Thin section

Specimens for (S)TEM have to be transparent to the electron beam. In order to get good contrast and resolution, they have to be thin enough to minimize inelastic scattering. The required thin sections of organic materials can be obtained by ultramicrotomy eitlier after embedding into suitable resms (mostly epoxy- or methacrylate resins [H]) or directly at low temperatures by cryo-ultramicrotomy [12]. 

Fig. 1. A "barred olivine" chondrule from the Allende-type CV chondrite that feU in Mexico in 1979. The transmitted polarized light image of the 0.5 mm-diameter chondrule was taken from a poHshed thin section. The bars are composed of oHvine, (Mg,Fe)2Si04. The interstitial material is glass quenched...

The Extrusion of Thin Sections, 3rd ed., technical service note, ICI, Wilmington, Del., 1969. 

Optical Properties and Radiation Effects. Within the range of wavelengths measured (uv, visible, and near-ir radiation), Teflon PFA fluorocarbon film transmits slightly less energy than FEP film (29) (Table 6). In thin sections, the resin is colorless and transparent in thicker sections, it becomes translucent. It is highly transparent to it radiation uv absorption is low in thin sections. Weather-O-Meter tests indicate unlimited outdoor life. 

The cycle can usually be estimated on the basis of about 30 s/3 mm of thickness most of it is devoted to ram-in-motion time (except for very thin sections). The mold temperature used with PFA 340 is often the highest temperature that allows the part to be ejected undamaged from the mold and retain its shape while cooling. 

The TEM is one of the most generally useful microscopes many thousands of them ate in daily use throughout the world. They ate appHcable to the study of ultrafine particles (eg, pigments abrasives and carbon blacks) as well as microtomed thin sections of plant and animal tissue, paper, polymers, composites of all kinds, foods, industrial materials, etc. Even metals can be thinned to sections thin enough for detailed examination. 

Soft x-rays with wavelengths of 1—10 nm ate used for scanning x-ray microscopy. A zone plate is used to focus the x-ray beam to a diameter of a few tens of nanometers. This parameter fixes and limits the resolution. Holographic x-ray microscopy also utilizes soft x-rays with photoresist as detector. With a strong source of x-rays, eg, synchrotron, resolution is in the 5—20-nm range. Shadow projection x-ray microscopy is a commercially estabflshed method. The sample, a thin film or thin section, is placed very close to a point source of x-rays. The "shadow" is projected onto a detector, usually photographic film. The spot size is usually about 1 ]lni in diameter, hence the resolution cannot be better than that. 

Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstmetures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, foUowed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

In thin sections natural graphite is translucent, strongly pleochroic, and uniaxial. It has a negative sign of birefringence and two extinctions per revolution under crossed Nicol prisms. The atomic number of carbon accounts for its low absorption coefficient for x-rays and electrons. 

A frequently cited example of protection from atmospheric corrosion is the Eiffel Tower. The narrow and, for that age, thin sections required a good priming of red lead for protection against corrosion. The top coat was linseed oil with white lead, and later coatings of ochre, iron oxide, and micaceous iron oxide were added. Since its constmction the coating has been renewed several times [29]. Modern atmospheric corrosion protection uses quick-drying nitrocellulose, synthetic resins, and reaction resins (two-component mixes). The chemist Leo Baekeland discovered the synthetic material named after him, Bakelite, in 1907. Three years later the first synthetic resin (phenol formaldehyde) proved itself in a protective paint. A new materials era had dawned. 

Variations on the basic lap joint are preferred to give Large contact areas using uniform thin sections. If at all, the joint should be loaded in shear and... 

Dispersion polymer, which leads to products with improved tensile strength and flex life, is not easily fabricated by the above techniques. It has, however, been found possible to produce preforms by mixing with 15-25% of a lubricant, extruding and then removing the lubricant and sintering. Because of the need to remove the lubricant it is possible to produce only thin-section extrudates by this method. 

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