Point defect: also structural consequences

Special Macromolecular Point Defects. End groups have a different chemical structure than that of the main-chain monomeric unit. They consequently produce a defect in the crystal lattice (see also Figure 5-21). 

The periodic arrangement of polymer chains in crystallites of semicrystalline polymers has been in the focus of early AFM investigations, in which known, but also unknown, crystal stmctures have been reported. Due to the local character of the experiment, ensemble averaging is avoided, unlike in conventional X-ray crystallographic methods. It is, however, important to note that most data published in the literature is contact mode AFM data, which represent, in most cases, lattice resolution only. One prominent example is the structure of poly(oxy methylene) unveiled by Snetivy and Vancso (Fig. 6.13) [24]. The information on the periodic arrangement of molecules is hence averaged over the length scale of the tip-sample contact. Consequently, point defects and other deviations in the periodic structure cannot be resolved. 

Some good papers have been published recently. Unfortunately the corresponding experimental data are most often lacking. The point-defect properties calculated from the electronic structure will have to be integrated in a proper thermodynamic theory. Such knowledge will also allow study in important fields that are practically unexplored up to now in intermetallic compounds point defect-impurity interaction, point defect-dislocation interaction, and consequences on the mechanical properties, etc. Considerable work is still required. 

Surface area is also directly proportional to the dissolution rate of a solute. Particle size reduction is another common and often efficient means by which to achieve higher levels of drug in solution at earlier time points.As particle size decreases, the surface area per unit volume of solute increases and consequently more drug is exposed to the solvent. Also, as particle size decreases the surface molecules are of higher free energy which increases dissolution. And finally, the processing of solid material can often lead to crystal defects within a particle or surface area where crystallinity is lost (amorphous), both of which can increase the apparent solubility. Mosharraf et al. have demonstrated the effect of crystal structure disorder on solubility and dissolution rate. ... 

It has already been pointed out that the undoped and acceptor-doped BaTi03 have essentially the same defect structure (see Figures 10.3c and/or 10.4b), in the oxygen partial pressure range that is experimentally viable in practice, namely —18 < log aoj < 0. Consequently, the equilibrium conductivities also have the same trend with oxygen activity, as shown in Figures 10.6a and b. Likewise, the relaxation kinetics appears also to be essentially the same [13, 16]. 

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

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