Diffraction measurements

X-ray diffraction measurements may be made on the fibrous material. Although the method is limited in application, it does enable definite statements to be made about the minimal length of the crystalline fiber component. Thus the minimal length of the anhydro-n-glucose polymer chain in ramie was found to be about 1000 A, and in wood, about 600 A. From these data it may be calculated that the degree of polymerization is greater than 200,and indeed may be found to be the approximate length of the basic molecular unit.  

Crystals with high symmetry allow for a collection of complete data sets within the accessible angle range in many cases. For compounds with low symmetry, the crystal can be re-oriented and re-measured after the first data collection. Since this process is in many cases fatal for the investigated specimen, it is often preferred to perform the least-squares refinements with a smaller number of reflections and, correspondingly, a reduced number of 

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Leadbetter A J and Norris E K 1979 Distribution functions in hree liquid crystals from x-ray diffraction measurements Moiec. Phys. 38 669-86... 

Amorphous silica, ie, silicon dioxide [7631-86-9] Si02, does not have a crystalline stmcture as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quart2 at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Structural data of a diaziridine come from gas phase electron diffraction measurements (74CC397). The N—N bond of 3-methyldiaziridine (24) is longer than in hydrazine (1.449 A) the C—N bond distances in (24) and in diazirine are nearly equal (1.479 versus 1.482 A),... 

Figure 4 Comparison of average distances from the bilayer center along the bilayer normal for deuterated methyl and methylene groups distributed throughout the DPPC molecule computed from constant-pressure MD calculations and neutron diffraction measurements on gel and liquid crystalline phase DPPC bilayers.

The simplest diffraction measurement is the determination of the surface or overlayer unit mesh size and shape. This can be performed by inspection of the diffraction pattern at any energy of the incident beam (see Figure 4). The determination is simplest if the electron beam is incident normal to the surface, because the symmetry of the pattern is then preserved. The diffraction pattern determines only the size and shape of the unit mesh. The positions of atoms in the surface cannot be determined from visual inspection of the diffraction pattern, but must be obtained from an analysis of the intensities of the diffracted beams. Generally, the intensity in a diffracted beam is measured as a fimction of the incident-beam energy at several diffraction geometries. These intensity-versus-energy curves are then compared to model calculations. ... 

It is difficult to attribute the capacity reduction which occurs above 700°C to structural effects because the structure of the samples is not changed significantly, as indicated by our X-ray diffraction measurements on the CRO samples. On the other hand, the hydrogen content of the samples is dramatically reduced over this temperature range. To investigate the importance of the hydrogen content, we made a series of cells from the other samples. 

Powder X-ray diffraction and SAXS were employed here to explore the microstructure of hard carbon samples with high capacities. Powder X-ray diffraction measurements were made on all the samples listed in Table 4. We concentrate here on sample BrlOOO, shown in Fig. 27. A weak and broad (002) Bragg peak (near 22°) is observed. Well formed (100) (at about 43.3°) and (110) (near 80°) peaks are also seen. The sample is predominantly made up of graphene sheets with a lateral extension of about 20-30A (referring to Table 2, applying the Scherrer equation to the (100) peaks). These layers are not stacked in a parallel fashion, and therefore, there must be small pores or voids between them. We used SAXS to probe these pores. 

The allyl radical would be expected to be planar in order to maximize n delocalization. Molecular structure parameters have been obtained from EPR, IR, and electron diffraction measurements and confirm that the radical is planar. ... 

As in other shock-modified powders, the x-ray diffraction measurements showed large values of residual strain resulting from extensive plastic defor-... 

The hydrogen treatment procedures, tensile, compression and torsion tests at fixed temperatures, transmission electron or optical microscopy at room temperature as well as X-ray diffraction measurements were detailed elsewhere All experiments were performed so as to compare properties of the same alloy, but modified using different treatment procedures. 

This mechanism is also confirmed by X-ray diffraction measurements [94, 100]. It is also mentioned that the absence of reflexes belonging to Li3NbC>4, in X-ray diffraction patterns obtained for mixtures treated at relatively low temperatures, could be explained by the formation of an amorphous material at the very beginning of the process [103]. 

In theory, electron diffraction measurements on gaseous molecules are somewhat sensitive to the thermal motions of the atoms,... 

X-Ray and electron diffraction measurements have been most usually used to characterize the phases present in any reactant mixture, and provide a means of identification of solid reactants, intermediates and products. In addition to such qualitative analyses, the method can also be used quantitatively, with suitable systems, to determine the amounts of particular solids present [111], changes in lattice parameters during reaction, topotactical relationships between reactants and products, the presence of finely divided or strained material, crystallographic transformations, etc. 

Direct kinetic measurements from the changes in diffracted beam intensities with time during heating of the reactant are illustrated in the work of Haber et al. [255]. Gam [126] has reviewed the apparatus used to obtain X-ray diffraction measurements in thermal analysis. Wiedemann [256] has designed equipment capable of giving simultaneous thermo-gravimetric and X-ray data under high vacuum. X-Ray diffraction studies enable the presence, or absence, of topotactic relationships between reactant and product to be detected [92,102,257—260], Results are sometimes considered with reference to the pseudomorphic shape of residual crystallites. 

Many studies have been made of the rates of water evolution from layer-type silicate minerals which contain structural hydroxyl groups (clays and micas). Variations in composition of mineral specimens from different sources hinders comparison of the results of different workers. Furthermore, the small crystallite sizes and poor crystallinity that are features of clays limit and sometimes prevent the collection of ancillary observations (e.g. microscopic examination and diffraction measurements). 

Baranowski [680] concluded that the decomposition of nickel hydride was rate-limited by a volume diffusion process the first-order equation [eqn. (15)] was obeyed and E = 56 kJ mole-1. Later, Pielaszek [681], using volumetric and X-ray diffraction measurements, concluded from observations of the effect of copper deposited at dislocations that transportation was not restricted to imperfect zones of the crystal but also occurred by diffusion from non-defective regions. The role of nickel hydride in catalytic processes has been reviewed [663]. 

No matter how thorough, a kinetic study does not really determine a mechanism in the same sense that single-crystal X-ray diffraction measurements determine a structure. The reaction mechanism is but a scientific postulate that is open to revision when new data, new insights, or new theories of reactivity emerge. 

Indications are that a C—D bond is slightly shorter than a corresponding C—H bond. Thus, electron-diffraction measurements of C2H6 and showed a C—H bond distance of 1.1122 0.0012 A and a C—D distance of 1.1071 0.0012... 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

Structure of Complexes in Solution Derived from X-Ray Diffraction Measurements Georg Johansson... 

The TEM images of 12 wt.% Co/MgO calcined at 873 K (Catalyst I) before and after reduction are shown in Fig. 1 (a) and (b), respectively. Although Co metal phase was detected in reduced Co/MgO by X-ray diffraction measurements (XRD) [7, 8], no Co metal particle was observed on both catalysts. EDS elemental analysis showed that primary particles contain both Mg and Co elements, whose concentrations were about the same as loaded amounts. Figure 2 shows TEM image of 12 wt.% Co/MgO calcined at 1173 K (Catalyst II). 

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