Phase property, structure sensitive

As in the case of the diffusion properties, the viscous properties of the molten salts and slags, which play an important role in the movement of bulk phases, are also very structure-sensitive, and will be referred to in specific examples. For example, the viscosity of liquid silicates are in the range 1-100 poise. The viscosities of molten metals are very similar from one metal to another, but the numerical value is usually in the range 1-10 centipoise. This range should be compared with the familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for the temperature dependence of the viscosity of liquids as an Arrhenius expression is... 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

To answer these questions requires some understanding of the properties of small metal particles, both structural and electronic. In this review we shall examine first the evidence relating to metal particles prepared by direct methods, e.g., vapour deposition or condensation in the gas phase. Then we shall consider whether this information can be applied to the case of supported metals where both precursor decomposition and support effects may add to the complexity of the total system. We shall then consider whether further changes in catalytic properties occur after preparation, i.e., during the catalytic reaction. Finally, we shall summarize some of the more recent evidence concerning the nature of structure sensitivity. 

Thus obtained results show that the polyamorphic transitions occur not only at compression (Si02, H20, etc.) but at extension as well (C) in the systems having stable or metastable crystal analogs with a different density and a different coordination number z. At the minimal z=2 (chain structures) the transitions may occurs only at compression, at the maximal z=12 (close-packed structures) - only at extension, at the intermediate z (2structure-sensitive properties change and new metastable phases can appear. Amorphization under radiation (crystal lattice extension) can be associated with a softening of phonon frequencies. The transitions in the molecular glasses consisted from the molecules with unsaturated bonds are accompanied by creation of atomic or polymeric amorphous systems. 

For bulk materials, all techniques based on structure-insensitive properties, as described in this section and elsewhere, yield closely similar data. The crystallinity model is thus a valid defect concept to describe structure-insensitive properties of semicrystalline polymers. It breaks down for three-phase systems, consisting, for example, of a crystalline phase, a mobile amorphous phase, and a rigid-amorphous fraction (see Chap. 6). In addition, one does not expect valid answers for structure-sensitive properties. 

The conundrum in designing membranes for high CO2 permeability is that the separation is enhanced by the solubility of CO2 in the polymer and that the polymer needs to be dissolved in solvents in order for the phase inversion process to occur and form the asymmetric structure. Properties that enhance CO2 performance also can make the structure sensitive to contaminants that over the long term can hurt performance and in worst case scenarios completely shut down performance. 

The interaction of liquid crystals with neighbor phases (gas, liquid, solid) is a very interesting problem relevant to their electrooptical behavior. The structure of liquid crystalline phases in close proximity to an interface is different from that in the bulk, and this surface structure changes boundary conditions and influences the behavior of a liquid crystal in bulky samples. The nematic phase is especially sensitive to external agents, in particular, to surface forces, and the majority of papers devoted to the surface properties of mesophases have been carried out on nematics. In addition, the nematic phase is of great importance from the point of view of applications in electrooptical devices. Thus, in this chapter, we will concentrate on surface properties of nematics, though the properties of the other phases will not be skipped either. 

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