubes gives rise to their structural changes. The structure of nanotubes obtained by carbon evaporation in the N₂-Ar atmosphere under high pressure is determined. A new type of nano-and microtubes with surface-modulated walls is revealed. Possible applications of carbon nanotubes are reviewed.     

B位取代PZT体系的电子结构与压电特性研究

采用自洽场离散变分Xα计算方法,分别计算了Pb(Zr1/2Ti1/2)O3(简称PZT)、Pb(Zn1/3Nb2/3)O3(简称PZN)、和Pb(Mn1/3Sb2/3)O3(简称PMS)体系的电子结构,研究了钙钛矿结构与烧绿石结构陶瓷的电子结构对压电性能的影响.结果表明,PZT铁电相较顺电相稳定,O的2p轨道与B位原子的最外层d轨道的杂化是铁电性的必要条件,杂化的强弱可表明铁电性的强弱;Mn1/3Sb2/3、Zn1/3Nb2/3取代(Zr, Ti)若生成四方钙态矿结构,体系总能量降低、轨道杂化增强,可以提高PZT体系的铁电性能,若生成立方烧绿石结构,由于B-O(//轴向)与B-O(⊥轴向)共价键强度差别太大,造成体系结构的不稳定,将导致铁电性的丧失.     

Electron spin resonance in glasses

The present paper reviews a quarter of a century of electron spin resonance (ESR) studies in glasses. Specific subtopics addressed include transition-group ions, radiation-induced defect centers, ferromagnetic precipitates, and photo-induced centers. For brevity, most of the examples in the first three categories are confirmed to the familiar oxide glass systems and only the most concrete conclusions are elaborated. The section treating radiation-induced defects is further abridged in light of several reviews of this topic in the current literature. Even within these limitations of scope, a great diversity of material is summarized. Most of the studies cited provide information concerning the atomic scale structure of the host glasses and several have immediate impact on practical problems ranging from glass technology to geochemistry.     

Structural Studies of Electron Deficient Titanacyclobutanes

Abstract  

Two 16 electron titanacyclobutanes of the formula Ti(C₅H₄R)₂(μ₂-CH₂)₂C(CH₃)(i-C₃H₇) (R=H, CH₃) have been prepared from the reaction of Ti(C₅H₅)₂(μ₂-CH₂)(μ₂-Cl)Al(CH₃)₂ or Ti(C₅H₄CH₃)₂(μ₂-CH₂)(μ₂-Cl)Al(CH₃)₂ with H₂C=C(CH₃)(i-C₃H₇). Structural parameters, most notably lengthened C–C bonds in the titanacyclobutane ring, for both complexes reveal the expected presence of (C–C)→Ti agostic interactions. The complexes are isomorphous, crystallizing in the monoclinic space group Cc. For Ti(C₅H₅)₂(μ₂-CH₂)₂C(CH₃)(i-C₃H₇), a = 11.3459(3) ?, b = 16.2108(4) ?, c = 8.1646(2) ?, β = 105.5276(16)°, V = 1446.87(6)°, D_calc = 1.268 at 150(1) K. For Ti(C₅H₄CH₃)₂(μ₂-CH₂)₂C(CH₃)(i-C₃H₇), a = 12.6591(2) ?, b = 16.2795(4) ?, c = 8.2462(2) ?, β = 107.2421(14)°, V = 1623.04(6) ?³, D_calc = 1.245 at 150(1) K.     

电子背散衍射花样的数学特征

电子背散衍射(EBSD)花样揭示了材料的物相成分、晶体结构、晶粒取向、晶粒大小和晶界的信息。EBSD花样非常复杂,通常需要借助专门的计算软件才能解析。本文系统地研究了EBSD花样的数学特征,建立了任意晶体取向与EBSD花样之间的数理关系,推导了面心立方、体心立方和六方晶体的基本晶带轴的理论EBSD花样的数学特征,以及面心立方晶体的(001)<110>取向和(001)<100>取向的理论菊池(Kikuchi)花样特征。在实测EBSD花样的分析中与各晶系各点阵的基本晶带轴的理论EBSD花样特征比较,即通过图像特征对比,就可以直接确定实测EBSD花样所属的晶系、点阵和Kikuchi线交点对应的晶带轴[uvw],再由基本晶带轴的坐标计算出晶体取向,还能提供基本晶面信息,如原子密排晶面在样品中的空间分布,这有利于晶体的变形或生长机理研究。EBSD为单晶芯片质量检验提供了新方法。     

C60分子的电子传导和量子流分布研究

利用基于Green's function的tight-binding方法,对由两条原子线电极连接C60分子远端构成的电子传导系统进行了理论计算和数值模拟,得出了入射电子通过C60分子传输到远端点的电子传输谱.其结果揭示了电子传导过程中C60分子的开关特性,并且得出了电子传输能量与分子轨道共振时传输概率峰值的出现及振荡特征.利用Fisher-Lee关系式和量子流密度理论,在传输概率峰值的能量点E=-1.38eV处获得了C60分子内的量子流分布,给出了键量子流的最大值和最小值.对全部分子键上的量子流数值进行了图形模拟,其结果符合量子流动量守恒定律.     

