Pyramide of growth

Figure 5.4 Pyramids of growth on an Ag(lOO) face obtained by applying a short overvoltage pulse on an initially flat crystal face in the standard system Ag(100)/AgNO3 [5.7]. The pyramids mark the emergence points of the screw dislocations. The quadratic symmetry of the pyramids corresponds to the (100) nature of the face. Face areay4(ioo) = 2 x 10" cm. ...

Figure 5.35 Striped pyramids of growth on an Ag(lll) crystal face obtained by appUcation of a pulsating current in the standard system Ag(l 11)/AgNOs 5.9, 5.55. Face areaA(iu) = 2 x 10 cm. ...

Figure 5.36 Slope of pyramids of growth (tan a) as a function of overpotential l l obtained in the standard system Ag(100)/AgNO3 [5.9,5.72].

Figure 15. Pyramids of growth on an Ag (100) face (a) and on an Ag (111) face (b). A triangular stripe is visible on each of the triangular pyramids on the (111) face (b) propagating down to the base of the pyramids, produced by a short change of the overpotential during growth at constant overpotential. ITie photograph is made 1 sec after the pulse application.

It was established that transition from pits to pyramids of growth was conditioned by the slowest stage of the electrocrystallization process and was... 

When the charge-transfer step in an electrodeposition reaction is fast, the rate of growth of nuclei (crystallites) is determined by either of two steps (I) the lattice incorporation step or (2) the diffusion of electrodepositing ions into the nucleus (diffusion in the solution). We start with the first case. Four simple models of nuclei are usually considered (a) a two-dimensional (2D) cylinder, (b) a three-dimensional (3D) hemisphere, (c) a right-circular cone, and (d) a truncated four-sided pyramid (Fig. 7.2). 

Figure 6.2. The crystalline habit of lactose a-hydrate. (A) Prism, formed when velocity of growth is very high. (B) Prism, formed more slowly than prism A. (C) Diamond-shaped plates transition between prism and pyramid. (D) Pyramids resulting from an increase in the thickness of the diamond. (E) Tomahawk, a tall pyramid with bevel faces at the base. (F) Tomahawk, showing another face which sometimes appears. (G) The form most commonly decribed as fully developed. (H) A crystal having 13 faces. The face shown in F is not present. (I) A profile view of H with the tomahawk blade sharpened. (From van Krevald and Michaels 1965. Reprinted with permission of the Journal of Dairy Science 48(3), 259-265.)...

The course of MDMA s history has recapitulated to a considerable degree that of LSD. The "tum-on" this time began among chemists and psychiatrists after a preliminary announcement from the team of Shulgin and Nichols in 1978 in a volume titled The Psychopharmacology of Hallucinogens. There followed a chain-reaction of growth in its distribution, which, in this instance, even manifested in a Tupperware party like pyramid scheme for sales. 

Scheme I. Average a/c ratios of developing pyramid crystals with transition into cubes and schematic drawing of growth process in the gel spheres.

When they are grown at sufficient dilution, the crystallites approximate to lamellae with a uniform thickness of about 12 nm, the precise value depending on the temperature of growth. Electron diffraction shows that the chain axes are approximately perpendicular to the planes of the lamellae. The crystals are not exactly flat, but have a hollow-pyramidal structure, with the chain axes parallel to the pyramid axis. This pyramidal structure is seen clearly in fig. 5.5, which shows a single crystal of polyethylene floating in solution. This should be compared with fig. 5.3(b), which shows similar crystals flattened on an electron-microscope grid. The dark lines on the crystals in fig. 5.3(b) show where the pyramid has broken when the crystal flattened. 

Some of the sodium chloride crystals had pyramidally shaped depressions in their crystal faces. These so called hopper crystals were most frequently encountered in the experiments with Xomipa = 0.1. Other crystal faces contained holes, which seemed to be partly overgrown (see Figure 9). Hopper crystals are generally formed in situations where the volume diffusion of growth units to the crystal surface is rate limiting for... 

Figure 14. Morphology of growth of an Ag (100) face intersected by screw dislocations. Pyramids with different slopes can be recognized. Step bunching effects are observable at the left-hand side of the picture.

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