TEM observations of SiC-SiC composites with a carbon interphase layer annealed in air at high temperatures

SiC–SiC composites with a carbon interphase layer have been annealed in air at high temperature (800–1400°C) in order to determine the oxidation mechanism. Transmission electron microscopy and high-resolution electron microscopy observations have been performed on thin slices of oxidized specimens. Observations show that, with oxidation above 800°C, the carbon interphase can be replaced totally by a thick silica layer, in agreement with previous work. However, in many cases the modified interphase is heterogeneous. The turbostratic carbon structure disappears and the carbon interphase layer is partially replaced by an amorphous silica layer found preferentially at the contact zone between the fibres and the carbon layer. A very thin silica layer can also be observed on the matrix side at the matrix-fibre interfaces. Some decohesion appears in areas where carbon is still present. For short-duration exposures, decohesion occurs between the fibres and the carbon interphase layer.     

Structure of nanometre-sized Xe particles embedded in Al crystals

The structure and lattice parameters of Xe particles about 1 nm to about 6 nm in size embedded in Al were investigated with off‐Bragg condition high‐resolution transmission electron microscopy. An Xe particle about 1 nm in size had different structural properties from those 2–6 nm in sizes. Some 1‐nm Xe particles had an face‐centred cubic (f.c.c.) structure with the same orientation as the Al matrix, whereas others of the same size had a non‐f.c.c. structure. The lattice parameters of a 1‐nm f.c.c. Xe particle were about 20% smaller than the average value obtained from electron diffraction, i.e. the particle was compressed by about 80%. The lattice parameters of Xe crystals about 2 nm to about 6 nm in size were almost the same as those obtained from diffraction results. One of the reasons for the extra compression seen with a 1‐nm Xe particle is the increase in pressure inside an Xe particle with decreasing particle size.     

Macroscopic elastic constants of pure metal based on the interactions between microscopic particles

The research activities of the calculation of the elastic constants of metal are mainly focused on the elastic constants of crystal at the micro level. To the calculation of the macroscopic elastic constants of metal, although molecular dynamics method and quasicontinuum method can be used, but there are shortcomings in them, such as a large amount of computation and that the spatial scale of the study model is limited. Therefore, with a pure metal thin plate composed of a single layer of microscopic particles as research object, a new mechanical model is established after the interactions between microscopic particles of the thin plate are applied on the continuum mechanics model of the thin plate. According to this model, the calculation formulas for the microscopic elastic constants, which are the elastic constants of any triangle region in the model, are obtained. After the concept of the ideal micro structure is presented, the calculation formulas for the macroscopic elastic constants, the elastic modulus and the Poisson’s ratio of pure metal are obtained, where the Poisson’s ratio is the constant that is equal to 1?3. As an example, the elastic constants and the elastic modulus of pure copper are solved, where c11 is 175.811 GPa, c12 is 58.604 GPa, c33 is 58.604 GPa and E is 156.277 GPa, the rationality and the correctness of the model are verified. The model presented fully embodies the discreteness of the microstructure of solid, is a development to the continuum model, and is more suitable to reality, more simplified and more new to the study of the macroscopic elastic constants of pure metal.     

An investigation of order/disorder in a chromia-doped rutile by high-resolution electron microscopy

Computer image simulations predict that the detailed atomic arrangements along extended CS defects in slightly-reduced rutile may be identified in 500 kV high-resolution electron micrographs. Experimental images from a chromia-doped rutile have generally confirmed these predictions although also indicating that considerable disorder remains. The need for very careful control of sample preparation, including heat treatments, is emphasized.     

负载型金属催化剂原子级表征及研究进展

负载型金属催化剂具有选择性高、活性高、稳定性高、腐蚀性小、可重复利用的特点,这些性能与催化剂结构存在很大关系。基于对负载型金属催化剂结构和作用机理的深入研究,本文介绍了几种负载型金属催化剂的原子级表征技术。主要归纳了几种表征技术的适用范围、不同表征数据的分析。通过这些表征技术以及多种技术的融合使用,为最终实现催化剂的实际工业价值提供可能性。     

Video microscopic image processing facilitates the evaluation of light microscopic autoradiography at high magnification

Light microscopic autoradiographs of H-thymidine labelled unstained semithin sections of Xenopus laevis embryonic nuclei were examined with conventional Nomarski differential interference contrast, phase-contrast and video microscopy. Whereas at low magnification it was possible to obtain a photograph of the nuclear structure and the silver grains in one focal plain, at high magnification, with small depths of focus, a satisfactory image was not attainable. Therefore, we stored the images of the two different focus levels with a digital image processing system and combined both images by an arithmetic operation. This video microscopic technique allows the use of high magnification light microscopy with oil immersion objectives and the application of additional electronic contrast enhancing methods for an adequate and rapid analysis of light microscopic autoradiographs.     

