A comparison of the microstructure of silicon nitride-silicon carbide composites made with and without deoxidized starting material

Two different types of silicon carbide (SiC) matrix composites, with either 10 wt% or 20 wt% silicon nitride (Si₃N₄) reinforcement, were fabricated to investigate the effect of pretreatment on the resulting composite micro-structure. The first type of composite was prepared from as-received α-SiC and α-Si₃N₄ powders, while the second type was prepared from powder compacts that had been deoxidized to eliminate surface silica on the powder particles. The composites were hot isostatically pressed in tantalum cans at 2373 K for 1h under a pressure of 200 MPa. Density measurements showed that full theoretical density was achieved for the composites prepared from the as-received powders, while much lower densities were obtained for the composites prepared from the deoxidized green compacts. Almost all of the α-SiC transformed into β-SiC, and almost all the α-Si₃N₄ transformed into α-Si₃N₄ in the composites made from the as-received powders, while in the composites made from the deoxidized material the α-SiC remained untransformed and both α-Si₃N₄ and β-Si₃N₄ phases were present in significant quantities. High-resolution transmission electron microscopy and Fresnel fringe imaging were used to identify the grain boundary and interphase boundary structure. Most interfaces were found to be covered with ? 1 nm thick amorphous intergranular films in the composites prepared from as-received powders, whereas most interfaces were found to be free of such amorphous intergranular films in the composites prepared from the deoxidized material. Taken together, the presence of intergranular films at the interfaces and the results from density measurements are consistent with the densification and reverse α → β-SiC transformation taking place in the composites made from as-received powders by a liquid-phase sintering route. An incomplete liquid-phase sintering mechanism is also able to explain the microstructure observed in the composites made from the deoxidized material.     

Buried amorphous-layer impact on dislocation densities in silicon

The impact of amorphous layers on dislocation densities in silicon piezoresistors was investigated by means of transmission electron microscopy and chemical etching. Mechanical bevel polishing at a shallow angle and selective etching were applied to assess the dislocation depth distributions. It was found that, despite the presence of additional defects after recrystallization, the initial presence of a buried amorphous layer reduced, after annealing, the dislocation density in the depletion region of a p–n junction, compared with the case of a shallower, surface amorphous layer.     

The remote electron beam-induced current analysis of grain boundaries in semiconducting and semi-insulating materials

When no charge collecting p-n junction or Schottky barrier is present in the specimen, but two contacts are applied, conductive mode scanning electron microscope (SEM) observations known as remote electron beam-induced current (REBIC) can be made. It was described as "remote" EBIC because the contacts to the specimen can lie at macroscopic distances from the beam impact point. In recent years, REBIC has been found to be useful not only for studies of grain boundaries in semiconducting silicon and germanium, but also in semi-insulating materials such as the wider bandgap II-VI compounds and electroceramic materials like varistor ZnO and positive temperature coefficient resistor (PTCR) BaTiO3. The principles of this method are outlined. Accounts are given of the five forms of charge collection and resistive contrast that appear at grain boundaries (GBs) in REBIC micrographs. These are (1) terraced contrast due to high resistivity boundary layers, (2) peak and trough (PAT) contrast due to charge on the boundary, (3) reversible contrast seen only under external voltage bias due to the beta-conductive effect in a low conductivity boundary layer, (4) dark contrast due to enhanced recombination, and (5) bright contrast apparently due to reduced recombination. For comparison, the results of the extensive EBIC studies of GBs in Si and Ge are first outlined and then the results of recent REBIC grain boundary studies in both semiconducting and semi-insulating materials are reviewed.     

'Five-parameter' analysis of grain boundary networks by electron backscatter diffraction

This paper describes state‐of‐the‐art analysis of grain boundary populations by EBSD, with particular emphasis on advanced, nonstandard analysis. Data processing based both on misorientation alone and customised additions which include the boundary planes are reviewed. Although commercial EBSD packages offer comprehensive data processing options for interfaces, it is clear that there is a wealth of more in‐depth data that can be gleaned from further analysis. In particular, determination of all five degrees of freedom of the boundary population provides an exciting opportunity to study grain boundaries by EBSD in a depth that was hitherto impossible. In this presentation we show ‘five‐parameter’ data from 50 000 boundary segments in grain boundary engineered brass. This is the first time that the distribution of boundary planes has been revealed in a grain boundary engineered material.     

