Crystallites single crystals

Tube wall Stacking of small crystallites Single crystal... 

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

Surface-sensitive diffraction is, for the most part, restricted to analysis of surfaces of single crystals and overlayers and films on such surfaces. If a polycrystalline sample is illuminated using a beam of low-energy electrons, each crystallite surfiice exposed will create its own diffraction pattern, all of which will be superimposed on the fluorescent screen detector. If more than a few orientations are illuminated by the beam, the pattern becomes too complicated to analyze. Flowever, if the size of the... 

Larger and better formed single crystals are obtained at higher T and with xs HCl in the gas phase. Dendritic TiB2 crystallites form at higher BCI3 concentrations . 

How does a support affect the morphology of a particle on top of it Which surface planes does the metal single crystal expose The thermodynamically most stable configuration of such small crystallites is determined by the free energy of the surface facets and the interface with the support, and can be derived by the so-called Wulff construction, which we demonstrate for a cross section through a particle-support assembly in two dimensions (Fig. 5.13). 

Grain boundaries form junctions between grains within the particle, due to vacancy and line-defect formation. This situation arises because of the 2nd Law of Thermodjmamics (Entropy). Thus, if crystallites are formed by precipitation from solution, the product will be a powder consisting of many small particles. Their actual size will depend upon the methods used to form them. Note that each crystallite can be a single-crystal but, of necessity, will be limited in size. 

Obviously, the major difference in the single-crystal and polycrystalline (crystallite) state is a matter of size. For the single-crysted, the size is leu ge (> 10 cm), wherecis in the polycrystalline state, the size of the cr3rstals is small (10 (im = 0.001 cm.) The methods for obfriining one or the other differ considerably. They include formation from ... 

What happens is that the crystallites melt and fuse into a small tip. If we do this carefully, we will have our "seed". The tip s small size limits regrowth of the remelted part to that of a single crystal. Then, when we return the seed to the melt, we can initiate the growth of a much Icurger single crystal, provided that growth-conditions are suitable. 

The extent to which small particles of Pd and Pt show evidence of oxidation after exposure to air Is also highly variable. It Is difficult to confirm the evidence of X-ray diffraction and EXAFS (25) that most particles In the 15-20A size range consist entirely of oxide. We have found that such particles usually give single crystal patterns attributable to the metals. There Is, however, considerable evidence that, in the case of Pt on alumina, the Pt crystals have a well-defined epitaxial relationship with the crystallites (20-50A diameter) of the nominally "amorphous" alumina substrate. 

Pd(lll). However, Pd(lll) shows little or no evidence for the stoichiometric 2Bi + L + 3L process. This could be due to the presence of longer range order on the single crystal than on the Pd particles, leading to processes more akin to two dimensional phase transitions on the Pd(lll) crystal surface, rather than a more local species conversion on the small metal crystallites. 

The kinetics of electrocrystallization conforms to the above description only under precisely defined conditions. The deposition of metals on polycrystalline materials again yields products with polycrystalline structure, consisting of crystallites. These are microscopic formations with the structure of a single crystal. 

In practice, commercially fabricated polymer items are generally oriented to some degree. The scattering patterns from such materials comprise arcs, which are parts of the full circles obtained from unoriented samples. The lengths and positions of these arcs reveal much about the orientation of crystallites within a sample. The shorter the arcs, the more oriented the sample. In cases of extreme orientation, as found in highly oriented fibers such as Kevlar , the scattering pattern can approach that of a single crystal. 

We have adapted a commercially available x-ray diffractometer normally used for structure determinations on single crystals to operate as a very flexible device for performing x-ray pole figure determinations and related studies on polymeric materials. Descriptions of crystallite orientations, as provided by pole figures, are useful in studying many aspects of the behavior of products made from semicrystalline polymers. This paper describes the software that we have written for our pole figure facility. Except for some vendor-provided routines to drive the hardware Interface all of our software is written in FORTRAN. Menu driven operation is provided to maximize user convenience. 

Fig. 9 Snapshot of a single crystal of lattice polymers viewed from the chain direction. The bonds are drawn as solid cylinders. The viewing angle is large for better observation of folds. The chain length is 512 units and the thickness of the crystallite is about 12 units. The dissolved chains are not shown for clarity [57]...

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