Noncrystalline materials

Toby B FI and Egami T 1992 Accuracy of pair distribution function analysis applied to crystalline and noncrystalline materials Aota Crystaiiogr.k 48 336-46... 

The description of the atomic distribution in noncrystalline materials employs a distribution function, (r), which corresponds to the probability of finding another atom at a distance r from the origin atom taken as the point r = 0. In a system having an average number density p = N/V, the probability of finding another atom at a distance r from an origin atom corresponds to Pq ( ). Whereas the information given by (r), which is called the pair distribution function, is only one-dimensional, it is quantitative information on the noncrystalline systems and as such is one of the most important pieces of information in the study of noncrystalline materials. The interatomic distances cannot be smaller than the atomic core diameters, so = 0. 

Benzamidomethyl-A -methylcysteine has been prepared as a crystalline derivative (H0CH2NHC0C6H 5, anhydr. CF3CO2H, 25°, 45 min, 88% yield as the tri-fluoroacetate salt) and cleaved (100% yield) by treatment with mercury(II) acetate (pH 4, 25°, 1 h) followed by hydrogen sulfide. Attempted preparation of S-acetamidomethyl-N-methylcysteine resulted in noncrystalline material, shown by TLC to be a mixture. ... 

Methyl-, 2-isopropyl-, 2,3-dimethyl-, 2-methyl-3-ethyl-, and 3-methyl-2-ethyl pyrrole all form crystalline salts of the corresponding dimers with dry HCl or picric acid in ether variable quantities of noncrystalline material are also produced. The dimers are quite stable even as the free bases. 

The development of the internal orientation in formation in the fiber of a specific directional system, arranged relative to the fiber axis, of structural elements takes place as a result of fiber stretching in the production process. The orientation system of structural elements being formed is characterized by a rotational symmetry of the spatial location of structural elements in relation to the fiber axis. Depending on the type of structural elements being taken into account, we can speak of crystalline, amorphous, or overall orientation. The first case has to do with the orientation of crystallites, the second—with the orientation of segments of molecules occurring in the noncrystalline material, and the third—with all kinds of structural constitutive elements. 

Restricted access phases are another approach to exploiting the differences in characteristics of analytes. Large analytes are excluded from an internal surface on which an adsorptive stationary phase is present. A herbicide analysis for Metsulfuron methyl, Bentazone, Bromoxynil, methylchlorophenoxy acid, and Mecoprop in the presence of humic acid was performed on restricted access reversed phase media.52 The cytostatic compound epirubicin and its metabolites were separated from plasma using a Pinkerton GFF II column.53 Gradient separations of polymers on reversed phase and on normal phase represent an alternative to gel permeation chromatography. Polyesters of noncrystalline materials were separated on a variety of such phases.54... 

Improve the tools for imaging and determining structure so that detailed chemical structures can be determined with tiny amounts of noncrystalline material. 

When considering reduction of particle size, one should be aware of potential instability enhancement. It is well known that grinding can reduce crystallinity [65] the resulting noncrystalline material may be more hygroscopic... 

Although rapid solidification may not produce a truly amorphous (noncrystalline) material for some alloy compositions, crystallite sizes of rapidly solidified crystalline... 

In crystalline semiconductors, the most common technique for the measurement of carrier mobility involves the Hall effect. However, in noncrystalline materials, experimental data are both fragmentary and anomalous (see, for example. Ref. [5]). Measured HaU mobility is typically of the order of 10 - 10 cm A /s and is frequently found to exhibit an anomalous sign reversal with respect to other properties providing information concerning the dominant charge carrier. Thus, apart from some theoretical interest, the Hall effect measurements are of minimal value in the study of macroscopic transport in amorphous semiconductors. 

These effects are described as weak localization . Experimental evidence for the effects predicted for amorphous metals is described in Chapter 10. It will be seen that the effects are only significant when l is small, and thus typically for noncrystalline materials. It is remarkable that, as a consequence of (51), inelastic scattering increases the conductivity. 

The measurements are, however, always difficult to interpret with certainty, since the true area of the adsorbing surface, which determines the number of molecules of the adsorbent actually exposed to the gas, is represented at best roughly by the apparent area, and often bears no relation to it. This is especially marked in the case of noncrystalline materials such as glass. 

Microscopic and mechanistic aspects of diffusion are treated in Chapters 7-10. An expression for the basic jump rate of an atom (or molecule) in a condensed system is obtained and various aspects of the displacements of migrating particles are described (Chapter 7). Discussions are then given of atomistic models for diffusivities and diffusion in bulk crystalline materials (Chapter 8), along line and planar imperfections in crystalline materials (Chapter 9), and in bulk noncrystalline materials (Chapter 10). 

Noncrystalline materials exist in many different forms. A huge variety of atomic and molecular structures, ranging from liquids to simple monatomic amorphous structures to network glasses to dense long-chain polymers, are often complex and difficult to describe. Diffusion in such materials occurs by a correspondingly wide variety of mechanisms, and is, in general, considerably more difficult to analyze quantitatively than is diffusion in crystals. 

The understanding of diffusion in many noncrystalline materials has lagged behind the understanding of diffusion in crystalline material, and a unified treatment of diffusion in noncrystalline materials is impossible because of its wide range of mechanisms and phenomena. In many cases, basic mechanisms are still controversial or even unknown. We therefore focus on selected cases, although some of the models discussed are still under development and not yet firmly established. 

We have striven to develop the subject in a systematic manner designed to provide readers with an appreciation of its analytic foundations and, in many cases, the approximations commonly employed in the field. We provide many extensive derivations of important results to help remove any mystery about their origins. Most attention is paid throughout to kinetic phenomena in crystalline materials this reflects the interests and biases of the authors. However, selected phenomena in noncrystalline materials are also discussed and, in many cases, the principles involved apply across the board. We hope that with the knowledge gained from this book, students will be equipped to tackle topics that we have not addressed. The book therefore fills a significant gap, as no other currently available text covers a similarly wide range of topics. 

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

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