Surface crystallization dendritic

Non-covalent thermotropic liquid crystal dendritic systems have been achieved recently by converting the amphiphihc surface of the dendrimers (-NH2) into a hydrophobic shell (alkanoate chains). Tomalia et al. reported on the non-aqueous lyotropic behavior of supramolecular complexes re-... 

Undoubtedly, the number of particles of a precipitate experimentally measured gives little indication of the number of effective nucleation sites. Primary particles normally have large surface energy, dendritic crystals in particular having many... 

Fig. 27a-b represents the SEM micrographs taken from the surface and the cross-section of the 0.90Te02-0.10W03 sample heat-treated at 410 °C, above the first crystallization onset temperature, respectively. Fig. 27a exhibits the presence of dendritic leaf-like crystallites differently oriented on the surface. However, in the cross-sectional micrograph (see Fig. 27b), a typical amorphous structure without any crystallization on bulk structure can be clearly observed following the crystallites on the surface. Based on the SEM investigations, it was determined that the crystallites formed on the surface and did not diffuse into the bulk structure proving the surface crystallization mechanism (Qelikbilek et al., 2011). 

Apart from the dendritic growth of crystals according to the mechanism of controlled crystallization, dendritic growth has also been observed for the mechanism of surface crystallization. When dendritic growth occurred, however, it produced an undesirable, disordered and defective structure, rather than... 

Figure 3-8 SEM image showing dendritic surface crystallization of leucite in leucite-apatite glass-ceramics at 900°C, heat-treated for 1 h, etched for 10 sec with 3% HF (Haiand et al., 2000c).

Growth theories of surfaces have received considerable attention over the last sixty years as summarized by Laudise et al. [53] and Jackson [54]. The well-known model of the crystal surface incorporating adatoms, ledges and kinks was first introduced by Kossel [55] and Stranski [56]. Becker and Doring [57] calculated the rates of nucleation of new layers of atoms, and Papapetrou [58] investigated dendritic crystallization. 

Different ways of the structural classification of deposits exist. In one system, the following structures are distinguished arbitrarily (1) fine-crystalline deposits lacking orientation, (2) coarse-crystalline deposits poorly oriented, (3) compact textured deposits oriented in field direction (prismatic deposits), and (4) isolated crystals with a predominant orientation in the field direction (friable deposits, dendrites). The structure of metal deposits depends on a large number of factors solution composition, the impurities present in the solntion, the current density, surface pretreatment, and so on. 

Figure 1.55. The relationships between the concentration product, (Ba " )i(S04 )i, at the initiation of barite precipitation, and morphologies of barite crystals (Shikazono, 1994). The dashed line represents the boundary between dendritic barite crystals and well-formed rhombohedral, rectangular, and polyhedral barite crystals. The 150°C data are from Shikazono (1994) the others from other investigations. D dendritic (spindle-like, rodlike, star-like, cross-like) barite Dp feather-like dendritic barite W well-formed rectangular, rhombohedral, and polyhedral barite. The boundary between the diffusion-controlled mechanism (Di) and the surface reaction mechanism (S) for barite precipitation at 25°C estimated by Nielsen (1958) The solubility product for barite in 1 molal NaCl solution at 150°C based on data by Helgeson (1969) and Blount (1977). A-B The solubility product for barite in 1 molal NaCl solution from 25 to 150°C based on data by Helgeson (1969).

It is usually believed that the growth of dendritic crystals is controlled by a bulk diffusion-controlled process which is defined as a process controlled by a transportation of solute species by diffusion from the bulk of aqueous solution to the growing crystals (e.g., Strickland-Constable, 1968 Liu et al., 1976). The appearances of feather- and star-like dendritic shapes indicate that the concentrations of pertinent species (e.g., Ba +, SO ) in the solution are highest at the corners of crystals. The rectangular (orthorhombic) crystal forms are generated where the concentrations of solute species are approximately the same for all surfaces but it cannot be homogeneous when the consumption rate of solute is faster than the supply rate by diffusion (Nielsen, 1958). 

Nunner [ 1.104] photographed with a special cryomicroscope the change of the planar front of a 0.9 ck NaCl solution during directional freezing in 360 s to a stable dendritic ice structure (Fig. 1.37). The concentrated NaCl (dark border) ean be seen on the surface of the ice crystals. 

Figure 2. (a) Schematic description for the growth of dendrite crystals on a Li surface. The film consisting of decomposition products as shown in Scheme 1 prevents the growth of large granular crystals but rather promotes the formation of treelike dendrites, (b) Schematic description for the formation of isolated lithium particles from Li dendrites. The uneven dissolution of the dendrites leaves lithium crystals detached from the lithium substrate. The isolated lithium crystals become electrochemically dead but chemically reactive due to their high surface area. 

Barnes et al. (44) observed similar results on copper single-crystal surfaces near the (100) face below 10-mV ridges, 40- to 70-mV platelets, 70- to 100-mV blocks, and fine platelets and above 100 mV, polycrystaUine depKJsit. The four basic structural forms are shown in Figure 7.19. Less frequently observed growth forms are p5namids, spirals, whiskers, and dendrites. The structure of depKJsits is discussed further in Chapter 16. 

The experimental conditions and results of the analysis of the purity of separated benzene crystals are shown in Table 1. In tests of No. 1-1 to 1-4, and 3-1 and 3-2, the melt was compressed to the pressure shown in Table 1 and kept on the same value, without seed crystals. Nucleation occured on the wall and crystals grew there. In tests of No.2-1, 2-2 and 3-3, seed crystals were made as described above they grew both inside the optical cell and on the wall. In these tests, since the melt around benzene crystals was replaced by the water, the crystals were taken out without serious destruction. The shapes of benzene crystals were dendritic, and purity of it was over 99.9 mole percent, independent from the operational conditions and the feed compositions as shown in Table 1. Therefore, crystals obtained by high pressxire crystallization is considered to be very pure due to the complete removement of mother liquid from crystal surface. 

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