Fe-25Al铝化物的电子结构及其本质脆性

铁铝金属间化合物是一类具有良好应用前景的新型材料,但室温脆性影响了其进一步应用.本文基于Fe-25Al各种物相的晶体结构和实际原子占位,运用固体与分子经验电子理论(EET)建立起Fe-25Al各相的价电子结构.通过比较不同相结构的价电子结构参数,对Fe3Al的室温脆性进行了阐述.认为较少的晶格电子、键络分布不均匀使Fe3Al具有本质脆性,添加固溶组元可以改变价电子结构参数从而改善强度和韧性.     

Electron microscopy of biomaterials based on hydroxyapatite

Three types of biomaterials based on hydroxyapatite are synthesized and investigated. Hydroxyapatite nanocrystals or microcrystals precipitated from low-temperature aqueous solutions serve as the initial material used for preparing spherical porous granules approximately 300–500 μm in diameter. Sintering of hydroxyapatite crystals at a temperature of 870°C for 2 h or at 1000°C (for 3 h) + 1200°C (for 2 h) brings about the formation of solid ceramics with different internal structures. According to the electron microscopic data, the ceramic material prepared at 870°C is formed by agglomerated hydroxyapatite nanocrystals, whereas the ceramics sintered at 1200°C (with a bending strength of the order of 100 MPa) are composed of crystal blocks as large as 2 μm. It is established that all the biomaterials have a single-phase composition and consist of the hydroxyapatite with a structure retained up to a temperature of 1200°C.     

基于非弹性电子隧穿的表面等离激元激发

表面等离激元是一种存在于金属(或掺杂半导体)-介质界面的电磁极化和振荡现象,可以显著增强纳米尺度光与物质的相互作用,在波导、生化传感、超快调制、探测以及非线性光学等领域具有重要应用前景。表面等离激元的激发主要采用受衍射极限限制的光学激发方式,通常需要棱镜、光栅等大尺寸光学元件的辅助,这极大限制了等离激元器件的小型化和片上高密度集成。通过将等离激元纳米结构和隧道结集成起来,低能量的隧穿电子可以直接激发该结构的等离激元模式,具有超小尺寸、超快调制速度等优点。本文将回顾基于电子隧穿效应的表面等离激元激发的研究历史,并着重介绍该领域的最新研究进展。     

Electron trap centers in Ag-borate glasses

The center created by silver trapping an electron in borate glasses are considered. It is shown by ESR techniques that on trapping an electron, silver ions may form four types of centers: the single Ag⁰ or (Ag⁺ + e) centers, and, at higher Ag contents, the Ag⁰(Ag⁺) or (Ag⁺ + e) (Ag⁺) centers in which a second silver is involved. The formation and stability of these centers is critically dependent on the silver content, the concentration of alkali and the temperature of irradiation and measurement. The Ag⁰(Ag⁺) center is generally enhanced by increasing silver and decreasing alkali content and does not show separate signals for the two isotopes ¹⁰⁷Ag, ¹⁰⁹Ag because the two silvers in proximity occur in three different isotope combinations which are superimposed.Above 20% alkali the hyperfine splitting constant for the silver—electron trap center was found to decrease with increasing alkali content, probably in connection with the increasing appearances of non-bridging oxygens and the trapping of the electron in an oxygen vacancy in the vicinity of Ag⁺, i.e. a change from Ag⁰ to (Ag⁺ + e) center. A similar decrease was observed in high alkali (30% R₂O) glasses with changes of alkali from Li⁺ to K⁺.     

X-ray Diffraction Analysis and Electron Microscopy of the Carbon Fiber Structure

Crystallography Reports - A technique for studying the carbon-fiber structure using X-ray diffraction and electron microscopy has been developed. Average crystal-structure parameters are determined...     

Electron Spin Resonance of Highly Anisotroic Organic Systems

After a brief discussion concerning the chemical nature and structural organization, ESR results obtained on low and highly conducting quasi one-dimensional organic systems, are presented. Related to the knowledge of the molecular properties, invaluable informations about the properties of the linear chains may be obtained on single crystals from the susceptibility, the g tensor and the line shape: existence of magnetically inequivalent sites, respective contributions of the different magnetic species (for low conducting systems) or of the different chains (for highly conducting systems). In particular cases, the existence of weak intrachain coupling has been pointed out and even some order of magnitude for the ratio of intra- to interchain exchange have been reached.     

