Optical microscopic

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

Kino G S and Xiao G Q 1990 Real time scanning optical microscope Confocai Microscopy ed T Wilson (New York Academic)... 

Figure Bl.19.38. Schematic of a scaimmg near-field optical microscope (SNOM). (Taken from [196]. figure 2.)...

Grober R D, Harris T D, Trautman J K and Betzig E 1994 Design and implementation of a low temperature near-field scanning optical microscope Rev. Sc/. Instrum. 65 626-31... 

Durand Y, Woehl J C, Viellerobe B, Gdhde W and Orrit M 1999 New design of a cryostat-mounted scanning near-field optical microscope for single molecule spectroscopy Rev. Sc/. Instrum. 70 1318-25... 

Gdhde W, Tittel J, Basche T, Brauchle C, Fischer U C and Fuchs H 1997 A low-temperature scanning confocal and near-field optical microscope Rev. Sc/. Instrum. 68 2466-74... 

Nie S 1998 A dual-beam optical microscope for observation and cleavage of single DNA molecules Anal. Chem. 70 1743-8... 

One of the most important uses of specific surface determination is for the estimation of the particles size of finely divided solids the inverse relationship between these two properties has already been dealt with at some length. The adsorption method is particularly relevant to powders having particle sizes below about 1 pm, where methods based on the optical microscope are inapplicable. If, as is usually the case, the powder has a raiige of particle sizes, the specific surface will lead to a mean particle size directly, whereas in any microscopic method, whether optical or electron-optical, a large number of particles, constituting a representative sample, would have to be examined and the mean size then calculated. 

Most tests of the validity of the BET area have been carried out with finely divided solids, where independent evaluation of the surface area can be made from optical microscopic or, more often, electron microscopic observations of particle size, provided the size distribution is fairly narrow. As already explained (Section 1.10) the specific surface obtained in this way is related to the mean projected diameter through the equation... 

Suppose a bulk-crystallized polymer sample is observed in an optical microscope with the sample placed between Polaroid filters oriented at right angles to each other. In the absence of any sample, the light would be attenuated owing to the 90° angle between the vectors describing the light transmitted by the two filters. With a crystalline sample of polymer in place, however, a display like... 

Near-field scanning optical microscope (NSOM)... 

Electrical trees consist of visible permanent hoUow channels, resulting from decomposition of the material, and show up clearly in polyethylene and other translucent soHd dielectrics when examined with an optical microscope. Eresh, unstained water trees appear diffuse and temporary. Water trees consist of very fine paths along which moisture has penetrated under the action of a voltage gradient. Considerable force is required to effect this... 

Nc.ar-Fi ld Scanning Optical Microscope.. The near-field scanning optical microscope (NSOM) should, strictiy speaking, be NSLM for near-field scanning light microscopy because "optical" includes electron optical as well as light optical and NSOM is a light microscope. 

Optical properties also provide useful stmcture information about the fiber. The orientation of the molecular chains of a fiber can be estimated from differences in the refractive indexes measured with the optical microscope, using light polarized in the parallel and perpendicular directions relative to the fiber axis (46,47). The difference of the principal refractive indexes is called the birefringence, which is illustrated with typical fiber examples as foUows. Birefringence is used to monitor the orientation of nylon filament in melt spinning (48). 

Most fractography can be conducted with simple instmments such as a pocket magnifying eyepiece or an optical microscope. As with any detective work, it is important to maintain careful records and to pay close attention to details in the reconstmctions of the conditions under which fabrication and failure occurred. Seemingly unimportant details of fabrication, service, and/or the conditions under which failure occurred can frequently be the key to determining the cause of failure. 

Fig. 10. Crack pinning by a SiC fiber in a glass matrix, photographed using an optical microscope and Nomarski contrast. Fiber ties perpendicular to plane of micrograph lines represent crack position at fixed intervals of time, crack mnning left to right.

Fig. 13. Optical microscope shows the presence of a Hquid crystal direcdy ia the emulsion.

Fig. 14. A sample of a lamellar liquid crystal between crosses polarized in an optical microscope gives a pattern of "oily streaks" and Maltese crosses (a) while the Hquid crystal consisting of an array of cylinders shows the characteristic sectional pattern (b).

Traditionally, the first instrument that would come to mind for small scale materials characterization would be the optical microscope. The optical microscope offered the scientist a first look at most samples and could be used to routinely document the progress of an investigation. As the sophistication of investigations increased, the optical microscope often has been replaced by instrumentation having superior spatial resolution or depth of focus. However, its use has continued because of the ubiquitous availability of the tool. 

For the purpose of a detailed materials characterization, the optical microscope has been supplanted by two more potent instruments the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM). Because of its reasonable cost and the wide range of information that it provides in a timely manner, the SEM often replaces the optical microscope as the preferred starting tool for materials studies. 

Optical CL microscopes are instruments that couple electron gun attachments to optical microscopes. Although such systems have a limited spatial resolution, they are used widely in the analysis of minerals. ... 

An optical microscope for precise positioning of the specimen relative to the X-ray spectrometers... 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

Run-of-the-mill instruments can achieve a resolution of 5-10 nm, while the best reach 1 nm. The remarkable depth of focus derives from the fact that a very small numerical aperture is used, and yet this feature does not spoil the resolution, which is not limited by dilfraction as it is in an optical microscope but rather by various forms of aberration. Scanning electron microscopes can undertake compositional analysis (but with much less accuracy than the instruments treated in the next section) and there is also a way of arranging image formation that allows atomic-number contrast, so that elements of different atomic number show up in various degrees of brightness on the image of a polished surface. 

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