HREM and STEM of defects in multiply-twinned particles

The structure of defects in multiply-twinned particles has been studied in detail using high-resolution lattice imaging, dark field and microdiffraction techniques. Icosahedral particles with sizes greater than about 15 nm were found to contain defects, in the form of stacking fault loops parallel with the external surface, which were extremely difficult to detect by conventional amplitude contrast techniques. Microdiffraction mappings correlated with these results, showing large rotations of the face-centred cubic segments. Results for decahedral particles indicated the presence of stacking faults running adjacent to, and parallel with, the twin boundaries. Microdiffraction maps confirmed that the particle structure was face-centred cubic, and also verified that the apparent epitaxy of these particles was highly variable. Models for the defects are proposed and discussed, and the relative merits of HREM and STEM for elucidating structural details in small particles is briefly considered. Finally, the potential for direct imaging at surfaces, as demonstrated by some recent images, is discussed.     

In-situ observation of shape and atomic structure of Xe nanocrystals embedded in aluminium

An imaging technique to determine in situ the shape and atomic structure of nanosized Xe crystals embedded in Al is described using high-resolution transmission electron microscopy (HRTEM). The Xe nanocrystals, with sizes less than 5 nm were prepared by the implantation of 30 keV Xe⁺ into Al at room temperature. The fcc Xe nanocrystals are mesotactic with the Al lattice and have a lattice parameter ≈ 50% larger than that of Al. HRTEM images of the Xe were not clear in [110] zone axis illumination because of the small number of Xe atoms relative to Al atoms in any atom column. An off-axial imaging technique that consists of tilting the specimen several degrees from a zone axis and defocusing to suppress the Al lattice fringes is employed for the 110 projection of the Xe/Al system and the structure of the Xe nanocrystals is successfully imaged. The Xe images clearly represent projections of cuboctahedra with faces parallel to eight Al {111} planes truncated by six {100} planes. The results of multislice image simulations using a three-dimensional atomic model agreed well with the results obtained by the off-axial imaging technique. The usefulness of the technique is demonstrated with observations of crystal defects introduced into the Xe under intense 1000 keV electron irradiation.     

High-angle annular dark-field STEM observation of Xe nanocrystals embedded in Al

High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) observation of Xe precipitates embedded in crystalline membranes has been made using electron probes of atomic dimensions and HAADF-STEM images of Xe precipitates qualitatively different from conventional TEM observation results have been obtained. Multislice-based HAADF-STEM simulation has been made and it has been revealed that the intensity of images of Xe atoms at positions displaced from Al matrix columns decreases rapidly as the thickness increases. Even in a thin specimen, the off-site Xe atoms of the precipitate at deep locations, were not observable. Therefore, different images are expected for specimens of different thicknesses or depths of these precipitates. These results indicate that the observation of precipitates in crystalline membranes requires some care.     

A transformation anomaly in a Cu–Zn–Al alloy

A comparison is described of the structures of an M9R Cu–Zn–Al alloy as quenched directly into the martensitic state, and as stabilized, with the structure of the material after up-quenching into the β₁ state after which treatment the martensitic memory transformation occurs thermoelastically. Preliminary results on the increased tendency for the stabilized alloy to form f.c.c. stacking sequence regions are given and discussed.     

High-resolution electron microscopy of human enamel crystals

The structure of enamel crystals obtained from four human premolars has been studied by high-resolution electron microscopy (HREM) in the [0001], [***2110], [***1540], [***0110] and [***1213] crystallographic directions at various microscope defocus and crystal thickness values. The resolution obtained has not previously been reported for human enamel crystals. In all cases, it was possible to match the experimental images to images calculated using the atomic positions of mineral hydroxyapatite. However, a deviation from hexagonal symmetry characterized by marked (***1010) planes of intensity different from the one of the (***3030) and {***1010}-type planes was observed. In this work, we present an improvement of Scherzer resolution of 0.25-0.20 nm over previous work on biological enamel crystals. This improvement of resolution has permitted the incorporation of crystallographic reflections of higher spatial frequencies into the imaging process of the microscope and has led to a more precise structure determination of the crystals studied.     

Analysis of nanometre-sized pyrogenic particles in the scanning transmission electron microscope

The chemical reactions that take place at surfaces as well as internally in nanoscale particles are of great scientific interest. Such reactions control the catalytic properties of small metal particles and thus are technologically very important. In order to allow enhancement of the performance of such catalytic systems, an understanding of the processes taking place at the atomic scale is necessary. Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the dedicated scanning transmission electron microscope can give atomic-scale information and thus provide a unique opportunity to study such nanoscale systems. Here, iron particles from known positions in a ferrocene-seeded flame and their effects on soot formation are studied. Using EELS, spatial variations in oxidation state of the metal atoms are detected across individual nanometre-sized particles collected from the post-flame region. The surfaces of these particles are found to be less oxidized than their centres. Additionally, the particles are imaged with atomic resolution allowing their structure to be identified. Using these results and those for particles collected from in-flame regions, direct evidence is provided for the catalytic role of Fe-containing particles in the removal of carbonaceous soot produced during combustion of hydrocarbon-based fuels.     