Facility for rapid preparation of silicon and silicide TEM specimens

A simple chemical jet polishing arrangement, for the thinning of semiconductors or metals for transmission electron microscopy (TEM), is described for the specific case of silicon and silicides. The effect of variation in three mechanical parameters on the profile and quality of the specimen is described, and the optimum conditions are determined. A proposed polishing solution is one part 48% HF mixed with one part fuming HNO₃. Reproducibility is only achieved in the presence of a relatively large concentration of nitrous acid in the polishing solution. The solution must also be relatively concentrated, as the reaction rate falls off rapidly with decreasing concentration.     

Segregation-induced hole drilling at grain boundaries

Grain boundaries in metals provide preferential sites for impurity atoms to segregate, forming an area of the metal with different chemical and physical properties compared to the bulk. This also means that the electron beam damage characteristics of the grain boundary might be different from that of the bulk. Segregation can be expected to make grain boundaries particularly vulnerable to electron-beaminduced hole drilling and beam damage, if the segregation produces a near two-dimensional compound with increased ionicity. Here we report preferential hole drilling at grain boundaries in an Fe-0–4 wt%P alloy. Such a phenomenon confirms evidence from energy-loss spectroscopy that charge transfer occurs between P and Fe at the grain boundary. It also explains why it is easier to detect phosphorus using EDX compared to PEELS, and suggests that the electron beam damage may be a major limiting factor in grain-boundary studies.     

磷在2.25CrlMo钢中的非平衡晶界偏聚动力学实验

采用2.25Cr1Mo工业用钢,通过俄歇电子能谱分析方法(AES),对磷在1050℃固溶处理、540℃恒温保持过程中的非平衡晶界偏聚浓度进行了测试,获得了磷在钢中的非平衡晶界偏聚动力学曲线,该曲线直接验证了非平衡晶界偏聚动力学理论中的Xu-Song模型。     

TEM study of chirality in MoS2 nanotubes

The dark-field diffraction contrast of helical nanotubes (NTs) is shown to be asymmetric when an NT is tilted at appropriate angle with respect to the incident electron beam. This phenomenon was used for the chirality determination of multi-shell NTs observed in MoS₂ layered compound. Both kinds of NT — helical and non-helical — were found. In the case of helical NTs only right-hand chirality was observed.     

High resolution electron microscopy of interfaces in chlorinated phthalocyanine molecular crystals

Molecular images of chlorinated copper phthalocyanine have shown the arrangement of molecules at the interface between two crystals. Mismatch does not cause distortion or recrystallization for low index junctions but for more irregular interfaces strain causes alteration of the molecular tilt angle and accommodation of the mismatch by the formation of edge dislocations whose Burger's vectors are parallel to the interface.     

New developments in the characterization of dislocation loops from LACBED patterns

The characterization of the Burgers vector of dislocations from large‐angle convergent‐beam electron diffraction (LACBED) patterns is now a well‐established method. The method has already been applied to relatively large and isolated dislocation loops in semiconductors. Nevertheless, some severe experimental difficulties are encountered with small dislocation loops. By using a 2 µm selected‐area aperture and a carbon contamination point to mark the loop of interest, we were able to characterize both the plane and the Burgers vector of dislocation loops of a few tens of nanometres in size present in Al‐Cu‐Mg alloys.     

Microstructural analysis of silicon carbide monofilaments

In the development of monofilaments, a good understanding of the process/property relationships is essential. Transmission electron microscopy (TEM) is a powerful tool but too slow and expensive to be used routinely. Alternative, cheaper techniques have therefore been investigated. The microstructures of three SiC monofilaments (DERA Sigma SM1140+, Textron SCS-6 and Ultra-SCS) and some experimental samples were studied using a combination of TEM, electron microprobe analysis, Raman microprobe analysis, thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). It was found that the Raman technique was complementary to TEM and easily identified the presence of amorphous C and Si. These could not be seen by electron or X-ray diffraction techniques. DSC indicated the presence of free Si in the DERA Sigma SM1140+ monofilament by a distinctive peak at ∼1400 °C. TGA showed the reaction of monofilament components with gaseous species. The Textron SCS-6 and Ultra species lost weight as C was oxidized to gaseous CO. By contrast, the Sigma monofilament gained weight from formation of SiO₂ from the free Si. The separations of the transverse optical phonon peaks in the Raman spectra were correlated with the density of stacking faults in the SiC crystallites. This was similar in all monofilaments. Analysis of the polarization of the Raman scattering gave information on the orientation of crystallites. The crystallites in SM1140+ and SCS-6 were orientated predominantly with the <111> parallel to the radius. Preliminary interpretation of the polarized Raman scattering from Ultra-SCS indicated more than one orientation of crystallite. One possibility was a mixture of <111> and <110> directions parallel to the radius.     

