Crystallization effects of orientation

Gupta [28] presents results on the effect of crystal orientation on shock propagation in LiF crystals. This work supports earlier studies and shows... 

The main effect of crystal orientation is caused by different barrier heights on different crystal faces. It is well known that Volta-potential differences are dependent on crystal orientation because the surface dipole differs for different faces. In the case of a semiconductor electrode this means that the flat band potential which can be determined experimentally depends... 

J.R. Ligenza. Effect of crystal orientation on oxidation rates of silicon in high pressure steam // J.Phys.Chem.- 1961.- V.65, No. 11.- P.2011-2014. 

The occurrence of different oxide orientations on different crystal faces is one of the most striking illustrations of the effect of crystal orientation on oxide growth. The determination of these epitaxial relationships between oxide and metal are important for an understanding of the mechanism of oxidation. Although numerous studies of die epitaxy of oxides on over a dozen different metals have been made (17-20,23,29,34-71), only five of diem have included more than a few major faces (18.34,35, 38, 53). 

Dick, J. J. (1984) "Effect of crystal orientation on shock initiation sensitivity of pentaerythritol tetranitrate explosive, Appl. Phys. Lett., 44, 859-861. 

The main problem in SIMS is quantification, because of the dependence of relative and absolute secondary-ion yields on matrix effects, on surface coverage by reactive elements (oxygen for instance), on background pressure in the sample chamber, on the effect of crystal orientation with respect to the directions of the primary- and secondary-ion beams, singular effects, etc. (see also Chapter 6). 

Effect of Molecular Orientation on Crystallization of Flexible-Chain Polymers. 217... 

The outer most levels in C60 are due to rc orbitals . These are formed by 2p electrons which have their orbitals oriented along the radius of the molecule. The different environment inside and outside the spherical molecule causes the double-peaked structure in the momentum densities. In graphite the n band is formed by 2p orbitals oriented perpendicular to the sheets of carbon atoms. Using single-crystal graphite films we have a unique opportunity to study the effects of the orientation of these 2p orbitals in detail. 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

Single crystals are of critical importance to the study of complex solids such as the new high temperature superconductors. Single crystals are free of grain boundary effects and allow measurements of physical properties as a function of crystal orientation. 

In bulk material, the resistivity is independent of crystal orientation because silicon is cubic. However, if the carriers are constrained to travel in a very thin sheet, eg, in an inversion layer, the mobility, and thus the resistivity, become anisotropic (18). Mobility is also sensitive to both hydrostatic pressure and uniaxial tension and compression, which gives rise to a substantial piezoresistive effect. Because of crystal symmetry, however, there is no piezoelectric effect. The resistivity gradually decreases as hydrostatic pressure is increased, and then abrupdy drops several orders of magnitude at ca 11 GPa (160,000 psi), where a phase transformation occurs and silicon becomes a metal (35). The longitudinal piezoresistive coefficient varies with the direction of stress, the impurity concentration, and the temperature. At about 25°C, given stress in a (100) direction and resistivities of a few hundredths of an O-cm, the coefficient values are 500—600 m2/N (50—60 cm2/dyn). 

We conclude from this discussion that a very complex correlation between structure and photoelectrochemical behavior is to be expected and it will often be difficult to decide what may be the main influence. The following examples are selected under the aspect to demonstrate some effects of surface orientation and crystal imperfections in systems where they are very pronounced. Materials with a large anisotropy of the crystal properties are the best candidates for this purpose. Therefore semiconductors with layer structure which have been introduced into photoelectrochemical studies by Tributsch (11,12,13) are predominantly used as examples. 

A growing number of reports are appearxng concerning %<3) of materials as determined by THG experiments. Among organics a variety of poly(diacetylenes), e. g., 4-BCMU (4-butoxycarbonylmethylurethane polydiacetylene) (15,109) have been studied as pure materials and as LB films, 110) Crystalline films of poly(4-BCMU) in the red form, were found to have higher %<3) than amorphous films, attributed to the orientational effect of crystallization.(109) The blue form also has been studied. 111) Maximum values of %<3) reported to date are 1 x 10 10 esu at 1.3 l. Poly (phenylacetylene) THG at 1.06 lhas recently been measured. 111) The 2- and 3-photon resonance enhanced value of %<3) determined is 7 x 10 12 esu. 

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