Analytical electron microscopy specimen

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

The origins of analytical electron microscopy go back only about 15 years when the first x-ray spectra were obtained from submicron diameter areas of thin specimens in an electron microscope [1]. Characterization of catalyst materials using AEM is even more recent[2,3] but is currently a very active research area in several industrial and academic laboratories. The primary advantage of this technique for catalyst research is that it is the only technique that can yield chemical and structural information from individual submicron catalyst particles. 

Although x-ray microanalysis in the STEM is the most developed form of analytical electron microscopy, many other types of information can be obtained when an electron beam interacts with a thin specimen. Figure 2 shows the various signals generated as electrons traverse a thin specimen. The following information about heterogeneous catalysts can be obtained from these signals ... 

Analytical electron microscopy by x-ray emission spectroscopy can be extremely useful as a qualitative analysis tool, e.g. to determine which elements are present in lOnm diameter areas of the specimen. However, the greatest impact of AEM comes from quantitative chemical profiles across minute regions or features in the specimen, information that usually cannot be obtained by other means. 

Analytical electron microscopy is the most sophisticated tool available for micro-structural analysis today. In this method, we can obtain both the high-resolution structure and elemental composition of a specimen. This is probably the best technique to obtain local elemental composition of small regions of heterogeneous solids. When a high-energy electron beam is incident on a specimen, we get elastically and inelastically... 

The size and distribution of pores and the size, distribution, and identity of minerals in coal specimens from an eastern Kentucky splint coal and the Illinois No. 6 coal seam were determined by means of transmission electron microscopy (TEM) and analytical electron microscopy (AEM). The observed porosity varies with the macerals such that the finest pores (<2-5 nm) are located in vitrinite, with a broad range of coarser porosity (40-500 nm) associated with the macerals exinite and inertinite. Elemental analyses, for elements of atomic number 11 or greater, in conjunction with selected area diffraction (SAD) experiments served to identify the source of the titanium observed in the granular material as the mineral rutile. Only sulfur could be de-tected in the other coal macerals. Dark-field microscopy is introduced as a means for determining the domain size of the coal macerals. This method should prove useful in the determination of the molecular structure of coal. 

Electron probe and X-ray fluorescence methods of analysis are used for rather different but complementary purposes. The ability to provide an elemental spot analysis is the important characteristic of electron probe methods, which thus find use in analytical problems where the composition of the specimen changes over short distances. The examination of the distribution of heavy metals within the cellular structure of biological specimens, the distribution of metal crystallites on the surface of heterogeneous catalysts, or the differences in composition in the region of surface irregularities and faults in alloys, are all important examples of this application. Figure 8.45 illustrates the analysis of parts of a biological cell just 1 pm apart. Combination of electron probe analysis with electron microscopy enables visual examination to be used to identify the areas of interest prior to the analytical measurement. 

Fe2-xCrx(Mo04)3 provides a good opportunity for the quantitative, comparison of two analytical techniques - "classical" atomic absorption analysis and x-ray microanalysis. X-ray microanalysis of thin samples using scanning transmission electron microscopy has become an effective quantitative technique in the last few years(86). as opposed to the well-known electron microprobe analyses of bulk specimens(87) ... 

The bulk of the results obtained in this study were obtained by analytical transmission electron microscopy. Observations were made on both grades of material in the as-received, untested condition and after tensile testing at 1250°C. Test samples selected for examination covered the range of observed creep behavior, and included samples that failed after times ranging from -10 to -200 hours depending on applied stress and also samples from interrupted tests that survived for up to 2000 hours under lower applied stresses. In addition, a comparison was made of non-reinforced samples tested with and without a 500 hour pre-anneal at the test temperature. In all cases, the gauge sections of crept samples were cut parallel to the stress axis to obtain both near-surface and raid-plane sections. Prior to final TEM specimen preparation, these sections were examined optically for evidence of distributed creep cavitation or crack damage. 

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