Studies of Material Reactions by In Situ High-Resolution Electron Microscopy
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work if studied conventionally. The information provided by in situ HREM is often unique and so it can become an important technique for fundamental materials investigations. Experimental Considerations As with any TEM study, specimens may be cut from any bulk material and thinned to electron transparency by standard, although time-consuming, procedures. Accordingly, as almost all materials are accessible to TEM examination, in situ investigation can equally be applied. These days, atomic resolution (e.g., 0.2 nm) is quite easily achieved, and television rate imaging and video recording are straightforward. We use a Philips EM430, 300 kV TEM equipped with a Gatan image pickup system, but equivalent machines are available commercially from several manufacturers. The quality, reproducibility, and stability of the specimen holder is also clearly important for in situ work, especially to realize the highest resolution (and magnification). Side-entry stages are the most convenient, and they can be either custom-built or supplied as standard accessories. Our best work has been performed using the standard single-tilt Philips heating holder. Some experimental facility needs to be acquired to achieve successful and meaningful results, but with a little practice it becomes possible to follow reactions for many hours, at a variety of sample temperatures, changed at will by the operator. The examples shown here are typical images from extensive recordings (e.g., 1-5 hours). A major concern with in situ HREM, as with any in situ TEM experiment, is the thin nature of the sample. It is neces-
sary to avoid thin-foil artifacts in order to deduce the real bulk behavior of the material. We have come up with a set of guidelines, many of which are common sense, to ensure that the results are truly representative.2^ 1. Compare in situ behavior in thin and thick areas of the samples. 2. Compare the reaction in areas under the electron beam illumination and away from it. 3. At a range of magnifications, compare the microstructural evolution in situ with equivalent samples ex situ. 4. Determine the activation energy of the reaction (to eliminate the possibility of a surface-controlled mechanism). 5. Re-thin an in situ TEM sample after completion of the experiment and compare the microstructure to that in the prior thin area. A combination of these tests can satisfy even the most skeptical researcher. However, a complete complement is extremely tedious, and we have found it to be necessary only when some new phenomenon is to be investigated. In our experience, the majority oi situations duplicate bulk behavior. Should it occur, the intercession of a specimen artifact is easily noticed. In those situations, the in situ approach might be abandoned, or alternatively used to advantage to establish thin-film behavior itself! We have often pondered why the in situ HREM approach has been so successful. Perhaps the specimen preparation by ion milling provides a "protective" surface layer to seal in the bulk material, but this is mere specula
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