Polycrystalline specimens, oriented

Oriented, polycrystalline specimens. If, for example, all Az = A = 0 the array would be a three-dimensionally ordered domain i.e. a "single crystal". The most highly organized fibers are ones in which many such domains are oriented with only the helix axes parallel. As I indicated above, such fibers provide X-ray diffraction patterns like those from a rotated single crystal. The not-quite-perfect parallelism of the domains causes the intensity to be distributed along arcs instead of concentrated in spots (see Fig. 2). 

Between the uniaxially oriented, non-crystalline specimens and the oriented polycrystalline materials, there is a whole range of ordering which yields different types of diffraction patterns in terms of detailed structures (3). 

The derivation of this equation can be found in various advanced texts, for example, those of Warren [G.30] and James [G.7]. It applies to a polycrystalline specimen, made up of randomly oriented grains, in the form of a flat plate of effectively infinite thickness, making equal angles with the incident and diffracted beams and completely filling the incident beam at all angles 6. The second factor in square brackets, containing F, p, and 0, will be recognized as Eq. (4-19), the approximate equation for relative line intensities in a Debye-Scherrer pattern. 

Frequently, polycrystalline specimens exhibit a preferred orientation of the crystallites or polycrystalline texture. In addition, many manufacturing processes of technological materials can induce texture. In comparison with specimens having randomly oriented crystallites, the relative intensities of the diffraction lines of textured samples are modified. As a consequence the structural and quantitative phase analysis of polycrystalline samples becomes impossible without proper modeling of the texture. 

Fig. 7. The logarithms of the pre-exponential factor versus the activation energy, for the decomposition of formic acid on single crystals of silver with surfaces oriented parallel to (111), (100), and (110). Points are taken from data of Sosnovsky (22, 38, 39). The numbers in the graph refer to the voltage applied during ion bombardment. X —samples without special pretreatment A—(100)orientation B—polycrystalline specimen C—(11 orientation. See further text in figure.

The observations in the field emission microscope establish that under the conditions of the adsorption studies of Section II, C, 1, a, xenon was mobile, and that the binding energy varies with the crystal orientation. The structure of the filament surface is not known in detail however, this is a polycrystalline specimen, the cylindrical surfaces of which are made up of planes with the orientation (hhk). The condensation coefficient must therefore be an average quantity, and energy transfer may thus not be the limiting step. 

Scanning a polycrystalline specimen results in changes of brightness forming the crystal orientation or channeling contrast (Fig. 87). 

However, not every crystalline substance can be obtained in the form of macroscopic crystals. This led to the Debye-Scherrer (16) method of analysis for powdered crystalline solids or polycrystalline specimens. The crystals are oriented at random so the spots become cones of diffracted beams that can be recorded either as circles on a flat photographic plate or as arcs on a strip of film encircling the specimen (see Figure 6.4) (17). The latter method permits the study of back reflections as well as forward reflections. 

Selected-area electron diffraction (SAD) is a basic TEM technique to obtain diffraction information from a part of the specimen. A selected-area aperture is inserted below the sample holder and in the image plane of the objective lens. Only the area selected by the aperture on the screen contributes to the SAD pattern. In case of polycrystalline specimens, if more than one crystal contributes to the SAD pattern, it can be difficult or impossible to analyze. As such, it is useful to select a single crystalline region for analysis at a time. It may also be useful to select two crystals at a time, in order to examine the crystallographic orientation between them. 

One common diffraction technique employs a powdered or polycrystalline specimen consisting of many fine and randomly oriented particles that are exposed to monochromatic x-radiation. Each powder particle (or grain) is a crystal, and having a large nnmber of them with random orientations ensmes that some particles are properly oriented snch that every possible set of crystallographic planes will be available for diffraction. 

Fig. 2. (continued)—(d) an aggregate of microcrystallites whose long axes are parallel, but randomly oriented (left), diffracts to produce a series of layer lines (right) and (e) a polycrystalline and preferentially oriented specimen (left) diffracts to give Bragg reflections on layer lines (right). The meridional reflection on the fourth layer line indicates 4-fold helix symmetry. 

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

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