Perfect crystalline structure

It should be noted that dielectric and optical properties of the near-the-surface layer of a semiconductor, which vary in a certain manner under the action of electric field, depend also on the physicochemical conditions of the experiment and on the prehistory of the semiconductor sample. For example, Gavrilenko et al (1976) and Bondarenko et al. (1975) observed a strong effect of such surface treatment as ion bombardment and mechanical polishing on electroreflection spectra. The damaged layer, which arises in the electrode due to such treatments, has quite different electrooptic characteristics in comparison with the same semiconductor of a perfect crystalline structure (see also Tyagai and Snitko, 1980). 

As in any electrode process, the potential applied to the electrode determines the reaction rate. In electrodeposition, we expect that it affects the rate of deposition and thence the structure of the deposit a low overpotential signifies more time available to form an electrodeposit of perfectly crystalline structure. This can be observed experimentally (Fig. 15.7). Another factor arises from differences in current density between different parts of the electrode owing to electrode shape, which affects mass transport and thus accessibility to the cations to be deposited. Generally, it is best to apply a potential corresponding to the formation of poly crystalline deposits. A more perfect crystalline structure would be desirable, but the low rate of electrodeposition does not compensate for using such low overpotentials. 

In the perfect crystalline structure of ordinary (hexagonal) ice, each water molecule is H-bonded to four tetrahedrally... 

By the first decade of this century it was established that material failures occur at such low stress levels, because real materials do not usually have a perfect crystalline structure and almost always some vacancies, interstitials, dislocations and different sizes of thin microcracks (having linear structure and sharp edges) are present within the sample. Since the local stress near a sharp notch may rise to a level several orders of magnitude higher than that of the applied stress, the thin cracks in solids reduce the theoretical strength of materials by similar orders, and cause the material to break at low stress levels. The failure of such (brittle or ductile) materials was first identified by Inglis (1913) to be the stress concentrations occurring near the tips of the microcracks present within the sample. 

Usually, the formation of nanoparticles proceeds under non-equilibrium conditions, and the free energy of nanoparticles may exceed the energetic state of the perfect crystalline structure. The higher energy can lead to an atomic disordering in the PbS structure [4]. One possible disordering mechanism could be the occupation of octahedral interstitial sites in the fee subblattice with an additional occupation of tetrahedral interstitial places. 

Typically, a formation of nanoparticles proceeds under the non-equilibrium conditions, and the free energy of nanoparticles exceeds the energies of perfect crystalline structures. The higher free energy can provide disordering of CdS structure. One of possible disordering mechanisms leading to an increase of free... 

Ternary blends from a thermotropic liquid crystalline polymer, PEN, and PET were prepared by melt blending and melt spinning to fibers. The mechanical properties of ternary blend fibers could be significantly improved by annealing at 180°C for 2 h. This is attributed to the development of more ordered crystallites and to the formation of more perfect crystalline structures. The interfacial adhesion between PEN and liquid crystalline polymer phases is enhanced when the blends are processed with dibutyl-tindilaurate as a reactive catalyst to promote transesterification. 

The host system is treated as a perfect crystalline structure, and the exploitation of periodicity or quasi-periodicity is an essential ingredient when treating the defect as an impurity. From a quantum-mechanical point of view, the defect is treated as a perturbation to the electronic structure of the perfect crystal environment. 

Considerations of the intra- and inter-molecular arrangements lead to a spectrum of order/ disorder. At one extreme of this spectrum we have a perfect crystalline structure where both the conformation of individual chain molecules and their mutual packing are exactly defined. The corresponding atoms of the repeating units form a perfect three-dimensional lattice. Needless to say, only polymer chains with a regular chemical structure (with respect to both constitution and configuration) can form a true three-dimensional lattice such polymers are crystallizable. 

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