Y-Shaped Materials

The terms Y -shaped [109], tripodal , 1,1-double-tailed or peg-shaped [110] describe the molecular geometry of this type of amphiphile. At the hydrophilic head two hydrophobic tails are joined close together or even by a common link to the headgroup. Very famous examples for this molecular architecture are the biologically active phospholipids and sphingolipids [111]. Lipids occur in all biological cells and have common solubility properties generally they are water-insoluble, amphiphilic molecules 

The degree of unsaturation of their fatty acyl chains and the chemically nature of their headgroups, the latter being responsible for the ability and strength of the hydrogen bond formation both between the lipid as well as the lipid and water molecules, do play a major role for the ampho-tropic behavior of such lipids. 

In water, these compounds spontaneously aggregate to bilayers in which the hydrophilic headgroups point outwards at each side whereas the lipophilic tails form the hydrophobic middle part of the layer. These bilayers may also form more complex aggregates, such as micelles, vesicles or lyome- 

X-ray investigations of especially the L phase of galactopyranosyl- and glucopyran-osyl-glycerols reveal that not simply the size or orientation of the hydrophilic head-groups regulate the supramolecular packing and, thus, the structure of the mesophases, but rather a combination of these supported interactions of their alkyl chains are responsible in this matter [113]. 

A discussion of the thermotropic properties of this type of compounds began in 1983 [119] with naturally occurring lipids. Again, the heterogeneity of the materials made syntheses of uniform derivatives desirable [114a]. Pure l,2-di-0-acyl-3-C)-(a-D-glu-copyranosyl)-5n-glycerols (32) were investigated in view of their thermotropic properties. The thermotropic mesophase of the members of series 32 has been denoted as 

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Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

Fig. 8.25 R-curves were normalized to the peak toughness to clearly show the change in shape with sintering additive. Values of Kq were deduced for two samples (La and Lu doped) from measured crack-opening profiles in R-curve samples, while Kq for Y-doped material was taken from Kruzic et al. [35] Also shown are typical data for a Y2O3 doped Si3N4 that exhibits transgranular fracture. With kind permission of John Wiley and Sons...

Recent developments of plastic stents have aimed to improve the resistance of plastic stents to external compression forces. Therefore, metal has been incorporated into the plastic material of the stent. One of the latest developments is the dynamic bifurcation stent made of silicone (Freitag et al. 1994). This Dynamic stent (Riisch, Kernen, Germany) is reinforced with horseshoe-shaped steel struts. A posteriorly located flexible membrane allows dynamic compression of the stent during coughing, whereas the steel struts prevent airway compression from external forces. Theoretically, this stent mimics the mechanical dynamics of the normal trachea. The distal end is a Y shape which rides on the carina to prevent distal migration. 

A DCP is formed when a gas (usually argon) is introduced into a high current flowing between two or three electrodes. Ionization of the gas produces a Y-shaped plasma. Unfortunately, early DCP instrumentation was prone to interference effects and also had some usability and reliability problems. For these reasons, the technique never became widely accepted by the analytical community. However, its one major benefit was that it could aspirate high dissolved or suspended solids because there was no restrictive sample injector for the solid material to block. This feature alone made it very attractive for some laboratories, and once the initial limitations of DCPs were better understood, the technique became more accepted. In fact, a DCP excitation source coupled to an optical emission instrument today, using an Echelle-based grating and a solid-state detector, has been commercially available for a number of years. ... 

Chan et al. (2005), have realised micro fuel cells through an approach that combines thin film materials with MEMS (micro-electro-mechanical system) technology. The membrane electrode assembly was embedded in a polymeric substrate (PMMA) which was micromachined through laser ablation to form gas flow channels. The micro gas channels were sputtered with gold to serve as current collectors. This cell utilized the water generated by the reaction for the humidification of dry reactants (H2 and O2). The peak power density achieved was 315 mW cm (901 mA cm" at 0.35 V) for the H2-O2 system with 20 ml min" O2 supply and H2 at 10 psi in dead ended mode of operation. A Y shaped microfluidic channel is depicted in Fig. 21. 

Huang, C.-Y. and Sheen, S.R., (1997) Synthesis of nanocrystalline and monodispersed copper particles of uniform spherical shape. Materials Letters, 30, 357-61. 

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