HREM and STEM of intergranular films at zinc oxide varistor grain boundaries

Grain boundaries in model ZnO–Bi₂O₃ and ZnO–Bi₂O₃–CoO varistors and a commercial multicomponent varistor have been characterized by high-resolution electron microscopy (HREM) and scanning transmission electron microscopy (STEM), in order to determine the relationship between Bi grain boundary segregation and formation of thin intergranular films. By controlling Bi₂O₃ content, applied pressure and temperature, the grain boundary Bi excess has been systematically varied from nearly zero to Γ_Bi = 1 × 10¹⁵ cm⁻² (≈ 1 monolayer), as measured by HB 603 STEM using an area-scan method. HREM shows that intergranular amorphous films are clearly distinguishable in samples with Γ_Bi > 8 × 10¹⁴ cm⁻². These films range in thickness, depending on the Bi excess, from 0.6 to 1.5 nm. Similar films of ≈ 1 nm thickness are widely observed in the commercial varistor. The composition of the films is a ZnO–Bi₂O₃ solid solution, which is in all cases more enriched in ZnO than the bulk eutectic liquid. The Bi-doped grain boundaries in ZnO varistors therefore contain an intergranular amorphous film which has not only an equilibrium thickness, but also a distinct equilibrium composition.     

Adaptive striping watershed segmentation method for processing microscopic images of overlapping irregular‐shaped and multicentre particles

Oversegmentation is a major drawback of the morphological watershed algorithm. Here, we study and reveal that the oversegmentation is not only because of the irregular shapes of the particle images, which people are familiar with, but also because of some particles, such as ellipses, with more than one centre. A new parameter, the striping level, is introduced and the criterion for striping parameter is built to help find the right markers prior to segmentation. An adaptive striping watershed algorithm is established by applying a procedure, called the marker searching algorithm, to find the markers, which can effectively suppress the oversegmentation. The effectiveness of the proposed method is validated by analysing some typical particle images including the images of gold nanorod ensembles.     

The frictional properties of carbides and borides at high temperatures

The frictional properties of the high-melting-point carbides and borides, and graphite have been measured over a temperature range from room temperature to 2000°C. The coefficient of friction of carbides on carbides and borides on borides first decreased with increasing temperature and then, at a definite and reproducible temperature between 800°–1400°C, increased rapidly. It was considered that the intercrystallite bonding was important and accounts for the initial decrease. At 800°C-1,400°C a change occurred which resulted in increased adhesion between the sliders and crystallites and consequently an increase in friction. For borides and carbides sliding on graphite no reaction was observed and the friction continued to decrease. The friction of the carbides and borides can be explained in terms of the adhesion theory of friction.     

High-resolution photon scanning tunneling microscopic investigations of NaCl crystal surfaces

Cleaved NaCl crystal surfaces were investigated with a photon scanning tunneling microscope. Steps 4 nm high and 3 nm wide could be resolved. The lateral resolution is coupled to the step height by the steepest measured slopes of about 50°. The measured stepwidth at shallow steps is noise-and slope-limited to 3 nm. The mapping of the decay coefficient of the evanescent field shows spatial inhomogeneities often coupled to the crystalline structure.     

Magnetic characterization of microscopic particles by MO-SNOM

This paper reports on the development of a magneto‐optical scanning near‐field optical microscope and the experimental near‐field study of the domain structure for a model magnetic particle of 16 × 16 µm² of a Co_70.4Fe_4.6Si₁₅B₁₀ amorphous thin film, deposited on a silicon substrate. We present the topographic, optical and magneto‐optical differential susceptibility (MODS) images of the particle. Imaging by using the local MODS reveals the domain structure. These images are also used for positioning the tip in order to acquire local hysteresis loops, with submicrometre spatial resolution.     

Solid particle erosion behaviour of metallic materials at room and elevated temperatures

The behaviour of metallic materials subjected to solid particle erosion has been studied extensively over the last few decades. It is not the purpose of this paper to provide a comprehensive review of the above body of work especially since many such reviews already exist. Rather, the aim of this paper is to describe briefly the salient features characteristic of room temperature and elevated temperature erosion of metallic materials and follow it up with a review of some of the recent results, which in our opinion, have enhanced our current understanding in the area of solid particle erosion of metallic materials. As a natural consequence, the paper concludes with a critical review of the areas which require further study.     