真空应用太阳模拟灯及其灯阵的研制

为了对某相机进行热平衡试验,并克服传统太阳模拟器结构复杂、成本较高、光效利用率低的缺点,提出了将太阳模拟灯阵整体放在真空罐内使用的研制方案。首先,对该方案所需解决的各种问题进行了理论分析,计算得出模拟灯阵的辐照度要求为635.2～905.4W/m2,光学系统效率为0.1444。分析了真空罐液氮冷却系统的导热能力,结果显示,该冷却系统温度升高ΔT为2.0741K,小于其过冷度4K,表明该冷却系统可以将太阳模拟灯阵灯的热量导出。对系统热设计进行了分析和讨论,结果表明,积分器和反射镜的温度在200℃左右,氙灯灯极温度在92℃左右,满足其长时间正常工作的条件。最后,测试了模拟灯阵的技术参数,结果显示其照度为490.37～1000.56W/m2连续可调,辐照不均匀度±5%,辐照不稳定度±5%/h,各项指标均满足设计指标要求。     

The high resolution electron microscopy of stacking defects in Cu–Zn–Al shape memory alloy

The characteristic defects in the martensitic phase of the memory alloy Cu–Zn–Al have been investigated using the Cambridge University 600 kV High Resolution Electron Microscope. Conditions are found under which the images can be quantitatively simulated when this involves the transfer of structural information at resolutions beyond the first zero of the contrast transfer function. Defects of sequence-type were positively identified by the comparison of high-resolution micrographs with full dynamical image simulations.     

High-resolution rlectron microscopic investigation of supported platinum particles reduced at high temperatures

High-resolution electron microscopy has been used to characterize the platinum particles supported on TiO₂ or ZnO. After reduction at elevated temperatures, the metallic particles display a regular, faceted shape, and several superstructures, Pt₃ Ti(C), Pt₃ Ti(H), PtTi, and PtZn, have been found. These results, which may involve strong metal-support interaction, have been confirmed by optical diffraction and image simulation.     

多孔铝及多孔Al/Al2O3复合材料的压缩行为

用粉末烧结法制备出了开孔型多孔铝及以Al2O3为增强相的铝基多孔复合材料,材料相对密度及孔径分别在0.25～0.40和100～400 μm范围内变化.对这两种多孔材料的压缩行为进行了研究.结果表明:Al/Al2O3复合材料有着比多孔铝更为有利的响应特征和更高的流动应力,该复合材料的压缩应力-应变曲线较为平坦,在与多孔铝相对密度相近时,屈服强度提高40%以上;经T6热处理,Al/Al2O3复合材料的屈服强度可进一步提高35%左右;此外,该复合材料的压缩行为具有明显的孔径依赖性,随孔径增大,流动应力升高,这主要与烧结过程中孔表面残留的气体有关.     

Enhancing AW/EP property of lubricant oil by adding nano Al/Sn particles

This paper presents results of experiments to enhance antiwear/extreme pressure (AW/EP) properties of a lubricant oil by adding metal nano particles. In this experiment, Al, Sn and Al + Sn nano-particles were selected as trial additives. The AW and EP properties were evaluated on Four-Ball test machine, while the feature and composition of the wear scar surface were investigated by scan electron microscope (SEM) and energy dispersion spectrum (EDS). The test results show that the AW and EP performance can be improved within a wide load range by adding Al + Sn nanoparticles. Analysis of the enhancement mechanism has also been conducted in this experiment and presented in this paper. It is found that nano-Sn particles can be deposited on the friction surface when the pressure was moderate and act as AW additive. It is also found that the nano-Al particles can be deposited under the condition of high load pressure and act as EP additive. Thus, the AW and EP properties of tested lubricant oil have been improved at the same time due to adding both Al and Sn.     

高温加热对于纯铝薄膜表面结构的影响研究

在半导体材料中,金属薄膜尺寸缩小会使通电时的电流密度升高而导致材料焦耳热迅速增加,从而影响金属薄膜表面结构。针对纯铝薄膜进行了高温加热试验,模拟实际运行中的温度影响,并分析了影响机理。试验结果表明随着加热温度和加热时间的增加会使铝膜表面原子迁移能力增强,单位时间内原子积聚数量增加,从而导致小丘数量和体积增大。     

Structure and composition of interfaces in an aluminium metal-matrix composite

The microstructure of a 2024 type aluminium alloy, reinforced with 13 vol.% SiC particles, and made by a spray technique, has been investigated by transmission electron microscopy and X-ray microanalysis in the as-extruded and T6 conditions. High resolution electron microscopy studies and energy-dispersive X-ray spectrometry microanalysis were performed on the interfaces between the SiC particles and the metal matrix. The interfaces were found to be abrupt: some exhibit an amorphous layer, rich in oxygen; in many others, silver-rich crystallites have grown coherently on the SiC particles.     

