Generation of Disorder

In this section brief consideration is given to mechanisms whereby colloidal metal may be produced in the small band gap azides by exposure to radiation. The mechanisms considered are those thought to be active in other inorganic crystals. No attempt is made here to be complete. 

In the silver halides Mott and Gurney suggested a mechanism for the formation of colloidal Ag [167]. A conduction-band electron produced by irradiation is first trapped at a lattice imperfection which may be a silver atom or ion, a chemical impurity, or a trapping site along a dislocation. The trapped electron then attracts a Ag interstitial ion to form a Ag atom. Following this, electrons and Ag interstitials are trapped at the site in proper sequence to cause the buildup of a colloidal silver particle. This mechanism requires the presence and mobility of silver ions, and it is further required that the hole motion be sufficiently small that trapped electrons are not annihilated by electron-hole recombinations. 

If this mechanism operates in the small band gap azides, interstitial metal ions must occur as intrinsic disorder in these materials, as in the silver halides, and be mobile at low temperatures, e.g., 12°K, because TIN3 and Pb(N3)2 can be photodecomposed at 12°K. While nothing is known about the properties of such defects in azides, it is unlikely that they are sufficiently mobile at such low temperatures. The silver halides are insensitive to coloration at low temperature because the silver ion is immobile. This mechanism could of course be operative at higher temperatures in the azides. 

Colloidal metal is produced in the alkali halides both by additive coloration and by irradiation [109]. In either case, the F center, an electron trapped at a negative ion vacancy, is the stable defect at room temperature. Clustering of F centers takes place during heat treatment or in some cases during irradiation. When a region consists almost exclusively of F centers, a coUapse of the lattice takes place and colloidal metal is formed. It is unlikely that colloids are formed in the small band gap azides in this manner. F centers have not been detected in these azides and are thus not dominant defects. In addition, to allow clustering, the mechanism requires F centers to be mobile at 12°K, which is unlikely. Colloids are not formed at low temperatures in the alkali halides, presumably because the F centers are not sufficiently mobile. 

Mechanisms requiring diffusion may nevertheless operate in the small band gap azides at higher temperatures. Colloidal metal has been observed in some of the alkali azides [26,42] however, irradiation and heat treatment are necessary in most cases to produce the colloids, indicating that diffusion is necessary. Since F centers have been observed in NaNs [17], it is possible that colloids are formed in the alkali azides by the clustering of F centers. Alternatively, alkali metal diffusion may take place. The properties of colloids in alkali azides are discussed elsewhere in this chapter (see Section C). 

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Figure 9.11. Generation of disorder in solids by a cascade of collisions (schematically).

In irreversible thermodynamics entropy was introduced and discussed as a result of a need for quantification of the degree of irreversibility of a process. The theory thus explain the way in which the generation of disorder reflected by entropy change results in conversion of potentially useful work energy into practically useless thermal energy. In this discipline of thermodynamics the continuum balance equation for entropy plays a central role [32]. This equation expresses the fact that the entropy of a volume element changes with time for... 

Generation of Disordered Phase in Isothermal Crystallization of Polyethylene We perform a similar experiment for PE. Figure 5.17 shows the time-resolved measurement of infrared spectra in the isothermal crystallization process from the melt, where we use a linear low-density PE with 17 ethyl branches per... 

The EXAFS technique is used primarily for investigations of disordered materials and amorphous solids. Figure 8.35(b) shows how interference occurs between the wave associated with a photoelectron generated on atom A and the waves scattered by nearest-neighbour atoms B in a crystalline material. 

The path from squalene (114) to the corresponding oxide and thence to lanosterol [79-63-0] (126), C qH qO, cholesterol [57-88-5] (127), and cycloartenol [469-38-5] (128) (Fig. 6) has been demonstrated in nonphotosynthetic organisms. It has not yet been demonstrated that there is an obligatory path paralleling the one known for generation of plant sterols despite the obvious stmctural relationships of, for example, cycloartenol (128), C qH qO, to cyclobuxine-D (129), C25H42N2O. The latter, obtained from the leaves of Buxus sempervirens E., has apparentiy found use medicinally for many disorders, from skin and venereal diseases to treatment of malaria and tuberculosis. In addition to cyclobuxine-D [2241-90-9] (129) from the Buxaceae, steroidal alkaloids are also found in the Solanaceae, Apocynaceae, and LiUaceae. 

Defining T(n) to be the density of triangular clearings with base length n , it has been found that, for large n and for patterns generated from disordered initial states, T(n) behaves as... 

Reactive compatibilization can also be accomplished by co-vulcanization at the interface of the component particles resulting in obliteration of phase boundary. For example, when cA-polybutadiene is blended with SBR (23.5% styrene), the two glass transition temperatures merge into one after vulcanization. Co-vulcanization may take place in two steps, namely generation of a block or graft copolymer during vulcanization at the phase interface and compatibilization of the components by thickening of the interface. However, this can only happen if the temperature of co-vulcanization is above the order-disorder transition and is between the upper and lower critical solution temperature (LCST) of the blend [20]. 

Charney, D. S. and Drevets, W. C. The neurobiological basis of anxiety disorders. In Neuropsychopharmacology the Fifth Generation of Progress. Ed. Nemeroff, C. Philadelphia Lippincott Williams Wilkins, 2002, pp901-930. 

This implies a modification of the chain direction and, finally, a generation of a helix with a larger cross section than that of the trans planar chain.218 Such phase transitions are often associated with the onset of some conformational disorder characterized by the occurrence of kink bands (Figure 2.48b), similar to those found in sPP samples quench crystallized from solutions189,190 (Figures 2.46 and 2.47) or in polyethylene.219 It is apparent from Figure 2.48... 

Some disorders are seen in more than one generation of a family. 

Two mutated copies of the gene are present in each cell when a person has an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers). Autosomal recessive disorders are typically not seen in every generation of an affected family. 

This type of inheritance, also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondria, which are structures in each cell that convert molecules into energy, each contain a small amount of DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial conditions to their children. Mitochondrial disorders can appear in every generation of a family and can affect both males and females, but fathers do not pass mitochondrial traits to their children. 

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