Structure onion-type

Figure 21.2 Assembly of mesosfructured hybrids, (a) Nanoparticles smaller than the darker gray block s R0 are miscible and assemble into a lamellar structure, (b) Nanoparticles larger than the darker gray block s R0 segregate, forming a nanoparticle-rich core around which lamellae assemble into an onion-type structure, (c) This can be used to generate compositionally heterogeneous nanostructures from tailored nanoparticle size distributions.10 (Reprinted with permission from S. C. Warren et al., Nature Mater. 2007, 6, 156-161. Copyright 2007 Macmillan Publishers Ltd.)...

Figure 21.4 Influence of nanoparticle size on silica-type mesostructuie. (a, b) TEM micrographs showing mesostructuies with a nanoparticle volume fraction

The preformed Pt core may be regarded as a living-metal polymer on which the Pd atoms are deposited to give onion-type bimetallic nanoparticles (5 nm), the structure of which has been characterized by a combination of analytical methods. 

FIGURE 11 Soot structure as (a) produced in the laboratory (Sergides et al, 1987), forming (b) basic structural units of 3-4 layers (Heidenreich et al, 1968), (c) randomly oriented basic structural units shown as a 2-dimensional schematic diagram, (d) onion-type particle with several condensation seeds (Heidenreich etal, 1968). 

FIGURE 3 Cross section SEM images of the lamellar (onion-type) structure formed in the mechanical alloying process of the Ti-15Zr-4Nb alloy particles (a, b - different magnifications)... 

The product is derived from a purified silicate liquor and has a much lower level of impurities than products made by the direct fusion route. By virtue of the granulating process, the spray-coated product has essentially an onion-type-layered structure and a superior particle size distribution and is relatively dust free and more readily soluble in water [1]. 

A slightly different approach to the formation of onion type micelles is that described by Prochazka etal. [275], Such structured micelles could be obtained by starting with PtBA-P2 VP... 

The advantages in tuning many physical and chemical properties using a bimetallic combination has triggered special interest in the synthesis and stabilization of bimetallic particles over monometallic particles. Here, bimetal refers to particles containing two different kinds of metals, which has either a core-shell or an alloy structure and the kind of structure is decided by the method of preparation. Bimetals can be prepared by either physical or chemical routes. Physical routes mainly consist of vapor deposition of one metal on top of the other, whereas chemical bonds involve simultaneous reduction of two metal ions or reduction of one after another in presence of a suitable stabilizer [238]. Additionally, bimetals generate properties that are different from monometallic components. After preparation of the desired colloid, the microdomains can be reloaded with precursor materials, which can subsequently be reacted to obtain intermetallic nanocolloids, sometimes in the form of onion-type clusters. 

Let us note in addition that the layered sulfides M0S2 and WS2 have been found to form nanotubes and other fullerene-type structures, on account of their highly folded and distorted nature that favors the formation of rag and tubular structures. Such materials have been synthesized by a variety of methods [78] and exhibit morphologies, which were described as inorganic fiillerenes (IF), single sheets, folded sheets, nanocrystals, and nested IFs (also known as onion crystals or Russian dolls ). 

Micelles of type (1) were the first investigated examples of ABC triblock copolymer micelles. These micelles are generally characterized by the so-called onion, three-layer, or core-shell-corona structures, i.e., the first insoluble A block forms the micellar core, the second insoluble B block is wrapped around the core, and the third soluble C block extends in the solution to form the micellar corona (Fig. 18). To the best of our knowledge, there are no known examples of ABC block copolymer micelles with A and C insoluble blocks and a B soluble block. 

As introduced previously, type 2 ABC triblock copolymer micelles are formed by triblock copolymers containing an insoluble A block while the B and C blocks are soluble in the considered solvent. The insoluble blocks can be located either between the two soluble blocks (BAC structure) or at one end of the triblock (ABC or ACB structures). Micelles of the latter type were discussed above for, e.g., PS-P2VP-PEO pH-responsive micelles and are indeed considered as core-shell-corona, onion, or three-layer structures since the heterogeneity in the micellar corona is observed in the radial direction (Fig. 18). Micelles formed by BAC triblock copolymers are different from the previous case because they can give rise in principle to a heterogenous corona in the lateral dimension (Fig. 18). This could induce the formation of noncentrosymmetric micelles as discussed in Sect. 7.3. 

Many nanomaterials can be made in different forms. We are familiar with the example of carbon, which we can find as diamond films, carbon black, fullerenes, and multi- and single-walled nanotubes. M0S2 can be made as nanotubes, onions (multi-walled fullerene-type structures), and thin films. 

Nested fullerene-type structures with other inorganic layered materials have been investigated recently. Tenne et al (1992) have shown that MoSj is formed in a nested (onion-like) structure when MoOj is reduced by HjS in the gas phase at 1070-1220K (Feldman et al, 1995). Similar results have been obtained with WSj. The idea seems to be more general, being applicable to other inorganic layered structures, as... 

Sketches of the interiors of chondritic asteroids, illustrating (a) onion shell and (b) rubble pile structures. The numbers and shadings identify petrologic types. 

Cooling rates for H chondrites of different petrologic types can be estimated from the blocking temperatures for diffusion in various radioisotope systems. The age measured by each radioisotope indicates the time the meteorite passed through that blocking temperature. Slower cooling rates for more highly metamorphosed H5 and H6 chondrites are evidence that the parent body had an onion shell structure. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

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