Crystals near the surface

Improvement of scratch resistance is one more positive effect of improved crystalline stracture by nucleation." The ultra-high injection speed resulted in the highest surface strength and scratch resistance as compared with lower injection speeds. The high scratch resistance was related to the presence of highly oriented molecules and crystals and the increase in the amoimt of the P-phase crystals near the surface, which were formed at high injection speeds. Crystalhnity and the effective number of entanglements increase scratch resistance of polymer. In addition to the effect of nucleation, there may be other resons for improvement of the scratch resistance of polymeric materials, which include reinforcement of surface layers and reduction of friction coefficient and fiiction wear of the surface. 

Since there is an excess of halide in the solution phase of a photographic emulsion, resulting in a depletion of Agf close to the surface of the crystal, the formation of the latent image near the surface is impeded relative to the interior of the crystal, as has been discussed in section 9.3.2. However, in the adsorption layer of the crystals, very mobile silver ions are expected to exist, which can, at least in part, counteract the depletion in the crystal near the surface. 

The intensity of the trunction rods is influenced by the structure of the crystal near the surface as well as that of the overlayer, or fluid near the electrode. The measurement of the truncation rods permits the determination of the structure of the adsorbed atoms relative to the positions of the substrate atoms, and also the rough features of the density distribution above the electrode. 

Face-centered cubic crystals of rare gases are a useful model system due to the simplicity of their interactions. Lattice sites are occupied by atoms interacting via a simple van der Waals potential with no orientation effects. The principal problem is to calculate the net energy of interaction across a plane, such as the one indicated by the dotted line in Fig. VII-4. In other words, as was the case with diamond, the surface energy at 0 K is essentially the excess potential energy of the molecules near the surface. 

There are two basic questions which can be decided only by experiments. First, we must know whether the metal or the oxygen is present in excess, and second, we must know how the excess component is incorporated in the oxide lattice. In connection with the latter question we have to remember that a non-stoichiometric crystal remains electrically neutral (except in narrow regions near the surfaces), so that if the excess component is present in the crystal as ions, lattice defects with charges of opposite sign must necessarily be present also (see Figs. 1.77 and 1.78). The most important defect structures will be discussed in this section. 

The crystallization of the D-enantiomer is therefore considered to be induced by crystal growth on the surface of the seed crystal and at the same time initial breeding may play a role that causes small crystals near the seed crystal. The propagation of nucleation in distance from the seed may be caused by convective flow of the solution due to density difference during the crystal growth. 

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

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