High resolution electron microscope observations of microstructures in A15 type Nb3X superconductors

Microstructures of superconducting Nb₃X(A15) compounds are studied by means of high resolution electron microscopy. Structure images are obtained with the 400kV high resolution electron microscope for both the annealed and ion-irradiated crystals. The images obtained from the annealed crystals show bright contrast patterns similar to the projections of Nb and X atoms comprising the A15 structure. Defects observed in the annealed crystal irradiated with 40 kV Nb⁺ ions show characteristic features in contrast. The dependence of image contrast on the element X and the atomic structures of the defects are discussed by comparison with the calculated images.     

Three-dimensional reconstruction of single particles embedded in ice.

Single particles embedded in ice pose new challenges for image processing because of the intrinsically low signal-to-noise ratio of such particles in electron micrographs. We have developed new techniques that address some of these problems and have applied these techniques to electron micrographs of the Escherichia coli ribosome. Data collection and reconstruction follow the protocol of the random-conical technique of Radermacher et al. [J. Microscopy 146 (1987) 113]. A reference-free alignment algorithm has been developed to overcome the propensity of reference-based algorithms to reinforce the reference motif in very noisy situations. In addition, an iterative 3D reconstruction method based on a chi-square minimization constraint has been developed and tested. This algorithm tends to reduce the effects of the missing angular range on the reconstruction, thereby facilitating the merging of random-conical data sets obtained from differently oriented particles.     

Structure analysis of embedded nano-sized particles by precession electron diffraction. eta'-precipitate in an Al-Zn-Mg alloy as example

The Vincent-Midgley precession technique has been used to collect three-dimensional electron diffraction intensity data from a dispersion of coherent precipitates in a matrix. In order to suppress severe effects from multiple diffraction via matrix reflections, a fairly large precession (tilt) angle had to be used. This implied a high background from the surrounding matrix, and limited the number of reflections that could be measured from patterns on image plates. The heavily faulted hexagonal eta'-precipitates (a = 0.496 nm, c = 1.405 nm) with thickness 3-5 nm occur in four equivalent orientations relative to the aluminium matrix; with frequent overlap of reflections. A model of the average structure in the space group P6(3)/mmc with assumed composition Mg(2)Zn(5-x)Al(2+x), have been derived by Patterson analysis and intensity comparisons.     

SiC和SiC/Al在TMA空间遥感器中的应用

简述了空间遥感器的空间应用环境和TMA轻型遥感器的特点,介绍了遥感器设计中材料选择的重要性及影响,在TMA轻型遥感器研制中选用了具有高导热性和高比刚度的SiC和SiC/Al复合材料新材料,计算及试验证明这些材料的应用提高了遥感器的机械性能和热性能,提高了TMA轻型空间遥感器的空间适应能力。     

高能球磨制备纳米级Al_2O_3/Al复合粉体过程中铝相晶粒尺寸和结构的变化

利用X射线衍射仪分析了在高能球磨制备纳米Al_2O_3/Al混合粉体过程中,球磨时间和纳米Al_2O_3含量对铝相晶粒尺寸和晶格应变的影响。结果表明:在初期短时间的球磨中,微米铝粉的晶粒尺寸迅速细化到纳米级,但随着球磨时间的进一步延长,高能球磨为晶粒融合和再生长提供了能量,使铝晶粒沿着某些晶向有长大的趋势;当Al_2O_3体积分数较低(5%)时可促进铝粉的破碎,但高含量的Al_O_3则对铝粉的球磨破碎不利。     

碳纳米管对Al/Al2O3复合相变蓄热材料蓄热与导热性能的影响

论文详细研究了碳纳米管对Al/A12O3复合相变蓄热材料蓄热、导热性能和循环稳定性的影响.利用XRD、TEM和DSC技术分别对碳纳米管的形貌和和蓄热材料的蓄热性能进行了表征.DSC结果表明碳纳米管的添加可略微提高材料的蓄热性能,蓄热值达到543kJ/kg.而添加量太大时,在空气中碳物种易发生氧化反应,在DSC中表现为明显的放热峰.另外,碳纳米管的添加大大提高了材料的导热性能,当碳纳米管的含量为5％时,材料的导热系数提高了30％.对蓄热材料进行升温-降温循环实验可有效提高蓄热材料的抗氧化能力,提高材料的循环稳定性.