Conical structure

The secondary and ternary islands will keep growing in approximately concentric fashion, thereby producing a conical structure above the original nucleation centers. This process of kinetic roughening supported by the Schwoebel effect makes a rather bumpy surface structure. Looking finally at a vicinal surface, this will grow rather smoothly when the width of the terraces is smaller than the typical distance between nucleation centers 4i (see below), and becomes bumpy in the opposite case [12,93]. 

Teeth, the hard conical structures embedded in their jaws that vertebrate animals use to chew food, consist of two layers of compact matter surrounding a core of soft, living tissue. The inner layer is composed of dentine, also known as ivory, whose composition is similar, but not identical, to that of bone it contains less collagen and is harder than bone. The thin outer layer of the teeth, the teeth s enamel, includes even less collagen and other organic matter than dentine and is the hardest substance produced by animals (Hilson 1986a Kurten 1986b 1982). 

Simple artificial examples of sinusoidal, helical and cycloid magnetic structures are given in Figure 3.4. In Figure 3.5 there is a real example of the incommensurate conical structure of DyMugGcg propagation vector ki = (0,0,0) and k2 = (0,0,5) at the interior of Brillouin Zone. 

Figure 3.5 Magnetic structure of DyMn6Ge6, space group F6lmmm, a 5.2 A, c 8.15A, propagation vectors ki = (0,0,0) and k2 = (0,0, ) with = 0.1651. This is a conical structure with a net magnetization along c. Details can be found in ref. 18.

Cauliflower structures and conical structures are usually observed from the CVD processes as shown in Figure 6.13. Generally speaking, the former is formed by a competition between randomly branching structures, and the conical-like column is the result of continuous and inherent competition in the evolution of the... 

Figure 6.13. Fractal structures in CVD deposits (a) cauliflower structure and (b) conical structure [28]...

Figure 5.9 Magnetic structures with net moment along the [11 l]-axis (a) and [001]-axis (b). The projections of the moments onto the plane perpendicular to these axes and the corresponding conical structures are shown. Reproduced from Knopfle etal. (2000). (2000), with permission from Elsevier Science.

Cyclodextrins are included here as water-soluble supports. They will also be mentioned in Chap. 7 on separations and in Chap. 8 on running reactions in water instead of organic solvents. Cyclodextrins221 are made by the enzymatic modification of starch.222 They are made commercially by Cer-estar and Wacker Chemie. They have conical structures of six, seven, and eight glucose units in rings, denoted a- /3-, and y-cyclodextrin, respectively. Mercian Corporation has a process for /3-cyclodextrin, which is more selective than usual, that produces no a- and only a small amount of the y-product.223 The insides are apolar and hydrophobic, whereas the outsides are hydrophilic (Table 5.2). 

The appearance of the craters obtained at low fluences varies partly from the morphologies of the craters formed at high fluences. Figure 6 shows the craters created on TM2 at fluences of 69 and 51 mj cm 2, respectively. Conical structures were observed in both cases at the bottom of the circular cra-... 

This is indeed close to reality as actually hemispheric caps are observed rather seldom. In most cases conical structures are found. These bear defects at the tip itself as well as at the transition to the nanotube. They aU have in common that the required curvature is brought about by pentagonal defects. The bevel angle of such conical caps depends on the number of five-membered rings present at their tips, as demonstrated by the values in Table 3.1. A cap with six pentagonal defects, for instance a hemispheric fullerene fragment, enables a direct transition to the tubular part, while a smaller number of five-membered rings causes a more or less conical shape. 

The ways of tube termination and further defects like kinks are another essential aspect of MWNT structure. Like in the single-walled tubes, five-membered rings are crucial to the formation of these domains. As for the asymmetric conical structure, a single pentagon defect at the end of the innermost tube is presumed to cause the generation of such a cap. Sometimes the inner tubes end at a considerable distance from the actual tip. This gives rise to larger voids. 

Carbon nanofibers are filamentous cylindrical or conical structures formed of various arrangements of stacked graphene sheets (as cones or cups), with a diameter ranging from one to several hundred nanometers, and lengths ranging from 1 pm to several millimeters. Nanostructured carbon foams... 

The MWNT end caps usually have quite varied forms Fig. 7, including the form of a cone with an angle of 20 degrees. In our investigations of carbon nanostructures obtained by the arc discharge method (39) we came across conical structures on one side, as well as structures conical on both sides (Fig.8). As these were not found to have any cylindrical segments they represent a new type of structure that essentially should be labeled as conical. Due to the specifics characteristic for the arc discharge method these structures are multilayer. 

During 1999 the research group of S. lijima (40), (41) by laser ablation of graphite without a catalyst, obtained single wall conical structures with a product yield of 50 g/h and a purity of 70-80%, sufficiently pure and not requiring further purification. The spatial angle of the conical surface is 20 degrees. In the literature these structures are termed nanohoms (SWNH). Sketches imitated closed (a) and open (b) SWNH are shown in Fig. 9. 

For the general focal conic structure, Kleman has shown that tfcii(l-e ) ln(a/r,),... 

When the isotropic melt is cooled, the smectic phase often makes its appearance in the form of needle-shaped particles showing evidence of focal conic structure. Examples of such needles, termed bdtonnets, can be seen in fig. 2.. 5 b). 

Smectic phases show a number of characteristic textures including (1) the focal conic and fan texture, characteristic of smectic A, and often formed from the coalescence of batoimets (2) the mosaic texture observed when the smectic B phase is formed and (3) the broken focal conic structure resulting from smectic C phases that can also show the Schheren patten described earho-. Examples of focal conic textnres arc shown in Figure 11.13. 

Holmium orders magnetically at 133 K to form a bassd plane spiral structure with an interplanar turn angle decreasing monotonically from 50° at Tn to 36° at 35 K, below which it drops steeply to 30° at Tc = 19 K. The neutron diffraction study of Koehler et al. (1966) revealed, below Tc, a ferromagnetic c-axis moment of IJ /ib - the moments lift out of the plane by some 10° to form a conical structure. 

Fig. 8-12. Double-walled conical structure uses either foam or ribs to effect stiffening separation and linking of walls.

Fig. 21 Amphiphiles with conical structures (a) form structures where the curvature of packing results in the formation of a spherical supramolecular ehtity (b), which self-organises to give a simple cubic mesophase (c).

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