On the accuracy of lattice-distortion analysis directly from high-resolution transmission electron micrographs

Lattice‐distortion analysis from high‐resolution transmission electron micrographs offers a convenient and fast tool for direct measurement of strains in materials over a large area. In the present work, we have evaluated the accuracy of the strain measurement when the effects of the realistic experimental variables are explicitly taken into account by the use of image simulation techniques. These variables are focal setting and variation, local thickness and orientation of the sample, as well as misalignments of the sample and the incident beam. The evaluation reveals that consistency of image features and contrast within the micrographs is desired for the analysis to eliminate effects of the variations of local focus value and specimen thickness. After proper orientation of a crystalline specimen, the misorientation of the object will not notably influence the strain measurement even though a local bending may exist within the sample. However, the incident beam of the microscope needs to be aligned carefully as the beam misalignment may introduce a notable artefact around the interface region.     

TEM analysis of centreline sulphide precipitates modified by titanium additions to low carbon steel

Elongated inclusions, particularly MnS, contribute significantly to reduced ductility and toughness in hot rolled steel but earlier research indicated that these properties can be improved by titanium additions. Such additions to a steel result in titanium being dissolved in manganese sulphide or MnS being replaced by TiS and/or titanium carbosulphides. In the present study, a steel was designed to decrease alloying element segregation and to evaluate the effect of titanium on centreline sulphide precipitates. Precipitates were identified by using scanning electron microscopy and characterized by the use of transmission electron microscopy following sample preparation by focused ion beam milling techniques. Iron–titanium‐sulphides form in close proximity to MnS precipitates that contain iron. Evidence is provided that an increase in the titanium content of steel leads to an increase in the percentage of titanium contained in the iron sulphides and a decrease in the iron content of MnS inclusions.     

TEM in materials science—past,present and future

The progress in transmission electron microscopy as applied in materials science is reviewed briefly, from the era of replica techniques in the 1940s and 1950s, through the development of the diffraction contrast technique in the late 1950s and 1960s, to the present day when instrumental resolution is sufficient to obtain structure images of a wide variety of crystalline solids. One of the most important advances has been the development of combined imaging and microanalytical techniques in a single instrument. The versatility of the TEM technique with atomic resolution and microanalytical facilities, the variety of signals and contrast effects available, and its universal application, establishes it as an essential tool in materials science for the foreseeable future.     

Pectin-like carbohydrates in the green alga Micrasterias characterized by cytochemical analysis and energy filtering TEM

Pectins are the major matrix polysaccharides of plant cell walls and are important for controlling growth, wall porosity and regulation of the ionic environment in plant cells. Pectic epitopes recognized by the monoclonal antibodies JIM5, JIM7 and 2F4 could be localized in the primary wall during development of the green alga Micrasterias. As the degree of pectin esterification determines the calcium-binding capacity and thus the physical properties of the cell wall, chemical and enzymatic in situ de-esterification was performed. This resulted in displacement of epitopes recognized by JIM5, JIM7 and 2F4, respectively, in changes in the intensity of the antibody labelling as visualized in CLSM. In addition, calcium-binding capacities of cell walls and components of the secretory apparatus were determined in transmission electron microscopy by electron energy loss spectroscopy and electron spectroscopic imaging. These analyses revealed that pectic polysaccharides are transported to the cell wall in a de-esterified form. At the primary wall, pectins get methyl-esterified at the inner side, thus allowing flexibility of the wall. At the outer side of the wall they become again de-esterified and bind high amounts of calcium which leads to cell wall stiffening. Mucilage vesicles possess the highest calcium-binding capacity of all structures observed in Micrasterias, indicating that the pectic polysaccharides of mucilage are secreted in a de-esterified, compact form. When mucilage is excreted through the cell wall, it loses its ability to bind calcium. The esterification of pectins involved is obviously required for swelling of mucilage by water uptake, which generates the motive force for orientation of this unicellular organism in respect to light. Incubation of Micrasterias in pectin methylesterase (PME), which de-esterifies pectic polymers in higher plants, resulted in growth inhibition, cell shape malformation and primary wall thickening. A PME-like enzyme could be found in Micrasterias by PME activity assays.     

Piezoelectric field around threading dislocation in GaN determined on the basis of high-resolution transmission electron microscopy image

A new method of determining the piezoelectric field around dislocations from high‐resolution transmission electron microscopy images is presented. In order to determine the electrical potential distribution near a dislocation core, we used the distortion field, obtained using the geometrical phase method and the non‐linear finite element method. The electrical field distribution was determined taking into account the inhomogeneous strain distribution, finite geometry of the sample and the full couplings between elastic and electrical fields. The results of the calculation for a transmission electron microscopy thin sample are presented.