Measurement of the effective thermal conductivities of molding sands at high temperatures

It is well known that the effective thermal conductivity of bonded molding sands depends on the volume fraction, thermal conductivity and arrangement of the components e.g., sand particles, bonding medium and air. The arrangement of components is known to be affected by particle size distribution, average size and shape. In this study, an experimental system using the line-heat-source method was designed and effective thermal conductivities of molding sands at temperatures up to 750°C were measured. The effects of binder content, initial moisture content, dry density and temperature were also investigated for four selected sand types: silica, olovine, zircon and chromite sands. The effect of dry density on the effective thermal conductivity of bintonite-bonded molding sands turned out to be more significant than the effect of either binder content or initial moisture content. The minimum effective thermal conductivity for bentonite bonded silica sand occurred at about 500°C. The effective thermal conductivity of silica sands bonded with western bentonite was found to be higher than that of silica sands bonded with southern bentonite up to 750°C.     

Influence of particle impact conditions and temperature on erosion-oxidation of steels at elevated temperatures

The behaviour of four steels that are used in fluidized-bed boilers (16Mo3, T91, 304L and 253MA) has been studied in air (oxidation) and under impacts by sand particles in air (erosion-oxidation) at 350-650 °C in an extensive test programme and their performance compared in terms of the resulting weight changes and surface appearances.The results show that the oxidation rates for the steels increase with temperature but decrease with increases in chromium contents of the steels, as expected. Through oxidation rate, temperature has a significant influence on overall weight changes during combined erosion-oxidation, with material losses often increasing with temperature. The lowest particle velocities often cause deposition of particle debris and some weight gains, whereas the highest velocities cause essentially loss of material. Ductile erosion behaviour is observed under most conditions, with higher material losses at a shallow than at a steep impact angle, but the overall angle-dependency slightly changes with increase in temperature. The explanations for these observations are discussed in this paper. Furthermore, materials selection maps are constructed from the results generated, demonstrating that choosing the most highly alloyed material available is not necessarily the best rationale for materials selection.     

Phenomenological investigation of cutting oils: Antifriction and antiwear behaviour at low contact temperatures

The basic reason for the use of cutting fluids in machining processes is to increase cutting tool life and to decrease machining costs. An experimental simulation of tribological processes representing rear cutting tool contact was carried out in a ‘block‐on‐disc’ (BOD) tribotester. The coefficient of friction and block wear were measured under boundary (discontinuous fluid film) and elastohydrodynamic (quasi‐continuous fluid film) lubrication conditions for neat and semi‐synthetic cutting oils. Interesting data were obtained concerning a further cutting fluid classification, which is helpful for optimal cutting fluid selection via their antiwear and antifriction characteristics.     

Tribological behavior of NiAl matrix composites with addition of oxides at high temperatures

NiAl, NiAl–Cr–Mo alloy and NiAl matrix composites with addition of oxides (ZnO/CuO) were fabricated by powder metallurgy route. It was found that some new phases (such as NiZn₃, Cu_0.81Ni_0.19 and Al₂O₃) are formed during the fabrication process due to a high-temperature solid state reaction. Tribological behavior was studied from room temperature to 1000 °C on an HT-1000 ball-on-disk high temperature tribometer. The results indicated that NiAl had high friction coefficient and wear rate at elevated temperatures, while incorporation of Cr(Mo) not only enhanced mechanical properties evidently but also improved high temperature tribological properties. Among the sintered materials, NiAl matrix composite with addition of ZnO showed the lowest wear rate at 1000 °C, while CuO addition into NiAl matrix composite exhibited the self-lubricating performance and the best tribological properties at 800 °C.     

A procedure for rapid studies of the metal oxidation kinetics at high temperatures

A procedure for rapid studies of the metal oxidation kinetics in one experiment with one sample in a wide high-temperature range is described. In this experiment, the temperature of the sample is sequentially raised step by step and its mass increment is continuously recorded in the isotherm. The proposed procedure differs from the thermogravimetric isothermic method for studying the kinetics of the high-temperature oxidation of metals (when it is necessary to conduct multiple isothermic experiments with a corresponding number of samples) in substantially decreased time and facilities required for executing the studies. To check this procedure, an iodide zirconium sample was oxidized in an air + argon mixture in a 1380-to 1720-K temperature range. The obtained results agree with the available reference data.