Environmental SEM

Electron microscopy (see section B1.18) is very valuable in characterizing particles (see, for instance, figure C2.6.1). The suspension stmcture is, of course, not represented well because of tire vacuum conditions in tire microscope. This can be overcome using environmental SEM [241. 

As time goes on, the ultimate resolution of the SEM operated in these modes will probably level out near 1 nm. The major growth of SEMs now seems to be in the development of specialized instruments. An environmental SEM has been developed that uses differential pumping to permit the observation of specimens at higher pressures. Photographs of the formation of ice crystals have been taken and the instrument has particular application to samples that are not vacuum compatible, such as biological samples. 

We thank Julius Chang and Jerome P. Downey for providing the iron carbide sample. This work was supported by DARPA via ONR contract. The purchase of the Raman spectrometer and environmental SEM was supported byNSF grants DMR-0116645 and BES-0216343. 

A Philips Environmental-SEM/EDX (Phihps XL30) was used to measure the morphology on untreated and plasma-treated powders. The powders were fixed on the sample holder by double conductive adhesive aluminum tape, and then gold-coated. Secondary electron images were recorded with the scanning electron microscope (SEM) using a 15 keV acceleration voltage. 

Some of the more noteworthy EM techniques which were first described prior to 1990, deserve mention because of their contribution to the advancement of food structure knowledge. The utility of ciyo-SEM in food systems was clearly illustrated by Saigent [57] including over 40 micrographs of diverse food systems as visualized by ciyo-SEM techniques. Since that time numerous papers can be found in the literature utilizing ciyo-SEM methods. An ever expanding list of unique and novel applications to new foods has come from the papers of Heertje and his co-workers and is summarized by Heertje [58]. Here we must focus on the newer techniques which have been developed in recent years the first of these is environmental SEM. 

Figure 19 Environmental SEM image of (a) Eudragit LlOO-55 and (b) Advantose 100 tablet in the fracture region after diametrical compression (reproduced from Yap et al., 2006). Permitted by Elsevier.

An imaging mode called "wet-STEM" recently developed in ESEM, and schematically presented in Figure 3.17, allows the observation of nano-objects suspended in a liquid phase, with a few nanometers resolution (87). The idea behind this technique is simply to perform STEM-in-SEM, that is SEM in transmission mode, in an environmental SEM, in the wet state. The ESEM is interesting for the ability to keep samples wet or liquid, and the STEM mode results in high contrast, resolution and thickness of transparency. 

Thiel, B.L., and Donald, A.M. (1998). In situ mechanical testing of fully hydrated carrots (Daucus carota) in the environmental SEM. Ann. Botany, 82,727 733. 

Schalek, R.L. Drzal, L.T. (2000) Characterisation of advanced materials using an environmental SEM. Journal of Advanced Materials 32, 32-38. 

Keddie, J.L. Meredith, P. Jones, R.A.L. Donald, A.M. Kinetics of film formation in acrylic latices studied with multiple angle-of-incidence ellipsometry and environmental SEM. Macromolecules 1995, 28, 2673-2682. 

The hot stage has not only been applied to optical and atomic force microscopes, but also to scanning electron microscopes. Hot-stage accessories are available on environmental SEMs that can collect ESEM images at elevated temperatures. Applications to the electronics industry include fluxless soldering, intermetallic growth studies, and copper thick-film sintering studies (92-94). 

A recent innovation in electron microscopy is environmental SEM, which allows samples to be studied at pressures and humidities that approach ambient conditions. To achieve this, several stages of differential pumping between the electron gun and the sample are used, and the sample itself is placed in a vacuum of a few hundred Pascals. Environmental SEM enables many materials to be examined without pretreatment, unlike conventional SEM, in which specimens must be solid, dry and usually electrically conductive. This now makes possible studies of the natural, unadulterated surfaces of specimens such as polymers, biological tissues and cells, food and drugs and forensic materials. 

As a special development in recent years, SEMs have been designed which no longer necessitate high vacuum (environmental SEM, ESEM variable pressure SEM, VPSEM). This development is important for the imaging of samples with a residual vapour pressure, such as aqueous biological or medical samples, but also samples in materials science (wet rock) or organic chemistry (polymers). 

Fig. 7. Averaged X-ray spectra showing the elemental composition inside a marine microgel with the large peak of calcium and the absence of phosphorus. Inset a shows an environmental SEM micrograph of a hydrated marine microgel (scale bar, 2 pm). Inset b reveals the presence of bound calcium in a microgel stained with chlortetracycline (reproduced from Chin et al. [41])...

A recent addition is the environmental SEM (ESEM) which operates with the sample chamber at greater than 20 torn. Water vapour can be maintained in the chamber by stabilizing the liquid water at pressures equal to the vapour pressure of water. At a given vapour pressure, the sample temperature can be adjusted to maintain, evaporate or condense water. The polymers can now be imaged wet and real-time studies can be carried out. 

Environmental SEMs allow operation at pressures of several torr (0.1-20 torr) in the sample chamber and at temperatures >1000°C. In addition to being able to examine insulators it is also possible to follow dynamic processes such as drying of cement and crystallization. 

NiO has been reduced to Ni metal while heating in the TEM at 1000°C. New microscope holders and stages are becoming available so we will see much more use of in situ TEM in the future. Most of the work has been carried out using the SEM environmental SEMs can heat the sample to 1500°C, but again the challenge is in controlling the local p02, etc. 

C A Wood and W L Bradley, Determination of the effect of sea-water on the interfacial strength of an interlayer E-glass/graphite/epoxy composite by in-situ observation of transverse cracking in an environmental SEM , Compos Sci Technol 1997 57 1033-1043. 

The specimen for SEM has to fulfill certain requirements. It needs to withhold vacuum and electron bombardment, it has to be electrically conductive and furthermore, and it needs to be clean and dry. A special version of an SEM is the environmental SEM (ESEM) which uses a different vacuum system and other types of detectors. The ESEM can be operated in higher, more natural pressures. 

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