Structure amorphous

We begin by looking at the smallest scale of controllable structural feature - the way in which the atoms in the metals are packed together to give either a crystalline or a glassy (amorphous) structure. Table 2.2 lists the crystal structures of the pure metals at room temperature. In nearly every case the metal atoms pack into the simple crystal structures of face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) or close-packed hexagonal (c.p.h.). 

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

A surface coating protects the substrate against abrasion, moisture, light, and corrosion. The binder for the pigment and extenders is fluid before application and rigid soon after. Natural binders range from gum arable to fish oil. The first varnishes were solutions of natural resins, having transparency, hardness, amorphous structure, and little permanence. 

Polyetherimides show no crystallinity as evidenced from calorimetry measurements. The heteroarylene like phenylquinoxaline [27], oxadiazole [30], and benzoxa-zole [56] activated polyethers show TgS from DSC thermograms, with no evidence of crystallization, indicating amorphous or glassy morphology. Furthermore, wide angle x-ray scattering measurements show no evidence of crystalline or liquid crystalline type morphologies, consistent with an amorphous structure. F polyether... 

The structure of PP-g-polystyrene was studied by wide and narrow angle x-ray diffractometry. The polystyrene component formed discrete amorphous structures, which were identified after chloromethylation [143]. Structure of Rayon-Styrene graft copolymer was characterized by IR, wide angle x-ray, and SEM, grafting occurred mainly in the amorphous regions of rayon [1441. 

The amorphous structure favours internal oxidation unless a protective oxide film is formed as, for example, under low oxygen partial pressures . 

One of the major drawbacks to many promising copolymers is their unsatisfactory electrochemical stability. Carbonyl groups which feature in many of the back-bone/chain linking groups are likely to cause stability concerns. Likewise, urethane, alcohol, and siloxane functions are sensitive to lithium metal. With this in mind, a recent trend has been to find synthetic routes to amorphous structures with... 

Processing conditions influence the performance of plastics. For example, heating a crystalline material above its melting point, then quenching it can produce a plastic that has a far more amorphous structure. Its properties can be significantly different than if it is cooled properly (slowly) and allowed to recrystallize during processing it becomes amorphous. The effects of time are similar to those of temperature in the sense that any... 

Oxidized regions are uniform in composition and consequently in charge density at every polarization time. Regions of neutral polymer have, as well, a uniform composition. Both oxidized and neutral regions have an amorphous structure. 

Silicate glasses have amorphous structures produced by addition of salts that disrupt the crystalline structure. They can be attacked by strong base and hydrofluoric acid. 

Amorphous Silicon (a-Si). Amorphous silicon is considered a promising new material.As mentioned above, only a very thin coating is necessary, since the amorphous structure is much better at absorbing sunlight than is the crystalline material. The most common process to produce a-Si is the decomposition of silane by plasma CVD (see Ch. 8). Thicknesses of a few micrometers can be deposited and,... 

X-ray diffraction patterns from dendrimers tend to lack sharp features and are similar to those from amorphous linear polymers. This suggests a molecular arrangement in dendrimers that is fairly disordered. There is also the problem that these molecules can exist in a large number of energetically equivalent conformations and that in solution there can be rapid interchange between these conformations. This contributes to the overall amorphous structure of dendrimers. 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

Since the diameters of both kinds of particles are about 200 mn, the thickness of a 10-layer assembly should be about 1.8 p,m. However, the cross section of a sample subjected to 10-fold dipping indicates a thickness of only 1.2 jim (Fig. 21). Similar observations were made by others [94,97] and can be ascribed to the low substrate coverage reached in each of the dipping steps. Serizawa et al. [97] used the second approach and determined the adsorption using QCM and SEM. An SEM image after two adsorption steps indicated a very irregular, amorphous structure, in agreement with Fulda and coworkers [93]. 

Further dehydration of boehmite at 600 0 produces y-alumina, whose spectrum is shown in Figure 3b. There is a loss in surface area in going from boehmite to y-alumina. The sample shown here has a surface area of 234 m /g (this sample was obtained from Harshaw A23945 the calcined Kaiser substrate gave an identical infrared spectrum). The y-alumina sample shows two major differences from o-alumina. First, there is a more intense broad absorption band at 3400 cm" due to adsorbed water on the y-alumina. Second, the y-alumina does not show splitting of the phonon bands between 400 and 500 cm" as was observed for o-alumina. The y-alumina is a more amorphous structure and has much smaller crystallites so the phonon band is broader. The y-alumina also shows three features at 1648, 1516 and 1392 cm" due to adsorbed water and carbonate. 

In addition to Au and noble metals, Ni-Zn nanoclusters with an amorphous structure were successfully deposited on Ti02 nanoclusters. The state of Ni was metallic. The catalytic activity of Ni-Zn/Ti02 in olefin hydrogenation was ca. 10 times higher than unsupported Ni nanoclusters. Selective deposition onto Ti02 and the addition of Zn seemed to play an important role to stabilize Ni nanoclusters and to decrease the size of Ni nanoclusters, respectively. Also, clearly Zn promoted the hydrogenation activity of Ni and inhibit the growth of the size, but did not substantially affect Ni nature itself... 

Differences in the absorption spectra of colloidal and macrocrystalline semiconductors were first recognized for CdS and AgBr The absorption of 3 nm particles of CdS in aqueous solution begins close to 515 nm, the wavelength at which bulk CdS starts to absorb however, the increase in absorption at shorter wavelengths is much less steep than for the macrocrystalline material (Fig. 6). The effect was first explained by a possible amorphous structure of the colloidal particles However, after it was shown by Brus and co-workers that the particles had an ordered struc-... 

A joint research project between Lummus Technology and the Delft University of Technology led to the discovery of a new mesoporous material, named TUD-1 (8). TUD-1 is a three-dimensional amorphous structure of random, intercoimecting pores. The original emphasis was on the silica version, which has since been extended to about 20 chemical variants (e.g., Al, Al-Si, Ti-Si, etc.). 

There is great interest in the development of methods that allow the identification of a reasonably good structure with which to start the simulation of dense atomistically detailed polymer systems. The problem of generating dense polymer systems is formidable due to the high density and the connectivity of polymer systems. For crystal structures this can be systematically achieved [33,34] for amorphous structures, however, there is no generally satisfactory method available. Two recent developments in methods for generating amorphous packing (Santos, Suter) are reviewed in Section 3. 

We now ask how well does the bond fluctuation model with these bond lengths and bond angle potentials reproduce the properties of real polymer melts quantitatively. First of all, it must be admitted that the model yields a qualitatively reasonable picture of the amorphous structure, as exemplified by... 

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