Crystallite orientations

Identification of crystalline phases determination of strain, and crystallite orientation and size accurate determination of atomic arrangements... 

The quantitative assessment of the degree of crystallite orientation by x-ray examination is not free of ambiguity. From a comparative analysis [23] in which results obtained from the consideration of (105) and from three different variations of equatorial reflection were compared, the conclusion was that the first procedure can lead to underrated results, i.e., to the underestimation of the orientation. However, it can be assumed that this does not result from an incorrect procedure, but from ignoring the fact that the adjacent (105) reflex can overlap. The absence of the plate effect of the orientation is characteristic of the orientation of crystallites in PET fibers. The evidence of this absence is the nearly identical azimuthal intensity distributions of the diffracted radiation in the reflexes originating from different families of lattice planes. The lack of the plate effect of orientation in the case of PET fiber stretching has to do with the rod mechanism of the crystallite orientation. 

The crystallite orientation in PET fibers depends first and basically on the applicated draw ratio and sec ond on the stretching rate. The values off. characteristics for PET fibers as established by the authors are in Table 5. 

Table 5 Crystallites Orientation Function (fc) of Differently Drawn PET Fibers...

Such simple considerations led Scholten and Konvalinka to confirm the form of the dependence of the reaction velocity on the pressure, as had been observed in their experiments. Taking into account a more realistic situation, on the polycrystalline hydride surface with which a hydrogen molecule is dealing when colliding and subsequently being dissociatively adsorbed, we should assume rather a different probability of an encounter with a hydride center of a /3-phase lattice, an empty octahedral hole, or a free palladium atom—for every kind of crystallite orientation on the surface, even when it is represented, for the sake of simplicity, by only the three low index planes. 

The order in PA can be studied by WAXS and SAXS. WAXS gives insights into the unit cell sttucture, the crystallinity, the crystallite size (if not too large), and the crystallite orientation. SAXS gives a more accurate measure of the lamella thickness. In a temperature sweep, the changes in structure with temperature can be followed. 

The metal surface is polycrystalline and has a rather complex profile. Because of different crystallite orientations at the surface, different crystaf faces are exposed, such as smooth fow-index faces and stepped high-index faces. Surface texture where a particufar kind of face is predominant can devefop in individual cases. Microcracks and various lattice defects (dislocations, etc.) will also emerge at the surface. 

Stretched Polymers MF membranes may be made by stretching (Fig. 20-68). Semicrystalline polymers, if stretched perpendicular to the axis of crystallite orientation, may fracture in such a way as to make reproducible microchannels. Best known are Goretex produced from Teflon , and Celgard produced from polyolefin. Stretched polymers have unusually large fractions of open space, giving them very high fluxes in the microfiltration of gases, for example. Most such materials are very hydrophobic. 

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. 

Processes such as film extrusion, fiber spinning, injection molding, and drawing tend to impart orientation to products made from semicrystalline polymers. Mechanical, dielectric, and optical properties, to mention only three, are often strongly influenced by orientation. X-ray diffraction offers a direct approach to studying crystallite orientation because the Intensity that is diffracted into a detector placed at an appropriate position is directly proportional to the number of crystal lattice planes that are in the correct orientation for diffraction. The principles of such measurements are well described in textbooks 0,2). 

There is now available a substantial amount of information on the principles and techniques involved in preparing evaporated alloy films suitable for adsorption or catalytic work, although some preparative methods, e.g., vapor quenching, used in other research fields have not yet been adopted. Alloy films have been characterized with respect to bulk properties, e.g., uniformity of composition, phase separation, crystallite orientation, and surface areas have been measured. Direct quantitative measurements of surface composition have not been made on alloy films prepared for catalytic studies, but techniques, e.g., Auger electron spectroscopy, are available. 

Fig. 22 (a) 14N-13C SQ-HMQC spectrum of Ala Gly Gly tripeptide with natural abundance of 13C, taken at 14.1 T under 25 kHz MAS. The corresponding 15N MAS spectrum is shown in the top right comer. (Reproduced with permission from reference [258].) (b) l4N- H SQ-HMQC and DQ-HMQC spectra of partly deuterated glycine-d2 NH3+CD2COO, taken at 9.4 T under 30 kHz MAS. The 14N projections were simulated using second-order quadrupolar powder patterns with Cq = 1.18 MHz and i/q = 0.5, assuming uniform excitation of all crystallite orientations. (Reproduced with permission from [264])... 

Elod and Schmid-Bielenberg86 observed that the speed of acetylation of dry native fibers increases with decreasing degree of micellar (crystallite) orientation. Arranged in order of increasing reactivity the dry native fibers were flax, hemp, ramie and cotton. On being pretreated with water or acetic acid, however, the fibers were alike in rates of reac-... 

It is probable that varying degrees of ordering of chains exist in a cellulosic material and that a sharp differentiation of crystalline and non-crystalline celluloses may not be feasible or even possible. Theoretically, the lateral surfaces of crystallites are amorphous but may have far less importance in determining such properties as strength, flexibility and extensibility than the non-crystalline cellulose which supplies continuity of structure in the direction of crystallite orientation. Yet properties like moisture absorption and swelling may be more dependent upon the amount of cellulose which exceeds a certain degree of disorder (permeability) than upon location. The definition of crystallinity may, therefore, be made ultimately in terms of practical objectives. 

Fig. 1 Comparison of single-crystal and powder XRD measurements. In powder XRD, the diffraction phenomenon for each individual crystallite in the powder is the same as the diffraction phenomenon in single-crystal XRD. However, the powder comprises a large collection of crystallites with (in principle) a random distribution of crystallite orientations, and as a consequence, the three-dimensional diffraction data are effectively compressed into one dimension (intensity vs diffraction angle 26) in the powder XRD measurement...

Organomagnesium alkoxides and aryloxides have been utilized in only a few applications. Methyhnagnesium f-butoxide 68 has been used in the chemical vapor deposition of MgO films onto silicon substrates . MgO films with good crystallinity were grown at 800 °C on Si(lll) surfaces, whereas polycrystalline films were formed at 400 °C. Intermediate temperatures produced multiple crystallite orientations. Similar results were obtained for deposition onto Si(lOO) surfaces over this range of temperatures. 

Usually it is difficult to separate the effect of ciystallite size on carbon reactivity from the effects of crystallite orientation and impurity content. However, Armington (62) attempted to do so by reacting a series of graphi-tized carbon blacks with oxygen and carbon dioxide, as discus.sed earlier in this article. Assuming that upon graphitization all the carbon blacks are converted to polyhedral particles with the surface composed almost completely of basal plane structure, it is possible to eliminate crystallite orientation as a variable. Spectroscopically, the total impurity content of all the graphitized carbon blacks is quite low and to a first approximation, the analyses of the individual constituents are similar. 

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