三端纳米分子桥的电子传输特性研究

利用基于Green Function的Tight-binding方法,对由平面共轭分子连结成的三端子纳米分子桥进行了理论研究和数值模拟,得出了入射电子通过纳米分子桥传输到各个端点的电子传输概率,揭示出传导电子与分子轨道共振时传输峰值的出现和电子传输振荡的物理机制。利用Fisher-Lee关系式和电子流密度理论,在传输概率出现峰值的两个能量点E=±1.89处计算了分子桥内的电子流分布,同时得出了键电子流的最大值,并且得出了数值模拟结果。     

Electron Microscopy in the Study of Lunar Regolith

Crystallography Reports - The results of long-term mineralogical studies of the lunar regolith, obtained using analytical scanning and transmission electron microscopy, are presented. These studies...     

Electron Spin Echo Study on Polyacetylene

Electron spin Echo (ESE) measurements on stretch-oriented films of polyacetylene are reported. The electron dipole-dipole interaction is estimated. Effect of oxygen on the phase memory decay has been studied.     

Electron microscopy of bulk metallic glass machining chips

Machined Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass (BMG) chips were characterized using high resolution electron microscopy. Compared to conventional processing techniques, machining produces very high heating/cooling, and deformation rates. It is therefore of interest to compare structural changes in machining chips with those produced by conventional processing. Large (～1 μm) crystalline grain, residual amorphous region, and phase separation in the amorphous–crystalline transition region were detected in bright field TEM images. Three equilibrium phases, Zr₂Cu, ZrAl₂, and Zr₂Ni, which have been identified in samples undergoing conventional annealing, were revealed from selected area electron diffraction patterns of the chips. High magnification TEM micrographs showed nanocrystallites, about 10 nm in size, in the amorphous–crystalline transition region.     

The Electron Microscopic Approach to the Structure and Reactivities of Organic Solids

In this paper we continue investigations started in Refs. 1 and 2 studying the effect of a magnetic field on electrohydrodynamics of liquid crystalling complex ethers with dielectric constant anisotropy close to zero. The investigations were made with 100 μm thick layers having initial planar “first” structure of B and C mixtures described in Ref. 2.

Figure 1 shows dependences of optical layer transmittance on the intensity of the magnetic field (H) directed normal to bearing surfaces, and in the absence of an electric field in the absence of an electric field (curves I and II). For some threshold values H = H_th all the samples showed an increased transmission arising due to Fredericks transition.³     

Electron diffraction study of the structural features of kaolinites

The structural variations of kaolinites have been investigated based on oblique-texture electron diffraction patterns. The features of manifestation of the 2D lattice of kaolinite layers in the geometry of the 20l and 13l reflections have been revealed. The manifestation of violations in the regular alternation of layers in kaolinites in the diffraction patterns has been analyzed using a numerical simulation of the diffraction profiles along the first (02l, 11l) and second (20l, 13l) ellipses of oblique-texture electron diffraction patterns. The simulation was performed for finite sequences of ten layers using the statistical Markov model in the quasihomogeneous approximation. It is shown that oblique-texture electron diffraction patterns can be used to reveal the coexistence of two phases with different structural perfections and particle morphologies in kaolinite samples.     

Electron and optical microscopy studies of extraterrestrial minerals in NWA4047 meteorite

Analytical electron microscopy and optical microscopy were used to determine the elemental and mineral composition of NWA4047 meteorite. The meteorite was found in 2005 in Morocco, and in 2006 was preliminary classified as H4‐5 ordinary chondrite. The main crystalline meteorite minerals: olivines, pyroxenes, kamacite and taenite, feldspars, as well as troilite identified in our sample represent extraterrestrial minerals typical of the ordinary chondrite of high iron content. Low‐Ca orthopyroxene called bronzite has been revealed. Single enstatite and clinopyroxene crystals have also been identified. Olivines in the meteorite contain 18 mol% of fayalite. The meteorite is a monomict breccia, in which fragments of petrographic types 4, 5, and 6 are present, with the groundmass being type 4. Some of clasts contain large grains of feldspars and high content of feldspars, which is typical of petrologic type 6. Chemical and mineral composition, petrologic types of chondrules, their abundance and sizes, the presence of troilite‐kamacite veins, and nonoxidised iron phases prove that the chondrite belong to H group, i.e. to the olivine‐bronzite group. Our results show that the NWA4047 meteorite is H4‐6 ordinary chondrite. The weathering grade of the meteorite W0/1 is the same as previously established, but the shock grade S3 seems to be higher in comparison with the previous classification. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)