Onion structure

Figure 10. Depiction of the (a) Onion structure, the (b) Radial structure, and (c) Actual product MCMB, [16],...

Nanofiber structures, which appear to grow spontaneously from carbon vapor under the appropriate experimental conditions, have also been identified (Figure 9(b)).They consist of concentric cylinders which are closed at the ends by capping groups similar in structure to one half of the onion structures described previously. Under specific experimental conditions (02/molten Pb) some endcaps appear to open, and the lead atoms flow inside the tubes. ... 

Very important factors in LCVD are (1) the location of the critically important layer, i.e., the dissociation glow, in a glow discharge, and (2) the location of the substrate with respect to the onion layer structure, i.e., in which layer of an onion structure the substrate is placed. The location of the critical layer depends on what kind of discharge system is employed to create a luminous gas phase. In a strict sense, it is impossible to uniformly coat a substrate placed in a fixed position in a reactor, and the relative motion of a substrate to the onion layer structure of luminous gas phase is a mandatory requirement if high uniformity of coating is required. 

Adaptability of an LCVD process in an industrial scale operation greatly depends on the nature of the onion structure of the luminous gas phase that could be accommodated in the operation. The change of reactor size inevitably changes the basic onion layer structure of the luminous gas phase, which constitutes the main (often insurmountable) difficulty in the scale-up attempt by increasing the size of reactor. (The scale-up principle is discussed in Chapter 19.)... 

In DC discharge for LCVD, the main core is the DG adhering to the cathode surface, and the anode is out of the onion structure in most cases. In DC discharge of Ar for glow discharge treatment or sputter deposition of the cathode material, the core is the IG, which does not touch the cathode surface. 

In alternating current discharge for LCVD, up to about 100 kHz, the DG adhering to the electrode surface (in the cathodic cycle) is the core. The discharge system has two cores, and the interelectrode space is filled with two onion structures overlapping in part, which approach each other when the value of Wjpd increases. 

Figure 35. Schematic representation of the Ru(bpy)3 /a-ZrP/viologen onion structure grown on colloidal silica particles. The sequence of fast (1, 2) and slow (3) electron-transfer steps that follow photoexcitation of Ru(Me-vpy)(bpy)2 " polymer is shown. Reproduced from D.M. Kaschak, S.A. Johnson, C.C. Waraksa, J. Pogue and T.E. Mallouk, Coord. Chem. Rev. 1999, 185-186, 403, with permission from Elsevier Science.

Scheme 10.3 Fabrication of drug-loaded pH-responsive three-layered onion-structured nanoparticles (3LNPs) via pH-controUed hierarchical self-assembly...

Zhan, Y Van Kirk, E. Xu, P Murdoch, W. J. Radosz, M. Shen, Y. pH-responsive three layer onion-structured nanoparticles for drug delivery. Polym. Mater. Sci. Eng. Prepr. 2006, 94, 139-140. 

In general, these methods are used for the production of nanocrystalline powders which may be further compacted via techniques such as hot-pressing [157, 158] or magnetic pulsed compaction [159, 160]. In addition, other types of nanoionic material maybe prepared, such as nanometer-thin films, using techniques including molecular beam epitaxy [161], pulsed laser deposition [162] or spin-coating methods [163]. Novel structures, such as core-shell [164—166] and multi-layered [167, 168] (so-called onion structures) materials, may also be produced in this way. 

Figure 4.31 The accretion of curved graphitic structures and of annealed aromatic compounds also leads to a formation of carbon onions. The inner shells are not always completely closed, which explains for defects of the onion structure ( ACS 1986).

The additional band at 1572 cm can be assigned to the onion structure. It originates from the in-plane vibration of the six-membered rings (E2g-mode) of the... 

Figure 4.42 Operation principle of carbon onions in tribological applications. The lubricating effect is conserved even upon destruction of the onion structure.

Little is known so far about the chemical properties, yet first results suggest a reactivity similar to that of multiwalled carbon nanotubes. Furthermore, a transformation of nano-onions into other forms of carbon can be achieved by heating (equihbration as faceted nanoparticles) or electron bombardment. In large carbon onions, a formation of small diamond clusters due to internal self-compression has been observed. These grow up to be nanoscale diamond particles under complete consumption of the onion structure. 

In order to probe the microstructure of onion structures containing polyelectrolytes 1 and 2, model composites containing the same components were grown on planar Si supports, and the thickness of each successive layer was... 

Interesting information relevant to this point can be obtained from ternary blends of two different diblock copolymers with one corresponding homopolymer. Riess et al. investigated the system PS/Cop 81 1/Cop SI2 where Cop SI- and Cop SI2 are diblock copolymers with Cop Sl-j > Cop Sl2 It was found that the Cop SI2, of higher can act as an emulsifier for Cop Sl-j which forms the dispersed phase of the blend. Multilayered structures ("onion" structures) were easily obtained for these systems. 

Onion structure In styrene-butadiene block copolymer blends with polystyrene, the copolymer forms spherical particles consisting of numerous concentric shells, with PS and PBD alternating 3.32, 3.33... 

Terrones, M., Terrones, G., Terrones, H. (2002). Structure, chirality, and formation of giant icosa-hedral fullerenes and spherical graphitic onions. Structural Chemistry, 13, 373-384. 

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