Oriented aggregate

It is also comparatively straightforward to-calculate P200, P220, P420 and P o for a biaxially oriented aggregate of transversely isotropic units in terms of the principal extension ratios Xx, X2 and (with X,X2 3 = 1). 

A rubber network which is deformed to three independent extension ratios Xt, X2 and X3 takes the form of a biaxially oriented aggregate defined by coefficients P,mo. For example, we have... 

P-chirally oriented aggregates /W-chirally oriented aggregates... 

Red hematite can be converted to purple hematite by increasing the size sufficiently, for example, by heating to >800 °C (von Steinwehr, 1969). There is some evidence that associations of small, platy, hematite crystals into orientated aggregates (e. g. in red beds) also cause a colour shift towards purple (Torrent Schwertmann, 1987). 

In terms of structure, the FF association leads to the build-up of successively larger stacks of partides called oriented aggregates, or tactoids [321-323], The EE and EF assodations produce floes and can lead to voluminous, three-dimensional assemblages, often described as card-house structures. Figure 1.7 provides an illustration of these modes of interaction. Descriptions of the details surrounding the transitions from one structure to another are given by van Olphen [1],... 

Oriented aggregate of surface-active molecules formed in solution when the solubility limit for single molecules (monomers) has been reached. Such aggregates may contain from approximately 5 to 100 molecules. Thus, in water, the hydrophilic portion of the surfactant of molecules is on the outside and this aggregation is named a normal micelle. In organic low-polar solvents, the lipophilic moieties are on the outside and the aggregation is termed an inverse or reverse micelle. 

Kimura, Shirai and coworkers used two chiral dimeric porphyrins 95 and 96 to investigate their self-assembling behavior [162,163]. While incorporation into fibers made of the alkylamide derivatives of (fl,fl)-DACH, 95 formed stable well-resolved fibrous assemblies as visualized by transmission electron microscopy, the fluorescence of which was not quenched by external electron acceptors [162]. However, the induced CD was not detected indicating an inability of 95 to form chirally orientated aggregates under the applied conditions. In contrast, 96 was able to produce optically active inter molecular self-assemblies with an enhanced chiroptical response through the //-oxo bridging in an alkali solution, while intramolecular //-oxo dimer formation was excluded on the basis of steric reasons [163]. 

The texture of unoriented filins of PBLG is interpreted to be composed of randomly oriented anisotropic rods which are again composed of oriented aggregates of the polymer molecules by means of the small-ai e li t scattering 17) and the electron micrt aph 93). 

Figure 10.21 TEM image of oriented aggregation of nanoplatelets of zeolite analcime. The inset (top) is the corresponding SAED pattern. Another inset (bottom) is TEM image of a single nanoplatelet.

Figure 19a illustrates oriented aggregation-based growth involving crystals that are >6 nm to tens of nanometers in diameter. Other data show the same processes involving particles 3 nm in diameter. Such observations (cited above also see Anastasio and... 

ZnS occurs as very small, crystalline nanoparticles and oriented aggregation-based crystal growth involving few nanometer-diameter ZnS nanoparticles occurs. Thus, we postulate that sphalerite nanoparticles form due to the aggregation of sphalerite-like Zri4S6(H20)4 clusters whereas wurtzite nanoparticles form by oriented aggregation of wurtzite-like Zn6S6(H20)9 clusters. These consideration may be relevant to many important minerals that are isostructural with sphalerite and wurtzite. 

Figure 22. Transmission electron microscope image of an single anatase crystal formed by oriented aggregation. Crystal margins are marked with arrows tips a subset of the particles that formed the aggregate are numbered. Dislocation positions are indicated by arrows (view at low angle). The diagram on the right illustrates formation of an edge dislocation at an interface where one particle (black) has a surface step (Perm and Banfield, impublished see Perm and Banfield 1998).

Penn and Banfield (1998b, 1999) and Banfield et al. (2000) demonstrated that twins and structurally distinct slabs are generated when oriented aggregation involves an interface for which coherence can be obtained in more than one orientation. Multiple aggregation events, perhaps in combination with screw dislocation-based growth, can generate complex polytypes and structural intergrowths (Penn and Banfield 1998). 

Figure 15. Aggregated nanoparticles of titania forming a single-crystal chain, an example of oriented aggregation. From Penn and Barrfield (1999) with permission from Elsevier, pubhshers of...

F, Huang, H, Z, Zhang, and J. E. Banfield. The role of oriented attachment crystal growth in hydrothermal coarsening of nanocrystaUine ZnS. J. Phys. Chem. B, 107 10470-10475,2003 R, L, Penn, Kinetics of oriented aggregation. J. Phys. Chem. B, 108 12707-12712, 2004... 

Mitchell. W. A., 1953. Oriented-aggregate specimens of clay for X-ray analysis made by pressure. Clay Mineral. Bull. 2 76-78. 

Williamson, W.O., 1955, Oriented aggregation, differential drying-shrinkage and recovery from deformation of a Kaolinite-Ilhte clay, Transactions of the British Ceramic Society, 54(7), 413-442. 

Oriented Aggregation The crystallographi-cally specific orientation among primary nanocrystals also results in the formation of self-assembled NP chains, which was first explored by Banfield and co-workers. Some concrete examples can be referred to the review on oriented aggregation of NPs [3]. 

Penn RL (2004) Kinetics of oriented aggregation. J Phys Chem B 108 12707-12712... 

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